Abstract
The carpus is commonly injured in racehorses and is one of the joints most affected by osteochondral damage. Hence, carpal lameness is a major cause of delays in training for Thoroughbred racehorses. Carpal conformation is considered to be a predisposing factor for the joint damage. However, few conformational studies have been reported that evaluate the carpal morphology. Consequently, the established parameters to quantify carpal conformation from these studies are limited and insufficient to represent such a complex joint. Radiography is considered to be the most accurate diagnostic tool to evaluate limb deformities including the carpus. From the five standard radiographic views of the carpus, the dorsopalmar view (DP) is the most relevant view to assess carpal deformity. Therefore, the centre aim of the present study was to identify a relationship between carpal morphometry and pathology in racing Thoroughbreds using DP radiographs. The initial phase of the study was to identify a DP view for the carpus at zero° (ZDP). The view was established by using two landmarks to evaluate the vertical and horizontal deviation of radiographs from the ZDP. Carpal morphometrical parameters were then developed. These were 26 angular parameters, individual and correlated, and 11 ratio parameters. Identifying any of these parameters was mainly based on the ability to identify its landmarks consistently. The parameters included most of the significant features which showed a high pathological incidence and morphometrical variations between individuals. Effects on the 37 carpal parameters of carpal rotation around vertical and horizontal axes of the carpus were tested when 195 radiographs from 15 carpi from different breeds were measured. All individual and most of the correlated angular carpal parameters showed significant changes during the vertical rotation. All correlated and the majority of the individual angles showed significant alterations during the horizontal rotation. The majority of ratio parameters were less affected by the carpal rotations. A number of carpal morphometrical comparisons were performed between sound and damaged Thoroughbred racehorses using the carpal parameters. Three reliable carpal parameters showed significant differences between normal horses and horses that were damaged at the middle carpal joint. They were the radial metaphyseal-radial facet C3 angle (Ra.meta-Rf.C3 angle), the radial carpal-radial facet C3 angle (RCJ-Rf.C3 angle) and the second carpal bone height to medial carpus ratio (C2/Mh ratio). The effects of ex vivo loading were tested in two carpi which were loaded and radiographed before and during loading. The carpus with good carpal conformation showed more stability during loading than the carpus which had a poor carpal conformation. In conclusion, a ZDP orientation in DP radiographs of the equine carpus was identified. Vertical and horizontal limb rotations had significant effects on most carpal parameters. Thoroughbred racehorses with an Ra.met-Rf.C3 angle smaller than 191.15 ± 1.84, an RCJ-Rf.C3 angle smaller than 188.68 ± 1.27 and a C2/Mh ratio of greater than 0.37 ± 0.01 seemed to have an increased susceptibility to carpal injuries. The carpus with preferable conformation was more stable during ex vivo loading of the joint.