Flexor Tendon Lacerations

Flexor tendon injuries are common hand-trauma injuries, typically in young working-age men from glass, knives and occupational sharp objects. Zone II is the most frequently injured and most studied zone. Understanding the zone system, repair biomechanics, and rehabilitation principles is essential for optimal outcomes — a digit that does not flex represents a time-sensitive surgical problem.

Tendon healing occurs by combined intrinsic (tenocyte-mediated, within the tendon) and extrinsic (from the sheath/surrounding tissue) pathways. Extrinsic healing forms adhesions that restrict gliding — the central biological problem in zone II. Repairs are weakest at roughly 7-10 days as collagen remodels, the period of peak rupture risk. Controlled tendon excursion biases healing toward the intrinsic pathway, which underpins early-motion rehabilitation.

Anatomy

Flexor Digitorum Profundus (FDP): Inserts on distal phalanx base. Flexes DIP joint.

Flexor Digitorum Superficialis (FDS): Splits around FDP (Camper's chiasm), inserts on middle phalanx. Flexes PIP joint.

Pulley System: A1-A5 (annular) and C1-C3 (cruciate) pulleys. A2 and A4 are critical for mechanical advantage. Others can be sacrificed if necessary.

Clinical Examination

FDP Test: Hold PIP extended, ask patient to flex DIP. Tests FDP independently.

FDS Test: Hold other fingers extended (blocking FDP contribution), ask to flex PIP. Tests FDS independently.

Cascading Posture: Normal resting hand has fingers in cascade of increasing flexion. Loss of cascade suggests laceration.

Wound Examination: Location indicates zone. Explore wound if tendon injury suspected (after tourniquet, in OR).

Flexor tendon laceration is largely a clinical and operative diagnosis — imaging is adjunctive.

• Plain radiographs: Mandatory after any sharp injury to exclude retained glass/foreign body, fracture or a bony avulsion fleck (suggests FDP avulsion / Jersey finger).
• Ultrasound: Useful for closed or doubtful injuries — confirms tendon continuity, identifies the level of a retracted proximal stump, and is operator-dependent.
• MRI: Reserved for complex, chronic or reconstructive planning (gap, sheath scarring, pulley integrity).
• Intraoperative assessment remains the reference standard.

Tendon Evaluation

Look for:

• Complete vs partial laceration (greater than 60% width = functionally complete)
• Level of laceration vs skin wound (tendons may have retracted with the digit position at injury)
• Associated injuries (digital nerves, vessels, bone)

A digit that will not flex is not always a tendon laceration. Distinguishing causes changes the operation, the timing and the consent.

Adhesions: Most common problem. Limit gliding. May require tenolysis.

Rupture: Peak at 7-10 days (weakest point in healing). Requires re-repair.

Stiffness: From adhesions or joint involvement. Address with therapy/tenolysis.

Bowstringing: If critical pulleys (A2, A4) not preserved or over-vented. Reduces mechanical advantage and excursion.

This topic repeatedly says to "preserve or reconstruct A2 and A4" and lists bowstringing as a complication, but the underlying mechanics — and what to actually do when a critical pulley is destroyed — decide whether a technically sound tendon repair becomes a working finger.

Why bowstringing matters

The annular pulleys hold the flexor tendon against the phalanges. This keeps the line of pull close to the bone, which minimises the moment arm at each joint and conserves tendon excursion — a given amount of muscle shortening is spent flexing the joints rather than lifting the tendon off the skeleton. When a long pulley (A2 or A4) is lost, the tendon lifts away from the bone and takes the chord across the flexing joint instead of following its arc:

• Excursion is wasted on the bowstring path, so the same FDP contraction produces less distal flexion — the fingertip can no longer reach the palm (loss of composite fist).
• The increased moment arm makes the proximal joint flex preferentially, producing an apparent flexion contracture and a weak, inefficient grip.
• A visible, palpable bowstring lifting off the volar finger is the late clinical sign.

A2 (over the proximal phalanx) and A4 (over the middle phalanx) matter most because they span the longest bony segments and resist the greatest bowstringing force; A1, A3 and A5 can be sacrificed with impunity.

How much pulley is enough — venting versus reconstruction

Modern practice (Tang, in the Evidence Base below) shows the system tolerates judicious venting: A2 may be partially incised over a length no greater than 1.5 to 2 cm, and A4 may be vented entirely, without observed bowstringing or functional loss. That defines the safe ceiling for incising a pulley to expose or deliver a repair. Beyond it — when both critical pulleys, or a long segment of one, are destroyed by trauma, infection or a failed repair — the pulley must be reconstructed to restore the tendon-to-bone relationship.

Reconstruction techniques

Pulley reconstruction recreates a smooth retinacular loop that holds the tendon (or, in staged cases, the Hunter rod) against the phalanx:

• Encircling free-graft loop — a strip of tendon graft (palmaris longus, a discarded slip of FDS, excised flexor tendon, or a toe extensor) is passed around the phalanx and sutured to itself. At the proximal phalanx the loop is passed deep to the extensor mechanism; over the middle phalanx it can lie superficial to it.
• Weave through the pulley rim (Weilby) — a graft is woven through the surviving rim remnants of the original pulley, useful when a partial pulley remains.
• Extensor-retinaculum graft (Lister) — a segment of dorsal extensor retinaculum is used, placing its smooth synovial surface against the gliding tendon.

Pulley reconstruction is integral to stage 1 of a two-stage Hunter-rod reconstruction, where the pulleys are rebuilt around the silicone rod so the later tendon graft glides correctly; the graft and staged technique themselves belong to flexor tendon reconstruction.

Outcome of a flexor repair is graded by active interphalangeal motion, because the whole point of repair is a finger that bends. The vivas and evidence on this page quote "good or excellent" results and a "TAM over 175" target without defining them — examiners expect the candidate to state the actual measurement.

Total Active Motion (TAM)

TAM is the global metric: the sum of active flexion at the MCP, PIP and DIP joints, minus any extension deficit (fixed flexion) at those joints. It captures both how far the finger flexes and how much it fails to straighten.

The Strickland–Glogovac formula

For a flexor repair, outcome is conventionally expressed as a percentage of normal interphalangeal motion, since the FDS and FDP act across the PIP and DIP. The Strickland–Glogovac formula is:

Percentage of normal = (active PIP flexion + active DIP flexion − PIP and DIP extension deficit) ÷ 175 × 100

The denominator 175 is the assumed normal combined IP arc (roughly 100 degrees at the PIP plus 75 degrees at the DIP). The result is then banded:

The modified Strickland system uses the same formula with more lenient bands, so the same finger scores a grade higher than under the original — which is why a paper must state which version it used when it reports "good/excellent" rates.

Tang criteria

The newer Tang grading (the system used in the zone-2 series cited in the Evidence Base) likewise sorts results into excellent, good, fair, poor and failure, but is built around the active flexion actually achieved and the residual extension loss, and reflects modern strong-core, early-active-motion outcomes. It is the system behind the "52 of 60 fingers good or excellent" result quoted on this page.

Global epidemiology: Flexor tendon lacerations are common hand-trauma injuries, predominantly affecting young working-age men, most often from glass, knives and occupational sharp objects. Zone II is the most frequently injured and most studied zone. The dominant injury pattern reflects manual work and domestic glass injuries worldwide, so the burden is highest in working populations and in regions with high rates of interpersonal and occupational sharp trauma.

Side-by-side guidance (recommendations converge more than they differ):

Registry note: There is no dedicated international flexor-tendon registry equivalent to the arthroplasty registries (NJR, AJRR, AOANJRR, SHAR). Best evidence therefore comes from multicentre cohorts and meta-analyses, which consistently report rupture rates of roughly 4-12% and good/excellent outcomes around 70-90% with modern strong-core repair and early active motion.

High- vs limited-resource practice variation: In well-resourced settings, repair is performed under loupe/microscope magnification with hand-therapy-supervised early active motion and, increasingly, WALANT. In limited-resource or remote settings, delayed primary or staged repair, simpler 2-strand techniques and immobilisation-based rehabilitation are more common because of restricted theatre access and limited specialist hand therapy — a recognised driver of higher adhesion and stiffness rates rather than a difference in principle.

Flexor tendon surgery is one of the most opinion-driven areas in hand surgery; examiners use these debates to separate safe from outstanding candidates.

• How critical are A2 and A4 really? The classic teaching that A2 and A4 must never be touched has softened. Tang's principle of judicious venting (A2 partially under 1.5-2cm, A4 entirely) reports no bowstringing or functional loss and is now mainstream, yet the safe limit of venting before mechanical disadvantage appears is still debated.
• Strand number — strong enough vs too bulky. More strands increase strength and resist gapping, but extra core passes add bulk and gliding resistance and prolong surgery. Many units now favour a well-tensioned 4- or 6-strand repair rather than chasing maximal strand counts.
• Suture caliber. Larger 3-0 suture intuitively seems stronger, but cadaver data show it fails by pulling through the tendon; 4-0 may give a more reliable suture-tendon interface in average-sized tendons.
• How much (and what kind of) peripheral suture? A formal circumferential epitendinous suture adds strength and smooths the repair, but tensioned strong-core series suggest only sparse peripheral stitches are needed, questioning routine elaborate epitendinous work.
• Should the FDS be repaired in zone II? Repairing both tendons restores independent PIP/DIP control but crowds the sheath and risks adhesions; some surgeons excise one FDS slip to make room. Evidence does not clearly favour either approach.
• Rehabilitation regime. True early active motion, place-and-hold, relative-motion-flexion orthoses and early passive motion all have advocates; the best protocol for a given repair strength and patient reliability is not settled.
• Anaesthesia. WALANT permits intraoperative active testing and is cost-effective, but high-quality comparative outcome data versus traditional anaesthesia remain limited.