The aim of this study was to evaluate primary stability of 3.7-mm diameter porous tantalum Trabecular Metal (TM) implant, and compare it to fully threaded implants, in the in vitro model of immediate implant placement in the anterior maxilla. A total of 60 implants were placed into bovine ribs using surgical guides. Implants were divided in 3 groups of 20 according to the design: TM, Tapered Screw-Vent (TSV), and NobelReplace. To simulate immediate placement in anterior maxilla, implants were placed under a sharp angle toward the ribs, not fully submerged. Placement angle of 20.7° was calculated after analysis of 148 virtually planned implants on cone beam computerized tomography scans of 40 patients. No statistically significant difference in implant stability quotient (ISQ) was found between TM (65.8 ± 2.6), TSV (64.7 ± 2.7), and NobelReplace (64.6 ± 2.7). TSV implants achieved higher insertion torque (37.0 ± 4.8 Ncm) than TM (32.9 ± 5.2 Ncm) and NobelReplace (23.2 ± 3.3 Ncm). TSV had the shortest insertion time of 13.5 ± 1.0 seconds, compared to 15.2 ± 1.2 seconds for TM, and 19.7 ± 1.7 seconds for NobelReplace. Pearson correlation analysis showed significantly correlated insertion torque and ISQ values for TM group (P = .011, r = .56), a nonsignificant correlation was found for TSV and NobelReplace. The results of the present study indicate that TM implant can achieve good primary implant stability in insertion torque and resonance frequency analysis.
Introduction
Mechanical implant stability is a prerequisite for successful osseointegration.1 It is determined by: (1) macro design of the implant in relation to the implant bed preparation, (2) vertical position of the implant relative to the bone crest, (3) surface morphology or roughness of the implant, and (4) local bone quality.2 Numerous methods have been proposed for evaluation and quantification of primary implant stability.3 Insertion torque (IT) and resonance frequency analysis (RFA) have shown to be reliable and are the most commonly used measures of implant stability.4 Both methods for stability measurements are noninvasive. IT is a parameter that defines implant stability at the time of the implant insertion, and it can be measured using manual torque wrench or implant motor. 5 IT value is expressed in Ncm. RFA is measured with a dedicated device, such as Osstell with SmartPeg (Osstell, Göteborg, Sweden). The Osstell consists of a wireless receptor known as SmartPeg. SmartPeg is attached to the implant and works like a tuning fork by emitting magnetic pulses from a probe, which makes the SmartPeg vibrate. The SmartPeg is excited by a hand-held pulse transmitter placed in close proximity to the SmartPeg. An algorithm-derived calculation of the resonance frequency is recorded as the implant stability quotient (ISQ). The SmartPeg measures the stiffness (rigidity) at the implant/bone interface. The value of RFA is expressed as the implant stability quotient (ISQ), ranging from 0 to 100.
Immediate implants in the anterior maxilla are positioned more palatal and intentionally avoiding contact with facial bone wall in coronal aspect.5 When an implant is placed in a fresh extraction socket of an anterior tooth, there should be at least 2 mm of space between the implant surface and the buccal bone wall. This is necessary to provide stability of peri-implant tissues. Therefore, placement of a reduced diameter implant provides additional space for grafting material in buccal gap.6 In immediate placement, primary implant stability is achieved by engaging the palatal wall and the bone 4–5 mm beyond the apex of the extraction socket.7,8 Implants placed in postextraction sockets are usually deeply submerged, making the primary stability dependent on the apical part of the extraction socket, which is called the primary stability rectangle.9 Thus, implant design plays a significant role in achieving immediate stability.
The Trabecular Metal (TM) implant has a unique design, with the middle part made of tantalum. It is designed to replicate natural bone with an open, interconnected, cancellous-like porous structure. This implant possesses excellent biological properties due to the: (1) high surface area and (2) the presence of pores that permits the ingrowth of blood vessels and bone inside the implant.10,11 The TM implant is not fully threaded, as there is an unthreaded portion made of porous tantalum material in the middle of the implant. The porous middle section has high frictional characteristics that may promote initial implant stability.12
Kim et al and Lee et al in animal studies compared primary stability of a 4.1-mm diameter TM implant to fully threaded implants in healed sites and fresh extraction sockets.13,14 These studies found the primary stability of TM implants to be like conventional fully threaded implants. Immediately placed 3.7-mm diameter TM implants have been proven as clinically effective in posterior regions, however, without supporting data regarding IT and ISQ values.8 Romanos et al investigated primary stability of the 3.7-mm diameter TM implant in an experimental study,15 but information on mechanical stability of the 3.7-mm TM implant in postextraction sockets is lacking in the literature.
The aim of this in vitro study was to evaluate and compare the primary stability of 3.7-mm diameter TM implants vs fully threaded implants in bovine ribs designed to simulate immediate implant placement in the anterior maxilla.
Materials and Methods
Placement angle
All implants were placed at an angle into the bovine ribs to simulate engagement of the palatal wall of extraction socket in the anterior maxilla. Determination of the angle to use was done by analyzing cone beam computerized tomography (CBCT) scans of patients and then virtually placing implants in the anterior maxilla. CBCT scans used for virtual implant planning were not specifically acquired for this study. The CBCT scans were randomly selected from the database at the Department of Implantology, Clinic of Dentistry, Military Medical Academy, Belgrade, Serbia. Examined teeth were maxillary cuspids, lateral incisors, and central incisors. Malpositioned, severely damaged, and surgically treated teeth were excluded from the study. If scattering from a metallic dental restoration disabled clear visualization of the tooth and neighboring structures, the tooth was excluded.
Measurements were performed with OnDemand3D (Cybermed, Korea) software. A cross-section image in the center of the midfacial aspect of the examined tooth was selected for measurements. A virtual 3.7-mm diameter × 11.5-mm length implant was placed in a prosthetically driven position; guided by the position of the existing crown of the tooth. The angle between the implant axis and inner surface of palatal bony wall was measured (Figure 1).
Virtual implant placement and measurement of the angle between implant axis and palatal bony wall on a sagittal slice of CBCT scan.
Virtual implant placement and measurement of the angle between implant axis and palatal bony wall on a sagittal slice of CBCT scan.
Specimen preparation
Twenty bovine ribs taken from multiple animals were obtained from a butcher shop. After removal of residual soft tissue, 3 radio-transparent markers were fixated to the ribs in a triangular pattern (Figure 2), and then a CBCT scan was performed (Scanora 3D, Soredex, Finland). Each rib was also scanned with an extraoral scanner (Autoscan-DS-EX, Shining 3D, Hangzhou, China). Specimens were kept frozen at –20°C in saline solution until the day of the experiment.
Three radio-transparent markers fixated to the rib in a triangular pattern prior to CBCT scan.
Three radio-transparent markers fixated to the rib in a triangular pattern prior to CBCT scan.
Implant characteristics
Sixty implants of 3 different designs belonging to 2 different implant systems were used for the experiment (Figure 3). Implants were divided into the following groups of 20: TM (Zimmer Biomet Dental, Palm Beach Gardens, Florida, USA), TSV (Zimmer Biomet Dental), and NobelReplace Tapered Groovy (Nobel Biocare, Zurich, Switzerland). An overview of implant characteristics are provided in Table 1.
Surgical guide design
Digital imaging and communications in medicine (DICOM) data and standard tessellation language (STL) file of each rib were merged in implant planning software (RealGUIDE 5.0, 3DIEMME, Serenza, Italy), using radio-transparent markers placed on a rib. Implant positions were planned according to the previously measured placement angle. Implants were positioned to a depth of 8.5 mm, leaving coronal 3 mm outside of the bone. Each surgical guide was designed for placement of 3 implants, and it was secured to a bone specimen with 2 fixation screws. The position of implants from each group was altered in one of 3 possible positions (middle position and the 2 lateral positions), using a blocked randomization list.
Drilling procedure and implant placement
Drilling was performed by a single operator (MM) under irrigation, using the surgical guide specific for a particular rib, and following the soft bone drilling protocol recommended by the implant manufacturer. Ribs were fixated with a metal clamp and surgical guides were secured to the ribs. The drilling speed (800 rpm) was the same for all implants. Drills were replaced after 20 uses, as recommended by the manufacturer.
Drilling sequence for TM and TSV implants consisted of 2 drills: 2.3 mm and 2.8 mm. The drilling sequence for NobelReplace implants consisted of a 2.0-mm guided twist drill and 3.5-mm guided tapered drill. During drilling the tube adapter suitable for the drill was placed in the sleeve incorporated in the surgical guide (Figure 4). The transfer mount was detached from each implant before placement.
Each rib received 3 implants that were placed after removal of the surgical guide (Figure 5). The depth of insertion was equalized among groups during virtual planning, where the portion of the implant inside and outside of the bone was measured prior to guide fabrication. After implant placement, a vernier caliper was used to confirm the insertion depth by measuring the part of the implant outside of the bone.
Primary stability and insertion time measurements
Peak IT was recorded during insertion using an implant motor (Implantmed, W&H, Bürmoos, Austria) calibrated at maximum IT of 50 Ncm. An Osstell ISQ device with suitable SmartPeg (Osstell) hand tightened to implant was used to determine the RFA. The probe was held 1 mm from the peg at the right angle. Stability of each implant was measured from 4 different directions by the operator who inserted implants, and the average value was calculated.
Insertion time was recorded to the surgical unit from the moment the implant started engaging osteotomy until it was fully seated.
Statistical analysis
The IT, ISQ, and insertion time are presented using mean values and standard deviations. The data obtained for IT, RFA, and insertion time were assessed for normality of distribution using the Shapiro-Wilk test and graphical evaluation of Q-Q plots. These tests revealed that data for IT and RFA were normally distributed, while insertion time data was not normally disturbed. The significance of the difference in IT and ISQ among groups was assessed by the 1-way analysis of variance test, followed by Tukey's multiple comparison test. The significance of the difference for the insertion time among groups was assessed by Kruskal-Wallis test, followed by Dunn's post-test. The strength of the association within the groups between IT and ISQ was assessed by Pearson correlation coefficient. Statistical analyses were performed with Prism 5.01 software (GraphPad, La Jolla, California, USA). Statistically significant value was set as P < .05. The research methodology and results were reviewed by an independent statistician.
Results
Placement angle and surgical guide design
The angle at which implants engaged the palatal wall of the alveolar socket was measured using CBCT scans of 40 patients with 148 teeth included (56 cuspids, 41 lateral incisors, 51 central incisors). The scans of 21 female and 19 male patients, with an average of 50.8 years were examined. The obtained average angle was 20.7°. A surgical guide was designed in the software by planning implant placement under the angle of 20.7° toward the ribs.
Primary implant stability and insertion time
TSV implants exhibited highest mean IT value of 37.0 ± 4.8 Ncm. TM had mean IT value of 32.9 ± 5.2 Ncm, while NobelReplace showed the lowest IT with average of 23.2 ± 3.3 Ncm. Statistically significant difference in IT values was observed between TSV and TM, TM and NobelReplace, and TSV and NobelReplace (P < .0001). Similar mean ISQ values were obtained for TM, TSV, and NobelReplace; 65.8 ± 2.6, 64.7 ± 2.7, and 64.6 ± 2.7, respectively. Differences in ISQ between tested implants were not statistically significant (P = .3108). Complete results of IT and RFA are listed in Table 2.
Statistically significant positive correlation between IT and ISQ was found for TM implants (P = .011, r = 0.56); no statistically significant correlation between IT and ISQ was found for TSV and NobelReplace (Table 3).
With insertion time of 13.5 ± 1.0 and 15.2 ± 1.2 seconds, TM and TSV implants needed significantly less time (P < 0.0001) for implant insertion compared to NobelReplace (19.7 ± 1.7 seconds). TSV implants were placed in osteotomy significantly faster than TM implants.
Discussion
In the present study, mechanical stability of TM implant was tested and compared with implants documented for immediate placement and immediate loading.7,16 TSV and TM implants have similar designs and share the same drilling protocol. TM implants are usually compared to TSV implants in experimental studies.13–15 In this study, in addition to comparing TM to TSV, the authors compared primary stability of the TM implant with implants of different designs and manufacturers. Proximal parts of the ribs served as a model of the human jawbone owing to the macroscopic composition of cortical and medullar bone that is similar to the bone found in the anterior maxilla.19
When placement angle was calculated only CBCT scans of teeth with the root positioned against the labial cortical plate20 were included. This present model of immediate implant placement differs from models used in previous studies. In previous studies the implants were placed at a right angle in controlled right angled circumferential bone preparations.21,22 In this experiment, the implants were placed at a sharp angle. The sharp angle of placement allowed the implants to benefit from anchorage in a layer of cortical bone. By engaging the layer of cortical bone, the implant anchorage in this model is like the palatal anchorage achieved when implants are placed at the time of tooth extraction in the anterior maxilla. This immediate implant placement sharp angle model more closely resembles the clinical scenario in which primary stability relies mostly on the engagement of palatal wall. After tooth extraction in the clinical setting, implant stability is obtained by engaging the palatal wall and bone beyond the apex of the tooth, and possibly part of the labial bony wall, depending on the post-extraction socket anatomy.8 In the present model the coronal threaded portion of implants did not fully engage bone, so we assumed that primary stability was mostly dependent on apical threads.23,24 Wu et al examined the influence of apical thread design on implant stability and concluded that conical implants with bowl flutes are associated with high stability.25 The TM implant matches this design with its slightly tapered apex combined with a bowl flute shape.
IT values were lowest for NobelReplace implants, while ISQ values did not differ significantly among the groups. Peak IT values are known to be highly dependent on bone quality.4 Since the bone quality was kept constant in this experiment, the difference in IT may be attributed to differences in implant design or drilling protocols. All implants in the present study had the same length, similar diameter, and tapered design. Self-tapping implants (TM and TSV) achieved higher IT values than non-self-tapping implants (NobelReplace), without difference in RFA, which is in accordance with the results of an in vitro study by Toyoshima et al.26
The undersized drilling technique was used for all implants. Multiple studies observed that under-dimensioned drilling has an obvious effect on insertion torque, but it is weakly associated with RFA.27,28 These findings are in correlation with our results for IT and RFA. Under preparation was more pronounced for TM and TSV, which achieved higher IT values than NobelReplace, while ISQ did not differ significantly among the groups. Increasing the discrepancy between the final drill and implant can improve primary stability.29 Difference between diameter of final drill and implant diameter was higher for TM and TSV, than for NobelReplace.
TM and TSV implants possess triple lead threads design, which enables faster implant insertion into osteotomy. Insertion of TM implant lasted longer than TSV because the porous part of implant engaged bone slower than the threaded part. This was noticed as the flattening of insertion torque curve on the graph recorded on the implant motor.
Correlation between IT and RFA was not found in clinical studies.4,5,31 On the contrary, in vitro studies found positive correlation between IT and ISQ values.32–34 In the present experiment, bone quality, length, and diameter of implants were consistent, and positive correlation between IT and ISQ was found only for the TM implant.
The limitation of the present study is that the in vitro study design is appropriate solely for investigating mechanical aspects of implant stability. Another limitation is that all tested TM implants had uniform dimensions, and the size of the porous tantalum area that affects stability varies by implant length and diameter. The results of the present study should be cautiously interpreted since patient- and operator-related factors were excluded from this experimental design. Further investigations on human subjects focusing not only on mechanical properties of the implant, but also on clinical outcome of the therapy, are required to confirm the present in vitro findings, and validate clinical performance of 3.7-mm diameter TM implant placed in fresh extraction sockets.
Conclusion
In conclusion, the tested 3.7-mm diameter TM implants achieved good primary implant stability in IT and RFA and performed like conventional fully threaded implants.
Abbreviations
Acknowledgments
The authors would like to thank Mr. Dejan Ivosevic for providing help with virtual implant planning and fabrication of surgical guides. Implants were kindly provided by Zimmer Biomet Dental (Zimmer Biomet, Palm Beach Gardens, Florida, USA). The authors report no conflicts of interest. There was no direct compensation for this study and Zimmer Biomet Dental had no role in the writing and publication of this manuscript.