In Vitro Comparison of the Retentive Properties of Ball and Locator Attachments for Implant Overdentures Pinar Eren Türk, PhD, DDS1/Onur Geckili, PhD, DDS2/ Yasin Türk, PhD, DDS1/Volkan Günay, PhD3/Tayfun Bilgin, PhD, DDS4 Purpose: To compare the retentive properties of ball and locator attachments during 5,000 insertionseparation cycles, corresponding to approximately 4.5 years of clinical use. Materials and Methods: Four dental implants (diameter, 3.8 mm; length, 12 mm) were inserted into the prepared beds of two polyethylene blocks. Twenty acrylic prosthetic components were fabricated and connected to the ball and locator abutments. Tensile force was applied to the prosthetic components until the attachments were separated from the abutments. All samples were subjected to 5,000 insertion-separation cycles. Retention forces were measured after 10, 100, 200, 300, 400, 500, 1,000, 1,500, 2,000, 3,000, 4,000, and 5,000 insertion-separation cycles. Additionally, the wear of the attachments was measured using scanning electron microscopy. Data were analyzed to determine statistical equivalence among the two different attachments using the Student t test procedure and the Mann-Whitney U test procedure (α = .05). Results: Ball attachments showed significant retention loss after 100, 200, 400, 500, 1,500, and 4,000 cycles, and the locator attachments showed significant retention loss after 100, 200, 300, 500, and 3,000 cycles as compared with the previous cycle (P < .05). Retention loss after 5,000 cycles was detected significantly more often for ball attachments than for locator attachments (P = .049). No significant difference was detected between the retention losses of the two attachment systems during the other cycles as compared with the initial retention values (P > .05). No significant difference was detected between the wear on the two attachment systems after 5,000 cycles (P > .05). Conclusion: Both attachment systems showed decreased retentive forces after 5,000 insertion-separation cycles. However, after 5,000 insertion-separation cycles, locator attachments showed better retentive properties than ball attachments. Int J Oral Maxillofac Implants 2014;29:1106–1113. doi: 10.11607/jomi.3621 Key words: edentulous mandible, implant, impression, overdenture
T
he necessity of suffering from uncomfortable dentures was eliminated with the introduction of dental implants to the field of dentistry in the early 1980s. In particular, the problems of lack of stability and the retention of mandibular dentures have been solved by fabricating a fixed prosthesis or retaining an overdenture (OVD) to implants when the implant number is limited because of anatomical or social complexities. Older individuals prefer an implant-retained OVD 1Private
Practice, Istanbul, Turkey. Professor, Department of Prosthodontics, Istanbul University, Faculty of Dentistry, Istanbul, Turkey. 3Associate Professor, TUBITAK-MRC, Materials and Chemical Technologies Research Institute Istanbul, Turkey. 4Professor, Department of Prosthodontics, Istanbul University, Faculty of Dentistry, Istanbul, Turkey. 2 Associate
Correspondence to: Dr Onur Geckili, Istanbul University, Faculty of Dentistry, Department of Prosthodontics, 2nd floor, Çapa-Istanbul Turkey. Fax: +90-212-535 25 85. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
over an implant-retained fixed prosthesis if given the choice.1,2 This clearly indicates that implant-supported OVD treatment should not be considered a substandard treatment. Mandibular two-implant–supported OVDs opposing conventional complete maxillary dentures have been proposed as the standard of care for edentulous patients.3 The problems observed with the use of conventional mandibular dentures have been eliminated with the fixation of OVDs to the implants. Moreover, with the insertion of implants, bone resorption is dramatically decreased as compared with conventional denture use.4 The average height reduction of the posterior mandibular residual ridge in patients with conventional dentures is 1.63 mm, whereas the reduction is 0.69 mm in patients with implant OVDs.4 In patients with mandibular atrophy, conventional complete mandibular dentures generally move 10 mm in function.5 Reduced residual ridges induce a decrease in the size of the denture-bearing areas, resulting in problems with denture stability. In these circumstances, repeatability of occlusal contacts is impossible, and controlling the direction of bite forces is
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Türk et al
difficult. With horizontal movement of the dentures, soft tissue impingements and rapid bone resorption may occur. With the connection of implants to OVDs, patients have a repeatable centric occlusion because of stabilization of the dentures and because lateral forces have a reduced effect on the dentures.6 Stabilization of dentures with implants provides a significant improvement in the guidance of mandibular movements and allows more harmonic and well-organized chewing movements.7 Moreover, nearly all patients subjectively report an improvement in their chewing functions when rehabilitated with implant OVDs.8 The use of two to four implants to support mandibular OVDs is a superior treatment as compared with conventional dentures. Clinical follow-up studies have reported predictable long-term treatment outcomes.1–3 Several attachment systems are used to connect implants to mandibular OVDs. Implants may be splinted or left unsplinted while connecting. All anchoring mechanisms used for retaining implant OVDs produce superior results as compared with complete dentures.8–10 However, implant-retained OVDs require routine maintenance and periodic repairs, because the implants and OVDs are subjected to biomechanical forces.11 When two implants are used in the anterior mandible to retain an OVD, the most common attachment used is the ball attachment.12,13 A prefabricated, self-aligning attachment system that maintains both vertical and hinge resiliency has recently been introduced and is called a locator attachment. The locator system has been promoted as an alternative to ball attachments, especially when the interarch distance or the height of the denture is inadequate for processing ball attachments. Locator attachments have become widely used and are marketed by most implant companies.13–15 Generally, the selection of an attachment system depends on the experience and preference of clinicians.16–18 It should also be noted that attachment systems clearly influence prosthodontic maintenance, particularly regarding the type of matrices used.18–20 It is vital for clinicians to be aware of the frequency of postinsertion maintenance requirements during the use of each specific attachment system to choose the treatment that requires the fewest repairs17 and will satisfy the retention expectation of the patient. The most frequent complications related to mandibular implant OVDs are loss of retention over time and damage to the retention mechanisms.16–20 Loss of retention negatively affects patient satisfaction, and a nonretentive implant OVD may cause bone resorption or soft tissue problems. Consequently, the authors designed this study to compare the retention loss of ball and locator attachments over a long period of time.
Fig 1 UHMWP blocks with the inserted implants. The UHMWP blocks are made with (left) Chirulen 1020x and (right) Chirulen 1020E.
The null hypothesis was that no difference exists in the retention properties of these two attachment types.
MATERIALS AND METHODS Two single attachment systems that are frequently used for the retention of implant OVDs were evaluated. The retentive properties of ball attachments were compared with those of locator attachments. Using a computer numerically controlled (CNC) machine (Haas VF-2SS, Haas Automation), two rectangular ultra-highmolecular-weight polyethylene (UHMWP) blocks were prepared (Chirulen 1020x, Chirulen 1020E, Quadrant) with dimensions of 15 × 45 × 20 mm. Two parallel holes (22 mm apart) with a diameter of 2 mm and length of 12 mm were prepared on each block using the CNC machine as a guide for implant placement. In each block, two implant beds were prepared according to the standard drilling protocol recommended by the manufacturer (Biohorizons). Two dental implants (TLX Laser-Lok, Biohorizons) with a diameter of 3.8 mm and length of 12 mm were inserted into the prepared beds. Surgical cover caps were then screwed to the implants. The UHMWP block fabricated with Chirulen 1020x was used for ball attachments, and the UHMWP block fabricated with Chirulen 1020E was used for locator attachments. There were no differences between the mechanical properties of the two blocks. Different materials were used for color identification of each block (Fig 1).
Preparation of Prosthetic Components
A polyvinylsiloxane putty material (Panasil, Kettenbach) was mixed within the mixing time recommended by the manufacturer and shaped into a rectangular box. The UHMWP blocks were embedded into the impression material, and after the impression material was set, the blocks were removed. Autopolymerizing The International Journal of Oral & Maxillofacial Implants 1107
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Türk et al
Fig 2 Twenty acrylic prosthetic components were fabricated. Fig 3 Biomechanical testing machine designed for creating insertion-separation cycles under reproducible conditions.
Fig 4 Universal testing machine used to measure the retentive force for each prosthetic component at a crosshead speed of 50 mm/min.
acrylic resin (Paladent, Heraeus Kulzer) was mixed and poured into the prepared silicon spaces of the UHMWP blocks. After polymerization, the acrylic blocks were removed and placed into a high-pressure curing unit for 15 minutes. Twenty prosthetic components, 10 blocks of each type, were fabricated using this method (Fig 2).
Connecting the Prosthetic Components aand the UHMWP Blocks
The surgical cover caps were removed from one of the UHMWP blocks, and ball abutments were screwed into the implants. On a second UHMWP block, locator abutments were screwed into the implants. All
abutments were fastened with 25 N of force using a torque wrench from the manufacturer. O-ring spacers were placed on the ball abutments, and locator spacers were placed on the locator abutments to prevent the flow of acrylic resin into the areas with undercuts.14 Clix Females (Biohorizons) were placed on the ball abutments, and locator processing caps were placed on the locator abutments. Approximately 5 mm of acrylic was removed from one surface of the prosthetic components with a round burr (#140. 277. 040, Acurata Imperial). An adequate amount of autopolymerizing acrylic resin was mixed and applied to the relief areas of the prosthetic components. The prosthetic components were applied to the UHMWP blocks. After the final polymerization, they were removed, and excess acrylic around the attachments was cleaned with a small round bur (#175. 001. 050, Acurata Imperial). The two holes in the block were widened slightly to ensure that the obtained values would represent the retentive values of the clips only.21 Ten prosthetic components were connected to the UHMWP block with ball abutments using matching ball attachments, and the remaining 10 prosthetic components were connected to the UHMWP block with locator abutments using matching locator attachments. For all abutments, the pink attachments, which are described as moderately retentive by the manufacturer, were used for testing. The prosthetic components and the UHMWP blocks were mounted onto a biomechanical testing machine designed for making reproducible insertion-separation cycles (Fig 3). The testing machine allowed a tensile force to be applied to the prosthetic components until the ball/locator attachments separated from the abutments. All samples were subjected to 5,000 insertion-separation cycles. Measurement of the retentive forces was started after 10 insertions. At the end of 10, 100, 200, 300, 400, 500, 1,000, 1,500, 2,000, 3,000, 4,000, and 5,000 insertion-separation cycles, a universal testing machine (Zwick Z250, Zwick/Roell; Fig 4) was used to test the retentive force for each prosthetic component at a crosshead speed of 50 mm/min. Five measurements were made for each sample, and the average was recorded as one value. Additionally, wear of the attachments was measured using scanning electron microscopy (SEM; Figs 5a and 5b) at the end of 5,000 cycles. The outer and inner diameters of the ball and locator attachments were measured and subtracted from the original diameters supplied by the manufacturer. The measured wear of the ball and locator attachments was then compared (Fig 6).
Statistical Analyses
For statistical analysis of the results, the NCSS 2007 and PASS 2008 statistical software (NCSS) were used.
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Türk et al
1471.8 μm 3.724 mm
2.016 mm
1453.1 μm 2.021 mm
1468.8 μm 3.781 mm
3.635 mm
2.344 mm
0.729 mm
2415.6 μm
a
2446.9 μm
4.066 mm
0.721 mm
0.737 mm
b
3.826 mm
Figs 5a and 5b Measurement of the wear of the (a) ball attachments and (b) locator attachments using SEM. The horizontal, vertical, and diagonal lines were drawn across the diameter of the attachments for measurement.
3.560 mm
2.314 mm
3.421 mm
2.194 mm
1.928 mm
1.934 mm
3.438 mm
2.246 mm
2.433 mm
1.852 mm
1.564 mm
1.840 mm
1.992 mm
3.677 mm
1.863 mm
1.904 mm
Fig 6 SEM photographs show the wear of (top) ball attachments, and (bottom) locator attachments at the end of 5,000 insertionseparation cycles.
Aside from descriptive statistics (means and standard deviations), the Student t test was used for the comparison of two groups with parameters of normal distribution. The percentage changes of the parameters not showing normal distribution were compared with the Mann-Whitney U test. The results were assessed at a 95% confidence interval, at a significance level of .05.
RESULTS The mean retentive values of ball attachments at each cycle and the significance of retention loss as compared with the previous measurement cycle are presented in Table 1. Statistically significant retention loss was observed after 100, 200, 400, 500, 1,500, and 4,000
cycles (P < .05; Table 1). No significant differences were detected after 300, 1,000, 2,000, 3,000, and 5,000 cycles (P > .05; Table 1). The mean retentive values of locator attachments at each cycle and the significance of retention loss as compared with the previous measurement cycle are presented in Table 2. Statistically significant retention loss was observed after 100, 200, 300, 500, and 3,000 cycles compared with the previous cycle (P < .05; Table 2). No significant differences were detected after 400, 1,000, 1,500, 2,000, 4,000, and 5,000 cycles compared with the previous cycle (P > .05; Table 2). A comparison of retention losses for the two attachment systems is presented in Table 3. Retention loss of ball attachments was significantly more frequent than that of locator attachments between cycles 100 and 200 The International Journal of Oral & Maxillofacial Implants 1109
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Türk et al
Table 1 Mean Retentive Values of Ball Attachments at Each Cycle and Significance of Retention Loss as Compared with the Previous Measurement Cycle Cycle
Retention Force (N) (mean ± SD)
P†
Table 2 Mean Retentive Values of Locator Attachments at Each Cycle and Significance of Retention Loss as Compared with the Previous Measurement Cycle Cycle 10
Retention Force (N) (mean ± SD)
P†
52.47 ± 6.70
10
32.91 ± 5.30
100
25.34 ± 5.33
.004*
100
43.71 ± 7.68
.026*
200
20.69 ± 7.26
.003*
200
40.38 ± 7.47
.027*
300
19.76 ± 7.66
.475
300
36.43 ± 7.57
.024*
400
21.60 ± 7.83
.038**
400
35.85 ± 9.48
.789
500
19.88 ± 8.15
.026*
500
32.24 ± 11.64
.023*
1,000
18.43 ± 8.91
.266
1,000
32.15 ± 12.76
.960
1,500
14.72 ± 7.54
.014**
1,500
29.83 ± 11.28
.152
2,000
13.84 ± 7.04
.177
2,000
29.40 ± 12.97
.681
3,000
12.73 ± 7.86
.111
3,000
25.58 ± 11.62
.032*
4,000
10.49 ± 7.59
.004*
4,000
19.74 ± 11.00
.106
5,000
9.70 ± 7.94
.310
5,000
21.70 ± 10.13
.220
†P
values are from a paired-sample t test with respect to the force generated for the previous measurement cycle. * P < .01. ** P < .05.
†P
Table 3 Comparison of Retention Losses of Ball and Locator Attachment Systems
Table 4 Retention Losses of Ball and Locator Attachment Systems as Compared with the Initial Retention Value
Retention loss (%) (mean ± SD) Cycle
Ball
10–100
21.92 ± 17.59
Locator
values are from a paired-sample t test with respect to the force generated for the previous measurement cycle. * P < .05.
Retention loss (%) (mean ± SD)
P†
15.09 ± 19.78
.326
Cycle
Ball
Locator
P†
100–200
19.83 ± 17.14
7.50 ± 9.50
.041*
10–100
21.92 ± 17.59
15.09 ± 19.78
.326
200–300
3.41 ± 23.04
9.68 ± 10.24
.545
10–200
36.42 ± 24.20
21.41 ± 19.62
.096
300–400
−11.05 ± 16.12
0.93 ± 18.81
.151
10–300
39.55 ± 24.20
29.13 ± 18.85
.290
11.72 ± 13.59
.999
10–400
34.21 ± 23.15
30.53 ± 21.36
.762
39.22 ± 25.72
37.38 ± 24.86
.705 .597
400–500
9.32 ± 9.90 7.95 ± 17.23
−0.76 ± 25.42
.705
10–500
1,000–1,500
16.37 ± 22.63
5.14 ± 12.54
.151
10–1,000
42.71 ± 30.00
36.73 ± 29.70
1,500–2,000
4.55 ± 15.38
3.28 ± 11.35
.821
10–1,500
53.66 ± 25.99
41.98 ± 23.82
.290
56.81 ± 24.04
42.70 ± 21.16
.174
500–1,000
2,000–3,000
11.18 ± 17.47
11.31 ± 20.53
.762
10–2,000
3,000–4,000
21.34 ± 20.03
17.61 ± 40.86
.821
10–3,000
59.90 ± 27.44
50.00 ± 24.15
.290
4,000–5,000
7.61 ± 27.89
−22.96 ± 42.65
.082
10–4,000
67.49 ± 25.08
60.39 ± 25.28
.406
10–5,000
69.43 ± 27.61
57.56 ± 21.65
.049*
†P
values are from a Mann-Whitney U test. * P < .05.
Mann-Whitney U test. *P < .05.
(P = .041). No significant differences were detected between the two attachment systems during the other cycles (P > .05; Table 3). The percent of retention loss of ball and locator attachments during each cycle as compared with the retention force measured after 10 cycles, which was regarded as the initial retention value, is presented in Table 4. At the end of the experiment, ball attachments had lost 69.43 ± 27.61% of the initial retention value, and locator attachments had lost 57.56 ± 21.65% of
the initial retention value. The retention loss of ball attachments at the end of 5,000 cycles was detected significantly more often than that of locator attachments (P = .049). No significant differences were detected between the retention losses of the two attachment systems during the other cycles as compared with the initial retention values (P > .05; Table 4). Additionally, for both attachment systems, significant retention loss was detected at every measurement cycle as compared with the initial measurement cycle (P < .05).
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No significant difference was detected between the wear of the two attachment systems as measured using SEM at the end of 5,000 cycles (P > .05; Table 5).
Table 5 Wear of the Ball and Locator Attachment Systems Measured with SEM Measurement
Ball*
Locator*
P†
Outer diameter difference (mm)
0.14 ± 0.07
0.17 ± 0.11
.686
DISCUSSION
Inner diameter difference (mm)
0.08 ± 0.08
0.11 ± 0.14
.936
Most in vitro studies that have investigated the retention forces of various attachment types have used implant analogs embedded in dental stones, aluminum, or acrylic resin blocks.21–26 In the present study, implants and UHMWP blocks were used instead of implant analogs and the above materials to simulate the osseointegration process. The experimental procedures were performed without simulating in vivo conditions, which is a limitation of this study. The presence of saliva and constant occlusal load may affect the rate of attachment wear and thus the retentive values. The presence of soft tissue resiliency may increase the load on the abutments and therefore affect the retentive values.21 However, artificial substitutes are not considered to be as efficacious as human saliva or soft tissue.27 Furthermore, simulation of such factors is difficult in an in vitro study, and those factors are better evaluated in clinical trials.22 Although in vitro studies differ from clinical studies, they allow standardization of the tests by excluding oral conditions, and therefore, they provide important information.28 Sarnat29 reported the speed of OVD removal as approximately 50 mm/min in vivo. Most in vitro studies25,30–33 that have investigated the retention forces of OVDs, including the current study, have used this value as the crosshead speed. Prefabricated attachments require a certain period of use before their function is optimized, and optimization of this function using laboratory procedures with multiple insertion-separation movements is recommended before intraoral use.34 Consequently, we measured the retentive forces after 10 cycles of insertion and separation. In most in vitro studies23–25,28,35–40 that have evaluated the retention of OVD attachments, the attachments were subjected to 540 to 10,000 insertion-separation cycles, which corresponds to 6 months to 10 years of clinical use, respectively, assuming OVD removal three times a day for oral hygiene procedures.36 With a few exceptions,23,24,39 all the studies found various degrees of retention loss at the end of the experimental procedures,25,28,35–38 which is in agreement with the results of the present study. In light of these studies, the samples of the present study were subjected to 5,000 insertion-separation cycles corresponding to approximately 4.5 years of clinical use.
*Measured diameters were compared with the original manufacturerprovided diameter data and are shown as the mean decrease ± SD. †P values are from a Mann-Whitney U test and compare the diameter decreases between the two systems.
Attachment retention forces from 5 to 7 N are sufficient to stabilize OVDs during function.41 Based on this information, the retention forces of both attachment systems tested in the present study would be acceptable after 4.5 years (mean of 9.7 N for ball attachments and 21.7 N for locator attachments after 5,000 insertion-separation cycles). The initial retention capability of an attachment system is essential for maintaining patient satisfaction.42 The minimum amount of retention that is sufficient for patient satisfaction is 8 to 20 N for removable dentures.33,43 Although these are not absolute values, if they are employed as a reference, the initial retention values that the authors measured—a mean of 32.91 N for ball and 52.47 N for locator attachments— appear to be higher than the stated range and adequate for patient satisfaction. Furthermore, after an analysis that approximated 4.5 years of clinical use, the mean retention force of both attachments was still in the range of the mentioned limits. The results of the present study showed a larger variation in the retention forces measured for both attachment systems between the initial cycle and cycle 300, which is in agreement with previous studies.25,34,44–47 This may be due to more changes in the nylon components during the initial cycles.36 In the present study, the retention force decreased over time for both attachment systems. This finding is not surprising and is in accordance with previous in vitro investigations.25,37,38 This retention loss can be described by surface changes in the nylon components during cyclic loading, as confirmed by SEM (Fig 7). Additionally, ball attachments showed significantly more retention loss between the initial cycle and cycle 100. This finding is important and may be due to the different geometric shapes of the ball and locator abutments. These cycles correspond to the first and second month of OVD usage, which may be regarded as the adaptation period of patients. The most interesting finding of this study is the significantly lower percentage of retention loss for The International Journal of Oral & Maxillofacial Implants 1111
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Türk et al
the locator attachments at the end of 5,000 cycles (P = .049). This result requires rejection of the null hypothesis that no difference exists in the retention properties of the two attachment types. Moreover, although the authors observed a significant difference between the percentages of retention losses of the two attachment systems for other cycles as compared with the initial retention values, the locator attachment showed a lower percentage of retention loss at the end of all cycle measurements. This difference may be related to the different dimensions and designs of the patrices and matrices of the two attachment systems. The higher retention capability of locator attachments as compared with Hader bar clips and galvanoformed milled bar attachments has been shown.21,30,42 The superior retention of locator attachments over ball attachments was shown by Sadig33 in a study that used one cycle load application to dislodge the attachments. The results of the present study at the end of 5,000 cycles strengthen Sadig’s33 results. Most complication types concerning implant OVDs involve the activation or replacement of the matrix in the prosthesis because of the wear on the plastic parts.48 The two attachment types tested in the present study showed comparable wear properties after 5,000 cycles. Of note, although not significant, locator attachments showed more wear on both the outer and inner diameter measurements, which may result from the different types of nylon used to fabricate the plastic components.49 Although relatively more wear effects were observed with locator attachments, the force decreased less than with ball attachments. Therefore, a connection between wear and retention force was not clearly detected, which is in accordance with a previous study.49 The nylon elements of the locator system are in the negative form of the abutments. Consequently, retention forces are supplied to both the inner and outer areas of the attachments.14 The inner areas of the locator system may have limited the ability of the wear patterns to affect the retentive forces of the locator. Accordingly, direct comparison of the wear properties and investigation of the connection between wear and retention for these two attachment systems are questionable. Metal components obviously would not develop wear patterns to the same extent as plastic components. Therefore, comparing the wear properties of attachments with metal and plastic housings may be of interest in future studies.
CONCLUSIONS Within the limitations of the present study, the following conclusions can be drawn.
The retention forces of the ball and locator attachments tested in the present study were acceptable after 5,000 insertion-separation cycles, which corresponds to approximately 4.5 years of clinical use. Between 100 and 200 insertion-separation cycles, locator attachments showed better retentive properties than ball attachments. After 5,000 insertion-separation cycles, locator attachments showed better retentive properties than ball attachments. Both attachment systems revealed a decrease in retentive forces at the end of the 5,000 insertion-separation cycles as compared with the initial cycle. Both attachment systems showed wear patterns after 5,000 insertion-separation cycles, but this wear did not differ significantly between the attachment systems.
ACKNOWLEDGMENTS The authors reported no conflicts of interest related to this study.
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13. Bilhan H, Geckili O, Sulun T, Bilgin T. A quality-of-life comparison between self-aligning and ball attachment systems for 2-implant-retained mandibular overdentures. J Oral Implantol 2011;37:167–173. 14. Geckili O, Bilhan H, Bilgin T. Locator attachments as an alternative to ball attachments in 2-implant retained mandibular overdentures. J Can Dent Assoc 2007;73:679–682. 15. Krennmair G, Seemann R, Fazekas A, Ewers R, Piehslinger E. Patient preference and satisfaction with implant-supported mandibular overdentures retained with ball or locator attachments: A crossover clinical trial. Int J Oral Maxillofac Implants 2012;27:1560–1568. 16. Kim HY, Lee JY, Shin SW, Bryant SR. Attachment systems for mandibular implant overdentures: A systematic review. J Adv Prosthodont 2012;4:197–203. 17. Kiener P, Oetterli M, Mericske E, Mericske-Stern R. Effectiveness of maxillary overdentures supported by implants: Maintenance and prosthetic complications. Int J Prosthodont 2001;14:133–140. 18. Rentsch-Kollar A, Huber S, Mericske-Stern R. Mandibular implant overdentures followed for over 10 years: Patient compliance and prosthetic maintenance. Int J Prosthodont 2010;23:91–98. 19. Watson GK, Payne AG, Purton DG, Thomson WM. Mandibular overdentures: Comparative evaluation of prosthodontic maintenance of three different implant systems during the first year of service. Int J Prosthodont 2002;15:259–266. 20. Mackie A, Lyons K, Thomson WM, Payne AG. Mandibular twoimplant overdentures: Prosthodontic maintenance using different loading protocols and attachment systems. Int J Prosthodont 2011;24:405–416. 21. Evtimovska E, Masri R, Driscoll CF, Romberg E. The change in retentive values of locator attachments and hader clips over time. J Prosthodont 2009;18:479–483. 22. Abi Nader S, de Souza RF, Fortin D, De Koninck L, Fromentin O, Albuquerque Jr RF. Effect of simulated masticatory loading on the retention of stud attachments for implant overdentures. J Oral Rehabil 2011;38:157–164. 23. Botega DM, Mesquita MF, Henriques GEP, Vaz LG. Retention force and fatigue strength of overdenture attachment systems. J Oral Rehabil 2004;31:884–889. 24. Fu CC, Hsu YT. A comparison of retention characteristics in prefabricated and custom-cast dental attachments. J Prosthodont 2009;18:388–392. 25. Rutkunas V, Mizutani H, Takahashi H. Influence of attachment wear on retention of mandibular overdenture. J Oral Rehabil 2007;34:41–51. 26. Setz J, Hyung S, Engel E. Retention of prefabricated attachments for implant stabilized overdentures in the edentulous mandible: An in vitro study. J Prosthet Dent 1998;80:323–329. 27. Turssi CP, Faraoni JJ, de Menezes M, Serra MC. Analysis of potential lubricants for in vitro wear testing. Dent Mater 2006;22:77–83. 28. Rodrigues RC, Faria AC, Macedo AP, Sartori IA, de Mattos Mda G, Ribeiro RF. An in vitro study of non-axial forces upon the retention of an O-ring attachment. Clin Oral Implants Res 2009;20:1314-1319. 29. Sarnat AE. The efficiency of cobalt samarium (Co5Sm) magnets as retention units for overdentures. J Dent 1983;11:324–333. 30. Chung KH, Chung CY, Cagna DR, Cronin Jr RJ. Retention characteristics of attachment systems for implant overdentures. J Prosthodont 2004;13:221–226.
31. Michelinakis G, Barclay CW, Smith PW. The influence of interimplant distance and attachment type on the retention of mandibular overdentures on 2 implants: Initial retention value. Int J Prosthodont 2006;19:507–512. 32. Doukas D, Michelinakis G, Smith PW, Barclay CW. The influence of interimplant distance and attachment type on the retention characteristics of mandibular overdentures on 2 implants: 6-month fatigue retention values. Int J Prosthodont 2008;21:152–154. 33. Sadig W. A comparative in vitro study on the retention and stability of implant-supported overdentures. Quintessence Int 2009;40:313–319. 34. Besimo CE, Guarneri A. In vitro retention force changes of prefabricated attachments for overdentures. J Oral Rehabil 2003;30:671–678. 35. Besimo CH, Graber G, Flühler M. Retention force changes in implant-supported titanium telescope crowns over long-term use in vitro. J Oral Rehabil 1996;23:372–378. 36. Branchi R, Vangi D, Virga A, Guertin G, Fazi G. Resistance to wear of four matrices with ball attachments for implant overdentures: A fatigue study. J Prosthodont 2010;8:1–6. 37. Fromentin O, Picard B, Tavernier B. In vitro study of the retention and mechanical fatigue behavior of four implant overdenture studtype attachments. Pract Periodont Aesthet Dent 1999;11:391–397. 38. Gamborena JI, Hazelton LR, Nabadalung D, et al. Retention of ERA direct overdenture attachments before and after fatigue loading. Int J Prosthodont 1997;10:123–130. 39. Pigozzo MN, Mesquita MF, Henriques GE, Vaz LG. The service life of implant-retained overdenture attachment systems. J Prosthet Dent 2009;102:74–80. 40. Wolf K, Ludwig K, Hartfil H, Kern M. Analysis of retention and wear of ball attachments. Quintessence Int 2009;40:405–412. 41. Lehmann KM, Arnim FV. Studies on the retention forces of snap-on attachments. Quintessence Dent Technol 1978;7:45–48. 42. Uludag B, Polat S. Retention characteristics of different attachment systems of mandibular overdentures retained by two or three implants. Int J Oral Maxillofac Implants 2012;27:1509–1513. 43. Trakas T, Michalakis K, Kang K, Hirayama H. Attachment systems for implant retained overdentures: A literature review. Implant Dent 2006;15:24–34. 44. Breeding LC, Dixon DL, Schmitt S. The effect of simulated function on the retention of bar-clip retained removable prostheses. J Prosthet Dent 1996;75:570–573. 45. Epstein DD, Epstein PL, Cohen BI, Pagnillo MK. Comparison of the retentive properties of six prefabricated post overdenture attachment systems. J Prosthet Dent 1999;82:579–584. 46. Svetlize CA, Bodereau Jr EF. Comparative study of retentive anchor systems for overdentures. Quintessence Int 2004;35:443–448. 47. Williams BH, Ochiai KT, Hojo S, Nishimura R, Caputo AA. Retention of maxillary implant overdenture bars of different designs. J Prosthet Dent 2001;86:603–607. 48. Gotfredsen K, Holm B. Implant-supported mandibular overdentures retained with ball or bar attachments: A randomized prospective 5-year study. Int J Prosthodont 2000;13:125–130. 49. Rutkunas V, Mizutani H, Takahashi H, Iwasaki N. Wear simulation effects on overdenture stud attachments. Dent Mater J 2011;30:845–853.
The International Journal of Oral & Maxillofacial Implants 1113 © 2014 BY QUINTESSENCE PUBLISHING CO, INC. PRINTING OF THIS DOCUMENT IS RESTRICTED TO PERSONAL USE ONLY. NO PART MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM WITHOUT WRITTEN PERMISSION FROM THE PUBLISHER.
T
he necessity of suffering from uncomfortable dentures was eliminated with the introduction of dental implants to the field of dentistry in the early 1980s. In particular, the problems of lack of stability and the retention of mandibular dentures have been solved by fabricating a fixed prosthesis or retaining an overdenture (OVD) to implants when the implant number is limited because of anatomical or social complexities. Older individuals prefer an implant-retained OVD 1Private
Practice, Istanbul, Turkey. Professor, Department of Prosthodontics, Istanbul University, Faculty of Dentistry, Istanbul, Turkey. 3Associate Professor, TUBITAK-MRC, Materials and Chemical Technologies Research Institute Istanbul, Turkey. 4Professor, Department of Prosthodontics, Istanbul University, Faculty of Dentistry, Istanbul, Turkey. 2 Associate
Correspondence to: Dr Onur Geckili, Istanbul University, Faculty of Dentistry, Department of Prosthodontics, 2nd floor, Çapa-Istanbul Turkey. Fax: +90-212-535 25 85. Email: [email protected] ©2014 by Quintessence Publishing Co Inc.
over an implant-retained fixed prosthesis if given the choice.1,2 This clearly indicates that implant-supported OVD treatment should not be considered a substandard treatment. Mandibular two-implant–supported OVDs opposing conventional complete maxillary dentures have been proposed as the standard of care for edentulous patients.3 The problems observed with the use of conventional mandibular dentures have been eliminated with the fixation of OVDs to the implants. Moreover, with the insertion of implants, bone resorption is dramatically decreased as compared with conventional denture use.4 The average height reduction of the posterior mandibular residual ridge in patients with conventional dentures is 1.63 mm, whereas the reduction is 0.69 mm in patients with implant OVDs.4 In patients with mandibular atrophy, conventional complete mandibular dentures generally move 10 mm in function.5 Reduced residual ridges induce a decrease in the size of the denture-bearing areas, resulting in problems with denture stability. In these circumstances, repeatability of occlusal contacts is impossible, and controlling the direction of bite forces is
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Türk et al
difficult. With horizontal movement of the dentures, soft tissue impingements and rapid bone resorption may occur. With the connection of implants to OVDs, patients have a repeatable centric occlusion because of stabilization of the dentures and because lateral forces have a reduced effect on the dentures.6 Stabilization of dentures with implants provides a significant improvement in the guidance of mandibular movements and allows more harmonic and well-organized chewing movements.7 Moreover, nearly all patients subjectively report an improvement in their chewing functions when rehabilitated with implant OVDs.8 The use of two to four implants to support mandibular OVDs is a superior treatment as compared with conventional dentures. Clinical follow-up studies have reported predictable long-term treatment outcomes.1–3 Several attachment systems are used to connect implants to mandibular OVDs. Implants may be splinted or left unsplinted while connecting. All anchoring mechanisms used for retaining implant OVDs produce superior results as compared with complete dentures.8–10 However, implant-retained OVDs require routine maintenance and periodic repairs, because the implants and OVDs are subjected to biomechanical forces.11 When two implants are used in the anterior mandible to retain an OVD, the most common attachment used is the ball attachment.12,13 A prefabricated, self-aligning attachment system that maintains both vertical and hinge resiliency has recently been introduced and is called a locator attachment. The locator system has been promoted as an alternative to ball attachments, especially when the interarch distance or the height of the denture is inadequate for processing ball attachments. Locator attachments have become widely used and are marketed by most implant companies.13–15 Generally, the selection of an attachment system depends on the experience and preference of clinicians.16–18 It should also be noted that attachment systems clearly influence prosthodontic maintenance, particularly regarding the type of matrices used.18–20 It is vital for clinicians to be aware of the frequency of postinsertion maintenance requirements during the use of each specific attachment system to choose the treatment that requires the fewest repairs17 and will satisfy the retention expectation of the patient. The most frequent complications related to mandibular implant OVDs are loss of retention over time and damage to the retention mechanisms.16–20 Loss of retention negatively affects patient satisfaction, and a nonretentive implant OVD may cause bone resorption or soft tissue problems. Consequently, the authors designed this study to compare the retention loss of ball and locator attachments over a long period of time.
Fig 1 UHMWP blocks with the inserted implants. The UHMWP blocks are made with (left) Chirulen 1020x and (right) Chirulen 1020E.
The null hypothesis was that no difference exists in the retention properties of these two attachment types.
MATERIALS AND METHODS Two single attachment systems that are frequently used for the retention of implant OVDs were evaluated. The retentive properties of ball attachments were compared with those of locator attachments. Using a computer numerically controlled (CNC) machine (Haas VF-2SS, Haas Automation), two rectangular ultra-highmolecular-weight polyethylene (UHMWP) blocks were prepared (Chirulen 1020x, Chirulen 1020E, Quadrant) with dimensions of 15 × 45 × 20 mm. Two parallel holes (22 mm apart) with a diameter of 2 mm and length of 12 mm were prepared on each block using the CNC machine as a guide for implant placement. In each block, two implant beds were prepared according to the standard drilling protocol recommended by the manufacturer (Biohorizons). Two dental implants (TLX Laser-Lok, Biohorizons) with a diameter of 3.8 mm and length of 12 mm were inserted into the prepared beds. Surgical cover caps were then screwed to the implants. The UHMWP block fabricated with Chirulen 1020x was used for ball attachments, and the UHMWP block fabricated with Chirulen 1020E was used for locator attachments. There were no differences between the mechanical properties of the two blocks. Different materials were used for color identification of each block (Fig 1).
Preparation of Prosthetic Components
A polyvinylsiloxane putty material (Panasil, Kettenbach) was mixed within the mixing time recommended by the manufacturer and shaped into a rectangular box. The UHMWP blocks were embedded into the impression material, and after the impression material was set, the blocks were removed. Autopolymerizing The International Journal of Oral & Maxillofacial Implants 1107
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Türk et al
Fig 2 Twenty acrylic prosthetic components were fabricated. Fig 3 Biomechanical testing machine designed for creating insertion-separation cycles under reproducible conditions.
Fig 4 Universal testing machine used to measure the retentive force for each prosthetic component at a crosshead speed of 50 mm/min.
acrylic resin (Paladent, Heraeus Kulzer) was mixed and poured into the prepared silicon spaces of the UHMWP blocks. After polymerization, the acrylic blocks were removed and placed into a high-pressure curing unit for 15 minutes. Twenty prosthetic components, 10 blocks of each type, were fabricated using this method (Fig 2).
Connecting the Prosthetic Components aand the UHMWP Blocks
The surgical cover caps were removed from one of the UHMWP blocks, and ball abutments were screwed into the implants. On a second UHMWP block, locator abutments were screwed into the implants. All
abutments were fastened with 25 N of force using a torque wrench from the manufacturer. O-ring spacers were placed on the ball abutments, and locator spacers were placed on the locator abutments to prevent the flow of acrylic resin into the areas with undercuts.14 Clix Females (Biohorizons) were placed on the ball abutments, and locator processing caps were placed on the locator abutments. Approximately 5 mm of acrylic was removed from one surface of the prosthetic components with a round burr (#140. 277. 040, Acurata Imperial). An adequate amount of autopolymerizing acrylic resin was mixed and applied to the relief areas of the prosthetic components. The prosthetic components were applied to the UHMWP blocks. After the final polymerization, they were removed, and excess acrylic around the attachments was cleaned with a small round bur (#175. 001. 050, Acurata Imperial). The two holes in the block were widened slightly to ensure that the obtained values would represent the retentive values of the clips only.21 Ten prosthetic components were connected to the UHMWP block with ball abutments using matching ball attachments, and the remaining 10 prosthetic components were connected to the UHMWP block with locator abutments using matching locator attachments. For all abutments, the pink attachments, which are described as moderately retentive by the manufacturer, were used for testing. The prosthetic components and the UHMWP blocks were mounted onto a biomechanical testing machine designed for making reproducible insertion-separation cycles (Fig 3). The testing machine allowed a tensile force to be applied to the prosthetic components until the ball/locator attachments separated from the abutments. All samples were subjected to 5,000 insertion-separation cycles. Measurement of the retentive forces was started after 10 insertions. At the end of 10, 100, 200, 300, 400, 500, 1,000, 1,500, 2,000, 3,000, 4,000, and 5,000 insertion-separation cycles, a universal testing machine (Zwick Z250, Zwick/Roell; Fig 4) was used to test the retentive force for each prosthetic component at a crosshead speed of 50 mm/min. Five measurements were made for each sample, and the average was recorded as one value. Additionally, wear of the attachments was measured using scanning electron microscopy (SEM; Figs 5a and 5b) at the end of 5,000 cycles. The outer and inner diameters of the ball and locator attachments were measured and subtracted from the original diameters supplied by the manufacturer. The measured wear of the ball and locator attachments was then compared (Fig 6).
Statistical Analyses
For statistical analysis of the results, the NCSS 2007 and PASS 2008 statistical software (NCSS) were used.
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Türk et al
1471.8 μm 3.724 mm
2.016 mm
1453.1 μm 2.021 mm
1468.8 μm 3.781 mm
3.635 mm
2.344 mm
0.729 mm
2415.6 μm
a
2446.9 μm
4.066 mm
0.721 mm
0.737 mm
b
3.826 mm
Figs 5a and 5b Measurement of the wear of the (a) ball attachments and (b) locator attachments using SEM. The horizontal, vertical, and diagonal lines were drawn across the diameter of the attachments for measurement.
3.560 mm
2.314 mm
3.421 mm
2.194 mm
1.928 mm
1.934 mm
3.438 mm
2.246 mm
2.433 mm
1.852 mm
1.564 mm
1.840 mm
1.992 mm
3.677 mm
1.863 mm
1.904 mm
Fig 6 SEM photographs show the wear of (top) ball attachments, and (bottom) locator attachments at the end of 5,000 insertionseparation cycles.
Aside from descriptive statistics (means and standard deviations), the Student t test was used for the comparison of two groups with parameters of normal distribution. The percentage changes of the parameters not showing normal distribution were compared with the Mann-Whitney U test. The results were assessed at a 95% confidence interval, at a significance level of .05.
RESULTS The mean retentive values of ball attachments at each cycle and the significance of retention loss as compared with the previous measurement cycle are presented in Table 1. Statistically significant retention loss was observed after 100, 200, 400, 500, 1,500, and 4,000
cycles (P < .05; Table 1). No significant differences were detected after 300, 1,000, 2,000, 3,000, and 5,000 cycles (P > .05; Table 1). The mean retentive values of locator attachments at each cycle and the significance of retention loss as compared with the previous measurement cycle are presented in Table 2. Statistically significant retention loss was observed after 100, 200, 300, 500, and 3,000 cycles compared with the previous cycle (P < .05; Table 2). No significant differences were detected after 400, 1,000, 1,500, 2,000, 4,000, and 5,000 cycles compared with the previous cycle (P > .05; Table 2). A comparison of retention losses for the two attachment systems is presented in Table 3. Retention loss of ball attachments was significantly more frequent than that of locator attachments between cycles 100 and 200 The International Journal of Oral & Maxillofacial Implants 1109
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Türk et al
Table 1 Mean Retentive Values of Ball Attachments at Each Cycle and Significance of Retention Loss as Compared with the Previous Measurement Cycle Cycle
Retention Force (N) (mean ± SD)
P†
Table 2 Mean Retentive Values of Locator Attachments at Each Cycle and Significance of Retention Loss as Compared with the Previous Measurement Cycle Cycle 10
Retention Force (N) (mean ± SD)
P†
52.47 ± 6.70
10
32.91 ± 5.30
100
25.34 ± 5.33
.004*
100
43.71 ± 7.68
.026*
200
20.69 ± 7.26
.003*
200
40.38 ± 7.47
.027*
300
19.76 ± 7.66
.475
300
36.43 ± 7.57
.024*
400
21.60 ± 7.83
.038**
400
35.85 ± 9.48
.789
500
19.88 ± 8.15
.026*
500
32.24 ± 11.64
.023*
1,000
18.43 ± 8.91
.266
1,000
32.15 ± 12.76
.960
1,500
14.72 ± 7.54
.014**
1,500
29.83 ± 11.28
.152
2,000
13.84 ± 7.04
.177
2,000
29.40 ± 12.97
.681
3,000
12.73 ± 7.86
.111
3,000
25.58 ± 11.62
.032*
4,000
10.49 ± 7.59
.004*
4,000
19.74 ± 11.00
.106
5,000
9.70 ± 7.94
.310
5,000
21.70 ± 10.13
.220
†P
values are from a paired-sample t test with respect to the force generated for the previous measurement cycle. * P < .01. ** P < .05.
†P
Table 3 Comparison of Retention Losses of Ball and Locator Attachment Systems
Table 4 Retention Losses of Ball and Locator Attachment Systems as Compared with the Initial Retention Value
Retention loss (%) (mean ± SD) Cycle
Ball
10–100
21.92 ± 17.59
Locator
values are from a paired-sample t test with respect to the force generated for the previous measurement cycle. * P < .05.
Retention loss (%) (mean ± SD)
P†
15.09 ± 19.78
.326
Cycle
Ball
Locator
P†
100–200
19.83 ± 17.14
7.50 ± 9.50
.041*
10–100
21.92 ± 17.59
15.09 ± 19.78
.326
200–300
3.41 ± 23.04
9.68 ± 10.24
.545
10–200
36.42 ± 24.20
21.41 ± 19.62
.096
300–400
−11.05 ± 16.12
0.93 ± 18.81
.151
10–300
39.55 ± 24.20
29.13 ± 18.85
.290
11.72 ± 13.59
.999
10–400
34.21 ± 23.15
30.53 ± 21.36
.762
39.22 ± 25.72
37.38 ± 24.86
.705 .597
400–500
9.32 ± 9.90 7.95 ± 17.23
−0.76 ± 25.42
.705
10–500
1,000–1,500
16.37 ± 22.63
5.14 ± 12.54
.151
10–1,000
42.71 ± 30.00
36.73 ± 29.70
1,500–2,000
4.55 ± 15.38
3.28 ± 11.35
.821
10–1,500
53.66 ± 25.99
41.98 ± 23.82
.290
56.81 ± 24.04
42.70 ± 21.16
.174
500–1,000
2,000–3,000
11.18 ± 17.47
11.31 ± 20.53
.762
10–2,000
3,000–4,000
21.34 ± 20.03
17.61 ± 40.86
.821
10–3,000
59.90 ± 27.44
50.00 ± 24.15
.290
4,000–5,000
7.61 ± 27.89
−22.96 ± 42.65
.082
10–4,000
67.49 ± 25.08
60.39 ± 25.28
.406
10–5,000
69.43 ± 27.61
57.56 ± 21.65
.049*
†P
values are from a Mann-Whitney U test. * P < .05.
Mann-Whitney U test. *P < .05.
(P = .041). No significant differences were detected between the two attachment systems during the other cycles (P > .05; Table 3). The percent of retention loss of ball and locator attachments during each cycle as compared with the retention force measured after 10 cycles, which was regarded as the initial retention value, is presented in Table 4. At the end of the experiment, ball attachments had lost 69.43 ± 27.61% of the initial retention value, and locator attachments had lost 57.56 ± 21.65% of
the initial retention value. The retention loss of ball attachments at the end of 5,000 cycles was detected significantly more often than that of locator attachments (P = .049). No significant differences were detected between the retention losses of the two attachment systems during the other cycles as compared with the initial retention values (P > .05; Table 4). Additionally, for both attachment systems, significant retention loss was detected at every measurement cycle as compared with the initial measurement cycle (P < .05).
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No significant difference was detected between the wear of the two attachment systems as measured using SEM at the end of 5,000 cycles (P > .05; Table 5).
Table 5 Wear of the Ball and Locator Attachment Systems Measured with SEM Measurement
Ball*
Locator*
P†
Outer diameter difference (mm)
0.14 ± 0.07
0.17 ± 0.11
.686
DISCUSSION
Inner diameter difference (mm)
0.08 ± 0.08
0.11 ± 0.14
.936
Most in vitro studies that have investigated the retention forces of various attachment types have used implant analogs embedded in dental stones, aluminum, or acrylic resin blocks.21–26 In the present study, implants and UHMWP blocks were used instead of implant analogs and the above materials to simulate the osseointegration process. The experimental procedures were performed without simulating in vivo conditions, which is a limitation of this study. The presence of saliva and constant occlusal load may affect the rate of attachment wear and thus the retentive values. The presence of soft tissue resiliency may increase the load on the abutments and therefore affect the retentive values.21 However, artificial substitutes are not considered to be as efficacious as human saliva or soft tissue.27 Furthermore, simulation of such factors is difficult in an in vitro study, and those factors are better evaluated in clinical trials.22 Although in vitro studies differ from clinical studies, they allow standardization of the tests by excluding oral conditions, and therefore, they provide important information.28 Sarnat29 reported the speed of OVD removal as approximately 50 mm/min in vivo. Most in vitro studies25,30–33 that have investigated the retention forces of OVDs, including the current study, have used this value as the crosshead speed. Prefabricated attachments require a certain period of use before their function is optimized, and optimization of this function using laboratory procedures with multiple insertion-separation movements is recommended before intraoral use.34 Consequently, we measured the retentive forces after 10 cycles of insertion and separation. In most in vitro studies23–25,28,35–40 that have evaluated the retention of OVD attachments, the attachments were subjected to 540 to 10,000 insertion-separation cycles, which corresponds to 6 months to 10 years of clinical use, respectively, assuming OVD removal three times a day for oral hygiene procedures.36 With a few exceptions,23,24,39 all the studies found various degrees of retention loss at the end of the experimental procedures,25,28,35–38 which is in agreement with the results of the present study. In light of these studies, the samples of the present study were subjected to 5,000 insertion-separation cycles corresponding to approximately 4.5 years of clinical use.
*Measured diameters were compared with the original manufacturerprovided diameter data and are shown as the mean decrease ± SD. †P values are from a Mann-Whitney U test and compare the diameter decreases between the two systems.
Attachment retention forces from 5 to 7 N are sufficient to stabilize OVDs during function.41 Based on this information, the retention forces of both attachment systems tested in the present study would be acceptable after 4.5 years (mean of 9.7 N for ball attachments and 21.7 N for locator attachments after 5,000 insertion-separation cycles). The initial retention capability of an attachment system is essential for maintaining patient satisfaction.42 The minimum amount of retention that is sufficient for patient satisfaction is 8 to 20 N for removable dentures.33,43 Although these are not absolute values, if they are employed as a reference, the initial retention values that the authors measured—a mean of 32.91 N for ball and 52.47 N for locator attachments— appear to be higher than the stated range and adequate for patient satisfaction. Furthermore, after an analysis that approximated 4.5 years of clinical use, the mean retention force of both attachments was still in the range of the mentioned limits. The results of the present study showed a larger variation in the retention forces measured for both attachment systems between the initial cycle and cycle 300, which is in agreement with previous studies.25,34,44–47 This may be due to more changes in the nylon components during the initial cycles.36 In the present study, the retention force decreased over time for both attachment systems. This finding is not surprising and is in accordance with previous in vitro investigations.25,37,38 This retention loss can be described by surface changes in the nylon components during cyclic loading, as confirmed by SEM (Fig 7). Additionally, ball attachments showed significantly more retention loss between the initial cycle and cycle 100. This finding is important and may be due to the different geometric shapes of the ball and locator abutments. These cycles correspond to the first and second month of OVD usage, which may be regarded as the adaptation period of patients. The most interesting finding of this study is the significantly lower percentage of retention loss for The International Journal of Oral & Maxillofacial Implants 1111
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Türk et al
the locator attachments at the end of 5,000 cycles (P = .049). This result requires rejection of the null hypothesis that no difference exists in the retention properties of the two attachment types. Moreover, although the authors observed a significant difference between the percentages of retention losses of the two attachment systems for other cycles as compared with the initial retention values, the locator attachment showed a lower percentage of retention loss at the end of all cycle measurements. This difference may be related to the different dimensions and designs of the patrices and matrices of the two attachment systems. The higher retention capability of locator attachments as compared with Hader bar clips and galvanoformed milled bar attachments has been shown.21,30,42 The superior retention of locator attachments over ball attachments was shown by Sadig33 in a study that used one cycle load application to dislodge the attachments. The results of the present study at the end of 5,000 cycles strengthen Sadig’s33 results. Most complication types concerning implant OVDs involve the activation or replacement of the matrix in the prosthesis because of the wear on the plastic parts.48 The two attachment types tested in the present study showed comparable wear properties after 5,000 cycles. Of note, although not significant, locator attachments showed more wear on both the outer and inner diameter measurements, which may result from the different types of nylon used to fabricate the plastic components.49 Although relatively more wear effects were observed with locator attachments, the force decreased less than with ball attachments. Therefore, a connection between wear and retention force was not clearly detected, which is in accordance with a previous study.49 The nylon elements of the locator system are in the negative form of the abutments. Consequently, retention forces are supplied to both the inner and outer areas of the attachments.14 The inner areas of the locator system may have limited the ability of the wear patterns to affect the retentive forces of the locator. Accordingly, direct comparison of the wear properties and investigation of the connection between wear and retention for these two attachment systems are questionable. Metal components obviously would not develop wear patterns to the same extent as plastic components. Therefore, comparing the wear properties of attachments with metal and plastic housings may be of interest in future studies.
CONCLUSIONS Within the limitations of the present study, the following conclusions can be drawn.
The retention forces of the ball and locator attachments tested in the present study were acceptable after 5,000 insertion-separation cycles, which corresponds to approximately 4.5 years of clinical use. Between 100 and 200 insertion-separation cycles, locator attachments showed better retentive properties than ball attachments. After 5,000 insertion-separation cycles, locator attachments showed better retentive properties than ball attachments. Both attachment systems revealed a decrease in retentive forces at the end of the 5,000 insertion-separation cycles as compared with the initial cycle. Both attachment systems showed wear patterns after 5,000 insertion-separation cycles, but this wear did not differ significantly between the attachment systems.
ACKNOWLEDGMENTS The authors reported no conflicts of interest related to this study.
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