Equine joint disease in the light of new developments in articular cartilage research

Abstract

Lesions of articular cartilage form a prominent source of wastage in athletic horses. This is largely due to the fact that repair of these lesions is known to be insufficient in mature individuals, which leads to chronic pathology and eventual invalidity. Recent research on basic biology and developmental aspects of equine cartilage has led to a more profound insight into the fundamental processes that underlie and influence joint pathology. This paper presents a synopsis of this work, placed in the context of related studies in other species. From biochemical studies on all main components of the extracellular matrix (ECM) of articular cartilages (collagen, proteoglycans and water), it has become clear that in mature individuals a marked topographic heterogeneity exists throughout the joint surface. Biomechanical studies on the tissue level have shown that there is a clear relationship between biochemical tissue characteristics and biomechanical performance, i.e. response to loading. On the other hand, biomechanical studies at the level of the entire joint show a close correlation between degree of loading under physiological conditions and biochemical composition. Main finding is that collagen content and degree of crosslinking is high at sites that are subjected to high, but intermittent loading, whereas proteoglycans are prominent at more constantly, but less heavily loaded sites. Studies into the developmental aspects of biochemical heterogeneity have shown that such heterogeneity does not exist at birth, but develops during early life. This development can be influenced (temporarily or permanently depending on the biochemical parameter) through manipulation of the exercise level the young individual is subjected to. From these findings a novel concept of the relationship of cartilage function and structure and of the development of the biochemical and biomechanical characteristics of the tissue emerges. In this concept it is the degree of exercise in the immature animal that largely determines the biochemical characteristics of the ECM of articular cartilage. In the mature individual no essential alterations of the collagen part of this ECM can be incited anymore by any means due to the rapid decrease in ECM (especially collagen) metabolism that starts after birth and brings the remodelling rate sharply down during the first year of life. As the biochemical characteristics of the collagen network are intricately linked to biomechanical behaviour, and thus to stress resistance, the foundation of the ultimate quality of articular cartilage is thus irreversibly laid at young age. New data on the developmental orthopaedic disease osteochondrosis (OC), in which a clear distinction is made between the pathogenesis of lesions and the ensuing repair process, are in line with this concept. In OC, lesions have been shown to be capable of complete repair until a certain age only. After that age, the decrease in cartilage metabolism precludes full repair and clinical lesions will remain. This concept of functional adaptation of articular cartilage as a once in a lifetime process strongly underlines the importance of the early postnatal period for the development of sound articular cartilage. A practical consequence is that young equines should be given ample opportunity for exercise, a requirement that conflicts with some developments in modern stud management. However, exercise should not be excessive as there are strong indications that too much or wrongly balanced exercise may be deleterious as well and may even have negative long-term effects.