Case Report: Oblique Fracture of the Ulnar Sesamoid Bone of the Metacarpophalangeal Joint, a Rare Pathology

INTRODUCTION Fractures of the sesamoid bones of the metacarpophalangeal joint (MCPJ) are a rare pathology in hand trauma. Only a few case reports and case series were found in literature. The diagnosis of an ulnar sesamoid bone fracture of the MCPJ is difficult and can pose a differential diagnostic problem with Stener lesions or ulnar collateral ligament (UCL) instability. This pathology is frequently missed with subsequent poor functional outcomes. Correct and early diagnosis is important as this will allow for correct treatment with full recovery of function without sequelae. CASE PRESENTATION We report a case of a 35-year-old male Caucasian patient after a thumb injury due to a fall with his mountain bike. X-ray showed a fracture of the ulnar sesamoid of the MCPJ of the pollex, which is rarely reported in this literature. Anamnesis, clinical examination, and X-ray evaluation contributed to our diagnosis. The ulnar sesamoid bone fracture of the MCPJ of the right pollex was initially treated conservatively with immobilization, rest and ice application. After 3-4 weeks, physiotherapy with thenar muscles exercises was started. 10 weeks after the injury, our patient reported a total recovery of function and pain-free thumb movement during daily activities. CONCLUSION Ulnar sesamoid fractures are rarely reported in current literature. This type of injury should be recognized as early as possible since it can be easily missed. Then a benign course can be expected after short immobilization of the MCPJ of the thumb during circa 2 weeks. UCL instability or a Stener lesion should be excluded at the emergency department by performing an ultrasound. With this case report, we underline the importance of early diagnosis and exclusion of an unstable Stener lesion.