The deep flexors of the forearm
Anatomy and supply
The deep flexors of the forearm are three muscles lying at the ventral/anterior forearm. They run deep to the flexor digitorum superficialis, very closely to the radius and ulna, and for that reason they are difficult to palpate. In detail they are:
Flexor digitorum profundus muscle
This muscle originates at the proximal half of the anterior ulna and the interosseous membrane. Its four tendons run through the carpal tunnel and between the split end tendons of the flexor digitorum superficialis at the height of the middle phalanges. Distally, they insert at the palmar side of the end phalanges of the second to fifth finger.
It is innervated by the median nerve (second and third finger, root values C8-T1) and ulnar nerve (fourth and fifth finger, root values C7-T1).
Flexor pollicis longus muscle
The FPL has its origin at the anterior side of the radius and the interosseous membrane, sometimes also at the medial epicondyle of the humerus. Its tendon runs also through the carpal tunnel and inserts at the palmar side of the distal phalanx of the thumb.
It is innervated by the median nerve (root values C8- T1).
Pronator quadratus muscle
The pronator quadratus arises from the distal end of the ulna and extends horizontally to the radius giving the muscle a square-shaped appearance. It is the deepest muscle in the anterior forearm.
It is innervated by the median nerve (C8-T1).
Innervation of the deep flexors
As outlined above, as with (almost) all flexors of the forearm, these three muscles are supplied by the median nerve. The specific innervating branch, the anterior interosseous nerve, arises approximately 5 cm underneath the medial epicondyle of the humerus from the median nerve.
From there, it courses between the flexor digitorum profundus and flexor pollicis longus along the interosseous membrane and ends distally at the pronator quadratus.
As an exception, the flexor digitorum profundus receives a double innervation through both the median and ulnar nerves.
At the wrist, the tendons of the flexor digitorum profondus and flexor pollicis longus run through the carpal tunnel, a passage formed by the carpal bones dorsally and a tight densification of the antebrachial fascia (flexor retinaculum) anteriorly. Along with these tendons, the carpal tunnel contains the median nerve as well as the four tendons of the flexor digitorum superficialis.
The deep flexors are mainly responsible for flexion of the wrist and finger joints.
- The contraction of the flexor digitorum profundus leads to flexion of the metacarpophalangeal (MCP), proximal interphalangeal (PIP) and distal interphalangeal joints (DIP) of the second to fifth fingers, as well as the wrist joint. As this muscle is permanently tensed, the fingers are always slightly bent while at rest.
- The flexor pollicis longus is responsible for flexion of the thumb at the metacarpophalangeal joint and interphalangeal joing of the first digit, and opposition of the thumb, which occurs primarily at the 'saddle-like' carpometacarpal joint and metacarpophalangeal joint of the thumb. Furthermore, it flexes and radially abducts the wrist joint.
- The pronator quadratus pulls the radius medially, thus causing a pronation at the radioulnar joint.
The deep flexors of the forearm can be paralyzed through a lesion of the anterior interosseous nerve (anterior interosseous syndrome or Kiloh-Nevin syndrome).
Common causes are an entrapment by the superficial flexors of the forearm (e.g. through hypertrophy) and accessory muscles, or more rarely, follwoing trauma to the arm/forearm (e.g. a fracture or elbow dislocation).
The affected patients complain about pain in the forearm and hand weakness. The most characteristic sign though is the inability of forming the “okay” sign with the fingers (pinch sign). This happens due to the fact that the flexor digitorum profundus and flexor pollicis longus are the only muscles which are able to bend the fingers at their distal interphalangeal joints.