Shoulder

Shoulder region connects arm with axial body.

Bony framework:

The underlying bony framework is composed of a shoulder girdle (or pectoral girdle) that connects upper limb with axial skeleton. Shoulder girdle consists of scapula, clavicle, and associated joints:

• Medially, clavicle articulates with sternum at sternoclavicular joint, which is synovial and saddle-shaped. It is reinforced by four ligaments: anterior and posterior sternoclavicular, interclavicular and costoclavicular ligaments. Laterally, clavicle articulates with acromial process of scapula at the acromioclavicular joint–a plain synovial joint. It is supported by coracoclavicular and acromioclavicular ligaments.

• Lateral angle of scapula has a glenoid cavity that articulates with head of humerus to form glenohumeral joint (a.k.a. shoulder joint).

• Glenohumeral joint is a ball and socket variety of synovial joint that allows wide range of movements including flexion, extension, adduction, abduction, circumduction, and internal and external rotation of arm. This extra mobility comes at the expense of reduced stability and increased chances of dislocation.

Stability factors for shoulder/shoulder joint:

To prevent dislocation, various factors provide stability to shoulder joint:

• Fibrocartilaginous rim–called glenoid labrum–around margins of glenoid cavity makes articulating surface less shallow, thus more stable.

• Fibrous capsule attached around anatomical neck of humerus and along margins of glenoid cavity offers additional joint stability. It is further strengthened by superior, middle, and inferior glenohumeral ligaments, coracohumeral and transverse humeral ligaments.

• Various synovial fluid-filled sacs or bursae–extensions of joint synovial membrane–are positioned around joint capsule and aid in mobility. Examples include subacromial, subdeltoid, subscapular, and subcoracoid bursae.

• Additional stability is provided by tendons of rotator cuff muscles (i.e., subscapularis, supraspinatus, infraspinatus, and teres minor muscles) whose tendons merge with capsule of shoulder joint to form a strong fibrotendinous rotator cuff sheath around joint. These muscles originate from scapula and insert into head of humerus.

• Posteriorly–behind shoulder–a group of three muscles connect medial border of scapula to vertebral column. These include levator scapulae, rhomboid major, and minor muscles, which pull scapula towards midline.

• Two additional superficial muscles include trapezius and deltoid. Trapezius is muscle of upper back, which crosses laterally behind shoulder to insert into acromian process of scapula and lateral part of clavicle. Next, deltoid muscle originates from same place of insertion of trapezius. Its muscle fibers cover shoulder and converge onto deltoid tuberosity in mid-arm.

• Lastly, a few other muscles in posterior shoulder region include teres major, latissimus dorsi (forming posterior axillary fold), and long head of triceps brachii muscle which ascend into shoulder to insert into inferior glenoid tubercle.

Neurovasculature:

• Arterial supply for shoulder region arises from axillary artery and its branches including superior thoracic artery, thoracoacromial artery, lateral thoracic artery, subscapular artery, anterior and posterior circumflex humeral arteries.

• Thyrocervical trunk of subclavian artery also gives off branches (subscapular, transverse cervical, and dorsal scapular arteries) that supply area of shoulder.

• Innervation of shoulder region and surrounding muscles comes from branches of brachial plexus.