Christopher H. Allan
DEFINITION
Flexor tendon injuries can occur in any of the five described zones within the finger, hand, wrist, or forearm. All such injuries require surgical repair to restore active finger flexion.
The most challenging injuries to manage are those in zone II, where two flexor tendons occupy a narrow fibro-osseous sheath. Successful repair requires meticulous technique and a careful postoperative therapy regimen balancing the risks of adhesion formation versus rupture.
ANATOMY
The flexor tendons form two layers in the forearm (zone V; FIG 1A), with the thumb’s flexor pollicis longus (FPL) and the fingers’ flexor digitorum profundus (FDP) muscles deep to the flexor digitorum superficialis (FDS) muscle. At the wrist the FPL and FDP tendons remain deep, with the index and small FDS tendons above, and the middle and ring FDS tendons most superficial.
The median nerve runs down the forearm beneath the fascia of the FDS on its undersurface, becoming superficial within the carpal tunnel just proximal to the volar wrist crease (zone IV), with the flexor tendons closely packed together.
Exiting the carpal tunnel, the flexor tendons cross the palm (zone III) toward the individual digits.
The two tendons of each non-thumb digit (the thumb has just the FPL) enter the fibro-osseous sheath (zone II) at the level of the metacarpophalangeal (MP) joint. The FDS then divides into two slips to form the decussation termed the chiasm of Camper, through which the FDP passes from deep to superficial (FIG 1B).
The two slips of the FDS insert along the proximal aspect of the volar surface of the middle phalanx, and the FDP proceeds distally to insert along the volar surface of the base of the distal phalanx.
The flexor sheath extends from the level of the MP joint to the distal interphalangeal (DIP) joint. Multiple condensations of discrete fibers are found along its course and are named as either annular or cruciate pulleys, reflecting the orientation of the fibers forming the pulley (FIG 1C).
The thicker, annular (A1 through A5, from proximal to distal) pulleys hold the tendons close to bone while the more slender cruciate (C1 through C3) pulleys collapse with digit flexion, allowing the sheath to shorten without buckling. Zone II is that part of the sheath where both FDS and FDP tendons are present, and zone I is distal to the FDS insertion.
Tendon nutrition within the sheath is provided indirectly via synovial fluid and directly via vascular inflow through mesenteric folds called vinculae, with one vinculum longus and one vinculum brevis to each flexor tendon.
PATHOGENESIS
Most acute flexor tendon injuries are the result of open trauma, with sharp transection of the tendon. In such cases, other structures are often injured as well. In particular, examination should include assessment of sensibility and capillary refill to identify injury to the digital nerves and vessels that would affect preoperative planning.
FIG 1 • A. Flexor tendon zones. B. Flexor tendon anatomy in zone II. C. Flexor sheath pulley anatomy and distribution in zones I and II.
A less common injury mechanism is closed avulsion of the FDP from its distal attachment to bone. The term “jersey finger” is sometimes used for this injury, as it is often the result of an athlete’s fingers forcibly flexing to grab an opponent’s jersey, followed by sudden and forceful extension of the DIP joint against resistance as the opponent pulls away. This avulsion injury is addressed elsewhere.
NATURAL HISTORY
Flexor tendon injuries require surgical repair to restore active digit flexion. Early repair is crucial, with several studies pointing to better results when repairs are performed within the first 7 days after injury.3,7
Outcomes aside, as a practical matter it is easiest to repair the tendon before proximal tendon retraction occurs, requiring additional incisions. Late repair with tendon retraction and muscle shortening can also result in tension at the repair site, leading to gapping of the repair (which increases the failure rate) or influencing the surgeon to splint the wrist or digits in excessive flexion, leading to joint contractures.
PATIENT HISTORY AND PHYSICAL FINDINGS
Methods for examining the hand or upper extremity with an acute flexor tendon injury
Isolate FDP: While maintaining the injured digit’s proximal interphalangeal joint extended, the examiner asks the patient to actively flex the DIP joint (FIG 2A).
Isolate FDS: While maintaining all uninjured digits in full extension, the examiner asks the patient to actively flex the injured digit’s proximal interphalangeal joint (FIG 2B).
Uncooperative or unresponsive patient
Tenodesis effect: The examiner extends the wrist; flexion is observed at the interphalangeal joints if the flexor tendons are intact.
Forearm compression: Pressure applied to flexor tendon muscle bellies results in interphalangeal joint flexion if flexor tendons are intact.
The examiner inspects for normal flexion arcade (FIG 2C).
Examination of the digit to rule out associated digital nerve injury is required.
PHYSICAL FINDINGS
Laceration
Affected digit held in unopposed extension
Inability to actively flex interphalangeal joints (if both tendons are cut), isolated DIP joint (if FDP only is cut) or isolated PIP joint (if FDS only is cut)
IMAGING AND OTHER DIAGNOSTIC STUDIES
The sudden loss of active flexion after a laceration overlying the flexor sheath almost always represents a tendon injury. Radiographs should be obtained to rule out associated fractures that would require treatment at the time of tendon repair. Lacerations due to glass, metal fragments, and so forth should be imaged to localize any residual foreign bodies for removal.
In the setting of a closed injury with the sudden loss of active flexion, one must consider the possibility of a tendon avulsion from its insertion. Radiographs may demonstrate an avulsed fleck of bone. In the more common cases of an FDP avulsion, this bone fragment may remain in the region of the distal phalanx or may be pulled proximally into the flexor sheath. If no bony fragment is seen on a plain radiograph and the diagnosis is still in doubt, ultrasound may help.
DIFFERENTIAL DIAGNOSIS
Pain after injury may cause a patient (especially a child) to hold a digit or hand immobile, mimicking tendon injury.
Testing for the tenodesis effect (digits passively flex with wrist extension) or compressing the flexor musculature in the forearm should help with diagnosis in these situations.
NONOPERATIVE MANAGEMENT
There is no nonoperative means of restoring active flexion to a digit whose flexors have been cut, as the tendon ends retract and do not heal to one another.
If a flexor tendon laceration is encountered within the first 4 weeks after injury, it is probably worthwhile attempting primary repair. After that time, discussion should be held with the patient regarding other options.
For late presentation of an isolated FDP laceration in zone I, it may be practical to do nothing, as full proximal interphalangeal motion should still be present. If the DIP joint is or becomes unstable, a DIP joint fusion or tenodesis of the distal FDP stump can be performed. One large series reported successful primary tendon grafting for isolated FDP lacerations even in zone II, but this is not widely performed.
FIG 2 • A. Isolation of distal interphalangeal flexion to test flexor digitorum profundus integrity. B. Isolation of proximal interphalangeal flexion to test flexor digitorum superficialis integrity. C. Loss of normal flexion cascade after tendon laceration in palm.
For late presentation of zone II injuries involving both tendons, staged tendon reconstruction (see Chap. HA-55) may be an option.
SURGICAL MANAGEMENT
The goal of flexor tendon surgery is a repair that will allow early motion, will not fail due to gap formation or suture pullout, and will not develop adhesions limiting final range of motion.
Several variables under control of the surgeon contribute to repair strength:
Number of strands (most important determinant; a fourstrand repair with an epitenon suture added has been shown in laboratory studies to withstand limited early active motion)6,11
Suture size (3-0 or 4-0 is sufficient; larger suture increases resistance to gliding)
Configuration of repair (cruciate repair requires only one knot, buries the knot within the repair site, and allows for equal distribution of force across all four strands)5
Use of locking stitches (adds resistance to suture pullout)
Addition of an epitenon suture (increases repair strength and decreases gap formation and gliding resistance)6
Presence of a gap (greater than 3 mm at any point will likely result in rupture)
Bunching of repair (due to taking too large a “bite” of tendon end; increases gliding resistance and therefore risk of rupture)
Integrity of pulleys (at least half of both A2 and A4 should be preserved to maintain tendon excursion and allow tip-to-palm contact)
Repair of one versus both tendons (if repair of both FDS slips impedes gliding, one slip should be resected, or the FDS not repaired at all)
It is well accepted that core suture strength is directly related to the number of suture strands crossing the repair site between proximal and distal tendon ends; all else being equal, using more strands means a stronger repair.11
This concept must be balanced against other factors: too many sutures crossing the repair site limits the available surface area for exposed tendon ends to heal; more sutures and knots increase the gliding resistance of the tendon; and the more sutures placed, the longer the surgical time, which is associated with risks such as infection and anesthesiarelated issues.
Strickland6 showed that a four-strand zone II repair with an epitenon suture is strong enough to tolerate an immediate light active range-of-motion protocol, which allows for early gliding of the repair and decreases the risk of adhesion formation.
Suture size contributes to repair strength, but one study showed 3-0 and 4-0 suture to resist repair rupture equally well and 2-0 suture to increase gliding resistance significantly.1
Adding at least one locking stitch (making an additional pass to capture more tendon fibers) has been shown to increase repair strength and minimize gap formation.
Multiple studies suggest that when both tendons are cut in zone II, repair of the FDP and only one slip of FDS rather than both slips results in decreased gliding resistance and improved range of motion.9,10,13
My present compromise is a four-strand cruciate repair using 3-0 nonabsorbable suture, with a 6-0 Prolene running epitenon suture, with repair of one slip of FDS and resection of the other slip when both tendons are lacerated in zone II.
Preoperative Planning
As noted previously, it is usually preferable to perform tendon repair early (if other circumstances allow).3,7 The upper limit of time past which proximal stump contracture is likely to cause technical difficulty is variable. Although 3 to 4 weeks is a commonly cited limit for primary tendon repair, in rare cases the vinculae may prevent retraction and allow repair even later.
Patients presenting late should be fully counseled regarding other options, including the potential for intraoperative changes in plan.
Positioning
Flexor tendon surgery, like most hand surgery, is generally performed with the affected extremity on a hand table, with the shoulder abducted 90 degrees and the elbow extended. The forearm is supinated, exposing the volar surface of the digits.
A positioning device such as a lead hand can be helpful in stabilizing digits for surgery (once tendon ends have been delivered into the wound) and keeping other digits out of the way.
Approach
Incisions should be planned so as to incorporate the laceration into the exposure.
Zigzag (Bruner) or midlateral approaches both work well; they can be combined if needed.
Midlateral incisions extending proximally on one side of the digit and distally on the other can give large flaps and excellent exposure.
The chief concern is not to cross a flexion crease at a right angle since the resultant scar will tend to contract and limit extension.
TECHNIQUES
PRIMARY REPAIR IN ZONE II
Retrieval of Tendon Ends
A zigzag or Bruner incision is made, sometimes in combination with mid-lateral incisions (TECH FIG 1A).
Often some manipulation is needed to bring the tendon ends into the wound; for the proximal tendon end, wrist flexion and “milking” of the forearm may succeed. The distal stump is best exposed by extending the incision so that the repair can be performed without holding the digit flexed.
Initial exposure should include both digital neurovascular bundles, whether or not they were injured along with the tendon or tendons.
If digital nerve or artery repair is needed, this should be done after the tendon repair so that the more delicate microsuture used is not disrupted with manipulation of the digit. Exposure of these bundles even when uninjured allows much more freedom for manipulating the cut tendon ends.
TECH FIG 1 • Exposure (A), retrieval (B), and temporary transverse pinning through sheath of cut tendon ends (C) to allow tension-free repair.
Once the neurovascular bundles are exposed and protected, the sheath should be cleared of overlying soft tissues and inspected. The sheath laceration can be extended with a sidecut to form an L-shaped flap, always preserving as much as possible of the A2 and A4 pulleys. Creating such flaps can facilitate retrieving the tendon ends.
Because most flexor tendon injuries occur with the digit in flexion, the skin laceration is generally more proximal than the tendon laceration. This means that exposure of the distal stump often requires considerable distal extension of the incision, often to the level of the DIP joint and obliquely across the pulp of the distal phalanx.
The proximal tendon end may be held in place near the laceration by its vinculae, but it will often have retracted well proximally.
It is reasonable to make several attempts to retrieve the proximal tendon end through the sheath with an appropriately small instrument (curved tendon passer, small hemostat, etc.), keeping in mind that the less damage to the tendon end, the easier the repair and the less scarring that will result. Flexing the wrist and “milking” the forearm will sometimes encourage a proximally migrated tendon to protrude into the wound.
If these measures fail, a short transverse incision along the distal palmar crease can be made, as if exposing the A1 pulley for a trigger finger release, and the tendon exposed at this level.
A pediatric feeding tube can be threaded from one wound to the other and sutured to the tendon in the proximal wound (TECH FIG 1B).
The tube and flexor tendon can then be retrieved into the distal wound and the tube and suture cut free.
Once the proximal end is in the planned repair site, the tendon can be pinned with a 25-gauge needle to prevent retraction back into the sheath (TECH FIG 1C).
Often the distal location of the distal stump requires that the proximal tendon end be threaded past the original laceration site to a more distal “window” made in the sheath for tendon repair. This, coupled with flexion of the DIP joint, should allow for apposition of the tendon ends and repair under minimal tension.
Tendon Repair
Four-strand cruciate repair is effected using 3-0 nonabsorbable suture, with a 6-0 Prolene running locking epitendon suture, and repair of one slip of FDS (TECH FIG 2).
Epitenon-First Repair
For very oblique lacerations it may be easier to perform an epitenon-first repair, coapting the cut tendon ends smoothly, and then performing the core stitch beginning through a slit on the outside of the tendon, burying the knot in this same slit (TECH FIG 3).
The repair is otherwise the same as a four-strand cruciate repair.
TECH FIG 2 • A. Four-strand cruciate repair with epitenon stitch. B,C. The hand is to the right and the fingertips to the left. B. Distal zone II flexor digitorum profundus laceration with tendon ends pinned in place and core stitch being placed. C. Completed repair with epitenon stitch.
TECH FIG 3 • A–D. The hand is to the right and the fingertips to the left. A. Oblique laceration of flexor tendons in zone II; tendon ends retrieved and pinned in place for repair. B. Epitenon repair performed first. C. Core suture begun via small incision in tendon proximal to repair; otherwise similar to standard cruciate repair. D. Core stitch completed and knot buried in small incision used for starting point.
POSTOPERATIVE CARE
If a primary repair has been performed as described above, an immediate light active “place and hold” therapy regimen can usually begin safely as long as the patient is reliable.
OUTCOMES
A recent meta-analysis of multiple studies over the past 15 years found a rupture rate of 4% to 10% and good to excellent results in about three quarters of patients.8 Present-day techniques should allow for continued improvement in these results.
Injury mechanism is a predictor of outcome and is beyond the surgeon’s control. An uncomplicated, isolated sharp flexor tendon laceration that is treated acutely represents the best possible scenario for a highly functional digit. Additional injury to bone, tendon, or nerve negatively affects outcome.
COMPLICATIONS
Two extremes of bad outcomes are tendon rupture and tendon adhesions. Ruptures are rare, but adhesions, and resultant limited motion, are common.
If ruptures are noticed immediately, a repeat repair should be performed, although the patient and surgeon should be prepared for the need to proceed intraoperatively with other reconstructive options.
The management of tendon adhesions is discussed in Chapter HA-55.
REFERENCES
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11. Thurman RT, Trumble TE, Hanel DP, et al. Two-, four-, and sixstrand zone II flexor tendon repairs: an in situ biomechanical comparison using a cadaver model. J Hand Surg Am 1998;23A:261–265.
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