Corticosteroids Cause Cartilage Deterioration in Exercised Horses
Corticosteroid injections into equine (horse) joints cause similar effects as those in the rabbit. Equine research has been consistent in that corticosteroids cause a breakdown of the cartilage matrix and protein synthesis.50-52 It is especially damaging to pony foals where corticosteroids caused joint damage either at the joint surface or deep within the cartilage. Signs of surface deterioration included edema, fibrillation, enlargement of lacunae, pitting, and shredding and erosions of the cartilage. Cartilage ulceration and fracture was common. Glycosaminoglycan content of articular cartilage decreased by 55% in three months. Corticosteroids inhibited articular chondrocyte metabolism which initiated cartilage degeneration. Surface destruction and osteochondrosis dissecans followed continued mechanical stress of compromised cartilage.53 (See Figure 7.)  In another study, articular cartilage and chondrocytes obtained from young adult horses ages 1.5–3.5 years of age were subjected to the corticosteroid methylprednisolone. Chondrocyte cytotoxicity was found as the steroid concentration was increased. This coincided with a decreased and altered chondrocyte expression of matrix proteins, which the authors felt likely contributed to the pathogenesis of corticosteroid-induced cartilage degeneration.54 Researchers at the University of Montreal showed that repeated intraarticular injections into the radiocarpal joint of horses free of OA, compared to controls, induced the breakdown of articular cartilage. Specifically, the biomarkers for proteoglycan and collagen breakdown were significantly elevated in the corticosteroid injected joint fluid.55 In a similar experiment, chromatographic analysis of joint fluid in corticosteroid injected joints showed fragments of the articular cartilage aggrecan. They were significantly elevated in the steroid injected joints, compared to control joints. The authors summarized their findings by saying, “these results indicate that the repeated use of intraarticular methylprednisolone acetate leads to potentially harmful inhibition of procollagen II synthesis and an increased release of degradation products of the proteoglycan aggrecan from articular cartilage.”56  To see what happens when you inject steroids into a joint and then exercise the joint, researchers at Kansas State University injected the contralateral middle carpal joints of healthy horses with either corticosteroid or diluents (control). The results showed that steroid injected cartilage was 24% thinner and had a 97% decrease in compressive stiffness. The authors concluded that repetitive intraarticular administration of corticosteroid in exercising horses alters the mechanical integrity of articular cartilage.57 A summary of the effects of the intraarticular corticosteroids as denoted by the above research can be seen in Table 1. (See Table 1.)
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