Flexor Digitorum Superficialis
The flexor digitorum superficialis courses along the volar aspect of the forearm, superficial to the flexor digitorum profundus and flexor pollicis longus muscles, and deep to the palmaris longus, flexor carpi radialis, flexor carpi ulnaris, and pronator teres.
It was previously also named as Flexor Digitorum Sublimis.
The flexor digitorum superficialis muscle has two origins/ heads:
- The humeroulnar head originates off the common origin of wrist flexors i.e. medial epicondyle of the humerus and the medial border of the coronoid process of the ulna.
- The radial head originates on the anterior oblique line of shaft of radius.
As it courses down the forearm, the flexor digitorum superficialis separates into two planes of muscular fibers, superficial and deep:
- The superficial plane further divides to supply tendons for the middle and ring fingers.
- The deep plane provides a muscular slip to join the part of the superficial plane that associates with the tendon of the ring finger and then divides to supply tendons for the index and little fingers
The four flexor digitorum superficialis tendons pass deep to the transverse carpal ligament, constituting four of the nine total tendons in the carpal tunnel. Continuing into the palm, the flexor digitorum superficialis tendinously slips into two parts to pass posteriorly around each side of the tendons of flexor digitorum profundus and ultimately insert onto the middle phalangeal bases of digits 2 through 5, on the volar surface of the hand. 
The primary blood supply to the flexor digitorum superficialis muscle is from the ulnar artery. Lymphatic drainage of the upper limb occurs via the axillary lymph nodes.
- It is the primary flexor of the proximal interphalangeal (PIP) joints of the middle phalanges of digits 2nd to 5th.
- It also assists in flexion of the metacarpophalangeal (MCP) joints of digits 2nd to 5th.
- It also helps in wrist flexion.
- Swan Neck Deformity: It is the hyperextension of the PIP joint with flexion posture of the DIP joint that most commonly occurs secondary to rheumatoid arthritis, cerebral palsy, connective tissue disorders such as Ehlers-Danlos syndrome, or trauma.
- Carpal Tunnel Syndrome: Carpal tunnel contains nine tendons and a nerve: the flexor pollicis longus, the four flexor digitorum superficialis, the four flexor digitorum profundus as well as the median nerve. The flexor pollicis longus has its own synovial sheath, whereas the flexor digitorum superficialis and profundus have a common synovial sheath. The variation in which instead of flexor digitorum superficialis tendon, it's muscle belly travel in the carpal tunnel. this variant can cause carpal tunnel syndrome. 
- Anterior interosseous nerve (AIN) syndrome: Anterior Interosseous nerve is the motor nerve that runs deep in between flexor digitorum profundus and flexor pollicis longus. A tight fibrous arch of the flexor digitorum superficialis was also found to compress the median nerve at the level of the AIN branch causing AIN syndrome.
- Variation of the absence of one tendon for the little finger of flexor digitorum superficialis: Knowledge of such variation may be important during the hand surgery and in the preoperative diagnosis related to the muscles in the front side of the forearm and the tendons passing through the carpal tunnel. It also plays an important role in the postoperative rehabilitation phase as this muscle is responsible for the skill actions of the fingers.
To test flexor digitorum superficialis, the patient is asked to flex PIP joint of one of the digits from 2nd to 5th while other remaining three digits held in extension so as to inactivate Flexor Digitorum Profundus.
Clinical assessment of the variation of flexor digitorum superficialis muscle to the fifth finger:
Two methods are common and accurate in practice:
- Modified BAker's test: In performing this test, the examiner asks the subject to flex the proximal interphalangeal (PIP) joint of the little finger alone while the other fingers were held extended by the examiner. Then the little and ring fingers were released to flex together to observe the improvement in the PIP joint flexion. If a person is unable to flex the PIP joint of a little finger while other fingers are restrained in extension but able to flex PIP joint of 5th digit while the 4th digit is released, then it shows absent of 5th tendon of Flexor digitorum superficial.
- Another test involves putting the bilateral palms and corresponding fingers face to face closely relied on the forces of their own hands. While the little finger is ordered to flex, the influence of the adjacent fingers can be observed apparently if there exists a connection between the little finger and the adjacent fingers.
Other related pages:
Flexor digitorum superficialis muscle weakness is associated with median nerve injury. Another common cause that can hamper FDS function is tendon injury. There are various treatment regimes based upon the causes of a decremental function of FDS affecting hand fine motor activities. So graded exercise program or rehabilitation program is necessary for physiotherapy treatment based on cause, assessment findings and patient-centered goal. For more details about the type of exercise and graded rehabilitation of flexion tendon injuries go for the following link below:
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