Chronic changes in rabbit retro-patellar cartilage and subchondral bone after blunt impact loading of the patellofemoral joint.

Animal models of acute joint injury are useful for study of changes in joint tissues that may eventually lead to degradative disease. Our laboratory has developed a joint trauma model using a single blunt impact to the patellofemoral joint of rabbits and documented softening of retro-patellar cartilage and thickening of its underlying bone out to 12 months post-trauma. In the present study, we examined changes in these joint tissues out to 36 months post-impact. Forty-nine Flemish giant rabbits were impacted on the right patellofemoral joint and sacrificed at one of six times: 0, 4.5, 7.5, 12, 24, and 36 months post-impact. A 30% reduction in the compressive modulus of traumatized retro-patellar cartilage occurred at 4.5 months versus the contralateral, non-impacted limb and remained at this reduced level out to 36 months. The fluid permeability of traumatized cartilage also increased over time from baseline and versus the non-impacted limb. Tissue thickness increased slightly at 4.5 months and then decreased over time to a 45% difference from baseline at 36 months post-trauma. While impacted cartilage revealed a significantly greater length of surface fissuring than contralateral, non-impacted cartilage, no time-dependent changes were evident in this study. Moreover, the number and depth of these impact surface lesions did not change as a function of time. Finally, the histological analyses indicated that the thickness of underlying subchondral bone increased over time from baseline and versus that in the non-impacted limb. This long-term study suggested an association between a decrease in the characteristic time constant of traumatized cartilage and thickening of the underlying subchondral bone. Any potential cause and effect relationship, however, must be investigated in future studies.

Impact orientation can significantly affect the outcome of a blunt impact to the rabbit patellofemoral joint.

This laboratory has developed a subfracture, joint trauma model in rabbits. Using a dropped impact mass directed onto a slightly abducted joint, chronic softening of retropatellar cartilage and thickening of underlying subchondral bone are documented in studies to 1 year post-insult. It has been hypothesized that these tissue changes are initiated by stresses developed during impact loading. A previous analytical study by this laboratory suggests that tensile strains in retropatellar cartilage can be significantly lowered, without significantly changing the intensity of stresses in the underlying subchondral bone, by reorientation of patellar impact more centrally on the joint. In the current study comparative experiments were performed on groups of animals after either an impact directed on the slightly abducted limb or a more central impact. One-year post-trauma in animals subjected to the central-oriented impact no degradation of the shear modulus for the retropatellar cartilage was documented, but the thickness of the underlying subchondral bone was significantly increased. In contrast, alterations in cartilage and underlying bone following impact on the slightly abducted limb were consistent with previous studies. The current experimental investigation showed the sensitivity of post-trauma alterations in joint tissues to slight changes in the orientation of impact load on the joint. Interestingly, for this trauma model thickening of the underlying subchondral plate occurred without mechanical degradation of the overlying articular cartilage. This supports the current laboratory hypothesis that alterations in the subchondral bone and overlying cartilage occur independently in this animal model.