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Dental implants have become part of daily practice. High survival and success rates have made implant dentistry a predictable treatment concept, offering patients function, esthetics, and, subsequently, confidence. In the early days of modern implantology, oral implants were mainly intended for the edentulous jaw to provide patients with fixed teeth. This was a time-consuming treatment that required a long integration period of several months.1 Nevertheless, outcomes were good, and implant survival rates of 78% in the maxilla and 86% in the mandible were reported after 15 years.2 Though esthetic results were often poor, patients' oral function and quality of life could be improved significantly. Also, suboptimal bone quality and quantity, unhealthy habits such as smoking, and/or poor health often led to implant failure.3
Today, a variety of surgical and prosthetic protocols, implant designs, and prosthetic devices are used, making it seem as if dental technology has finally outsmarted biology. Bone quality and quantity, systemic conditions, and patient-related factors appear to play less significant roles than previously. New implants emerge on the market every year with claims of being better than previous versions, though scientific data may be lacking. With a plethora of options available, for practitioners, choosing an implant system has become highly challenging. Having an understanding of the role of different implant design properties may help clinicians make informed decisions.
Since the start of modern implantology, the position of the bone level around the implant neck has been considered influential in the success of an implant.4 The presence and stability of the surrounding bone is not only a prerequisite for the longevity of the implant but also necessary to support the mucosa and help maintain the soft-tissue profile. Consequently, with regard to implant design, the implant collar has been a focal point for the maintenance of bone.
In 2006, Lazzara and Porter introduced the concept of platform switching.5 By using wide-platform implants with regular-diameter healing abutments, a mismatch in diameter is created at the connection, which results in less bone loss over time. Since then, their findings have been supported by numerous other studies, leading to a widespread acceptance and application of the platform-switching concept. Canullo et al demonstrated that not only was the mismatch significant but so was its size.6 The larger the difference in diameter, the better the bone level could be maintained. It is hypothesized that the larger offset keeps the bacteria and inflammatory cell infiltrate that comes from the implant-abutment junction further away from the bone.7 This could also explain the good outcomes of some ultra-wide-diameter implants, which feature a large built-in platform switch and have been shown to demonstrate very little bone loss.8,9 A study comparing the effect of platform switching using an experimental ultra-wide-diameter implant with an off-centric connection demonstrated less bone loss at the platform-switched side compared to the non-switched side.10 But, overall, bone loss was limited to 0.80 mm after 1 year.
Today, numerous implant connections are available on the market. While the early Brånemark implant had an external hexagonal connection, internal-connection implants have become the standard in many parts of the world. It is believed that the internal connection is more stable, leading to less screw loosening and less bacterial infiltration.11 In turn, this helps to preserve the bone surrounding the implant. There is, however, very little evidence to support these claims.
Glibert et al compared internal- and external-connection implants, with or without coronal microthreads.12 All four designs were placed in the edentulous maxilla of the same patient to support a bar-retained overdenture. After 1 year, there was no difference in bone loss between implant types or connections. The authors concluded that bone loss is multifactorial, but mostly biologically determined by patient and site-specific factors.
Studies have demonstrated that once the initial bone remodeling has taken place in the first 6 months after implant placement, the bone level stabilizes.13,14 This early bone remodeling is a biologic response to the placement of the implant and a consequence of the establishment of the biologic width. From this perspective, the correct vertical positioning of the implant is crucial to limit the amount of bone loss. Studies have shown that implants should not be placed flush with the crest, but in relation to the soft-tissue thickness.15-17 In other words, a thin mucosa requires sub-crestal placement of the implant to provide space for the settling of the biologic width. In the case of a thick mucosa, the necessary space required for biologic width is already present, and, thus, the position of the implant shoulder is less crucial. If subcrestal implant placement is not possible, changing the mucosal biotype by thickening the tissues with a connective tissue graft will also help to limit bone loss.18
Thus, it seems that biology is predominantly steering the bone remodeling process, rather than implant design. In a study by De Bruyn et al, 10% of the implants showed a bone level located 2 mm below the interface at baseline, and after 3 years 20% had lost a further 2 mm or more of bone.19 However, less than 5% of the implants showed a significant increase in bone loss over time. When evaluating the risk for peri-implantitis, the prevalence is between 0 and 75% depending on the success criteria used. Over the years, different criteria and prevalences have been reported. Roos-Jansaker et al defined peri-implantitis as 3 mm or more bone loss with bleeding on probing.20 After 9 to 14 years, 16% of the patients in their study had peri-implantitis. Rinke et al considered peri-implantitis to be bone loss in conjunction with bleeding on probing and pocket depths of ≥5 mm, which turned out to be the case for 11.2% of the patients they studied.21 On the other hand, Ostman et al22 and Buser et al23 reported much lower figures of 1.6% and 1.8%, respectively, of the implants after 10 years. Long-term studies have shown that bleeding on probing can barely be correlated with bone loss and, thus, is not a reliable predictor for peri-implantitis.24,25 However, the prevalence of mucositis proved to be very high, at 90% of implants after 5 years.26
Peri-implant disease is a consequence of bacterial contamination. Analyses have pointed out that several bacteria can be found in large amounts in the implant sulcus. However, the intracoronal and abutment compartments are heavily contaminated with the same bacterial flora, such as Fusobacterium nucleatum, Aggregatibacter actinomycetemcomitans, Leptotrichia buccalis, Parvimonas micra, Prevotella melaninogenica, and Treponema denticola.27 This may be a source for recontamination, which could explain why peri-implantitis treatments have very little effect. Not surprisingly, the microbial composition of the neighboring teeth also resembles that of the implant surface. Prevention and treatment of periodontitis prior to implant placement is, thus, critical to prevent contamination of the implant surface and peri-implantitis.28
Early implants had a machined titanium surface, which successfully facilitated osseointegration. Nevertheless, in compromised or demanding cases, failure rates were significantly higher. Smoking or uncontrolled diabetes were often associated with higher implant failure rates.29 In cases of immediate loading and immediate placement, implants failed to osseointegrate more frequently.
The introduction of new and rougher surfaces was a turning point, as this new generation of implants significantly improved outcomes in these challenging situations. Based on surface roughness, Wennerberg et al introduced a classification categorizing implants as smooth, minimally rough, moderately rough, and rough.30 Further research pointed out that the best integration was achieved with a moderately rough or rough implant, although the latter surface also turned out to be more conducive to plaque and bacterial colonization.31 A meta-analysis evaluating the clinical survival of machined and dual-acid-etched implants in poor bone quality demonstrated that the moderately roughened dual-acid-etched implant performed significantly better.32 However, the roughened surface also holds a risk in terms of plaque adhesion and bacterial colonization. There is a significant higher risk of developing peri-implantitis in case of a rough surface topography compared to a turned surface.33
Although moderately rough implants yield a comparable outcome to turned surfaces in terms of peri-implantitis, they are more strongly associated with late implant failure. In a long-term study, Vandeweghe et al examined 218 implants with a mean follow-up of 161 months.34 About a third of the implants had a machined surface, while the others had a moderately rough surface obtained by sand-blasting and solvent cleaning. Although there was no significant difference in bone loss, it was interesting that all the outliers were found in the moderately rough group. Thus, it seems like a minority of these implants demonstrated progressive bone loss, which was not the case in the machined group. In a meta-analysis, Doornewaard et al found that moderately rough implants yielded the best survival rate, but minimally rough implants displayed less advanced bone loss of more than 2 mm and 3 mm.35
Therefore, the recent introduction of hybrid implant surfaces, featuring a moderately rough apical portion for improved integration and survival and a minimally rough coronal part to decrease the risk for peri-implantitis, is highly intriguing. There is, however, some concern that machining the coronal part could lead to more bone loss. Glibert et al (in preparation) compared a moderately rough implant with a hybrid surface implant and found no significant differences in bone loss during the first year.
In the early days, wide-diameter implants failed more often compared with conventional regular-diameter implants.36,37 However, these disappointing results were primarily due to an inappropriate drilling protocol and the use of these implants as "rescue implants" when regular-diameter implants were not a viable option.38 More recent research does not show any significant differences between regular- and wide-diameter implants.39-41 Today, even ultra-wide-diameter implants up to 9 mm in diameter are available. These implants have demonstrated a survival rate of 95.7% after 1 year with limited bone loss (0.46 mm).8 Their main indication is the molar extraction socket, where they provide good primary stability due to their large diameter. When used for immediate placement, implant success was 97.9% after a mean follow-up of 20 months.9
One of the main advantages of the wide-diameter implant is the large contact surface area with the bone. Short ultra-wide implants have an equal or even larger surface area compared with long regular-diameter implants. Thus, the shorter length is compensated for by a larger diameter. These implants can achieve ideal primary stability in limited bone height and demonstrate survival rates of greater than 96% after more than 1 year.42 Growing evidence on the success of short implants has led to the assumption that advanced surgical procedures such as sinus lifting can be avoided in many cases, especially because the load-bearing capacity of sinus-lifted bone is limited and the need to place a long implant is questionable.
Influence of the Restorative Approach
The restorative approach may also influence the implant outcome. Implant restorations can be screw-retained or cemented. There is overwhelming evidence that advises screw-retention whenever possible, mainly to avoid cement remnants.44,45 When cemented prostheses exhibit fistula formation,46 this is often a sign of cement being trapped subgingivally around the implants. Although many clinicians assume they have control over the cementation procedure, this is not the reality. Linkevicius et al evaluated the efficacy of removing residual cement after the cementation procedure and found that all samples still contained traces of cement.47 Wasiluk et al removed already-cemented crowns and abutments to examine for possible cement remnants and found that 73.3% of the restorations demonstrated excess cement that was not removed after the initial crown cementation.48 This excess cement may lead to mucositis and peri-implantitis if not removed,49 especially in patients with a history of periodontal disease.50 Fortunately, removal will lead to an improvement in most cases.
Despite these issues, cementation often is the only solution when the implant is not in the ideal prosthetic position. In such cases, the procedure should be performed with great caution. A convex abutment design with the margin only slightly subgingival may help to avoid cement from becoming trapped underneath the soft tissue.51 Also, the use of a copy-abutment to perform the cementation procedure will help to avoid excess cement.52
Another option is the use of a subcrestally angled implant, which allows a correction of the implant direction at the implant neck. This simplifies the restorative treatment significantly, as no further correction or special components are necessary, and it allows for the use of a screw-retained prosthesis in all cases. The use of this type of implant has been reported in several studies, using different treatment protocols. In a prospective study, Vandeweghe et al reported 1.2 mm bone loss after 1 year for single, angled prosthetic platform correction implants (Co-Axis®, Southern Implants, southernimplants.com) that were loaded immediately with the final restoration.13
Van Weehaeghe et al compared straight and angled implants when used for all-on-4 treatment in the mandible.53 Although bone loss was not significantly different, more complications were reported with the angulated abutment connected to a straight implant, while none were reported for the subcrestally angle-corrected implant.
In conclusion, innovations in implant design and surfaces have improved the outcomes of dental implant treatments. However, biology will always prevail when it comes to osseointegration and preservation of bone. Therefore, surgical and prosthetic treatment should respect the biologic width and avoid interventions such as cementation that may disturb the surrounding soft and hard tissues.
About the Authors
Stefan Vandeweghe, DDS, PhD
Faculty of Medicine and Health Sciences, Dental School, Ghent University, Gent, Belgium
Hugo De Bruyn, DDS, MSc, PhD
Faculty of Medicine and Health Sciences, Dental School, Ghent University, Gent, Belgium; Department of Dentistry, Radboud University Medical Center, Nijmegen, The Netherlands
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