Stress Reactions to the Plantarolateral Condyles of MtIII in UK Thoroughbreds: 26 Cases

Stress remodeling is a normal physiological response of bone. This is induced by changing forces as a result of increased exercise.1 A stress reaction represents an extreme of remodeling in which this physiological response becomes pathological. This term describes a bone injury that is detectable scintigraphically but not radiographically.2,3 A stress reaction within the plantarolateral condyle of MtIII describes the pathological condition occurring at this anatomical site. In more advanced cases there can be subchondral bone collapse with resultant disruption to the integity of the overlying cartilage.4 Stress reactions to the plantarolateral condyle of MtIII are often bilateral. They are also commonly seen in the McIII. Affected horses may display a poor action as opposed to an overt lameness. This phenomenon appears to be related to the bilateral presentation. Histological studies of normal metacarpal and metatarsal condyles consistently show an increase in bone density or hypermineralization in the plantarolateral and plantaromedial condyles.5 This increase in bone density reduces dramatically as the sagittal ridge is approached. It has been proposed that this adjacent variation in bone density and inherent variations in mechanical properties may predispose to parasagittal condylar fractures.5 This stress reaction involving the plantarolateral condyle of MtIII appears to show similarities to stress fractures in other regions of the body. The cyclical nature of fast exercise results in the repeated bone damage exceeding the reparative capacity of the bone tissue. A significant difference in this region is the close anatomical relationship between bone and articular cartilage. This appears to affect the course and prognosis when compared with a diaphyseal stress fracture. Previous studies have shown the articular cartilage of the plantar condyle to show gross abnormalities.5,6 It is postulated that this cartilage degeneration occurs secondary to subchondral bone pathology.4 The term traumatic osteochondrosis has been used to describe this observation.4