A tendon is a strong, fibrous cord of connective tissue that links muscle to bone. This structure acts as a biological lever, translating the force generated by a contracting muscle into skeletal movement. A flexor tendon specifically facilitates the bending or flexing of a joint, bringing two connected body parts closer together. Composed primarily of collagen fibers, these tendons are built to withstand immense tension and flexibility, producing joint-bending motion such as making a fist.
Anatomy Locating Flexor Tendons
The most intricate location for flexor tendons is within the upper extremity, particularly the hands and fingers. The muscles responsible for finger flexion originate in the forearm, on the palm-side of the arm. These muscles taper into long flexor tendons that travel toward the hand. The tendons pass through the carpal tunnel at the wrist, a narrow passageway formed by bones and a thick ligament.
Once through the wrist, these structures fan out across the palm and extend into each digit. Each finger, except the thumb, receives two main flexor tendons: the flexor digitorum superficialis (FDS) and the flexor digitorum profundus (FDP). The FDS is the more superficial, while the FDP is deeper, attaching farther down the finger bone. The FDP passes through a split in the FDS before attaching at the fingertip. This arrangement allows for the precise, coordinated bending of the different finger joints.
The Mechanics of Finger Movement
Finger movement begins when the muscle in the forearm shortens, or contracts. This contraction exerts a pulling force on the attached flexor tendon, which pulls on the bone where it inserts, causing the finger joint to bend. For this motion to be smooth and efficient, the flexor tendons glide within specialized tunnels called synovial sheaths. The sheath produces a lubricating fluid that significantly reduces friction as the tendon moves during flexion and extension.
A sophisticated system of fibrous bands, known as pulleys, is arranged along the length of the fingers, wrapping around the tendons and bone. These pulleys hold the tendons close to the phalangeal bones, preventing a phenomenon called “bowstringing.” Without the pulleys, the tendon would lift off the bone, resulting in a loss of mechanical efficiency and grip strength. The pulley system ensures that the linear force of the contracting muscle is effectively converted into the rotational movement of the joint.
Recognizing Flexor Tendon Damage
Flexor tendons are often injured by sharp objects, such as glass or knives, causing a laceration on the palm side of the hand or finger. Injuries can also occur from crushing forces or from a forceful, sudden pulling away of the finger while attempting to grip an object. When a flexor tendon is cut, the tension in the connected muscle causes the two ends of the tendon to recoil and separate, making natural healing impossible.
The most telling sign of a flexor tendon injury is the inability to fully bend one or more joints of the affected finger. A person may still be able to passively move the finger with the other hand, but the ability to move it on their own is compromised. Associated symptoms include localized pain, noticeable swelling, and sometimes numbness if nearby nerves are also damaged. Any suspected flexor tendon injury requires immediate medical evaluation to determine the extent of the damage and restore function.