Flexor Digitorum Superficialis Index Slip

musculus flexor digitorum superficialis — fasciculus indicis

The index finger slip of FDS is the most independently controllable of the four FDS slips, reflecting the index finger's requirement for precision. It can be independently activated in most individuals, forming the basis of the FDS test for individual finger assessment.

Nerve: Median nerve (C7, C8, T1) — AIN to… Blood Supply: Ulnar artery Region: Forearm

Anatomical Data

Origin, Insertion & Supply

Origin	Anterior oblique line of the radius (radial head) and medial epicondyle (humeral head)
Insertion	Base of the middle phalanx of the index finger — two slips straddling the FDP tendon
Nerve Supply	Median nerve (C7, C8, T1) — AIN to the radial head portion
Blood Supply	Ulnar artery

Biomechanics

Function & Actions

Actions	Flexes the PIP joint of the index finger — the only PIP flexor; Assists index MCP flexion and wrist flexion

Clinical Relevance

Clinical Notes

The FDS index slip is tested by holding all other fingers in full extension and asking the patient to flex the index PIP — the index FDS can produce PIP flexion independently due to its separate radial head origin. Zone II flexor tendon injuries ("no man's land") to the FDS index slip require careful repair within the fibro-osseous sheath.

Palpation

Tested by isolated index PIP flexion with other fingers held extended by the examiner.

Pathology

Common Injuries & Conditions

FDS Index Slip Laceration in Zone II

Isolated FDS index slip laceration in the fibro-osseous sheath requiring primary repair using modified Kessler technique with early controlled mobilisation.