The flexor digitorum superficialis (FDS) tendon splits into two slips in zone 2 (the digital fibro-osseous sheath within the finger, between the A1 and A4 pulleys) to decussate around the FDP tendon at the Camper chiasm, then reunites and inserts on the middle phalanx base. This chiasmatic arrangement allows the FDP to pass through the FDS to reach the distal phalanx. The FDS is primarily tested by flexing a single finger PIP joint while the adjacent fingers are held in extension (isolating the FDS from the FDP which has a common muscle belly for the index-small fingers).
FDS repair in zone 2 (no man's land, between the proximal digital crease and the middle phalanx) was historically considered hazardous due to adhesion formation within the fibro-osseous sheath; Kleinert and Verdan revolutionised hand surgery by demonstrating reliable results with primary FDS and FDP repair in zone 2 using the Kessler core suture technique. The Camper chiasm split is an important anatomical detail in staged tendon reconstruction (Hunter-Zancolli technique): in stage 1, a silicone Hunter rod occupies the sheath and in stage 2 a tendon graft replaces the rod, threading through the FDS chiasm.
Sharp laceration across the palm-finger crease divides the FDS (and often FDP) within zone 2 of the fibro-osseous sheath, producing loss of PIP flexion; primary repair within 24-72 hours using core Kessler suture and running epitendinous suture followed by Kleinert early controlled mobilisation provides the best functional outcome.
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