Subtalar joint middle facet agenesis: a case report and literature review.

Articular facet morphology plays a fundamental role in subtalar joint biomechanics and stability, and likely influences the development of hindfoot osteoarthritis. While multiple anatomical studies have shown wide variation in articular facet configuration, the clinico-radiological findings are rarely presented. We illustrate a case of bilateral subtalar joint middle facet agenesis in a 45-year-old woman, which was missed despite several presentations. We demonstrate the imaging findings to enable clinicians to distinguish this from the more common middle facet coalition. We summarise the developmental anatomy and discuss the potential implications on biomechanical function. Recognition of middle facet agenesis within the complex subtalar joint is important to prevent misdiagnosis and unnecessary surgery.