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

• IntroductionWhen an implant is placed in the bone the body responds to thetrauma by encapsulating the implant and its survival depends onthe ability for hard tissue encapsulation. The stability of the implantduring the healing phase is essential to achieve a good result[1]. Biological, physiological and mechanical phenomena affectimplant stability. To achieve sufficient stability during the initialhealing phase the implant has to provide sufficient static interactionwith the bone. The static interaction might affect the biologicalprocesses that in turn affect implant stability. Although, numerousstudies on the effect of dynamic interaction on implant stabilityand bone remodeling exist, the effect of static strain has yetto be clarified.As the healing progresses it may result in bone formation in closecontact with the implant (i.e osseointegration) that stabilizes theimplant. It has been found that implant surface modifications atthe micro level promote osseointegration and that moderatelyroughened implants provide rapid and strong bone response [2, 3].In addition, the application of nanostructures to an implant surfacehas been shown to elicit an initial complex gene response that mayresult in further enhancement in bone formation around the implant[4]. Furthermore the implant surface structure interlocks mechanicallywith the bone that affects the stability of the implant.The implant surface design has to take into account both biologicaland mechanical behavior of the tissues.Materials and methodsTo investigate how implant stability and the biological responseare affected by an induced static load to the bone an in vivo studywas performed. Two types of controlled static loads, excessive andmoderate, were induced by specially designed implants. Two typesof surface structure, turned and blasted, were applied on the implants.The implants were inserted in rabbits and healed for 3-84days before the stability was measured by removal torque.To simulate how the pressure changes, due to biological and mechanicalphenomena, on an implant surface that was subjected toan initial pressure, a constitutive model was developed that wascomprised 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 responsesare affected by the surface structure an in vivo study and a finiteelement analysis of the theoretical interfacial shear strength wereperformed. In the pre-clinical study, three groups of implants withdifferent nano- and microstructures were compared to an implantwith a control surface structure.The theoretical interfacial strength at different healing times wasestimated by simulating the surface structure interlocking capacityto bone using an explicit finite element method. Simulations wereperformed for different surface structures and for different pressures,simulating visco-elastic and remodeling phenomena.ResultsImplants that induced a moderate bone condensation in the bonehad a significantly higher removal torque value at the implantationtimes of 3-24 days compared to implants that did not induce condensation.The effect the induced moderate bone condensation hadon 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 condensationresulted in significant increased implant stability at implantationtimes of 3-24 days compared to implants that induced no condensation.However, when they were placed in femur it provided nosignificant difference in removal torque at an implantation time of24 days compared to implants that induced no condensation.The developed constitutive model is able to capture visco-elasticmaterial behavior and remodeling phenomena of cortical bonewhich can be used to simulate how the pressure changes on an implantsurface that is subjected to an initial pressure caused by condensation.The implant nano- and microsurface structure affects the magnitudeof the removal torque value. It was found that implants, withno significant difference in surface roughness parameters (Sa, Ssk,Sdr) on micro level, can present a significant difference in removaltorque value at 4 weeks of implantation time. In addition, it wasalso found that implants with a significant difference in surfaceroughness parameters (Sa, Ssk, Sdr) can present no significant differencein removal torque value at 4 weeks of implantation times. Thedifference may be due to various biological responses from thenano- and microstructure surfaces.The simulated interfacial strength for the different surfaces did notreach the interfacial strength that corresponds to the removaltorque obtained in the in vivo study. Comparing the two surfaces in respect of removal torque ratio, suggests that during the earlyhealing phase the difference is caused by different bone formationrates from biological processes. As the healing progresses the effectof structural interlocking capacity is more pronounced.ConclusionsThe results suggest that increased static strain in the bone not onlycreates higher implant stability at the time of insertion, but alsogenerates increased implant stability throughout the observationperiod of 3-24 days. The proposed constitutive material model consists of three differentcomponents: a visco-elastic component, a visco-plastic componentand a remodeling component. The model captures with goodagreement the experimental behavior of cortical bone during differentlongitudinal 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 microstructurealteration on a blasted implant might enhance the initial biomechanicalperformance, while for longer healing times, the surfaceinterlocking capacity seems to be more important.Simulation of the interfacial shear strength by means of finite elementanalysis seems to be a promising method to estimate the loadbearing capacity of the bone-to-implant interface for different surfacestructures at stable healing conditions i.e. longer healing times.Furthermore, it is a promising method to estimate the implant stabilityfor different magnitudes of condensation.

Ämnesord

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Odontologi (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Dentistry (hsv//eng)

Nyckelord

animal model

bone resorption

implant stability

cortical bone

viscoelastic-viscoplastic

creep remodeling

constitutive model

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