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Spontaneous degeneration of the knee joint cartilage in male Hartley guinea pigs was studied by light microscopy in animals aged S 1 8 months. Unilateral focal degeneration, characterized by chondrocyte death and proteoglycan loss with surface fibrillation, was observed on the medial tibial plateau in 2 of 5 guinea pigs that were 3 months old. The incidence and severity of the lesions increased with age, and by 12-18 months of age, all animals had moderate-to-severe degeneration of the medial tibial plateau, femoral condyle, and meniscus. Lesions were not present on the lateral aspect of the knee joint in any of the animals.Naturally occurring and experimentally induced cartilage degeneration has been described in a variety of laboratory and domestic animal species (1-3). Recently, several different models of experimentally induced osteoarthritis have been described in mature guinea pigs, but in none of these has mention been made of any significant incidence of spontaneous cartilage degeneration (4-9).Over the course of experimentation with various surgical and nonsurgical methods of inducing cartilage degeneration in mature male and female Hartley guinea pigs, we observed a high incidence of spontaneous cartilage degeneration of the medial tibial plateau, meniscus, and femoral condyle. This occurred not only in the contralateral joints of animals

Hartley albino guinea pigs develop spontaneous osteoarthritis (OA) of the knee joint. A study was done to determine the importance of body weight in the pathogenesis of this disease. Two groups of 20 male guinea pigs each were maintained on the same diets. The control group was allowed ad libitum feed consumption and the other group was restricted to 30-35 gm of feed per day. Ten animals from each group were killed at 9 months of age to evaluate histologic features of the knee joints. The severity of the OA lesions was reduced by 40%, in conjunction with a 28% decrease in body weight, in the diet-restricted group. The remaining animals were killed at 18 months of age. Those in the diet-restricted group had a 56% reduction in severity of lesions, with a 29% decrease in body weight. These results indicate that body mass in guinea pigs, as in humans, is an important predisposing factor for the development of spontaneous OA of the knee.Osteoarthritis (OA) affects nearly 10% of the population over the age of 60, with knee OA being one of the most common causes of pain and disability in the elderly (1). Various predisposing factors including conformational abnormalities, knee injury, and obesity have been associated with the disease (2-7). A body mass index >30 has been strongly associated with bilateral knee OA (8).Spontaneous OA of the knee joint commonly occurs in Hartley albino guinea pigs (9,lO). The disease progression is similar to that in other laboratory animals that develop OA of the knee joint (1 1-13), and the lesions resemble those of medial compartment knee OA in humans (14). The cause or causes of this naturally occurring OA in guinea pigs are unknown. However, laboratory-raised guinea pigs on ad libitum feeding regimens typically become quite obese with increasing age (10). The incidence and severity of the degenerative changes in their knee joints also increase with age. We hypothesized that obesity might be an important predisposing factor in the development of spontaneous OA in guinea pigs and questioned whether diet restriction of sufficient magnitude to significantly reduce body weight compared with that of ad libitum-fed controls, yet maintain good health, would influence disease progression. Our findings are presented here. MATERIALS AND METHODSExperimental animals. Forty 30-day-old male Hartley albino guinea pigs from Charles River (Wilmington, MA) were housed individually in 38 x 34 x 18-cm wire cages. The animals were assigned to control (n = 20) and diet-restricted (n = 20) groups. Food (Purina Guinea Pig Chow #5025; Ralston Purina, Richmond, IN) and water were available for ad libitum consumption by the control animals from the beginning to the end of the study. Guinea pigs in the diet-restricted group were fed 30 gm of food (Purina Guinea Pig Chow #5025) from day 42 to day 180, and then 35 gm for the remainder of study. The body weights of all guinea pigs were recorded at approximately monthly intervals.

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