Chapter 49: Shoulder Girdle & Arm Anatomy

The joints of the upper limb show a proximodistal increase in complexity; stability is gained through ‘close-packing’ mechanisms, most extensively in the hand's carpus, though glenohumeral stability is almost entirely dependent on muscular activity. The skeletal framework is comprised of the crank-shaped clavicle, which begins ossifying before any other bone, transmitting limb weight to the axial skeleton. Fractures frequently occur at the transition zone between the antecurve and retrocurve, potentially displacing the lateral fragment downwards and forwards. The large, triangular scapula functions via interconnected suspension, motion, and articulation mechanisms. It features three robust columns for load-bearing and several fossae for muscle attachment: subscapular, supraspinous, and infraspinous. Developmental anomalies like congenital undescended scapular syndrome (Sprengel deformity) involve altered development of the scapular body but generally unaffected processes. The humerus, the longest upper limb bone, features a head that is posteriorly rotated relative to the inter-epicondylar axis in the anatomical position, allowing for a great range of external rotation compared to quadrupeds. Key joints include the sternoclavicular joint, the sole skeletal articulation to the axial skeleton, stabilized largely by strong ligaments and the articular disc; the acromioclavicular joint, stabilized primarily by the coracoclavicular ligaments; and the highly mobile glenohumeral joint. Glenohumeral stability relies heavily on concavity compression created by the rotator cuff muscles (subscapularis, supraspinatus, infraspinatus, teres minor, and functionally including teres major). Muscles are organized into functional groups, where thoracobrachial muscles like deltoid and pectoralis major primarily position the elbow and hand in space. Injuries to the accessory nerve (trapezius palsy) or long thoracic nerve (serratus anterior palsy) result in characteristic scapular winging, which can be clinically distinguished. The axillary artery is divided into three parts by pectoralis minor, supplying the ventral compartments, while two branches of the subclavian artery, the dorsal scapular and suprascapular arteries, perfuse the dorsal suspension and rotator cuff muscles. Crucial neurovascular bundles pass through the quadrangular and triangular spaces. The axillary nerve innervates deltoid and teres minor, and suprascapular nerve injuries result in severely reduced abduction, often with proximal humeral head migration.