Peri-implant tissue conditions following transcrestal and lateral sinus floor elevation: 3-year results of a bi-center, randomized trial

Maxillary sinus floor elevation with transcrestal and lateral access (tSFE and lSFE, respectively) are two validated options for vertical bone augmentation in the atrophic posterior maxilla. Both techniques were associated with high implant survival rates at long-term follow-up intervals [1, 2].

Radiographic analyses consistently indicate that, after its displacement with tSFE, the dimensions of the endo-sinusal grafted area tend to reduce overtime, although this tendency did not consistently reach statistical significance in all studies [3‐8]. Also, it was shown that this reduction may lead to the exposure of the endo-sinusal portion of the implant [4, 6, 8]. Similar findings were reported for sites undergoing lSFE [9].

To date, the clinical outcomes of tSFE and lSFE have been compared in several trials [7, 10‐21]. Among these, few randomized trials comparatively evaluated the impact of the dimensional reduction of the grafted area on peri-implant bone support at a follow-up of at least 2 years following tSFE and lSFE [14, 22]. In particular, endo-sinus bone–implant contact rates (as assessed radiographically) were similarly high and stable through the observation period, and amounted to 97.19% and 98.15% for tSFE and lSFE groups, respectively, at 2 years. Also, the 2-year position of the crestal bone level was found always coronal to the implant shoulder except for one patient in lSFE group [22]. Interestingly, a recent multivariate analysis conducted within a multicenter cross-sectional study identified the surgical access to the maxillary sinus as a factor influencing the risk for peri-implantitis occurrence [23]. In particular, lSFE was associated with significantly higher risk (OR = 6.75) for peri-implantitis compared to tSFE at a mean follow-up of 67.65 months from surgery. In the interpretation of their findings, the authors attributed increased peri-implantitis risk for lSFE to the combined effect of factors that may have determined early marginal bone loss: (i) extensive full thickness flap and a prolonged surgical time, (ii) compromised blood supply to the residual bone crest, and (iii) mechanical and thermal stress of the bone during underpreparation of the implant bed to obtain implant primary stability [23]. The evaluation of the incidence of peri-implant biological complications around implants undergoing tSFE vs lSFE within the context of a randomized trial remains confined to a limited number of studies [19, 22].

Recently, we performed a bi-center, parallel-arm, randomized trial comparatively evaluating tSFE and lSFE when applied concomitantly with implant placement at sites with limited (3–6 mm) residual bone. The results related to the clinical outcomes, morbidity, costs, and aspects of quality of life have already been published [15‐17]. Based on the same cohort, the present study presents data related to (i) peri-implant bone stability and (ii) the conditions of the peri-implant marginal tissues at 3 years following tSFE and lSFE.

As in previous analyses conducted on the present patient cohort [16] or other retrospective and prospective studies on sinus floor elevation procedures [8, 22], the proportion of the implant surface in direct contact with the radiopaque area was used to quantify the peri-implant bone support following sinus floor elevation. In our study, totCON% encompasses both portions of the implant which are in contact with (1) the pristine bone and (2) the endo-sinusal grafted space, which may become indistinguishable during the process of endo-sinusal bone formation and graft remodeling. At 3 years, the median totCON% was 100% in both groups, indicating that the peri‐implant bone support obtained at 1 year was fully maintained at 3 years for both tSFE and lSFE.

Some considerations may be advanced to explain the stability of peri-implant bone support overtime following sinus floor elevation. The high proportion of patients showing totCON% of 100% at 1 year may be partly ascribed to the considerable sinus floor elevation that can be obtained with DBBM in combination with tSFE [27, 28, 30] and lSFE [37, 38]. Also, a limited incidence of intra- and postoperative complications was observed in both groups [15]. In this respect, a sub-analysis conducted on the present cohort showed that the appropriate intraoperative management of membrane perforation resulted in a limited impact of membrane perforations on the 1-year radiographic outcomes, in general, and totCON%, in particular [16]. The stability in totCON% observed from 1 to 3 years may be partly explained by the use of DBBM that was shown to undergo slow resorption/degradation rate following sinus lift [39‐42]. In particular, previous analyses showed that 72.6% of the vertical dimension of the area grafted with DBBM is maintained from 6 months to 3 years post-surgery [8]. Consistently with our results, a recent randomized trial reported that the use of DBBM resulted in similarly high and stable values of endo-sinus bone–implant contact rates (as assessed radiographically) for both tSFE and lSFE over a 2-year observation period [22].

Some technical aspects of the investigated interventions may have accounted for the minimal to null incidence and extent of MBL loss observed at 3 years post-surgery in both treatment groups. All patients received tissue-level implants with an intraoperatively conditioned hydrophilic surface. For this type of implants, stable levels of peri-implant marginal bone were reported at 3 [43] and 5 years [44] post-surgery, and similarly low apical migration of the peri-implant bone crest was reported at 2 years following their placement concomitantly with tSFE (0.35 mm) and lSFE (0.40 mm) in a randomized trial [14]. In the present material, implants were placed with the 1.0‐mm polished collar above the bone crest. Data from a prospective trial on the same implant system showed that the displacement of the implant-abutment junction at least 0.5 mm coronal to the bone crest had a positive impact on peri-implant bone loss at 1 year, which amounted to 0.2 mm and 0.4 mm for implants with 0.5-mm and 2.5-mm polished collar, respectively [45]. Interestingly, data from another randomized study where bone-level implants were used showed that the crestal bone level was always coronal to the implant shoulder in all tSFE cases and all but one lSFE cases at 2 years [22].

In the present study, the healing protocol (submerged or transmucosal) was left at the operator discretion. Due to different healing conditions and the need for additional surgery for implant uncovering in the submerged approach, it could be hypothesized that the healing protocol may have partly influenced our findings on MBL. A meta-analysis conducted on 11 clinical studies, however, reported a non-significant estimate of the difference in 12-month MBL of − 0.01 mm between submerged and transmucosal implant placement protocols [46]. Also, the same review indicated a limited impact of the healing protocol on the outcomes of bone augmentation procedures, although this conclusion was based on a limited amount of data [46].

At 3 years, peri-implant mucositis occurred in tSFE and lSFE groups with a similar prevalence (52.4% and 66.7%, respectively) and extent (33% BoP + sites per implant in both groups). Within the limitations due to the heterogeneity in case definitions of peri-implant mucositis that have been used through the years, our findings confirm that peri-implant mucositis is a highly prevalent biological complication at dental implants, including those placed concomitantly with sinus floor elevation procedures in the atrophic posterior maxilla. In this respect, subject-based summary estimates for the prevalence of peri-implant mucositis as reported in influential systematic reviews were 42.9% [47] and 46.83% [48]. Also, clinical trials conducted on implants placed after staged lSFE indicate that this complication is prevalent even at short-term (1 year) follow-up, reaching incidence higher than 60% [49]. The extent of peri-implant mucosal inflammation (expressed in terms of prevalence of BoP + sites per implant with mucositis) is also rather consistent with other studies, where implant-level mean BoP values amounted to 43% [50]. Among the factors that have been associated with BoP around dental implants [51, 52], some may explain the high prevalence of BoP in our cohort. Given the statistically significant positive relationship between BoP and PD [50, 51, 53, 54], the presence of sites with moderate PD in both treatment groups (Table 3) may have contributed the high BoP prevalence. Since BoP assessments were performed maintaining the implant-supported prosthesis in situ, some BoP-positive sites may also have been caused by improper use/angulation of the probe with consequent tissue trauma.

Despite the high prevalence of peri-implant mucositis, no peri-implantitis cases (or implant failures/loss due to peri-implantitis progression) were observed in either the tSFE or lSFE group. Some hypotheses can be advanced to explain the absence of peri-implantitis cases in the present study cohort. First, the follow-up (3 years) may have been too short for peri-implantitis to become clinically manifest. In this respect, the incidence of peri-implantitis was showed to increase at the increasing of the mean function time [47]. The exclusion of heavy smokers from the study during the screening phase and the low prevalence of current and former smokers in our treatment groups may have limited the negative effect of smoking on the risk for peri-implantitis [23, 55].

A potential limitation of the study may reside in the lack of data on 9 patients who participated in the 1-year visit, but were not available for the 3-year visit. Also, differently from a previous analysis [16], totCON% was assessed on bidimensional (periapical) radiographs taken without a customized film holder at both 1 and 3 years. Although periapical radiographs may suffer from dimensional distortion due to deformation of the film on the palate and allow for the evaluation of the mesial, distal, and apical implant aspect only, recent findings showed a high level of agreement between linear measurements of peri-implant bone anchorage performed on non-standardized periapical radiographs and CBCTs [56]. However, whether and to what extent a bidimensional evaluation of totCON% may reflect the peri-implant bone condition when assessed 360° around the implant by a CT/CBCT is currently undetermined, and the lack of a customized film holder remains a potential limitation of the present study that may have partly affected the reliability of within- and between-group comparisons. Another limitation is the lack of information on the amount of plaque deposits as well as the characteristics of supportive periodontal care from 1 to 3 years. In this respect, the onset and severity of peri-implant diseases has been previously associated with the amounts of plaque deposits [57, 58] as well as the efficacy of a supportive periodontal care regimen [59, 60].

In conclusion, the results of the present study indicate that, at 3 years following surgery, implants placed concomitantly with tSFE and lSFE fully maintain peri-implant bone support. Peri-implant mucositis was the most prevalent condition, with a similar prevalence between groups.