Ideal primary stability has been determined in the early 80s by Branemark et al1 as being around 35 Ncm in submerged healing for smooth surface implants. This set number was considered to be a guaranty for the implants' success. Since the emergence of moderately rough surface implants, a shift of this concept has emerged, and no fixed threshold value of primary stability seems to be mandatory to obtain osseointegration. However, in immediate loading protocols, most authors agree that a high primary stability is a necessity.
Different companies recommend different initial insertion torques to achieve primary stability.2 This insertion torque would range between 20 and 50 Ncm. However, some implants would offer even higher insertion torques.
Bashutski et al,3 in a case report, postulated that high insertion torque (HIT) led to implant failure, with the so-called compression necrosis. However, in an animal study, Trisi et al4 demonstrated that HIT in dense cortical bone proved to increase primary stability and did not induce bone necrosis or implant failure, but lead to higher bone apposition and higher reverse torque. In a human control study, comparing high (up to 176 Ncm) and normal (30–50 Ncm) insertion torques, Khayat et al5 showed both similar implant success rates and marginal bone levels at the time of loading and 1 year later. Data comparing the effect of high- and low-insertion torque on marginal bone levels in the dental literature are scarce.6–8 Linkevicius et al,9,10 in a series of articles, have shown that the initial gingival thickness at the crest influenced the marginal bone stability around implants, and tissue thickness of less than 2 mm would eventually induce crestal bone loss. On the other hand, buccal bone thickness is known to play a major role in buccal bone stability after implant placement, and thus is detrimental for long-term stability of the esthetic result. Moreover, no study has evaluated the effect of soft-tissue thickness or buccal bone thickness, as confounding factors, when applying HIT.
Although soft-tissue thickness is easily accessed via different available means, including clinical and ultrasonic sounding, the only noninvasive means of evaluating the buccal bone thickness after implant loading remain the cone-beam computed tomography (CBCT).
Therefore, the purpose of the study was to compare prospectively the effect of different insertion torques on implant survival and marginal bone stability, considering soft-tissue and buccal bone thickness before implant insertion as confounding factors.
Material and Methods
In the present prospective study, patients attending the Department of Periodontology at Saint Joseph University seeking implant therapy between December 2014 and March 2015 were consecutively invited to participate.
Patients were accepted to be included in the study when they met the following criteria: >18 years old, healthy systemically and periodontally, with good oral hygiene levels (Full mouth plaque scores < 20% and full mouth bleeding index < 20%), seeking an implant placement in “pristine bone,” willing to sign an informed consent form, and to show-up for their 1-year follow-up visit.
Patients were excluded when they were heavy smokers (>10 cig/day), had a systemic disease, (uncontrolled diabetes, degenerative disease, patients medicated with substances that would negatively influence wound and/or bone healing, or patients treated with radiotherapy for head and neck cancer), and presented with an unhealed or augmented ridge.
This study was conducted in full accordance with expressed ethical principles, (USJ-2017–110) including the Declaration of Helsinki (as revised, amended, and clarified in its version of 2008). Thirty-nine patients were enrolled for this study, and qualifying patients were treated according to the following protocol.
The surgical procedure was performed under local anesthesia using Septanest (4% articaine with adrenaline 1:100,000, Septodont Paris, France). A mid-crestal incision was used, and a full-thickness buccal flap was reflected. Soft-tissue thickness at the unseparated palatal/lingual flap was measured at the center of the future implant site using a periodontal probe (PCP-UNC15; Hū-Friedy, Chicago, IL). Thereafter, a full-thickness palatal reflection was peformed, implant site preparation performed according to the manufacturer recommendation, and buccal and palatal/lingual bone thickness measured using the Iwanson caliper (KLS Martin Group, Umkirch, Germany) at ≈1 mm from the crest. Straumann bone-level implants were inserted at bone crest. The final seating torque was recorded using a manual hand wrench (JDTorque), which included insertion torque values ranging between 15 and 80 Ncm. Healing abutments were subsequently connected, and the flaps sutured using single interrupted resorbable sutures (ACE 5–0 PGA Sutures; ACE Surgical Supply Co., Inc., Houston, TX).
Each patient received 1 g of amoxicillin twice a day for 5 days, ibuprofen 600 mg 3 times daily for 3 days, and 0.12% chlorhexidine twice a day for 2 weeks. Three months after implant placement, impressions were taken and ceramo-metallic crowns were fabricated and cemented.
XCP-Rinn (Dentsply Rinn, York, PA) mounts were individualized using an acrylic resin, and periapical radiographs were taken at final cementation and 1 year later. A CBCT was also taken at 1 year after loading. Radiographs were digitized using a digital camera (EOS 700D; Canon, Tokyo, Japan). Both interproximal marginal and buccal bone loss were measured from the implant shoulder to the most apical bone to implant contact, using, respectively, the UTHSCA Image Tool (Version 3.00 for Windows; University of Texas Health Science Center, San Antonio, TX) and SIMPLANT software (Dentsply, York, PA).
The radiographic cut going through the central portion of the implant's long axis was used for buccal bone measurements.
Implants were subsequently divided into 3 groups according to the final seating torque recorded: group I included implants inserted at less than 30 Ncm, group II between 30 and 45 Ncm, and group III above 45 Ncm.
Statistical analyses were performed using Statistical Package Software for Social Science (Version 17.0; SPSS for Windows, Chicago, IL). The level of statistical significance was set at P value <0.05, and all results were provided as mean ± SD. Primary outcome variable of the study was the bone-level variation calculated by comparing the bone level between implant placement and 1 year after loading. The Kolmogorov-Smirnov test was used to determine normality of continuous variables. Because variables were not normally distributed, nonparametric tests were accomplished; Kruskall–Wallis tests and Mann–Whitney tests were used to compare continuous variables between 2 or 3 groups. Wilcoxon tests were used to compare continuous variables within time.
In this prospective pilot study, we assessed longitudinally over 12 months the effect of implant stability on the marginal bone level. Thirty-nine patients initially participated in this study. Thirteen patients not fulfilling the inclusion criteria were excluded (Fig. 1). Of the 26 included patients who received a total of 44 implants, 4 were lost to follow-up. Thus, the study was conducted on a total of 22 patients receiving 39 implants. All 39 implants were integrated successfully and were subsequently restored with ceramo-metallic fixed prosthesis.
Soft-tissue thickness, measured with the periodontal probe on the unseparated palatal flap, was ≤2 mm (Thin biotype) in 10 implants (25.6%) and >2 mm (Thick biotype) in 29 implants (74.4%). Buccal plate thickness, measured with the caliper at implant site preparation, was <2 mm in 18 patients (46.2%) and ≥2 mm in 21 patients (53.8%). Palatal/lingual plate thickness was <2 mm in 16 patients (41%) and ≥2 mm in 23 patients (59%) (Table 1).
Five implants had an insertion torque <30 Ncm (group I), 20 were inserted between 30 to 45 Ncm (group II), and 14 had an insertion torque >45 Ncm (group III). (Table 2). Mean (±SD) interproximal bone loss around implants was 0.33 ± 0.39 mm. Mean (±SD) buccal-lingual/palatal bone loss as assessed on CBCTs was 1.00 ± 1.06 mm.
The Mann–Whitney correlation test did not show any statistically significant positive correlation between soft-tissue thickness and mesial-distal bone loss in both thick (>2 mm) and thin (≤2 mm) biotypes (P = 0.692) (Table 3). No significant relation was found between soft-tissue thickness and buccal-lingual/palatal bone loss (Data not shown).
The Kruskall-Wallis test did not show any statistically significant correlation between insertion torque and buccal and lingual/palatal marginal bone loss, with respective P values of 0.78 for buccal and 0.66 for palatal (Table 4). No significant relation was found between insertion torque and mesial-distal bone loss (Data not shown).
The Wilcoxon signed rank-sum test showed a statistically significant correlation between initial buccal bone thickness after last drilling and buccal bone loss “P < 0.0001.” When buccal bone thickness was less than 2 mm, a mean buccal bone loss of 2.34 ± 2.16 was observed, whereas a minimal buccal bone loss was observed (0.31 ± 0.63) when the initial buccal bone thickness was more than 2 mm (Table 5). Similar results were observed for lingual/palatal bone loss (Data not shown).
In this prospective study, 39 implants were placed in 22 patients and assessed over a period of 12 months. Average marginal bone loss at mesial-distal site was 0.33 ± 0.39 mm. Results obtained were in accordance with numerous previously reported results of bone-level implants.11 Buccal and lingual/palatal marginal bone loss was 1.01 ± 1.06 mm. This result is in agreement with the previously reported data, though relatively sparse.12,13 To our knowledge, none of the previously reported data have taken into consideration the buccal bone thickness and soft-tissue biotype nor primary stability of implants and buccal/palatal bone loss.
Mucosal tissue thickness did not influence marginal bone loss in the present report (P value 0.692). This result was in accordance with a recently published study by Canullo et al14 using bone-level platform switching implants. The initial mucosa thickness surrounding the 68 implants placed in 26 patients, and followed for up to 3 years, did not influence the pattern of physiologic bone loss. On the contrary, Linkevicius et al, in a series of articles, reported on the correlation between soft-tissue thickness and crestal bone loss. According to those studies, a soft-tissue thickness of 2 mm or less may lead to a crestal bone loss of 1.45 mm (as measured interproximally) in platform switching implants.9,10,15 More recently, van Eekeren et al16 confirmed the influence of initial mucosal thickness on crestal bone changes in non–platform switching implants. Although platform switching was doomed unable to prevent crestal bone loss when soft tissue was thin (<2 mm),17 thickening of the mucosal tissue by means of an allogeneic membrane during implant placement was applied as a preventive measure.15,17,18 In a systematic review meta-analysis, Suarez-Lopez del Amo et al19 also demonstrated that implants placed with initially thicker periimplant soft tissue had less radiographic marginal bone loss in the short term.
Our results were not in full accordance with most published articles. This could be explained by the small sample size and the presence of other confounding factors such as the buccal bone plate thickness and implant insertion torque that were noted at implant placement to assess their influence on bone loss.
Our main objective was to particularly evaluate the effect of high- versus low-insertion torque on periimplant bone loss. No statistical difference between low, regular, and HIT, when comparing periimplant bone loss (M/D B/PL), was found in our study. In a case report, Bashutski et al3 presumed that compression of bone beyond its physiologic tolerance due to HIT may result in ischemia with subsequent necrosis, or sequestrum formation, leading to implant failure. Conversely, Trisi et al4 used HIT (mean 110 Ncm) and low-insertion torque (mean 10 Ncm) when placing implants in a dense cortical bone, using sheep mandibles. Histological results showed a higher bone apposition at all times in the high-insertion group, whereas reverse torque tests were consistently higher in the same group. These results were confirmed in human studies by different authors using HITs. When Khayat et al placed implants with an insertion torque up to 176 Ncm, they reported a marginal bone loss at 1 year after loading of 1.24 ± 0.75 mm. No significant difference between high (>70 Ncm) and regular (30–50 Ncm) insertion torque groups, when comparing bone stability, and implant success rates, at one-year after loading was found.5 Similar results were reported by Grandi et al20 1 year after implant placement using HIT (up to 80 Ncm) versus regular insertion torque (30–45 Ncm). Minimal marginal bone loss in both high (0.41 ± 0.26 mm) and regular insertion torque groups was found (0.45 ± 0.29 mm). Our study confirmed the aforementioned results. To our knowledge, none of the studies compared the effect of low versus HIT on marginal bone loss. The originality of our study was the evaluation of the effect of different insertion torques on the buccal plate using the CBCT radiographs as it has been rarely used in recent publications.12,13 Interestingly, our results failed to show any relation between insertion torque (high or low) and buccal bone loss, whereas buccal bone thickness had a major effect on the preservation of the buccal plate when the initial thickness was ≥2 mm. The latter result was in accordance with previously published data. In fact, Spray and Grunder found that the 2-mm threshold had a tremendous effect in preserving marginal bone and esthetics as defined by soft-tissue stability.21,22 This was questioned by a recent publication where no statistical difference was found between implant placed with a buccal bone thickness of 1 mm or less versus implants placed with a buccal bone thickness of more than 1 mm.23 However, the results of this study should be carefully interpreted because they evaluated 24 subjects up to 12 months.
Moreover, one should not generalize our particular results (no relation between insertion torque and bone loss) to all cases, or to all other implants, because this study included only 1 type of implant (Straumann, bone-level implants), with a relatively reduced number of cases (n = 39 implants). This is certainly a limitation to this study, and applying a similar protocol comparing different implants and the correlation between insertion torque and bone loss is a subject for future research.
Within the limits of the present prospective study, insertion torque and mucosal tissue thickness did not influence implant survival or the three-dimensional marginal bone loss. Buccal bone thickness of 2 mm or more was associated with a minimal marginal bone remodeling. Future well-designed randomized controlled clinical trials should be aimed at evaluating the importance of soft-tissue thickness versus buccal thickness on the long-term maintenance of esthetic results.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
ERC: Ref. USJ-2017 to 110 Approval was granted by the Ethical Research Committee (ERC). Approval number USJ-2017 to 110.
Roles/Contributions by Authors
Johnny Nohra: main investigator and writer. Maroun Dagher: main nvestigator and writer. Ghassan Matni: coinvestigator. Nadim Mokbel: contributed to proof reading, editing, and project tutelage. Elie Jobaili: coinvestigator. Nada Naaman: contributed to proof reading, editing, and project tutelage.
The authors thank Dr. Nada Osta for her contribution to the statistical analysis. Johnny Nohra, DDS, MScD, and Maroun Dagher, DDS, MScD, contributed equally to this work.
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