Upper Limb Anatomy

Understanding upper limb anatomy is crucial for diagnosing common musculoskeletal and neurological conditions, from fractures and dislocations to nerve entrapments and palsies. It's a high-yield area for SBAs and OSCEs, particularly regarding nerve injury presentations and their anatomical basis. Expect questions on brachial plexus lesions, specific nerve deficits, and common fracture sites with associated neurovascular risks.

The brachial plexus (C5-T1) is a complex network formed by ventral rami of spinal nerves, passing through the posterior triangle of the neck and axilla. Its terminal branches (Musculocutaneous, Axillary, Radial, Median, Ulnar) supply motor and sensory innervation to the entire upper limb. The arrangement of muscles into anterior (flexor) and posterior (extensor) compartments in the arm and forearm dictates their primary actions and innervation. For example, the anterior arm compartment (biceps, brachialis, coracobrachialis) is primarily innervated by the musculocutaneous nerve, while the posterior compartment (triceps) is by the radial nerve. The arterial supply follows a clear path: subclavian -> axillary -> brachial -> radial/ulnar, with important anastomoses around the elbow. Venous drainage involves superficial (cephalic, basilic, median cubital) and deep systems. The carpal bones (8 in two rows) provide flexibility and stability to the wrist; their specific arrangement is critical for hand function and vulnerability to injury.

Nerve injuries are a common exam topic. A mid-shaft humerus fracture puts the radial nerve at risk (wrist drop). A supracondylar humerus fracture can injure the median nerve and brachial artery, leading to Volkmann's ischaemic contracture. Shoulder dislocations (usually anterior) can damage the axillary nerve, causing deltoid weakness and sensory loss over the 'regimental badge' area. Ulnar nerve compression at the elbow (cubital tunnel) or wrist (Guyon's canal) results in 'claw hand' deformity and sensory loss in the medial 1.5 digits. Median nerve compression in the carpal tunnel presents with paraesthesia in the lateral 3.5 digits and thenar muscle wasting. Scaphoid fractures require high suspicion, especially with anatomical snuffbox tenderness, due to the risk of avascular necrosis and non-union if missed. Rotator cuff tears (supraspinatus most common) present with shoulder pain and weakness, particularly on abduction.

For suspected fractures, plain X-rays (AP and lateral views) are the first line. For scaphoid fractures, specific scaphoid views or repeat X-rays after 7-10 days are often needed if initial films are normal but clinical suspicion is high. Nerve injuries may require Electromyography (EMG) and Nerve Conduction Studies (NCS) to confirm diagnosis, localisation, and severity. Ultrasound can be used for soft tissue injuries (e.g., rotator cuff tears, tendinopathies) or to assess nerve compression. MRI is excellent for detailed soft tissue assessment, including ligaments, tendons, and cartilage (e.g., labral tears, complex rotator cuff pathology).

Management involves immobilization (casts, splints, slings) for fractures and some soft tissue injuries, physiotherapy for rehabilitation, and sometimes surgical intervention (e.g., ORIF for displaced fractures, carpal tunnel release for severe Carpal Tunnel Syndrome, rotator cuff repair).