On a biomechanical approach to analysis of stability and load bearing capacity of oral implants

Sammanfattning: Introduction When an implant is placed in the bone the body responds to the trauma by encapsulating the implant and its survival depends on the ability for hard tissue encapsulation. The stability of the implant during the healing phase is essential to achieve a good result [1]. Biological, physiological and mechanical phenomena affect implant stability. To achieve sufficient stability during the initial healing phase the implant has to provide sufficient static interaction with the bone. The static interaction might affect the biological processes that in turn affect implant stability. Although, numerous studies on the effect of dynamic interaction on implant stability and bone remodeling exist, the effect of static strain has yet to be clarified. As the healing progresses it may result in bone formation in close contact with the implant (i.e osseointegration) that stabilizes the implant. It has been found that implant surface modifications at the micro level promote osseointegration and that moderately roughened implants provide rapid and strong bone response [2, 3]. In addition, the application of nanostructures to an implant surface has been shown to elicit an initial complex gene response that may result in further enhancement in bone formation around the implant [4]. Furthermore the implant surface structure interlocks mechanically with the bone that affects the stability of the implant.The implant surface design has to take into account both biological and mechanical behavior of the tissues. Materials and methods To investigate how implant stability and the biological response are affected by an induced static load to the bone an in vivo study was performed. Two types of controlled static loads, excessive and moderate, were induced by specially designed implants. Two types of surface structure, turned and blasted, were applied on the implants. The implants were inserted in rabbits and healed for 3-84 days before the stability was measured by removal torque. To simulate how the pressure changes, due to biological and mechanical phenomena, on an implant surface that was subjected to an initial pressure, a constitutive model was developed that was comprised of visco-elastic, visco-plastic and remodeling components. The pressure on the surface in turn affects the implant stability. To investigate how the biomechanical and the biological responses are affected by the surface structure an in vivo study and a finite element analysis of the theoretical interfacial shear strength were performed. In the pre-clinical study, three groups of implants with different nano- and microstructures were compared to an implant with a control surface structure. The theoretical interfacial strength at different healing times was estimated by simulating the surface structure interlocking capacity to bone using an explicit finite element method. Simulations were performed for different surface structures and for different pressures, simulating visco-elastic and remodeling phenomena.Results Implants that induced a moderate bone condensation in the bone had a significantly higher removal torque value at the implantation times of 3-24 days compared to implants that did not induce condensation. The effect the induced moderate bone condensation had on implant stability decreases over time until the pressure has vanished, which approximately occurred after 28-30 days. Turned implants, placed in tibia, that induced excessive bone condensation resulted in significant increased implant stability at implantation times of 3-24 days compared to implants that induced no condensation. However, when they were placed in femur it provided no significant difference in removal torque at an implantation time of 24 days compared to implants that induced no condensation. The developed constitutive model is able to capture visco-elastic material behavior and remodeling phenomena of cortical bone which can be used to simulate how the pressure changes on an implant surface that is subjected to an initial pressure caused by condensation. The implant nano- and microsurface structure affects the magnitude of the removal torque value. It was found that implants, with no significant difference in surface roughness parameters (Sa, Ssk, Sdr) on micro level, can present a significant difference in removal torque value at 4 weeks of implantation time. In addition, it was also found that implants with a significant difference in surface roughness parameters (Sa, Ssk, Sdr) can present no significant difference in removal torque value at 4 weeks of implantation times. The difference may be due to various biological responses from the nano- and microstructure surfaces. The simulated interfacial strength for the different surfaces did not reach the interfacial strength that corresponds to the removal torque obtained in the in vivo study. Comparing the two surfaces in respect of removal torque ratio, suggests that during the early healing phase the difference is caused by different bone formation rates from biological processes. As the healing progresses the effect of structural interlocking capacity is more pronounced. Conclusions The results suggest that increased static strain in the bone not only creates higher implant stability at the time of insertion, but also generates increased implant stability throughout the observation period of 3-24 days. The proposed constitutive material model consists of three different components: a visco-elastic component, a visco-plastic component and a remodeling component. The model captures with good agreement the experimental behavior of cortical bone during different longitudinal loading situations i.e. in vitro stress-strain relationship, in vivo relaxation, in vitro creep and in vivo remodeling. The results of the present study suggest that nano- and microstructure alteration on a blasted implant might enhance the initial biomechanical performance, while for longer healing times, the surface interlocking capacity seems to be more important. Simulation of the interfacial shear strength by means of finite element analysis seems to be a promising method to estimate the load bearing capacity of the bone-to-implant interface for different surface structures at stable healing conditions i.e. longer healing times. Furthermore, it is a promising method to estimate the implant stability for different magnitudes of condensation.