Imaging and histopathological findings of a third metacarpal aneurysmal bone cyst and pathological proximal suspensory ligament avulsion fracture

A 10-year-old Quarter Horse mare presented with acute and severe lameness, persisting despite initial farriery intervention. Upon physical examination, palpation of the proximal thirds of the third and fourth metacarpal bones elicited marked pain. Radiographic evaluation revealed a well-defined radiolucency within the proximal third of the left third metacarpal bone, localised to the enthesis of the proximal suspensory ligament. Ultrasonographic findings demonstrated moderate thickening and increased vascularity of the suspensory ligament at this level. Surgical intervention was considered; however, its feasibility was constrained by the atypical nature and location of the lesion. Subsequent magnetic resonance imaging (MRI) provided a detailed characterisation of the lesion, confirming its expansile nature and the suspected associated avulsion fracture presence. Histopathological examination corroborated the diagnosis of an aneurysmal bone cyst (ABC) with an accompanying bone fracture and bone remodelling. Cases like the one described here define the importance of utilising advanced imaging techniques in combination with conventional modalities, especially when diagnosing and assessing the extent of unusual or complex lesions, to inform prognosis and guide treatment decisions. Due to the lesion's intramedullary location, expansile nature and involvement of critical anatomical structures, surgical interventions were limited which led to a recommendation for humane euthanasia.