Biomechanical bases for tendon transfers at the shoulder.

Paralysis of the muscles around the shoulder is a debilitating condition that continues to be a very challenging problem. It leads to an inability to position one's hand in space. This greatly compromises the function of the upper limb and can lead to chronic shoulder pain due to inferior glenohumeral subluxation. Management of these complex problems has two main objectives: (i) stabilize the glenohumeral joint to decrease pain related to inferior glenohumeral subluxation; (ii) restore active range of motion in external rotation, abduction, and internal rotation. All the shoulder muscles contract in a coordinated and complex manner to allow the shoulder to move through a complete range of motion. Understanding how the different muscle groups coordinate their contractions and the basic biomechanical principles of tendon transfers is paramount before considering doing a tendon transfer around the shoulder. To function properly, a tendon transfer should have a similar line of pull (similar moment arm), similar tension and similar excursion to that of the muscle it replaces; one tendon transfer should replace only one function and the donor (transferred) muscle should have normal muscle strength (at least M4).