Recognizing a Zone II Flexor Tendon Injury
Missing a flexor tendon injury, or misjudging why zone II lacerations heal worse than injuries elsewhere in the finger, leaves a patient with a digit that cannot flex, and recognizing the exam findings and the anatomy behind them is what keeps that from happening.
Takeaways
- Zone II earns the name no man's land because early primary repair attempts there failed so consistently that pioneering hand surgeons abandoned the approach. Sterling Bunnell's own figure legend defined the zone for primary tendon suture as the region between the distal palmar crease and the middle crease of the finger, and surgeons before 1960 favored secondary grafting over primary repair there (Kotwal and Ansari, Indian J Orthop 2012;46:608-615, PMID 23325961).
- Zone II runs from the A1 pulley to the FDS insertion on the middle phalanx. Zone I lies distal to that insertion and contains only FDP, zone III spans the palm proximal to A1, zone IV sits inside the carpal tunnel, and zone V runs from the carpal tunnel to the musculotendinous junction in the forearm (Alkandari et al, Cureus 2023;15:e34360, PMID 36874729).
- FDS flexes the PIP joint and FDP flexes the DIP joint, the basis for testing each tendon independently at the bedside. FDS has a separate muscle belly for each finger and acts fairly independently, while all four FDP tendons share a common muscle belly in the forearm, which is why isolating one finger's DIP flexion against the others is part of the standard exam.
- Inside zone II, the FDP tendon threads through a split in the FDS tendon at Camper's chiasm, which is why a laceration there can sever FDP completely while leaving FDS intact. The deep tendon lies directly beneath the wound at this level even though the more superficial FDS survives (Alkandari et al, Cureus 2023;15:e34360, PMID 36874729).
- A fully lacerated FDP tendon does not always sit at the skin wound. Once cut, the tendon can retract proximally into the palm, so a surgeon exploring the wound may not find the cut end without extending the exploration, and a missed retracted tendon is a recognized cause of delayed diagnosis (Alkandari et al, Cureus 2023;15:e34360, PMID 36874729).
- Zone II heals worse than the rest of the finger because two tendons are forced to glide through one tight fibro-osseous tunnel with a limited blood supply. Injury to that shared sheath produces adhesions that restrict tendon gliding long after the laceration itself has healed (Alkandari et al, Cureus 2023;15:e34360, PMID 36874729).
- Early controlled motion after repair replaced weeks of cast immobilization because immobilization produced unacceptably stiff, scarred fingers. Modern rupture rates after zone II repair with an early motion protocol run about 4% to 6%, and Kleinert's early report of 87% good to excellent results helped drive the shift away from splinting (Kotwal and Ansari, Indian J Orthop 2012;46:608-615, PMID 23325961).
- Flexor tendon injuries are uncommon but disproportionately feared. A population-based study measured an incidence of 7.0 per 100,000 person-years, higher in men, yet these injuries remain among the most challenging hand injuries a surgeon manages (Manninen et al, Scand J Surg 2016;106:278-282, PMID 27550244; Klifto et al, J Am Acad Orthop Surg 2018;26:e26-e35, PMID 29303923).