Abstract
The carpus is one of the most complicated regions in the limbs. The term carpus or wrist refers to the carpal bones, joint and ligaments. The seven constant carpal bones are arranged in two rows. The proximal row includes the radial, intermediate, ulnar and accessory carpal bones and articulates with the distal end of the radius to form the antebrachial joint. The distal row includes the second, third and fourth carpal bones, and articulates with the proximal carpal row to form the intercarpal joint and with proximal end of the metacarpal bones to form the carpometacarpal joint. The accessory and inconstant first carpal bones are not part of the column of bones that directly supports the body weight. Although the carpal bones have been described in a general manner, descriptions of variations between equine breeds is still insufficient, neither are there descriptions available for normal variations between the two limbs within an individual. Therefore, the purpose of this study was to identify the important morphological variations of the carpal bones between and within the Thoroughbreds, Standardbreds, Quarter Horses (large breeds) and the Ponies (small breed). Paired carpi were collected from twenty eight horses. Ten horses were Thoroughbreds, thirteen were Ponies, three were Standardbreds and two were Quarter Horses. The horses were killed for non-orthopaedic reasons. Carpi were boiled at 98.5C˚ for 48 hours. The right and left carpus from each pair were treated individually to identify the presence of the inconstant first carpal bone. The weight of each carpal bone was measured after air drying for 24 hours. The bones were then heated at 49.5C˚ for 8 hours, then re-weighed to give the dry weight. A complete morphological description was reported for the bones to include the general and specific features within and between the breeds. A number of measurements were taken including lateromedial width, dorsopalmar distance, proximodistal height, relative density and volume. The ANOVA test was used to analyze the statistical data. This study showed that there were remarkable variations in the articular facets of the intermediate, ulnar, third and fourth carpal bones between the Ponies and the other breeds. The relative density of all the carpal bones except for the intermediate carpal bone, was significantly higher in the Ponies. The proportionate volume of the intermediate and right third carpal bones was significantly larger in the Ponies than the others. The spine-like structure of the palmar end of the proximal articular surface of the intermediate bone was more dominant and proportionately larger in the large breeds than in the Ponies. The palmar tubercle of the palmar surface of the ulnar carpal bone was also more dominant and proportionately larger in the large breeds than in the Ponies. The inconstant first carpal bone was proportionately larger in size in the Ponies than in the large breeds. There were considerable morphological variations in the carpal bones between the Thoroughbreds, Ponies, Standardbreds and Quarter Horses. Further studies are needed to identify the reasons and mechanical effects of these variations.