Osteochondral properties and relationships in the synovial joint

Abstract

The overall goal of this study was to examine the relationship between the properties of articular cartilage and its underlying subchondral bone using an equine model. Thus, the objectives of this study were: (1) to determine the structural and mechanical properties of equine cartilage, (2) to determine the bone mineral density of the underlying subchondral bone, (3) to determine the relationship between these cartilage and bone properties in different joint regions, (4) to develop methodology for subchondral bone plate thickness measurement, (5) to develop experimental protocols and novel equipment for loading opposing osteochondral surfaces and to quantify intratissue cartilage strains within the loaded samples, and (6) to determine any regional relationships between the cartilage strains and underlying subchondral bone plate thickness and bone mineral density. This study presents the first comparative body of data on equine structural/material articular cartilage properties and underlying subchondral bone density for the palmar and dorsal aspects of the medial condyle of the 3rd metacarpal bone, medial trochlear ridge, and the medial femoral condyle. A positive correlation between cartilage tensile equilibrium modulus and underlying bone mineral density was discovered. The measurement of subchondral bone plate thickness is important to the study of osteoarthritis and joint adaptation. A subchondral bone plate thickness measurement technique using high-resolution contact radiographs and image processing was developed with minimal systematic error and high repeatability. The ability to quantify intra-tissue cartilage strains in loaded physiologically opposing surfaces and underlying subchondral bone characteristics should help elucidate the etiology and progression of osteoarthritis. A custom-designed loading system and associated sample preparation procedure were developed and validated to quantify two-dimensional strain fields using video image correlation analysis between two opposing articular cartilage surfaces from the equine metacarpophalangeal joint. The error involved in this process, including the image acquisition system, data analysis, and induced friction, was on the order of 7600 μstrain (0.76%). The present study demonstrated that superficial cartilage strains were larger than deeper cartilage strains, subchondral bone plate thickness and volumetric bone mineral density across the joint regions were positively correlated, and cartilage strain did not correlate with subchondral bone plate thickness or volumetric bone mineral density in the normal equine metacarpophalangeal joint. Although there are some limitations, testing opposing osteochondral surfaces in compression has tremendous benefits; (1) strain distributions across articular surfaces with complex geometry can be obtained, (2) provides the capability to relate depth-dependent articular cartilage strains with underlying subchondral bone properties or pathology changes, and (3) emulates physiological boundary conditions better than platen loading experiments. The benefits of the current method could be applied to a variety of applications where opposing articular surface loading would be important, such as, determining osteochondral implant discontinuities with surrounding host tissue, determining the effects of cartilage and/or bone defects on overlying cartilage strain distributions, and determining the effects of impact loading on strain distributions between articular cartilage strains.