The Microtomographic Evaluation of Marginal Bone Resorption of Immediately Loaded Scalloped Design Implant With Various Microthread Configurations in Canine Mandible: Pilot Study

The creation of an esthetic implant restoration with a harmonized gingival architecture is the main topic of contemporary implant dentistry.1 To achieve long-term stable function and esthetics, only minimal marginal bone resorption should be allowed; therefore improvements in various aspects of dental implants, including implant geometry, surface modification, and surgical technique, have been tried and confirmed.2 

It is proposed that the scalloped nature of the dentogingival complex is an essential element for attaining long-lasting esthetic treatment outcomes and for integrating the restoration completely and harmoniously into the existing anterior dentition.3 Several clinical studies have supported this modification of platform design, despite a few controversies.4–7 

On the other hand, Hansson8 introduced retention elements located at the implant neck, and these microthreads or microgrooves have been shown in several studies to have the effect of protecting marginal bone resorption theoretically and clinically.9–11 

Currently, a scalloped implant platform and the microthread have been combined in some implant designs to maximize the stability of marginal bone and soft tissues; however, the mechanism for microthread preservation of marginal bone support remains unclear, and whether the topography of the microthread has some effect on this mechanism is also unknown.

The possibility of using X-ray microtomography for noninvasive evaluation of the bone–implant interface was first suggested by Sennerby et al.12 Correlation between microtomography and histomorphometry for assessment of implant osseointegration was investigated in the previous study, and the results showed some promise of using microtomograms for noninvasive osseointegration assessment.13 

The aim of this study was to use microtomography to evaluate the effects on marginal bone resorption of immediately loaded implants of a microthread configuration located in the scalloped design implant neck portion.

One-piece–type implants that had an abutment portion on the implant body were used. A total of 8 grade IV titanium screw-type implants were used; these had rough surfaces on the fixture with a top diameter of 4.3 mm and a gradual 2 degree taper to the end (Warrantec, Seoul, Korea). The fixtures were blasted with Al₂O₃ particles (mean particle size of 50 µm) at a pressure of 4 to 5 kg, and this was followed by acid etching in H₂SO₄ and HCl solution.

Each implant had a scalloped conical portion, and 4 different types were used for this experiment. As shown in Figure 1a, the type 1 implant had a machined scalloped collar, type 2 had a sandblasted and acid-etched scalloped collar, type 3 had horizontal microthreads, and type 4 had parabolic microthreads parallel to the scalloped conical margin. In types 3 and 4, the blasted and etched surface treatment was done on the microthreaded portion. Definitive restorations were made with InCeram (Vita Zahnfabrick, Bad Säckingen, Germany) (Figure 1b). Two implants of the same type were used and installed separately to each side.

One beagle dog, about 1 year old, was used. General anesthesia was induced with propofol (10 mg/mL, 0.6 mL/kg) intravenously and sustained with N₂O∶O₂ (1∶1.5–2) and isoflurane employing endotracheal intubation during all surgical procedures. A local anesthetic agent was administered to the mandibular left and right quadrants. A crevicular incision with short mesial and distal vertical incisions was made for flap access, and full-thickness, mucoperiosteal flaps were raised on both sides of the mandible. All mandibular premolars and first mandibular molars were hemisected with carbide burs under a continuous stream of sterile saline. Once separated, the roots were removed with elevators and forceps, and the sockets were carefully debrided. Surrounding alveolar bone is reduced to the expected level of the scalloped conical portions of fixtures to house the fixtures properly. Immediately after extraction and reduction, each of the 4 types of implants was installed at each side randomly. Eight total implants were installed to the level where the tops of implants were submerged (Figure 2a). All implants installed had primary stability that could accommodate the restoration. InCeram restorations previously made were fully seated and luted with Rely-X cements (3M ESPE, St Paul, Minn) (Figure 2b). After the manufacturer-recommended setting time had passed, excess cement material was removed. Wound closure was achieved with interrupted 4/0 Vicryl sutures. Control of postsurgical infection and swelling was accomplished with 1.0 mg of dexamethasone given the day after surgery and amoxicillin 500 mg twice a day for 10 days. After 12 weeks, the animal was sacrificed by induction of deep anesthesia followed by intravenous sodium pentobarbital euthanasia. The mandible was removed, and blocks containing the implant specimen were prepared so X-ray microtomography could be performed.

Microtomography was performed with a Skyscan 1072 X-ray microtomograph (Skyscan, Antwerpern, Belgium). The system consisted of a sealed X-ray tube, 20–80 kV/100 mA, an 8-mm spot size, and a precision object manipulator with 2 translations and 1 rotation direction. Furthermore, the system included a 12-bit digital-cooled CCD camera (1024 × 1024 pixels) with fiber optics.

For microtomographic reconstruction, the transmission of X-ray images was required from 200 rotation views through 180 degrees of rotation (rotation step  =  0.9), with a 1 mm aluminum filter. The magnification rate for the present study was ×15 because of the sample size. The microtomographic reconstruction of each cross-section was made with a Dual Intel Xeon 1.7 GHz computer under Windows XP Professional. All constructed cross-sections contained 1024 pixels, with a cross-section pixel size of 15.95 mm and cross-section-to-cross-section distances of 15.95 mm. A specialized reconstruction program “Ant” was used for this purpose. Detailed observation of each reconstructed sample was performed.

All implants were clinically osseointegrated. No implants were mobile or turned under the tightening torque of 35 Ncm by the manual torque wrench, Torque-Lock (Intra-lock International Inc, Boca Raton, Fla). X-ray microtomographic results are displayed in Figure 3. Type 1 specimens showed that the scalloped conical portion completely lost the marginal alveolar bone. A similar pattern of bone resorption was seen in type 2 specimens. Both specimens showed that the marginal bone level is located below the scalloped conical portion and around the first thread. In type 3 specimens, the marginal bone level is located on the microthreaded portion, which means that substantial marginal bone resorption progressed. Type 4 specimens showed the marginal bone loss pattern definitively analogous to the scalloped margin. The alveolar bone top is located near the microthreaded portion.

Surgical trauma, biologic width, and implant microdesign are thought to be the causes of crestal bone resorption around the implants; however, the precise mechanism is not still clearly understood. To achieve stable osseointegration for implant restoration, the generation of high-stress concentration in bone, which is supposed to be one of the causes of the bone loss, should be avoided. A number of finite element studies show that shear stress concentrated at the crestal bone area was lessened by modification of the implant design, such as microthreads. The rough surfaced neck was also shown to promote low resorption of crestal bone, which was not incorporated into the original implant because several authors believed that a smooth surface prevents plaque accumulation; however, it is not fully known whether or how the microdesign of retention elements affects the bone loss rate. It was the purpose of this study to elucidate whether the geometry of microthreads affects the bone resorption pattern around the scalloped design implant.

Results of the present study showed that bone resorption around the scalloped neck implant with parabolic microthread displayed a peculiar pattern that approximates the microthreads or scalloped margin. Scalloped design alone and scalloped design with rough surface did not show a similar pattern with the type 4 implant. The scalloped neck implant with horizontal microthreads also showed a substantial amount of bone resorption, but the marginal bone level was slightly higher than those of implants without microthreads. The level was located somewhere on the microthreaded portion. Because of the small sample size of this preliminary study, it is inappropriate to consider this as a general phenomenon. This might be affected by the regional inflammation reaction or other factors. In addition, horizontal microthreads on the scalloped neck do not form closed curves; it is suspected that this may affect the resorption results.

In a compromised buccal bone situation, Carmagnola et al14 reported that some bone regrowth and osseointegration occurred at the buccal wall, but at the lingual wall, substantial resorption of marginal bone occurred. This might mean that marked modeling and remodeling of the bone tissue takes place to level the marginal bone during the process of healing. If marginal bone resorption stops around the level of horizontal threads, it is inferred that parabolic microthread geometry mimicking the cementoenamel junction could hold the resorption level parabolically, not horizontally. On the other hand, Botticelli et al15 demonstrated that bone modeling and remodeling at an implant placed in a fresh socket differ from those in a healed ridge. In addition, Araújo et al16,17 reported that implant installation failed to preserve the hard tissue dimension of the ridge following tooth extraction; this might be due to the unavoidable bundle bone resorption.

In the present study, immediate installation was performed following extraction and subsequent test site preparation; the possibility of modeling cannot be excluded, and remodeling of marginal bone appeared in the fresh extraction socket. Thus, this modeling and remodeling might affect marginal bone resorption around the scalloped design implant used, and the condition of buccal and lingual bone thickness was not uniformly controlled in this experiment. Therefore, further study that involves a greater number of specimens and more controlled conditions should be performed.