420 - Anatomical Versus Reverse Shoulder Arthroplasty

Like the spine of the scapula and the acromion, it also is easily palpable on a patient. The clavicle articulates at one end with the sternum (chest bone) and with the acromion at the other end. This articulation between the clavicle and the acromion (acromioclavicular or AC joint) forms the roof of the shoulder. The proximal end of the humerus consists of the head, neck, greater and lesser tuberosities (or tubercles) and the shaft. The head is hemispherical in shape and projects onto the glenoid cavity. The anatomical neck lies between the head and the greater and lesser tuberosities, which are prominent landmarks on the humerus, and serve as attach- ment sites for the rotator cuff muscles. There are four joints located within the shoulder region: • The shoulder joint is known as the glenohumeral joint. This is the ball and socket joint between the humeral head and the glenoid cavity of the scapula • The acromioclavicular (AC) joint – where the clavicle meets the acromion of the scapula • The sternoclavicular (SC) joint – where the clavicle meets the sternum • The scapulothoracic joint – where the scapula meets the ribs at the back of the chest The head of the humerus does not fit into the glenoid, as the glenoid cavity is too shallow. This is what helps to allow for the wide range of movement provided by the shoulder. The increased mobility, however, leads to decreased skeletal stability. Joint stability is provided instead by the labrum (a soft, fibrous tissue rim that surrounds the shoulder socket), the contour of the humeral head and glenoid, and the rota- tor cuff muscles. The rotator cuff consists of four distinct muscles and their tendons that keep the head of the humerus in the shoulder socket. It also helps a person to raise, lower and rotate their arm. These muscles are commonly referred to as the SITS muscles, which is an acronym for supraspina- tus, infraspinatus, teres minor and subscapularis. Each one of these muscles is a part of the rotator cuff and plays an important role: • Supraspinatus – holds the humerus in place and keeps the upper arm stable, while also helping to lift the arm • Infraspinatus – is the main muscle that allows for exter- nal rotation and extension of the shoulder • Teres minor – is the smallest rotator cuff muscle whose main job is to assist with rotation of the arm away from the body • Subscapularis – holds the humerus to the shoulder blade and helps internally rotate the arm, hold it straight out, and lower it The tendons associated with each of these four muscles originate on the scapula and inserts into the humerus. A N A T O M I C A L V E R S U S R E V E R S E S H O U L D E R It is important to distinguish the similarities and differenc- es between an anatomic and reverse shoulder arthroplasty. For example, the stem can be the same regardless of which type of shoulder arthroplasty is performed. If a surgeon begins a procedure planning for an anatomic total shoul- der and changes over to a reverse arthroplasty after seeing the patient’s anatomy, the choice of stem probably will not change. The positioning and implantation of the stem, how- ever, may be different. What does change is the role of the glenoid and humeral head. The shoulder is a ball-and-socket joint, just like the hip, although the shoulder is less constrained. Where the femoral head is the ball and the acetabulum is the socket in the hip joint, the humeral head is the ball and the glenoid is the socket in the shoulder. Unlike the hip, where the ball and socket roles are never reversed, they can be reversed during a shoulder replacement. In a reverse shoulder arthroplasty, the humerus houses the socket while the glenoid becomes the site of attachment for the ball of the shoulder joint. The new socket component on a reverse shoulder will fit into the humeral stem exactly the same way a head would on an anatomic total shoulder. This is why the same stem can be used regardless of which type of procedure is being performed. The integrity of the rotator cuff will determine wheth- er a surgeon will perform an anatomic or reverse shoulder arthroplasty. The integr ity of the rotator cuff will determine whether a surgeonwill performan anatomic or reverse shoulder arthroplasty. | The Surgical Technologist | DECEMBER 2018 548