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Basic Science-Anatomy and Biomechanics

The humerus is the largest bone of upper extremity. The proximal humerus is composed of the humeral head, the greater and lesser tuberosities, the anatomic and surgical necks of the humerus. The humeral head is the most proximal, ball-like region of the humerus that is retroverted (28 to 40 degrees) and articulates with the glenoid cavity of the scapula.

The head of the humerus is significantly larger than the glenoid fossa. There is no bony structural stability to the joint. The motion of the glenohumeral joint completely relies on the balance of tension and compression of soft tissues attached to the proximal humerus and the scapula. After a fracture occurs, the pull of shoulder muscles becomes the deforming force dictating the pattern of the fracture.

In addition, understanding a relationship between bony fragment position, soft tissue balancing and glenohumeral joint function is essential for successful outcome after proximal humerus injury. The greater and lesser tuberosities are close to the humeral head and they are insertion sites for the rotator cuff. Both tuberosities are separated by the intertubecular groove in which lies the tendon of the long head of the biceps muscle and the arcuate artery, a branch of the anterior circumflex humeral artery. The intertubercular groove is covered by the transverse humeral ligament. The humeral head and tuberosities are separated by a narrow region which represents the anatomic neck of the humerus.

Distal to anatomic neck proximal humerus narrows connecting to humeral shaft. This area is called surgical neck and represents most vulnerable part of the proximal humerus. This area also contains the main arterial supply to the proximal humerus, anterior and posterior circumflex arteries that arise from axillary artery. Both circumflex arteries make numerous extra-osseous collaterals that help to supply the most proximal areas of the proximal humerus.

Despite the rich blood supply, avascular necrosis (AVN) of the humeral head is still a common sequelae of the proximal humerus fractures particularly if majority of the bony and soft tissue have been detached from the humeral head. The shoulder’s innervation is derived from brachial plexus. From a clinical perspective, there are three important nerves in close proximity of the proximal humerus: These include the axillary, suprascapular, and musculocutanneous nerve.

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