Shock absorbing ability of articular cartilage and subchondral bone under impact compression

Despite the important role of subchondral bone in maintaining the integrity of the overlying articular cartilage, little research has focused on measuring its mechanical behavior, particularly under injurious load conditions such as impact compression. In this study, the stiffness and the absorbed energy of subchondral bone were compared to that of its overlying cartilage by applying impact compression to equine cartilage–bone specimens. Deformations of the cartilage and subchondral bone were examined independently within the cartilage–bone unit by analyzing real-time images of cartilage–bone explants. Peak subchondral bone and cartilage stiffness (mean±SD) were 800.7±250.0 MPa and 119.9±50.8 MPa respectively. The maximum absorbed energy per unit volume of subchondral bone was approximately 4 times lower than that of cartilage. Micro-computed tomography (μCT) images at 9 μm resolution revealed oblique fissures at the cartilage articular surface. At the cartilage–bone interface, micro-cracks as thin as 30 μm in width and micro-fractures of width 200 μm could be seen in the μCT images. The relative energy loss in bone was 76.5±6.8% in specimens with bone fracture and 23.0±20.4% in specimens without bone fracture. Our results indicate that both articular cartilage and subchondral bone absorb shock under impact compression, but the energy absorption of bone is much higher in specimens that fracture. This may spare the overlying cartilage from immediate injury, but is a potential risk for subsequent post-traumatic osteoarthritis (PTOA).