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 dimen sions, and body mass. The size and shape of limb joint surfaces must theref ore also change throughout post-natal development in order to maintain norm al joint function. Frost's (1979, 1999) chondral modeling theory proposed t hat 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 chon drocytes to mechanically induced deformation, and the development of articu lar 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 hydrostat ic pressure within the articular cartilage of limb joints. The model also p redicts that peak levels of hydrostatic pressure in articular cartilage inc rease between birth and adulthood. Finally, the chondral modeling theory pr oposes that the cell-cell and cell-extracellular matrix interactions within immature articular cartilage resulting from mechanically induced changes i n hydrostatic pressure regulate the metabolic activity of chondrocytes. Sit e-specific rates of articular cartilage growth are therefore regulated in p art by the magnitude, frequency, and orientation of prevailing loading vect ors. 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 lim b joint curvature observed in mammals. The chondral modeling response is th erefore an important physiological mechanism that maintains the match betwe en skeletal structure, function, and locomotor performance throughout mamma lian ontogeny and phylogeny. (C) 1999 Academic Press.