Lateral condylar fractures in thoroughbred racehorses: investigation of premonitory radiographic changes in distal metacarpal III

Reasons for performing this study: Metacarpal/metatarsal III condylar fractures are the most common type of fracture associated with Thoroughbred horseracing in the UK, and the most common reason for euthanasia on the racecourse. Prediction of fracture through quantification of exercise-related radiographic changes could enable modification of horse management to prevent this injury, improving animal welfare and reducing wastage. Objectives: The general aims were to validate objective radiographic measurement of exercise-related modelling in distal metacarpal III of Thoroughbred racehorses and to explore their utility for fracture prediction by comparing measurements from horses that sustained a fracture of the lateral condyle of distal metacarpal III with those from controls. The initial objective was to establish methods to measure objectively the following variables in distal metacarpal III: condyle opacity, cortical thickness and metaphyseal angle. Radiographic measurements were correlated with equivalent measurements derived from computed tomography (CT) to determine their accuracy. The second objective was to explore the ability of the radiographic measurements to identify horses at risk of fracture. This was done initially by comparing measurements made from radiographs taken at a single time point between Thoroughbred racehorses with and without a lateral condylar fracture, and subsequently by analysis of measurements from series of radiographs obtained prior to and at diagnosis of lateral condylar fracture as part of routine clinical practice. For the final objective, the maximum density of the palmar condyles of distal metacarpal III was determined using a quantitative CT-based technique. Maximum density was compared between horses with fractures and those without, and comparison made with the maximum opacity measured from radiographs, to investigate whether these measurements could be used to identify horses at risk of fracture. Materials and Methods: Fifty-five metacarpal III bones obtained from Thoroughbred racehorses euthanized on UK racecourses (age range 3-11 years) were divided in to three groups: control (no distal limb fracture, n=30), non-fractured (contralateral limb lateral condylar fracture, n=11) and fractured (lateral condylar fracture, n=14). All limbs were subjected to radiography and computed tomography for measurement of diaphyseal cortical thickness 2.5cm distal to the nutrient foramen (expressed as dorsal cortex:medulla width) and metaphyseal angle (deviation of the metaphysis relative to the diaphysis). Measurement of radiopacity of the medial and lateral condyles was calibrated using an aluminium phantom i (condyle:phantom ratio). Correlation between radiography and CT measurements and comparison of radiography measurements between groups was performed. For the longitudinal study, medical records of the Singapore Turf Club were reviewed. Thoroughbred racehorses diagnosed with a lateral condylar fracture that had also been radiographed in the previous 24 months were identified and placed in the ‘high risk’ group (n=16). Horses radiographed and then raced for a minimum of 2 years without suffering a fracture were placed in the ‘low risk’ group (n=15). Twenty-seven metacarpal III bones obtained from Thoroughbred racehorses euthanized on UK racecourses (age range 3-10 years) were used in the investigation of maximum density of the palmar condyles. They were divided in to three groups: control (no distal limb fracture, n=10), non-fractured (contralateral limb lateral condylar fracture, n=8), and fracture (lateral condylar fracture, n=9). Computed tomography measurement of condyle density was calibrated using a potassium phosphate phantom to enable comparison between age-matched bones. Results: Diaphyseal cortical thickness and metaphyseal angle measurements made by radiography and CT were significantly correlated (r=0.74, 0.73; p=0.007, 0.004) but there was no significant difference in measurements between groups. Radiopacity of medial and lateral condyles was significantly greater in non-fracture (0.67±0.04, 0.68±0.04) and fracture (0.82±0.07, 0.68±0.09) groups than the control (0.59±0.08, 0.57±0.09) group (p=0.003). The medial condyle (0.82±0.07) was significantly more radiopaque than the lateral condyle (0.68±0.09) in the fracture group only (p=0.001). In the longitudinal study, the dorsal cortex was thicker in high risk than low risk horses, the mean ± SD cortical thickness ratio of the high and low risk groups was 1.36 ± 0.26 and 1.08 ± 0.19 respectively (p=0.008). Metaphyseal angle measurement displayed a similar pattern and the difference was statistically significant, the mean ± SD metaphyseal angle of the high and low risk groups was 9.65° ± 2.21 and 6.95° ± 1.65 respectively (p=0.000). Computed tomography measurement of maximum density of distal metacarpal III, found no significant difference in maximum density of the lateral and medial palmar condyle between groups. There was a significant correlation between age and maximum density of the lateral condyle (r=0.44, p=0.019) and medial condyle (r=0.54, p=0.003). ii Conclusions: Objective measurement of dorsal cortical thickness and metaphyseal angle of distal metacarpal III can be performed accurately and precisely using radiography in Thoroughbred horses. Radiographic measurement of condyle opacity was precise but conclusions about the accuracy of this measurement await comparison with an appropriate reference standard. Increased radiopacity of the distal condyles was identified in horses that sustained lateral condylar fracture. However, this finding was not reflected by data from the horses represented by the clinical radiographs, suggesting that this measurement may not be useful in identifying horses at risk of fracture. Evidence supportive of this conclusion was provided by maximum density measurements made by CT. Significant differences in dorsal cortical thickness and metaphyseal angle were found between horses that sustained a lateral condylar fracture and horses that underwent radiography for clinical reasons but subsequently raced without fracture, suggesting that these variables could be of value in determining the risk of fracture and merit further investigation