Osseous cyst-like lesions in an unusual location in the equine distal phalanx

Abstract

Osseous cyst-like lesions (OCLLs) of the distal phalanx commonly occur around midline. The dorsopalmar location can be challenging to identify radiographically; however, advanced imaging has shown they are typically located in the central and palmar thirds of the bone. This study reports unusually located OCLLs in the dorsal, axial aspect of the palmar processes of the distal phalanx in three horses, with communication with the distal interphalangeal (DIP) joint confirmed by contrast arthrography in two horses. All horses were lame and had lameness localised to the distal limb with diagnostic anaesthesia. A rounded radiolucency with a surrounding rim of increased opacity was identified in the dorsal aspect of a palmar process on radiographs, with the palmaroproximal–palmarodistal oblique (PaPrPaDiO) view being most useful. On magnetic resonance imaging (MRI), the lesions were seen as hyperintense structure on T2*, T2 FSE and STIR sequences; a thin rim of intermediate signal surrounding a low signal centre was seen on T1 images. A rim of low T1/T2 FSE signal was seen in the surrounding bone in all cases, consistent with sclerosis, and mildly increased STIR signal was present in the surrounding trabecular bone in two cases. Two horses underwent standing cone-beam computed tomography (CT) examination of the affected foot, revealing a well-defined hypoattenuating region within the dorsal, axial aspect of the palmar process of the distal phalanx, with surrounding rim of increased attenuation. Contrast arthrography was performed in two cases, one using radiographs and the other CT, and confirmed communication of the cystic lesion with the DIP joint in both cases. Although advanced imaging provided further information and allowed other possible causes of lameness to be identified, careful assessment of radiographs allowed these unusually located OCLLs to be identified in all cases.