Magnetic resonance imaging of the equine stifle: normal anatomy

Introduction: The equine stifle is a complex joint that may be the site of equine lameness, but diagnosis of pain can be frustrating using standard imaging techniques. Magnetic resonance imaging (MRI) of the equine distal limb has led to improvements in understanding of lameness pathogenesis. Until recently, MR imaging has been limited to the distal limb, but recently the use of MRI for clinical evaluation of the equine stifle has been reported. However there is little published information regarding the normal variation in MR anatomy of the equine stifle with which to compare horses with stifle lameness. The aim of this study was to describe normal MRI anatomy and variation in immature and adult equine stifles using comparison between anatomical cross-sectional slices and MR images. Materials and Methods: Post-mortem stifles from 9 immature ponies and 9 adult horses without hind limb lameness underwent MRI using a 1.5 Tesla GE Signa Echospeed MRI system with spoiled gradient echo, T2* gradient echo, short Tau inversion recovery, proton density, and fast dual echo sequences in 3 planes. Stifles were then sectioned in sagittal, dorsal or transverse planes, to obtain anatomical slices for comparison with MR images. MR images from different sequences and planes were described and compared with digital photographs of the anatomical sections. Results:Soft and osseous tissues of the immature and adult equine stifle could be clearly defined on MR images and corresponded well with macroscopic anatomy of the cadaver sections and the anatomy described in literature. Incidental findings included small lesions in cartilage, subchondral, cortical and cancellous bone, and ligamentar tissues likely indicative of normal variation. Conclusions: This study provides an atlas of normal anatomy of the equine stifle, which could assist in future clinical evaluation and as a reference for improving understanding of stifle anatomy.