A method for delivering variable impact stresses to the articular cartilage of rabbit knees

Objective: To develop a method by which a single impact force of controlled magnitude and rate could be applied uniformly to an area on the posterior aspect of the medial femoral condyle of adult rabbits. Design: An in-vivo animal model using a pendulum device, designed and manufactured to supply the kinetic energy necessary to apply different impact loads to the posterior aspect of the medial femoral condyle of a rabbit. Setting: Biomechanical laboratory, University Medical Center. Subjects: A total of thirty-six femoral condyles from 3-kilogram New Zealand White (NZW) rabbits were used during this evaluation. Intervention: An aluminum impactor was made based on the sagittal and coronal radii of curvature of six matched pairs (n = 12) of femurs from three-Mogram NZW rabbits. This impactor was coupled with the pendulum and used to apply different impact loads to both of the medial femoral condyle of the knees of NZW rabbits (n = 24). Main Outcome Measurements: Peak impact force, time to peak impact force, and average contact area between impactor and medial femoral condyle, were measured for each group of animals tested. Results: The pendulum delivered a consistent impact force to the rabbit condyle of 120.0 N (;+/-18.1; coefficient of variance, 15 percent) with 400 grams attached to the pendulum arm, at an average time to peak force of 0.021 seconds (+/-0.001, co. var. 4.8 percent). The peak impact force was significantly different for each of the three impact mass groups of animals (p < 0.001). By contrast, time to peak force for each mass group averaged approximately 0.020 seconds and the average contact area was 6.26 mm(2) (+/-0.51). Quantitative assessment of the exposed medium pressure-sensitive film confirmed uniform impact force intensity within each specimen. Conclusions: An in-vivo animal model was developed to deliver a controlled and rapid impact force to a specific area of the weight-bearing surface of die adult rabbit knee. These loads were applied at a rate comparable to the clinical setting of falling onto an outstretched hand, thus simulating a common clinical scenario by which cartilage is often injured. This model can be used in future experiments to investigate mechanism by which posttraumatic arthritis develops after articular injuries.