Ligament · Hand

Annular pulleys of the fingers

Rings of dense fibrous tissue that arch over the flexor tendons of each finger and bolt them down to the skeleton, five to a finger and numbered A1 to A5 from the palm outwards. Two of them do most of the work: A2 over the first bone of the finger and A4 over the second arise directly from the periosteum of those bones and are much the strongest, while A1, A3 and A5 arise from the palmar plates at the joints. Their job is geometric rather than muscular — as the finger curls, the tendon inside wants to lift away from the bone and take the shortest line across the arc, and the pulleys hold it against the skeleton so the pull is spent bending the joints instead of being lost in slack. Without them a full effort at the muscle produces a fraction of the curl.