Arm & Forearm
The upper arm is comprised of the middle section of the humerus and the flexor and extensor muscles of the elbow.
This session will describe the relevant important landmarks on the middle section of the humerus and the origin of the flexor and extensor muscles of the elbow.
The structure of the flexor and extensor muscles will then be covered in detail and the course of the musculocutaneous and radial nerves described as they pass through these muscles. The brachial artery will be followed through the arm, accompanied by the median and ulnar nerves.
The radial nerve is particularly vulnerable to injury in surgical approach to the humerus and the principles of safe surgical approach will be discussed.
The humerus consists of a proximal humeral head, shaft and distal condylar region.
The humeral shaft is circular in cross-section proximally but becomes triangular distally above the elbow (Fig 1). The bicipital groove (for the long head of biceps) peters out at the deltoid tuberosity laterally where the deltoid muscle inserts. Opposite the deltoid tuberosity on the medial side there is a slightly roughened area for the insertion of the coracobrachialis muscle.
On the posterior aspect there is a faint groove which spirals from proximal (medially) to distal (laterally). This is the spiral groove (Fig 2) which separates the origins of the medial and lateral heads of the triceps muscle. The radial nerve follows the groove but may be separated from it by fibres of the medial head of triceps. Distally, the humeral shaft flares forming medial and lateral supracondylar ridges, from which muscles of the forearm and the medial and lateral intermuscular septae originate.
The flexor muscles comprise the biceps, coracobrachialis and brachialis muscles. The biceps does not have a bony attachment in the upper arm. The coracobrachialis muscle inserts on a slightly roughened area on the medial aspect of the upper humerus, opposite the deltoid tuberosity. The brachialis arises from the lower anterior surface of the middle and distal shaft of the humerus. Its origin ascends up and around the deltoid insertion in a V shaped manner. Distally the brachioradialis and extensor carpi radialis muscles arise from the lateral supracondylar ridge and the pronator teres arises from the medial supracondylar ridge.
The triceps consists of three heads, the longest of which arises from the inferior aspect of the glenoid at the shoulder. The medial and lateral heads arise from the posterior surface of the humerus separated by the spiral groove (Fig 2). The lateral head arises from the proximal edge of the spiral groove in a line that extends down to the origin of the brachialis as it surrounds the deltoid tuberosity posteriorly. The medial head arises from a large surface on the posterior aspect of the distal humerus, extending from the lower margin of the spiral groove to the supracondylar area.
- Biceps, as it name suggests, has two heads or proximal origins.
The short head arises from the coracoid process of the scapula while its longer head arises from the superior portion of the glenoid.
Distally it inserts into the radial bicipital tuberosity and via an aponeurosis to the subcutaneous border of the proximal ulna.
The nerve supply is by the musculocutaneous nerve (C5,6) which lies between the biceps and brachialis muscles. The action of the biceps is to flex the elbow and supinate the forearm.
2. Coracobrachialis also arises from the coracoid process; the muscle lies medial and posterior to the short head of biceps before inserting on the medial side of the proximal humerus, opposite the deltoid insertion. It is also supplied by the musculocutaneous nerve. It is a functionally unimportant muscle but weak adductor of the shoulder.
3. Brachialis arises from the front of the lower two thirds of the humerus and its proximal origin lies around the deltoid insertion.
It lies deep to biceps, the musculocutaneous nerve lying between. It inserts on the coronoid process of the ulna and acts as a strong flexor of the elbow. It has a dual nerve supply. The lateral portion is supplied by the radial nerve but the main medial portion is supplied by the musculocutaneous nerve (C5,6).
An anterior approach to the humerus splits the brachialis muscle longitudinally thus preserving nerve supply to each part.
Triceps is the muscle of the extensor compartment. The long head arises from the inferior part of the glenoid. It is joined by the lateral head which arises from a linear origin on the proximal portion of the spiral groove. The long and lateral heads lie superficial to the deeper medial head which arises from the inferior distal part of the spiral groove, the whole of the posterior surface of the distal humerus and both intermuscular septae.
The radial nerve and the profunda brachii artery follow the spiral groove lying on the upper fibres of the medial head and deep to the conjoined fibres of the long and lateral heads.
The triceps tendon inserts on the olecranon and the posterior capsule of the elbow joint. It act primarily as an extensor of the elbow and is supplied by the radial nerve (C7,8).
The brachial artery is a continuation of the axillary artery and passes though the arm in a relatively superficial position accompanied by the median and ulnar nerves, just deep to the deep fascia. It enters the upper arm medially from the axilla and leaves the upper arm, entering the cubital fossa medial to the biceps insertion and lateral to the median nerve.
The brachial artery has the ulnar nerve behind it in the upper arm before this leaves it by passing posteriorly through the intermuscular septum. The median nerve is initially lateral to the artery, then crosses in front of the artery before passing medial to it by the time it enters the cubital fossa.
Profunda brachii artery
The brachial artery has a main branch, the profunda brachii which leaves the axilla though the lower triangular space and accompanies the radial nerve in the spiral groove. The artery supplies the triceps. Further braches from the brachial artery supply the flexor muscles. Distal branches, the ulnar collaterals and branches from the profunda brachii form an anastomosis around the elbow.
The brachial artery is accompanied by the venae comitantes which form the axillary vein proximally. The basilic vein joins the venae comitantes by perforating the deep fascia medially, opposite the deltoid insertion.
Nerves of the Upper Arm
The median nerve is formed by the joining of the medial and lateral heads of the median nerve on the front of the axillary artery. The nerve passes through the upper arm initially on the lateral side of the brachial artery before crossing anteriorly to its medial side by the time it enters the cubital fossa. No branches are given off in the upper arm.
The ulnar nerve lies posterior to the brachial artery and leaves it to pass through the medial intermuscular septum above the elbow. It is accompanied by an ulnar collateral branch from the brachial artery and also a branch from the radial nerve to the medial head of triceps. It gives no branches in the upper arm.
The musculocutaneous nerve passes through the coracobrachialis muscle before crossing the upper arm, lying between biceps and brachialis. It supplies all of these muscles (except the lateral portion of the brachialis). It exits laterally from between biceps and brachialis becoming the lateral cutaneous nerve of the forearm.
The radial nerve leaves the axilla through a triangular space whose boundaries are the lower border of teres major. and the long head of triceps and the humerus. It winds around the posterior aspect of the humerus, following the spiral groove lying between the lateral and the deeper, medial, heads of the triceps. It supplies the three heads of triceps as well as the posterior cutaneous nerve of the arm, the lower lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the forearm. The nerve exits the arm by piercing the lateral intermuscular septum and comes to lie between brachioradialis and brachialis. The radial nerve is vulnerable to injury in fractures of the mid shaft of the humerus and in the posterior surgical approach to the humerus.
Principles of Safe Surgical Approach to the Humerus
The humeral shaft can be approached surgically in a safe manner if the courses of the main neuro-vascular structures are borne in mind. The humeral shaft can be approached posteriorly and anterolaterally.
In the posterior approach the long and lateral heads of triceps are gently split in a longitudinal fashion, taking care proximally to identify the radial nerve and profunda brachii vessels as they follow the spiral groove. The nerve will be found at the level of the deltoid insertion in the midline, passing laterally distal to this level. The nerve and vessels will lie on the deep medial head of triceps. Having identified the radial nerve, the part of the medial head distal to the nerve can be split safely to allow access to the posterior aspect of the humeral shaft.
In the anterolateral approach, the humeral shaft is approached by displacing the biceps muscle medially to reveal the brachialis muscle. The musculocutaneous nerve needs to be identified distally and protected. The brachialis muscle is then split directly anteriorly over the humerus so that the nerve supply to the lateral and medial parts are preserved.