Journal of Investigative and Clinical Dentistry (2011), 2, 229–235
REVIEW ARTICLE Implant Dentistry
Do dental implants preserve and maintain alveolar bone? Jessica E. O’Neill & Stephen C. Yeung Faculty of Dentistry, The University of Sydney, Sydney, NSW, Australia
Keywords alveolar bone, alveolar bone loss, alveolar bone remodeling, dental implant, extraction socket remodeling. Correspondence Dr Jessica O’Neill, Faculty of Dentistry, Westmead Centre for Oral Health, Westmead Hospital, University of Sydney, C24 Darcy Road, Westmead, NSW 2145, Australia. Tel: +61-2-9845-5951 Fax: +61-2-9893-8671 Email: [email protected] Received 6 February 2010; accepted 20 May 2010.
Abstract Dental implants have been touted as capable of playing an active role in the maintenance of alveolar bone height, despite the lack of a sound biological basis to support this proposition. This paper reviews the biology of bone loss, the literature concerning alveolar bone remodeling in both the postextraction period and long term, and discusses the literature regarding the influence dental implant placement has on this process. Based on current evidence, implants do not have an active role in the preservation and maintenance of alveolar bone height. Following extraction of a tooth, no external influence has been identified that will prevent loss of bundle bone or alveolar bone remodeling. Additionally, the magnitude of change is patient, site, and time dependant. Further supporting evidence is required before it can be concluded that dental implants are capable of influencing alveolar bone remodeling.
doi: 10.1111/j.2041-1626.2011.00074.x
Introduction Implant dentistry has developed dramatically in the past 30 years. Since the discovery of osseointegration in 1977, the application of dental implants has progressed from being the support of a fixed prosthesis for an edentulous mandible1,2 to the sophisticated use of dental implants in the replacement of single or multiple missing teeth in partially-edentulous dentitions.3,4 In more recent times, immediate implant placement following tooth extraction had been advocated by some practitioners as a viable and predictable clinical technique.5,6 Clinical follow-up studies have yielded mixed results. Implant survival studies reported comparable data of implant survival, whether an implant was placed into an extraction socket immediately following extraction or placed into a healed edentulous alveolus.7,8 Furthermore, based on clinical and radiographic observations, claims had been made that early placement of dental implants could assist in preserving alveolar bone.9,10 The clinical importance of preserving interproximal bone between the tooth and ª 2011 Blackwell Publishing Asia Pty Ltd
implant or between two adjacent implants is obvious to most clinicians, particularly when considering esthetics in the anterior regions. Paradoxically, clinical studies have now identified a consistent loss of labial bone on anterior implants placed immediately into extraction sockets,11,12 resulting in unsightly and unwanted gingival retraction/recession.12,13 This finding is not unexpected, considering the labial bone consists primarily of ‘‘bundle bone’’,14 which is biologically programmed to undergo remodeling following tooth loss or extraction. However, the relatively ‘‘stable’’ interproximal bone between the tooth and implant might present a different set of circumstances in contrast to that between an implant and the free-standing labial alveolus. The presence of a neighboring tooth with its intact periodontium might have contributed to supporting and maintaining the interproximal bone. In this paper, we reviewed the current literature in search of a biological explanation for the bone remodeling behavior around dental implants, in general, and in extraction sockets, in particular. Specifically, we sought evidence to support the proposition that 229
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osseointegrated dental implants will support or maintain alveolar bone height. Bone mineral density There have been reports suggesting that implants might play a role in the maintenance of alveolar bone mineral density. The hypothesis that increased biomechanical load on bone causes strain leading to stimulation of bone growth15,16 has been supported by meta-analyses of the effects of exercise and increasing load on the maintenance of bone mineral density in both men and women.17–19 von Wowern and Gotfredsen20 demonstrated that the bone mineral content (BMC) of mandibular bone decreases at a similar rate as skeletal BMC, which is measured using forearm BMC. Additionally, they showed in two studies that BMC of peri-implant bone reduced significantly less than either skeletal BMC or mandibular BMC, suggesting that dental implants had a stabilizing effect on peri-implant bone.20,21 These data suggest that ‘‘increased function leads to load-related positive bone remodeling that minimizes, or in some cases may counteract, the physiologic age-related changes in the bone remodeling process leading to BMC loss’’.20 However, while plausible, it must be considered an unproven extrapolation to assert that these changes to bone mineral density can be translated to the positive influence on vertical bone height changes. Reducing postextraction alveolar bone remodeling using dental implants The alveolar process has been described as the portion of the mandible or maxilla containing the sockets of the teeth,22 and consists of outer cortical walls, central cancellous bone, and bundle bone lining the socket walls. Bundle bone and the periodontal ligament share embryological origin, function, and blood supply, and are physically united at the cellular level via extrinsic collagen fibers.23,24 This interdependency of biological structure and the loss of bundle bone following tooth extraction are part of the natural process of wound healing and bone modeling.14,25 These changes can be observed clinically and radiographically as a reduction of alveolar bone height and width.26–28 Morphological changes occurring following tooth extraction have been studied using dental casts,29 cadaver specimens,30 and radiographs,5,31 and clinically,26,28,32,33 in both partially- and fully-edentulous patients. These studies have conclusively shown that following tooth extraction, the wound-healing process results in reduction in the dimensions of alveolar bone.29–31 Lekovic et al.32,33 conducted a series of studies reporting vertical changes in 230
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buccal bone height over a 6-month period following single tooth extraction, showing a mean reduction of 1.0 ± 2.25 mm and 1.50 ± 0.26 mm. Similar findings were also reported by Iasella et al.28 The significant variation in the range of these dimensional bone changes reported in these studies suggest that it might be difficult to predict the amount of variation to expect in each case, as the magnitude of reduction in bone height and width is patient, site, and time dependent. For example, in their studies, Lekovic et al.,32 Iasella et al.,28 and Johnson27 reported individual postextraction alveolar bone reductions ranging between 0 and 4 mm. Likewise, multiple studies demonstrate the site-specific nature of alveolar bone loss, occurring to a greater extent at buccal than palatal or lingual aspects of the tooth socket,28,29 and occurring to a greater extent at multiple than single extraction sites.5,27 Finally, the expected variation might be influenced by pathology or traumatic processes that might have damaged the bony walls,8 or influenced by the duration since extraction, as the majority of alveolar bone height loss has repeatedly been shown to occur in the first 3 months following both extraction and surgery.5,6,27 The precise long-term behavior in jaw bone remodeling following tooth extraction has not been well studied. Currently, there is insufficient data to quantify changes in alveolar height in the long term. One longitudinal radiographic study of patients 15–25 years after tooth loss demonstrated the edentulous alveolar process perpetually models and remodels, albeit slowly, with a rate of resorption estimated at 0.05 mm/year for the maxilla and 0.2 mm/year for the mandible.31 It is unclear if there is an end-point to jaw resorption and remodeling, and what outside influences might play a part in the rate of dimensional changes in edentulous jaws. Changes to alveolar bone following immediate implant placement Opinion has been divided on the ability of immediate implants to maintain alveolar bone following tooth extraction, and thus, interfere with hard tissue changes that would otherwise occur.34 The apparent maintenance of alveolar bone after extraction has been used as a rationale for immediate implant placement,9 although, as discussed later, this was not supported by conclusive evidence. One of the few clinical trials used to support this controversial claim, albeit weakly, are data from a splitmouth study by Paolantonio et al.10 in which patients received bilateral implants, one in mature bone, and another in a contralateral fresh extraction socket. The histological and radiographic results showed no difference in ª 2011 Blackwell Publishing Asia Pty Ltd
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vertical bone height lost between the groups over a 12-month period, suggesting immediate implants preserve alveolar bone dimensions and counteract tissue modeling. The authors thus stated that ‘‘early implantation may preserve the alveolar anatomy and that the placement of a fixture in a fresh extraction socket may help to maintain the bony crest structure’’. Interestingly, several of the immediate implants in this study were placed into palatal root sockets of upper molar teeth; an experimental site surrounded by thick, bony plates, and not subject to the extent of resorption found with buccal alveolar bone. Additionally, Wheeler et al.35 claimed to be able to ‘‘preserve the osseous structure surrounding the socket’’ using a novel immediate implant technique. However, this statement was supported by descriptive results of only two cases, both of which involved an atraumatic extraction technique, which in itself might reduce alveolar bone loss. Conversely, reduction in alveolar bone height has been a consistent feature in the majority of immediate implant trials.11,36–40 For example, Arau´jo et al.37 in an experimental study, found vertical resorption 3 months after immediate implantation of 2.6 ± 0.4 mm for buccal and 0.2 ± 0.5 mm for lingual plates. Similarly, Botticelli et al.11 reported a mean 1.9 ± 0.9 mm reduction in the vertical height of the buccal crest over a 4-month period. This amount of bone loss is similar to the aforementioned extraction studies, and is greater than the reduction in vertical height reported by similar studies of implants placed into healed sites (0.3–1.10 mm).41,42 These observations suggested that the remodeling behavior of buccal bone could be substantially different from that of interproximal bone when (an) adjacent natural tooth/teeth was/were present. Zitzmann et al.43 suggested that these peri-implant bone changes, occurring despite immediate insertion of the dental implants, were as a result of the incongruity between dental implant diameter and the morphology of the alveolus. This suggestion was used to sustain a niche for the use of guided bone regeneration, a technique using grafting materials and/or membranes to reduce such defects. The ability of this technique to reduce socket defects is apparent;43 however, there is no evidence to suggest that this technique, nor dental implants, is capable of maintaining bundle bone. Delayed implant placement, in the period immediately following initial resorption (6–8 weeks), was advocated as ‘‘preventive implant therapy’’ by Denissen et al.44,45 as a method to prevent or delay loss of the alveolar ridge, yet waiting for the inevitable bundle bone resorption to have occurred. However, these authors supported their statement with a cross-sectional study of cadaver mandibles, providing little information on the time since tooth loss, ª 2011 Blackwell Publishing Asia Pty Ltd
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and the reason for extraction or postextraction socket morphology. Thus, the conclusion that alveolar bone loss was prevented by dental implants was not sustained by the data. The passivity of dental implants in the maintenance of alveolar bone height is further supported by a group of studies comparing peri-implant alveolar bone height reductions of immediate, delayed, and late placement of implants. Watzek et al.46 analyzed these three groups of patients over a 27-month period, reporting that periimplant bone loss was greatest in the immediate group (mean: 1 mm), less in the delayed group (0.8 mm), and least in the late group (0.5 mm). Similar findings were also reported by Mensdorff-Pouilly.47 Taken together, these data suggest that the modeling of alveolar bone following extraction was uninfluenced by the presence of a dental implant, thus refuting the proposition that dental implants could maintain alveolar bone in the immediate healing phase. Maintaining alveolar bone morphology The classification of bone loss into that occurring in the first postsurgical year (0.30–1.64 mm)48–50 and mean annual bone loss in the period after the first postsurgical year (0.00–0.15 mm)48–50 permits analysis of the effects implants have on alveolar bone maintenance once the influence of wound healing has receded. Unfortunately, the lack of available evidence so far has prevented a conclusion being drawn on this question. Studies quantifying long-term changes to edentulous alveolar bone failed to provide a baseline for change, as they either frequently combined soft and hard tissue measurements;29,51,52 the study participants wore mucosal-borne prostheses at the site, which itself influences alveolar bone maintenance;31 or the duration since tooth extraction was not provided.29,30 The radiographic study of edentulous patients by Tallgren31 reported a mean annual bone height reduction of 0.05 mm in the maxilla and 0.2 mm in the mandible, similar to the amount of bone height loss found adjacent to implants.49,53 Extrapolation of this data suggests that if dental implants did play a passive role in alveolar bone maintenance, it would take 200 years to lose alveolar bone along the length of a 10-mm maxillary implant, and 50 years at mandibular sites. This slow and gradual rate of change is unable to be effectively studied. In 1990, Adell et al.54 presented mean annual marginal bone loss measurements for three groups of patients, with observation times of 1–4, 5–9, and 10–15 years. The mean annual marginal bone loss in all groups was 0.1 mm, suggesting that the rate of alveolar bone height reduction is constant over time. 231
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By comparing the data from Tallgren and Adell, it is evident that long-term crestal resorption occurs to a similar extent, whether it is adjacent to dental implants, or as an edentulous saddle area under a mucosal-borne prosthesis. Insufficient data exist to suggest a rate of change for completely edentulous sites without prostheses.54 Bone loss in the partially-edentulous site appears to be less, and it is influenced by the presence of the adjacent teeth. Wyatt and Zarb42 proposed the theory that ‘‘implants may serve to retain the bone by applying tensional forces, while the mucosal borne prosthesis may accelerate bone resorption due to compressive forces’’, and presented 1–12-year data, with a mean annual bone loss of 0.00 mm per year ()0.26 mm to +0.42 mm). Although the patients in this study were younger than those in Tallgren’s paper, age-related physiological changes had not been taken into consideration. It is accepted that dental implants increase load to alveolar bone, evidenced both by increased radio-opacity in bone surrounding implants,48 and by studies assessing biting force, which have repeatedly shown increased bite force from patients with implant-supported overdentures, when compared to patients with conventional dentures.55–57 These changes have been described as ‘‘successive load related bone remodeling’’.48 However, there is insufficient evidence to conclude that dental implants actively maintain alveolar bone height simply because alveolar bone has been shown to react to loading stresses. Taken together, the above data suggest that jaw bone remodeling after tooth loss continues unabated throughout life, irrespective of the presence of a foreign object (dental implant). At this point in time, there is insufficient evidence to suggest that the presence of a dental implant, osseointegrated with the jaw bone and functionally loaded, has an impact on modifying the rate of bone remodeling. Factors affecting alveolar bone remodeling Alveolar bone is a dynamic tissue influenced by multiple factors. Assessing the role played by a single factor, such as dental implants, in the maintenance of bone height is therefore fraught with difficulty. The following factors have been suggested to play a role in alveolar bone height: Patient-related factors The presence of an implant could pose a risk for the loss of alveolar bone height as a direct result of peri-implantitis; a disease process defined by the loss of alveolar bone.58 Recent evidence suggests peri-implantitis is preva232
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lent, occurring in 12–3% of sites,59 and thus poses a clinically-significant risk. There is evidence to suggest that poor oral hygiene is strongly correlated with peri-implant bone loss.50 Bone metabolism might be influenced by metabolic bone diseases, such as osteoporosis, demonstrated in the study by von Wowern and Gotfredsen,20 whereby osteoporotic patients were shown to have lost more vertical bone height (0.47 mm) over a 5-year period, compared to non-osteoporotic patients (0.01 mm). Individual patient response appears to influence the magnitude of change following tooth extraction, evident in the apparent wide range of initial alveolar bone height reductions. However, longitudinal data suggest that in the long term, the maintenance of peri-implant alveolar bone is fairly constant for most individuals.1 Local factors An experimental study by Vignoletti et al.60 supports the passivity of dental implants in alveolar bone maintenance, using histological data to suggest that the amount of alveolar bone lost adjacent to immediate implants was influenced to a greater extent by socket morphology and arch position than by surface topography of the implant. Strietzel et al.61 also supports the influential role of local morphology and bone quality on the final position of the peri-implant alveolar bone crest. A range of local factors have been suggested as influential on dimensional changes to alveolar bone height.8,51,60,61 Influencing factors include: (a) reasons for tooth extraction; (b) number and proximity of teeth to be extracted; (c) condition of the socket before and after tooth extraction; (d) influence of tissue biotype on healing; (e) local differences between sites in the mouth and the dental arches; (f) type of interim prosthesis used; (f) presence of pathological or traumatic processes; (g) extent of surgical trauma; (h) bone quality; and (i) width of the extraction socket wall. Furthermore, a consistent finding following surgery is that raising a mucoperiosteal flap will lead to bone remodeling and loss of alveolar bone height.62–64 Once again, the lack of confirmatory data on each of the above factors makes it difficult to separate fact from fiction. Implant-related factors Additionally, several implant-related factors have been suggested as influential on alveolar bone height maintenance. First, present evidence supports the influential role of loading conditions on peri-implant bone height. Lindquist et al.50 found that distal implants of implantsupported over-dentures maintained more peri-implant ª 2011 Blackwell Publishing Asia Pty Ltd
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bone than mesial implants. Evidence taken from biteforce studies comparing full denture patients with ‘‘sufficient’’ mandibular height (>16 mm) versus ‘‘reduced’’ alveolar height (9–15 mm) suggested that the amount of force transferred to alveolar bone was dependent on the residual alveolar bone height.55 Bite-force studies analyzing a range of residual alveolar bone heights supporting dental implants have not been conducted, thus the force transferred to alveolar bone by dental implants cannot be quantified, nor can the role this force has on the maintenance of alveolar bone. Second, Zollner et al.65 suggested that the immediate loading of implants resulted in statistically-significantly greater bone height reduction than using an early-loading protocol over a 5-month period. However, the differences in bone level changes reported by Zollner et al. were found to be center dependent and not related to the loading protocol. Interestingly, Galli et al.’s study41and a systematic review66 found that the timing of implant loading was not influential on changes to peri-implant bone height. Third, it was suggested that implant surface roughness was capable of modulating osteoblastic function with clinically-significant results. This controversial yet popular theory among implant manufacturers is supported by few studies. Most studies have shown negligible difference in peri-implant bone height changes between different implant surfaces.67,68 Meta-analyses of available clinical trials have suggested that neither the type of implant nor the surface characteristics influence
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peri-implant bone levels. However, the author of the meta-analyses highlighted that this was based on a limited number of trials with short follow-up times.69 Changes to implant surfaces, such as the recentlyreleased SLActive implant from Straumann, might prove to be more influential on alveolar bone maintenance; however, no published long-term, clinical data currently exist. Thus, the role of implant surfaces on alveolar bone maintenance remains unclear. Conclusion The current available data suggest that following the extraction of a tooth, no known factor can prevent loss of bundle bone or alveolar bone remodeling. Additionally, the magnitude of change is patient, site, and time dependent. Some evidence suggests that alveolar bone remodels at a constant slow rate in both the edentulous and periimplant sites, resulting in a gradual loss of alveolar bone height in the long term. Taken together, current evidence suggests that implants do not have an active role in the maintenance of alveolar bone height. Further evidence is required before it can be concluded that dental implants alter these processes. The role of the neighboring or adjacent teeth on the maintenance of bone height is also not entirely clear because of the lack of conclusive data to answer this question. It might be prudent for the dentist to take this information into consideration when planning complex restorative cases using dental implants for support.
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diate denture treatment. Aust Dent J 1969; 6: 370–6. Weber HP, Crohin CC, Fiorellini JP. A 5-year prospective clinical and radiographic study of nonsubmerged dental implants. Clin Oral Impl Res 2000; 2: 144–53. Adell R, Eriksson B, Lekholm U, Bra˚nemark PI, Jemt T. Long-term follow-up study of osseointegrated implants in the treatment of totally edentulous jaws. Int J Oral Maxillofac Implants 1990; 4: 347– 59. Fontijn-Tekamp F, Slagter A, Van Der Bilt A et al. Biting and chewing in overdentures, full dentures, and natural dentitions. J Dent Res 2000; 7: 1519–24. Haraldson T, Zarb G. A 10-year follow-up study of the masticatory system after treatment with osseointegrated implant bridges. Scand J Dent Res 1988; 3: 243– 52. Haraldson T, Jemt T, Stalblad P, Lekholm U. Oral function in subjects with overdentures supported by osseointegrated implants. Scand J Dent Res 1988; 3: 235–42. European Workshop on Periodontology (ed.). Proceedings of the 1st European Workshop on Periodontology. Ittingen Thurgau, Switzerland: Quintessence, 1993. Zitzmann N, Berglundh T. Definition and prevalence of peri-implant diseases. J Clin Periodontol 2008; s8: 286–91. Vignoletti F, Johansson C, Albrektsson T, De Sanctis M, San Roman F, Sanz M. Early healing of implants placed into fresh extraction sockets: an experimental study in the beagle dog. De novo bone formation. J Clin Periodontol 2009; 3: 265– 77. Strietzel FP, Nowak M, Kuchler I, Friedmann A. Peri-implant alveolar bone loss with respect to bone quality after use of the osteotome technique: results of a retrospective study. Clin Oral Impl Res 2002; 5: 508–13.
62 Yaffe A, Fine N, Binderman I. Regional accelerated phenomenon in the mandible following mucoperiosteal flap surgery. J Periodontol 1994; 1: 79–83. 63 Fickl S, Zuhr O, Wachtel H, Bolz W, Huerzeler M. Tissue alterations after tooth extraction with and without surgical trauma: a volumetric study in the beagle dog. J Clin Periodontol 2008; 4: 356–63. 64 Blanco J, Nunez V, Aracil L, Munoz F, Ramos I. Ridge alterations following immediate implant placement in the dog: flap versus flapless surgery. J Clin Periodontol 2008; 7: 640–8. 65 Zollner A, Ganeles J, Korostoff J, Guerra F, Krafft T, Bragger U. Immediate and early non-occlusal loading of Straumann implants with a chemically modified surface (SLActive) in the posterior mandible and maxilla: interim results from a prospective multicenter randomized-controlled study. Clin Oral Impl Res 2008; 5: 442–50. 66 Esposito M, Grusovin MG, Achille H, Coulthard P, Worthington HV. Interventions for replacing missing teeth: different times for loading dental implants. Cochrane Database Syst Rev 2009; Issue 1. Art. No.: CD003878. DOI: 10.1002/14651858.CD003878.pub4. 67 Tawse-Smith A, Perio C, Payne AG, Kumara R, Thomson WM. One-stage operative procedure using two different implant systems: a prospective study on implant overdentures in the edentulous mandible. Clin Implant Dent Relat Res 2001; 4: 185–93. 68 Kemppainen P, Eskola S, Ylipaavalniemi P. A comparative prospective clinical study of two single-tooth implants: a preliminary report of 102 implants. J Prosthet Dent 1997; 4: 382–7. 69 Esposito M, Murray-Curtis L, Grusovin MG, Coulthard P, Worthington HV. Interventions for replacing missing teeth: different types of dental implants. Cochrane Database Syst Rev 2007; Issue 4. Art. No.: CD003815. DOI: 10.1002/14651858.CD003815. pub3.
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REVIEW ARTICLE Implant Dentistry
Do dental implants preserve and maintain alveolar bone? Jessica E. O’Neill & Stephen C. Yeung Faculty of Dentistry, The University of Sydney, Sydney, NSW, Australia
Keywords alveolar bone, alveolar bone loss, alveolar bone remodeling, dental implant, extraction socket remodeling. Correspondence Dr Jessica O’Neill, Faculty of Dentistry, Westmead Centre for Oral Health, Westmead Hospital, University of Sydney, C24 Darcy Road, Westmead, NSW 2145, Australia. Tel: +61-2-9845-5951 Fax: +61-2-9893-8671 Email: [email protected] Received 6 February 2010; accepted 20 May 2010.
Abstract Dental implants have been touted as capable of playing an active role in the maintenance of alveolar bone height, despite the lack of a sound biological basis to support this proposition. This paper reviews the biology of bone loss, the literature concerning alveolar bone remodeling in both the postextraction period and long term, and discusses the literature regarding the influence dental implant placement has on this process. Based on current evidence, implants do not have an active role in the preservation and maintenance of alveolar bone height. Following extraction of a tooth, no external influence has been identified that will prevent loss of bundle bone or alveolar bone remodeling. Additionally, the magnitude of change is patient, site, and time dependant. Further supporting evidence is required before it can be concluded that dental implants are capable of influencing alveolar bone remodeling.
doi: 10.1111/j.2041-1626.2011.00074.x
Introduction Implant dentistry has developed dramatically in the past 30 years. Since the discovery of osseointegration in 1977, the application of dental implants has progressed from being the support of a fixed prosthesis for an edentulous mandible1,2 to the sophisticated use of dental implants in the replacement of single or multiple missing teeth in partially-edentulous dentitions.3,4 In more recent times, immediate implant placement following tooth extraction had been advocated by some practitioners as a viable and predictable clinical technique.5,6 Clinical follow-up studies have yielded mixed results. Implant survival studies reported comparable data of implant survival, whether an implant was placed into an extraction socket immediately following extraction or placed into a healed edentulous alveolus.7,8 Furthermore, based on clinical and radiographic observations, claims had been made that early placement of dental implants could assist in preserving alveolar bone.9,10 The clinical importance of preserving interproximal bone between the tooth and ª 2011 Blackwell Publishing Asia Pty Ltd
implant or between two adjacent implants is obvious to most clinicians, particularly when considering esthetics in the anterior regions. Paradoxically, clinical studies have now identified a consistent loss of labial bone on anterior implants placed immediately into extraction sockets,11,12 resulting in unsightly and unwanted gingival retraction/recession.12,13 This finding is not unexpected, considering the labial bone consists primarily of ‘‘bundle bone’’,14 which is biologically programmed to undergo remodeling following tooth loss or extraction. However, the relatively ‘‘stable’’ interproximal bone between the tooth and implant might present a different set of circumstances in contrast to that between an implant and the free-standing labial alveolus. The presence of a neighboring tooth with its intact periodontium might have contributed to supporting and maintaining the interproximal bone. In this paper, we reviewed the current literature in search of a biological explanation for the bone remodeling behavior around dental implants, in general, and in extraction sockets, in particular. Specifically, we sought evidence to support the proposition that 229
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osseointegrated dental implants will support or maintain alveolar bone height. Bone mineral density There have been reports suggesting that implants might play a role in the maintenance of alveolar bone mineral density. The hypothesis that increased biomechanical load on bone causes strain leading to stimulation of bone growth15,16 has been supported by meta-analyses of the effects of exercise and increasing load on the maintenance of bone mineral density in both men and women.17–19 von Wowern and Gotfredsen20 demonstrated that the bone mineral content (BMC) of mandibular bone decreases at a similar rate as skeletal BMC, which is measured using forearm BMC. Additionally, they showed in two studies that BMC of peri-implant bone reduced significantly less than either skeletal BMC or mandibular BMC, suggesting that dental implants had a stabilizing effect on peri-implant bone.20,21 These data suggest that ‘‘increased function leads to load-related positive bone remodeling that minimizes, or in some cases may counteract, the physiologic age-related changes in the bone remodeling process leading to BMC loss’’.20 However, while plausible, it must be considered an unproven extrapolation to assert that these changes to bone mineral density can be translated to the positive influence on vertical bone height changes. Reducing postextraction alveolar bone remodeling using dental implants The alveolar process has been described as the portion of the mandible or maxilla containing the sockets of the teeth,22 and consists of outer cortical walls, central cancellous bone, and bundle bone lining the socket walls. Bundle bone and the periodontal ligament share embryological origin, function, and blood supply, and are physically united at the cellular level via extrinsic collagen fibers.23,24 This interdependency of biological structure and the loss of bundle bone following tooth extraction are part of the natural process of wound healing and bone modeling.14,25 These changes can be observed clinically and radiographically as a reduction of alveolar bone height and width.26–28 Morphological changes occurring following tooth extraction have been studied using dental casts,29 cadaver specimens,30 and radiographs,5,31 and clinically,26,28,32,33 in both partially- and fully-edentulous patients. These studies have conclusively shown that following tooth extraction, the wound-healing process results in reduction in the dimensions of alveolar bone.29–31 Lekovic et al.32,33 conducted a series of studies reporting vertical changes in 230
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buccal bone height over a 6-month period following single tooth extraction, showing a mean reduction of 1.0 ± 2.25 mm and 1.50 ± 0.26 mm. Similar findings were also reported by Iasella et al.28 The significant variation in the range of these dimensional bone changes reported in these studies suggest that it might be difficult to predict the amount of variation to expect in each case, as the magnitude of reduction in bone height and width is patient, site, and time dependent. For example, in their studies, Lekovic et al.,32 Iasella et al.,28 and Johnson27 reported individual postextraction alveolar bone reductions ranging between 0 and 4 mm. Likewise, multiple studies demonstrate the site-specific nature of alveolar bone loss, occurring to a greater extent at buccal than palatal or lingual aspects of the tooth socket,28,29 and occurring to a greater extent at multiple than single extraction sites.5,27 Finally, the expected variation might be influenced by pathology or traumatic processes that might have damaged the bony walls,8 or influenced by the duration since extraction, as the majority of alveolar bone height loss has repeatedly been shown to occur in the first 3 months following both extraction and surgery.5,6,27 The precise long-term behavior in jaw bone remodeling following tooth extraction has not been well studied. Currently, there is insufficient data to quantify changes in alveolar height in the long term. One longitudinal radiographic study of patients 15–25 years after tooth loss demonstrated the edentulous alveolar process perpetually models and remodels, albeit slowly, with a rate of resorption estimated at 0.05 mm/year for the maxilla and 0.2 mm/year for the mandible.31 It is unclear if there is an end-point to jaw resorption and remodeling, and what outside influences might play a part in the rate of dimensional changes in edentulous jaws. Changes to alveolar bone following immediate implant placement Opinion has been divided on the ability of immediate implants to maintain alveolar bone following tooth extraction, and thus, interfere with hard tissue changes that would otherwise occur.34 The apparent maintenance of alveolar bone after extraction has been used as a rationale for immediate implant placement,9 although, as discussed later, this was not supported by conclusive evidence. One of the few clinical trials used to support this controversial claim, albeit weakly, are data from a splitmouth study by Paolantonio et al.10 in which patients received bilateral implants, one in mature bone, and another in a contralateral fresh extraction socket. The histological and radiographic results showed no difference in ª 2011 Blackwell Publishing Asia Pty Ltd
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vertical bone height lost between the groups over a 12-month period, suggesting immediate implants preserve alveolar bone dimensions and counteract tissue modeling. The authors thus stated that ‘‘early implantation may preserve the alveolar anatomy and that the placement of a fixture in a fresh extraction socket may help to maintain the bony crest structure’’. Interestingly, several of the immediate implants in this study were placed into palatal root sockets of upper molar teeth; an experimental site surrounded by thick, bony plates, and not subject to the extent of resorption found with buccal alveolar bone. Additionally, Wheeler et al.35 claimed to be able to ‘‘preserve the osseous structure surrounding the socket’’ using a novel immediate implant technique. However, this statement was supported by descriptive results of only two cases, both of which involved an atraumatic extraction technique, which in itself might reduce alveolar bone loss. Conversely, reduction in alveolar bone height has been a consistent feature in the majority of immediate implant trials.11,36–40 For example, Arau´jo et al.37 in an experimental study, found vertical resorption 3 months after immediate implantation of 2.6 ± 0.4 mm for buccal and 0.2 ± 0.5 mm for lingual plates. Similarly, Botticelli et al.11 reported a mean 1.9 ± 0.9 mm reduction in the vertical height of the buccal crest over a 4-month period. This amount of bone loss is similar to the aforementioned extraction studies, and is greater than the reduction in vertical height reported by similar studies of implants placed into healed sites (0.3–1.10 mm).41,42 These observations suggested that the remodeling behavior of buccal bone could be substantially different from that of interproximal bone when (an) adjacent natural tooth/teeth was/were present. Zitzmann et al.43 suggested that these peri-implant bone changes, occurring despite immediate insertion of the dental implants, were as a result of the incongruity between dental implant diameter and the morphology of the alveolus. This suggestion was used to sustain a niche for the use of guided bone regeneration, a technique using grafting materials and/or membranes to reduce such defects. The ability of this technique to reduce socket defects is apparent;43 however, there is no evidence to suggest that this technique, nor dental implants, is capable of maintaining bundle bone. Delayed implant placement, in the period immediately following initial resorption (6–8 weeks), was advocated as ‘‘preventive implant therapy’’ by Denissen et al.44,45 as a method to prevent or delay loss of the alveolar ridge, yet waiting for the inevitable bundle bone resorption to have occurred. However, these authors supported their statement with a cross-sectional study of cadaver mandibles, providing little information on the time since tooth loss, ª 2011 Blackwell Publishing Asia Pty Ltd
Dental implants effects on alveolar bone
and the reason for extraction or postextraction socket morphology. Thus, the conclusion that alveolar bone loss was prevented by dental implants was not sustained by the data. The passivity of dental implants in the maintenance of alveolar bone height is further supported by a group of studies comparing peri-implant alveolar bone height reductions of immediate, delayed, and late placement of implants. Watzek et al.46 analyzed these three groups of patients over a 27-month period, reporting that periimplant bone loss was greatest in the immediate group (mean: 1 mm), less in the delayed group (0.8 mm), and least in the late group (0.5 mm). Similar findings were also reported by Mensdorff-Pouilly.47 Taken together, these data suggest that the modeling of alveolar bone following extraction was uninfluenced by the presence of a dental implant, thus refuting the proposition that dental implants could maintain alveolar bone in the immediate healing phase. Maintaining alveolar bone morphology The classification of bone loss into that occurring in the first postsurgical year (0.30–1.64 mm)48–50 and mean annual bone loss in the period after the first postsurgical year (0.00–0.15 mm)48–50 permits analysis of the effects implants have on alveolar bone maintenance once the influence of wound healing has receded. Unfortunately, the lack of available evidence so far has prevented a conclusion being drawn on this question. Studies quantifying long-term changes to edentulous alveolar bone failed to provide a baseline for change, as they either frequently combined soft and hard tissue measurements;29,51,52 the study participants wore mucosal-borne prostheses at the site, which itself influences alveolar bone maintenance;31 or the duration since tooth extraction was not provided.29,30 The radiographic study of edentulous patients by Tallgren31 reported a mean annual bone height reduction of 0.05 mm in the maxilla and 0.2 mm in the mandible, similar to the amount of bone height loss found adjacent to implants.49,53 Extrapolation of this data suggests that if dental implants did play a passive role in alveolar bone maintenance, it would take 200 years to lose alveolar bone along the length of a 10-mm maxillary implant, and 50 years at mandibular sites. This slow and gradual rate of change is unable to be effectively studied. In 1990, Adell et al.54 presented mean annual marginal bone loss measurements for three groups of patients, with observation times of 1–4, 5–9, and 10–15 years. The mean annual marginal bone loss in all groups was 0.1 mm, suggesting that the rate of alveolar bone height reduction is constant over time. 231
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By comparing the data from Tallgren and Adell, it is evident that long-term crestal resorption occurs to a similar extent, whether it is adjacent to dental implants, or as an edentulous saddle area under a mucosal-borne prosthesis. Insufficient data exist to suggest a rate of change for completely edentulous sites without prostheses.54 Bone loss in the partially-edentulous site appears to be less, and it is influenced by the presence of the adjacent teeth. Wyatt and Zarb42 proposed the theory that ‘‘implants may serve to retain the bone by applying tensional forces, while the mucosal borne prosthesis may accelerate bone resorption due to compressive forces’’, and presented 1–12-year data, with a mean annual bone loss of 0.00 mm per year ()0.26 mm to +0.42 mm). Although the patients in this study were younger than those in Tallgren’s paper, age-related physiological changes had not been taken into consideration. It is accepted that dental implants increase load to alveolar bone, evidenced both by increased radio-opacity in bone surrounding implants,48 and by studies assessing biting force, which have repeatedly shown increased bite force from patients with implant-supported overdentures, when compared to patients with conventional dentures.55–57 These changes have been described as ‘‘successive load related bone remodeling’’.48 However, there is insufficient evidence to conclude that dental implants actively maintain alveolar bone height simply because alveolar bone has been shown to react to loading stresses. Taken together, the above data suggest that jaw bone remodeling after tooth loss continues unabated throughout life, irrespective of the presence of a foreign object (dental implant). At this point in time, there is insufficient evidence to suggest that the presence of a dental implant, osseointegrated with the jaw bone and functionally loaded, has an impact on modifying the rate of bone remodeling. Factors affecting alveolar bone remodeling Alveolar bone is a dynamic tissue influenced by multiple factors. Assessing the role played by a single factor, such as dental implants, in the maintenance of bone height is therefore fraught with difficulty. The following factors have been suggested to play a role in alveolar bone height: Patient-related factors The presence of an implant could pose a risk for the loss of alveolar bone height as a direct result of peri-implantitis; a disease process defined by the loss of alveolar bone.58 Recent evidence suggests peri-implantitis is preva232
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lent, occurring in 12–3% of sites,59 and thus poses a clinically-significant risk. There is evidence to suggest that poor oral hygiene is strongly correlated with peri-implant bone loss.50 Bone metabolism might be influenced by metabolic bone diseases, such as osteoporosis, demonstrated in the study by von Wowern and Gotfredsen,20 whereby osteoporotic patients were shown to have lost more vertical bone height (0.47 mm) over a 5-year period, compared to non-osteoporotic patients (0.01 mm). Individual patient response appears to influence the magnitude of change following tooth extraction, evident in the apparent wide range of initial alveolar bone height reductions. However, longitudinal data suggest that in the long term, the maintenance of peri-implant alveolar bone is fairly constant for most individuals.1 Local factors An experimental study by Vignoletti et al.60 supports the passivity of dental implants in alveolar bone maintenance, using histological data to suggest that the amount of alveolar bone lost adjacent to immediate implants was influenced to a greater extent by socket morphology and arch position than by surface topography of the implant. Strietzel et al.61 also supports the influential role of local morphology and bone quality on the final position of the peri-implant alveolar bone crest. A range of local factors have been suggested as influential on dimensional changes to alveolar bone height.8,51,60,61 Influencing factors include: (a) reasons for tooth extraction; (b) number and proximity of teeth to be extracted; (c) condition of the socket before and after tooth extraction; (d) influence of tissue biotype on healing; (e) local differences between sites in the mouth and the dental arches; (f) type of interim prosthesis used; (f) presence of pathological or traumatic processes; (g) extent of surgical trauma; (h) bone quality; and (i) width of the extraction socket wall. Furthermore, a consistent finding following surgery is that raising a mucoperiosteal flap will lead to bone remodeling and loss of alveolar bone height.62–64 Once again, the lack of confirmatory data on each of the above factors makes it difficult to separate fact from fiction. Implant-related factors Additionally, several implant-related factors have been suggested as influential on alveolar bone height maintenance. First, present evidence supports the influential role of loading conditions on peri-implant bone height. Lindquist et al.50 found that distal implants of implantsupported over-dentures maintained more peri-implant ª 2011 Blackwell Publishing Asia Pty Ltd
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bone than mesial implants. Evidence taken from biteforce studies comparing full denture patients with ‘‘sufficient’’ mandibular height (>16 mm) versus ‘‘reduced’’ alveolar height (9–15 mm) suggested that the amount of force transferred to alveolar bone was dependent on the residual alveolar bone height.55 Bite-force studies analyzing a range of residual alveolar bone heights supporting dental implants have not been conducted, thus the force transferred to alveolar bone by dental implants cannot be quantified, nor can the role this force has on the maintenance of alveolar bone. Second, Zollner et al.65 suggested that the immediate loading of implants resulted in statistically-significantly greater bone height reduction than using an early-loading protocol over a 5-month period. However, the differences in bone level changes reported by Zollner et al. were found to be center dependent and not related to the loading protocol. Interestingly, Galli et al.’s study41and a systematic review66 found that the timing of implant loading was not influential on changes to peri-implant bone height. Third, it was suggested that implant surface roughness was capable of modulating osteoblastic function with clinically-significant results. This controversial yet popular theory among implant manufacturers is supported by few studies. Most studies have shown negligible difference in peri-implant bone height changes between different implant surfaces.67,68 Meta-analyses of available clinical trials have suggested that neither the type of implant nor the surface characteristics influence
References 1 Adell R, Lekholm U, Rockler B, Bra˚nemark PI. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 1981; 10: 387–416. 2 Bra˚nemark PI, Adell R, Albrektsson T, Lekholm U, Lundkvist S, Rockler B. Osseointegrated titanium fixtures in the treatment of edentulousness. Biomaterials 1983; 1: 25–8. 3 Jung RE, Pjetursson BE, Glauser R, Zembic A, Zwahlen M, Lang NP. A systematic review of the 5-year survival and complication rates of implant-supported single crowns. Clin Oral Impl Res 2008; 2: 119–30. 4 Pjetursson BE, Bragger U, Lang NP, Zwahlen M. Comparison of survival and complication rates of tooth-
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peri-implant bone levels. However, the author of the meta-analyses highlighted that this was based on a limited number of trials with short follow-up times.69 Changes to implant surfaces, such as the recentlyreleased SLActive implant from Straumann, might prove to be more influential on alveolar bone maintenance; however, no published long-term, clinical data currently exist. Thus, the role of implant surfaces on alveolar bone maintenance remains unclear. Conclusion The current available data suggest that following the extraction of a tooth, no known factor can prevent loss of bundle bone or alveolar bone remodeling. Additionally, the magnitude of change is patient, site, and time dependent. Some evidence suggests that alveolar bone remodels at a constant slow rate in both the edentulous and periimplant sites, resulting in a gradual loss of alveolar bone height in the long term. Taken together, current evidence suggests that implants do not have an active role in the maintenance of alveolar bone height. Further evidence is required before it can be concluded that dental implants alter these processes. The role of the neighboring or adjacent teeth on the maintenance of bone height is also not entirely clear because of the lack of conclusive data to answer this question. It might be prudent for the dentist to take this information into consideration when planning complex restorative cases using dental implants for support.
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