The effect of body mass and physical activity on the development of guinea pig osteoarthrosis.

We quantitatively evaluated the morphological and biochemical effects of body mass and physical activity on spontaneously developing guinea pig osteoarthrosis (OA). 6-month-old male guinea pigs were allocated to 3 groups: controls (C) living under standard laboratory conditions with food ad libitum; mobilized animals (M) allowed unrestricted motion in large rooms with food ad libitum; and a diet group (D) weight-matched with the M-group. At 9- and 12-months of age they were killed and the left proximal tibia was processed for quantitative histology and the right tibial articular cartilage for analyses of glycosaminoglycan (GAG). OA mostly occurred on the medial condyle's central part not covered by the meniscus. The thinnest cartilage was found in 12-month-old M-animals (M12), which had 60% of the central cartilage surface affected by lesions that extended down to the mineralized cartilage. C12 had 25% exposed mineralized cartilage and D12, 2%. Subchondral bone density followed the loading patterns--the highest in M12 and lowest in D12. M12 had the lowest cartilage GAG concentrations. Load appears to be a key external factor in guinea pig OA. An increase in physical activity may be chondroprotective in the early phase, but harmful when fibrillations eventually have developed. This is underscored by the extensive OA changes in M12, although these animals weighed about the same as D12 (which had the least extensive OA). Therefore, a reduction in body mass seems to retard the progression of OA in animals, which are mainly subjected to a static load (C12 and D12), but not sufficiently in animals with a more dynamic load (M12). Changes in morphological patterns are paralleled by changes in GAG concentration, which probably reflect the metabolic capacity of the cartilage.