Subchondral bone density on the fetlock joint of the horse. Part 1: Copputertomographic osteoabsorptiometry (CT-OAM)

The fetlock joint (articulatio metacarpophalangea) ploys an exceptional rol e within the limb, as it is the only joint which is in hyperextension in th e standing horse. It is therefore exposed to special loading situations com pared to other joints. The objective of this study is to describe the distr ibution of the subchondral bone density as a measure for the longterm loadi ng of the fetlock joint. The method of Computed Tomographic Osteoabsorptiom etry (CT-OAM, Moller-Gerbi et al., 1989) was used. 45 joint preparations of the fetlock joint from 26 horses of different breeds and sizes, male and f emale were examined. The horses ranged from foal to 30 years of age. The joints were scanned in the sagittal plane in a CT-scanner Somatom AR (S iemens, Erlangen, Germany). Using these CT-data sets the bones of the fetlo ck joint were reconstructed three-dimensionally and the subchondral bone de nsity was displayed topographically in a colour coded picture. In the dorso -distal aspect of the trochlea a broad transversal bond of high bone densit y can be observed. Palmar there are two almost triangular areas of high bon e density at the trochlea. On the articular surface of the, proximal phalan x band shaped areas close to the dorsal and abaxial edges are heavily miner alized. The sesamoid bones display an average level of bone density through out the articular facet. A relatively low bone density can be found in the dorsal and palmar part close to the border of the facet as well as in the a xial area distally. This also applies to the palmar border of the phalangea l articular surface. Bone density increases significantly during growth of the animals. The density distribution of subchondral bone refers to areas o f the articular surface which are exposed to extremely high loading. These are parts of the trochlea which correspond to the proximal phalanx and the proximal sesamoid bone at the angle of stance. CT-OAM is a non-invasive met hod, which allows the investigation of the articular longterm loading. It c an be put to use in the living horse for follow-up studies after surgery or for scientific purposes.