The tendons will always travel in a straight line unless restrained by a pulley. Ie they take the shortest line between the base and the tip of the finger.

A Pulley is defined as “A wheel with a grooved rim in which a rope, chain, belt … or tendon can run, in order to change the direction or point of application of a force applied to the … tendon”

       

The primary role of the flexor pulley system is to convert linear translation and force in the muscle-tendon into rotation and torque at the finger joints.

The sheath holds the flexor tendons close to the bone allowing them to effectively ‘turn a corner’ and transfer the force developed in the muscle-tendon unit into movement at the phalanges

The tendon has a variable radius as the finger flexes. The force vector is constant.

Longitudinal tendon motion is converted into angular joint motion.

 

            

Loss of the pulley function can result in a fixed flexion deformity of the finger due to bowstringing of the tendons and also a limitation in full flexion.