The fourth-compartment syndrome: its anatomical basis and clinical cases.

We propose a new term, the "fourth-compartment syndrome" to describe chronic dorsal wrist pain of the fourth compartment. Five main causes responsible for this syndrome are thought to be as follows: 1. Ganglion involvement, including an occult ganglion; 2. Extensor digitorum brevis manus muscle; 3. Abnormal extensor indicis muscle; 4. Tenosynovialitis; 5. Anomaly or deformity of carpal bones. Should the above mentioned conditions occur in the fourth compartment, pressure within the fourth compartment increases, ultimately compressing the posterior interosseous nerve directly or indirectly. Anatomical studies of the fourth compartment of the wrist and the posterior interosseous nerve are presented and the fourth-compartment syndrome is summarized with twelve clinical cases (six cases of occult ganglions, two cases of extensor digitorum brevis manus, two cases of tenosynovialitis, one case of abnormal extensor indicis muscle, and one case of carpal bossing).