Flexor tendon injury and reconstruction

Tendons are composed of dense connective tissues that transmit forces generated by muscles to move the joints or to create action power. Functionally, the hand is dependent upon the integrity and ample gliding of the tendons.

Difficulties in restoration of function of digital flexor tendons relate chiefly to the intricate anatomy of flexor tendon systems: the coexistence of superficialis and profundus tendons within a tight fibro-osseous tunnel.

( Box 24.1 )

Whenever possible, acutely lacerated flexor tendons in the hand and forearm should be treated primarily or at the delayed primary stage.

• Primary tendon repair is the end-to-end repair performed immediately after wound cleaning and debridement, usually within 24 h of trauma.

• Delayed primary repair is defined as repair performed within 3 or even 4 weeks after tendon lacerations.

• The tendon injured in critical areas (such as zone 2) should not be repaired by an inexperienced surgeon. Rather, tendon repair can be delayed until an experienced surgeon is available.

• My preferred period of deliberate delay is 4–7 days, when the risk of infection can be properly addressed and edema has reduced substantially.

• Delay of the repair beyond 3–4 weeks may cause myostatic shortening of the muscle–tendon unit; for these late cases, lengthening the tendon within the muscles in the forearm can ease the tension ( Fig. 24.1 ) .

  

Rupture of the repaired flexor tendons after surgery can be re-repaired if the rupture occurs within a few weeks up to a month after surgery; secondary tendon grafts may be the only choice for ruptured cases in the presence of obvious retraction of the tendon end or extensive scarring in the intact flexor digitorum superficialis (FDS) tendon when the flexor digitorum profundus (FDP) tendon ruptures.

Traumatic FDP tendon avulsion from the tendon–bone junction accounts for a major portion of closed rupture cases. The tendon disrupts at its insertion to the distal phalanx.

• The injury mechanism is hyperextension of the distal interphalangeal (DIP) joint, which subjects the FDP tendon to excessive load.

• Athletic injuries can lead to this type of injury. When one player grabs another's jersey, a finger may be caught and pulled, resulting in disruption of flexor tendons. This injury (“jersey finger”) is seen most commonly in the ring finger.

Closed tendon ruptures at the wrist can be associated with fractures in carpal bones.

Flexor pulleys are prone to sprains and ruptures during climbing. Rupture of the pulleys occurs in up to 20% of climbers. The A2 pulley of the ring finger is the most often injured. Closed pulley ruptures are treated conservatively or by surgical reconstruction.

Closed tendon ruptures can be classified into the following types:

• Class I: the FDP tendon is avulsed from the phalanx and retracts into the palm. The vincula of the FDP tendon are disrupted. There is no active flexion of the DIP joint. A tender mass is present in the palm.

• Class II: the FDP tendon retracts to the level of the proximal interphalangeal (PIP) joint. This is the most common type. The sheath is not compromised, and muscle contracture does not develop easily.

• Class III: a large bone fragment is attached to the FDP tendon. This bone fragment frequently prevents the tendon from retracting proximal to the A4 pulley.

• Class IV: the FDP tendon avulses from the bony fragment and retracts beyond the middle phalanx and even into the palm.

Early recognition of closed tendon ruptures is of paramount importance. In cases where there is late diagnosis, primary repair is difficult or even impossible. Chronic cases require free tendon grafting.

Tendon grafting is indicated: (1) when the lacerated tendons are not treated during primary or delayed primary stage; (2) when the primary repairs have ruptured and cannot be re-repaired directly; and (3) in the cases not indicative of primary tendon repairs because of severe contamination, infection, lengthy loss of tendon substance, extensive destruction of the pulleys, or accompanying injuries.

Flexor tendon injuries are open in most cases, resulting from a sharp cut or a crush, but they can also present as closed injuries.

• Open injuries due to extensive trauma are frequently associated with neurovascular deficits.

• Closed injuries often relate to forced extension during active flexion of the finger.

• Flexor tendon rupture can also occur as a result of chronic attrition in rheumatoid disease, Kienbock disease, scaphoid nonunion, or hamate or distal radius fracture.

Careful attention to the patient's history and the mechanism of injury can alert the surgeon to the extent of the tendon trauma and associated injuries.

The natural resting posture of the wounded digits is important for evaluation.

• Complete lacerations of both FDP and FDS tendons are easily diagnosed when the affected fingers are seen in a relatively extended position with loss of active finger flexion at PIP and DIP joints.

• If the patient can actively flex the DIP joint while the motion of the PIP joint is blocked, no injuries or only partial injuries to the FDP tendon can be diagnosed.

• To assess the continuity of the FDS tendon, the adjacent fingers are held in full extension by the examiner. If the patient cannot actively flex the PIP joint, the FDS tendon is completely severed.

• Variations in the FDS tendons in the little finger are frequent. The FDS in 30–35% of the little fingers is connected with the FDS in the ring or middle fingers. Some little fingers (10–15%) are missing an FDS tendon. These patients have limited or no PIP flexion of the little finger during testing.

• Weakness during resisted finger flexion indicates a possible partial tendon cut.

• To test the FPL tendon, the thumb metacarpophalangeal joint (MCP) is stabilized in a neutral position. The patient is asked to flex the IP joint. Loss of active flexion at the joint indicates complete severance of the FPL tendon.

Nerve and vascular function should be assessed routinely because accompanying injuries in the neurovascular bundles in one or both sides of the fingers or median and ulnar nerves at the carpal tunnel or distal forearm are common.

• Loss of sensation in the finger pulps or loss of function of intrinsic muscles in the hand is indicative of such accompanying injuries; treatment of neurovascular injuries must be included when planning surgical strategies.

• If fingers or hands are found to be hypovascular or avascular due to vascular lacerations, vascular anastomosis should be a surgical emergency.

• Otherwise, after wound debridement, either the lacerated flexor tendons can be repaired (when experienced surgeons are readily available), or the skin can be closed to allow for delayed primary repairs within days by experienced surgeons.

Radiographs should always be taken. Associated fractures are not infrequent and require treatment.

CT or MRI should be prescribed for the cases suspicious of closed tendon ruptures. Ultrasonographic examination may also reveal rupture of the tendons.

Primary or delayed primary end-to-end tendon repairs are mainly indicated in clean-cut tendon injuries with limited damage to peritendinous tissues.

Serious crush injuries, severe wound contamination, loss of extensive soft tissues, or extensive destruction of pulleys and tendon structures are contraindications for primary tendon repairs.

Fractures involving multiple bones, particularly at different levels or not yielding stable internal fixation, are contraindications for primary tendon repairs ( Box 24.2 ).

Flexor tendon repairs in children have a better prognosis than those in adults. As children may be less compliant with instructions to limit movement, the repaired digits are usually immobilized for 3–3.5 weeks after surgery. Either a two-strand or a four-strand repair can be used.

There are 12 flexor tendons in the hand and forearm regions, including finger and thumb flexors and wrist flexors.

• Finger flexor tendons are the FDS, FDP, and flexor pollicis longus (FPL).

• The FDS and FDP originate from muscles at about the midforearm, while the FPL tendon arises from the volar aspect of the midportion of the radial shaft and from its adjacent interosseous membrane.

• The tendons of the FDP come from a common muscle belly, while the tendons of the FDS originate from separate muscle bellies, which allows more independent finger flexion.

• The wrist flexors are the flexor carpi radialis (FCR) and ulnaris, and the palmaris longus (PL).

• The PL is absent in about 15–20% of the normal population, and wrist flexion power is not affected by its absence.

Within the carpal tunnel, nine tendons exist – four FDS, four FDP, and one FPL.

• The relationship of these tendons within the carpal tunnel is fairly constant.

• The FDS tendons to the ring and middle fingers lie superficially, deeper are the FDS tendon to the index and small fingers, and deeper still are the FDP tendons.

• The FPL tendon is located deep and radially adjacent to the scaphoid and the trapezium.

• After emerging from the carpal tunnel, the tendons enter the palm. At about the level of the superficial palmar artery arch, the lumbrical tendons originate from the FDP tendons.

The most intricate portions of the flexor tendons are within the fingers, where the tendons glide within a closed fibro-osseous sheath with segmental, semirigid, constrictive dense connective tissue bands present.

• The digital sheath forms a closed synovial compartment extending from the distal palm to the middle of the distal phalange.

• The FDS tendons lie superficial to the FDP tendons up to the bifurcation of the FDS tendon at the level of the MCP. There, in the A2 pulley area, the FDS tendons become two slips coursing laterally and then deeper to the FDP tendons.

• Now deep to the FDP tendon, the FDS slips rejoin to form Camper chiasm (a fibrous interweaved connection between two FDS slips), and distally insert on the proximal and middle parts of the middle phalanx as two separate slips.

• The FDP tendon inserts into the volar aspect of the distal phalanx.

• The FPL tendon is the only tendon inside the flexor sheath of the thumb and inserts at the distal phalanx.

The synovial sheath is a thin layer of continuous smooth paratenon covering the inner surface of the fibrous sheath, providing a smooth surface for tendon gliding and nutrition to the tendons.

The pulley system of the digital flexor tendon is unique; it consists of annular pulleys (condensed, rigid, and heavier annular bands) and cruciate pulleys (filmy cruciform bands) ( Fig. 24.2 ) . The annular pulleys maintain the anatomical paths of tendons close to bones and phalangeal joints, thus optimizing the mechanical efficiency of digital flexion. The more compressible cruciate pulleys allow for digital flexion to occur with condensation of the fibro-osseous sheath at the inner part of flexed fingers.

• There are five annular pulleys (A1–A5), three cruciate pulleys (C1–C3), and one palmar aponeurosis pulley.

• The A1, A3, and A5 pulleys originate from the palmar plates of the MCP, PIP, and DIP joints, and the A2 and A4 pulleys originate from the middle portion of the proximal and middle phalanges, respectively.

• The broadest annular pulley is the A2 pulley, which covers the proximal two-thirds of the proximal phalanx and encompasses the bifurcation of the FDS tendon at its middle part.

• The A4 pulley is located at the middle third of the middle phalanx.

• The A2 and A4 pulleys are the largest among five annular pulleys and have the most important function.

In the thumb, there are three pulleys (A1, oblique, and A2) with no cruciate pulleys ( Fig. 24.3 ) .

• The A1 and oblique pulleys are functionally important.

• The A1 pulley is located palmar to the MCP joint, while the oblique pulley spans the middle and distal parts of the proximal phalanx.

• The A2 pulley is near the site of insertion of the FPL tendon.

The FDP tendon has two vincula: a fanlike, short vinculum and a cordlike, long vinculum. The short vinculum is located at the insertion of the FDP tendon ( Fig. 24.4 ) . The long vinculum connects the FDP tendon through the short vinculum of the FDS tendon to the floor of the palmar surface of the phalanges.

The FDS tendon also has two vincula: one connecting to the proximal phalanx and another at the insertion of the FDS tendon.

Vincula carry blood vessels to the dorsum of these tendons, providing limited nutrition. Tendon insertion sites to bones also carry vessels into tendons over a very short distance.

According to anatomical features, the flexor tendons in the hand and forearm are divided into five zones, which offer the fundamental nomenclature for flexor tendon anatomy and surgical repairs ( Figs. 24.5 , 24.6 ) :

• Zone 1: from the insertion of the FDS tendon to the terminal insertion of the FDP tendon.

• Zone 2: from the proximal reflection of the digital synovial sheath to the FDS insertion.

• Zone 3: from the distal margin of the transverse carpal ligament to the digital synovial sheath.

• Zone 4: area covered by the transverse carpal ligament.

• Zone 5: proximal to the transverse carpal ligament.

In the thumb, zone 1 is distal to the interphalangeal (IP) joint, zone 2 is from the IP joint to the A1 pulley, and zone 3 is the area of the thenar eminence.