Emergency Medicine Hub / Trauma & Orthopedics • 3 min read • 2 sections

Hand Tendon & Nerve Injuries: Flexor & Extensor Systems

Comprehensive emergency evaluation and protocolized surgical management of acute hand trauma: flexor tendon system (FDP vs. FDS testing, flexor zones I through V; Zone II 'No Man's Land'; 'Jersey finger' avulsion of FDP at zone I; Kanavel's four cardinal signs of infectious flexor tenosynovitis); extensor tendon zones I through VIII (Zone I 'Mallet finger' terminal extensor disruption and continuous extension splinting x 6–8 weeks; Zone III 'Boutonnière deformity' central slip rupture); digital nerve anatomy (two-point discrimination < 6 mm); and emergency tendon repair vs. primary delayed repair protocols.

Resuscitation Quick Actions • First 2 Minutes

High-Acuity

FDP Isolated Test

Hold PIP in complete extension: patient flexes DIP joint independently; inability to flex DIP = FDP laceration or 'Jersey finger'

FDS Isolated Test

Hold ALL other fingers in flat extension (cancels FDP mass action): patient flexes PIP joint independently = tests FDS

Zone II 'No Man's Land'

Distal palmar crease to mid-phalanx: DO NOT attempt bedside primary tendon repair! Clean, close skin only, and refer to hand surgery

Mallet Finger Rule

DIP hyper-extension splint CONTINUOUSLY for 6–8 weeks; if DIP flexes for even one second, the entire 8-week clock restarts!

Kanavel's Four Signs

1) Symmetrical sausage digit, 2) Semi-flexed posture at rest, 3) Tenderness along entire flexor sheath, 4) PAIN ON PASSIVE EXTENSION

Two-Point Discrimination

Normal digital nerve two-point discrimination is < 6 mm at the fingertips; > 10 mm indicates digital nerve laceration

Bottom-Line Clinical Pearl

Hand injuries require structured anatomical testing under tension. For flexors: test Flexor Digitorum Profundus (FDP) by immobilizing the PIP and MCP joints and having the patient flex ONLY the DIP joint; test Flexor Digitorum Superficialis (FDS) by holding all other fingers flat in complete extension to cancel FDP and testing PIP flexion. Zone II ('No Man's Land', from distal palmar crease to middle phalanx) contains both FDS and FDP tendons tightly enclosed within the fibrous sheath; lacerations here mandate operative hand surgical repair, NOT bedside primary closure. 'Jersey Finger' is an avulsion of the FDP tendon from the distal phalanx (inability to flex DIP) requiring surgical reattachment within 7–10 days. 'Mallet Finger' is an extensor disruption at Zone I (inability to actively extend DIP); manage with STRICT CONTINUOUS DIP EXTENSION SPLINTING FOR 6 TO 8 WEEKS (if the DIP flexes for even one second during splint change, the 8-week clock restarts!). Kanavel's four cardinal signs identify surgical infectious flexor tenosynovitis.

1. Flexor Tendon Zones & Physical Examination Mechanics

The flexor tendon system comprises the Flexor Digitorum Profundus (FDP) (inserts on the base of the distal phalanx; flexes the DIP, PIP, and MCP joints) and the Flexor Digitorum Superficialis (FDS) (bifurcates around FDP to insert on the middle phalanx; flexes the PIP and MCP joints).

Flexor Tendon Zone	Anatomical Boundaries	Surgical Characteristics & Emergency Management
Zone I	Distal to FDS insertion on middle phalanx to the distal phalanx base	Contains FDP only. Injury: 'Jersey Finger' (forced hyperextension during active DIP flexion, e.g., grabbing opponent's jersey). FDP tendon retracts into the palm, avulsing vincula blood supply. Requires operative re-insertion within 7–10 days.
Zone II ('No Man's Land')	Distal palmar crease to the mid-portion of the middle phalanx (FDS insertion)	Most treacherous anatomical zone. Both FDS and FDP travel together inside a narrow, tight fibro-osseous tunnel (A1–A5 annular pulleys). Primary bedside repair in the ED is strictly contraindicated (inevitable scarring and adhesions freeze the finger). Clean wound, close skin loosely, splint in dorsal blocking splint, and refer for delayed primary repair by a hand surgeon within 7 days.
Zone III (Palm)	Distal edge of transverse carpal ligament to the distal palmar crease	Lumbrical muscle origins; associated with common digital nerve and superficial palmar arch injuries.
Zone IV (Carpal Tunnel)	Beneath the transverse carpal ligament	Contains 9 flexor tendons (4 FDS, 4 FDP, 1 FPL) and the median nerve.
Zone V (Distal Forearm)	Proximal border of transverse carpal ligament to the musculotendinous junctions	Extensive tendon, ulnar/median nerve, and radial/ulnar artery lacerations ('spaghetti wrist').

2. Kanavel's Four Cardinal Signs of Infectious Flexor Tenosynovitis

Infectious flexor tenosynovitis is a closed-space surgical emergency where puncture wounds inoculate bacteria (Staphylococcus aureus, MRSA) into the synovial flexor sheath, rapidly destroying tendon gliding and causing tendon necrosis within 24–48 hours:

Kanavel's Cardinal Sign	Clinical Physical Exam Finding	Diagnostic Significance
1. Uniform, Symmetrical Swelling	The entire length of the involved finger is diffusely swollen ('fusiform' or 'sausage digit').	Fluid accumulation throughout the closed synovial flexor sheath.
2. Flexed Finger Posture at Rest	The digit is held in slight, involuntary flexion at rest compared to adjacent fingers.	Minimizes hydrostatic pressure inside the closed synovial sheath to reduce pain.
3. Tenderness Along Flexor Sheath	Exquisite point tenderness elicited along the entire anatomic course of the flexor sheath (from DIP to proximal palm).	Direct inflammation of the visceral/parietal synovia.
4. Pain on Passive Extension (MOST SENSITIVE)	Excruciating pain elicited by gently extending the DIP or PIP joint while supporting the rest of the hand.	The single most sensitive and earliest physical finding! Tenses the inflamed flexor sheath over the phalanges. Mandates STAT Emergent Hand Surgery Consult for OR Sheath Washout + IV Vancomycin + Ampicillin-Sulbactam.

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