Carpal coalition: 4 cases of a rare bony coalition between the capitate and trapezoid bone, with MRI findings in a symptomatic case.

Clinical History

A 49-year-old female patient with persistent pain during several months around the third carpometacarpal joint was referred for a radiological control and MRI analysis with the clinical suspicion of carpal boss. The patient was treated conservatively with anti-inflammatory medication. In three other patients the same variant was a coincidental finding.

Imaging Findings

The radiography revealed a capitate-trapezoid coalition (CTC). The capitate-trapezoid joint (CTJ) was narrowed with irregular, sclerotic margins. There were no arguments for a carpal boss. MRI moreover showed a CTJ filled with fibrovascular tissue, a surrounding kissing bone marrow oedema, the presence of small subchondral cysts and irregular joint margins around the complete joint (Minnaar type 1 coalition). No other bony or soft tissue anomalies were detected.

Discussion

Carpal coalition may occur in two or more adjacent bones in almost any combination (most frequently lunotriquetral 89,3%, rarely capitate-trapezoid 2,9% [1]) Most coalitions occur between carpals within the same carpal row [2]. Coalitions between carpals from different rows are rare [3]. Fusion of carpal bones is hereditary (transmitted as a dominant factor which is not sex linked [4]). The coalition is caused by a failure of cavitation of the cartilaginous hand bud precursor (during the fourth to eighth week of gestation) or of cartilaginous segmentation of a common cartilaginous carpal precursor [5].The malsegmentation results in a carpal coalition, which may be fibrous (syndesmosis), cartilaginous (synchondrosis), frequently a mixture of both or osseous. CTC - a rare coalition type - is usually considered asymptomatic. In very rare cases however fibrocartilaginous (type 1) CTC may be a cause of focal pain due to the pseudarthrosis or a post-traumatic disruption of CTC. The most frequently used classification on plain film is of de Villiers Minnaar [6] (four types). Minnaar type 1 resembles a pseudarthrosis due to its incomplete fibrocartilaginous coalition. The type 1 coalition may become symptomatic due to the pseudarthrosis or - as for all structures - in case of fracture. The weaker fibrocartilaginous coalition appears to be more susceptible to stress or trauma. The deficient intra-articular cartilage formation at the joint results in a clinical and anatomic condition similar to degenerative osteoarthritis [7]. The cartilage of the surrounding joints may become damaged as well due to disturbed motion. Although carpal fusion is mostly an asymptomatic condition, symptoms may appear after intense and repeated movements. A possible alteration of the normal biomechanics of the wrist may cause an abnormal stress. CTC type 1 resembles a pseudarthrosis on plain film radiography with irregular sclerotic margins, a narrowed joint space and eventually subcortical cysts. This non-osseous coalition may result in degenerative osteoarthritis [8]. MRI shows without the use of radiation more clearly the pseudarthrosis with irregular sclerotic margins, subchondral cysts and a narrowed joint space filled with fibrocartilage. It may in symptomatic cases additionally show bone marrow edema adjacent to joint on fluid-sensitive sequences. 3D-gradient echo sequences also offer a more accurate classification than plain films due to the elimination of bony superposition. MRI may give the necessary information about the condition of the surrounding articular cartilage and may also exclude concomitant (soft tissue) pathology of the wrist.

Differential Diagnosis List

Final Diagnosis

A rare case of symptomatic capitate-trapezoid carpal coalition, Minnaar-type 1.

URL:	https://www.eurorad.org/case/12910
DOI:	10.1594/EURORAD/CASE.12910
ISSN:	1563-4086