Diagnostic exercise: intraosseous epidermoid cysts in the third phalanx of a dressage mare.

Summary

# Editorial Summary: Intraosseous Epidermoid Cysts in the Equine Pedal Bone An 11-year-old dressage mare presenting with lameness and suspected foot abscess underwent radiographic and computed tomographic imaging that revealed two distinct intraosseous epidermoid cysts within the third phalanx—one sizeable lesion near the apex and a smaller cyst at the lateral wing—ultimately confirmed through histological and histochemical examination. The case illustrates how epidermoid cysts, rare benign lesions derived from ectopic epithelial tissue within bone, can mimic more common foot pathologies and complicate clinical diagnosis when presenting with vague lameness and radiographic evidence of lytic lesions. Although epidermoid cysts are uncommon findings in equine practice, their presence within the pedal bone warrants inclusion in differential diagnoses for chronic or recurring foot lameness, particularly when radiographic lucencies don't correspond to typical abscessation patterns or respond poorly to conventional farriery and therapeutic interventions. Computed tomography proved superior to conventional radiography for characterising the cystic lesions and determining their precise location and extent, making it an invaluable diagnostic tool when pedal bone pathology remains ambiguous. Clinicians should consider that benign bone cysts, whilst rare, may explain persistent foot problems unresponsive to standard treatments, potentially guiding more targeted therapeutic approaches or prognostic discussions with owners.

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Key Findings

• •Two intraosseous epidermoid cysts identified in the third phalanx of an 11-year-old dressage mare
• •Cystic lesions located at the apex and lateral wing of the pedal bone
• •Radiographic and CT imaging successfully identified lesions that were confirmed histologically as epidermoid cysts

Conditions Studied