DOCTORAL DISSERTATION ON SUBCRESTAL IMPLANT PLACEMENT: ANALYSIS OF MARGINAL BONE LEVELS RELATED TO SURGICAL AND PROSTHETIC FACTORS

ABSTRACT BACKGROUND: The subcrestal placement of dental implants is becoming increasingly common. Modern biomedical devices with features to make them employable in such a manner have been developed to reduce the implant exposure within the oral environment. Indeed, it is universally recognized that, after placement, dental implants undergo a process of bone remodeling that has the greatest clinical impact at the level of the bony ridge. The exposure of the implant surface into the oral environment is potentially harmful because the accumulation of bacterial plaque on such surfaces can trigger inflammatory processes that, over time, may potentially lead to the loss of the implant itself. Subcrestal placement is based on the rationale that, provided a certain amount of crestal bone resorption cannot be avoided, this phenomenon remains superficially confined. According to this, the implant remains integrally within the bone envelope. Nevertheless, not all commercially available implant systems can be used in such a manner. Moreover, there are only a few clinical studies with short-to-medium-term follow-up in the literature. The primary outcome of this thesis is to investigate the prosthetic and surgical factors that may influence MBL of dental implants designed to be placed subcrestally. OBJECTIVES: General objectives of this thesis are: to present the clinical validation of subcrestal implants and to identify surgical and prosthetic factors that may affect MBL. Secondarily, the relationship between subcrestal placement and peri-implant biometric parameters is evaluated. Specific objectives of each study: (I.) To retrospectively evaluate 410 subcrestal dental implants placed in 93 patients in order to identify potential factors associated with MBL variation by multiple linear regression analysis. (II.) To investigate a possible correlation between the prosthetic emergence angle (EA) (greater or less than 30 degrees) of fixed rehabilitations screwed onto subcrestal implants and the peri-implant probing depth. Secondarily, to evaluate the correlation between emergence angle and gingival and plaque indices. (III.) To retrospectively evaluate the impact of vestibular emergence angle (EA) in the anterior zone with respect to peri-implant biometric indices of subcrestally placed implants. (IV.) systematic review of the literature to investigate the role of emergence angle (EA) on the development of peri-implantitis; (V.) A randomized controlled trial (RCT) with 3-year follow-up was conducted in order to evaluate the MBL of subcrestal implants placed by static computer-guided surgery versus freehand placement. Secondarily, to evaluate biological and prosthetic complications as well as peri-implant biometric parameters. MATERIALS & METHODS, RESULTS: Study IV.: A narrative review of the literature was performed to investigate the role of implant emergence angle on the development of peri-implantitis. Electronic search was performed on MEDLINE (PubMed) and Scopus by combination of MeSH terms and free text words combined through Boolean operators (AND or OR). Only English-language studies published up to June 2022 were selected. Prospective studies, retrospective studies, and case series with at least 10 patients were considered as eligible. All type of fixed implant-supported rehabilitations were evaluated: implant-supported single crowns, fixed partial restorations, and fixed complete restorations. By electronic search, 264 articles were identified; duplicate articles and those having only the abstract were removed. After title/abstract screening and full-texts reading, 5 studies were included. The studies selected for review employed different methods for measuring implant emergence angle. Katafuchi and Yi measured the angle of emergence (EA) radiographically from the mesio-distal aspect. The result was a higher prevalence of peri-implantitis in cases of EA >30°. Inoue et al. measured EA at the mesial, distal, buccal and lingual directions on each dental implant. The result that emerged was that marginal bone loss tends to be less when EA is between 20°-40°. On the other hand, studies conducted by Lops and Hentenaar reported that EA >30 degrees may not affect peri-implant tissue health in the short to medium term (3-5 years). Study I.: 410 implants were retrospectively evaluated in 93 patients with an observation period ranging from 2 to 9 years (mean 2.72). A total of 164 prosthetic rehabilitations including single crowns, partial and/or complete implant-supported prostheses were evaluated. A mean MBL of -1.09 ± 0.65 mm was calculated at baseline (day of delivery of final prosthetic restoration) and -1.00 ± 0.37 mm at t1 (last follow-up visit). The MBL change between the two timepoints (t0 and t1) was found to be 0.09 ± 0.68 mm. Multiple linear regression analysis showed that MBL change (MBL change) was associated with: observation period (cumulative follow-up), subcrestal implant position at t0 (baseline), and presence of type 2 diabetes (controlled). Study II.: A retrospective evaluation of 312 implants in 74 patients with a follow-up period greater than 3 years (mean 3.8 ± 1.3 years) was performed). Thirty-four single crowns, 65 fixed partial dentures, and 12 fixed full-arch rehabilitations were evaluated. A comparative evaluation was then performed on the basis of prosthetic emergence angle (EA). Group 1 including 175 implants with mean EA of 45 ± 4 degrees versus group 2 including 137 implants with mean EA of 22 ± 7 degrees. The MBL change between the two groups was found to be similar (MBL change group 1: 0.06 ± 0.09 mm ; MBL change group 2: 0.06 ± 0.10 mm) and no statistically significant differences were observed. In parallel, linear regression analysis showed no association between MBL and variables such as type of prosthetic rehabilitation, emergence angle and site (anterior versus posterior). It was possible to observe small values in terms of modified bleeding index change (0.2 for group 1 and 0.1 for group 2) and modified plaque index change (0.2 for group 1 and 0.2 for group 2). No statistically significant difference was found between the groups. Study III.: Retrospective evaluation of 220 dental implants placed in 57 patients with follow-up of more than 3 years. The vestibular emergence angle of implants placed in the incisor, canine and premolar regions was evaluated digitally. The prosthetic rehabilitations consisted of 34 single crowns and 62 fixed partial dentures. The mean vestibular emergence angle (EA) measured 46.4 ± 12.2 degrees in group 1 and 24.5 ± 4.7 degrees in group 2. A difference in probing depth of 0.062 mm was found to be greater for group 1 than for group 2. However, no statistically significant differences in probing depth, gingival index and plaque index were observed between the two groups. Study V.: A randomized, controlled parallel-group study on 60 implants was conducted. Implants placed in the Control group were positioned freehand while implants placed in the Test group were inserted by static computer-guided surgery. The primary objective was to evaluate the MBL annually from baseline (T0, day of delivery of the final prosthesis) to 3 years of observation. No statistically significant differences were found between the groups in terms of MBL, PPD, mBI and mPI. No biological or prosthetic complications were detected. Marginal bone resorption was evaluated as a function of tissue thickness at the time of implant placement (STH). Implants placed in sites characterized by thick tissues (>3mm) showed, on average, 0.33mm less marginal bone resorption than implants placed in sites with thin tissues (<3mm) between the day of implant placement and the prosthetic loading (T0). CONCLUSIONS: The present thesis supports the clinical success of subcrestally placed implants (1mm) in the short to medium term (3-year follow-up). From the observational and interventional studies which were mentioned above, it seems that the MBL is contained within more than acceptable values when compared to the proposed success criteria for modern implant systems. Interproximal and vestibular prosthetic emergence angle seems not to be associated with MBL and peri-implant biometric parameters related to plaque and bleeding. Subcrestal placement by static computer-guided surgery is comparable to freehand placement for the same implant system investigated in this set of studies.