INFLUENCE OF METHYLPREDNISOLONE ACETATE ON OSTEOCHONDRAL HEALING IN EXERCISED TARSOCRURAL JOINTS OF HORSES

Objective-To evaluate joint function and healing of surgically created full-thickness articular cartilage defects in exercised horses after intra-articular administration of methylprednisolone acetate (MPA; 120 mg) and sterile saline solution in the contralateral limb. Design-Exp erimental investigation. Sample Population-12 healthy, sound, radiogra phically normal horses with induced full-thickness osteochondral lesio ns on the medial and lateral trochlear ridges of the tali. Procedure-T wo 8.4-mm-diameter full-thickness articular cartilage lesions were cre ated in each tarsocrural joint (12 horses [24 tarsocrural joints]); 1 was in a weight-bearing (WB) position and the other in a less weight-b earing (LWB) position. Each horse was maintained on a standardized exe rcise protocol (stall rest, days 0-6; walking, days 7-12; and treadmil l, days 13-42) and evaluated throughout the study for changes in joint circumferences, synovial fluid, radiographs, lameness, and scintigrap hy. 6 horses were euthanatized on day 42, and 6 on day 180. Gross morp hometric assessment was performed, using an image analysis system on a projected color slide of the defect. The type of repair tissue, based on gross appearance, was expressed as a percentage of the total defec t for each osteochondral defect. Histochemical assessment was performe d, using safranin-O staining for proteoglycans and an image analysis s ystem to express the area of stain uptake. Histomorphometric assessmen t was performed on H&E-stained sections, using an image analysis syste m. The repair tissue filling the defect was categorized as to tissue t ype and expressed as a percentage of the total defect area. Synovial m embrane specimens were assessed semiquantitatively on H&E-stained sect ions for changes in character. Significance was established at P < 0.0 5, Results-Joint circumference was significantly increased in the sali ne, compared with the MPA-treated, limbs on days 7, 12, and 42. Synovi al fluid WBC counts were significantly increased in the MPA-treated li mbs on day 42. Gross osteochondral defects had a greater percentage of mature repair tissue in saline-treated joints (30.8% LWB, 23% WE), co mpared with MPA-treated joints (0% LWB, 0% WE) at 42 days. Histomorpho metric assessment of the repair tissue indicated significant differenc es with regard to the quality of repair in the saline-treated (34.% fi brous tissue LWB, 19.4% fibrous tissue WE) versus MPA-treated (2.5% fi brous tissue in LWB and WE) joints at 42 days, Microscopically, the pe rcentage of fibrocartilage in the LWB (MPA, 23.7%; saline, 24.8%) was significantly greater than that in the WE (MPA, 14.6%; saline, 15.4%) site al day 180; The MPA-treated limbs had greater villous hyperplasia , edema, ana extent of inflammation within the synovial membrane than did saline-treated limbs (days 42 and 180), Conclusion-MPA inhibits th e development and maturation of repair tissue at 42 days and incites p otential long-term (180 days) detrimental synovial membrane inflammati on. Furthermore, a single dose of MPA does not cause long-term detrime ntal effects (180 days) in quality of repair tissue.