The deep flexors of the forearm
Anatomy and supply
The deep flexors of the forearm are three muscles lying at the ventral forearm. They run under 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: 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. Innervation: median nerve (second and third finger, C8,Th1) and ulnar nerve (fourth and fifth finger, C7-Th1).
- Flexor pollicis longus muscle: 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. Innervation: Median nerve (C8,Th1).
- Pronator quadratus muscle: 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. Innervation: Median nerve (C8-Th1).
As (almost) all flexors of the forearm, these three muscles are supplied by the median nerve. The 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 hand and finger joints. The contraction of the flexor digitorum profundus leads to a flexion in 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 the flexion of the thumb at the MCP and DIP, and opposition at the saddle joint. Furthermore, it bends and radially abducts the hand 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, rarely through trauma (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.