Use of standing MRI and a new MRI grading system of the subchondral bone of the distal third metacarpal bone to identify bone changes in the Thoroughbred racehorse when comparing 21 cases of catastrophic biaxial proximal sesamoid bone fracture with 53 controls

Reasons for performing study:

Biaxial fracture of the proximal sesamoid bones (PSB) is the leading cause of fetlock failure resulting in euthanasia in Thoroughbred racehorses in California, Kentucky and Hong Kong. Identifying alterable risk factors to reduce biaxial PSB fractures could have a significant impact on reducing the occurrence of catastrophic failure of the fetlock (CFF).

Objectives:

We hypothesised that standing MRI (sMRI) would identify a greater degree of bone change in Thoroughbred racehorses with CFF from biaxial PSB fracture when compared to controls. Study design: Retrospective cohort study.

Methods:

Bone changes from Florida Thoroughbred racehorses with CFF due to biaxial fracture of the PSB (n = 21) were compared to controls (n = 53) using the sMRI. Data for control limbs were Florida racehorses that were subjected to euthanasia for reasons other than CFF (enteritis, pneumonitis, etc.). All forelimb fetlocks underwent sMRI imaging within 72 h of euthanasia. Bone scores were assigned to the subchondral bone (SCB) on the distal palmar aspect of the lateral and medial condyle of the third metacarpal bone (McIII) and the lateral and medial PSB. All information for analysis from sMRI images for cases and controls for all Thoroughbred racehorses was reviewed and collected by the lead author. The association between the dichotomous outcomes of PSB fracture with independent variables was assessed using bivariable and multivariable logistic regression analysis with significance set at P<0.05.

Results:

The proportion of cases with >75% sclerosis of the ipsilateral lateral PSB (43%, 9/21) was significantly greater than that among controls (6%, 3/53). The proportion of cases with SCB lesion grade >3 of the contralateral palmar lateral condyle (29%, 6/21) was significantly greater than that among controls (0%, 0/53).

Conclusions:

The sMRI identified a significantly greater degree of bone change in racehorses with PSB fracture compared to control horses. These identified changes may be useful as a screening tool to identify horses at increased risk of fetlock fracture during racing.