Ultrasonographic evaluation of sites of osteochondrosis in young Thoroughbred horses

Radiography is the main imaging modality used in the diagnosis of osteochondrosis (OC) in horses. Radiography relies on the detection of subchondral bone changes, whereas OC may only affect the articular cartilage. Radiography also involves the exposure of the horse and any human bystanders to ionising radiation. Contrast arthrography, particularly computed tomographic arthrography (CTA) is a more invasive procedure compared to computed tomography which requires injecting positive contrast into the joint, as well as using ionising radiation, usually necessitating general anaesthesia and the availability of CTA is limited to larger centres. Ultrasonography (US) is a commonly used imaging modality in equine veterinary practice, particularly in musculotendinous imaging. Despite the availability of good quality, reasonably priced US equipment, equine veterinary practitioners underutilise US in the assessment of OC. It was hypothesised that US is a valid screening tool in the assessment of the common sites of OC of juvenile equine articular cartilage and that US results would agree with radiographic and CTA imaging findings. Firstly, a systematic review of the literature was performed to assess the use of ultrasound in the identification of osteochondrosis in the horse, compared to arthroscopy, imaging or post mortem findings. There was marked variation in the studies between study population, breed, horse age, number of horses, experience of ultrasonographers, reference standard used and risk of bias. Overall there was no strong evidence confirming the diagnostic accuracy and validity of US in the detection of OC in horses.The ultrasonographic technique for articular cartilage assessment is poorly described in the peer reviewed literature, with only a few descriptions of OC lesions. A screening protocol for young horses was described in step-by step detail. Two additional observers with various US experience performed the examination twice, in a timely manner. The ultrasonographic examination was shown to be a viable examination which can be performed in the field using commonly available equipment, yielding high quality images. The ultrasonographic screening protocol was performed with 44 Thoroughbred horses enrolled from stud-farms. Each horse also had a concurrent “yearling set” of screening radiographs taken, and 26 of these horses also had an additional set of radiographs acquired for sale approximately six months later. The initial ultrasonographer and a second reviewer, both board-certified radiologists, assessed the anonymised images. Overall, ultrasonographic screening showed substantial agreement with concurrent and yearling sale radiographs, as well as moderate interobserver agreement. In cases of OC, articular cartilage can show pathologic thickening. A study of cartilage thickness measurement agreement between US and CTA was undertaken using 33 cadaveric metacarpophalangeal joints (MCPJs) from 18 young horses. US showed the strongest agreement with histology at the sagittal ridge, which is the main site of OC lesions in the horse, and strong interobserver agreement between US and histology measurements.