Flexor Tendons

1. Overview

Flexor tendons are fibrous connective tissue structures that connect flexor muscles to bones, transmitting force to produce joint flexion. These tendons play a vital role in bending the fingers, wrist, toes, and ankle. They are essential in enabling gripping, walking, and other flexion-based movements of the limbs. Located primarily on the anterior aspect of the upper and lower limbs, flexor tendons are critical for fine and gross motor activities.

2. Location

Flexor tendons are located on the anterior (palmar or plantar) side of the limbs:

• Upper limb:

  • Pass through the carpal tunnel at the wrist into the fingers and thumb.

  • Originate from flexor muscles in the forearm (e.g., flexor digitorum profundus, flexor digitorum superficialis, flexor pollicis longus).

• Lower limb:

  • Extend from the posterior compartment of the leg, pass behind the medial malleolus, and insert on the foot and toes.

  • Include tendons such as flexor hallucis longus and flexor digitorum longus.

3. Structure

Flexor tendons are composed of dense regular connective tissue, primarily collagen type I, and are enclosed in tendon sheaths for smooth movement:

• Tendon fibers: Parallel bundles of collagen fibers with elongated fibroblasts (tenocytes).

• Synovial sheath: Double-layered tube containing synovial fluid that reduces friction between the tendon and surrounding structures.

• Fibrous sheath: Anchors tendons to bone in the fingers and toes and guides tendon motion.

• Pulleys (hand/foot): Annular and cruciate ligaments in the fingers and toes maintain tendon alignment during movement.

• Vascular supply: Limited; receives nutrients from vincula (small vascular structures), diffusion, and synovial fluid.

4. Function

Flexor tendons transmit muscular contractions that result in joint flexion:

• Finger flexion: Flexor digitorum superficialis (PIP joint) and flexor digitorum profundus (DIP joint) tendons.

• Thumb flexion: Flexor pollicis longus tendon flexes the interphalangeal joint of the thumb.

• Wrist flexion: Tendons of flexor carpi radialis and ulnaris flex the wrist.

• Toe flexion: Flexor digitorum longus and flexor hallucis longus tendons flex toes and great toe respectively.

• Plantar flexion (ankle): Assists in pointing the foot downward during gait and jumping.

5. Physiological role(s)

Flexor tendons contribute to numerous functional and physiological processes:

• Grip strength: Essential for grasping and manipulating objects; fine motor control in the hand is highly dependent on intact flexor tendons.

• Locomotion: Toe and ankle flexors stabilize the foot and assist with propulsion during walking and running.

• Hand precision: Allow independent finger movement critical for writing, typing, and tool use.

• Shock absorption: Flexion during impact activities helps reduce stress on joints and ligaments.

• Energy efficiency: Tendons store elastic energy during movement, aiding in efficient repetitive motion like running or grasping.

6. Clinical Significance

Flexor tendons are commonly affected by injuries, overuse, and systemic diseases:

• Flexor tendon lacerations:

  • Often occur from deep cuts to the palm or fingers; may result in loss of finger flexion and require surgical repair.

• Trigger finger (stenosing tenosynovitis):

  • Inflammation and narrowing of the flexor tendon sheath causes painful snapping during finger movement.

• Jersey finger:

  • Avulsion of the flexor digitorum profundus tendon from the distal phalanx, typically during forceful finger extension (e.g., grabbing a jersey).

• Tendinopathies:

  • Overuse or repetitive strain can lead to tendinitis, particularly in the wrist or ankle flexors.

• Rheumatoid arthritis:

  • May lead to attrition and rupture of flexor tendons, particularly in the hands.

• Surgical and rehabilitation relevance:

  • Flexor tendon injuries require precise surgical repair and prolonged physical therapy due to risk of adhesions and limited healing capacity.