Composition and mechanical properties of osteoarthritic subchondral trabecular tibial bone.

Early-stage primary osteoarthritic bone samples, clinically-defined using the Mankin scoring system, were harvested from both medial and lateral compartments of diseased human tibiae.  Only the medial compartments exhibited clinical evidence of early-stage OA.  Subchondral trabecular bone from both the medial and lateral compartments were dehydrated and embedded in PMMA and subsequently examined using nanoindentation and quantitative backscatter electron imaging (QBEI).  Stiffness, hardness and mineralisation were measured as a function of depth below the sample surface. A novel calibration technique was developed to facilitate damage-free analysis using qbei. Significant differences in intrinsic tissue stiffness and mineralisation were observed in all samples presenting with clinically-defined, early-stage OA.  Non-presenting lateral samples also exhibited significant differences in stiffness and mineralisation but these were opposite in direction to those seen in their medial counterparts.  Hence, the presence of an early, non-presenting stage of OA initiation was observed.  Additionally, differences in the measured tissue properties were difficult to explain as secondary to changes in the overlying cartilaginous matrix, implying an exclusively biological equilibrium breakdown within affected knee joints.  No significant relationship between stiffness or hardness and corresponding mineralisation density was observed, disproving our hypothesis that changes in the intrinsic mechanical properties are predominantly linked with corresponding changes in tissue mineralisation density in primary knee OA, strongly implicating the contribution of collagen and/or its subsequent effect on cohesive mineralisation. In conclusion, subchondral bone plays a significant role in the pathogenesis of human primary knee OA, conclusively proving that the classic 'wear' and 'tear' definition of this disease is no longer applicable to human knee OA.