34th Annual Scientific Meeting proceedings

Introduction
Recently, the central (CTB) and third tarsal bones (TIII) of 23 foals were examined using micro-computed tomography. Radiological changes including incomplete ossification and focal defects interpreted as osteochondrosis were detected in 16/23 cases. The geometry of the osteochondrosis defects suggested that they were the result of vascular failure, but this must be validated. The study aim was to examine CTBs and TIIIs from the 16 cases histologically to describe tissues present, cartilage canals and pathology including suspected osteochondrosis lesions. 

Materials and Methods
Samples from nine males and seven females from 0-150 days old were examined histologically, comprising 11 Icelandic Horses, two Standardbreds, two Warmbloods and one Coldblooded Trotter. 

Results
Until four days old, all aspects of the bones were covered by growth cartilage but from 105 days, the dorsal and plantar aspects were covered only by fibrous tissue undergoing intramembranous ossification. Patent cartilage canal vessels gradually decreased but were present in most cases up to 122 days old, and absent in the next available case at 150 days. Radiological osteochondrosis defects were captured in histological sections from three cases and confirmed to consist of necrotic vessels surrounded by ischaemic chondronecrosis (articular osteochondrosis) and areas of retained, morphologically viable hypertrophic chondrocytes (physeal osteochondrosis). 

Discussion/Conclusions
The CTB and TIII grew by both endochondral and intramembranous ossification. The blood supply to the growth cartilage of the CTB and TIII regressed between 122-150 days old. Radiological osteochondrosis defects represented vascular failure, chondrocyte necrosis and retention, i.e., a combination of articular and physeal osteochondrosis.