Bipartite trapezoid: a rare entity

Clinical History

Trapezoid is a very well protected carpal bone. Its dislocation and fractures are documented in the literature but are very rare. We present a case of bipartite trapezoid, where there was a diagnostic dilemma on the basis of clinical and radiological findings.

Imaging Findings

A 17 year old boy presented to Casualty with pain in the base of right thumb. He had a recent history of minor fall in outstretched hand. On examination, there was mild tenderness at the base of the right thumb. Neurovascular status was intact. A plain X-ray of the right wrist showed a suspected undisplaced fracture of the right trapezoid. In suspicion of a fracture, a plaster cast was applied. On review at the fracture clinic, 3 days later, we found that the wrist was almost pain-free. The plaster was removed. On examination, there was no external injury, bruising or swelling. There was mild tenderness at the base of the right thumb in the anatomical snuffbox. Movements of the wrist did not elicit any discomfort or pain. X-ray showed that there was a clear, smooth line along the radial 1/3^rd of the right trapezoid passing through the entire length of the bone. There was no other fracture or 'fat pad sign' that excluded bony or soft tissue injury. As the boy was in his late teen-age, the epiphyseal fusion was yet not complete. The distal end of the radius clearly showed the epiphyseal line. Axial CT scan in coronal plane showed that the trapezoid bone was in 2 parts and the margins were corticated, indicating either established non-union or a bipartition of trapezoid, which is considered as a normal but rare variant. As he was asymptomatic with almost normal clinical picture a diagnosis of bipartite trapezoid was made.

Discussion

Bipartite trapezoid is a natural but rare variant. As far as we have found from the literature search such a case was not reported. However, a few cases of fracture of the trapezoid reported ^{2, 3,4,5}. The main concerns about our patient were the age (17 years) and the nature of the presentation, which raised the suspicion of a fracture. Trapezoid is situated in the radial side of the distal row of carpal bones surrounded by trapezium, 2^nd metacarpal, scaphoid and capitate. Because of its anatomy, its wedge shape, with the dorsal part being twice as broad as the volar part, and its tight ligamentous connections to the other carpal bones, the trapezoid is extremely well protected ¹. The carpal bones develop from secondary centres of ossification. The fusion of different parts of such bones occurs after teen-age. The x-rays and the CT scan clearly demonstrated that the line between the two parts of the trapezoid is full length, highly suggestive of a bipartite trapezoid. . Finally, the pain that was there after the fall might be due to soft tissue injury like sprain of ligaments. The boy became symptom-free with conservative treatment and was discharged from the clinic.

Differential Diagnosis List

Final Diagnosis

Bipartite right trapezoid

URL:	https://www.eurorad.org/case/5887
DOI:	10.1594/EURORAD/CASE.5887
ISSN:	1563-4086