Influence of Exercise and Intra-articular Site on Canals in Articular Calcified Cartilage of Equine Third Carpal Bones

Abstract

The third carpal bone (C3) responds to exercise by adaptive modeling of bone and articular calcified cartilage along the dorsal load path. Canals penetrating articular calcified cartilage, thought to contain vascular tissue, are reported in numerous species. Their significance remains unclear. Our objective was to determine if the number of canals was significantly different in strenuously exercised and control young horses and in a site of intermittent high loading compared to sites sustaining lower habitual loads. Volumetric bone mineral density in the radial facet of C3 of strenuously exercised and gently exercised (control) 19-month-old thoroughbred horses (n ¼ 6/group) was determined by peripheral quantitative computed tomography. The hyaline cartilage was corroded to expose the surface of articular calcified cartilage. The number of canals penetrating the articular calcified cartilage surface in en face scanning electron microscopy images was compared in 4 regions. Volumetric bone mineral density of C3 was significantly greater (P ¼ .004) in strenuously exercised horses. There were 2 morphologically distinct groups of canals and significantly fewer (P ¼ .006) large canals in the dorsal than in the palmar aspect of C3 in control but not in exercised horses. Roughly circular depressions in the articular calcified cartilage surface around apparently forming canals were visible in some samples and have not been previously described in the literature. The canals may be evidence of chondroclastic activity reaching the interface of hyaline and calcified cartilage. Further work is needed to elucidate the relationships between presence of canals and the responses to exercise and to joint disease.

A diagnostic pathologist’s guide to carpal disease in racehorses

“…13,23,46,82 A separate study in a different set of samples confirmed the presence of similar extruded mineralized matrix extending from cracks in the calcified cartilage into the deeper regions of hyaline cartilage, corresponding to sites of superficial hyaline cartilage erosion and fibrillation. 31 Other studies performed in carpal bones of both Thoroughbred and Standardbred racehorses identified subchondral bone sclerosis with increased mineral density, and increases in biochemical markers of bone turnover, corresponding to gross and histologic lesions of cartilage degeneration and osteoarthritis. 22,79 Additional studies attempting to mimic early stages of "naturally occurring" repetitive impact trauma induced by conditioning exercise in 1.5-2-y-old Thoroughbred fillies showed significantly higher bone mineral density in C3, 24 as well as mechanical and morphologic differences in the hyaline and mineralized articular cartilage within sites that withstand high intermittent loads (e.g., dorsodistal Cr and Ci, and dorsoproximal C3).…”

“…13,23,46,82 A separate study in a different set of samples confirmed the presence of similar extruded mineralized matrix extending from cracks in the calcified cartilage into the deeper regions of hyaline cartilage, corresponding to sites of superficial hyaline cartilage erosion and fibrillation. 31 Other studies performed in carpal bones of both Thoroughbred and Standardbred racehorses identified subchondral bone sclerosis with increased mineral density, and increases in biochemical markers of bone turnover, corresponding to gross and histologic lesions of cartilage degeneration and osteoarthritis. 22,79…”

“…Additional studies attempting to mimic early stages of “naturally occurring” repetitive impact trauma induced by conditioning exercise in 1.5–2-y-old Thoroughbred fillies showed significantly higher bone mineral density in C3, 24 as well as mechanical and morphologic differences in the hyaline and mineralized articular cartilage within sites that withstand high intermittent loads (e.g., dorsodistal Cr and Ci, and dorsoproximal C3). 31 48,49 Yet a different study performed in young juvenile Thoroughbreds comparing the effects of pasture exercise versus controlled, conditioning exercise instituted from birth through 18 mo of age on opposing surfaces of cartilage and subchondral bone taken from Cr and C3 identified gross and histologic lesions involving osteochondral junctions and mineralized cartilage within both exercised and control groups, thus suggesting that “early” microscopic osteochondral lesions (e.g., localized disruption of the osteochondral junction, thickening of the mineralized cartilage with matrix texture changes and microcracks) may arise within sites predisposed to developing more advanced osteoarthritis, regardless of exercise program. 37,38 Although much more is known regarding lesions associated with the development of equine osteoarthritis, clearly the results of the studies listed above incite additional questions necessitating further research into the predisposition and pathogenesis of equine osteochondral disease.…”