Operative Techniques in Hand, Wrist, and Elbow Sur 2023, Nov, 12 | 12:00 am

Flexor Tendon Injuries - A Comprehensive Guide

Learn about the causes, diagnosis, treatment, anatomy, and classification of flexor tendon injuries. Find out about the symptoms, physical examination, imaging techniques, and postoperative rehabilitation. Discover the potential complications and treatment options for flexor tendon injuries.

Introduction

Flexor Tendon Injuries are traumatic injuries to the flexor digitorum superficialis and flexor digitorum profundus tendons that can be caused by laceration or trauma. Diagnosis is made clinically by observing the resting posture of the hand to assess the digital cascade and the absence of the tenodesis effect. Treatment is usually direct end-to-end tendon repair.

Epidemiology

Incidence: Rare

Occurs in: 4.83 per 100,000

Etiology

Pathophysiology: The mechanism of injury commonly results from volar lacerations and may have concomitant neurovascular injury. Tendon healing occurs via 2 pathways:

  • Intrinsic: Produced by tenocytes within the tendon.
  • Extrinsic: Stimulated by surrounding synovial fluid and inflammatory cells, implicated in the formation of scarring and adhesions; occurs in 3 phases.

Anatomy

Muscles

  • Flexor digitorum profundus (FDP): Functions as a flexor of the DIP joint, assists with PIP and MCP flexion; shares a common muscle belly in the forearm; has dual innervation.
  • Flexor digitorum superficialis (FDS): Functions as a flexor of the PIP joint, assists with MCP flexion; individual muscle bellies exist in the forearm; innervated by the median nerve.
  • Flexor pollicis longus (FPL): Located within the carpal tunnel as the most radial structure; innervated by the AIN of the median nerve.
  • Flexor carpi radialis (FCR): Primary wrist flexor, inserts on the base of the second metacarpal; closest flexor tendon to the median nerve; innervated by the median nerve.
  • Flexor carpi ulnaris (FCU): Primary wrist flexor, inserts on the pisiform, hook of hamate, and the base of the 5th metacarpal; innervated by the ulnar nerve.

Camper Chiasm

The Camper chiasm is located at the level of the proximal phalanx where the FDP splits the FDS.

Pulley System

The pulley system in digits 2-5 contains:

  • 5 annular pulleys (A1 to A5): These pulleys are thicker and stiffer than cruciate pulleys. A2 and A4 arise from the periosteum, and are the most important pulleys to prevent flexor tendon bowstringing. A1, A3, and A5 arise from the volar plate.
  • 3 cruciate pulleys (C1 to C3): These pulleys are collapsible and flexible. They allow the annular pulleys to approximate each other during digital flexion.

The thumb contains:

  • 3 annular pulleys (A1, Av, A2): A2 contributes least to the arc of motion of the thumb.
  • 1 interposed oblique pulley: This is the most important pulley to prevent flexor tendon bowstringing (along with the A1 pulley).

Blood Supply

There are 2 sources of blood supply:

  • Diffusion through synovial sheaths: This occurs when flexor tendons are located within a sheath, and it is the more important source distal to the MCP joint.
  • Direct vascular perfusion: This nourishes flexor tendons located outside of synovial sheaths. It is supplied by the vincular system, osseous bony insertions, reflected vessels from the tendon sheath, and longitudinal vessels from the palm.

Classification

Flexor Zones of Injury

A classification system based on the zones of injury has been developed to guide the treatment plan. The following table provides information on the zones of injury, their definitions, characteristics, and treatment approaches:

Zone Definition Characteristics Treatment
I Distal to FDS insertion Jersey finger Direct tendon repair
II FDS insertion to distal palmar crease/proximal A1 pulley The zone is unique in that FDP and FDS are in the same tendon sheath (both can be injured within the flexor retinaculum). If the vincula are disrupted, tendons can retract. Historically, this zone had poor results, but advances in postoperative motion protocols have improved outcomes. Direct tendon repair followed by early ROM (such as the Duran or Kleinert protocols).
III Palm (A1 pulley to distal aspect of carpal ligament) Often associated with neurovascular injury which carries a worse prognosis. Direct tendon repair. Good results from direct repair can be expected due to the absence of retinacular structures (if no neurovascular injury). May require A1 pulley release to avoid impingement of the repaired tendon on the pulley.
IV Carpal tunnel Often complicated by postoperative adhesions due to the close quarters and synovial sheath of the carpal tunnel. Direct tendon repair. The transverse carpal ligament should be repaired in a lengthened fashion if tendon bowstringing is present.
V Carpal tunnel to forearm Often associated with neurovascular injury which carries a worse prognosis. Direct tendon repair
Thumb TI, TII, TIII Outcomes are different than those for fingers. Direct end-to-end repair of FPL is advocated. Try to avoid Zone III to avoid injury to the recurrent motor branch of the median nerve. The oblique pulley is more important than the A1 pulley; however, both may be incised if necessary. Attempt to leave one pulley intact to prevent bowstringing.

Presentation

Symptoms

The main symptom of a flexor tendon injury is the loss of active flexion strength or motion of the involved digit(s).

Physical Exam

  • Inspection: Observe the resting posture of the hand and assess the digital cascade. Evidence of malalignment or malrotation may indicate an underlying fracture. Assess skin integrity to help localize potential sites of tendon injury. Look for evidence of traumatic arthrotomy.
  • Motion: Make sure to perform passive wrist flexion and extension, which allows for the assessment of the tenodesis effect (normally, wrist extension causes passive flexion of the digits at the MCP, PIP, and DIP joints. Maintenance of extension at the PIP or DIP joints with wrist extension indicates flexor tendon discontinuity). Active PIP and DIP flexion is tested in isolation for each digit.
  • Neurovascular: It is important to evaluate the digital neurovascular bundles, as flexor tendons are in their close proximity.

Imaging

  • Radiographs: These may have an associated fracture.
  • Ultrasound: This is used to assess suspected lacerations.

Treatment

Nonoperative

Nonoperative treatment includes wound care and early range of motion. It is indicated for partial lacerations less than 60% of tendon width.

Operative

Operative treatment is required for lacerations greater than 60% of tendon width. There are 2 main approaches:

  • Flexor tendon repair and controlled mobilization: This is indicated for lacerations greater than 75% or ≥50-60% with triggering.
  • Flexor tendon reconstruction and intensive postoperative rehabilitation: This is indicated for failed primary repair, chronic untreated injuries, or for FDS4 transfer to the thumb.

Techniques

Flexor Tendon Repair

The following are the fundamental principles and techniques of flexor tendon repair:

  • Indications: >75% laceration, ≥50-60% laceration with triggering. Epitendinous suture at the laceration site is sufficient; there is no benefit of adding a core suture.
  • Timing of repair: The repair should be performed within three weeks of injury (2 weeks is ideal); delayed treatment leads to difficulty due to tendon retraction.
  • Approach: The incisions should always cross flexion creases transversely or obliquely to avoid contractures (never longitudinal). Meticulous atraumatic tendon handling minimizes adhesions.
  • Core sutures: The number of suture strands that cross the repair site is more important than the number of grasping loops. There is a linear relationship between the strength of the repair and the number of sutures crossing the repair. Four to six strands provide adequate strength for early active motion. High-caliber suture material increases strength and stiffness and decreases gap formation, while locking-loops decrease gap formation. The ideal suture purchase is 10mm from the cut edge, and core sutures placed dorsally are stronger.
  • Circumferential epitendinous suture: This improves tendon gliding by reducing the cross-sectional area. It also improves the strength of the repair (adds 20% to tensile strength) and allows for less gap formation (the first step in repair failure). A simple running suture is recommended as it produces less gliding resistance than other techniques.
  • Sheath repair: Theoretically, this improves tendon nutrition through the synovial pathway. Clinical studies show no difference with or without sheath repair. Most surgeons will repair if it is easy to do.
  • Pulley management: Historically, pulley preservation was considered critical. Recent biomechanical studies have shown that 25% of A2 can be incised with little resulting functional deficit, and 100% of A4 can be incised with little resulting functional deficit.
  • FDS repair: In zone 2 injuries, repair of one slip alone improves gliding compared to repair of both slips.

Repair Failure

The following are important factors to monitor for repair failure:

  • Tendon repairs are weakest between postoperative day 6 and 12.
  • Repair usually fails at suture knots.
  • Repair site gaps > 3mm are associated with an increased risk of repair failure.

Tenolysis

Tenolysis may be required to treat localized tendon adhesions with minimal to no joint contracture and full passive digital motion. The procedure should be performed after waiting for soft tissue stabilization (> 3 months) and full passive motion of all joints. Careful technique should be used to preserve A2 and A4 pulleys.

Postoperative Rehabilitation

Postoperative controlled mobilization has been the major reason for improved results with tendon repair, especially in zone II. It leads to improved tendon healing biology, limits restrictive adhesions, and leads to increased tendon excursion.

Protocols

  • Immobilization: This is indicated for children and non-compliant patients. Casts/splints are applied with the wrist and MCP joints positioned in flexion and the IP joints in extension.
  • Early passive motion: This includes the Duran protocol, the Kleinert protocol, and the Mayo synergistic splint. These protocols involve low force and low excursion, and aim to achieve active finger extension with passive finger flexion and/or a static or dynamic splint.
  • Early active motion: This protocol involves moderate force and potentially high excursion, and dorsal blocking splint limiting wrist extension. "Place and hold" exercises should be performed with individual digits.

Complications

The following are potential complications of flexor tendon injuries:

  • Tendon adhesions: This is the most common complication following flexor tendon repair, with a higher risk in zone 2 injuries. Treatment includes physical therapy and tenolysis.
  • Rerupture: There is a 15-25% rerupture rate. Treatment includes primary repair or tendon grafting.
  • Joint contracture: Rates can be as high as 17%.
  • Swan-neck deformity.
  • Trigger finger.
  • Lumbrical plus finger.
  • Quadrigia.