This study investigated the effect of undersized preparations with two different implant macrogeometries. There were four experimental groups: group 1, conical implant with an undersized osteotomy of 3.2 mm; group 2, conical implant with an undersized osteotomy of 3.5 mm; group 3, cylindrical implant with an undersized osteotomy of 3.2 mm; group 4, cylindrical implant with an undersized osteotomy of 3.5 mm. Implants were placed in one side of the sheep mandible ( n = 6). After 3 weeks, the same procedure was conducted on the other side; 3 weeks later, euthanasia was performed. All implants were 4 mm × 10 mm. Insertion torque was recorded for all implants during implantation. Retrieved samples were subjected to histological sectioning and histomorphometry. Implants of groups 1 and 2 presented significantly higher insertion torque than those of groups 3 and 4 ( P < 0.001). No differences in bone-to-implant contact or bone area fraction occupied were observed between the groups at 3 weeks ( P > 0.24, and P > 0.25, respectively), whereas significant differences were observed at 6 weeks between groups 1 and 2, and between groups 3 and 4 ( P < 0.01). Undersized drilling affected the biological establishment of bone formation around both dental implant macrogeometries.
Numerous studies have suggested the importance of surgical procedures and the macrogeometry of the implants utilized. An osteotomy preparation that is smaller than the diameter of the implant provides a press-fit situation in which strain propagates into the supporting bone. This so-called undersized drilling technique supposedly compresses the bone so that implants achieve higher insertion torque, which has been reported as one of the indications for primary stability of some implant systems. This surgical innovation is commonly applied by surgeons, as the level of torque can be controlled by modifying the drilling protocol. It is reported that the insertion torque values of implants placed in differently sized sites show variation; naturally, the most undersized defect presents the highest insertion torque. In general, there has been a tendency for undersized sites to present higher histomorphometric values at the time of initial implantation, which provides strong evidence for the initial biomechanical competence of the implant-in-bone system. However, it has been speculated that high torque levels may result in high degrees of compression in bone, resulting in extensive bone remodeling over time.
A potential drawback that can cause implant failure is excessive compression. It has been reported that if the compression level is beyond the theoretical yield limit, the bone will fracture or will present compression osteonecrosis. It can be said that adequate stress levels are necessary to protect the surrounding bone from ischemia and crestal bone resorption.
The impact of macrogeometry on primary stability of the implant has been investigated in numerous studies. The macrogeometry of the implant influences the tangential velocity and centrifugal force during insertion, static strain levels in the bone, as well as dynamic strain in the bone during functional loading. Studies have also suggested that the degree and location of the stress distribution is also dependent on the macrogeometry, and that both the thread design and the bulk design of the implant significantly influence primary stability. It has been suggested that conical implants present higher insertion torque than cylindrical implants. Since insertion torque may be interpreted as a barometer for initial stability, the conical implant design has been preferred by oral surgeons when initial torque is necessary, especially in the maxilla.
Although undersized drilling has been proven to present higher insertion torque values, the biological impact of this methodology has not been fully clarified. In fact, determining the degree of undersized drilling to achieve a high insertion torque is dependent on the surgeon’s perception of bone quality in clinical reality. Furthermore, different implant designs present different initial and long-term biological outcomes and the information may be valuable in the selection of an appropriate drilling protocol for different implant designs. Thus, the objective of the present study was to observe the initial mechanical implant interlocking to bone and the eventual osseointegration of conical and cylindrical implants placed in two different undersized defects.
Materials and methods
The present study utilized a total of 24 conical and 24 cylindrical implants, both types 4 mm in diameter and 10 mm in length (Implacil De Bortoli, Sao Paulo, Brazil). The conical implant demonstrates progressive trapezoidal threads and the cylindrical implant demonstrates triangularly shaped threads ( Fig. 1 ). All implants possessed the same surface topography, presenting a textured surface that is fabricated by a proprietary grit-blasting/acid-etching protocol, as stated by the manufacturer.
Animals and surgery
Animal experiments were conducted after receiving the required ethical approval from the Institutional Animal Care and Use Committee (IACUC). Six Finnish Dorset cross-bred sheep (each weighing approximately 70 kg) were utilized for this study. There were four experimental implant groups (all implants 4.0 mm in diameter and 10 mm in length): (1) group 1, conical implant with an undersized osteotomy of 3.2 mm; (2) group 2, conical implant with an undersized osteotomy of 3.5 mm; (3) group 3, cylindrical implant with an undersized osteotomy of 3.2 mm; (4) group 4, cylindrical implant with an undersized osteotomy of 3.5 mm. Implants were placed in the sheep mandibular base.
Prior to surgery, the mandibular regions were shaved using aseptic procedures. The animals were then monitored continuously for heart rate, oxygen saturation, respiratory rate, temperature, and tissue coloration prior to the complete shaving of the intended surgical site. The relevant adjacent areas were also accessed prior to the application of a povidone–iodine solution. Monitoring was then transferred to an automated system and the animal draped aseptically. Anesthesia was induced with sodium pentothal (15–20 mg/kg) in Normasol solution in the jugular vein. Anesthesia was maintained with isoflurane (1.5–3%) in O 2 /N 2 O (50/50). Preoperatively and postoperatively, 500 mg of cefazolin was administered intravenously. Throughout anesthesia, body temperature was maintained with a circulating hot water blanket placed underneath the sheep. Vital signs were intermittently monitored with electrocardiography, end-tidal CO 2 , and SpO 2 .
Two surgical procedures were performed at 3 and 6 weeks prior to euthanasia (on the left and right sides, respectively) by means of a 10-cm extraoral incision located 2 cm distally from the most distal position of the masseter. After mandibular bone exposure, the osteotomy was prepared using the drills provided by the manufacturer, and standard gradual expanding drilling sequences were utilized starting with a 2.0-mm drill. Drilling was performed under abundant saline irrigation at a drilling speed of 900 rpm. Two types of osteotomy site were prepared using final drills with diameters of 3.2 mm and 3.5 mm, respectively ( Fig. 2 ). Thereafter, one implant from each group was randomly placed in the sites in a proximal-to-distal order at 2-cm intervals (site 1 to site 4) from the adjacent implant centers.
The implant groups were interpolated as a function of implantation site to minimize site bias throughout the study. During insertion, the maximum insertion torque was recorded with a portable digital torque meter (Tohnichi, Tokyo, Japan), with a 200 N cm load cell used for each implant placed. Since a threadless component and a microthread component were respectively present for the cylindrical and conical implants, these were placed at the maximum cervical depth at which the triangular threads and progressive trapezoidal threads were present ( Fig. 1 ).
Postoperative antibiotic and anti-inflammatory medications included a single dose of benzylpenicillin benzathine (20,000 IU/kg) intramuscularly and ketoprofen 1% (1 ml/5 kg). The sheep were euthanized by anesthesia overdose, and the mandibles were retrieved by sharp dissection. The soft tissue was removed using surgical blades, and an initial clinical evaluation was performed to determine implant stability. If an implant was clinically unstable, it was excluded from the study.
Histological processing and histomorphometry
The bones containing the implants were reduced to blocks and immersed in 10% buffered formalin solution for 24 h. The blocks were then washed in running water for 24 h, and steadily dehydrated in a series of alcohol solutions ranging from 70% to 100% ethanol. Following dehydration, the samples were embedded in a methacrylate-based resin (Technovit 9100; Heraeus Kulzer GmbH, Wehrheim, Germany) in accordance with the manufacturer’s instructions. The blocks were then cut into slices (approximate thickness, 300 μm) with a precision diamond saw (IsoMet 2000; Buehler Ltd, Lake Bluff, IL, USA), aiming at the center of the implant along its long axis, and these were glued to acrylic plates with an acrylate-based cement; a setting time of 24 h was allowed prior to grinding and polishing. The sections were then reduced to a final thickness of approximately 30 μm by means of a series of SiC abrasive papers (400, 600, 800, 1200, and 2400 grit; Buehler Ltd) in a grinding/polishing machine (MetaServ 3000; Buehler Ltd) under water irrigation. The sections were then stained with Stevenel’s blue and observed by optical microscopy at 50–200× magnification (Leica DM2500M; Leica Microsystems GmbH, Wetzlar, Germany) for histomorphological evaluation.
The bone-to-implant contact (BIC) was determined at 50–200× magnification (Leica DM2500M) by means of a computer software program (Leica Application Suite, Leica Microsystems GmbH). The regions of bone-to-implant contact along the implant perimeter were subtracted from the total implant perimeter, and calculations were performed to determine the BIC. The bone area fraction occupied (BAFO) between the threads in the trabecular bone regions was determined at 100× magnification (Leica DM2500M) by means of the computer software program (Leica Application Suite). The areas occupied by bone were subtracted from the total area between the threads, and calculations were performed to determine the BAFO (reported as percentage values of the bone area fraction occupied).
The statistical analysis was performed by a repeated measures analysis of variance. Statistical significance was set at α = 0.05, and the data are presented as means ± the 95% confidence interval.
No complications regarding procedural conditions or other immediate clinical concerns accompanied the surgical procedures or follow-up. No postoperative complication was detected, and no implant was excluded from the study because of clinical instability of the implant after euthanasia.
The insertion torque recorded for each implant design and drilling group is presented in Fig. 3 ; the insertion torque decreased non-significantly ( P > 0.65) as a function of drilling diameter from 3.2 mm to 3.5 mm for both designs. However, significant insertion torque differences were observed between the conical and cylindrical designs, regardless of the drilled site diameter ( P < 0.001).
Histological observation and histomorphometry
A qualitative evaluation of the histological sections demonstrated contact between all experimental groups and the cortical/trabecular bone at both implantation times ( Figs 4 and 5 ).