Lateral patellar instability in Standardbred horses at the weanling age: Clinical and morphological features in four cases (2017-2019)

The objective of this study was to describe lateral patellar instability (LPI) in a population of weanling Standardbreds and characterise the prevalence, clinical features, imaging findings and long-term follow-up. A total of 222 weanling Standardbreds were examined between June 2017 and December 2019 for presales examination, and LPI was identified in four horses. Presumptive diagnosis was made on clinical and dynamic examination at walk and confirmed by radiography. One horse was subject to euthanasia for causes related to LPI (Case 3), and computed tomography (CT) of both hindlimbs was performed post-mortem in order to better characterise the anatomical variations of the affected stifle. The intertrochlear groove angle (IGA), the angle of anteversion (AA) of the femoral neck, the anatomical lateral distal femoral angle (aLDFA) and the degree of tibial torsion (TT) were measured. LPI prevalence in Standardbreds weanlings was 1.8%, and it carried a poor athletic prognosis. Radiographic abnormalities were observed in the shape of the patella, characterised by a rounder profile and a less pronounced sagittal crest. The abnormal limb also presented a wider IGA, an outward rotation of the proximal aspect of the tibia and varus deformity of the femorotibial joint compared with the contralateral limb. LPI seems to occur with a high prevalence in the Standardbred weanling population, and our findings suggest that LPI is related to a combination of epiphyseal dysplasia and rotational and angular abnormalities of the affected limb. No effective treatments have been reported.