Flexor tendons are strong, cord-like structures originating from forearm muscles, extending through the wrist, and into the fingers and thumb. They are responsible for bending the digits, enabling precise hand movements like gripping and making a fist. An injury, most commonly a deep cut or laceration, immediately compromises the ability to move the affected finger.
The Immediate Answer: Why Flexor Tendons Do Not Heal Spontaneously
Flexor tendons, when completely severed, cannot heal spontaneously. The primary reason is the constant tension exerted by the forearm muscles. When the tendon is cut, this muscle tension pulls the two severed ends apart, often creating a gap of several centimeters. This separation prevents the physical contact required for the body’s natural repair mechanisms to bridge the injury site and form a stable connection.
The unique environment of the flexor tendons further complicates spontaneous repair. They glide within the synovial sheath, a confined tunnel that produces lubricating fluid to minimize friction. While essential for normal hand function, this environment actively inhibits the formation of a strong, stable fibrin clot—the initial scaffold needed for tissue regeneration. Without surgical repair to bring the ends together, the resulting gap leads to permanent loss of function, as the finger cannot bend.
Understanding Flexor Tendon Anatomy and Injury Zones
The complexity of the flexor tendon system significantly contributes to the difficulty of repair and recovery. Tendons travel within the digital flexor sheath, a narrow tunnel that protects them and provides lubrication for gliding. Reinforcing this sheath is the pulley system, a series of annular and cruciate structures that keep the tendon close to the bone during flexion.
The A2 and A4 annular pulleys are important for efficient mechanical function, preventing the tendon from “bowstringing” away from the bone when the finger is bent. Injuries are classified into five distinct zones based on anatomical location. Zone II, extending from the distal palmar crease to the middle of the finger, is historically referred to as “No Man’s Land” due to the poor prognosis for healing.
This zone is complex because it contains both the superficial (FDS) and deep (FDP) flexor tendons within the same narrow sheath. Injury or repair in this tightly packed area creates a high risk of scar tissue formation, known as adhesions. Adhesions cause the repaired tendon to stick to the sheath or the other tendon, severely impeding gliding and leading to a stiff finger and loss of movement.
Standard Medical Intervention and Recovery
Since tendon ends cannot meet or heal spontaneously, treatment for a complete flexor tendon laceration requires a specialized, two-phase approach starting with surgical repair.
Phase I: Surgical Repair
Surgery is necessary to locate the retracted tendon ends and suture them together. This procedure is typically performed within 7 to 10 days of injury to prevent further retraction and achieve the best outcome. The surgeon secures the ends, often using a multi-strand core suture technique to provide maximum strength during early healing.
If the injury is severe, a tendon graft may be required to bridge the gap using tissue from another part of the body. Following repair, the hand and wrist are immobilized in a protective splint, holding the wrist and fingers slightly bent to minimize tension. This surgical reattachment is only the first step, as the repair site is weakest immediately post-operatively.
Phase II: Post-Operative Rehabilitation
The success of the entire treatment hinges on a demanding, lengthy, and strictly controlled program of physical therapy. The primary goal of rehabilitation is to encourage the repaired tendon to glide within its sheath while protecting the delicate repair from re-rupture. The tensile strength of the repair is lowest between days five and fifteen post-surgery, making this period the most vulnerable.
Therapy protocols, such as controlled passive motion or early active motion (EAM), begin as early as 48 to 72 hours after the operation. These exercises involve specific movements to achieve controlled tendon excursion and prevent the formation of adhesions. A common early exercise is the “place and hold” technique, where the therapist passively moves the finger into a bent position, and the patient holds that position using minimal muscle power.
The splint is typically worn for five to six weeks, and the patient must perform a high volume of repetitions multiple times daily. Compliance with this rigorous schedule is paramount, as non-compliance raises the risk of re-rupture or permanent stiffness. Although the splint is discontinued around six weeks, the tendon does not regain full strength for three months, and movement improvements can continue for up to six months. If stiffness persists due to excessive scar tissue, a second surgical procedure called tenolysis may be required later to release the tendon.