Magnetic Resonance Imaging, Computed Tomographic and Radiographic Findings in the Metacarpophalangeal Joints of 40 Non-Lame Thoroughbred Yearlings

SIMPLE SUMMARY: Most catastrophic musculoskeletal injuries in Thoroughbred racehorses involve the fetlock region. Magnetic resonance imaging (MRI) and computed tomography (CT) aid detection of potentially serious abnormalities, before they become identifiable on radiographs. However, no data exist in live horses regarding how these injuries develop and progress. The aim of this study was to describe MRI, CT and radiographic findings in the front fetlock of Thoroughbreds entering racehorse training. Forty non-lame yearlings underwent low-field MRI, CT and radiographic examinations of both front fetlocks. The most common finding was a lesion consistent with osteochondrosis (developmental abnormality) in the cannon bone. Of the 33 lesions seen in CT images, about two-thirds were detected on radiographs. In the majority of the cannon bones, there was some increase in bone density and enlargement of vascular channels, consistent with the bone’s response to exercise. Small defects in the joint surface were seen in both the cannon bone (11 limbs) and the long pastern bone (19 limbs) which may indicate developmental abnormalities or subtle injuries. In conclusion, variants or abnormalities occur in the cannon bone in the fetlock region of yearling Thoroughbreds and may reflect early changes as the bone adapts to exercise or changes secondary to osteochondrosis. ABSTRACT: Most catastrophic injuries in Thoroughbred racehorses involve the fetlock. There is no description of comparative imaging in Thoroughbreds entering racehorse training. The aim was to describe MRI, CT and radiographic findings in the metacarpophalangeal joint of non-lame Thoroughbred yearlings. Forty Thoroughbreds underwent low-field MRI, fan-beam CT and radiographic examinations of both metacarpophalangeal joints. Images were assessed subjectively. A hypoattenuating lesion of the sagittal ridge of the third metacarpal bone (McIII) was identified in 33/80 limbs in CT reconstructions. Cone-shaped mineralisation in the sagittal ridge was detected in MR images (n = 17) and in CT images (n = 5). Mild hyperattenuation was common in trabecular bone in the dorsomedial (36/80) and palmarolateral (25/80) metacarpal condyles in CT reconstructions. A focal lesion in the subchondral bone was seen in the proximal phalanx (n = 19) and in McIII (n = 11). Enlarged vascular channels were detected in the metacarpal condyles in 57/80 limbs and in the proximal sesamoid bones in all limbs. Signs of bone modelling are seen in yearling Thoroughbred fetlocks. Sagittal ridge lesions were common and are likely associated with osteochondrosis or other developmental osteochondral defects. Focal lesions in the subchondral bone of McIII and proximal phalanx can indicate developmental abnormalities or subtle subchondral bone injuries.