Digital Fibrous Sheath
Proximal to the metacarpophalangeal joint, the tendon pair of the superficial and deep flexor enter a Fibrous Digital Sheath. The sheath extends from the palmar ligament of the metacarpophalangeal joint to the insertion of the deep tendon into the base of the terminal phalanx. There are five fibrous tissue bands overlying the synovial sheath of the flexor tendons. They are numbered from proximal to distal (A-1 through A-5). The A-2 and A-4 pulleys are the strongest (annular in form, rigid and thick), and are associated with the shafts of the proximal and intermediate phalages. The A-1, A-3, and A-5 are the weakest (cruciate in form, pliable and thin), and associated with the metacarpophalangeal and the proximal and distal interphalangeal joints.
"No man's land" of the hand is the area where the tendon pair of the superficial and deep flexors of the fingers pass through a tight fibrous tunnel. Designated as Zone II, it is the area between the distal palmar crease and the midpoint of the intermediate (middle) phalanx. Repair of tendon lacerations in this region requires meticulous technique and knowledgeable postoperative management.