# The Evolution of Lesions on Follow-Up Magnetic Resonance Imaging of the Proximal Metacarpal Region in Non-Racing Sport Horses That Returned to Work (2015–2023)

## TL;DR

This study examines how MRI lesions in the proximal metacarpal region of sport horses evolve over time as they return to work.

## Contribution

The study provides new insights into the MRI evolution of proximal metacarpal lesions in horses returning to soundness.

## Key findings

- Hyperintense signals in the PSL on T1W/T2*W GRE sequences decreased or remained stable in most cases.
- Resolution of McIII hyperintense STIR signals was observed in all patients, with a mean resolution time of 94 days.
- The size of the PSL did not change significantly between initial and follow-up MRI scans.

## Abstract

Proximal metacarpal pain is an important cause of lameness, and its diagnosis can be challenging. In some patients, magnetic resonance imaging (MRI) is needed for the evaluation of the proximal metacarpal region, and it is valuable for diagnosis. A challenge for these patients is to perform follow-up monitoring if the lesion cannot be visualized using radiography and/or ultrasonography, which is where rescan MRIs come into play. Various injuries have been visualized within the proximal metacarpal region, but it is currently unknown how they evolve in horses returning to soundness. This small case series evaluates various features of the proximal metacarpal region on MRI, which appear important to monitor, and includes hyperintense signals within the dorsal collagenous part of the proximal suspensory ligament (PSL) on T1W/T2*WGRE images, STIR hyperintense signals within the dorsal collagenous part of the PSL, and the third metacarpal bone (McIII). Complete normalization of the dorsal collagenous part of the PSL does not appear necessary for a return to soundness. A resolution of the McIII hyperintense STIR signal is expected for horses returning to soundness, and in this population of horses, there was a mean of 94 days (range: 47–202 days). A rescan time frame of 120 days for the proximal metacarpal region is suggested. The size of the proximal suspensory ligament does not change between the initial and follow-up MRI examinations.

Background: This study evaluates the change in an MRI of the proximal metacarpal region in a group of sport horses that returned to work. This retrospective analysis evaluated 18 limbs represented by 17 horses. Results: The hyperintense signal within the dorsal collagenous part of the proximal suspensory ligament (PSL) on T1W/T2*W GRE sequences decreased or stayed the same in the majority of cases. The hyperintense STIR signal within the dorsal collagenous part of the PSL resolved in the majority of the patients, and the third metacarpal bone (McIII) hyperintense STIR signal resolved in all patients. The dorsal margin irregularity of the PSL stayed the same, and McIII sclerosis and resorption of the palmar margin of McIII stayed the same in the majority of cases. McIII hyperintense STIR signal resolution carries a broad time range, with a mean of 94 days and a range of 47–202 days. Conclusions: Complete normalization of the dorsal collagenous part of the PSL does not appear necessary for a return to soundness, but a resolution of the McIII hyperintense STIR signal is expected for horses returning to soundness. A rescan period of 120 days for the proximal metacarpal region is suggested. In addition, there was no significant change in the size of the PSL between the initial and final MRI.