Implant surface topography is one of the decisive factors for osseointegration, and notable evolution of implant microsurface has been noted over the past 30 years.1,2 Different surface texturing procedures have been proposed, and many of them are well documented. Moreover, it has been shown in recent studies that different parameters can be controlled to generate desired surface microtopographies through microrobotized blasting procedures.3,4
Compared with the extensive research conducted on implant surface topography, implant macrogeometry has been of less focus, and evidence with regard to the interplay between different implant macrogeometries and different surface topographies has been scarce. However, the interplay between the 2 parameters is an important aspect because the effect of surface topography may present different outcomes depending on the way it is initially in contact to the bone. It has been suggested in recent studies that press-fit type implants, in which the implant thread is completely interlocked in the bone, would undergo a remodeling phase at the interface.5 Namely the interfacial remodeling healing pathway involves bone resorption of the bone initially in contact and thereafter bones apposition to the implant. It is suggested that with this type of healing, the effect of surface modification is more prominent at the bone apposition phase and its contribution is limited during the bone resorption phase.
Thus, another type of healing mode has been of some interest, where a healing chamber is generated between the implant and the wall of the osteotomy.6,7 Namely, the intramembranous-like bone healing pathway, allows direct bone apposition to the textured implant surface without undergoing bone resorption within the healing chamber. Although in vivo studies have presented enhanced achievement of osseointegration within the healing chamber, the primary stability of the implant is naturally of concern because the interlocking to the bone is only at the tip of the implant threads, which could be of some concern for clinical application.8 Therefore, a further modification in the instrumentation and macrogeometry has been suggested to be effective so that maximized primary stability can be achieved and at the same time, generate healing chambers for intramembranous-like healing. The so-called hybrid-healing pathway concept has been introduced in different studies and has shown improved implant stability, while maintaining direct bone apposition generated by the healing chamber.5,9
In a previous study conducted by Yoo et al8 in which this type of implant with 3 different surface topographies were placed in the sheep ilium, it was shown that the bone-to-implant contact (BIC) and biomechanical removal torque values were significantly higher for the acid-etched surface compared with the grit-blasted surface or the machined surface. This suggested that the interplay between the acid-etched surface and the macrogeometry of the implant presented the highest degree of osseointegration when placed in bone with low density.
Because the quality of the bone is one of the important factors for the successful achievement of osseointegration,10 it is also important to consider whether the interplay between the surface texturing and macrogeometry occurs in the same manner for all bone types. For instance, bone with thick cortical layers tends to have high bone density rich in lamellar bone, which is often represented by the anterior mandible.11 However, bone in the anterior maxilla is known to possess more bone marrow than cortical bone.11 In a finite element analysis conducted by Tada et al,12 it was shown that the maximum equivalent stress/strain in bone increased with a decrease in cancellous bone density. Friberg et al13 have reported in a clinical study to investigate the causes for early implant failures that the most frequent bone type observed in relation to implant losses was in type 1 bone. Therefore, such difference in bone composition should influence the degree of osseointegration and the same setup of implant surface and implant macrogeometry that presented the best outcomes may not be the case in high-density bone.
Thus, in this study, the same set of implant groups used in the study conducted by Yoo et al (2015)8 were placed in the external body of the sheep mandible to determine the optimal combination of macrogeometry and microtopography for high-density bone. The degree of osseointegration was evaluated histologically and through histomorphometrical evaluation.
Materials and Methods
A total of 15 implants (ø4.0, 10 mm length, FGM, Joinville, SC, Brazil) were divided into the following 3 groups depending on the surface treatment: dual acid-etched (AA), grit-blasted/acid-etched (GB), and machined surface (control) (n = 5 per group). Topological and morphological characterization of the identically surface-treated implants has been conducted in the previous study published by Yoo et al8 (Fig. 1).
The animal experiments were performed after approval of Ethical Committee from École Nationale Vetérináiré d'Alfort (approval number 00391.01, Maisons-Alfort, France). Five male sheep (mean weight 54 kg) were used in this study. The animals were anesthetized with sodium pentothal (15–20 mg/kg) in Normosol solution into the jugular vein, and anesthesia was maintained with isoflurane (1.5%–3%) in O2/N2O (50/50). Vital signs were monitored throughout the surgery.
Before surgery, skin over the external mandibular body was shaved thereafter; iodine solution was applied to disinfect the surgical site. After incision of the skin, dissection of the muscular plane was performed with a periosteal elevator so that the bone was exposed. After exposure of the bone, osteotomy was performed following the manufacturers' instructions, thereafter; the implants were seated with a torque wrench. The muscle layers were sutured with Vicryl 2-0, and the skin layers were sutured with 2-0 nylon. Preoperative and postoperative Cefazolin (500 mg) was intravenously administered.
After 6 weeks in vivo, the sheep were killed by anesthesia overdose, and the mandibles were dissected from the animals. The soft tissue was carefully removed, and an initial clinical evaluation was performed to determine implant stability. If an implant was clinically unstable, it was excluded from the study.
Histological Preparation and Histomorphometry
The dissected bone-implant blocks were fixated in 4% formaldehyde for 24 hours. Thereafter, all samples were subjected to dehydration in a series of ethanol (70%–100%) and infiltration in light curing methacrylate resin (Technovit 7200 VLC, Heraeus Kulzer, Wehrheim, Germany, 30%–100%) under constant vacuuming and thereafter were embedded and polymerized under ultraviolet light. The embedded resin blocks were subjected to nondecalcified cut and grind sectioning. In brief, a central section of each sample was prepared using the EXAKT cutting and grinding equipment (EXAKT, Norderstedt, Germany) to a final thickness of approximately 20 μm. After polishing, the sections were stained with a mixed solution of toluidine blue and pyronin G.
The histological observation was performed using a light microscope (Eclipse ME600, Nikon, Sendai, Japan), and the histomorphometrical data were analyzed using image analysis software (ImageJ v. 1.43u, National Institute of Health, Bethesda, MD); the outcomes were calculated in an excel software (Microsoft Co, Redmond, WA) as follows:
- Bone-to-implant contact (BIC): Percentage of the bone in direct contact to the implant surface and was divided by the entire length of the implant placed in the bone.
- Bone area fraction occupancy (BAFO): Percentage of the area inside a thread occupied by newly formed bone.
Statistical analyses were performed using IBM SPSS v.20 (IBM Inc., Armonk, NY) software. To verify the effect of implant type on the histomorphometrical parameters, a mixed model with surface as a within subject factor was applied with 0.05 set as significance level.
The animal surgical and postoperative procedures yielded no procedural complications, postoperative infection, or other clinical concerns. At the time of euthanasia, 1 implant in the control group was excluded because of lack of osseointegration.
Histomorphometric and Histologic Analysis
The histomorphometric results demonstrated that there were no significant differences between all groups tested for both BIC and BAFO (Fig. 2, A and B). Tendency for higher mean BAFO percentages were noted when the control group was compared with the AA and the GB groups (53.9%, 68.0%, and 68.2%, respectively). The histologic observation showed new bone formation along and in the vicinity of the implant threads for all groups tested in which healing chamber was generated between the cortical bone and the implant threads. Bone resorption and bone apposition could be noted at the thread top areas where the implants were initially in contact to the bone at the time of implant installation. No negative responses such as inflammation, infection, or excessive bone resorption were noted (Fig. 3, A and B).
This study evaluated the synergetic effect of different implant surface topographies and implant macrogeometry in a specific bone type in which blood circulation is restricted (type I) and was conducted to determine whether a specific textured surface would provide benefits in terms of initial osseointegration in this type of bone. Aforementioned from the study by Friberg et al, implants placed in type I bone presented the highest percentage of failures compared with the other bone types.13 Based on the results obtained from our previous studies, one of the reasons for this phenomenon could be that the implants placed underwent an interfacial remodeling osseointegration pathway in which bone remodeling occurs at sites where bone is in intimate contact to the implant. With the press-fit type implant used in the Friberg study, osseointegration is achieved through constant bone remodeling, and different clinical parameters can provoke negative responses that may have led to the loss of the implants. Especially when the press-fit implants are placed in type I bone, the risk for bone resorption could increase because of the high bone density and lack of sufficient blood microcirculation. Namely, the ischemic bone necrosis is a pathologic status in which the bone loses its vitality and will result in high levels of osteoclastic activity.14,15 Therefore, it can be suggested that modification of the implant hardware (macrogeometry and drill protocol) in high-density bone type to obtain a healing chamber situation, at the same time obtain rigid initial stability, is beneficial for the achievement of osseointegration.
The results of the histomorphometric analysis presented no significant differences between the 3 groups for both BIC and BAFO. Accordingly, the histologic micrographs presented similar bone healing patterns, showing new bone formation within the healing chamber. As expected, the initially interlocked regions presented active bone remodeling presumably as a result of the interfacial strain generated between the implant thread tip and the cortical bone.
The results of this study are in disagreement with the previous parallel study in which the AA presented the highest histomorphometrical and biomechanical values. The ilium possesses a thin cortical layer and thereunder the bone marrow, which is classified as low-density bone, or bone with poor quality. However, the bone marrow is rich in osteogenic stem cells and the microtextured implant surface generated by the dual acid etching may have improved the surface's ability to recruit the migration of these cells to the implant surface within the healing chamber relative to the other experimental groups. Therefore, in sites with low bone density, both implant macrogeometry drilling protocols and microtextured surface had significant influence on osseointegration.
The results of this study are of interest because they suggested that in high-density bone, the influence of microtextured surfaces on measured osseointegration parameters were less prominent and were mainly implant hardware dependent. This was evident by the fact that the measured histomorphometric parameters presented statistical insignificances, which suggest that the effect of implant macrogeometry and instrumentation were the decisive factors for the achievement of osseointegration.
It can be speculated that in such a clinically challenging situation in which the number of cells and vasculature are compromised, the degree of compression and the degree of surgical trauma to the bone is a primary influential factor for osseointegration. In a previous study, implants possessing cutting flutes on all threads were tested in the same animal model and site, and the results showed that this specific macrodesign significantly lowered the insertion torque, resulting in less necrotic bone around the implant compared with the implants with identical geometry without cutting flutes.16 The implants used in this study did not possess cutting flutes; however, the macrodesign of the implant generated a healing chamber, and the implant surface was in intimate contact to the bone only at the thread top areas, which reduced the total interfacial strain. It goes without saying that micro-/nano-textured surface topography is of great importance during the course of osseointegration; however, within the limitation of our 6-week-study, this effect was minimum and osseointegration was macrogeometry driven.
This study investigated the interplay between implant surface topography and implant macrogeometry possessing a hybrid-healing capacity in high-density bone. The results showed that the effect of surface topography was not significant in this type of bone, and it was suggested that osseointegration was macrogeometry dependent.
The authors claim to have no financial interest, either directly or indirectly, in the products or information listed in the article.
Roles/Contributions by Authors
Yohei Jinno: Histological processing and manuscript construction. Ryo Jimbo: Project planning, surgery, statistics, and manuscript approval. Nick Tovar: Surgery, data analysis, and manuscript construction. Hellen S. Teixeira: Surgery and data analysis. Lukasz Witek: Planning, analysis, and statistics. Paulo G. Coelho: Planning, surgery, and manuscript approval.
This study was partially funded by FGM, Joinville, Brazil, and partially funded by FAPESP grant # 2010/06152-9, and CNPq # 309475/2014-7.
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