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Low-grade right fore fetlock lameness in a 3 year old racehorse.

Fig 1 A dorsopalmar radiograph of the right metacarpophalangeal joint taken as part of the initial radiographic series the day before referral for MRI. No flexed dorsopalmar radiograph was included in the initial study.

Diagnostic interpretation:

Focal T1/T2* hyperintensity is present within the subchondral bone plate of the lateral metacarpal condyle consistent with an articular defect (focal subchondral bone collapse). This defect is not detected on STIR images likely due to volume averaging. The STIR image clearly shows well defined zone of fluid-based signal intensity material (yellow arrowhead) deep to the articular defect, which is present, though less well visualised on T1 and T2* weighted sequences. A wider zone of T1 hypointensity, T2* heterogeneous intensity and STIR hyperintensity is present with in the lateral metacarpal condyle consistent with a mixed pattern of fluid and increased bone mineral density. Focal T1, T2* and linear STIR hyperintensity located toward the proximal margin of the diffuse zone of fluid-based signal represents a vascular channel.

Fig 2: T1 frontal, T2 * transverse oblique and STIR frontal MR images of the right metacarpophalangeal joint (lateral is to the left). A focal defect in the subchondral bone plate is visible on the T1 and T2* weighted images (red arrowhead) which is not clearly visualised on the STIR images. A zone of increased signal on all imaging sequences (yellow arrowheads) is present within the more generalised zone of condylar sclerosis. A prominent vascular channel is positioned more proximally within the cancellous bone (green arrowheads)

Conclusions:

An articular defect in the palmar aspect of the lateral metacarpal condyle with a deeper zone of subchondral bone collapse / necrosis / surrounded by a wider zone of condylar densification (sclerosis) and interosseous fluid signal (inflammation / haemorrhage) common to palmar osteochondral disease (POD).

The radiographs were repeated following the MRI examination to attempt to localise the lesion radiographically. Several flexed dorsopalmar radiographs of the right metacarpophalangeal joint were acquired. This revealed a focal lucent zone in the lateral metacarpal condyle, the conspicuity of which changes significantly with alterations in the angle of the projection. This area is poorly visualised on weight-bearing dorsopalmar radiographs of the metacarpophalangeal joint due to the overlying proximal sesamoid bones, which illustrates the importance of including several flexed dorsopalmar projections as standard when radiographing the metacarpo/metatarsophalangeal joints of the racing Thoroughbred.


Fig3 Flexed dorsopalmar radiographs of the right metacarpophalangeal joint. The limb is brought forward onto a block at a height of 25-30cm. The joint is not in full flexion, the third metacarpal bone is perpendicular to the floor with the cassette is rested on the heel bulbs. The image on the left is with the tube head angled 20o distoproximally and the images on the right with the tube head angled 20o proximodistally. This allows more complete evaluation of the articular surface. A radiolucent region in the lateral metacarpal condyle (white arrow) is clearly visible on both images, more readily in the image on the right. The vascular channel seen on the MR images is also seen radiographically (green arrowheads).

Update:

The joint was medicated with a short acting corticosteroid preparation and the colt was given a period of time out of full training for the bone to remodel as far as possible. The colt returned to full training within 6 weeks with close clinical and radiographic monitoring of the joint advised.

A meeting of the international Fracture Risk Assessment for Thoroughbreds (FRAT) Group in March 2020, of which the author is a member, concluded that “Currently, radiography remains the most important imaging modality in fetlock bone risk assessment. With wide availability and the knowledge gained by more advanced imaging techniques refining the most appropriate projections to use; radiography represents a relatively untapped resource that through education of primary care vets could immediately have a profound impact on injury mitigation”.

An action point of that meeting was to produce a field guide for The Flexed Dorsopalmar / Plantarodorsal Radiographic Projection of the fetlock joint. Which is now available as a free download via the British Equine Veterinary Association Bookshop website.

References:

Editorial Equine Vet J. 2020 Jul;52(4):482-488.

Science-in-brief: Risk assessment for reducing injuries of the fetlock bones in Thoroughbred racehorses. Victoria A Colgate, FRAT Group; Celia M Marr

PMID: 32525619 doi: 10.1111/evj.13273.

Ramzan PHL. The BEVA Field Guide to: The flexed dorsopalmar

radiographic projection of the fetlock. https://www.beva.org.uk/Guidance-and-Resources/Imaging

Accessed April 22, 2020. Available as a free download.