Victor Forge
The human hand is a complex system of joints and muscles that work together to allow for a wide range of motions and dexterity. One of the key components of hand movement is the interphalangeal joints, which facilitate flexion and extension of the fingers. The proximal interphalangeal joint (PIP) is responsible for flexion towards the palm, and its range of motion varies across different fingers. The muscles that control PIP flexion include the flexor digitorum superficialis and flexor digitorum profundus, with the latter being the only muscle capable of flexing the distal interphalangeal joints.
| Characteristics | Values |
|---|---|
| Muscle controlling PIP flexion | Flexor digitorum superficialis (FDS) |
| Other names for FDS | N/A |
| Other muscles involved in PIP flexion | Flexor digitorum profundus (FDP), intrinsic muscle group |
| Range of motion at PIP joint | 30–70°, increasing from the index finger to the little finger |
| Other joints involved in FDS-induced flexion | MCP, PIP |
| Other joints involved in FDP-induced flexion | MCP, PIP, DIP |
| Useful stability information | The PIP joint exhibits great lateral stability |
| Function of interphalangeal joints | Permit fine motor movements in the digits |
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What You'll Learn
The flexor digitorum superficialis (FDS) muscle
The FDS has two heads: a humeroulnar head and a radial head. The muscle's large belly extends distally towards the wrist, where it splits into four tendons that attach to the middle phalanges of the second to fifth digits of the hand. These tendons can be easily felt on the distal part of the forearm. The FDS is innervated by the median nerve (C7, C8, T1) and receives arterial blood supply from the ulnar artery and its anterior recurrent branch.
The primary function of the FDS is flexion of the middle phalanges of the four fingers (excluding the thumb) at the proximal interphalangeal joints. With continued action, the FDS also flexes the metacarpophalangeal joints and wrist joint. The FDS works together with the flexor digitorum profundus, whose tendons pass through a split (Camper's Chiasm) at the end of the FDS tendons.
It is common for the FDS to be missing from the little finger, which can cause challenges in diagnosing little finger injuries. The median nerve passes between the two heads of the FDS, and compression of the nerve at this site can result in pronator syndrome.
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The flexor digitorum profundus (FDP) muscle
The FDP is a composite muscle, innervated by the anterior interosseous nerve and ulnar nerves. The medial aspect of the muscle, which flexes the fourth and fifth digits, is supplied by the ulnar nerve, while the lateral aspect, which flexes the second and third digits, is supplied by the median nerve. The FDP is the only muscle capable of flexing the distal interphalangeal joints of digits 2-5.
The FDP is the main gripping muscle of the hand, and its strength can be measured using a dynamometer (handgrip ergometer). Any injury causing weakness or decreased function in the hand should focus on strengthening the FDP through an appropriate exercise regime.
The FDP is associated with several conditions that can affect hand function. For example, Quadrigia occurs when the FDP tendons to the third, fourth, and fifth fingers are unable to move independently due to interconnectedness with the tendon running to the index finger. This can lead to a weak grasp and difficulty making a full fist. Jersey finger is another condition caused by a rupture of the FDP tendon from its distal attachment at the base of the distal phalanx, resulting in a loss of active flexion at the distal interphalangeal joint.
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The extensor digitorum communis (EDC) muscle
The EDC is innervated by the posterior interosseous nerve (C7, C8), which is a continuation of the deep branch of the radial nerve (C7, C8). This muscle plays a role in limiting the independent extension of the ulnar three digits. It also contributes to the flexion of the middle and little fingers at the MCP joint, preventing the independent extension of the ring finger.
A cadaveric study investigated the role of individual extrinsic muscles in finger joint motion. The study found that the EDC generated simultaneous motion at the MCP and PIP joints, with relatively small motion at the DIP joint. The EDC produced an average extension of 18.3 degrees at the MCP joint, 15.2 degrees at the PIP joint, and 4.0 degrees at the DIP joint.
The EDC works in conjunction with other muscles to facilitate precise finger movements required for various manual tasks, such as grasping, pinching, and gripping objects. Understanding the role of the EDC in finger joint motion is essential for comprehending the complex functionality of individual muscles and their contributions to dexterous finger maneuvers.
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The extensor indicis proprius (EIP) muscle
The extensor indicis proprius (EIP) is a muscle in the human forearm that is responsible for the movement of the index finger. It is a narrow, elongated skeletal muscle situated in the deep layer of the dorsal forearm, specifically in the posterior compartment. Its tendon goes to the index finger, which it extends.
The EIP arises from the distal third of the dorsal part of the body of the ulna and from the interosseous membrane. It runs through the fourth tendon compartment together with the extensor digitorum, from where it projects into the dorsal aponeurosis of the index finger. Opposite the head of the second metacarpal bone, it joins the ulnar side of the tendon of the extensor digitorum, which belongs to the index finger.
The EIP is frequently used as a donor for tendon transfers, particularly for the reconstruction of the extensor pollicis longus (EPL). It is also known to have several anatomical variants, including split tendons and variations in tendon arrangement. The EIP tendon is often harvested at the level of the extensor retinaculum for tendon transfer procedures.
The EIP muscle exhibits some variations among individuals. It typically exists as a single tendon, but there have been reports of double tendons. It inserts into the index finger on the ulnar side of the extensor digitorum, although insertion on the radial side is infrequently observed. The extensor indicis proprius plays a crucial role in extending the index finger and, by its continued action, assists in extending the wrist and midcarpal joints.
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The flexor pollicis longus muscle
The flexor pollicis longus (FPL) is a long muscle located in the forearm and hand that flexes the thumb. It is unique to humans, being either rudimentary or absent in other primates. The FPL is the only muscle that flexes the interphalangeal joint of the thumb, which makes it essential for gripping actions. It also flexes the thumb at the metacarpophalangeal joint and assists in wrist flexion.
The FPL arises from the anterior surface of the radius and adjacent structures, such as the interosseous membrane and the lateral border of the coronoid process of the ulna. It crosses three joints and inserts onto the distal phalanx of the thumb. The muscle forms a flattened tendon that courses through the carpal tunnel, attaching at the base of the distal phalanx. This tendon can sometimes act more like a ligament, restricting extension of the interphalangeal joint of the thumb.
The FPL is supplied by the anterior interosseous branch of the median nerve. It receives dual blood supply, with its medial part supplied by the anterior interosseous artery (a branch of the ulnar artery) and its lateral part receiving blood from the radial artery. The FPL may also receive contributions from the median artery if the muscle is well-developed.
The FPL is classified as an extrinsic muscle of the hand, as it lies in the forearm but inserts in the hand and influences thumb movement. It is located in the same plane as the flexor digitorum profundus, with which it shares an origin point. The FPL can occasionally blend with the flexor digitorum superficialis, flexor digitorum profundus, or pronator teres.
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Frequently asked questions
PIP flexion is controlled by the flexor digitorum superficialis (FDS) and the flexor digitorum profundus (FDP).
The FDS originates in the forearm and inserts into the intermediate phalanx, therefore, it does not cross the DIP joint.
The FDP originates outside the hand, inserts into the distal phalanx, and crosses the wrist, the MCP, PIP, and DIP joints.
