The gleno-humeral joint is formed by the hemispherical surface of the humeral head and the pear-shaped glenoid cavity. The shoulder’s impressive range of motion is possible due to the disproportion between the size of the humeral head and the glenoid cavity. The disproportion is quantified by the gleno-humeral index, which is defined as normal when accounting for 0.86 in the sagittal and 0.58 in the axial axis [1, 2]. Besides the articular surface the greater tuberosity and the lesser tuberosity also have specific biomechanical functions. The greater tuberosity is the insertion site of the supraspinatus and the infraspinatus muscle’s tendons. The subscapularis tendon inserts at the lesser tuberosity. Further bony structures of the shoulder joint that should be considered are the acromion and the coracoid process, which are both located cranial to the gleno-humeral joint [3, 4]. Two muscles, the coracobrachialis and the biceps brachii muscle, arise from the coracoid process. In addition, the pectoralis minor muscle inserts at the coracoid process. The coracoid process is located on the lateral edge of the antero-superior portion of the scapula. The acromion is also a bony process and together with the clavicle it forms the top of the shoulder. The acromio-clavicular joint is of importance for raising the arm above the shoulder level. [5]

The circularly shaped glenoidal labrum enlarges the articular surface of the glenoid. The articular capsule arises from the medial glenoid surface and spherically encases the gleno-humeral joint [6, 7]. In neutral position and adduction of the arm the caudal parts of the capsule form the axillary recess. There are three gleno-humeral ligaments, the superior-, middle- and inferior gleno-humeral ligament,, which are all located ventral to the gleno-humeral joint to stabilize the articular capsule [8]. The subacromial space with the subacromial bursa is located between the articular capsule und the acromion. [5]