The flexor sheath is a smooth tunnel running up the front of each finger. It holds the flexor tendons down to the underlying bones.
It is like the eyelets in a fishing rod.
Thickenings of the sheath are called pulleys.
There are two types of pulleys.
The annular pulleys are relatively strong. There are five separate annular pulleys.
The A2 and A4 pulleys are tough and relatively inflexible and arise from the underlying bone.
The A1 , A3 and A5 pulleys arise from the ligamentous tissues ( Volar Plate ) on the front of the respective joints.
The thumb has only two annular pulleys.
The cruciate pulleys are soft and flexible so that they can get out of the way during finger flexion.
The A2 and A4 pulleys are considered to be the most important.
Bowstringing
Bowstringing occurs when there is a complete pulley rupture.
When a finger bends it rotates around the so-called centre of rotation of the joint.
Any tendon which is lying below the centre of rotation is an extensor of the finger and any tendon above the centre of rotation is a flexor. See diagram.
There is a fine balance between the forces of the finger extensor ( straightening ) and flexor (bending ) tendons.
It is similar to the forces on a seesaw.
If the same weight on a seesaw is moved further away from the centre of the seesaw it has a relatively greater effect.
This explains the typical development of a flexion deformity of the finger when there has been a significant pulley rupture.
It also results in loss of full finger flexion. The flexor tendons move away from the joint and although the muscle contraction force has not changed the relative force across the joint has increased.
Most commonly significant bowstringing will only occur if at least two sequential pulleys have been completely ruptured but there have been documented cases of it occurring in isolated A2 ruptures.
Bowstringing leads to flexion contracture of the fingers and grip strength weakness.
04 / 08 / 2021