Journal of Cranio-Maxillo-Facial Surgery xxx (2015) 1e8
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Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study € lzle a, T. Steiner a T. Torsiglieri a, *, S. Raith a, b, A. Rau c, H. Deppe c, F. Ho €tsklinikum Aachen, RWTH Aachen, Germany Department of Oral and Maxillofacial Surgery, Universita €tsklinikum Aachen, RWTH Aachen, Germany Dental Materials and Biomaterials Research, Universita c €t München, Germany Department of Oral and Maxillofacial Surgery, Klinikum rechts der Isar, Technische Universita a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Paper received 10 July 2014 Accepted 2 March 2015 Available online xxx
Objectives: Fractures of the atrophic edentulous mandible are a rare complication that can become severe after the insertion of dental implants. This in vitro study investigated the effects of different implant settings varying in number, diameter, and length. and the influence of a fixed bar. Materials and methods: In biomechanical experiments on artificial mandibles, an unmodified reference group, four implant settings with two different implants, and the effect of adding a fixed bar to these settings were tested. All specimens were loaded with incisal biting forces until failure due to fracture. Results: Implants weakened all specimens significantly compared with those in the reference group. Without a fixed bar, four short and thick implants showed the best results, with high significance. With a fixed bar, four long and thin implants withstood the highest loads. The addition of fixed bars reduced the differences between the implant settings. Fixed bars did not show increased stability for all groups; however, these groups showed a higher mean strength. Conclusions: Four implants with a short and thick design should be the first choice when implants are placed without a fixed bar in an atrophic mandible. With a fixed bar, four long and thin implants should be used. © 2015 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.
Keywords: Dental implant Mandibular fracture Mandibular atrophy
1. Introduction Bone remodeling of the alveolar crest is a lifelong process. In edentulous jaws, the lack of physiological stress on the bone induces bone resorption. Pressure exerted on the bone by a conventional prosthesis and the duration of edentulism are factors that can accelerate this process. Implant-supported prostheses in edentulous mandibles have led to substantially reduced bone loss in comparison with those in conventional denture-wearing jaws (Carlsson, 2004). The remodeling is subject to high individual variations and is not sufficiently understood (Carlsson, 2004). Several studies have even detected bone apposition in patients treated with these prostheses (Davis et al., 1999; Reddy et al., 2002; von Wowern and Gotfredsen, 2001; Wright et al., 2002). In 1998, Fontijn et al. reported that patients with mandibular implanteretained overdentures had
* Corresponding author. Mauerstrasse 26, 52064 Aachen, Germany. Tel.: þ49 1755864986. E-mail address: [email protected] (T. Torsiglieri).
significantly higher maximum bite forces than conventional complete-denture wearers. In 2006, Fueki et al. reported, in their literature review covering more than 50 years, objective benefits in the masticatory performance of implant-supported overdentures compared with conventional dentures in edentulous patients with resorbed mandibles (Fueki et al., 2007). These overdentures appeared to yield higher patient satisfaction scores, even with patients who had undergone preprosthetic surgery (Sadowsky, 2001). In 2000, Awad et al. observed, in a randomized clinical trial, that treatment with implant-supported overdentures was associated with a significantly better quality of life. The additional costs of implant insertion seem to be supported by the patients, because up to 77% of conventional denture wearers were willing to pay even three times more than the current cost of conventional dentures for implant-retained prostheses (Esfandiari et al., 2009). Although the insertion of dental implants has become a standard treatment in recent years, the treatment of the severely atrophic mandible remains challenging. Due to the weakened mandibular bone lacking the alveolar crest, the insertion of
http://dx.doi.org/10.1016/j.jcms.2015.03.001 1010-5182/© 2015 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Torsiglieri T, et al., Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study, Journal of Cranio-Maxillo-Facial Surgery (2015), http://dx.doi.org/10.1016/j.jcms.2015.03.001
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T. Torsiglieri et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2015) 1e8
implants decreases the stability of the bone and may lead to fractures either during or shortly after implant insertion. Numerous case reports have indicated the risk of mandibular fractures during cavity preparation or implant insertion (Almasri and El-Hakim, 2012; Fontijn-Tekampl et al., 1998; Karlis et al., 2003; Luhr et al., 1996; Oh et al., 2010; Raghoebar et al., 2000). A fracture of the mandible is the most feared of all complications related to endosseous implants in the extremely resorbed mandible. Mandibular fractures after implant placement are a rare complication in conjunction with severely resorbed mandibles (Goodacre et al., 1999; Raghoebar et al., 2000; Soehardi et al., 2011). However, several studies have pointed out that fractures of the atrophic mandible have severe consequences for the patient; therapy remains challenging and thus should be avoided by all means (Ellis and Price, 2008; Madsen et al., 2009, 2011; Melo et al., 2011; Soehardi et al., 2011; Wittwer et al., 2006). Atrophic mandibular fractures are often treated with an open reduction and internal fixation technique using a reconstruction plate via a submandibular transcutaneous approach (Soehardi et al., 2011; Toma et al., 2003). In the literature, complications such as non-union, osteomyelitis, and infection occur in up to 48% of patients with fractures after the seating of implants in atrophic mandibles (Soehardi et al., 2011). The best treatment for implant-related fracture of the mandible is prevention. There are surgical precautions and guidelines to minimize the weakening of the bone and risk of fracture, such stepwise drilling, sufficient irrigation, and avoiding the inferior cortex, among others (Mason et al., 1990). The site of an implant that has not yet been osseointegrated is characterized by tensile stress concentration and weakness. Repeated submaximal functional forces in such an area of bony weakness may lead to a spontaneous fracture without associated trauma (Mason et al., 1990). The use of immediately loaded implants in the anterior mandible for overdenture design is a promising treatment concept and has shown success rates of 95.6e100% (Chiapasco and Gatti, 2003; Payne et al., 2002). However, atrophic mandibles require special attention. The above mentioned negative consequences indicate that the weakening of the bone should be minimized. Different implant settings are proposed for the treatment of the atrophic mandible. In the moderately resorbed edentulous mandible, fabrication of an overdenture for two or four interforaminal implants is currently an accepted, widespread treatment modality for improving the function of a mandibular prosthesis (Batenburg et al., 1998; Thomason et al., 2012). Although there are many studies that have focused on the topic of implant-supported overdentures in the edentulous mandible, statements relating to the impacts of implant design, number, diameter, and length on the weakening of the atrophic bone are rare and not based on biomechanical evidence. No large prospective studies have been performed; most publications are retrospective single-case studies. To address the open clinical question regarding which implants place the least strain on the atrophic mandible, we performed biomechanical experiments on a self-developed test bench. This allowed for the simulation of physiological loading of artificial, atrophic mandibular models in the laboratory. Prior to these experiments, no commercially available atrophic mandibular model was on the market. Therefore, our working group developed artificial mandibles derived from computed tomographic data. To our knowledge, no biomechanical data have been published on the stability of edentulous, atrophic mandibles. Previous studies have examined the effect of implant cavity preparation on the stability of the jaw. The results have shown that the number and dimensions of dental implant cavities have a significant impact on mechanical stability (Steiner et al., 2015). We hypothesized that different implant settings could have a significant influence on the weakening of the jaw.
The aim of this in vitro study was to investigate the effects of implant settings, differing in number, diameter, and length, on the stability of the jaw. Furthermore, we investigated whether the use of a fixed bar would deliver a favorable outcome. 2. Material and methods 2.1. Experimental setting The experimental testing device used was the same as that introduced by Steiner et al. (2015) for experiments on atrophic mandibular models weakened by implant cavities but without inserted implants. The incisal biting forces were modeled with a rope, pulled by a controlled testing device (Zwick i-line 5 kN, Zwick Messtechnik, Ulm, Germany). Accordingly, the masticatory muscle forces were modeled by ropes acting at the mandibular angles to represent the physiological actions of the pterygomasseteric sling (Fig. 1). A specially developed platform was used to perform the tests with repeatable accuracy. The temporomandibular joints were modeled through bearings made of concavely lathed spherical boxes to represent the anatomical shape of the temporomandibular fossae (Steiner et al., 2015). This load setting has proved its applicability in various biomechanical studies for investigations on the stability of mandibular reconstructions with autologous bone grafts (Grohmann et al., 2013; Steiner et al., 2012; Trainotti et al., 2014). In our tests, force was applied continuously until the test bodies failed due to fracture. Incisal load and incisal movement were recorded by the Zwick device at a sampling rate of 4 Hz. 2.2. Artificial mandibular specimens Standardized conditions are a crucial requirement for in vitro biomechanical experiments. High interindividual differences in atrophy, bone quality, and morphology in human mandibles would make the use of cadaver specimens nearly impossible for testing, because the statistical power is reduced by these overlaid scattering factors. However, prior to these experiments, no commercially available atrophic mandibular model was on the market. Therefore, our working group developed artificial mandibles. These artificial, biomimetic mandibular specimens (Synbone #8570, Synbone, Malans, Switzerland) were fabricated with two individualized polyurethane foam materials. The density of the foam was adjusted to mimic the biomechanical behavior of the cortical and spongiose part of the mandibular bone, respectively. The geometry of these specimens was designed according to a mean shape based on 27 atrophic mandibles, derived with an algorithm that has previously been published (Steiner et al., 2015). 2.3. Specimen preparation and implantation procedure All implants, abutments, fixed bars, and tools were used as recommended by the manufacturer. The implant cavities were prepared under standardized conditions in a box column drill with a tapping machine (Steiner et al., 2015). This was executed stepwise with steel drills of different diameters (800e200 rpm), and tabs were cut (15 rpm) by one experienced implantologist, according to the standardized procedure described by Steiner et al. (2015). Straumann (Straumann AG, Basel, Switzerland) tools were used for all steps of preparation. To ensure a very high level of standardization, all specimens were drilled in parallel by means of the same drilling rig and templates in each group (Fig. 2). The implants were placed with Straumann tools in all test bodies. All implants were placed interforaminally equidistant from each other, with equal bone remaining to the labial and lingual borders. The distance
Please cite this article in press as: Torsiglieri T, et al., Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study, Journal of Cranio-Maxillo-Facial Surgery (2015), http://dx.doi.org/10.1016/j.jcms.2015.03.001
T. Torsiglieri et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2015) 1e8
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Fig. 1. (a) Testing device with mandible and ropes used in the study (front view). (b) Testing device with mandible and ropes used in the study (side view). (c) Mechanical principle of load application with mandible mounted in the test rig and ropes to simulate incisal-chewing forces. Red arrows show simulated pterygomasseteric sling.
Fig. 2. Fixed drilling rig and templates used in the dental laboratory. (a) Template for four-implant groups. (b) Template for two-implant groups.
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T. Torsiglieri et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2015) 1e8 Table 1 Straumann implants and parts suitable for fabricating and connecting the fixed bars. Item
In the following named as
Item no.
Bone Level Implant, Ø 4.8 mm RC, SLActive 8 mm, TiZr Bone Level Implant, Ø 3.3 mm NC, SLActive 14 mm, TiZr NC Multi-Base abutment straight, D4.5 mm, GH 4 mm RC Multi-Base abutment straight, D6.5 mm, GH 4 mm NC Burn-out coping for bridge, Multi-Base, D4.5 mm, Pom RC Burn-out coping for bridge, Multi-Base, D6.5 mm, Pom
Short and thick Long and thin
021.6308 021.2314 022.2744 022.4764 023.2744 023.4764
between the foramina was divided equally, and the holes were drilled at the same distance from each other. Two different types of Straumann bone level implants were used (Table 1). To hold the fixed bar in place, corresponding abutments were fixed to the implants with a torque of 35 Ncm, with 15 Ncm to hold the fixed bar as recommended by the manufacturer. One dental technician manufactured all fixed bars manually by using an individually waxed-up model. The cobaltechromium alloy used was Girobond nb (Amann Girrbach AG, Koblach, Austria). 2.4. Mandibular groups investigated Ninety specimens divided into nine different groups were examined in the scope of the present study, with 10 specimens per group. A reference group with unaltered mandibles and two different placements of the implant, each in combination with the two different implant types that were used, yielded the four different implant settings that were investigated (Fig. 3aed, Table 2, Fig 5). Furthermore, all groups were tested with and without a fixed bar (Fig. 4aec).
modify these threshold values accordingly. The sample size of 10 specimens per group is based on a 5% level of significance and a power of 80%, with the resulting minimum sample size of seven specimens per group. 3. Results 3.1. Reference group The reference group withstood, in mean, 729.5 N ± 59.9 N incisal loading until failure due to fracture. Four specimens broke at the condyle, one at the angle, and five in the interforaminal area of the jaw. 3.2. Groups with implants All test bodies broke in the region where implants were placed and were highly significantly debilitated compared with those in the reference group. All p values were under the threshold of high significance according to BHC.
2.5. Statistical evaluations
3.3. Groups without fixed bar
The recorded experimental failure loads were compared by the WilcoxoneManneWhitney U-test to quantify the differences, with values of p 0.05 and p < 0.001 as threshold levels for statistical significance and high significance, respectively. For multiple comparisons, the BonferronieHolm correction (BHC) was applied to
This group showed a larger span of results (mean, 509.3 Ne267.5 N) than the group with a fixed bar (mean, 479.1 Ne413.5 N). Groups with short and thick implants showed a significantly higher resistance to loading than those with long and thin implants.
Fig. 3. Ten specimens per group. (a) Two short and thick implants. (b) Four short and thick implants. (c) Two long and thin implants. (d) Four long and thin implants.
Please cite this article in press as: Torsiglieri T, et al., Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study, Journal of Cranio-Maxillo-Facial Surgery (2015), http://dx.doi.org/10.1016/j.jcms.2015.03.001
T. Torsiglieri et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2015) 1e8 Table 2 Mean failure loads due to fracture and standard deviations of the tested groups. Group Specimens, implant characteristics no.
Breaking load; Standard mean, N deviation, N
X 1 2 3 4 1a 2a 3a 4a
729.5 445.6 369.8 509.3 267.5 429.0 413.5 449.5 479.1
Reference 2 Implants 2 Implants 4 Implants 4 Implants 2 Implants 2 Implants 4 Implants 4 Implants
short and thick long and thin short and thick long and thin short and thick with fixed-bar long and thin with fixed-bar short and thick with fixed-bar long and thin with fixed-bar
±59.9 ±39.4 ±53.6 ±38.7 ±18.0 ±92.9 ±46.5 ±46.7 ±36.4
3.3.1. Four implants Four long and thin implants showed the lowest fracture loads (mean, 267.5N ± 18.0 N) with significant differences compared with all specimens with implants (significant to all other groups without fixed bar, according to BHC). In contrast, the group with four short and thick implants showed the highest failure loads of all specimens after implant insertion (mean, 509.3N ± 38.7 N). Data for this group were highly significantly better (with consideration of BHC) than those from all other groups except the group with four long and thin implants connected with a bar, which was the strongest group within its group of fixed-bareconnected implants. 3.3.2. Two implants The specimens with two short and thick implants showed mean failure loads of 445.6 N ± 39.4 N and were highly significantly stronger (p < 0.001) than those with two long and thin implants (mean, 369.8N ± 53.6 N).
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3.4. Groups with fixed bar None of the specimens showed failure of the implant structure, either at the connection between the abutment and the implant or between the abutment and the fixed bar. Four long and thin implants showed the highest values of fracture loads of this group (mean, 479.1 N ± 36.4 N), followed by mandibles with four short and thick implants (mean, 449.5 N ± 46.7 N); however, this difference is not significant with consideration of BHC (p ¼ 0.04, compared to a BHC threshold of 0.017). In the group of two implants connected with a fixed bar, short and thick implants (mean, 429.0 N ± 92.9 N) showed slightly, but not significantly, better results than long and thin implants (mean, 413.5 N ± 46.5 N) (p ¼ 0.2). The only significant difference between either group of the fixed bar specimens was the comparison between two long and thin implants that were inferior with respect to four long and thin implants (p ¼ 0.0014). 3.5. Comparison of groups without and with a fixed bar The group with fixed bars showed a smaller span of results (mean, 479.1 Ne413.5 N) than the group without fixed bars (mean, 509.3 Ne267.5 N). The respective strongest specimens of each group did not show significantly differing results for failure loads (p ¼ 0.08), i.e., four short and thick implants without fixed bars and four long and thin implants with fixed bars. Interestingly, the strongest group without a fixed bar was significantly more resistant to incisal force than the same implant setting connected with a fixed bar (p ¼ 0.003), i.e., four short and thick implants. The strongest setting with a fixed bar
Fig. 4. Manually manufactured fixed bars used in the study. (a) Fixed bar mounted on two implants inserted into specimen. (b) Fixed bar mounted on four implants inserted into specimen. (c, Left) Two long and thin implants with fixed bar attached. (c, Right) Two short and thick implants with fixed bar attached.
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Fig. 5. Results of the different groups investigated for failure load due to fracture.
was yet to the weakest setting without a fixed bar, i.e., four long and thin implants. The union of implants significantly reinforced particular implant settings, namely, four long and thin implants. 4. Discussion Edentulous patients treated with conventional overdentures suffer from insufficient retention, severely reduced biting forces, intolerance to loading by the mucosa, pain, difficulty with eating and speech, loss of soft-tissue support, altered facial appearance, and a reduced quality of life (Awad et al., 2000; Fontijn-Tekampl et al., 1998; Sadowsky, 2001; Stellingsma et al., 2004). These factors are improved with the aid of implant-retained overdentures, and the application of endosseous implants has changed treatment concepts enormously. Therefore, dental implantology is currently a valuable treatment modality for the edentulous patient (Stellingsma et al., 2004), because it can reduce bone loss and may even stimulate the formation of new bone (Carlsson, 2004; Davis et al., 1999; Reddy et al., 2002; von Wowern and Gotfredsen, 2001; Wright et al., 2002). Many prosthodontic and surgical treatments have been attempted in cases of severe residual ridge resorption, but none have been completely predictable (Carlsson, 2004). Different treatment recommendations for implant-retained or -supported overdentures of the lower jaw have been published (Ferreira et al., 2010; Sadowsky, 2001; Stellingsma et al., 2004). In 2012, Atieh et al. found, in a systematic review, that there is no significant difference in survival rates between short and long implants if immediate loading is avoided (Atieh et al., 2012). In 2002, the McGill consensus statement suggested that treatment with two implants to retain the overdenture should become the treatment of first choice for most edentulous patients, taking into account performance, patient satisfaction, cost, and clinical time (Liu et al., 2013; Thomason et al., 2012). In 2006, Fitzpatrick stated that there is no evidence for a single, universally superior treatment modality for the edentulous mandible (Fitzpatrick, 2006). Because
treating the edentulous, highly atrophic mandible is challenging, and because concepts have thus far been evaluated only in retrospective clinical studies and case reports (Ellis and Price, 2008; Madsen et al., 2009, 2011; Melo et al., 2011; Wittwer et al., 2006), analysis of the data generated by this study could help to improve planning and treatment options. The data on fractures of the atrophic mandible as a complication during or shortly after the insertion of interforaminal implants are limited, and many cases have probably been unreported (Lamas Pelayo et al., 2008; Raghoebar et al., 2000). Nevertheless, this is a severe and serious complication. In 2011, Soehardi et al. reported that the main reason for fractures is that bone height is too low or that the dimensions of the jawbone are too narrow (63%). Iatrogenic factors are mentioned in 18% of the reported cases (Soehardi et al., 2011). Treatment of fractures of highly atrophic mandibles is challenging, with possible associated complications, and different treatment options have been published (Almasri and El-Hakim, 2012; Raghoebar et al., 2000; Stellingsma et al., 2004). Several authors agree that the best treatment is prevention (Almasri and ElHakim, 2012; Mason et al., 1990). To achieve this goal, a sophisticated treatment concept with a predictable classification of fracture risks would be very useful. Three-dimensional radiological imaging should be part of clinical treatment planning. The time of fracture is not uniform, because there are two biomechanical considerations: either when implants are placed, or when bone is weakened and breaks secondarily when loaded by chewing forces. The risk of this severe complication can be minimized by careful consideration of certain aspects of treatment such as correct presurgical planning, the use of adequate surgical techniques, postsurgical follow-up, respect for the osseointegration period, appropriate design of the superstructure, the study and correct distribution of occlusal loads, and meticulous hygiene during the maintenance phase (Lamas Pelayo et al., 2008). When implants are placed in an atrophic mandible, periodic follow-ups, including clinical and radiographic examinations, become even more necessary to identify complications such as osteomyelitis or periimplantitis at an early stage, as well as to instruct the patient on how to avoid occlusal overloading during osseointegration (Lamas Pelayo et al., 2008). However, new concepts regarding immediate loading in patients with severely atrophic mandibles, albeit with promising results, have not yet been published (Chiapasco and Gatti, 2003; Ferreira et al., 2010). In 1990, Mason et al. had already published surgical tips to minimize the weakening of bone when dental implants are placed in atrophic mandibles. They recommended stepwise drilling, sufficient irrigation, primary bone grafting in the atrophic mandible, avoiding inferior cortex engagement if possible, fixture tap, avoiding force when the implant is seated (by use of a torque force indicator), and a postoperative soft diet. Some authors have reported that the placement of wide-diameter implants in conjunction with bicortical penetration in a severely atrophic edentulous mandible could increase the risk of mandibular fracture (Oh et al., 2010; Schug et al., 1999). The coherence of implant diameter and design and the degree of atrophy are vital in countering the risk of fracture. One should also bear in mind that, in atrophic mandibles, bone quality is not consistent throughout the bone, and functional demands are highly variable (Fitzpatrick, 2006; Lamas Pelayo et al., 2008). This leads to unequal implant sites and therefore different degrees of primary implant stability. We examined the influence of implant settings differing in number, diameter, and length, as well as the influence of a fixed bar in combination with these settings. All implants were placed interforaminally and equidistantly, with equal quantities of bone up
Please cite this article in press as: Torsiglieri T, et al., Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study, Journal of Cranio-Maxillo-Facial Surgery (2015), http://dx.doi.org/10.1016/j.jcms.2015.03.001
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to the labial and lingual borders. The implants were chosen with differing lengths and diameters to establish a more comparable contact surface between the implant and the bone. Since a longer implant weakens the bone foreseeably more, a smaller-diameter implant leaves more bone to the labial and lingual borders. This should partially equalize this difference. Implants with a length of 14 mm are marginally long in atrophic mandibles. This was chosen deliberately to determine the quantitative difference between a rather short and a rather long implant. Interestingly, the differences were equalized as the implants were connected by a fixed bar, despite the marginal length. We investigated patients with an atrophic, edentulous mandible who were treated elsewhere with dental implants. Some patients received marginally long implants and showed complications most likely due to this parameter. Thus, we wanted to investigate and quantify the effect of marginally long implants. The number of implants did not show a uniform tendency. Four implants shoed the highest as well as the lowest failure loads. Mandibles with four short and thick implants demonstrated the highest fracture loads of all settings. Interestingly, this group was significantly more resistant to incisal force than was the same implant setting connected with a fixed bar. The strongest setting with a fixed bar was coeval the weakest setting without a fixed bar, i.e., four long and thin implants. It can be concluded that both numbers and positions of implants have a mechanical impact on the mandible and should not be considered separately. The fact that specimens with four implants were, in part, significantly stronger than specimens with two implants can be explained by the locations of the cavities. Two implants appeared to be placed at positions with higher tension on the bone. The union of implants significantly reinforces particular implant settings, namely, four long and thin implants. Thus, distinct weakening of this configuration without the fixed bar could be compensated for by the equal load distribution provided by the fixed bar. All specimens were highly significantly debilitated (p < 0.001) compared with the reference group and broke in the region where implants were placed. This confirms the assumption that drilling holes and inserting implants significantly weakens the bone. None of the specimens showed failure of the implant structure, either at the connection between the abutment and the implant or between the abutment and the fixed bar. Thus, it is evident that bone is the weakest link in biomechanical experiments, confirming reports in the literature (Sadowsky, 2001; Stellingsma et al., 2004). Together, these results indicate that a fixed-bar design changed the mechanical conditions and particularly the load distribution within the mandible. The finding that it decreased the differences among implant settings may be attributed to the more uniform load distribution caused by the relatively stiff fixed bar. The fixed bar balanced the differences among settings and provided a mechanical situation that decreased the influence of implant-related parameters such as number of implants and their position, length, and diameter compared with parameters in settings without a fixed bar. The biomechanical study design was executed in artificial mandibular models. Because these do not feature real bone, and because the loading scenarios in the experimental device are artificial, the results of the presented experiments may differ from those of clinical applications in patients. Further in vivo tests are necessary to examine whether the findings of this in vitro study also apply to clinical observations. However, because the results differed significantly among the different groups and because of the high degree of standardization in the test settings resulting from the use of geometrically identical
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specimens, the study findings should have an impact on clinical treatment recommendations. Nevertheless, the next step should be experiments using cadaver mandibles in a similar setting, as well as clinical trials. This test, however, has the disadvantage that each body is accessible only once. Due to the high variability in shapes and properties of the mandible, the degrees of atrophy, and the interindividual differences in the mechanical properties of bone tissue, the scattering in those tests is assumed to be higher than in the present study. Hence, larger test groups should be selected to obtain statistical power sufficient for significant findings to be drawn based on these studies. 5. Conclusions The dimensions of an implant determine the stability of both the implant itself and the overdenture. However, implant size influences the risk of fracture as well. Careful pre- and postinsertion planning and care can help to minimize complications such as implant-associated mandibular fractures. A sophisticated treatment concept also requires a predictable classification of fracture risks. Three-dimensional radiological imaging should be part of the clinical treatment planning for severely atrophic, edentulous mandibles. From the tested implant settings, the following conclusions can be drawn within the limitations of this in vitro study: Four implants with a short and thick design and not connected by a fixed bar might minimize the risk of fracture when implants are placed interforaminally to stabilize the overdenture in an atrophic edentulous mandible. If a fixed bar was used, the differences among the four tested implant settings were equalized. The fixed-bar design showed greater strength over all groups (mean, 442.8 N; without fixed bar: mean, 398.1 N). Using a fixed bar, the authors suggest four implants with a long and thin design to minimize the risk of fracture; however, the within-group differences were less distinct. Applying these recommended implant settings might reduce the risk of immediate post-insertion fracture due to chewing forces. However, other implant dimensions should be investigated in further studies to determine whether there are even more boneand stability-preserving implant setting. Conflicts of interest On behalf of all authors of this manuscript, we certify that there is no actual or potential conflict of interest relative to this article. Additionally, we declare that there is no financial or personal relationship with other individuals or organizations that could inappropriately influence this work. Acknowledgments The authors are grateful to the ITI Foundation for financial support (ITI Research Grant No.739_2010); Antigone Kirchartz for manufacturing the drilling rig, drilling templates, and fixed bars; and Stefan Eichhorn for his support in building the device to fix the mandible in the test rig. References Almasri M, El-Hakim M: Fracture of the anterior segment of the atrophic mandible related to dental implants. Int J Oral Maxillofac Surg 41: 646e649, 2012 Atieh MA, Zadeh H, Stanford CM, Cooper LF: Survival of short dental implants for treatment of posterior partial edentulism: a systematic review. Int J Oral Maxillofac Implant 27: 1323e1331, 2012
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Awad MA, Locker D, Korner-Bitensky N, Feine JS: Measuring the effect of intra-oral implant rehabilitation on health-related quality of life in a randomized controlled clinical trial. J Dent Res 79: 1659e1663, 2000 Batenburg RH, Meijer HJ, Raghoebar GM, Van Oort RP, Boering G: Mandibular overdentures supported by two Branemark, IMZ or ITI implants. A prospective comparative preliminary study: one-year results. Clin Oral Implant Res 9: 374e383, 1998 Carlsson GE: Responses of jawbone to pressure. Gerodontology 21: 65e70, 2004 Chiapasco M, Gatti C: Implant-retained mandibular overdentures with immediate loading: a 3- to 8-year prospective study on 328 implants. Clin Implant Dent Relat Res 5: 29e38, 2003 Davis WH, Lam PS, Marshall MW, Dorchester W, Hochwald DA, Kaminishi RM: Using restorations borne totally by anterior implants to preserve the edentulous mandible. J Am Dent Assoc 130: 1183e1189, 1999 Ellis E, Price C: Treatment protocol for fractures of the atrophic mandible. J Oral Maxillofac Surg 66: 421e435, 2008 Esfandiari S, Lund JP, Penrod JR, Savard A, Thomason JM, Feine JS: Implant overdentures for edentulous elders: study of patient preference. Gerodontology 26: 3e10, 2009 Ferreira EJ, Kuabara MR, Gulinelli JL: “All-on-four” concept and immediate loading for simultaneous rehabilitation of the atrophic maxilla and mandible with conventional and zygomatic implants. Br J Oral Maxillofac Surg 48: 218e220, 2010 Fitzpatrick B: Standard of care for the edentulous mandible: a systematic review. J Prosthet Dent 95: 71e78, 2006 Fontijn-Tekampl E, Slagter AP, van’t Hof MA, Geertman ME, Kalk W: Bite forces with mandibular implant-retained overdentures. J Dent Res 77: 1832e1839, 1998 Fueki K, Kimoto K, Ogawa T, Garrett NR: Effect of implant-supported or retained dentures on masticatory performance: a systematic review. J Prosthet Dent 98: 470e477, 2007 Goodacre CJ, Kan JY, Rungcharassaeng K: Clinical complications of osseointegrated implants. J Prosthet Dent 81: 537e552, 1999 Grohmann I, Raith S, Kesting M, Rau A, Mücke T, Lethaus B, et al: Experimental biomechanical study of the primary stability of different osteosynthesis systems for mandibular reconstruction with an iliac crest graft. Br J Oral Maxillofac Surg 51: 942e947, 2013 Karlis V, Bae RD, Glickman RS: Mandibular fracture as a complication of inferior alveolar nerve transposition and placement of endosseous implants: a case report. Implant Dent 12: 211e216, 2003 ~ arrocha Diago M, Martí Bowen E, Pen ~ arrocha Diago M: IntraLamas Pelayo J, Pen operative complications during oral implantology. Med Oral Patol Oral Cir Bucal 13: E239eE243, 2008 Liu J, Pan S, Dong J, Mo Z, Fan Y, Feng H: Influence of implant number on the biomechanical behaviour of mandibular implant-retained/supported overdentures: a three-dimensional finite element analysis. J Dent 41: 241e249, 2013 Luhr HG, Reidick T, Merten HA: Fractures of the atrophic mandibleda challenge for therapy. Fortschr Kiefer Gesichtschir 41: 151e154, 1996 Madsen MJ, Haug RH, Christensen BS, Aldridge E: Management of atrophic mandible fractures. Oral Maxillofac Surg Clin North Am 21: 175e183, 2009 Madsen MJ, Kushner GM, Alpert B: Failed fixation in atrophic mandibular fractures: the case against miniplates. Craniomaxillofac Trauma Reconstr 4: 145e150, 2011
Mason ME, Triplett RG, Van Sickels JE, Parel SM: Mandibular fractures through endosseous cylinder implants: report of cases and review. Oral Maxillofac Surg 48: 311e317, 1990 Melo AR, de Aguiar Soares Carneiro SC, Leal JL, Vasconcelos BC: Fracture of the atrophic mandible: case series and critical review. J Oral Maxillofac Surg 69: 1430e1435, 2011 Oh W, Roumanas ED, Beumer J: Mandibular fracture in conjunction with bicortical penetration, using wide-diameter endosseous dental implants. J Prosthodont 19: 625e629, 2010 Payne AG, Tawse-Smith A, Duncan WD, Kumara R: Conventional and early loading of unsplinted ITI implants supporting mandibular overdentures. Clin Oral Implant Res 13: 603e609, 2002 Raghoebar GM, Stellingsma K, Batenburg RH, Vissink A: Etiology and management of mandibular fractures associated with endosteal implants in the atrophic mandible. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 89: 553e559, 2000 Reddy MS, Geurs NC, Wang IC, Liu PR, Hsu YT, Jeffcoat RL, et al: Mandibular growth following implant restoration: does Wolff's law apply to residual ridge resorption? Int J Periodontics Restorative Dent 22: 315e321, 2002 Sadowsky SJ: Mandibular implant-retained overdentures: a literature review. J Prosthet Dent 86: 468e473, 2001 Schug T, Dumbach J, Rodemer H: Mandibular fracture. An unusual implantation complication. Mund Kiefer Gesichtschir 3: 335e337, 1999 Soehardi A, Meijer GJ, Manders R, Stoelnga PJ: An inventory of mandibular fractures associated with implants in atrophic edentulous mandibles: a survey of Dutch oral and maxillofacial surgeons. Int J Oral Maxillofac Implant 26: 1087e1093, 2011 Steiner T, Raith S, Eichhorn S, Doebele S, Trainotti S, Müller S, et al: Evaluation of a new optical measuring system for experiments on fractured human mandibles: a biomechanical feasibility study in maxillofacial surgery. Clin Oral Investig 16: 1535e1542, 2012 € lzle F, et al: How much Steiner T, Torsiglieri T, Rau A, Eichhorn S, Deppe H, Ho impairment tolerates an atrophic mandible? A biomechanic study about implant-cavities. Br J Oral Maxillofac Surg, 2015 [unpublished] Stellingsma C, Vissink A, Meijer HJ, Kuiper C, Raghoebar GM: Implantology and the severely resorbed edentulous mandible. Crit Rev Oral Biol Med 15: 240e248, 2004 Thomason JM, Kelly SA, Bendkowski A, Ellis JS: Two implant retained overdenturesda review of the literature supporting the McGill and York consensus statements. J Dent 40: 22e34, 2012 Toma VS, Mathog RH, Toma RS, Meleca RJ: Transoral versus extraoral reduction of mandible fractures: a comparison of complication rates and other factors. Otolaryngol Head Neck Surg 128: 215e219, 2003 Trainotti S, Raith S, Kesting M, Eichhorn S, Bauer F, Kolk A, et al: Locking versus nonlocking plates in mandibular reconstruction with fibular graft-a biomechanical ex vivo study. Clin Oral Investig 18: 1291e1298, 2014 Von Wowern N, Gotfredsen K: Implant-supported overdentures, a prevention of bone loss in edentulous mandibles? A 5-year follow-up study. Clin Oral Implant Res 12: 19e25, 2001 Wittwer G, Adeyemo WL, Turhani D, Ploder O: Treatment of atrophic mandibular fractures based on the degree of atrophydexperience with different plating systems: a retrospective study. J Oral Maxillofac Surg 64: 230e234, 2006 Wright PS, Glantz PO, Randow K, Watson RM: The effects of fixed and removable implant-stabilised prostheses on posterior mandibular residual ridge resorption. Clin Oral Implant Res 13: 169e174, 2002
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Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study € lzle a, T. Steiner a T. Torsiglieri a, *, S. Raith a, b, A. Rau c, H. Deppe c, F. Ho €tsklinikum Aachen, RWTH Aachen, Germany Department of Oral and Maxillofacial Surgery, Universita €tsklinikum Aachen, RWTH Aachen, Germany Dental Materials and Biomaterials Research, Universita c €t München, Germany Department of Oral and Maxillofacial Surgery, Klinikum rechts der Isar, Technische Universita a
b
a r t i c l e i n f o
a b s t r a c t
Article history: Paper received 10 July 2014 Accepted 2 March 2015 Available online xxx
Objectives: Fractures of the atrophic edentulous mandible are a rare complication that can become severe after the insertion of dental implants. This in vitro study investigated the effects of different implant settings varying in number, diameter, and length. and the influence of a fixed bar. Materials and methods: In biomechanical experiments on artificial mandibles, an unmodified reference group, four implant settings with two different implants, and the effect of adding a fixed bar to these settings were tested. All specimens were loaded with incisal biting forces until failure due to fracture. Results: Implants weakened all specimens significantly compared with those in the reference group. Without a fixed bar, four short and thick implants showed the best results, with high significance. With a fixed bar, four long and thin implants withstood the highest loads. The addition of fixed bars reduced the differences between the implant settings. Fixed bars did not show increased stability for all groups; however, these groups showed a higher mean strength. Conclusions: Four implants with a short and thick design should be the first choice when implants are placed without a fixed bar in an atrophic mandible. With a fixed bar, four long and thin implants should be used. © 2015 European Association for Cranio-Maxillo-Facial Surgery. Published by Elsevier Ltd. All rights reserved.
Keywords: Dental implant Mandibular fracture Mandibular atrophy
1. Introduction Bone remodeling of the alveolar crest is a lifelong process. In edentulous jaws, the lack of physiological stress on the bone induces bone resorption. Pressure exerted on the bone by a conventional prosthesis and the duration of edentulism are factors that can accelerate this process. Implant-supported prostheses in edentulous mandibles have led to substantially reduced bone loss in comparison with those in conventional denture-wearing jaws (Carlsson, 2004). The remodeling is subject to high individual variations and is not sufficiently understood (Carlsson, 2004). Several studies have even detected bone apposition in patients treated with these prostheses (Davis et al., 1999; Reddy et al., 2002; von Wowern and Gotfredsen, 2001; Wright et al., 2002). In 1998, Fontijn et al. reported that patients with mandibular implanteretained overdentures had
* Corresponding author. Mauerstrasse 26, 52064 Aachen, Germany. Tel.: þ49 1755864986. E-mail address: [email protected] (T. Torsiglieri).
significantly higher maximum bite forces than conventional complete-denture wearers. In 2006, Fueki et al. reported, in their literature review covering more than 50 years, objective benefits in the masticatory performance of implant-supported overdentures compared with conventional dentures in edentulous patients with resorbed mandibles (Fueki et al., 2007). These overdentures appeared to yield higher patient satisfaction scores, even with patients who had undergone preprosthetic surgery (Sadowsky, 2001). In 2000, Awad et al. observed, in a randomized clinical trial, that treatment with implant-supported overdentures was associated with a significantly better quality of life. The additional costs of implant insertion seem to be supported by the patients, because up to 77% of conventional denture wearers were willing to pay even three times more than the current cost of conventional dentures for implant-retained prostheses (Esfandiari et al., 2009). Although the insertion of dental implants has become a standard treatment in recent years, the treatment of the severely atrophic mandible remains challenging. Due to the weakened mandibular bone lacking the alveolar crest, the insertion of
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Please cite this article in press as: Torsiglieri T, et al., Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study, Journal of Cranio-Maxillo-Facial Surgery (2015), http://dx.doi.org/10.1016/j.jcms.2015.03.001
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implants decreases the stability of the bone and may lead to fractures either during or shortly after implant insertion. Numerous case reports have indicated the risk of mandibular fractures during cavity preparation or implant insertion (Almasri and El-Hakim, 2012; Fontijn-Tekampl et al., 1998; Karlis et al., 2003; Luhr et al., 1996; Oh et al., 2010; Raghoebar et al., 2000). A fracture of the mandible is the most feared of all complications related to endosseous implants in the extremely resorbed mandible. Mandibular fractures after implant placement are a rare complication in conjunction with severely resorbed mandibles (Goodacre et al., 1999; Raghoebar et al., 2000; Soehardi et al., 2011). However, several studies have pointed out that fractures of the atrophic mandible have severe consequences for the patient; therapy remains challenging and thus should be avoided by all means (Ellis and Price, 2008; Madsen et al., 2009, 2011; Melo et al., 2011; Soehardi et al., 2011; Wittwer et al., 2006). Atrophic mandibular fractures are often treated with an open reduction and internal fixation technique using a reconstruction plate via a submandibular transcutaneous approach (Soehardi et al., 2011; Toma et al., 2003). In the literature, complications such as non-union, osteomyelitis, and infection occur in up to 48% of patients with fractures after the seating of implants in atrophic mandibles (Soehardi et al., 2011). The best treatment for implant-related fracture of the mandible is prevention. There are surgical precautions and guidelines to minimize the weakening of the bone and risk of fracture, such stepwise drilling, sufficient irrigation, and avoiding the inferior cortex, among others (Mason et al., 1990). The site of an implant that has not yet been osseointegrated is characterized by tensile stress concentration and weakness. Repeated submaximal functional forces in such an area of bony weakness may lead to a spontaneous fracture without associated trauma (Mason et al., 1990). The use of immediately loaded implants in the anterior mandible for overdenture design is a promising treatment concept and has shown success rates of 95.6e100% (Chiapasco and Gatti, 2003; Payne et al., 2002). However, atrophic mandibles require special attention. The above mentioned negative consequences indicate that the weakening of the bone should be minimized. Different implant settings are proposed for the treatment of the atrophic mandible. In the moderately resorbed edentulous mandible, fabrication of an overdenture for two or four interforaminal implants is currently an accepted, widespread treatment modality for improving the function of a mandibular prosthesis (Batenburg et al., 1998; Thomason et al., 2012). Although there are many studies that have focused on the topic of implant-supported overdentures in the edentulous mandible, statements relating to the impacts of implant design, number, diameter, and length on the weakening of the atrophic bone are rare and not based on biomechanical evidence. No large prospective studies have been performed; most publications are retrospective single-case studies. To address the open clinical question regarding which implants place the least strain on the atrophic mandible, we performed biomechanical experiments on a self-developed test bench. This allowed for the simulation of physiological loading of artificial, atrophic mandibular models in the laboratory. Prior to these experiments, no commercially available atrophic mandibular model was on the market. Therefore, our working group developed artificial mandibles derived from computed tomographic data. To our knowledge, no biomechanical data have been published on the stability of edentulous, atrophic mandibles. Previous studies have examined the effect of implant cavity preparation on the stability of the jaw. The results have shown that the number and dimensions of dental implant cavities have a significant impact on mechanical stability (Steiner et al., 2015). We hypothesized that different implant settings could have a significant influence on the weakening of the jaw.
The aim of this in vitro study was to investigate the effects of implant settings, differing in number, diameter, and length, on the stability of the jaw. Furthermore, we investigated whether the use of a fixed bar would deliver a favorable outcome. 2. Material and methods 2.1. Experimental setting The experimental testing device used was the same as that introduced by Steiner et al. (2015) for experiments on atrophic mandibular models weakened by implant cavities but without inserted implants. The incisal biting forces were modeled with a rope, pulled by a controlled testing device (Zwick i-line 5 kN, Zwick Messtechnik, Ulm, Germany). Accordingly, the masticatory muscle forces were modeled by ropes acting at the mandibular angles to represent the physiological actions of the pterygomasseteric sling (Fig. 1). A specially developed platform was used to perform the tests with repeatable accuracy. The temporomandibular joints were modeled through bearings made of concavely lathed spherical boxes to represent the anatomical shape of the temporomandibular fossae (Steiner et al., 2015). This load setting has proved its applicability in various biomechanical studies for investigations on the stability of mandibular reconstructions with autologous bone grafts (Grohmann et al., 2013; Steiner et al., 2012; Trainotti et al., 2014). In our tests, force was applied continuously until the test bodies failed due to fracture. Incisal load and incisal movement were recorded by the Zwick device at a sampling rate of 4 Hz. 2.2. Artificial mandibular specimens Standardized conditions are a crucial requirement for in vitro biomechanical experiments. High interindividual differences in atrophy, bone quality, and morphology in human mandibles would make the use of cadaver specimens nearly impossible for testing, because the statistical power is reduced by these overlaid scattering factors. However, prior to these experiments, no commercially available atrophic mandibular model was on the market. Therefore, our working group developed artificial mandibles. These artificial, biomimetic mandibular specimens (Synbone #8570, Synbone, Malans, Switzerland) were fabricated with two individualized polyurethane foam materials. The density of the foam was adjusted to mimic the biomechanical behavior of the cortical and spongiose part of the mandibular bone, respectively. The geometry of these specimens was designed according to a mean shape based on 27 atrophic mandibles, derived with an algorithm that has previously been published (Steiner et al., 2015). 2.3. Specimen preparation and implantation procedure All implants, abutments, fixed bars, and tools were used as recommended by the manufacturer. The implant cavities were prepared under standardized conditions in a box column drill with a tapping machine (Steiner et al., 2015). This was executed stepwise with steel drills of different diameters (800e200 rpm), and tabs were cut (15 rpm) by one experienced implantologist, according to the standardized procedure described by Steiner et al. (2015). Straumann (Straumann AG, Basel, Switzerland) tools were used for all steps of preparation. To ensure a very high level of standardization, all specimens were drilled in parallel by means of the same drilling rig and templates in each group (Fig. 2). The implants were placed with Straumann tools in all test bodies. All implants were placed interforaminally equidistant from each other, with equal bone remaining to the labial and lingual borders. The distance
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Fig. 1. (a) Testing device with mandible and ropes used in the study (front view). (b) Testing device with mandible and ropes used in the study (side view). (c) Mechanical principle of load application with mandible mounted in the test rig and ropes to simulate incisal-chewing forces. Red arrows show simulated pterygomasseteric sling.
Fig. 2. Fixed drilling rig and templates used in the dental laboratory. (a) Template for four-implant groups. (b) Template for two-implant groups.
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T. Torsiglieri et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2015) 1e8 Table 1 Straumann implants and parts suitable for fabricating and connecting the fixed bars. Item
In the following named as
Item no.
Bone Level Implant, Ø 4.8 mm RC, SLActive 8 mm, TiZr Bone Level Implant, Ø 3.3 mm NC, SLActive 14 mm, TiZr NC Multi-Base abutment straight, D4.5 mm, GH 4 mm RC Multi-Base abutment straight, D6.5 mm, GH 4 mm NC Burn-out coping for bridge, Multi-Base, D4.5 mm, Pom RC Burn-out coping for bridge, Multi-Base, D6.5 mm, Pom
Short and thick Long and thin
021.6308 021.2314 022.2744 022.4764 023.2744 023.4764
between the foramina was divided equally, and the holes were drilled at the same distance from each other. Two different types of Straumann bone level implants were used (Table 1). To hold the fixed bar in place, corresponding abutments were fixed to the implants with a torque of 35 Ncm, with 15 Ncm to hold the fixed bar as recommended by the manufacturer. One dental technician manufactured all fixed bars manually by using an individually waxed-up model. The cobaltechromium alloy used was Girobond nb (Amann Girrbach AG, Koblach, Austria). 2.4. Mandibular groups investigated Ninety specimens divided into nine different groups were examined in the scope of the present study, with 10 specimens per group. A reference group with unaltered mandibles and two different placements of the implant, each in combination with the two different implant types that were used, yielded the four different implant settings that were investigated (Fig. 3aed, Table 2, Fig 5). Furthermore, all groups were tested with and without a fixed bar (Fig. 4aec).
modify these threshold values accordingly. The sample size of 10 specimens per group is based on a 5% level of significance and a power of 80%, with the resulting minimum sample size of seven specimens per group. 3. Results 3.1. Reference group The reference group withstood, in mean, 729.5 N ± 59.9 N incisal loading until failure due to fracture. Four specimens broke at the condyle, one at the angle, and five in the interforaminal area of the jaw. 3.2. Groups with implants All test bodies broke in the region where implants were placed and were highly significantly debilitated compared with those in the reference group. All p values were under the threshold of high significance according to BHC.
2.5. Statistical evaluations
3.3. Groups without fixed bar
The recorded experimental failure loads were compared by the WilcoxoneManneWhitney U-test to quantify the differences, with values of p 0.05 and p < 0.001 as threshold levels for statistical significance and high significance, respectively. For multiple comparisons, the BonferronieHolm correction (BHC) was applied to
This group showed a larger span of results (mean, 509.3 Ne267.5 N) than the group with a fixed bar (mean, 479.1 Ne413.5 N). Groups with short and thick implants showed a significantly higher resistance to loading than those with long and thin implants.
Fig. 3. Ten specimens per group. (a) Two short and thick implants. (b) Four short and thick implants. (c) Two long and thin implants. (d) Four long and thin implants.
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T. Torsiglieri et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2015) 1e8 Table 2 Mean failure loads due to fracture and standard deviations of the tested groups. Group Specimens, implant characteristics no.
Breaking load; Standard mean, N deviation, N
X 1 2 3 4 1a 2a 3a 4a
729.5 445.6 369.8 509.3 267.5 429.0 413.5 449.5 479.1
Reference 2 Implants 2 Implants 4 Implants 4 Implants 2 Implants 2 Implants 4 Implants 4 Implants
short and thick long and thin short and thick long and thin short and thick with fixed-bar long and thin with fixed-bar short and thick with fixed-bar long and thin with fixed-bar
±59.9 ±39.4 ±53.6 ±38.7 ±18.0 ±92.9 ±46.5 ±46.7 ±36.4
3.3.1. Four implants Four long and thin implants showed the lowest fracture loads (mean, 267.5N ± 18.0 N) with significant differences compared with all specimens with implants (significant to all other groups without fixed bar, according to BHC). In contrast, the group with four short and thick implants showed the highest failure loads of all specimens after implant insertion (mean, 509.3N ± 38.7 N). Data for this group were highly significantly better (with consideration of BHC) than those from all other groups except the group with four long and thin implants connected with a bar, which was the strongest group within its group of fixed-bareconnected implants. 3.3.2. Two implants The specimens with two short and thick implants showed mean failure loads of 445.6 N ± 39.4 N and were highly significantly stronger (p < 0.001) than those with two long and thin implants (mean, 369.8N ± 53.6 N).
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3.4. Groups with fixed bar None of the specimens showed failure of the implant structure, either at the connection between the abutment and the implant or between the abutment and the fixed bar. Four long and thin implants showed the highest values of fracture loads of this group (mean, 479.1 N ± 36.4 N), followed by mandibles with four short and thick implants (mean, 449.5 N ± 46.7 N); however, this difference is not significant with consideration of BHC (p ¼ 0.04, compared to a BHC threshold of 0.017). In the group of two implants connected with a fixed bar, short and thick implants (mean, 429.0 N ± 92.9 N) showed slightly, but not significantly, better results than long and thin implants (mean, 413.5 N ± 46.5 N) (p ¼ 0.2). The only significant difference between either group of the fixed bar specimens was the comparison between two long and thin implants that were inferior with respect to four long and thin implants (p ¼ 0.0014). 3.5. Comparison of groups without and with a fixed bar The group with fixed bars showed a smaller span of results (mean, 479.1 Ne413.5 N) than the group without fixed bars (mean, 509.3 Ne267.5 N). The respective strongest specimens of each group did not show significantly differing results for failure loads (p ¼ 0.08), i.e., four short and thick implants without fixed bars and four long and thin implants with fixed bars. Interestingly, the strongest group without a fixed bar was significantly more resistant to incisal force than the same implant setting connected with a fixed bar (p ¼ 0.003), i.e., four short and thick implants. The strongest setting with a fixed bar
Fig. 4. Manually manufactured fixed bars used in the study. (a) Fixed bar mounted on two implants inserted into specimen. (b) Fixed bar mounted on four implants inserted into specimen. (c, Left) Two long and thin implants with fixed bar attached. (c, Right) Two short and thick implants with fixed bar attached.
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Fig. 5. Results of the different groups investigated for failure load due to fracture.
was yet to the weakest setting without a fixed bar, i.e., four long and thin implants. The union of implants significantly reinforced particular implant settings, namely, four long and thin implants. 4. Discussion Edentulous patients treated with conventional overdentures suffer from insufficient retention, severely reduced biting forces, intolerance to loading by the mucosa, pain, difficulty with eating and speech, loss of soft-tissue support, altered facial appearance, and a reduced quality of life (Awad et al., 2000; Fontijn-Tekampl et al., 1998; Sadowsky, 2001; Stellingsma et al., 2004). These factors are improved with the aid of implant-retained overdentures, and the application of endosseous implants has changed treatment concepts enormously. Therefore, dental implantology is currently a valuable treatment modality for the edentulous patient (Stellingsma et al., 2004), because it can reduce bone loss and may even stimulate the formation of new bone (Carlsson, 2004; Davis et al., 1999; Reddy et al., 2002; von Wowern and Gotfredsen, 2001; Wright et al., 2002). Many prosthodontic and surgical treatments have been attempted in cases of severe residual ridge resorption, but none have been completely predictable (Carlsson, 2004). Different treatment recommendations for implant-retained or -supported overdentures of the lower jaw have been published (Ferreira et al., 2010; Sadowsky, 2001; Stellingsma et al., 2004). In 2012, Atieh et al. found, in a systematic review, that there is no significant difference in survival rates between short and long implants if immediate loading is avoided (Atieh et al., 2012). In 2002, the McGill consensus statement suggested that treatment with two implants to retain the overdenture should become the treatment of first choice for most edentulous patients, taking into account performance, patient satisfaction, cost, and clinical time (Liu et al., 2013; Thomason et al., 2012). In 2006, Fitzpatrick stated that there is no evidence for a single, universally superior treatment modality for the edentulous mandible (Fitzpatrick, 2006). Because
treating the edentulous, highly atrophic mandible is challenging, and because concepts have thus far been evaluated only in retrospective clinical studies and case reports (Ellis and Price, 2008; Madsen et al., 2009, 2011; Melo et al., 2011; Wittwer et al., 2006), analysis of the data generated by this study could help to improve planning and treatment options. The data on fractures of the atrophic mandible as a complication during or shortly after the insertion of interforaminal implants are limited, and many cases have probably been unreported (Lamas Pelayo et al., 2008; Raghoebar et al., 2000). Nevertheless, this is a severe and serious complication. In 2011, Soehardi et al. reported that the main reason for fractures is that bone height is too low or that the dimensions of the jawbone are too narrow (63%). Iatrogenic factors are mentioned in 18% of the reported cases (Soehardi et al., 2011). Treatment of fractures of highly atrophic mandibles is challenging, with possible associated complications, and different treatment options have been published (Almasri and El-Hakim, 2012; Raghoebar et al., 2000; Stellingsma et al., 2004). Several authors agree that the best treatment is prevention (Almasri and ElHakim, 2012; Mason et al., 1990). To achieve this goal, a sophisticated treatment concept with a predictable classification of fracture risks would be very useful. Three-dimensional radiological imaging should be part of clinical treatment planning. The time of fracture is not uniform, because there are two biomechanical considerations: either when implants are placed, or when bone is weakened and breaks secondarily when loaded by chewing forces. The risk of this severe complication can be minimized by careful consideration of certain aspects of treatment such as correct presurgical planning, the use of adequate surgical techniques, postsurgical follow-up, respect for the osseointegration period, appropriate design of the superstructure, the study and correct distribution of occlusal loads, and meticulous hygiene during the maintenance phase (Lamas Pelayo et al., 2008). When implants are placed in an atrophic mandible, periodic follow-ups, including clinical and radiographic examinations, become even more necessary to identify complications such as osteomyelitis or periimplantitis at an early stage, as well as to instruct the patient on how to avoid occlusal overloading during osseointegration (Lamas Pelayo et al., 2008). However, new concepts regarding immediate loading in patients with severely atrophic mandibles, albeit with promising results, have not yet been published (Chiapasco and Gatti, 2003; Ferreira et al., 2010). In 1990, Mason et al. had already published surgical tips to minimize the weakening of bone when dental implants are placed in atrophic mandibles. They recommended stepwise drilling, sufficient irrigation, primary bone grafting in the atrophic mandible, avoiding inferior cortex engagement if possible, fixture tap, avoiding force when the implant is seated (by use of a torque force indicator), and a postoperative soft diet. Some authors have reported that the placement of wide-diameter implants in conjunction with bicortical penetration in a severely atrophic edentulous mandible could increase the risk of mandibular fracture (Oh et al., 2010; Schug et al., 1999). The coherence of implant diameter and design and the degree of atrophy are vital in countering the risk of fracture. One should also bear in mind that, in atrophic mandibles, bone quality is not consistent throughout the bone, and functional demands are highly variable (Fitzpatrick, 2006; Lamas Pelayo et al., 2008). This leads to unequal implant sites and therefore different degrees of primary implant stability. We examined the influence of implant settings differing in number, diameter, and length, as well as the influence of a fixed bar in combination with these settings. All implants were placed interforaminally and equidistantly, with equal quantities of bone up
Please cite this article in press as: Torsiglieri T, et al., Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study, Journal of Cranio-Maxillo-Facial Surgery (2015), http://dx.doi.org/10.1016/j.jcms.2015.03.001
T. Torsiglieri et al. / Journal of Cranio-Maxillo-Facial Surgery xxx (2015) 1e8
to the labial and lingual borders. The implants were chosen with differing lengths and diameters to establish a more comparable contact surface between the implant and the bone. Since a longer implant weakens the bone foreseeably more, a smaller-diameter implant leaves more bone to the labial and lingual borders. This should partially equalize this difference. Implants with a length of 14 mm are marginally long in atrophic mandibles. This was chosen deliberately to determine the quantitative difference between a rather short and a rather long implant. Interestingly, the differences were equalized as the implants were connected by a fixed bar, despite the marginal length. We investigated patients with an atrophic, edentulous mandible who were treated elsewhere with dental implants. Some patients received marginally long implants and showed complications most likely due to this parameter. Thus, we wanted to investigate and quantify the effect of marginally long implants. The number of implants did not show a uniform tendency. Four implants shoed the highest as well as the lowest failure loads. Mandibles with four short and thick implants demonstrated the highest fracture loads of all settings. Interestingly, this group was significantly more resistant to incisal force than was the same implant setting connected with a fixed bar. The strongest setting with a fixed bar was coeval the weakest setting without a fixed bar, i.e., four long and thin implants. It can be concluded that both numbers and positions of implants have a mechanical impact on the mandible and should not be considered separately. The fact that specimens with four implants were, in part, significantly stronger than specimens with two implants can be explained by the locations of the cavities. Two implants appeared to be placed at positions with higher tension on the bone. The union of implants significantly reinforces particular implant settings, namely, four long and thin implants. Thus, distinct weakening of this configuration without the fixed bar could be compensated for by the equal load distribution provided by the fixed bar. All specimens were highly significantly debilitated (p < 0.001) compared with the reference group and broke in the region where implants were placed. This confirms the assumption that drilling holes and inserting implants significantly weakens the bone. None of the specimens showed failure of the implant structure, either at the connection between the abutment and the implant or between the abutment and the fixed bar. Thus, it is evident that bone is the weakest link in biomechanical experiments, confirming reports in the literature (Sadowsky, 2001; Stellingsma et al., 2004). Together, these results indicate that a fixed-bar design changed the mechanical conditions and particularly the load distribution within the mandible. The finding that it decreased the differences among implant settings may be attributed to the more uniform load distribution caused by the relatively stiff fixed bar. The fixed bar balanced the differences among settings and provided a mechanical situation that decreased the influence of implant-related parameters such as number of implants and their position, length, and diameter compared with parameters in settings without a fixed bar. The biomechanical study design was executed in artificial mandibular models. Because these do not feature real bone, and because the loading scenarios in the experimental device are artificial, the results of the presented experiments may differ from those of clinical applications in patients. Further in vivo tests are necessary to examine whether the findings of this in vitro study also apply to clinical observations. However, because the results differed significantly among the different groups and because of the high degree of standardization in the test settings resulting from the use of geometrically identical
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specimens, the study findings should have an impact on clinical treatment recommendations. Nevertheless, the next step should be experiments using cadaver mandibles in a similar setting, as well as clinical trials. This test, however, has the disadvantage that each body is accessible only once. Due to the high variability in shapes and properties of the mandible, the degrees of atrophy, and the interindividual differences in the mechanical properties of bone tissue, the scattering in those tests is assumed to be higher than in the present study. Hence, larger test groups should be selected to obtain statistical power sufficient for significant findings to be drawn based on these studies. 5. Conclusions The dimensions of an implant determine the stability of both the implant itself and the overdenture. However, implant size influences the risk of fracture as well. Careful pre- and postinsertion planning and care can help to minimize complications such as implant-associated mandibular fractures. A sophisticated treatment concept also requires a predictable classification of fracture risks. Three-dimensional radiological imaging should be part of the clinical treatment planning for severely atrophic, edentulous mandibles. From the tested implant settings, the following conclusions can be drawn within the limitations of this in vitro study: Four implants with a short and thick design and not connected by a fixed bar might minimize the risk of fracture when implants are placed interforaminally to stabilize the overdenture in an atrophic edentulous mandible. If a fixed bar was used, the differences among the four tested implant settings were equalized. The fixed-bar design showed greater strength over all groups (mean, 442.8 N; without fixed bar: mean, 398.1 N). Using a fixed bar, the authors suggest four implants with a long and thin design to minimize the risk of fracture; however, the within-group differences were less distinct. Applying these recommended implant settings might reduce the risk of immediate post-insertion fracture due to chewing forces. However, other implant dimensions should be investigated in further studies to determine whether there are even more boneand stability-preserving implant setting. Conflicts of interest On behalf of all authors of this manuscript, we certify that there is no actual or potential conflict of interest relative to this article. Additionally, we declare that there is no financial or personal relationship with other individuals or organizations that could inappropriately influence this work. Acknowledgments The authors are grateful to the ITI Foundation for financial support (ITI Research Grant No.739_2010); Antigone Kirchartz for manufacturing the drilling rig, drilling templates, and fixed bars; and Stefan Eichhorn for his support in building the device to fix the mandible in the test rig. References Almasri M, El-Hakim M: Fracture of the anterior segment of the atrophic mandible related to dental implants. Int J Oral Maxillofac Surg 41: 646e649, 2012 Atieh MA, Zadeh H, Stanford CM, Cooper LF: Survival of short dental implants for treatment of posterior partial edentulism: a systematic review. Int J Oral Maxillofac Implant 27: 1323e1331, 2012
Please cite this article in press as: Torsiglieri T, et al., Stability of edentulous, atrophic mandibles after insertion of different dental implants. A biomechanical study, Journal of Cranio-Maxillo-Facial Surgery (2015), http://dx.doi.org/10.1016/j.jcms.2015.03.001
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Awad MA, Locker D, Korner-Bitensky N, Feine JS: Measuring the effect of intra-oral implant rehabilitation on health-related quality of life in a randomized controlled clinical trial. J Dent Res 79: 1659e1663, 2000 Batenburg RH, Meijer HJ, Raghoebar GM, Van Oort RP, Boering G: Mandibular overdentures supported by two Branemark, IMZ or ITI implants. A prospective comparative preliminary study: one-year results. Clin Oral Implant Res 9: 374e383, 1998 Carlsson GE: Responses of jawbone to pressure. Gerodontology 21: 65e70, 2004 Chiapasco M, Gatti C: Implant-retained mandibular overdentures with immediate loading: a 3- to 8-year prospective study on 328 implants. Clin Implant Dent Relat Res 5: 29e38, 2003 Davis WH, Lam PS, Marshall MW, Dorchester W, Hochwald DA, Kaminishi RM: Using restorations borne totally by anterior implants to preserve the edentulous mandible. J Am Dent Assoc 130: 1183e1189, 1999 Ellis E, Price C: Treatment protocol for fractures of the atrophic mandible. J Oral Maxillofac Surg 66: 421e435, 2008 Esfandiari S, Lund JP, Penrod JR, Savard A, Thomason JM, Feine JS: Implant overdentures for edentulous elders: study of patient preference. Gerodontology 26: 3e10, 2009 Ferreira EJ, Kuabara MR, Gulinelli JL: “All-on-four” concept and immediate loading for simultaneous rehabilitation of the atrophic maxilla and mandible with conventional and zygomatic implants. Br J Oral Maxillofac Surg 48: 218e220, 2010 Fitzpatrick B: Standard of care for the edentulous mandible: a systematic review. J Prosthet Dent 95: 71e78, 2006 Fontijn-Tekampl E, Slagter AP, van’t Hof MA, Geertman ME, Kalk W: Bite forces with mandibular implant-retained overdentures. J Dent Res 77: 1832e1839, 1998 Fueki K, Kimoto K, Ogawa T, Garrett NR: Effect of implant-supported or retained dentures on masticatory performance: a systematic review. J Prosthet Dent 98: 470e477, 2007 Goodacre CJ, Kan JY, Rungcharassaeng K: Clinical complications of osseointegrated implants. J Prosthet Dent 81: 537e552, 1999 Grohmann I, Raith S, Kesting M, Rau A, Mücke T, Lethaus B, et al: Experimental biomechanical study of the primary stability of different osteosynthesis systems for mandibular reconstruction with an iliac crest graft. Br J Oral Maxillofac Surg 51: 942e947, 2013 Karlis V, Bae RD, Glickman RS: Mandibular fracture as a complication of inferior alveolar nerve transposition and placement of endosseous implants: a case report. Implant Dent 12: 211e216, 2003 ~ arrocha Diago M, Martí Bowen E, Pen ~ arrocha Diago M: IntraLamas Pelayo J, Pen operative complications during oral implantology. Med Oral Patol Oral Cir Bucal 13: E239eE243, 2008 Liu J, Pan S, Dong J, Mo Z, Fan Y, Feng H: Influence of implant number on the biomechanical behaviour of mandibular implant-retained/supported overdentures: a three-dimensional finite element analysis. J Dent 41: 241e249, 2013 Luhr HG, Reidick T, Merten HA: Fractures of the atrophic mandibleda challenge for therapy. Fortschr Kiefer Gesichtschir 41: 151e154, 1996 Madsen MJ, Haug RH, Christensen BS, Aldridge E: Management of atrophic mandible fractures. Oral Maxillofac Surg Clin North Am 21: 175e183, 2009 Madsen MJ, Kushner GM, Alpert B: Failed fixation in atrophic mandibular fractures: the case against miniplates. Craniomaxillofac Trauma Reconstr 4: 145e150, 2011
Mason ME, Triplett RG, Van Sickels JE, Parel SM: Mandibular fractures through endosseous cylinder implants: report of cases and review. Oral Maxillofac Surg 48: 311e317, 1990 Melo AR, de Aguiar Soares Carneiro SC, Leal JL, Vasconcelos BC: Fracture of the atrophic mandible: case series and critical review. J Oral Maxillofac Surg 69: 1430e1435, 2011 Oh W, Roumanas ED, Beumer J: Mandibular fracture in conjunction with bicortical penetration, using wide-diameter endosseous dental implants. J Prosthodont 19: 625e629, 2010 Payne AG, Tawse-Smith A, Duncan WD, Kumara R: Conventional and early loading of unsplinted ITI implants supporting mandibular overdentures. Clin Oral Implant Res 13: 603e609, 2002 Raghoebar GM, Stellingsma K, Batenburg RH, Vissink A: Etiology and management of mandibular fractures associated with endosteal implants in the atrophic mandible. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 89: 553e559, 2000 Reddy MS, Geurs NC, Wang IC, Liu PR, Hsu YT, Jeffcoat RL, et al: Mandibular growth following implant restoration: does Wolff's law apply to residual ridge resorption? Int J Periodontics Restorative Dent 22: 315e321, 2002 Sadowsky SJ: Mandibular implant-retained overdentures: a literature review. J Prosthet Dent 86: 468e473, 2001 Schug T, Dumbach J, Rodemer H: Mandibular fracture. An unusual implantation complication. Mund Kiefer Gesichtschir 3: 335e337, 1999 Soehardi A, Meijer GJ, Manders R, Stoelnga PJ: An inventory of mandibular fractures associated with implants in atrophic edentulous mandibles: a survey of Dutch oral and maxillofacial surgeons. Int J Oral Maxillofac Implant 26: 1087e1093, 2011 Steiner T, Raith S, Eichhorn S, Doebele S, Trainotti S, Müller S, et al: Evaluation of a new optical measuring system for experiments on fractured human mandibles: a biomechanical feasibility study in maxillofacial surgery. Clin Oral Investig 16: 1535e1542, 2012 € lzle F, et al: How much Steiner T, Torsiglieri T, Rau A, Eichhorn S, Deppe H, Ho impairment tolerates an atrophic mandible? A biomechanic study about implant-cavities. Br J Oral Maxillofac Surg, 2015 [unpublished] Stellingsma C, Vissink A, Meijer HJ, Kuiper C, Raghoebar GM: Implantology and the severely resorbed edentulous mandible. Crit Rev Oral Biol Med 15: 240e248, 2004 Thomason JM, Kelly SA, Bendkowski A, Ellis JS: Two implant retained overdenturesda review of the literature supporting the McGill and York consensus statements. J Dent 40: 22e34, 2012 Toma VS, Mathog RH, Toma RS, Meleca RJ: Transoral versus extraoral reduction of mandible fractures: a comparison of complication rates and other factors. Otolaryngol Head Neck Surg 128: 215e219, 2003 Trainotti S, Raith S, Kesting M, Eichhorn S, Bauer F, Kolk A, et al: Locking versus nonlocking plates in mandibular reconstruction with fibular graft-a biomechanical ex vivo study. Clin Oral Investig 18: 1291e1298, 2014 Von Wowern N, Gotfredsen K: Implant-supported overdentures, a prevention of bone loss in edentulous mandibles? A 5-year follow-up study. Clin Oral Implant Res 12: 19e25, 2001 Wittwer G, Adeyemo WL, Turhani D, Ploder O: Treatment of atrophic mandibular fractures based on the degree of atrophydexperience with different plating systems: a retrospective study. J Oral Maxillofac Surg 64: 230e234, 2006 Wright PS, Glantz PO, Randow K, Watson RM: The effects of fixed and removable implant-stabilised prostheses on posterior mandibular residual ridge resorption. Clin Oral Implant Res 13: 169e174, 2002
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