Myotendinous junction tear of the anterior bundle of the supraspinatus muscle—a rare pattern of injury involving rotator cuff muscles

Myotendinous junction injuries are rare and often present with distinctive imaging findings that should be differentiated from purely tendinous degenerative ruptures. Myotendinous junction tears are common in the lower limb but rarely involve rotator cuff muscles. Considering rotator cuff muscles, the infraspinatus and supraspinatus muscles are the most frequently implicated. The intrinsic anatomy of the supraspinatus muscle gives it a greater contractile force and consequently a propensity for rupture. It is composed of two bundles: anterior and posterior (with the latest further divided in a deep anterior, a medial and a superficial posterior portion). These two components have distinctive anatomy with the anterior bundle having a long intramuscular tendon and bipennate configuration and the posterior bundle having a smaller intramuscular tendon and parallel muscle fibres. This distinctive anatomy grants a greater contractile force to the anterior bundle of the supraspinatus muscle and for this reason it is more prone to myotendinous rupture. This type of injury has been associated with a rapid progression to severe fatty infiltration and should be differentiated from purely tendinous tears that are more frequent and associated with degenerative changes. Myotendinous tears occur centrally located in the muscle belly and are not associated with full thickness tears of the distal tendon attachment.