Radiographic assessment of bone morphometry, alignment and loading stability of the equine carpal joint in racehorses

Abstract

The carpus is the most complex joint of the equine forelimb and lameness secondary to carpal injuries accounts for up to 41% of forelimb lameness in racehorses. However, despite suggestions that carpal conformation is a contributory factor to the orthopedic health, performance and forelimb lameness, few attempts have been made to objectively measure radiographic variations of carpal bone morphometry and alignment in horses due to insufficient measurable carpal parameters. Furthermore, non-physiologic loading of carpal bones is believed to result in osteochondral fractures, ligament ruptures and axial instability of the equine forelimb; however, the mechanism of carpal damage due to non-physiologic loading of the carpus is largely unknown. The aims of this thesis were: 1. To investigate the radiographic anatomy of the equine carpus and develop reliable measurable parameters that can be used to consistently and objectively measure carpal conformation in horses 2. To provide baseline data for the developed parameters from radiographs of a group of racing thoroughbred horses. 3. To use some of the developed parameters to assess the stability of the equine carpal joint under incremental load. 4. To observe the roles of the equine carpal ligaments to the load redistribution within the carpus and stability of the carpal joint during axial compressional loads. A pilot study on 6 cadaveric equine forelimbs from 3 adult horses (5.67±2.08 years), was used to investigate the radiographic anatomy of the carpus in “Zero Lateromedial” (ZLM) and “Zero Dorsopalmar” (ZDP) views and 17 measurable parameters with validated anatomical landmarks were developed. Six parameters were developed from the ZLM view and 11 parameters from the ZDP views consisting of angles, ratios and linear measurements. Subsequent studies established: i.) baseline data of these parameters from carpal radiographs of a group of 20 two-year old thoroughbred racehorses in training; ii.) effects of limb postural changes, vertical rotations of radiographic plate, vertical and horizontal rotations of projection angles of primary X-ray beam on the quality of radiographs and carpal measurements; iii.) changes in positions of carpal bones during flexion, extension and incremental load and iv.) a 3-D finite element model of the bony components of the equine carpus. The proximo-distal gliding movement within the carpus enabled transverse movement of the proximal carpal row which in turn allowed the proximal and distal articular surfaces of the radial (Cr), intermediate (Ci) and ulna (Cu) carpal bones to slide into and out of congruity with the distal articular surfaces of the radius and the proximal articular surfaces of the distal carpal row during extension (loading) and flexion. Increased load on the carpus produced carpal hyperextension with measurable radiographic changes in the position and alignment of the carpal bones. A relaxed intercarpal ligament between Cr and Ci (Cr-Ci ICL) during loading, as indicated by decrease in the width of the groove diameter of Cr-Ci ICL (GD.Cr-Ci ICL), signifies minimal or no stretch (strain) on Cr-Ci ICL. This would facilitate absorption and redistribution of concussion forces within the carpal joint during loading, thereby providing a useful mechanism to minimize carpal damage. In conclusion, the carpal bone geomorphometric and loading data, along with information generated on the ligaments of the proximal carpal row in this study, will allow reliable quantitative assessment of carpal conformation and eliminate judgmental errors or variation between observers using subjective visual assessment for the carpus. This data will improve our understanding of carpal biomechanics and pathogenesis of injury. The measurement protocols will require further investigation on large groups of different breeds of horses for wider acceptance, adaptability and validation.