Contact Pressure Properties of Osteochondral Defects of the Knee: The Effect of Non-Vertical Walls (SS-57)

The purpose of this study was to compare the mechanical behavior and load transmission of full thickness osteochondral defects of articular cartilage in human knee cadaveric specimens. In addition, the change in behavior was measured after a cylindrical lesion of known dimension was converted to a conical shaped defect with beveled walls. Also, it was determined what part of the lesion size is more critical: the outer diameter or the inner diameter in relation to load transmission. Ten fresh-frozen cadaveric knees were mounted at 30° of flexion in a materials testing machine. Digital electronic pressure sensors (Tekscan) were placed in the medial and lateral compartments. Each knee was first loaded to 700 N at a rate of 100 N/s and then held for 5 seconds. Dynamic pressure readings were recorded throughout the loading and holding phases. Loading was repeated over full thickness osteochondral defects with vertical walls (6, 8, 10, 12, 14, and 16 mm). These defects were created using OATS devices in the medial and lateral femoral condyles. The well-shouldered lesions were then transformed into beveled lesions by using a countersink and the loading was repeated. Also the center to peak pressure distance (the distance from the measured diameter of the defect to the location of highest pressure) in each compartment was calculated. The vertically oriented osteochondral defects have a cylindrical shape and therefore retain the same diameter throughout. The beveled defects have a conical shape and therefore have an outer diameter that is larger than the inner diameter by 2 mm. An important comparison is the difference in center to peak pressure distance between well-shouldered and beveled defects when using outer diameter as a metric (e.g. 10 mm beveled with 12 mm outer diameter vs. 12 mm well shouldered). In the lateral condyle 4 out of the 6 test pairs showed an increased distance from well shouldered to beveled lesions, with an average distance of 0.2174 mm. In the medial condyle 5 out of 6 test pairs showed an increased distance from well shouldered to beveled lesions, with an average distance of 0.5231 mm. Beveled lesions were observed to act more like well-shouldered lesions of a larger size. (10 mm beveled acted more like a 12 mm well shouldered) This comparison demonstrated in beveled lesions the more critical value was the outer diameter. The outer diameter of beveled defects in knee articular cartilage is a better predictor of pressure distribution on surrounding cartilage than the inner diameter. Therefore, beveled defects of the articular cartilage in the human knee should be evaluated and treated based on their outer diameter measurement.