Anconeus Anatomy

Origin

• Posterior (and posteroinferior) aspect of the lateral epicondyle of the humerus, arising by a short thick tendon deep to the common extensor origin and deep to the lateral collateral ligament complex.
• The origin is intimately related to the lateral ulnar collateral ligament and the posterolateral capsule — a relationship that matters enormously in lateral elbow approaches.

Insertion

• Lateral surface of the olecranon and the proximal quarter of the posterior (dorsal) surface of the ulna, along and just lateral to the subcutaneous border.
• The muscle fans out from a narrow humeral origin to a broad ulnar insertion — hence its triangular shape.
• The fascia over the anconeus is continuous with the triceps aponeurosis proximally, which is the anatomical basis for using it to reinforce a deficient triceps.

Dimensions

• Ng and colleagues measured the anconeus in eight cadaveric upper extremities: mean fibre length 88.0 plus or minus 9.9 mm and mean muscle area 1341.9 plus or minus 230.4 mm squared — larger on both counts than the abductor pollicis brevis it was being assessed to replace.
• Mean fibre angle 70.5 plus or minus 11.9 degrees.
• Practically, the muscle is a triangle roughly 5 to 7 cm along the ulna and 3 to 4 cm from epicondyle to ulna, capable of covering a defect of similar dimensions when rotated.

Bony Landmarks to Quote

• Origin: posterior lateral epicondyle, deep to the common extensor origin.
• Insertion: extends from the lateral olecranon along the proximal quarter (approximately 5 to 7 cm) of the posterior ulna.
• The subcutaneous border of the ulna forms the medial edge of the muscle and is the palpable landmark for its entire length.

Radial Nerve (C7, C8)

• Supplied by the terminal continuation of the nerve to the medial head of triceps — the most distal motor branch of the radial nerve in the arm.
• This branch arises from the radial nerve proximally, in the axilla or upper arm, runs distally on the medial side of the arm alongside the ulnar nerve, then passes through the substance of the medial head of triceps to reach the anconeus.
• The nerve enters the muscle on its deep surface, proximally, close to the vascular pedicle.
• Nerve diameter: measured at 1.7 plus or minus 0.3 mm in cadaveric study — adequate for microsurgical coaptation in a free functioning muscle transfer.

Clinical Consequences of the Innervation Pattern

• Because the branch arises proximal to the spiral groove, the anconeus is typically spared in a radial nerve palsy at the groove, in company with the medial head of triceps.
• Because the branch passes through the medial head of triceps, a deep dissection or split of the medial head carried too far distally can denervate the anconeus.
• Electromyography of the anconeus is technically easy (the muscle is superficial and easily located two fingerbreadths distal to the lateral epicondyle along the ulnar border) and is used in electrodiagnostic assessment of the C7-C8 roots and the radial nerve. Because the branch is long, the anconeus is a sensitive site for detecting radial neuropathy.

Root Values Compared

Dominant Pedicle: The Recurrent Posterior Interosseous Artery

• The recurrent posterior interosseous artery arises from the posterior interosseous artery shortly after it emerges from the supinator and passes proximally to enter the deep surface of the anconeus distally.
• Measured parameters (Ng and colleagues, 8 cadaveric limbs):
  • Pedicle length: 31.3 plus or minus 6.9 mm
  • Artery diameter: 1.9 plus or minus 0.2 mm
  • Venae comitantes diameter: 1.0 mm
• These are the numbers that matter: a pedicle of that length and calibre is sufficient for microsurgical anastomosis, which is why the anconeus can be used not only as a local rotation flap but as a free functioning muscle transfer.

Secondary Contributions

• Middle collateral branch of the profunda brachii supplies the proximal portion near the epicondylar origin.
• Interosseous recurrent and posterior ulnar recurrent anastomoses around the elbow contribute.
• The dual proximal and distal supply is what permits the muscle to be raised either proximally based (on the middle collateral contribution, for triceps reconstruction) or distally based (on the recurrent posterior interosseous artery, for cover over the olecranon and proximal ulna).

Flap Classification and Options

Superficial

• Skin and subcutaneous tissue — the anconeus is essentially subcutaneous, covered only by the antebrachial fascia. This makes it easy to find, easy to raise, and easy to sample for electromyography.
• The olecranon bursa lies over its medial edge.

Deep

• Posterolateral capsule of the elbow joint and the annular ligament.
• Lateral ulnar collateral ligament — this is the critical relation. The ligament runs from the isometric point on the lateral epicondyle (at the centre of the capitellar arc of curvature) posteriorly and distally around the radial head to insert on the supinator crest of the ulna. It lies deep to the anconeus origin and at the floor of the Kocher interval.
• Radiocapitellar joint and the proximal radioulnar joint.

Anterior (Radial Side)

• Extensor carpi ulnaris — the interval between them is the Kocher interval.
• Further anteriorly, extensor digiti minimi and extensor digitorum communis.

Posterior and Medial

• Subcutaneous border of the ulna.
• Triceps proximally, with which the anconeus fascia is continuous.

The Posterior Interosseous Nerve

• Not directly related to the anconeus itself, but it is the structure endangered by any dissection carried distally along the proximal radius from a lateral or posterolateral approach.
• Diliberti and colleagues performed exactly the posterolateral approach between anconeus and extensor carpi ulnaris in 32 cadaveric specimens and measured the safe zone from the capitellum to where the posterior interosseous nerve crosses the radial shaft.

Aetiology

• Trauma: the commonest cause. Elbow dislocation, or a fall on the outstretched hand with valgus, supination and axial load. The lateral ulnar collateral ligament fails first (Horii stage 1).
• Iatrogenic: the second commonest, and the one under a surgeon's control.
  • Overly aggressive lateral epicondylitis release extending posterior to the midline of the radial head, taking the lateral ulnar collateral ligament with the extensor carpi radialis brevis origin.
  • Kocher approach with the capsulotomy carried posterior to the equator of the radial head.
  • Repeated corticosteroid injection into the lateral epicondyle, attenuating the ligament complex.
  • Radial head excision without appreciating that the radial head is a secondary stabiliser to valgus and posterolateral forces.
• Chronic cubitus varus after a malunited paediatric supracondylar fracture, shifting the mechanical axis and progressively attenuating the lateral complex — the so-called tardy posterolateral rotatory instability.

Presentation

• Mechanical symptoms — clicking, clunking, snapping or locking on extension with the forearm supinated.
• Difficulty pushing up out of a chair or down a flight of stairs, both of which load the elbow in the provocative position.
• Recurrent lateral elbow pain misattributed to tennis elbow, which is the classic diagnostic trap.

Imaging

• Radiographs: may be normal, or show subtle posterior radial head subluxation on the lateral view, or a stress lateral view under fluoroscopy demonstrating the subluxation.
• MRI: best for the lateral ulnar collateral ligament, particularly with the elbow in extension and supination; look for discontinuity or attenuation of the ligament at its humeral origin.
• Examination under anaesthesia with fluoroscopy remains the definitive assessment.

Management

• Acute ligament avulsion: repair to the isometric point on the lateral epicondyle (the centre of the arc of curvature of the capitellum on a true lateral) with an anchor or transosseous sutures.
• Chronic insufficiency: reconstruction with a tendon graft (palmaris longus, gracilis, or triceps fascia) from the isometric point to the supinator crest of the ulna, usually in a docking or figure-of-eight configuration.
• Preserve and reattach the anconeus and the common extensor origin at closure — the dynamic restraints matter.

The Problem of the Posterior Elbow

The skin over the olecranon is thin, mobile, poorly vascularised and under tension in flexion. Any breakdown rapidly exposes the olecranon, the triceps insertion, hardware or the joint itself. The options are limited: local fasciocutaneous flaps are unreliable in this territory, free tissue transfer is a large undertaking for a small defect, and the radial forearm flap sacrifices a major vessel.

The Anconeus Rotation Flap

Elhassan and colleagues reported 20 elbows in 20 patients with chronic posterior elbow soft-tissue defects managed with a local pedicled anconeus flap:

• Defect size: mean 4 by 3 cm, range 2 by 3 cm to 7 by 4 cm.
• Chronicity: mean symptom duration 11 months (range 4 to 96); a mean of 3 previous debridements and attempts at closure (range 2 to 7) before the flap.
• Wound healing: achieved in all 20 elbows, at a mean of 3 weeks (range 2 to 5).
• Complications: none, either at surgery or postoperatively.
• Outcome at a mean 17 months: subjective elbow value improved from 70 per cent to 95 per cent; Mayo Elbow Performance Score from 74 to 90 (both p equals 0.001). All patients had full range of motion and all were satisfied.

Anconeus Rotation for Triceps Insufficiency

Triceps insufficiency after total elbow arthroplasty is the characteristic late soft-tissue failure of that operation. In the Mayo series (Celli, Arash, Adams and Morrey), 16 elbows in 14 patients out of 887 total elbow arthroplasties required reconstruction, and anconeus rotation was used in 4 of the 16.

The Reconstruction Algorithm

Anconeus Epitrochlearis

• An accessory muscle running from the medial epicondyle to the olecranon, across the roof of the cubital tunnel, present in roughly 10 to 30 per cent of limbs depending on the series (it is genuinely common).
• Despite the shared name it is not part of the anconeus and is not innervated by it — it is supplied by the ulnar nerve.
• It replaces or overlies Osborne's ligament (the cubital tunnel retinaculum) and can be a cause of cubital tunnel syndrome, particularly in a muscular patient, by dynamic compression of the ulnar nerve in flexion.
• Management: if it is found at cubital tunnel decompression, it is excised. Its presence is one reason to consider in situ decompression rather than transposition, since removing the muscle alone often suffices.
• MRI appearance: a muscle of normal signal intensity in the cubital tunnel, which must not be mistaken for a mass.

Anconeus Compartment Syndrome

• Rare, described after fracture, exertion or reperfusion injury, presenting as severe posterolateral elbow pain with a tense, tender anconeus and pain on passive elbow flexion.
• Diagnosed on clinical suspicion with pressure measurement; treated by fasciotomy through a direct posterolateral incision.

Snapping Anconeus and Radiocapitellar Plica

• A posterolateral plica or synovial fringe at the radiocapitellar joint, deep to the anconeus, can produce painful snapping in extension and is a differential for lateral epicondylitis and for posterolateral rotatory instability. It is treated arthroscopically by excision through the soft spot and posterolateral portals.

Anconeus Interposition Arthroplasty

• Described for radiocapitellar and proximal radioulnar impingement after radial head excision, and for painful proximal radioulnar convergence. The muscle is interposed as a vascularised spacer on its pedicle. A niche salvage procedure with modest evidence.

The Standard Lateral Approach to the Elbow

• Interval: anconeus (radial nerve) and extensor carpi ulnaris (posterior interosseous nerve) — a true internervous plane.
• Skin incision: from just proximal to the lateral epicondyle, curving distally toward the subcutaneous border of the ulna, roughly 6 to 8 cm.
• Identifying the interval: look for the fine fat stripe running obliquely between the anconeus and extensor carpi ulnaris — this is the reliable landmark and it is easy to miss if the dissection is too proximal (the two muscles are fused near the epicondyle).
• Deep dissection: the joint capsule is entered anterior to the equator of the radial head with the forearm pronated.
• What it exposes: the radial head and neck, the capitellum, the lateral collateral ligament complex and the anterolateral coronoid (with additional anterior capsular release).

Structures at Risk with Distances

The Pronation Rule and Why It Is Opposite to Henry

Diliberti and colleagues performed the posterolateral approach between anconeus and extensor carpi ulnaris in 32 cadaveric specimens and measured the posterior interosseous nerve safe zone from the capitellum:

• Pronation: safe zone mean 52.0 plus or minus 7.8 mm, minimum 38 mm. The nerve-to-shaft angle fell to 27.8 plus or minus 6.7 degrees.
• Supination: safe zone mean 33.4 plus or minus 5.7 mm, as little as 22 mm. The nerve-to-shaft angle was 47.4 plus or minus 6.8 degrees.
• Conclusion: approaching the lateral proximal radius is safest in PRONATION.

The exam trap: the volar Henry approach requires SUPINATION to move the nerve away, and the posterolateral Kocher or Thompson approach requires PRONATION. The directions are opposite because you are approaching from opposite sides. Candidates confuse this constantly.

Comparison of Lateral and Posterolateral Approaches

The Boyd Caveat

The Boyd approach strips both the anconeus and extensor carpi ulnaris subperiosteally from the proximal ulna, exposing the interosseous space. This is precisely the setting in which proximal radioulnar synostosis develops, and the approach has consequently fallen out of favour for anything other than complex Monteggia lesions where simultaneous access to both bones is genuinely required.

The Anconeus and the Soft Spot

• The direct lateral (soft spot, midlateral) portal is placed in the centre of the triangle formed by the lateral epicondyle, olecranon tip and radial head, traversing the anconeus to enter the posterolateral joint.
• It is the safest portal at the elbow — no major neurovascular structure crosses it — and is the standard site for diagnostic aspiration and injection.
• It gives a view of the radiocapitellar joint, the proximal radioulnar joint and the posterolateral gutter.

Portal Safety Overall

General Elbow Arthroscopy Safety

• Insufflate the joint with 20 to 30 mL of fluid before placing any portal — this displaces the anterior neurovascular structures forward and increases the capsule-to-nerve distance.
• Flex the elbow to about 90 degrees for anterior work, which relaxes the anterior structures away from the capsule.
• Use retractors, not suction shavers, near the anterior capsule.
• A contracted capsule pulls the neurovascular structures closer to the joint — the stiffer the elbow, the smaller the margin, and the greater the argument for open release.

Raising the Anconeus Rotation Flap

• Position: lateral decubitus or supine with the arm across the chest; tourniquet.
• Incision: posterolateral, along the lateral border of the proximal ulna, extended proximally as needed for the defect.
• Elevation: the muscle is detached from its ulnar insertion and elevated from distal to proximal, preserving the recurrent posterior interosseous pedicle entering the deep surface distally for a distally based flap, or preserving the proximal contributions for a proximally based flap into a triceps defect.
• Arc: rotates comfortably over the olecranon and posterior elbow; defects up to 7 by 4 cm have been covered.
• Cover: a split-thickness skin graft is applied over the muscle if the skin cannot be closed directly.
• Donor site: closes primarily; no functional deficit.

As a Free Functioning Muscle Transfer

Ng and colleagues assessed the anconeus against the abductor pollicis brevis for thenar reconstruction in eight cadaveric limbs and found:

• Anconeus fibre length 88.0 mm versus abductor pollicis brevis 57.7 mm; area 1341.9 mm squared versus 987.7 mm squared — the anconeus is the larger muscle on both counts.
• No significant difference in fibre angle (70.5 versus 78.4 degrees), artery diameter (1.9 versus 2.0 mm) or nerve diameter (1.7 versus 2.1 mm).
• Pedicle length 31.3 plus or minus 6.9 mm with venae comitantes of 1.0 mm — sufficient for microsurgical anastomosis.
• The authors concluded the anatomical rationale is sound, with the advantages of reliable matching anatomy, minimal donor morbidity, and single-stage restoration of opposition.

This remains a cadaveric feasibility study rather than established practice, but it is a legitimate answer to "what else could the anconeus be used for" and it supplies the pedicle numbers.