Ultrasonographic diagnosis of caudoproximal humeral stress fracture in thoroughbred racehorses.

Background: Humeral stress fractures in racehorses can progress to catastrophic fracture if unrecognised. Scintigraphy is the gold standard diagnostic technique but is limited by accessibility and cost. It was hypothesised that ultrasonography could be used to visualise caudoproximal humeral stress fractures.

Objectives: To determine the utility of ultrasonography to identify caudoproximal humeral stress fractures in racehorses.

Study design: Clinical case series.

Methods: Seven racehorses that had a clinical history consistent with the presence of a humeral stress fracture were examined using humeral ultrasonography and radiography with or without scintigraphy from June 2013 to June 2021. Clinical and imaging findings, outcomes and descriptive statistics are reported.

Results: Seven Thoroughbred racehorses aged 2-4 years had a history of acute onset of a severe lameness, with four returning to training 3-12 months after layup for an unrelated reason. Nine of 10 humeral stress fractures (2 bilateral, 3 left, 2 right) were identified with ultrasonography. Ultrasonographic abnormalities included a step defect (5 humeri, 5 horses), periosteal callus/roughening (7 humeri, 4 horses) and/or an abnormally convex contour of the caudal aspect of the humeral neck (6 humeri, 5 horses). Radiographs revealed periosteal (8 humeri, 6 horses) and/or endosteal (6 humeri, 4 horses) proliferation adjacent to the caudoproximal aspect of the humeral cortex. Scintigraphy of five horses identified increased radiopharmaceutical uptake in the caudoproximal aspect of seven humeri. Serial recheck radiography and ultrasonography (5 horses) revealed bone remodelling. Horses were returned to intended use as racehorses (4) riding horses (2) or were retired (1).

Main limitations: Small case series in only Thoroughbred racehorses with a clinical indication of possible humeral stress fracture. Ultrasonographic examination was limited to the caudoproximal aspect of the humerus. Not all cases underwent all imaging modalities.

Conclusions: Ultrasonography is useful for caudoproximal humeral stress fracture detection and can be used to monitor healing.