Flexor Tendon Laceration - Complete Clinical Guide

A "cat cut" (sharp laceration, often from glass, knife, or cat scratch/bite) over the palmar side of the hand or finger that severs a flexor tendon produces inability to flex (bend) the finger because the muscle-to-bone force transmission is physically interrupted. This is a true hand surgery emergency.

Anatomy - Why the Finger Can't Move

Two flexor tendons run through each finger inside a fibro-osseous tunnel (the flexor sheath):

Tendon	Insertion	Action
Flexor Digitorum Profundus (FDP)	Base of distal phalanx	Flexes DIP joint
Flexor Digitorum Superficialis (FDS)	Base of middle phalanx	Flexes PIP joint

When either or both are cut, the relevant joint cannot bend. The finger loses its natural resting cascade posture (the slightly curved "waterfall" alignment of the fingers at rest).

Clinical Signs

Loss of the digital cascade - the injured finger sits extended/straight while others remain in natural flexion at rest
FDP test: Hold MCP and PIP joints in extension - ask patient to flex the DIP joint only. Inability = FDP cut
FDS test: Hold all other fingers in full extension (to neutralize FDP), then ask to flex at PIP. Inability = FDS cut
Weakness or pain with resisted flexion may indicate a partial laceration (which can be clinically silent but still dangerous if untreated)

Tendon Injury Zones

The hand is divided into 5 zones, which determine repair difficulty and prognosis:

Zone	Location	Key Notes
Zone 1	Distal to FDS insertion (fingertip)	Only FDP injured; "jersey finger"; blood supply at risk
Zone 2	Distal palm to mid-middle phalanx	Historically "no man's land" - tight sheath, technically demanding repair
Zone 3	Distal to carpal tunnel → proximal flexor sheath	Generally good outcomes
Zone 4	Carpal tunnel	Median nerve injury common
Zone 5	Proximal to carpal tunnel	Often multiple tendons + nerve/vessel involvement

Sabiston Textbook of Surgery, p. 1187-1194

Emergency Management

Immediate steps in the ED:

Do NOT close the wound and discharge without tendon assessment - partial lacerations can be clinically silent yet still catastrophic if missed
Examine position-dependently - test the tendon with the hand in the same position as when the injury occurred (the cut tendon end moves with the finger)
Splint in blocking position to prevent further retraction:
- Wrist neutral to slightly flexed
- MCP joints flexed 70-90°
- IP joints minimally flexed 10-15°
- Thumb spica if thumb involved
Antibiotics for open wounds: Cephalexin 500 mg TID x 7-10 days (doxycycline 100 mg BID x 7-10 days if penicillin-allergic)
X-ray to rule out avulsion fractures

Urgency of hand surgery referral:

Situation	Timing
Open fracture, dislocation, arterial injury, grossly contaminated, or bite wound	Emergency consult same day
Clean laceration with confirmed tendon injury	Follow-up within 3 days
Zone 1 avulsion with disrupted blood supply	Surgery within 7-10 days
Delayed presentation	Within 2-3 weeks maximum (after this, scarring + retraction require tendon grafting)

Rosen's Emergency Medicine, pp. 3040-3049

Surgical Repair

Primary repair (within 24 hours): end-to-end tenorrhaphy using a core strand suture technique
Delayed primary repair (up to 3 weeks): acceptable when experienced specialist unavailable and wound is clean
Secondary repair / tendon grafting: required if > 14 days have passed and tendon retraction/scarring have occurred; may need silicone rod spacer placement as a first stage
Repairs in Zone 2 are hardest and most prone to adhesion formation; require early protected mobilization therapy post-op

A 2025 systematic review (PMID 41187290) confirms ultrasound and MRI are both useful for pre- and post-operative assessment of flexor tendon injuries.

Complications if Untreated or Poorly Managed

Permanent loss of finger flexion
Tendon retraction making repair impossible without grafting
Tendon adhesions (commonest complication)
Extensor lag or joint contracture
In zone 1 avulsions: avascular necrosis of the tendon end

Key Takeaway

Any palmar-side laceration over a finger or hand that produces loss of active finger flexion = flexor tendon laceration until proven otherwise. The patient needs hand surgery referral urgently, proper splinting, antibiotics, and should not be reassured that the wound is "just a cut." Partial lacerations can look deceptively minor but still propagate or cause triggering.

Roberts and Hedges' Clinical Procedures in Emergency Medicine
Rosen's Emergency Medicine, Chapter 42
Sabiston Textbook of Surgery, Chapter 119
Campbell's Operative Orthopaedics, 15th Ed 2026

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