HEALING AND REMODELING OF ARTICULAR INCONGRUITIES IN A RABBIT FRACTURE MODEL

Healing of 0.5 or 1.0-millimeter step-off defects associated with disp laced intra-articular fractures of the medial femoral condyle was exam ined in fifty-four adult New Zealand White rabbits. The rabbits were t reated with either immobilization for three weeks, intermittent active motion, or continuous passive motion for seven days. At twelve weeks, the healing and remodeling of the step-off defects were examined with use of contact-pressure maps on pressure-sensitive film, light micros copy (with hematoxylin and eosin or safranin-O staining), and scanning electron microscopy. Macroscopically, the sharp profile that had been present initially with both sizes of step-off defect had rounded off; however, there was less residual incongruity with the 0.5-millimeter step-offs than with the 1.0-millimeter step-offs. Among step-off defec ts of the same size, the method of treatment had no discernible effect on the macroscopic appearance of the surface of the joint. With fresh step-offs (the control group), the contact pressure of the cartilage on the elevated side was approximately three times greater than that a t a distance from the step-off. On the depressed side, an unloaded zon e extended approximately three times the height of the step-off, with an average width of 3.4 millimeters for the 1.0-millimeter step-offs a nd 1.6 millimeters for the 0.5-millimeter step-offs. After healing and remodeling, the unloaded zone still averaged 2.5 millimeters in width for the 1.0-millimeter step-offs but had decreased to only 0.35 milli meter in width for the 0.5-millimeter step-offs. For seven of the nine 0.5-millimeter step-offs, the contact pressure in the previously unlo aded zone ranged from 0.5 to 1.5 megapascals, with a mean of 0.8 megap ascal (40 per cent of the normal mean contact pressure at this locatio n). Under light microscopy, the cartilage on the elevated side of the healed step-offs had decreased in thickness, was displaced toward the defect and tapered toward the depressed side, and ended in a hypocellu lar tissue flap. In contrast, the cartilage on the depressed side had thickened as a result of hyperplasia of the chondrocytes and hypertrop hy of the cartilage and had failed to establish continuity between the sides of the defect. There was a marked increase in the subchondral v ascular bed and re-establishment of the subchondral plate. With the ex ception of the aforementioned hypocellular tissue flap, safranin O sta ined the cartilage on both levels of the step-off uniformly, which ind icated the absence of glycosaminoglycan depletion. With the use of sca nning electron microscopy, the 1.0-millimeter step-offs were seen to h ave severe fibrillation of the cartilage on the central portion of the condyle on the elevated side, peripheral synovial invasion, and expos ure of subchondral bone. In contrast, the 0.5-millimeter step-offs had healing at the extremes of the osteotomy site, only mild fibrillation of the cartilage on the central portion of the condyle on the elevate d side, and no synovial invasion or exposure of subchondral bone. Thes e findings were comparable in each treatment group. CLINICAL RELEVANCE : Cartilage and subchondral bone exhibit multiple adaptive mechanisms that may partially restore congruity and load transmission to a joint with a step-off defect. The healing and remodeling patterns of step-of fs, demonstrated in this animal model, are substantially different fro m those reported for full-thickness lesions; this suggests that there should be different guidelines for clinical treatment. For example, co ntinuous passive motion did not appear to improve healing or remodelin g compared with the results seen with free activity, particularly for large step-off defects. In general, concern regarding the ability of t he step-off to remodel successfully should increase as the size of the step-off exceeds the local thickness of the articular cartilage.