Three dimensional structure of the distal condyles of the third metacarpalbone of the horse

This study examined the three-dimensional (3D) microarchitecture of regions of the equine third metacarpal bone (McIII) commonly involved in distal co ndylar fractures. Limbs were obtained from Thoroughbred horses (neonates to age 24 years) destroyed for inoperable fractures and a variety of other co nditions. Beams, blocks and sections were cut in the principal axes, some e mbedded in PMMA and others examined unembedded, Several methods were used t o study the 3D structure, including conventional and confocal optical micro scopy, scanning electron microscopy (SEM) and radiography, The mineralised articular cartilage tends to cleave in the sagittal plane. Proximal to the subchondral bone, the main trabeculae are robust plates running in the sagi ttal direction with less significant mediolateral connections. Small blood vessel canals lie inside the sagittal plates, This structure gives maximum strength and protection in the sagittal plane in which the bone rotates, bu t offers minimal resistance to fracture propagation in this plane. The anat omical course of the common distal condylar fractures of the third metacarp al bones can be explained by underlying anisotropic structural features of the mineralised tissues.