Study Of The Fluid Behavior Of The Bone-cartilage Unit In Osteoarthritis

Objective:The pathogenesis of osteoarthritis(OA)is still not well understood.The fluid behavior within the bone-cartilage unit(BCU),the basic structural unit of the joint,is important for the biomechanical stability,metabolism and nutrient supply of the joint.The functional relationship between the subchondral bone(SB)and the articular cartilage(AC)is unclear.The purpose of this study was to investigate the mechanical,electrical,permeability and morphological structural alterations of osteoarthritic cartilage and subchondral bone,to establish a finite element model of fluid transport in the bone-cartilage unit,and to characterize the effects of structural alterations of osteoarthritic subchondral bone on fluid flow in the bone-cartilage unit under simulated physiological motion.Methods:In this study,an ACLT-induced rabbit model of osteoarthritis was used to evaluate the morphological structure of subchondral bone and cartilage in four regions of the tibial plateau and femoral condyle using Micro CT and histological assessments.The biomechanical properties of cartilage were measured using nanoindentation testing,while cartilage permeability was calculated using Darcy’s law and a custom-designed permeability tester.The electrical impedance characteristics of cartilage were determined using an electrochemical workstation.Based on Micro CT data,three-dimensional finite element models of normal and osteoarthritic bone-cartilage units were established.Physiological compressive loading was simulated by applying unrestricted time-varying displacement loads to the models,and the stress,displacement,pore pressure,and fluid velocity of cartilage and subchondral bone were quantified to characterize the effects of structural changes in subchondral bone on fluid flow in the bone-cartilage unit in osteoarthritis.Results:ACLT rabbits all exhibited early osteoarthritis.Compared to the control group,the subchondral bone volume fraction and trabecular thickness were significantly reduced in the osteoarthritis group,while the ratio of bone surface area to volume was significantly increased.In addition,cartilage elastic modulus was significantly decreased in the osteoarthritic group,and tibial plateau and femoral condyles were significantly decreased medially relative to laterally.Cartilage and subchondral bone permeability were significantly increased in the osteoarthritic group,while cartilage resistivity was decreased.The mean cartilage and subchondral bone stress,pore pressure and fluid flow velocity of the tibial plateau increased in the osteoarthritis group.Conclusion:The results of this study indicate that osteoarthritis leads to changes in the morphological structure of cartilage and subchondral bone.During the progression of osteoarthritis,the mechanical properties of cartilage decrease,while its permeability increases and electrical resistivity decreases.Moreover,the changes in subchondral bone structure in osteoarthritis alter the stress,pore pressure,and fluid velocity of cartilage and subchondral bone.The changes in subchondral bone structure may be an important factor in the pathogenesis of osteoarthritis.