Arm, Elbow and Forearm

Part 2 · The upper limb

Compartments, the cubital fossa, pronation and supination, and the two epicondyles

What this region is for

The shoulder points the arm; the hand does the work; the elbow and forearm position the hand in space and set its orientation. Two functions, two mechanisms:

• The elbow adjusts the hand’s distance from the body — a hinge with one degree of freedom, and near-perfect bony congruence, so it is the most stable major joint in the upper limb.
• The forearm sets the hand’s orientation — pronation and supination, a uniquely human refinement that lets you turn a key, a doorknob, a screwdriver or a page without moving the shoulder.

Because of that second function, the forearm is functionally a joint, not a segment: two bones rotating about each other, connected proximally, distally and throughout by the interosseous membrane. Treat it as a single mechanical unit and its injuries make sense.

Learning outcomes

• Describe the fascial compartments of the arm and forearm, their contents and their nerve supply.
• Describe the elbow joint complex: surfaces, capsule, ligaments, stability, movements and carrying angle.
• Describe the proximal and distal radio-ulnar joints and the mechanism of pronation and supination.
• Name the muscles of the arm and forearm with attachments, nerve supply and actions.
• Describe the cubital fossa: boundaries, contents and clinical significance.
• Describe the course of the median, ulnar and radial nerves through the region and predict the deficits of injury at each level.
• Describe the arterial supply and the anastomoses around the elbow.
• Explain lateral and medial epicondylalgia, cubital and radial tunnel syndromes, and their management.
• Recognise supracondylar fracture, elbow dislocation, biceps and triceps rupture, and pulled elbow.

The arm: two compartments

Figure 1 · Compartments of the arm

The brachial fascia, with the medial and lateral intermuscular septa running from it to the humeral supracondylar ridges, divides the arm into two.

Anterior compartment muscles

Why biceps is a poor elbow flexor when pronated

Biceps inserts on the radial tuberosity. In supination that tuberosity faces medially and biceps has a good flexion moment arm; in pronation the tuberosity has rotated to face laterally and posteriorly, and biceps’ line of pull first has to unwind the pronation. Test elbow flexion in full pronation and you are testing brachialis and brachioradialis; test it in supination and biceps contributes fully. This is exactly why brachialis, not biceps, is the muscle that never fails you.

Posterior compartment

The elbow joint complex

Figure 2 · The elbow and radioulnar joints

Three articulations within one joint capsule and one synovial cavity — which is why an effusion here distends all three and why septic arthritis of the elbow involves the whole complex.

Capsule and ligaments

• The capsule is thin anteriorly and posteriorly (permitting flexion and extension) and thickened at the sides into the collateral ligaments. It encloses the coronoid, radial and olecranon fossae.

Stability

The elbow is often quoted as the most inherently stable joint in the upper limb. Its stability comes from three primary and several secondary restraints:

Primary: the humero-ulnar (trochlear notch) articulation, the anterior band of the UCL, and the LUCL. Secondary: the radial head (a critical valgus stabiliser when the UCL is deficient — the reason a radial head is now preserved or replaced rather than excised), the common flexor and extensor origins, and the capsule.

Close-packed position: full extension with supination. Resting position: ~70° flexion, 10° supination. Capsular pattern: flexion more limited than extension.

Movements and range

The functional range for most activities of daily living is approximately 30–130° of flexion and 50° each of pronation and supination — a useful target when full range is not achievable.

Carrying angle: 5–15° valgus in men, 10–20° in women, present in full extension and supination and disappearing in flexion and pronation. Increased = cubitus valgus (risk of tardy ulnar nerve palsy); reduced or reversed = cubitus varus / gunstock deformity, the classic sequel of a malunited supracondylar fracture.

Pronation and supination

The only movement in the body in which one bone rotates around another along its length.

The mechanism, precisely:

• The ulna is essentially fixed (it can abduct slightly during pronation, mainly through the anconeus, to keep the hand’s axis stable).
• The radius rotates about a longitudinal axis running from the centre of the radial head proximally to the centre of the ulnar head distally — an oblique axis, which is why the hand’s axis of rotation passes through the little finger when the elbow is fixed but through the thumb when the whole arm participates.
• Proximally, the radial head spins within the ring formed by the radial notch and the anular ligament.
• Distally, the concave ulnar notch of the radius swings around the convex head of the ulna — so the radius carries the hand around the ulna.
• The TFCC binds the two bones at the wrist and is the principal stabiliser of the distal radio-ulnar joint.

Test it yourself: turning a screw clockwise (supination for a right-handed person) is powerful; anticlockwise is weak. Right-hand screw threads are designed around this asymmetry, which exists because biceps — a large arm muscle — is a supinator, while pronation depends on two smaller forearm muscles.

The forearm: compartments and muscles

Figure 3 · Compartments of the forearm

The antebrachial fascia, the interosseous membrane and two intermuscular septa divide the forearm into anterior (flexor–pronator) and posterior (extensor–supinator) compartments, with a lateral “mobile wad” often described separately.

11.5.1 Anterior compartment — three layers

The rule: the anterior compartment is median territory, with two ulnar exceptions — FCU and the medial half of FDP.

11.5.2 Posterior compartment

The rule: the posterior compartment is entirely radial nerve territory — brachioradialis, ECRL and (usually) anconeus from the radial nerve proper, and everything else from the posterior interosseous nerve.

Anatomical snuffbox: anterior/lateral border = APL and EPB; posterior/medial border = EPL; floor = scaphoid and trapezium; crossed by the radial artery and the superficial branch of the radial nerve.

The cubital fossa

Figure 4 · The cubital fossa

A triangular hollow in front of the elbow — the region you palpate, inject into and read fractures across.

Contents, lateral to medial — mnemonic “TAN” (Tendon, Artery, Nerve):

• Tendon of biceps brachii
• Artery — the brachial artery, dividing at the level of the radial neck into radial and ulnar arteries
• Nerve — the median nerve

The radial nerve lies just outside the fossa, deep to brachioradialis, where it divides into its superficial (sensory) and deep (posterior interosseous) branches.

Clinical significance:

• Blood pressure measurement — the stethoscope goes over the brachial artery, medial to the biceps tendon
• Venepuncture — the median cubital vein in the roof, with the bicipital aponeurosis protecting the brachial artery and median nerve beneath it. This is why the aponeurosis was called the “grace of God” fascia in the era of routine bloodletting
• Supracondylar fracture — the displaced fragment threatens the brachial artery and median nerve here
• Distal biceps rupture — loss of the palpable tendon and the “hook test”: you can no longer hook a finger under the tendon from the lateral side

Nerves through the region

Median nerve (C6–T1)

Course: runs on the lateral side of the brachial artery in the arm, then crosses to lie medial to it in the mid-arm. It passes through the cubital fossa medial to the artery, then between the two heads of pronator teres, then deep to the fibrous arch of flexor digitorum superficialis. It descends between FDS and FDP to reach the wrist between the tendons of FDS and FCR, before entering the carpal tunnel. in the arm, crosses to lie medial to it in the mid-arm, passes through the cubital fossa medial to the artery, then between the two heads of pronator teres, then deep to the fibrous arch of flexor digitorum superficialis, and descends between FDS and FDP to reach the wrist between the tendons of FDS and FCR, before entering the carpal tunnel.

Branches in the forearm:

• muscular branches to all superficial flexors except FCU
• the anterior interosseous nerve (FPL, FDP to index and middle, pronator quadratus)
• the palmar cutaneous branch, which arises proximal to the flexor retinaculum and therefore passes superficial to it — which is why palmar sensation over the thenar eminence is spared in carpal tunnel syndrome

Lesions:

High median nerve palsy produces the “hand of benediction” on attempted fist-making: the index and middle fingers cannot flex (FDP lateral half and FDS), while the ring and little fingers do.

Ulnar nerve (C8, T1)

Course: descends medially in the anterior compartment, pierces the medial intermuscular septum in the mid-arm (the arcade of Struthers) to enter the posterior compartment, passes behind the medial epicondyle in the ulnar (cubital) groove — subcutaneous, palpable, and the “funny bone” — then between the two heads of FCU, and descends on FDP, becoming superficial in the distal forearm before entering Guyon’s canal.

Branches in the forearm: FCU, the medial half of FDP, and the dorsal cutaneous branch, which arises ~5–7 cm proximal to the wrist and supplies the dorsomedial hand — spared in Guyon’s canal lesions but lost in cubital tunnel lesions, a useful localising sign.

Cubital tunnel syndrome — the second commonest upper-limb entrapment after carpal tunnel. Compression sites: the arcade of Struthers, the medial epicondyle, the cubital tunnel retinaculum (Osborne’s ligament), and the two heads of FCU. Symptoms worsen with elbow flexion, which stretches and compresses the nerve (tunnel volume falls markedly beyond 90°). Sleeping with the elbow flexed is a classic aggravator; night splinting in extension is a first-line intervention.

The ulnar paradox

A distal ulnar lesion (at the wrist) produces more marked clawing of the ring and little fingers than a proximal lesion (at the elbow), even though the proximal lesion is more extensive. The reason: a proximal lesion also paralyses the medial half of FDP, so the distal interphalangeal joints cannot flex, and the claw is less pronounced. As a proximal lesion recovers and FDP reinnervates before the intrinsics, the claw worsens — a sign of recovery that patients find alarming and must have explained.

Radial nerve (C5–T1)

Course: passes through the triangular interval into the posterior compartment, spirals around the humerus in the radial groove with the profunda brachii, pierces the lateral intermuscular septum ~10 cm above the lateral epicondyle to re-enter the anterior compartment, lies deep to brachioradialis anterior to the lateral epicondyle, and divides into:

• Superficial branch — purely sensory; runs under brachioradialis, emerges to supply the dorsolateral hand and dorsum of the lateral 3½ digits proximal to the DIP joints.
• Deep branch → posterior interosseous nerve (PIN) — passes between the two heads of supinator through the arcade of Frohse (a fibrous arch, the commonest PIN compression site), then supplies all the deep extensors.

Lesions:

Vessels

The brachial artery continues from the axillary artery at the lower border of teres major, lies medial then anterior in the arm on brachialis, and divides in the cubital fossa into radial and ulnar arteries. Branches: profunda brachii (with the radial nerve), nutrient artery to the humerus, superior and inferior ulnar collateral arteries.

The anastomosis around the elbow links the collateral branches (from brachial and profunda brachii) above with the recurrent branches (from radial, ulnar and common interosseous) below. It maintains flow when the brachial artery is occluded or when the elbow is fully flexed and the vessel kinked.

The ulnar artery gives the common interosseous (dividing into anterior and posterior interosseous arteries) and runs with the ulnar nerve deep to FCU. The radial artery runs on the lateral side, is superficial at the wrist between FCR and brachioradialis (the radial pulse), then crosses the anatomical snuffbox to enter the palm and form the deep palmar arch. The ulnar artery forms the superficial palmar arch.

Allen’s test assesses the patency of both arches before radial artery cannulation or harvest — and is worth knowing because a false-negative can cost a hand.

Clinical conditions

Lateral epicondylalgia (“tennis elbow”)

The commonest elbow condition, peaking at 35–55 years, with only a minority of sufferers playing tennis.

• Site: the origin of extensor carpi radialis brevis at the lateral epicondyle, sometimes with extensor digitorum communis involvement.
• Pathology: not inflammation. Histology shows angiofibroblastic tendinosis — disorganised collagen, increased ground substance, immature vascular ingrowth, absent inflammatory cells (Nirschl). It is a tendinopathy (Chapter 7).
• Clinical: tenderness ~1 cm distal to the lateral epicondyle; pain on resisted wrist extension, resisted middle-finger extension (Maudsley’s test), and gripping; reduced pain-free grip strength (the most useful objective outcome measure).
• Management: the natural history is favourable but slow (most resolve within 1–2 years). Corticosteroid injection gives short-term relief but worse 6–12-month outcomes and higher recurrence than exercise or wait-and-see (Bisset et al., Coombes et al.) — a finding worth quoting to patients who request injection. Progressive loading (isometric then eccentric/heavy slow resistance for the wrist extensors), grip and forearm strengthening, addressing shoulder and kinetic-chain deficits, load modification, and counterforce bracing as a short-term adjunct.
• Always screen the cervical spine and the radial nerve. Radial tunnel syndrome produces tenderness ~4–5 cm distal to the epicondyle over the supinator, and coexists with lateral epicondylalgia in a proportion of cases.

Medial epicondylalgia (“golfer’s elbow”)

Same pathology at the common flexor origin, principally pronator teres and flexor carpi radialis. Roughly a fifth as common as the lateral form. Always examine the ulnar nerve, which is involved in up to 20–50% of cases; and in throwing athletes, assess the UCL for valgus instability.

Cubital tunnel and radial tunnel syndromes

Covered in §11.7. Note that radial tunnel syndrome produces pain without weakness and without sensory loss (compression of the PIN before its motor branches, or of the nerve to the radial wrist extensors), whereas PIN syndrome produces weakness (finger drop) without pain. They are ends of the same spectrum and both are frequently misdiagnosed as resistant tennis elbow.

Supracondylar fracture of the humerus

The commonest elbow fracture in children (peak 5–8 years), usually from a fall on the outstretched hand with an extension-type displacement (~95%).

• Structures at risk: the brachial artery and the median nerve (particularly the anterior interosseous branch); also radial and ulnar nerves.
• The emergency: compartment syndrome of the forearm, whose end result is Volkmann’s ischaemic contracture — a fixed flexion deformity of the wrist and fingers from fibrotic replacement of the flexor compartment. Pain on passive finger extension is the cardinal early sign; a palpable radial pulse does not exclude it (Chapter 7).
• Late complication: cubitus varus (gunstock deformity) from malunion.
• The epicondyle–olecranon triangle is preserved (unlike in dislocation) — a bedside discriminator.

Elbow dislocation

The second most commonly dislocated major joint in adults after the shoulder; posterior or posterolateral in ~90%, from a fall on an outstretched hand with valgus, supination and axial load.

• The “terrible triad” — dislocation + radial head fracture + coronoid fracture — is unstable and usually surgical.
• Injury proceeds from lateral to medial (Horii circle): LUCL first, then anterior and posterior capsule, then the UCL.
• Posterolateral rotatory instability (PLRI) is the commonest chronic pattern, from LUCL insufficiency; tested by the lateral pivot-shift test and the table-top relocation test.
• The elbow stiffens readily. It is one of the joints most prone to post-traumatic contracture and heterotopic ossification (particularly with associated head injury or burns). Rehabilitation is early active motion within a stable arc; passive forced stretching and vigorous passive movement are contraindicated and will make it worse.

Tendon ruptures and pulled elbow

Where students consistently go wrong

• Calling biceps the main elbow flexor. Brachialis is, and it works in every forearm position.
• Forgetting the two ulnar exceptions in the anterior forearm. FCU and the medial half of FDP.
• Confusing FDS and FDP. Superficialis → middle phalanges (PIP); profundus → distal phalanges (DIP).
• Forgetting the median palmar cutaneous branch is superficial to the retinaculum. It is why thenar-eminence sensation is spared in carpal tunnel syndrome.
• Missing the ulnar paradox. A distal lesion claws more.
• Confusing radial groove palsy with PIN palsy. Wrist drop vs finger drop, and sensory loss vs none.
• Calling tennis elbow an inflammation. It is a tendinosis; steroid injection worsens 12-month outcomes.
• Forgetting the elbow triangle in distinguishing supracondylar fracture from dislocation.
• Applying vigorous passive stretch to a stiff post-traumatic elbow. This provokes heterotopic ossification.
• Ignoring the wrist after a radial head fracture. Essex-Lopresti (Chapter 9).

Check yourself

15 questions on this chapter. Tap one to see the answer and the reasoning.

Quick review

Further reading

Chapter 11 of 24 · Human Anatomy · Physiotherapist India Next: Chapter 12 — Wrist and Hand: the carpal tunnel, three nerve territories, the arches, and the four grips.

Reviewed by the Physiotherapist India Team. · Human Anatomy contents