The Flexor Digitorum Profundus (FDP) tendon bends the distal joint of the four fingers, allowing for fine motor tasks like pinching and gripping. Originating deep in the forearm, this powerful tendon travels the full length of the hand and finger to attach at the fingertip bone. An injury to the FDP tendon effectively paralyzes the last segment of the finger, requiring specialized surgical intervention to restore movement. Successful FDP repair depends highly on both the surgeon’s technical skill and the patient’s commitment to post-operative rehabilitation.
What Happens When the FDP Tendon is Injured
An injury to the FDP tendon immediately results in the inability to flex the fingertip at the distal interphalangeal (DIP) joint. This deficit is clinically checked by stabilizing the middle joint and asking the patient to bend the tip. Common mechanisms of injury include sharp lacerations, such as those from glass or a knife, and avulsion injuries, where the tendon is forcefully pulled off its attachment point.
The classic avulsion injury is known as a “jersey finger,” occurring when the finger is forcibly extended while the tendon is contracted, pulling the tendon off the bone. When the tendon is cut, the muscle belly in the forearm contracts and retracts the tendon proximally, sometimes into the palm. Injuries are classified into five distinct zones; Zone 1 contains only the FDP tendon at the fingertip.
Injuries in Zone 2, spanning from the middle crease to the distal palmar crease, are particularly challenging due to the intricate anatomy, historically called “No Man’s Land.” In this tight space, the FDP tendon runs beneath the Flexor Digitorum Superficialis (FDS) tendon and is constrained by fibrous pulleys. Trauma here risks injury to both tendons, and the confined space makes repaired tendons highly prone to scarring and adhesion formation.
How Surgeons Repair the Flexor Digitorum Profundus
The primary goal of FDP repair is to reconnect the severed tendon ends, creating a repair strong enough for early motion and smooth enough to glide without friction. The surgeon must extend the original wound, often using zigzag-shaped incisions to avoid joint contractures. The retracted proximal tendon end must first be retrieved, sometimes requiring an additional incision in the palm, and then carefully brought back to the repair site.
Surgical repair uses two types of sutures: the core suture and the peripheral suture. The core suture is the primary repair, providing the tensile strength needed to hold the two tendon ends together. Modern repairs use multi-strand techniques, typically involving four to six strands of material, to maximize connection strength. The number of strands must be balanced to avoid excessive bulk that could impede gliding.
A running peripheral suture is placed circumferentially around the repair site to smooth the junction. This reduces friction as the tendon moves through the tight pulley system, preventing adhesions and improving excursion. The entire repair must preserve the integrity of the annular pulleys, which keep the tendon close to the bone. Failure to preserve the pulleys causes the finger to “bowstring” when flexed. Specialized hand surgeons are required due to the technical difficulty of operating in the constrained space of the flexor sheath, especially in Zone 2.
Managing Recovery Through Post-Operative Rehabilitation
Post-operative rehabilitation determines the long-term success of an FDP repair. Without immediate, controlled motion, the repaired tendon will adhere to surrounding tissues, resulting in a stiff finger. Hand therapy begins almost immediately after surgery to encourage tendon gliding while protecting the repair from rupture.
The patient is fitted with a dorsal blocking splint, which prevents full extension of the fingers and wrist, keeping tension off the repaired tendon. Controlled motion protocols are initiated within this splint, typically using the Kleinert or Duran regimens. The Kleinert protocol uses a dynamic splint with rubber bands that passively pull the fingers into flexion. The patient actively extends the fingers against the bands, which then passively flex them back down.
The Duran protocol is a controlled passive motion technique where the patient or therapist passively moves the finger joints through a specific arc of motion. This movement is performed multiple times a day to promote tendon gliding. Both protocols encourage intrinsic tendon healing while mitigating the risk of rupture, which is highest in the first three weeks. Active motion, contracting the flexor muscles, usually begins around four to six weeks post-surgery when the tendon has gained sufficient strength. The splint is typically discontinued around six weeks, shifting focus to regaining full range of motion and strength. Full return to light activities is expected around eight to twelve weeks, but heavy gripping can take four to six months. If stiffness persists due to adhesion formation, a secondary procedure called tenolysis may be necessary.