Flexor Tendon Injuries of the Hand: Flexor Digitorum Superficialis (FDS), Flexor Digitorum Profundus (FDP), Flexor Pollicis Longus (FPL)

Flexor tendon injuries of the hand are common, particularly in trauma settings, and present significant challenges in diagnosis, management, and rehabilitation. These injuries often result from sharp lacerations, crush mechanisms, or avulsion injuries. Given the critical role of the flexor tendons in hand function, prompt and precise management is essential to ensure optimal recovery and prevent long-term functional deficits. This article provides an in-depth review of flexor tendon anatomy, injury mechanisms, clinical assessment, surgical techniques, and postoperative care.

1. Anatomy of the Flexor Tendon System

The flexor tendons are integral to finger flexion and grasping, functioning through a complex interplay with surrounding structures:

• Flexor Digitorum Superficialis (FDS): Inserts on the middle phalanx of each finger and is responsible for flexion at the proximal interphalangeal (PIP) joints.
• Flexor Digitorum Profundus (FDP): Inserts on the base of the distal phalanx, flexing the distal interphalangeal (DIP) joints.
• Flexor Pollicis Longus (FPL): Responsible for flexing the thumb's interphalangeal (IP) joint.

These tendons are supported by a system of fibro-osseous pulleys (A1-A5), which prevent bowstringing (tendons pulling away from the bones) and ensure smooth movement. The A2 and A4 pulleys are particularly crucial for maintaining normal tendon function, and their disruption leads to the "bowstring effect," impairing finger flexion.

2. Types and Mechanisms of Flexor Tendon Injury

2.1. Sharp Laceration

This is the most common mechanism, often caused by knives, glass, or other sharp objects. Lacerations may involve complete or partial tendon transection and are frequently accompanied by injuries to nearby neurovascular structures, particularly in Zone V injuries.

2.2. Avulsion Injuries (Jersey Finger)

In a Jersey Finger (most commonly affecting the ring finger), the FDP tendon avulses from its insertion on the distal phalanx, typically occurring when the finger is forcefully extended while gripping, such as when a player grabs an opponent’s jersey. Radiographs may reveal an avulsion fracture of the distal phalanx.

2.3. Crush Injuries

Crush mechanisms, often industrial accidents, result in more complex injuries that may involve multiple tendons, bone fractures, and neurovascular damage. These injuries are more difficult to manage due to extensive tissue damage and a higher risk of complications such as adhesions or tendon rupture.

3. Zones of Flexor Tendon Injury

Flexor tendon injuries are classified by zone, as outlined by Verdan’s classification. Each zone has unique anatomical and functional considerations, which dictate surgical strategy and prognosis:

• Zone I: Extends from the insertion of the FDP at the distal phalanx to the middle phalanx. Injuries here typically involve FDP avulsion (e.g., Jersey Finger).
• Zone II: Known as "No Man’s Land," this zone extends from the middle phalanx to the distal palmar crease. It contains both FDP and FDS tendons and is notorious for poor outcomes due to the risk of adhesions between the two tendons.
• Zone III: Extends from the distal palmar crease to the flexor retinaculum. Injuries here have better outcomes due to good vascular supply and less anatomical complexity.
• Zone IV: Includes the carpal tunnel. Injuries in this zone may involve multiple tendons, the median nerve, and accompanying blood vessels.
• Zone V: Extends proximal to the carpal tunnel, often involving neurovascular structures, and carries the risk of significant functional deficits.

4. Clinical Evaluation of Flexor Tendon Injuries

A thorough physical examination is critical for diagnosing flexor tendon injuries. The following techniques are essential for confirming tendon function and assessing the extent of injury:

4.1. Observation

• Finger Cascade: When the hand is in a relaxed position, the fingers should naturally flex in a cascading manner. Any finger that appears straightened or does not follow this cascade should raise suspicion for tendon injury.

4.2. Tenodesis Effect

• Passive Finger Flexion with Wrist Extension: Ask the patient to extend their wrist. In a normal hand, the fingers will passively flex. A lack of flexion indicates a disruption in the flexor tendon mechanism.

4.3. Tendon Function Tests

• FDP Test: Stabilize the middle phalanx and ask the patient to flex the DIP joint. Inability to flex the DIP suggests FDP injury.
• FDS Test: To isolate the FDS, hold the other fingers in extension and ask the patient to flex the finger at the PIP joint. Inability to flex at the PIP joint suggests FDS rupture. Notably, many individuals lack independent FDS function in the small finger.
• FPL Test: Stabilize the proximal phalanx of the thumb and ask the patient to flex the IP joint. Inability to do so suggests FPL injury.

4.4. Neurovascular Assessment

Always assess the neurovascular status in cases of flexor tendon injuries. Testing for 2-point discrimination, light touch, and pinprick sensation can reveal accompanying nerve damage.

5. Surgical Management of Flexor Tendon Injuries

5.1. Timing of Surgery

• Early Primary Repair (within 24 hours): This is ideal to minimize adhesion formation and tendon retraction.
• Late Primary Repair (24 hours to 2 weeks): Sometimes necessary if the patient has life-threatening conditions or delayed presentation.
• Secondary Repair (> 2 weeks): Tendon retraction and scarring make secondary repairs more complex, with worse functional outcomes.

5.2. Surgical Techniques

The goal of flexor tendon repair is to restore continuity, prevent adhesions, and allow early mobilization:

• Core Sutures: Techniques like the Modified Kessler suture are commonly used, providing strong repair while minimizing gapping at the repair site. Non-absorbable sutures such as 3-0 or 4-0 are preferred.
• Epitendinous Sutures: Placing running epitendinous sutures around the tendon improves strength and smooths the tendon surface, reducing friction and adhesion formation.

5.3. Pulley Preservation

Whenever possible, especially in Zones II and IV, preservation of the A2 and A4 pulleys is crucial to prevent bowstringing and preserve tendon function.

6. Postoperative Rehabilitation

Postoperative care is as critical as the surgery itself to achieve optimal results:

6.1. Splinting

The standard postoperative position includes:

• Wrist flexion (approximately 30°),
• MCP joints flexed (about 70°),
• PIP and DIP joints allowed relative freedom.

6.2. Early Motion Protocols

Early controlled motion reduces adhesion formation and encourages tendon healing:

• Kleinert Protocol: Uses rubber bands for passive flexion while allowing active extension.
• Duran Protocol: Involves passive motion exercises of the affected fingers by the patient.

These protocols must be carefully supervised to prevent tendon rupture during the vulnerable early healing phase (typically 10-14 days postoperatively).

7. Complications

7.1. Tendon Adhesion

Adhesions, especially in Zone II, are the most common complication, severely limiting finger flexion. Adhesions can be minimized through meticulous surgical technique and early mobilization protocols.

7.2. Tendon Rupture

Early rupture of the tendon repair, particularly during the first two weeks, is a devastating complication. This can occur due to aggressive rehabilitation or inadequate surgical technique.

7.3. Joint Contracture

Prolonged immobilization or inadequate rehabilitation can lead to contractures of the finger joints, limiting overall hand function.

8. Prognosis

Prognosis varies by injury location, severity, and the timing of intervention. Injuries in Zone I and Zone III typically have good outcomes, while those in Zone II are more prone to complications such as adhesions. Proper surgical technique, early motion protocols, and careful postoperative management are key to optimizing recovery.

9. Conclusion

Flexor tendon injuries are complex and require a thorough understanding of anatomy, injury mechanisms, surgical techniques, and rehabilitation protocols. For clinicians, mastering the evaluation and management of these injuries is crucial for ensuring optimal functional recovery for patients. Early recognition, meticulous surgical repair, and adherence to postoperative rehabilitation protocols are the foundations of successful treatment.