Metacarpal synostosis: A simple classification and a new treatment technique

The current classification of metacarpal synostosis is based on the extent of the synostosis. The authors propose a new classification that takes into account the shape of the metacarpal bones, the curvature of the epiphysis, and the discrepancy in length between the two bones. This classification p rovides better guidelines for the correction of all components of the defor mity. The classification is based on the authors' observations of and exper ience with 36 cases of metacarpal svnostosis; 13 of the deformities were su rgically corrected. The I-shaped deformity, whether with distinct (type d) or fused (type f) metacarpophalangeal joints, does not require surgical cor rection. The U-shaped deformity has parallel epiphysis and does not require surgery unless the two metacarpals are asymmetrical in length (type a) or tightly fused (type t); in these cases, simple lengthening or widening of t he space with a bone graft is sufficient. Y-shaped synostosis should be sep arated whether the branches are symmetrical or asymmetrical, the latter hav ing one branch shorter than the other. Because the epiphyses are already di vergent, simple separation does not effectively correct Y-shaped synostosis . The authors propose an osteotomy to isolate a trapezoidal segment of bone from the bifurcation. The isolated bone segment is then reversed in the pr oximal-distal direction to provide a "plateau" upon which the two distal me tacarpals can be realigned. Two cases of Ys (symmetrical) synostosis were s uccessfully treated with this technique; one case of Ya (asymmetrical) syno stosis also required distraction lengthening of the shorter metacarpal to a chieve an excellent result. One of the most difficult types of metacarpal s ynostosis to treat is k-shaped synostosis, observed only between the fourth and fifth metacarpals; in this type, the head of the short fifth metacarpa l abuts the metaphysis of the fourth. Osteotomy and distraction lengthening provide predictable results for correction of this deformity. The authors suggest that k-shaped synostosis might represent a late evolution of untrea ted Ua synostosis.