Marginal bone response around preloaded dental implants – A histological investigation in rabbits

Background

Implant treatment is highly influenced by marginal bone (MB) stability that is usually thought to be influenced by occlusal load application and or peri-implant inflammation. However, the sole effect of abutment preload installation on MB around implant had not been studied yet. The contact force that brings the abutment and implant clamped together is called preload, which is a tensile force. Finite element analysis was conducted and revealed the existence of preload stress distributed in MB.

Aim/Hypothesis

The purpose was to investigate histological MB alternations around osseointegrated dental implants caused by abutment screw preload using recommended and high tightening torque values in-vivo.

Material and Methods

Sixteen Japanese white male rabbits were used. Each animal received two implants in each right and left femur. After eight weeks, test and control implants were randomly selected. A 35Ncm torque was applied to tighten abutment screws (n = 16) as recommended preload group (RP). Other abutment screws (n = 16) were subjected to 70Ncm tightening torque as high preload group (HP). Tightening group (HT) received only 70Ncm tightening torque without preload (n = 8) as screw was untightened immediately. Control group (Cont) implants remained in-situ (n = 24). Animals were sacrificed at 4, 6, 8, 10 weeks after abutment screw attachment. MicroCT images were taken, undecalcified ground sections were prepared, stained with Toluidine Blue and investigated under light microscope and polarized light. Bone volume fracture (BVF), bone-to-implant contact (BIC), and bone area (BA) were calculated and two-way ANOVA test was performed as statistical analysis. The ethical approval obtained as No.28-189-2 Niigata Uni

Results

Cross sections of cortical bone showed bone remodeling activities adjacent to the implants in all specimens of all groups. While bone marrow spaces were relatively small in Cont and HT groups, RP and HP groups showed a larger area of bone marrow spaces, especially after 10 weeks. Those spaces were defined as Bone Multicellular Units (BMUs), which is responsible for cortical bone remodeling activities. Moreover, lamellar bone thickness appeared to be larger in Cont and HT groups compared to RP and HP groups. Under polarized light, lamellar bone in Cont and HT groups displayed alignments of orderly homogeneous collagen fibers perpendicular to the implant axial plane. In contrast, predominant transverse or alternated collagen fibers appeared in RP and HP groups. BIC was significantly lower in RP and HP groups compared to Cont groups at 6, 8, 10 weeks (P ˂ 0.05). Furthermore, RP and HP groups showed significantly less BVF and BA compared to other groups especially at 8 and 10 weeks (P ˂ 0.05).

Conclusion and Clinical Implications

The findings indicated the possible transfer of preload stress from the implant-abutment joint to the surrounding MB even without occlusal loading. This study demonstrated active bone remodeling, the appearance of BMUs in the interested area and decrease of BIC in response to preload. We may have to take the finding into consideration upon abutment screw tightening. Needs for further in-vivo investigations for optimal torque of screw tightening and initiation of occlusal load application were raised