Flexor Tendon Injuries of the Hand

Flexor tendon injuries of the hand can be debilitating, impacting your ability to grasp, hold, and perform everyday tasks. Understanding these complex injuries, from their anatomical basics to advanced rehabilitation strategies, is crucial for both medical students and anyone interested in hand health. This article breaks down everything you need to know about flexor tendon injuries, ensuring you grasp the key concepts for your studies or personal understanding.

Understanding Flexor Tendon Injuries in the Hand

Flexor tendons are vital cords that connect muscles in your forearm to bones in your fingers, allowing you to bend them. Injuries to these tendons can vary widely in severity and location, necessitating different treatment approaches. Early and accurate diagnosis, followed by appropriate surgical repair and rehabilitation, are essential for restoring hand function.

Anatomy of Flexor Tendons in the Hand

The hand's complex structure includes several key flexor tendons. The Flexor Digitorum Superficialis (FDS) tendons allow for isolated PIP joint flexion, originating from the medial epicondyle, ulna, and radius. The Flexor Digitorum Profundus (FDP) tendons enable DIP joint flexion, arising from the ulna and interosseous membrane. The Flexor Pollicis Longus (FPL) tendon is responsible for thumb flexion. These tendons pass through specific anatomical zones within the hand and wrist.

Nutritional supply to these tendons is dual: through vascular perfusion via the vincular system and synovial diffusion within the digital sheath. The specialized epitenon surface covering the tendons is crucial for smooth gliding.

Flexor Tendon Injury Zones

To standardize diagnosis and treatment, flexor tendon injuries are classified into five anatomical zones:

• Zone 1: Distal to the FDS tendon insertion, involving only the FDP tendon.
• Zone 2: Extends from the A1 pulley (entrance to the fibro-osseous sheath) to the insertion of the FDS tendon. This zone is often called “no man’s land” due to the challenges of primary repair here.
• Zone 3: Encompasses the origin of the lumbricals from the FDP tendons.
• Zone 4: Describes the flexor tendons within the carpal tunnel.
• Zone 5: From the muscle-tendon junction to the proximal aspect of the carpal tunnel.

Flexor Digitorum Profundus Avulsion Classifications

Avulsions of the FDP tendon are specific Zone 1 injuries. Leddy and Packer classified them into three types, with a fourth identified later:

• Type I: FDP tendon retracts into the palm, with disrupted blood supply. Requires urgent surgical repair.
• Type II: Tendon stump retracts to the PIP joint level, with some blood supply preserved. Primary repair may be done later.
• Type III: A large bone fragment remains attached to the tendon stump, preventing retraction beyond the A4 pulley. Fracture repair with Kirschner wire or miniature screw fixation is necessary.
• Type IV: Involves a fracture and avulsion of the FDP tendon from the fracture fragment, which may be within the tendon sheath or palm. This type is generally more severe.

Diagnosing Flexor Tendon Injuries: Preoperative Evaluation

A thorough history and physical examination are paramount before any surgical intervention. This includes assessing the skin, musculoskeletal system, neurological status, and vascular supply of the injured digit.

Systematic Examination:

• Integument: Examine skin integrity and record any injuries.
• Musculoskeletal: Check for angular or rotational deformity. Lacerated flexor tendons result in an extended posture of PIP and DIP joints and a loss of the tenodesis effect.
• FDS Tendon Isolation: To test FDS, hold adjacent fingers in full extension/hyperextension, then ask the patient to actively flex the PIP joint of the affected finger. Active flexion indicates intact FDS fibers, though partial injury is still possible.
• FDP Tendon Isolation: To test FDP, hold the middle phalanx firmly and ask the patient to actively flex the DIP joint.
• Neurologic: Assess light touch and static two-point discrimination. Sensation loss in a digital nerve distribution suggests a nerve transection until proven otherwise.
• Vascular: Evaluate capillary refill of the volar digital pulp and nail bed. Delayed refill may indicate a digital artery laceration. A digital Allen’s test can also be performed.

Plain radiographs are usually obtained initially, and intraoperative fluoroscopic images may be useful in specific cases, especially for FDP avulsions.

Surgical Treatment Approaches for Flexor Tendon Injuries

Optimal timing for flexor tendon repair depends on the evaluation findings. Early repair is generally preferable to prevent changes in tendon ends and proximal muscle that complicate primary repair.

Optimal Timing for Repair

• Emergency Repair: Indicated in cases of altered digital perfusion requiring microvascular repair or reconstruction, or injury to both arteries.
• Prompt Repair: Definitive repair done within days of injury ensures optimal conditions and easier wound management.
• Delayed Presentation: For unknown injury intervals, consider the need for tendon reconstruction or primary grafting.
• Post-3 Weeks: Primary repair becomes less likely due to scarring and retraction.

General Principles of Flexor Tendon Repair

Successful repair aims to accurately coapt tendon ends, allow postoperative gliding, inhibit adhesions, and restore the gliding surface and normal range of motion. Key principles include:

• Extensile Exposure: Use a controlled setting with optical magnification to define the injury.
• Atraumatic Tendon Retrieval: Employ gentle techniques through non-crucial areas of the sheath.
• Minimal Tendon Handling: Limit contact with the epitenon to prevent adhesions.
• Accurate Orientation: Ensure correct anatomical relationship of tendons.
• Stable, Smooth Repair: Core sutures for accurate coaptation and free gliding.
• Circumferential Suture: Augments repair strength and smooths the repair site.
• Sheath Closure: Only if it improves tendon gliding.

Repair Techniques by Zone

Zone 1 Lacerations or Avulsions: Tendon to Bone Repair

• FDP Tendon Advancement: If the distal stump is less than 1 cm, FDP tendon advancement and primary repair to bone are indicated.
• Primary Tenorrhaphy: If more than 1 cm of stump is available, this is usually done to avoid the “quadrigia effect,” which is diminished flexion in other fingers due to excessive tension.
• Techniques:
• Traditional Pull-Out Suture Methods: Core suture is placed in the proximal tendon, and free ends secure it into a trough in the distal phalanx, tied over the fingernail. Removed around 6 weeks post-procedure.
• Internal Suture Methods: Use suture anchors or other methods to affix the tendon directly to the bone, suitable for miniature anchors in the distal phalanx. Corner sutures can guide and reinforce the repair.

Zone 2 Lacerations: Core and Epitendinous Sutures

Zone 2 repairs are challenging due to the fibro-osseous sheath. Improvements focus on suture materials, caliber, and technique.

• Core Sutures: Multiple sites of tendon-suture integration provide greater tensile strength and resistance to gapping.
• Multistrand Techniques: (e.g., Strickland, cruciate, Becker, Savage, Winters) are preferred over two-strand methods (Kessler, modified Kessler, Tajima) for improved strength and reduced gapping.
• Suture Material & Caliber: Stronger sutures (e.g., 3-0 braided caprolactam) are used based on tendon caliber.
• Suture Configuration: “Locking” loops show greater time-zero strength than “grasping” loops.
• Epitendinous Suture (Circumferential Suture): Augments repair site strength and smooths the repair. It is an adjunct to core tendon suture, with placement often facilitating core suture tensioning.
• Gap Formation: Repair site gaps greater than 3 mm negatively affect healing and increase rupture risk. Aim for less than 3 mm gap.
• Retrieval of Proximal Tendon Stumps: Methods include gentle grasping of the endotenon, milking the tendon, or using a pediatric feeding tube (Sourmelis and McGrouther technique) to deliver the stump through the sheath.

Zones 3, 4, and 5 Lacerations

Repairs in these zones are similar to Zone 2, but often have an improved prognosis due to more spacious areas for tendon movement. An extensile approach is vital to identify all injuries, including common arterial and nerve damage. Arterial repairs are often done first, followed by deepest to most superficial tendon repairs, then nerve repairs. A modified Kessler four-strand technique with a 3-0 looped, braided caprolactam suture is often preferred, sometimes with simple corner sutures for approximation.

Partial Tendon Lacerations

For lacerations greater than 50% of the tendon cross-sectional area, repair using a core and circumferential suture is usually done. If less than 50%, the tendon may be repaired or debrided to prevent entrapment or triggering.

Flexor Pollicis Longus (FPL) Lacerations

Intrasynovial FPL lacerations can be challenging if the proximal stump retracts deep to the thenar musculature. Retrieval may require a separate incision in the carpal canal or distal forearm.

Postoperative Management and Rehabilitation

Rehabilitation is critical for successful outcomes after flexor tendon repair. It must be individualized, considering patient compliance, edema, suture quality, and any complications.

Importance of Rehabilitation and Early Motion Protocols

Tendon rehabilitation aims to improve overall hand function, promote tendon excursion, and prevent restrictive adhesions. The first 4 weeks are the most important. Low force, moderate excursion programs (e.g., 6-9 mm) are effective. Controlled passive motion and