An in vivo rabbit model for cartilage trauma: a preliminary study of the influence of impact stress magnitude on chondrocyte death and matrix damage.

OBJECTIVE: This study was designed to evaluate the postimpact response of the articular cartilage in the rabbit knee after a single traumatic episode.
DESIGN: A novel servo-controlled Rabbit Impact Test System (RITS) was developed to apply a well-defined trauma to the femoral condyle in the rabbit knee. The RITS was first used in an in vitro study to determine an appropriate stress to cause cartilage damage without bone fracture. Viable rabbit knees (n = 18) were impacted with stresses of 15 to 50 MPa at a stress rate of 420 MPa/s, the latter corresponding to joint impact rates commonly seen in sports injuries and vehicular accidents. Based on the in vitro study, we performed an in vivo study by impacting the knees of rabbits (n = 9) with a 35 MPa peak stress at a stress rate of 420 MPa/s. The articular cartilage in these knees was analyzed at 0 and 3 weeks after impaction.
SETTING: Center for Laboratory Animal Services, Hospital for Special Surgery.
SUBJECTS: A total of 27 New Zealand White rabbits were used in this study. INTERVENTION: A rabbit's knee was rigidly immobilized in the adjustable frame of the RITS. A small incision on the knee exposed the lateral condyle and the impactor was positioned perpendicular to the surface of the condyle. The lateral femoral condyle of the left knee was impacted, whereas the right knee was sham operated and used as a control. MAIN OUTCOME MEASUREMENTS: Visual matrix damage, cell viability, and microscopic matrix damage was assessed.
RESULTS: In the in vitro study, matrix damage was observed at stress magnitudes > or =30 MPa. However, cell death was initiated at approximately 20 MPa at the articular surface and increased in depth with increasing stress magnitude (2.8 +/- 2% thickness/MPa,). In the in vivo study, visible surface damage was observed immediately after impaction but not at 3 weeks after impaction. At 3 weeks, the articular cartilage showed significant arthritic changes (matrix damage, chondrocyte death, and proteoglycan loss) typical of late-stage osteoarthritis.
CONCLUSIONS: Our novel impact test system was able to accurately apply a quantifiable stress magnitude at a constant stress rate to rabbit femoral condyles in the in vitro and in vivo settings. At the time of impaction, the extent of cell death depended with the intensity of trauma (stress magnitude) in which complete cell death was observed in the impacted site at >40 MPa. Under in vivo conditions, the test system was able to consistently produce superficial matrix damage and cell death at 35 MPa stress magnitude at the time of impaction. This resulted in cartilage "arthritic" changes by 3 weeks postinjury.