A distal-to-proximal 5-zone (I-V) classification system has been developed based on location, treatment considerations and prognosis . The thumb flexor tendon injury zones differ from the fingers as the thumb has one less phalanx.
Zone I contains only the FDP tendon and extends from the insertion of the FDP to the insertion of the FDS tendon.
Zone II is the area extending from the insertion of the FDS tendon to the distal palmar crease (proximal end of the A1 pulley). This area is also known as 'No-Man's land', due to the shared flexor sheath and a higher risk of adhesions.
Zone III is the palm area from the distal palmar crease (proximal end of the A1 pulley) to the distal border of the transverse carpal ligament.
Zone IV is within the carpal tunnel.
Zone V is proximal to the carpal tunnel in the distal forearm.
Zone TI is from the insertion of the flexor pollicis longus (FPL) to the proximal part of the A2 pulley.
Zone TII is from the proximal part of the A2 pulley to the distal part of the A1 pulley.
Zone TIII is proximal to the A1 pulley as far as the carpal tunnel.