Tendons are strong, flexible bands of connective tissue that link muscle to bone. The dexterity and strength of the human hand depend entirely on these structures, which transmit the force generated by the forearm muscles into finger movement. Without the precise action of these tendons, complex motions like grasping, bending, and straightening the fingers would be impossible.
The Two Main Tendon Systems
The tendons responsible for finger movement are organized into two distinct groups, located on opposite sides of the hand. The flexor tendons, which enable gripping and bending, run along the palm side (volar aspect) of the hand and fingers. These structures originate from muscle bellies located in the forearm and pass through the wrist into the hand. In each finger, two primary flexor tendons exist: the flexor digitorum superficialis and the flexor digitorum profundus.
The superficialis tendon attaches to the middle bone of the finger, allowing bending at the middle joint. The profundus tendon continues past it, inserting onto the fingertip bone to bend the outer joint. To facilitate this arrangement, the superficialis tendon splits at the base of the finger, creating a tunnel for the deeper profundus tendon to pass through. Conversely, the extensor tendons, which allow the fingers to straighten, are situated on the back (dorsal aspect) of the hand. These tendons are flatter and more complex as they spread over the knuckles.
At the knuckle, the extensor tendon broadens into the extensor mechanism or extensor hood. This hood divides into a central slip, which extends the finger at the middle joint, and two lateral bands that continue to the fingertip joint. This intricate arrangement allows for the coordinated straightening of all three finger joints. The muscles controlling these extensor tendons are located in the forearm, with the tendons running across the back of the wrist.
How Finger Movement Works
Finger movement is not generated by muscles within the hand itself but is powered remotely by muscles in the forearm. The tendons function as transmitters of this muscular force, acting like pull-cables to move the skeletal structure. When a forearm muscle contracts, it shortens and pulls on the attached tendon. This pulling action is translated into the bending or straightening of the corresponding finger joint.
For example, when you clench your fist, the flexor muscles in your forearm contract, pulling the flexor tendons through the wrist. This tension causes the finger bones to pivot at their joints, resulting in a grasping motion. The efficiency of this system depends on the tendons gliding smoothly without friction. This process is an efficient lever system, designed to maximize the force of the forearm muscles over the bones of the hand.
The Importance of the Pulley System
The flexor tendons, responsible for powerful gripping, are stabilized by the pulley system. This system consists of a series of fibrous rings that wrap around the tendons and anchor them against the finger bones. Without these rings, the tendons would lift away from the bone when the finger flexed, a phenomenon known as bowstringing. Bowstringing reduces the mechanical advantage and force of the grip, making effective flexion impossible.
There are five primary annular pulleys (A1 through A5), along with smaller cruciate pulleys. The A2 and A4 pulleys are the most mechanically significant because they are located over the long shafts of the finger bones and prevent bowstringing. The A1 pulley is situated at the base of the finger, near the knuckle, and is often involved in common hand conditions. The pulley system ensures that the tendons remain close to the joints, maintaining the necessary leverage for strong movement.
Common Problems Affecting Finger Tendons
The highly specialized anatomy of the finger tendons makes them susceptible to several common conditions. One frequent issue is Trigger Finger (stenosing tenosynovitis), which affects the flexor tendons and the A1 pulley. This condition occurs when the tendon or its protective sheath becomes inflamed and thickens, or when the A1 pulley narrows. The thickened tendon sheath struggles to pass smoothly through the constricting A1 pulley, leading to a noticeable catching, clicking, or locking sensation as the finger is straightened or bent.
Another common ailment is general tendonitis, which is inflammation of the tendon, or tenosynovitis, which is inflammation of the protective sheath. These conditions often result from repetitive strain or overuse, causing pain and swelling that impede the smooth gliding of the tendon. Sudden, forceful actions or deep lacerations can cause catastrophic injury, such as a complete tendon rupture or severance. Because tendons act as uninterrupted cables from muscle to bone, a laceration, even a partial one, can severely compromise the finger’s ability to flex or extend, often requiring surgical repair to re-establish the connection.