Subcrestal versus Bone Level One Stage Implants: Early Bone and Soft Tissues Modification. One Year Randomized Clinical Trial.

1. Introduction

Dental implants are a widespread and reliable treatment for the replacement single and multiple missing teeth. Whilst short term clinical success is commonly achieved, the real challenge is to maintain peri-implant hard and soft tissue stability in the long-term, which is crucial for functional and esthetic outcomes, as well as for the prevention of biological complications and, ultimately, implant success [1,2,3,4,5,6,7].

Early implant bone loss, defined as the loss of marginal bone occurring during the first year of the implant prosthetic function, is a multifactorial process which has traditionally been linked to factors such as the surgical procedure, the establishment of a peri-implant biological width, the restorative protocol and the presence of microgaps between the various components [8,9,10]. Whilst historically an early bone loss up to 1.5mm in the first year and 0.2mm for every subsequent year has been deemed acceptable [11,12], recent evidence suggests that an early bone loss exceeding 0.5 mm represents a risk factor for future peri-implantitis development and implant failure [7]. Therefore, several strategies and protocols have been tested with the aim of limiting implant bone loss.

Sub-crestal implant placement has been introduced with the rationale that anticipating the expected early bone loss by adjusting the apico-coronal position of the implant could prevent the exposure of the treated and threaded area of the fixture, lowering the risk of biological and esthetic complications [2,13,14,15]. Sub-crestal placement seems to be particularly important for patients with thin soft tissues at the time of implant placement: a thin biotypes seems to lead to more bone loss, possibly due to the space required for the establishment of a biological width of appropriate dimension [2,9,13,16].

The benefits of sub-crestal implant placement are still controversial, as recent publications found no significant difference in the bone loss around sub-crestal and equi-crestal implants [2,17,18]. However, some authors have recently started arguing that not all bone loss bear the same clinical weight: bone resorption occurring below the implant neck increases the risk of thread exposure and complications, whilst bone resorption above the implant neck leaves the fixture safely covered. Therefore, Linkevicious et al. [19] and Spinato et al. [15] introduced the distinction between bone remodeling observed above the implant collar (smooth or machined titanium surface), and bone loss which instead exposes the implant neck. It is worth mentioning that only implants placed sub-crestal present bone above the collar which could buffer the initial bone modification as bone remodeling, whilst equi-crestal implants inevitably fall into the bone loss category. With this distinction in mind, it seems that sub-crestal implant placement might not prevent bone modification, but it mostly occurs as bone remodeling, with minimal observed bone loss [19].

Sub-crestal implant placement is often used in conjunction with a One Time Abutment (OTA). An OTA consists of an immediate abutment placement at time of implant placement, which will be left in situ for all the restorative procedures, avoiding the multiple disconnection and reconnection of regular abutments during traditional workflows [20,21,22]. The use of an OTA may reduce the risk to damage the supra-crestal soft tissues complex and the implant to abutment connection wear and tear [20,23,24,25].

Other factors that seem to positively influence the initial bone modification relate to the implant design, in specific the choice of an internal conical connection and the platform switching. An internal conical connection between the fixture and the abutment seems to be less prone to bacterial infiltration and more stable under static and dynamic loads, when compared to flat-to flat connection systems [4,26,27,28,29,30] Platform switching of the abutment diameter is thought to provide an horizontal space for soft tissue attachment during the formation of the biological width therefore reducing bone modification [5,31,32]. Finally, the choice of a screw-retained restoration over a cemented one [33,34,35] and the careful positioning of the restorative margin through the correct prosthetic abutment height [36,37,38] are important considerations for the peri-implant tissues stability.

The aim of the present study was to compare the early bone changes around conical connection, platform-switched implants placed at bone level or 2mm sub-crestal. These changes were measured as Marginal Bone Modification (MBM), a term describing the apical repositioning of the peri-implant marginal bone, within which a distinction was made between Bone Remodeling (BR) above the implant neck and Bone Loss (BL) below it. Secondary outcome was the evaluation of peri-implant tissue health through Probing Depth (PD), Bleeding on Probing (Bop) and Plaque Index (PI).

2. Materials and Methods

2.1. Patient Selection

The present study is a single blind, parallel randomized controlled prospective clinical trial, which protocol was reviewed and approved by the local University/Hospital ethical committee (N4078) and recorded in public registry of clinical trials (www.clinicaltrials.gov - NCT06182670). The study was conducted according to the principles of Helsinki Declaration (as revised in Fortaleza 2013), following CONSORT (Consolidated Standards of Reporting Trials) guidelines. Patients were recruited and treated by one experienced operator from April 2021 to May 2023; the study flow-chart is reported in Figure 1.

Patients needing a single tooth replacement in the upper and lower posterior areas (premolar and molar) by means of an implant fixed rehabilitation were included in the present study. The implant site was assessed for bone volume and space based on the following requirements: 1,5 millimeter of residual buccal and lingual bone around the selected implant diameter, a minimum bone height of 10.5mm, in order to place an implant with a minimum length of 7.5mm, and at least 1.5 millimeters of bone between the fixture and the adjacent tooth. Patients with systemic diseases, history of radiation therapy in the head and neck region, current treatment with steroids, neurological or psychiatric impairments that could interfere with good oral hygiene, immuno-compromised status, including infection with human immunodeficiency virus, severe clenching or bruxism, smokers (more than 10 cigarettes per day), drug or alcohol abuse and inadequate compliance were excluded. All included patients gave their written consent after being informed in detail about the objectives of the study.

Before treatment, patients were clinically and radiographically evaluated. An orthopantomography and peri-apical radiographs were used as a first level exam to evaluate the bone quantity before implant placement. A cone beam TC was also performed to plan for a prosthetically guided implant placement..

2.2. Surgical Procedures

All surgical and prosthetic procedures were performed by a single operator (ES), and All cases have been treated with the same implant system (Advan GTB-Tzero, Advan S.r.l. Via Linussio, 1 33020 – Amaro, Italy) provided with platform switching and conical hybrid connection. Under local anesthesia, an incision along the center of the ridge was performed. After elevating a mucoperiosteal buccal envelope flap, supra-crestal soft tissue height (STH) of the undetached lingual flap was measured using a periodontal probe (PC-PUNC 15 HU-Friedy, Milan, Italy). An independent investigator then opened the randomization envelope and communicated patient’s group allocation to the surgeon. Control group received a bone level implant placement (BL) whilst Test group received a 2 millimeters sub-crestal implant placement (SC). (Figure 2, Figure 3 and Figure 4)

Implant site preparation was performed by following manufacturer's instructions. Insufficient implant primary stability (< 35 Ncm) and therefore the necessity to bury the implant was considered an exclusion criteria for the present study.

Trans-mucosal healing abutments were inserted on the BL implants, whilst a Gingival Former Abutment (GFA) for the One Time Abutment technique was connected to the implants belonging to the SC group. The flaps were repositioned and sutured, and post-operatory instructions provided. Ibuprofen 600 mg every 4-6 hours was prescribed for pain relief.

2.3. Prosthetic Protocol

Three months after surgery, implant-level and abutment-level impressions were taken to provide for single crown screw-retained restorations for the Control and Test group respectively (Figure 5 and Figure 6).

The final crowns were inserted two weeks after impressions.

All restorations margins were placed from 1mm below the gingival margin to juxta-gingival level by using the appropriate Ti-base and GFA abutment transmucosal height (Figure 7 and Figure 8).

The crowns were tightened to the implants with a torque of 25 N/cm (Figure 9, Figure 10 and Figure 11). An intraoral periapical radiograph of the restored implant site was taken and peri-implant PPD, BoP and PI were assessed.

2.4. Radiographic Evaluations

All peri-apical radiographs for the measurement of the Marginal Bone Modification (MBM) were taken with a standardized parallel technique (FONA™ Dental Image Plates). Images were then analyzed with a computer software (Image J, National Institute of Health, Bethesda, Rockville, MA, USA). Before measurement, each radiograph was calibrated by using the implant diameter as a reference to correct any distortion. Measurements were repeated twice by a single operator (MZ). MBM measurements were taken at both mesial and distal crest.

For each implant, radiographs performed at the time of surgical implant placement (T0), restoration delivery (T1), and after 6 (T2) and 12 (T3) months of prosthetic function were analyzed. The most coronal extension of the crestal bone at the time of the surgical placement was identified as Marginal Bone Level (MBL). MBL in the BL group corresponds to the level of the implant neck, whilst it is positioned coronal to it in the SC group. In the assessment of the MBM, Bone Remodeling (BR) and Bone Loss (BL) were distinguished. Bone loss was measured as the distance between the initial MBL and first radiographically visible bone-to-implant contact apical to the implant neck, whilst Bone Remodeling as the distance from the initial MBL to first point of contact coronal to the implant neck. The mean value of the mesial and distal MBM measurements was pooled for each implant. Any apical MBM was expressed as negative number. (Figure 12)

MBM was measured at baseline (T0), at crown delivery (T1), and at 6 (T2) and 12 (T3) months follow-up visits. (Figure 13 and Figure 14).

2.5. Clinical Evaluations

Peri-implant soft tissues parameters were collected using a calibrated probe (Vivacare TPS probe Vivadent, Scahaan, Liechtenstein) (Figure 15).

Probing depth (PD) was measured at four aspects (mesial, distal, buccal, palatal) following the Mombelli and Lang (1994) classification [39]. Bleeding on probing (BOP) and Plaque Index (PI) were measured at implant site by following Trombelli et al. (2018) [40] and O’Leary (1972) classifications [41]. PD, BoP and PI indexes were measured at different at each follow-up visits: 6 and 12 months after restoration delivery (Figure 1).

2.6. Sample Size and Randomization

Sample size was computed assuming a two parallel groups design with balanced groups and an independent sample t-test. We assumed a single implant and a single measurement for patient, a standard deviation of 0.5 mm and an average difference of at least 0.5mm. A total sample size of 34 patients provides a power of at least 80% at a 5% significance level. Assuming a 15% drop-out, the total sample size is N=40 patients.

2.7. Statistical Analysis

Data is described using standard statistics such as, mean, median standard deviation and IQR for quantitative variable, and counts and percentages for qualitative variables.Variation in MBL is modelled using both a simple t-test for independent samples (after averaging values within patients) as well as a multilevel model using GEE (Generalised Estimating Equations) on individual site measurements and accounting for within subject correlation.

Secondary continuous outcomes are modelled using GEE with ad hoc error distribution according to outcome (Gaussian for PPD, CAL, REC, binomial for BOP).Results are expressed as estimates and relative 95% confidence intervals. A significance level of 5% is used for all the comparisons and all analysis are performed using R (version 3.6.3 or higher).

3. Results

Thirty-eight patients (15 males and 23 females, respectively) aged from 23 to 72 years (mean age 49.4±32.4 years) were recruited in the present study, and 36 completed the study: two implants were considered as drop-outs due to insufficient implant primary stability (< 35 Ncm) during the implant placement. Patients demographic characteristics are reported in Table 1.

The implant and surgical site characteristics are reported in Table 2: 12 (33,3%) implants were positioned in the upper jaw. 21 implants (58,3%) were in molar region. 23 implants (63,9%) were 3,6x 9 mm and 13 (36,1%) 4,3x 7,5 mm. Observed STH was 3 mm in 21 patients (58,4%) and 2 mm in 12 (33,3%). No patient had STH equal to 1 mm and only 3 patients (8,3%) had 4 mm.

MBM for both groups from the baseline (T0) to the prosthetic loading (T1), six months follow-up visit (T2) and 1 year follow-up visit (T3) were statically significant (p<0.01) as shown in Table 3. Particularly, after 1-year of loading, the mean MBM in the Control group was -0,52 mm compared to the baseline (T0), and -0.60 mm in Test group. However, no significant difference was observed when the MBM values in two study groups were compared (p>0.05) (Table 3). The Control Group showed a mean Bone Loss (BL) equivalent to the MBM values, whilst no MBM below the implant neck was noted for the Test Group, therefore the Bone Loss for this group equals to zero at all time points.

Clinical peri-implant parameters modification over time are reported in Table 4. In the control group, a mean PD of 3.04 mm was observed after 1 year (T3), while mean PD was 2.38 mm in test group, with a statistically significant difference (p<0.05). The mean BoP at 1 year (T3), was 43.06% in the control group and 50% in the test group, with no statistically significant intergroup difference (p>0.05). Comparably, control and test group PI indexes after 1 year were 19.44% and 33.33%, respectively (p>0.05).

4. Discussion

The present study evaluate the Marginal Bone Modification around one stage, platform-switched, internal conical connection implants positioned 2 mm sub-crestally or equi-crestal. The results show that the mean MBM or the Test and Control group was comparable with values of 0.61mm and 0.52mm respectively at 12 months of prosthetic function. However, following the distinction made by Linkevicious et al. [19] and Spinato et al. [15], all the MBM in the Control group equi-crestal implants developed as Bone Loss below the implant neck level, whilst the MBM in the Test group was limited to the area above the implant neck, due to the 2mm of sub-crestal depth of the planned fixture position. Therefore, MBM in the Test group resulted in zero bone loss. This leaves on average 1.5mm of coronal marginal bone that protects the treated and threaded portion of the fixture, helping to prevent future biological and esthetic complications [2,13,14,15]. In fact, in a recent 10-years prospective study, Windael and colleagues [7] demonstrated that implants with Early BL >0,5mm during the first year of function showed 5.43 times higher odds of future peri-implantitis development than implants with Early BL <0,5mm.

Bone loss seems to have mostly occurred in the first 3 months after surgery for both groups, in agreement with other histological studies on animals and humans conducted [42,43,44]. In those studies, they also demonstrated that implant supra crestal soft tissues around implants then tend to stabilize after 8-12 weeks, achieving an average height of 3.6mm height on average, which constitutes the new biological width. MBM seems to be a necessary step for the re-establishment of such dimension [19]. As a consequence, mucosal thickness at the time of implant placement is a significant influencing factor on peri-implant marginal bone stability, and the soft tissue height seems to be inversely proportional to the early bone resorption. This is likely due to the necessary space for the formation of the 3.6mm of tissue height aforementioned, which occurs at the expense of the bone [7,9,13,16,45,46]. In the present study a soft tissue thickness ranged between 1 to 4 mm, with 2 and 3 mm being the most frequent values. Linkevicious et al. [45] analyzed the bone loss around bone-level implants in patient with thin (<2mm) and thick (>2mm) soft tissue and found a mean bone loss of 1.17mm in the first group at 12 months after restoration. Despite one third of the patients in our control group showing a STH of 2mm, none of the implants exceeded 0.75mm of bone loss. This difference might be due to other factors such as implant type and design.

Our bone loss results are in line with the one obtained by Vervake et al. [16] and Linkevicious et al. [19], who measured 0.04mm and 0.11mm of bone loss respectively around sub-crestally placed platform-switched implants. Linkevicious et al. [19] also observed 0.33mm of bone loss around equi-crestal implants, more than in the current study. However, they also performed a soft tissue augmentation procedure which could have provided the extra space for the biological width re-establishment.

When assessing the soft tissues, the mean PD value in the test group was 2.38 mm at 12 months after loading and 3.04 mm in the control group, respectively. That difference was both statistically and clinically significant considering the confidence interval of 2.13-2.62 for the test group and 2.64-3.45 in the control group. It means that in the test group more than 2.62mm of PD could not be found, while in the control group values even up to 3.45mm could be observed. The recent literature highlighted that the depth of the mucosal tunnel modifies can be a crucial factor in the treatment peri-implant mucositis, should it occur [25]. Chan et al. [25] showed that deeper mucosal tunnel depth (>= 3mm) is associated to a chronic inflammation. Crown removal and professional hygiene were needed to revert such mucositis. Differently, in the shallow tunnel depth <= 1mm just the oral hygiene practice was sufficient to revert the mucositis. Accessibility for biofilm removal around implant prosthesis is crucial for preventing and managing peri-implant diseases [47].

An additional observation was made in the test group: one year after loading, in 18% of cases BoP without presence of plaque was observed. The authors’ assumption is that the bleeding was due to compression of the soft tissues by the crown contour. In those cases, the crowns were unscrewed from the GFA, therefore without interfering with the integrity of the mucosal barrier, remodeled and polished to reduce the soft tissues compression. It was also possible to check the health of the free gingival sulcus around the GFA. The mean PD was 1.22 mm (IC 1.07-1.37mm), and juxtagingival position of the restoration margin was confirmed. This aspect may be important in patients with a history of periodontitis and high risk of implant diseases [48] in the posterior sectors where home hygiene procedures are difficult, and aesthetics is less important. Heitz-Mayfield and colleagues, in a recent randomized controlled clinical trial [36] on the management of peri-implant mucositis, showed that implants with supra-mucosal restoration margins yelded significantly greater reductions in probing depth following treatment compared to those with submucosal margins.

A limitation of the current study is that the BL control group was restored with crowns directly screwed to the implant with Ti-base abutment while in the test group a definitive GFA was used, following a One Time Abutment (OTA) prosthetic workflow. The rationale behind an OTA is to limit the disconnection and reconnection of the abutment from the fixture, which is shown to possibly negatively affect marginal bone levels [49]. However, the topic is still controversial: in fact, some papers seem to associate OTA technique to improved marginal bone stability when compared to traditional restorative procedures [20,21,22,49,50,51], whilst others failed to find significant advantages [23,52,53].

Linkevicious et al. [54] found that immediate abutments significantly reduce the amount of early bone loss at 1 month after prothesis delivery on sub-crestal implants, however no difference is found with traditional abutments at 12 months. It also appears that bone gain was obtained around traditional implant-level restorations. Conversely, a slight bone gain was observed in the test group of the present study, where the average Bone Remodeling at 6 months was -0.72mm, but becase -0.60mm at 12months.

On the other hand, Molina et al. [50] found the OTA significantly reduced the early bone loss around equi-crestal implants. Therefore, it appears that minimal disturbance of soft tissues might be of greater importance for equi-crestal implants. The bone loss observed by Molina et al. [50] around implants with traditional prosthetic workflow was 1.21mm on average, higher than the one observed in the present group. This again, could be due to differences in implant design. The choice to use the definitive GFA in the present study was due to the manufacturing company recommendations, and to facilitate the impression on deep transmucosal tunnel. However it would be interesting to replicate in a future trial both protocols of Molina et al. and Linkevicious et al. [50,54].

Nevertheless, the authors believe it would be misleading to consider separately every single factor associated to implant failure or success. A more global comprehension of different clinical parameters interacting together with the peri-implant hard and soft tissues would be more effective way to understand the crucial balance between patient biology and the implant [9,36,45].

5. Conclusions

The use of internal conical connection, platform switching implants placed 2mm sub-crestal and restored though a One Time Abutment workflow with screw-retained crowns, could result in marginal bone modifications with zero bone loss. Further clinical studies will be necessary to better understand the role of the OTA and the relationship between the STH and bone modification. Only long-term follow ups will be able to demonstrate the theoretical reduction in risk of biological complications.