MINI-IMPLANT SUPPLEMENT
Journal of Orthodontics, Vol. 41, 2014, S8–S14
Translational mini-screw implant research Emile Rossouw Department of Orthodontics, University of North Carolina School of Dentistry at Chapel Hill, North Carolina, USA
It is important to thoroughly test new materials as well as techniques when these innovations are to be utilized in the human clinical situation. Translational research fills this important niche. The purpose of translational research is to establish the continuity of evidence from the laboratory to the clinic and in so-doing, provide evidence that the material is functioning appropriately and that the process in the human will be successful. This concept applies to the mini-screw implant; which, has been very successfully introduced into the orthodontic armamentarium over the last decade for application as a temporary anchorage device. The examples of translational research that will be illustrated in this paper have paved the way to ensure that clinicians have evidence to confidently utilize mini-screw implants in orthodontic practice. Needless to say, more studies are needed to ensure a safe, effective and efficient manner to practice orthodontics. Key words: Mini-screw implants, orthodontics, anchorage, bone-implant-contact, translational research Received 30 May 2014; accepted 31 May 2014
Introduction The logical requirements for an optimal mini-screw implant (MSI) anchorage system include stability during utilization; small dimensions; minimal surgical morbidity; easy placement and removal; simple and reliable attachment; capability of immediate loading; economical; broad range of application (not site specific) and withstand loads greater than clinically required. Oral and maxillofacial surgeons have utilized bone screws, which are similar to MSIs, for many years to provide stabile surgical fixation and immobilization of bone segments. We learnt from their surgical experience that MSI must be immobile, biocompatible, easily usable and independent of patient compliance. The MSI as used in orthodontics today, provides the greatest theoretical versatility and meets the aforementioned criteria. However, in their systematic review of anchorage, Feldmann and Bondemark (2006) suggested that further clinical and laboratory research studies were needed to evaluate new approaches, such as MSIs to enhance mechanics, and establish the potentials and limitations of these devices. This article aims to demonstrate how MSI studies have provided new evidence that has translated to the clinical environment to enhance orthodontic outcomes. Translational research is an important component of orthodontic research, which can have different meanings to different individuals. Simply put, it can be interpreted Address for correspondence: Dr. P. Emile Rossouw, Department of Orthodontics, 277 Brauer Hall, Campus Box 7450, University of North Carolina School of Dentistry, Chapel Hill, NC 27599-7450, USA Email: [email protected] # 2014 British Orthodontic Society
as the research information translated or moved from one area to enhance or improve the outcomes of another. Thus, data from a research laboratory translated for clinical implementation are often referred to as from ‘bench-to-bedside’. This transfer of new knowledge gained in the laboratory of how a specific apparatus, such as MSIs, function can be utilized for the development of new methods for diagnosis, treatment, and prevention of problems during treatment. Often this leads or translates to the first testing in humans (Fontanarosa and DeAngelis, 2002). In addition, the clinician can help shape the research protocol by seeking answers to difficult problems to which applied and/or basic research would only offer minimal answers, but the translational research effect could lead to significant improvements. The influence of MSI design on stability Torque is the rotational resistance to MSI insertion or removal. Using computerized tomography scanning Cha et al. (2010) determined that clinical placement and removal torque values were influenced by such factors as screw position, screw type and bone density. In addition, they observed that tapered MSIs had a higher placement torque than the tested cylindrical type. Artificial bone blocks serve well for this type of study over animal or cadaver bone use as the artificial bone exhibits uniform cortical bone density and depth and is
DOI 10.1179/1465313314Y.0000000109
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Figure 1 An example of a split mouth design (Owens et al. 2007). The four quadrants received randomly determined force loading which were delayed or immediate and consisted of 25 or 50 g. All quadrants had control MSIs
unaffected by desiccation, all factors important to ensure that standardized information can be obtained. The translational influence on the stability of MSI insertion into artificial bone blocks (Sawbones, Sawbones AB, Malmo, Sweden) was well-illustrated by Holm et al. (2012). An interesting phenomenon was observed in the designs; 1.5 mm diameter cylindrical design was inserted with a significantly lower maximum insertion torque compared to the 1.5 mm tapered and the 2.0 mm cylindrical designs. It was concluded in this laboratory study that MSIs attained greater primary stability in higher-density cortical bone, and the 1.5 mm diameter tapered and 2.0 mm cylindrical designs offered greater primary stability than the 1.5 mm cylindrical design. Moreover, Morarend et al. (2009) reported that a larger diameter screw body increased anchorage resistance (2.5 mm compared to 1.5 mm MSI) in both jaws. They also showed that bicortical engagement of 1.5 mm screws provided anchorage force resistance at least equal to larger diameter 2.5 mm monocortical screws. However, both larger diameter and bicortical screws have the potential to increase tissue damage and the clinician must be cognizant of the limited bone stock available between roots (Poggio et al., 2006; Schnelle et al., 2004). These latter few study examples indicate why it is important to test the characteristics of design on the stability of the MSI utilization, as this design information translated from the ‘bench to the clinic’ to enhance superior clinical outcomes.
The experimental animal model The Beagle dog has served as a useful experimental model in various studies to assess effectiveness of the MSI. A split mouth design (Owens et al., 2007) is the most appropriate format to prospectively study the effects of the inserted MSI (Figure 1). This information can often be directly translated to the human patient. Owens et al. (2007) studied the effect of space closure following tooth extraction utilizing 661.8 mm IMTEC MSIs (IMTEC, Ardmore, Oklahoma, USA) as anchorage. This model provided an opportunity to study the impact of delayed and immediate loading on MSI stability. In addition, variable force loading was compared to that of a control unloaded MSI. A very favourable outcome was shown with an overall success rate of 93%. If three partial failures, which appeared mobile but were still clinically functional over the observation period of 131 days were included, then the success rate could be measured as 98% (55/56). Interestingly, the MSI exposed to trauma became mobile and tipped into the direction of the force vector, then maintained its position and remained a functional anchor unit throughout the tooth movement (Figure 2). This valuable information is clinically relevant should a similar situation present itself in a human subject (Rossouw et al., 2008). Certainly this translational evidence has influenced my own clinical management of delayed MSI mobility/movement, such that I first assess the mobility and eliminate the aetiology. Stability
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Figure 2 (a) Trauma resulting in complete loss of the MSI; note the arch wire insult and remaining tissue injury; (b) MSI tipped then maintained its position as a functional anchor (Owens et al. 2007).
can then be reinforced by a re-screw action until the MSI is stable. Also, immobilize the MSI to prevent further vertical or horizontal ‘wiggling’ (Figure 3). The MSI need only be replaced if this sequence of action fails, pain is experienced when force is applied and/or peri-implantitis persists. In the Owens et al. (2007) study the immediately loaded MSI showed promising results. The immediately loaded data from this investigation were enhanced by Mortensen et al. (2009) who showed in beagle dogs that the success rate of immediately loaded MSIs was affected by their length (3 mm success was significantly lower than 6 mm). In order to test if the angle of loading, especially if an MSI has moved and potentially influenced its stability as an anchor, a laboratory study by Pickard et al. (2010) investigated the vector of force application that would be the most efficient in respect to the MSI inclination. It appeared that the highest force to displace an MSI is the
MSI immobilized by a stainless steel ligature to provide either direct anchorage to the MSI or indirect anchorage to the teeth; in addition, the latter negated traumatic insults to the MSI
Figure 3
tensile force perpendicular to the long-axis followed by the force application in the direction of tipped movement. Notably it was not the force which one would expect to yield high values in the form of the tent-peg resistance concept. This study was an excellent example of a translational effect from the clinic to the laboratory to establish evidence for future clinical application. Finite element models to assess bone quality Animal models are expensive and have rigorous approval rules, which often make this type of research impractical. However, a finite element model was developed by Dalstra et al. (2004) to evaluate the influence of both the cortical thickness and the underlying trabecular bone density. This finite element model revealed that the thickness of the cortical bone determined the overall load transfer from the MSI to bone and that the stiffness (or density) of the trabecular bone played only a minor role. It appeared that bone strains can reach values associated with the pathological overload window only in sites with thin cortical bone (0.5 mm) and low-density trabecular bone. For medium and high-density trabecular bone, this risk was not present. This model therefore indicated that trabecular bone, although of less importance, should not be overlooked in the overall stability assessment. Cortical thickness. In a cone beam computerized tomography study to determine the impact of cortical bone thickness on the decision of where and how to place the MSI, Farnsworth et al. (2011) measured the posterior and infra-zygomatic crest regions in adults and adolescents (Figure 4). The most important information translating from this data was that adults and the mandibular posterior region exhibited thicker cortical bone. Thus, pre-drilling a pilot hole should be
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(a) Means and ranges of cortical bone thickness in the maxillary buccal regions; (b) Means and ranges of cortical bone thickness in the mandibular buccal region (Farnsworth et al., 2011).
Figure 4
considered in the thicker bone situations to avoid unnecessary bone and MSI fractures due to excessive force generated during the MSI placement.
Osseointegration versus bone-to-implant contact (BIC) The original definition of osseointegration indicated that a structural and functional connection exists between living bone and a load-carrying implant. Moreover, histological evidence of BIC confirmed this osseointegration (Bra˚nemark, 1983; Albrektsson and Eriksson,
1985). Bra˚nemark (1983) described this process in the early animal studies as the titanium specimens ‘were inseparably incorporated within the bone tissue, which actually grew into very thin spaces in the titanium’. This microscopically close contact between the bone and implant surface is also true for MSIs as exhibited by the histologic study of Woods et al. (2009). The MSI BIC thus meets the requirements of Bra˚nemark’s definition (Bra˚nemark, 1983). This issue has been debated since the introduction of MSIs. The use of the term biocompatible is clear; however, other terms can be confusing, such as the term biochemical integration,
Figure 5 Bone volume in relation to total volume illustrating the bone-implant contact when comparing machine
polished versus a surface treatment utilizing micro-CT images (Ikeda et al. 2011).
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which was suggested as the true meaning of osseointegration versus only mechanical retention (Cope, 2005). One should distinguish between the terms biointegration (bio-chemically enhanced surfaces of implants) and pure mechanical contact. Irrespective of how one wishes to define this process, the intimate BIC for the MSI has now been well-illustrated through histology (Woods et al., 2009), as well as micro-CT evidence (Ikeda et al., 2011). In addition, the surface interaction of the MSI throughout the bone (BIC) is important for ultimate success during anchorage utilization. Ikeda et al. (2011) showed in a micro-CT study that the three-dimensional BIC of smooth surface versus surface-treated MSIs appeared to favour the latter MSI. It was also shown that the loaded implant showed an enhanced BIC throughout the length of the MSI a concept which is in agreement with earlier studies on loaded implants by Berglundh et al. (2005). The latter reinforced the importance of the cortical bone contact, but also that the medullary bone played an important role (Figure 5). From the Woods et al. (2009) and Ikeda et al. (2011) studies it can be translated that all bone is important when it comes to MSI stability. Cortical bone served an important role in the initial or primary stability. However, the MSI was mostly utilized over extended periods and the long-term (secondary stability) from all bone was important in enhancing the overall stability of the MSI. Tissue damage due to MSI placement The insertion of the original self-tapping, but not selfdrilling MSIs, indicated that the MSI could not damage tooth structure (Herman and Cope, 2005) but, on closer inspection one realized that this was only true for the original blunt-tipped MSI’s (Herman and Cope, 2005; Rossouw et al., 2008). However, the newest MSIs are self-drilling (and self-tapping). Consequently, the clinician now has to be very cautious, as the sharp tip can fracture with tooth contact, and it could penetrate the tooth (like a pilot drill) with possible severe deleterious effects, which can lead to pulpal destruction (Brisceno et al., 2009; Hembree et al., 2009). The translational effect of this information has led to an evolution of the insertion technique. Clinicians realized that a thorough knowledge of the anatomy of the periodontium and surrounding structures is imperative to avoid tissue damage and knowledge of the healing process to ensure a successful outcome with the use of MSIs. If a root is contacted, most believed that damage with MSIs was minimal (Melsen, 2005; Melsen and Verna, 2005). However, the incidence of root damage was shown to range from 0.47 to 43.3% (Borah and
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Ashmead, 1996; Fabbroni et al., 2004; Farr and Whear, 2002). Root contact is possible as a result of directly inserting MSIs into root; moving the tooth onto the MSI; or tipping the MSI into the root (Liou et al., 2004; Mortensen et al., 2009). The likelihood of damage was systematically tested by Brisceno et al. (2009) and Hembree et al. (2009) and the consequences of the damage and healing were well-illustrated by their studies. Healing following MSI trauma depended on the extent of damage: healing can be uneventful, repaired by cementum, or with severe damage the pulpal tissue will disintegrate showing only an infiltrate of inflammatory cells. Minor damage would lead to healing within six weeks. Torque measurements during MSI insertion can be a valuable indicator of root contact. The results of the latter two studies (Brisceno et al., 2009; Hembree et al., 2009) showed that MSI breakage resulted with high torque values. This translational evidence indicated that a significant increase in torque serves as an indication of root contact. In this situation, the clinician should immediately stop the insertion in order to avoid tooth damage, back-up and change the direction of insertion or if indicated re-insert the MSI in a different site. The increased vertical problem – molar intrusion effects Patients with anterior open bites and class II skeletal hyper-divergent patterns often seek non-surgical treatment possibilities. Molar intrusion or relative intrusion (by preventing eruption of maxillary molars) (Erverdi et al., 2004; Rossouw et al., 2008) is supported by translational experimental and histological studies (Carrillo et al., 2007; Ramirez-Echave et al., 2011). This has opened the possibility of using MSIs for posterior intrusion and indirect mandibular autorotation. For example, a prospective, feasibility study was designed by Buschang et al. (2011) to produce vertical facial reductions by intruding segments of teeth in a controlled fashion using MSIs. This novel approach using MSIs and growth to treat retrognathic, hyperdivergent adolescents showed a very favourable response, with advancement of the chin, the SNB angle increased, the mandibular plane angle decreased, and facial convexity decreased. However, orthodontists have historically considered tooth intrusion to cause root resorption and this question needed to be addressed with translational research. To begin with, radiographically detectable root resorption did not appear to be related to the amount of tooth movement (Carrillo et al., 2007). However, Ramirez-Echave et al. (2010) histologically evaluated root resorption and repair after
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orthodontic intrusion utilizing specimens from the same sample as Carrillo et al. (2007). Significant intrusion (1.2–3.3 mm) of multiradicular teeth was measured following the application of constant intrusive forces from MSI anchorage (Carrillo et al., 2007). Histology showed that the root apices and inter-radicular areas were most affected by resorption and dentine involvement was detected at the root furcation. Cementum repair occurred in 24.14% of the lacunae observed. Signs of ankylosis were rare and appeared in association with cellular cementum repair of lacunae. Thus, light forces seem adequate for intrusive movements using this technique and should be the choice during treatment in order to prevent traumatic root resorption. Conclusions The MSI is a valuable temporary anchorage device and is not dependent on patient compliance. Translational research has enabled the successful development of the MSI for use during contemporary orthodontic treatment, resulting in its transformation in numerous anchorage situations. Disclaimer statements Funding None. Conflicts of interest None. Ethics approval The paper is an overview paper on translational research. All the material cited has been previously published in respected journals. All the papers cited where I am also a co-author have been from projects which received Institutional Review Board approval.
Acknowledgements Colleagues and residents with whom I had the pleasure to work with in obtaining the information presented.
References Albrektsson T, Eriksson R. Thermally induced bone necrosis in rabbits: relation to implant failure in humans. Clin Orthop 1985; 195: 311–312. Berglundh T, Abrahamsson I, Lindhe J. Bone reactions to longstanding functional load at implants: an experimental study in dogs. J Clin Periodontol 2005; 32: 925–932. Borah GL, Ashmead D. The fate of teeth transfixed by osteosynthesis screws. Plast Reconstr Surg 1996; 97: 726–729. Bra˚nemark PI. Osseointegration and its experimental background. J Prosthet Dent 1983; 50: 399–410. Brisceno CE, Rossouw PE, Carrillo R, Spears R, Buschang PH. Healing of the roots and surrounding structures following intentional damage with miniscrew implants. Am J Orthod Dentofac Orthop 2009; 135: 292–301. Buschang PH, Carrillo R, Rossouw PE. Orthopedic correction of growing hyperdivergent, retrognathic patients with miniscrew implants. J Oral Maxillofac Surg 2011; 69: 754–762.
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Carrillo R, Rossouw PE, Franco PF, Opperman LA, Buschang PH. Intrusion of multiradicular teeth and related root resorption with mini-screw implant anchorage: a radiographic evaluation. Am J Orthod Dentofac Orthop 2007; 132: 647–655. Cha JY, Kil JK, Yoon TM, Hwang CJ. Miniscrew stability evaluated with computerized tomography scanning. Am J Orthod Dentofacial Orthop 2010; 137: 73–79. Cope JB. Temporary Anchorage Devices in Orthodontics: A paradigm shift. Semin Orthod 2005; 11: 3–9. Dalstra M, Cattaneo PM, Melsen B. Load transfer of mini screws for orthodontic anchorage. Orthodontics 2004; 1: 53–62. Erverdi N, Keles A, Nanda R. The use of skeletal anchorage in open bite treatment: A cephalometric evaluation. Angle Orthod 2004; 74: 381. Fabbroni G, Aabed S, Mizen K, Starr DG. Transalveolar screws and the incidence of dental damage: a prospective study. Int J Oral Maxillofac Surg 2004; 33: 442–446. Farnsworth D, Rossouw PE, Ceen RF, Buschang PH. Cortical bone thickness at common miniscrew implant placement sites. Am J Orthod Dentofac Orthop 2011; 139(4): 495–503. Farr DR, Whear NM. Intermaxillary fixation screws and tooth damage. Br J Oral Maxillofac Surg 2002; 40: 84–85. Feldmann I, Bondemark L. Orthodontic anchorage: a systematic review. Angle Orthod 2006; 76: 493–501. Fontanarosa PB, DeAngelis CD. Basic science and translational research in JAMA 2002; 287(13): 1728. Hembree M, Buschang PH, Carrillo R, Spears R, Rossouw PE. The effects of intentional damage of the root and surrounding structures with miniscrew implants. Am J Orthod Dentofac Orthop 2009; 135: 280.e1– 280.e9. Herman R, Cope JB. Miniscrew implants: IMTEC mini ortho implants. Semin Orthod 2005; 11: 32–39. Holm L, Cunningham SJ, Petrie A, Cousley RRJ. An in vitro study of factors affecting the primary stability of orthodontic mini-implants. Angle Orthod 2012; 82: 1022–1028. Ikeda H, Rossouw PE, Campbell PM, Kontogirgos E, Buschang PH. Threedimensional analysis of peri-bone-implant contact of rough-surface miniscrew implants. Am J Orthod Dentofac Orthop 2011; 139(2): e153– e163. Liou EJ, Pai BC, Lin JC. Do miniscrews remain stationary under orthodontic forces? Am J Orthod Dentofacial Orthop 2004; 126: 42–47. Melsen B. Mini-implants: where are we? J Clin Orthod 2005; 39: 539–547. Melsen B, Verna C. Miniscrew implants: the Aarhus anchorage system. Semin Orthod 2005; 11: 24–31. Morarend C, Qian F, Marshall SD, Southard KA, Grosland NM, Morgan TA, et al. Effect of screw diameter on orthodontic skeletal anchorage. Am J Orthod Dentofacial Orthop 2009; 136: 224–229. Mortensen MG, Buschang PH, Oliver DR, Kyung HM, Behrents RG. Stability of immediately loaded 3- and 6-mm miniscrew implants in beagle dogs—a pilot study. Am J Orthod Dentofacial Orthop 2009; 136: 251–259. Owens S, Buschang PH, Cope J, Franco P, Rossouw PE. Experimental evaluation of tooth movement in the beagle dog utilizing the mini-implant for orthodontic anchorage. Am J Orthod Dentofac Orthop 2007; 132: 639– 646. Pickard MB, Dechow P, Rossouw PE, Buschang PH. Effects of miniscrew orientation on implant stability and resistance to failure. Am J Orthod Dentofac Orthop 2010; 137(1): 91–99. Poggio PM, Incorvati C, Velo S, Carano A. ‘Safe Zones’: a guide for miniscrew positioning in the maxillary and mandibular arch. Angle Orthod 2006; 76: 191–197. Ramirez-Echave JI, Buschang PH, Carrillo R, Rossouw PE, Nagy WW, Opperman LA. Histologic evaluation of root response to intrusion in mandibular teeth in beagle dogs. Am J Orthod Dentofacial Orthop 2011; 139: 60–69. Rossouw PE, Buschang PH, Carrillo R. The Baylor experience with using miniimplants for orthodontic anchorage: Clinical and experimental evidence. In McNamara Jr JA, (ed), Microimplants as Temporary Orthodontic Anchorage. Craniofacial Growth Series. Ann Arbor, MI: University of Michigan, 2008, Vol. 45. pp 407–459.
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Schnelle MA, Beck FM, Jaynes RM, Huja SS. A radiographic evaluation of the availability of bone for placement of miniscrews. Angle Orthod 2004; 74: 832–837.
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Woods PW, Buschang PH, Owens SE, Rossouw PE, Opperman LA. The effect of force, timing, and location on bone-to-implant contact of miniscrew implants. Eur J Orthod 2009; 31(3): 232–240.
Journal of Orthodontics, Vol. 41, 2014, S8–S14
Translational mini-screw implant research Emile Rossouw Department of Orthodontics, University of North Carolina School of Dentistry at Chapel Hill, North Carolina, USA
It is important to thoroughly test new materials as well as techniques when these innovations are to be utilized in the human clinical situation. Translational research fills this important niche. The purpose of translational research is to establish the continuity of evidence from the laboratory to the clinic and in so-doing, provide evidence that the material is functioning appropriately and that the process in the human will be successful. This concept applies to the mini-screw implant; which, has been very successfully introduced into the orthodontic armamentarium over the last decade for application as a temporary anchorage device. The examples of translational research that will be illustrated in this paper have paved the way to ensure that clinicians have evidence to confidently utilize mini-screw implants in orthodontic practice. Needless to say, more studies are needed to ensure a safe, effective and efficient manner to practice orthodontics. Key words: Mini-screw implants, orthodontics, anchorage, bone-implant-contact, translational research Received 30 May 2014; accepted 31 May 2014
Introduction The logical requirements for an optimal mini-screw implant (MSI) anchorage system include stability during utilization; small dimensions; minimal surgical morbidity; easy placement and removal; simple and reliable attachment; capability of immediate loading; economical; broad range of application (not site specific) and withstand loads greater than clinically required. Oral and maxillofacial surgeons have utilized bone screws, which are similar to MSIs, for many years to provide stabile surgical fixation and immobilization of bone segments. We learnt from their surgical experience that MSI must be immobile, biocompatible, easily usable and independent of patient compliance. The MSI as used in orthodontics today, provides the greatest theoretical versatility and meets the aforementioned criteria. However, in their systematic review of anchorage, Feldmann and Bondemark (2006) suggested that further clinical and laboratory research studies were needed to evaluate new approaches, such as MSIs to enhance mechanics, and establish the potentials and limitations of these devices. This article aims to demonstrate how MSI studies have provided new evidence that has translated to the clinical environment to enhance orthodontic outcomes. Translational research is an important component of orthodontic research, which can have different meanings to different individuals. Simply put, it can be interpreted Address for correspondence: Dr. P. Emile Rossouw, Department of Orthodontics, 277 Brauer Hall, Campus Box 7450, University of North Carolina School of Dentistry, Chapel Hill, NC 27599-7450, USA Email: [email protected] # 2014 British Orthodontic Society
as the research information translated or moved from one area to enhance or improve the outcomes of another. Thus, data from a research laboratory translated for clinical implementation are often referred to as from ‘bench-to-bedside’. This transfer of new knowledge gained in the laboratory of how a specific apparatus, such as MSIs, function can be utilized for the development of new methods for diagnosis, treatment, and prevention of problems during treatment. Often this leads or translates to the first testing in humans (Fontanarosa and DeAngelis, 2002). In addition, the clinician can help shape the research protocol by seeking answers to difficult problems to which applied and/or basic research would only offer minimal answers, but the translational research effect could lead to significant improvements. The influence of MSI design on stability Torque is the rotational resistance to MSI insertion or removal. Using computerized tomography scanning Cha et al. (2010) determined that clinical placement and removal torque values were influenced by such factors as screw position, screw type and bone density. In addition, they observed that tapered MSIs had a higher placement torque than the tested cylindrical type. Artificial bone blocks serve well for this type of study over animal or cadaver bone use as the artificial bone exhibits uniform cortical bone density and depth and is
DOI 10.1179/1465313314Y.0000000109
JO September 2014
Mini-implant Supplement
Translational mini-screw implant research
S9
Figure 1 An example of a split mouth design (Owens et al. 2007). The four quadrants received randomly determined force loading which were delayed or immediate and consisted of 25 or 50 g. All quadrants had control MSIs
unaffected by desiccation, all factors important to ensure that standardized information can be obtained. The translational influence on the stability of MSI insertion into artificial bone blocks (Sawbones, Sawbones AB, Malmo, Sweden) was well-illustrated by Holm et al. (2012). An interesting phenomenon was observed in the designs; 1.5 mm diameter cylindrical design was inserted with a significantly lower maximum insertion torque compared to the 1.5 mm tapered and the 2.0 mm cylindrical designs. It was concluded in this laboratory study that MSIs attained greater primary stability in higher-density cortical bone, and the 1.5 mm diameter tapered and 2.0 mm cylindrical designs offered greater primary stability than the 1.5 mm cylindrical design. Moreover, Morarend et al. (2009) reported that a larger diameter screw body increased anchorage resistance (2.5 mm compared to 1.5 mm MSI) in both jaws. They also showed that bicortical engagement of 1.5 mm screws provided anchorage force resistance at least equal to larger diameter 2.5 mm monocortical screws. However, both larger diameter and bicortical screws have the potential to increase tissue damage and the clinician must be cognizant of the limited bone stock available between roots (Poggio et al., 2006; Schnelle et al., 2004). These latter few study examples indicate why it is important to test the characteristics of design on the stability of the MSI utilization, as this design information translated from the ‘bench to the clinic’ to enhance superior clinical outcomes.
The experimental animal model The Beagle dog has served as a useful experimental model in various studies to assess effectiveness of the MSI. A split mouth design (Owens et al., 2007) is the most appropriate format to prospectively study the effects of the inserted MSI (Figure 1). This information can often be directly translated to the human patient. Owens et al. (2007) studied the effect of space closure following tooth extraction utilizing 661.8 mm IMTEC MSIs (IMTEC, Ardmore, Oklahoma, USA) as anchorage. This model provided an opportunity to study the impact of delayed and immediate loading on MSI stability. In addition, variable force loading was compared to that of a control unloaded MSI. A very favourable outcome was shown with an overall success rate of 93%. If three partial failures, which appeared mobile but were still clinically functional over the observation period of 131 days were included, then the success rate could be measured as 98% (55/56). Interestingly, the MSI exposed to trauma became mobile and tipped into the direction of the force vector, then maintained its position and remained a functional anchor unit throughout the tooth movement (Figure 2). This valuable information is clinically relevant should a similar situation present itself in a human subject (Rossouw et al., 2008). Certainly this translational evidence has influenced my own clinical management of delayed MSI mobility/movement, such that I first assess the mobility and eliminate the aetiology. Stability
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Figure 2 (a) Trauma resulting in complete loss of the MSI; note the arch wire insult and remaining tissue injury; (b) MSI tipped then maintained its position as a functional anchor (Owens et al. 2007).
can then be reinforced by a re-screw action until the MSI is stable. Also, immobilize the MSI to prevent further vertical or horizontal ‘wiggling’ (Figure 3). The MSI need only be replaced if this sequence of action fails, pain is experienced when force is applied and/or peri-implantitis persists. In the Owens et al. (2007) study the immediately loaded MSI showed promising results. The immediately loaded data from this investigation were enhanced by Mortensen et al. (2009) who showed in beagle dogs that the success rate of immediately loaded MSIs was affected by their length (3 mm success was significantly lower than 6 mm). In order to test if the angle of loading, especially if an MSI has moved and potentially influenced its stability as an anchor, a laboratory study by Pickard et al. (2010) investigated the vector of force application that would be the most efficient in respect to the MSI inclination. It appeared that the highest force to displace an MSI is the
MSI immobilized by a stainless steel ligature to provide either direct anchorage to the MSI or indirect anchorage to the teeth; in addition, the latter negated traumatic insults to the MSI
Figure 3
tensile force perpendicular to the long-axis followed by the force application in the direction of tipped movement. Notably it was not the force which one would expect to yield high values in the form of the tent-peg resistance concept. This study was an excellent example of a translational effect from the clinic to the laboratory to establish evidence for future clinical application. Finite element models to assess bone quality Animal models are expensive and have rigorous approval rules, which often make this type of research impractical. However, a finite element model was developed by Dalstra et al. (2004) to evaluate the influence of both the cortical thickness and the underlying trabecular bone density. This finite element model revealed that the thickness of the cortical bone determined the overall load transfer from the MSI to bone and that the stiffness (or density) of the trabecular bone played only a minor role. It appeared that bone strains can reach values associated with the pathological overload window only in sites with thin cortical bone (0.5 mm) and low-density trabecular bone. For medium and high-density trabecular bone, this risk was not present. This model therefore indicated that trabecular bone, although of less importance, should not be overlooked in the overall stability assessment. Cortical thickness. In a cone beam computerized tomography study to determine the impact of cortical bone thickness on the decision of where and how to place the MSI, Farnsworth et al. (2011) measured the posterior and infra-zygomatic crest regions in adults and adolescents (Figure 4). The most important information translating from this data was that adults and the mandibular posterior region exhibited thicker cortical bone. Thus, pre-drilling a pilot hole should be
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(a) Means and ranges of cortical bone thickness in the maxillary buccal regions; (b) Means and ranges of cortical bone thickness in the mandibular buccal region (Farnsworth et al., 2011).
Figure 4
considered in the thicker bone situations to avoid unnecessary bone and MSI fractures due to excessive force generated during the MSI placement.
Osseointegration versus bone-to-implant contact (BIC) The original definition of osseointegration indicated that a structural and functional connection exists between living bone and a load-carrying implant. Moreover, histological evidence of BIC confirmed this osseointegration (Bra˚nemark, 1983; Albrektsson and Eriksson,
1985). Bra˚nemark (1983) described this process in the early animal studies as the titanium specimens ‘were inseparably incorporated within the bone tissue, which actually grew into very thin spaces in the titanium’. This microscopically close contact between the bone and implant surface is also true for MSIs as exhibited by the histologic study of Woods et al. (2009). The MSI BIC thus meets the requirements of Bra˚nemark’s definition (Bra˚nemark, 1983). This issue has been debated since the introduction of MSIs. The use of the term biocompatible is clear; however, other terms can be confusing, such as the term biochemical integration,
Figure 5 Bone volume in relation to total volume illustrating the bone-implant contact when comparing machine
polished versus a surface treatment utilizing micro-CT images (Ikeda et al. 2011).
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which was suggested as the true meaning of osseointegration versus only mechanical retention (Cope, 2005). One should distinguish between the terms biointegration (bio-chemically enhanced surfaces of implants) and pure mechanical contact. Irrespective of how one wishes to define this process, the intimate BIC for the MSI has now been well-illustrated through histology (Woods et al., 2009), as well as micro-CT evidence (Ikeda et al., 2011). In addition, the surface interaction of the MSI throughout the bone (BIC) is important for ultimate success during anchorage utilization. Ikeda et al. (2011) showed in a micro-CT study that the three-dimensional BIC of smooth surface versus surface-treated MSIs appeared to favour the latter MSI. It was also shown that the loaded implant showed an enhanced BIC throughout the length of the MSI a concept which is in agreement with earlier studies on loaded implants by Berglundh et al. (2005). The latter reinforced the importance of the cortical bone contact, but also that the medullary bone played an important role (Figure 5). From the Woods et al. (2009) and Ikeda et al. (2011) studies it can be translated that all bone is important when it comes to MSI stability. Cortical bone served an important role in the initial or primary stability. However, the MSI was mostly utilized over extended periods and the long-term (secondary stability) from all bone was important in enhancing the overall stability of the MSI. Tissue damage due to MSI placement The insertion of the original self-tapping, but not selfdrilling MSIs, indicated that the MSI could not damage tooth structure (Herman and Cope, 2005) but, on closer inspection one realized that this was only true for the original blunt-tipped MSI’s (Herman and Cope, 2005; Rossouw et al., 2008). However, the newest MSIs are self-drilling (and self-tapping). Consequently, the clinician now has to be very cautious, as the sharp tip can fracture with tooth contact, and it could penetrate the tooth (like a pilot drill) with possible severe deleterious effects, which can lead to pulpal destruction (Brisceno et al., 2009; Hembree et al., 2009). The translational effect of this information has led to an evolution of the insertion technique. Clinicians realized that a thorough knowledge of the anatomy of the periodontium and surrounding structures is imperative to avoid tissue damage and knowledge of the healing process to ensure a successful outcome with the use of MSIs. If a root is contacted, most believed that damage with MSIs was minimal (Melsen, 2005; Melsen and Verna, 2005). However, the incidence of root damage was shown to range from 0.47 to 43.3% (Borah and
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Ashmead, 1996; Fabbroni et al., 2004; Farr and Whear, 2002). Root contact is possible as a result of directly inserting MSIs into root; moving the tooth onto the MSI; or tipping the MSI into the root (Liou et al., 2004; Mortensen et al., 2009). The likelihood of damage was systematically tested by Brisceno et al. (2009) and Hembree et al. (2009) and the consequences of the damage and healing were well-illustrated by their studies. Healing following MSI trauma depended on the extent of damage: healing can be uneventful, repaired by cementum, or with severe damage the pulpal tissue will disintegrate showing only an infiltrate of inflammatory cells. Minor damage would lead to healing within six weeks. Torque measurements during MSI insertion can be a valuable indicator of root contact. The results of the latter two studies (Brisceno et al., 2009; Hembree et al., 2009) showed that MSI breakage resulted with high torque values. This translational evidence indicated that a significant increase in torque serves as an indication of root contact. In this situation, the clinician should immediately stop the insertion in order to avoid tooth damage, back-up and change the direction of insertion or if indicated re-insert the MSI in a different site. The increased vertical problem – molar intrusion effects Patients with anterior open bites and class II skeletal hyper-divergent patterns often seek non-surgical treatment possibilities. Molar intrusion or relative intrusion (by preventing eruption of maxillary molars) (Erverdi et al., 2004; Rossouw et al., 2008) is supported by translational experimental and histological studies (Carrillo et al., 2007; Ramirez-Echave et al., 2011). This has opened the possibility of using MSIs for posterior intrusion and indirect mandibular autorotation. For example, a prospective, feasibility study was designed by Buschang et al. (2011) to produce vertical facial reductions by intruding segments of teeth in a controlled fashion using MSIs. This novel approach using MSIs and growth to treat retrognathic, hyperdivergent adolescents showed a very favourable response, with advancement of the chin, the SNB angle increased, the mandibular plane angle decreased, and facial convexity decreased. However, orthodontists have historically considered tooth intrusion to cause root resorption and this question needed to be addressed with translational research. To begin with, radiographically detectable root resorption did not appear to be related to the amount of tooth movement (Carrillo et al., 2007). However, Ramirez-Echave et al. (2010) histologically evaluated root resorption and repair after
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orthodontic intrusion utilizing specimens from the same sample as Carrillo et al. (2007). Significant intrusion (1.2–3.3 mm) of multiradicular teeth was measured following the application of constant intrusive forces from MSI anchorage (Carrillo et al., 2007). Histology showed that the root apices and inter-radicular areas were most affected by resorption and dentine involvement was detected at the root furcation. Cementum repair occurred in 24.14% of the lacunae observed. Signs of ankylosis were rare and appeared in association with cellular cementum repair of lacunae. Thus, light forces seem adequate for intrusive movements using this technique and should be the choice during treatment in order to prevent traumatic root resorption. Conclusions The MSI is a valuable temporary anchorage device and is not dependent on patient compliance. Translational research has enabled the successful development of the MSI for use during contemporary orthodontic treatment, resulting in its transformation in numerous anchorage situations. Disclaimer statements Funding None. Conflicts of interest None. Ethics approval The paper is an overview paper on translational research. All the material cited has been previously published in respected journals. All the papers cited where I am also a co-author have been from projects which received Institutional Review Board approval.
Acknowledgements Colleagues and residents with whom I had the pleasure to work with in obtaining the information presented.
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