A chondral modeling theory revisited.

The mechanical environment of limb joints constantly changes during growth due to growth-related changes in muscle and tendon lengths, long bone dimensions, and body mass. The size and shape of limb joint surfaces must therefore also change throughout post-natal development in order to maintain normal joint function. Frost's (1979, 1999) chondral modeling theory proposed that joint congruence is maintained in mammalian limbs throughout postnatal ontogeny because cartilage growth in articular regions is regulated in part by mechanical load. This paper incorporates recent findings concerning the distribution of stress in developing articular units, the response of chondrocytes to mechanically induced deformation, and the development of articular cartilage in order to expand upon Frost's chondral modeling theory. The theory presented here assumes that muscular contraction during post-natal locomotor development produces regional fluctuating, intermittent hydrostatic pressure within the articular cartilage of limb joints. The model also predicts that peak levels of hydrostatic pressure in articular cartilage increase between birth and adulthood. Finally, the chondral modeling theory proposes that the cell-cell and cell-extracellular matrix interactions within immature articular cartilage resulting from mechanically induced changes in hydrostatic pressure regulate the metabolic activity of chondrocytes. Site-specific rates of articular cartilage growth are therefore regulated in part by the magnitude, frequency, and orientation of prevailing loading vectors. The chondral modeling response maintains a normal kinematic pathway as the magnitude and direction of joint loads change throughout ontogeny. The chondral modeling theory also explains ontogenetic scaling patterns of limb joint curvature observed in mammals. The chondral modeling response is therefore an important physiological mechanism that maintains the match between skeletal structure, function, and locomotor performance throughout mammalian ontogeny and phylogeny. Copyright 1999 Academic Press.