Definition
By: Gregory R. Waryasz, MD
The flexor digitorum superficialis muscle of the musculoskeletal system is characterized by being part of the intermediate layer of the flexor-pronator muscles.
It is part of the forearm. It consists of skeletal muscle fibers.
Its unique structural features include its humeroulnar and radial head. The median nerve and ulnar artery enter the forearm by passing through the humeroulnar and the radial head of the flexor digitorum superficialis. At approximately mid-forearm, the muscle divides into four bundles. The superficial bundle contains the tendon to the long and ring fingers. The deep bundle contains the tendons to the index and little finger. The flexor digitorum superficialis tendons are covered by synovial tissue. At the mid-palmar area, the flexor digitorum superficialis tendons are more volar than the flexor digitorum profundus. As the tendons become more distal, the flexor digitorum superficialis becomes more dorsal than the flexor digitorum profundus as the flexor digitorum profundus continues on to its more distal insertion site. The flexor digitorum superficialis splits so as to insert onto the radial and ulnar aspects of the middle phalanges.
The origin of the humeroulnar head is the medial epicondyle. The origin of the radial head is the superior half of the anterior border of the radius bone.
The insertion is the shafts of the middle phalanges of the medial 4 fingers.
The blood supply is from the ulnar artery and venous drainage is from the accompanying veins.
The innervation is from the median nerve.
The flexor digitorum superficialis muscle as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in the embryo.
The function of the flexor digitorum superficialis is to flex the middle phalanges at the proximal interphalangeal joints of the middle four fingers. It also flexes the proximal phalanges at the metacarpophalangeal joints. It works synergistically with the flexor digitorum profundus. The flexor digitorum superficialis also helps to flex the hand at the wrist and to flex the forearm at the elbow.
Common diseases include tendon rupture, tenosynovitis, and Swan Neck deformity of the finger.
Tendon rupture can occur with trauma.
Tenosynovitis of the flexor tendons can lead to the trigger finger phenomenon.
The Swan Neck deformity can occur if there is excision of the flexor digitorum superficialis for harvest. The swan neck refers to hyperextension of the proximal interphalangeal joint and flexion of ht edistal interphalangeal joint. The typical cause of this deformity is a disruption of the volar plate.
Commonly used diagnostic procedures include clinical history, physical exam, x-ray, and MRI.
It is usually treated with rest, NSAIDs, physical therapy, splinting, and corticosteroid injection for tenosynovitis. Tenosynovitis causing a triggering phenomenon may require a release of the A1 pulley. Tendon rupture may require surgery. The Swan Neck deformity due to excision of the flexor digitorum superficialis can be with observation or surgery to correct the deformity.
References
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Wheeless? Textbook of Orthopaedics: Flexor Digitorum Superficialis (http://www.wheelessonline.com/ortho/flexor_digitorum_superficialis)