Arm and Humerus — Surgical Anatomy and Approaches
Introduction
The arm — extending from the shoulder to the elbow — contains the humerus and its surrounding muscular envelope, with the principal neurovascular structures (musculocutaneous nerve, radial nerve, median nerve, ulnar nerve, brachial artery, brachial vein) traversing the segment in characteristic relationships to the bone. The surgical approaches to the humerus are dictated by the location of pathology along its length and by the proximity of the radial nerve to the bone in the spiral groove, the principal neurological consideration in approaches to the middle and distal thirds of the humerus. The two principal approaches — the anterolateral approach for proximal and middle thirds and the posterior approach for middle and distal thirds — together provide access to most regions of the humeral shaft and to the surrounding muscular structures. This chapter, drawing on Orthopaedic Surgical Approaches, Netter’s Concise Orthopaedic Anatomy, and Gray’s Anatomy, addresses the surgical anatomy and principal approaches to the arm and humerus.
Bony Anatomy and Compartments
The humerus is the largest long bone of the upper extremity, extending from the glenohumeral joint proximally to the elbow joint distally. The principal anatomical regions are the proximal humerus (head, anatomical neck, greater and lesser tuberosities, surgical neck; see Topic Trauma-13 for fracture anatomy), the humeral shaft (diaphysis) with its characteristic anterior bow and the spiral groove on the posterior surface, and the distal humerus (medial and lateral columns, trochlea, capitellum; see Topic Trauma-15). The arm has two principal muscular compartments: The anterior compartment contains the biceps brachii (with its short head from the coracoid and long head from the supraglenoid tubercle, both inserting via the bicipital aponeurosis on the radial tuberosity), the coracobrachialis (from the coracoid to the medial humerus), and the brachialis (from the lower humerus to the coronoid process of the ulna). The compartment is supplied by the musculocutaneous nerve (C5-C7) — which pierces coracobrachialis approximately 5 to 8 cm distal to the coracoid, then runs between biceps and brachialis to emerge as the lateral antebrachial cutaneous nerve. The posterior compartment contains the triceps brachii with three heads (long head from the infraglenoid tubercle, lateral head from the upper posterior humerus, medial head from the lower posterior humerus). The triceps inserts on the olecranon process of the ulna. The compartment is supplied by the radial nerve (C5-T1), which crosses through the spiral groove of the humerus from medial-superior to lateral-inferior, supplying branches to the triceps before becoming superficial laterally and passing anterior through the lateral intermuscular septum.
Neurovascular Structures
Radial Nerve in the Arm The radial nerve is the principal neurological structure at risk in approaches to the humerus. It branches from the posterior cord of the brachial plexus, descends posterior to the brachial artery, and enters the spiral groove of the posterior humerus, where it passes obliquely from medial-superior to lateral-inferior. In the spiral groove, the nerve lies immediately adjacent to the periosteum (within 5 mm), with no muscular protection — this is the anatomical fact underlying the high rate of radial nerve injury in humeral shaft fractures (Topic Trauma-14) and the demand for meticulous identification and protection during surgical approaches to the posterior and lateral humerus. After traversing the spiral groove, the radial nerve pierces the lateral intermuscular septum at the junction of the middle and distal thirds of the humerus and enters the anterior compartment of the distal arm, where it lies between the brachialis (medial) and the brachioradialis / extensor carpi radialis longus (lateral). At the level of the elbow, the radial nerve divides into the superficial sensory branch and the deep motor branch (posterior interosseous nerve) — the PIN passing through the arcade of Frohse at the proximal margin of the supinator and into the forearm.
Musculocutaneous Nerve The musculocutaneous nerve branches from the lateral cord of the brachial plexus and pierces the coracobrachialis muscle (typically 5 to 8 cm distal to the coracoid). It then runs between the biceps and brachialis muscles in the anterior compartment, supplying both muscles and the coracobrachialis. It emerges from the lateral aspect of the biceps as the lateral antebrachial cutaneous nerve (sensory) at the level of the elbow. Brachial Artery and Median/Ulnar Nerves The brachial artery descends along the medial bicipital groove between the biceps and the medial intermuscular septum, accompanied by the median nerve (which runs lateral to the artery proximally and crosses to the medial side at the elbow). The ulnar nerve is in the posterior compartment for most of the arm but is more medial than the radial nerve and is generally not at risk in standard humeral approaches. Lateral and Medial Intermuscular Septa The lateral intermuscular septum separates the anterior and posterior compartments laterally and runs from the deltoid tuberosity to the lateral epicondyle. The medial intermuscular septum is the medial counterpart from the coracobrachialis insertion to the medial epicondyle. The radial nerve pierces the lateral intermuscular septum at the junction of the middle and distal thirds of the humerus, a critical anatomical landmark for identifying the nerve during surgical approaches.
The Anterolateral Approach to the Humerus
The anterolateral approach provides access to the proximal and middle thirds of the humeral shaft for plate fixation of fractures, tumor resection, infection drainage, and biopsy. The approach uses the natural plane between the biceps and brachialis muscles. Patient Positioning The patient is supine with the affected arm draped freely, often supported on an arm board or hand table. The shoulder is positioned in slight abduction.
Surface Landmarks and Incision The deltopectoral groove marks the proximal limit of the approach; the lateral margin of the biceps marks the line of the incision distally. The skin incision extends from just below the deltoid insertion (deltoid tuberosity) distally along the lateral border of the biceps for the desired length. Approach The skin and subcutaneous tissue are incised. The fascia of the arm is opened in line with the skin incision. The biceps muscle is identified and retracted medially. The brachialis muscle is now visualized; the muscle is split longitudinally in line with its fibers at the midline (the lateral two-thirds of the brachialis is supplied by the musculocutaneous nerve, the medial third by the radial nerve, so the midline split preserves at least partial innervation of both sides). Beneath the brachialis split, the humerus is exposed subperiosteally. The radial nerve must be specifically identified and protected if the dissection is to extend into the distal third of the humeral shaft — the nerve pierces the lateral intermuscular septum at this level and enters the brachialis from the radial side. Extension Distally The anterolateral approach can be extended distally to expose the elbow joint by curving the incision laterally over the lateral epicondyle, developing the interval between brachialis (medial) and brachioradialis (lateral), with the radial nerve protected in this interval. Complications The complications of the anterolateral approach include musculocutaneous nerve injury (rare; preserved within the muscle split), radial nerve injury (the principal concern when the approach is extended into the distal third), and stiffness of the elbow if the approach is extended distally with extensive soft-tissue disturbance.
The Posterior Approach to the Humerus
The posterior approach provides access to the middle and distal thirds of the humerus for plate fixation of distal-third fractures, radial nerve exploration, distal humerus fracture fixation, triceps tendon procedures, and tumor or infection access. The
approach uses the natural plane between the lateral and medial heads of the triceps, or the triceps-splitting approach, depending on the desired exposure. Patient Positioning The patient is positioned in lateral decubitus with the affected side up and the arm draped over a padded support, or prone with the arm on a small platform. The arm is positioned with the elbow flexed approximately 90 degrees. Surface Landmarks and Incision The olecranon distally and the midline of the posterior arm proximally mark the line of the incision. The skin incision extends along the midline of the posterior arm from approximately 10 to 15 cm proximal to the olecranon distally to the olecranon. Approach — Triceps-Splitting Technique The skin and subcutaneous tissue are incised. The triceps fascia is opened in the midline. The triceps muscle is split longitudinally in the midline between the lateral and medial heads (or, alternatively, the long head can be split centrally). The radial nerve must be specifically sought and protected at the lateral aspect of the proximal third of the humerus, where it traverses the spiral groove — palpation along the posterior humerus identifies the nerve, which lies in the deep groove against the periosteum. The split is carried down to the periosteum, and subperiosteal dissection exposes the humerus. The profunda brachii artery accompanies the radial nerve and must be protected. Approach — Paratricipital Technique (Bryan-Morrey) The paratricipital (Bryan-Morrey) approach retracts the triceps medially or laterally rather than splitting it. The lateral version develops the interval between the triceps and the lateral intermuscular septum, retracting the triceps medially. The medial version develops the interval between the triceps and the ulnar nerve / medial intermuscular septum, retracting the triceps laterally. The approach preserves triceps function but provides less direct access to the distal humerus. Approach — Triceps-Reflecting Technique (Bryan-Morrey) The triceps-reflecting (Bryan-Morrey) approach detaches the triceps insertion from the olecranon and reflects it proximally. The approach provides extensive exposure of the distal humerus but creates a triceps insertion that must be repaired at closure; postoperative triceps weakness is a concern. Approach — Olecranon Osteotomy For the most extensive exposure of the distal humeral articular surface, the olecranon osteotomy (chevron-shaped osteotomy of the olecranon) is performed. The olecranon is osteotomized at its narrowest point, and the proximal fragment with the attached triceps is reflected proximally. The osteotomy is fixed at closure with tension-band wiring or a plate.
The approach provides excellent visualization of the distal humeral articular surface for complex intra-articular fractures and is the standard approach for distal humerus ORIF (Topic Trauma-15). Complications The complications of the posterior approach include radial nerve injury (the principal concern; the nerve is identified and protected in the spiral groove), profunda brachii artery injury, triceps weakness (with the reflecting and olecranon osteotomy approaches), ulnar nerve injury (with medial paratricipital approach if performed without explicit ulnar nerve identification and protection), and nonunion of the olecranon osteotomy (with the olecranon osteotomy approach).
The Anterior Approach to the Distal Humerus
The anterior approach to the distal humerus is used for selected procedures involving the distal humerus, the elbow joint, or the brachial artery. The approach is less commonly used than the anterolateral or posterior approaches but has specific indications. Approach A longitudinal incision is made over the anterior aspect of the distal arm in the line of the brachial artery. The brachialis is identified and retracted laterally; the biceps is retracted medially. The brachial artery and median nerve are identified and retracted as needed. The anterior aspect of the distal humerus is exposed through this approach. The approach is occasionally extended distally as the Henry approach to the proximal forearm and elbow (see Topic Anatomy-5).
Compartment Anatomy and Fasciotomy of the Arm
The arm compartments are the anterior compartment (biceps, brachialis, coracobrachialis) and the posterior compartment (triceps). Arm compartment syndrome is uncommon but can occur after high-energy trauma, vascular injury with reperfusion, or prolonged immobilization. Fasciotomy of the arm involves: Anterior compartment release through a longitudinal anterior incision with division of the fascia overlying the anterior compartment. Posterior compartment release through a longitudinal posterior incision with division of the fascia overlying the triceps. Both incisions can be combined with carpal tunnel release if hand compartment syndrome is also present.
Approach to the Brachial Artery and Median Nerve
The brachial artery and median nerve descend along the medial bicipital groove. Surgical access to these structures uses a longitudinal incision along the medial bicipital
groove, with the biceps retracted laterally and the medial intermuscular septum and ulnar nerve protected medially. The artery and nerve are then directly identified. The approach is used for vascular injury repair, exploration of the brachial artery, and decompression of the median nerve (rarely; median nerve compression in the arm is uncommon).
Approach to the Radial Nerve in the Arm
Direct surgical exposure of the radial nerve in the arm is required for nerve exploration after humeral shaft fracture (particularly for failure of recovery or for secondary palsy after manipulation), traumatic laceration repair, and neurolysis or transfer procedures. Posterior Approach to the Radial Nerve The radial nerve in the spiral groove is approached through the posterior triceps- splitting approach as described above, with specific identification of the nerve at the lateral aspect of the proximal third of the humerus. Anterolateral Approach to the Radial Nerve The radial nerve in the distal third of the humerus (where it pierces the lateral intermuscular septum) is approached through an anterolateral incision along the lateral border of the biceps, with development of the interval between brachialis (medial) and brachioradialis (lateral). The radial nerve emerges through the lateral intermuscular septum at this level and is directly identified.
Summary and Take-Home Points
The arm provides relatively straightforward surgical approaches compared with the shoulder, with the principal challenge being the radial nerve in the spiral groove of the posterior humerus. The two principal approaches — anterolateral (using the biceps- brachialis interval with midline brachialis split) and posterior (using triceps-splitting, paratricipital, triceps-reflecting, or olecranon osteotomy techniques) — together provide access to most regions of the humeral shaft. The choice of approach depends on the location of pathology (proximal and middle thirds favoring anterolateral; middle and distal thirds favoring posterior; distal articular surface favoring olecranon osteotomy). The radial nerve is identified specifically and protected in any approach that extends to the middle or distal thirds of the humerus, with the junction of the middle and distal thirds of the humerus being the critical landmark where the nerve pierces the lateral intermuscular septum to enter the anterior compartment. The musculocutaneous nerve in the anterior compartment, the ulnar nerve medially, and the brachial artery with median nerve in the medial bicipital groove are the additional neurological structures requiring awareness during surgical approaches to the arm.
The arm compartments (anterior and posterior) are uncommon sites of compartment syndrome but require recognition and fasciotomy when present. Surgical access to the brachial artery and median nerve uses a longitudinal medial incision; surgical access to the radial nerve uses either the posterior triceps-splitting approach (for the spiral groove segment) or the anterolateral approach (for the distal segment as it pierces the lateral intermuscular septum). The chapter that follows turns to the elbow, where the complex articular anatomy of the trochlear notch, capitellum, and radial head requires careful operative approaches.