SSMB syndrome. Symptomatic supernumerary muscle belly syndrome.

Supernumerary muscle bellies (SMBs) of the forearm, common anatomic variant, are usually asymptomatic but can produce a debilitating pain syndrome that is secondary to a tendon-muscle shear phenomenon. This shear phenomenon seems to occur when a SMB is attached to another muscle and the excursion of the two units are different, thus parts of a muscle unit are prevented from migrating proximally on contraction of the entire muscle. The anomalous muscle may have either greater excursion, as in an anomalous palmaris longus attached to the flexor digitorum superficialis, or less excursion, as in an anomalous superficialis muscle attached to the carpus. The shear at the interface between different muscle-tendon units produces a burning pain that usually is localized to the distal one-third of the forearm. Surgical excision of the abnormal restricting muscle or tendon component relieves the symptom complex and restores the ability to apply full power. Observation on typical cases, diagnostic methods, operative technique, and results of the syndrome suggest that SSMB may be responsible for undiagnosed and untreated instances of distal forearm complaints.