Associations between the radiographic appearance of vascular channels in proximal sesamoid bones, their microstructural characteristics and past racing performance in thoroughbreds

Abstract

Reasons for performing the study: ‘Sesamoiditis’ is classically defined by equine practitioners as the presence of and abnormalities in vascular channel appearance within the proximal sesamoid bones (PSB). It is the most common finding in Thoroughbred yearling presale radiographs, as well as often being evaluated on radiographs of adult racehorses with lameness and poor performance. Despite this, the pathogenesis and clinical significance of changes in vascular channel morphology are poorly understood, and the association of ‘sesamoiditis’ with racing performance is inconsistently reported.

Objectives: To determine the microstructural characteristics of the PSB associated with the radiographic appearance of vascular channels in Thoroughbred racehorses using micro-computed tomography (µCT), and to determine whether the presence, number and size of vascular channels has an association with past racing performance.

Methods: Study design was cross-sectional. One pair of PSB were isolated from a randomly selected forelimb of 59 Thoroughbred racehorses presenting for post-mortem examination over the study period. Each PSB (n=118) was radiographed, assigned a vascular channel grade using previously published and novel radiographic grading systems, then imaged using µCT and similarly assessed. Racing history for each horse was collected. Uni- and multi-variable generalized linear models accounting for clustering at the horse level were generated to investigate associations between radiographic, µCT and performance variables.

Results: All PSB had vascular channels observed on µCT originating from the abaxial border (mean 3.6, s.d 0.89), yet in only 63.6% (75/118) were channels observed radiographically. PSB with a higher bone volume fraction (OR 1.08; P=0.031) and wider channel diameter on µCT (OR 20.67; P=0.001) were more likely to have vascular channels identified on radiographs. Radiographic channel number (OR 0.96; P=0.043) and channel size (OR 0.96; P=0.049) were negatively associated with career placings.

Main Limitations: Only the forelimb proximal sesamoid bones were collected, radiographs of isolated bones avoided normal superimposition of soft tissue encountered in the live horse, and a cross-sectional study design meant changes in sesamoid vasculature over time and work load could not be assessed.

Conclusions: The ability to identify vascular channels radiographically indicates widening of channels and densification of the bone. Increased radiographic channel number and size is associated with poorer measures of past performance suggesting that these changes are not desirable.