Pronator Teres Anatomy

The humeral head. The larger, superficial head forms most of the muscle bulk. It arises from the medial supracondylar ridge just above the medial epicondyle, from the common flexor origin on the medial epicondyle, and from the medial intermuscular septum and deep antebrachial fascia.

The ulnar head. The smaller, deep head arises from the medial side of the coronoid process, just distal to the sublime tubercle where the anterior band of the medial collateral ligament inserts. It is absent in roughly 5 to 10 per cent of limbs. That variation has direct clinical relevance: with no deep head, the median nerve passes deep to the whole muscle, and compression at this site cannot occur in the classical manner.

The insertion. A broad, flat, roughened area roughly 4 to 5 cm long on the middle third of the lateral surface of the radius, at the point of maximum convexity of the radial bow. It is sometimes called the pronator tuberosity or pronator teres tubercle, and lies immediately distal to the supinator insertion and proximal to the middle of the bone. Its position at the apex of the bow gives the muscle a good pronation moment arm and makes it a deforming force in fractures at that level.

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Motor supply. The median nerve (C6, C7) gives its motor branches to pronator teres proximal to the muscle, typically at or just above the level of the elbow, before it passes between the two heads. There are usually one to three branches entering the deep surface of the humeral head and, when the deep head is present, a separate small branch to it.

Why pronator teres works in pronator syndrome. Compression between the two heads is distal to the muscle's own innervation, so its function is preserved when the nerve is trapped within it.

Electrodiagnostic use. Pronator teres is the classic C6-C7 median-innervated muscle, and is used electrodiagnostically to distinguish a C6-C7 radiculopathy from a more distal median nerve lesion.

The median nerve's course through the muscle. From the cubital fossa to the wrist, the nerve:

1. Enters the cubital fossa medial to the brachial artery.
2. Passes deep to the lacertus fibrosus (bicipital aponeurosis).
3. Passes between the humeral (superficial) and ulnar (deep) heads of pronator teres.
4. Passes deep to the fibrous proximal arch of flexor digitorum superficialis, the sublimis bridge.
5. Continues distally on the deep surface of flexor digitorum superficialis, sandwiched between it and flexor digitorum profundus.
6. Emerges at the wrist between the tendons of flexor digitorum superficialis and flexor carpi radialis, just deep and radial to palmaris longus.

The anterior interosseous nerve. It arises from the posterior (dorsal) aspect of the median nerve, typically 5 to 8 cm distal to the lateral epicondyle, at or just distal to the level of pronator teres. It supplies flexor pollicis longus, the radial half of flexor digitorum profundus (index and usually middle finger) and pronator quadratus.

It is purely motor to the hand, but it carries afferent fibres from the volar wrist capsule and the interosseous membrane. Those fibres are the basis for anterior interosseous nerve neurectomy in wrist denervation, and part of the explanation for the deep forearm ache that often precedes the palsy.

Motor branch order in the forearm. The sequence of branches, and where each leaves the nerve:

Named vessels. Four arteries contribute:

• The brachial artery gives muscular branches to the proximal muscle, before its bifurcation in the cubital fossa.
• The ulnar artery is the dominant supply. It arises from the brachial artery in the cubital fossa and gives muscular branches to the deep surface as it passes beneath both heads.
• The anterior ulnar recurrent artery, a branch of the ulnar artery ascending to the medial epicondyle, supplies the proximal muscle and the common flexor origin.
• The radial artery gives small branches to the distal muscle near its radial insertion.

The pedicle. The dominant pedicle enters the deep surface in the proximal third, close to the median nerve's passage between the heads. Because it enters proximally, the muscle can be mobilised extensively from its origin distally and its insertion detached from the radius without devascularising it, which is what makes the pronator teres to extensor carpi radialis brevis transfer possible.

Not a flap. Pronator teres is not used as a flap. It is not particularly bulky, its pedicle is short and shared with the median nerve's course, and it is more valuable as a tendon transfer donor.

The cubital fossa. Pronator teres forms one of its borders, and the rest of the fossa is built around it:

• Medially, the lateral border of pronator teres.
• Laterally, the medial border of brachioradialis.
• The base (superior) is an imaginary line between the medial and lateral epicondyles.
• The floor is brachialis and supinator.
• The roof is deep fascia reinforced by the lacertus fibrosus, with the median cubital vein and the medial and lateral cutaneous nerves of the forearm superficial to it.

Its contents. From lateral to medial, the fossa holds the radial nerve (in the floor, between brachioradialis and brachialis, often omitted but present), the biceps tendon, the brachial artery and the median nerve. The mnemonic TAN (Tendon, Artery, Nerve), lateral to medial, is the version most often quoted.

Superficial and deep. The muscle is essentially subcutaneous in the proximal medial forearm and is easily palpated; over it lie only the lacertus fibrosus proximally, the median cubital and superficial veins, and skin. Deep to it lie brachialis proximally, the anterior capsule of the elbow and the medial coronoid, then flexor digitorum superficialis and its proximal fibrous arch.

Between and beneath the heads. The median nerve passes between the two heads, and that is the whole point of the muscle. The ulnar artery passes deep to both heads, a point examiners like because the nerve goes between and the artery goes under.

Medial and lateral. Medially lie flexor carpi radialis, then palmaris longus, then flexor carpi ulnaris, the superficial flexor group, all arising from the common flexor origin. Laterally lies brachioradialis, with the brachioradialis-flexor carpi radialis interval of the Henry approach and the radial artery deep in that interval.

Presentation. The Hartz series, gathered over seven years, gave the classical picture: aching discomfort in the forearm, weakness in the hand, and numbness in the thumb and index finger. Cyclic stress usually brought on the symptoms, meaning repetitive pronation and supination at work or in sport. The distinctive physical finding was tenderness over the proximal part of pronator teres, aggravated by resisted pronation of the forearm, flexion of the elbow, and occasionally resisted contraction of the flexor superficialis of the long finger.

The four compression sites. From proximal to distal, with their provocative tests in the examination table above:

What Hartz found at operation. They explored 36 forearms in 32 patients. Besides the tendinous bands and the flexor superficialis indentation, vascular and muscular abnormalities were found occasionally, and intra-operative recordings showed improvement shortly after release in 6 of the 10 forearms tested. The results were 28 good or excellent, 5 fair and 3 unchanged.

Management. Non-operative treatment comes first, and for a prolonged period: activity modification to avoid repetitive pronation and supination, splinting, and physiotherapy with nerve gliding. Surgical decompression is for symptoms that persist, and it releases all four sites (technique under Surgical Relevance).

The syndrome. Anterior interosseous nerve syndrome is a pure motor syndrome affecting flexor pollicis longus, the radial half of flexor digitorum profundus (index and usually middle finger), and pronator quadratus. No sensory loss whatsoever is the defining negative.

Signs. The patient cannot make a round circle with the thumb and index finger, the OK sign (Kiloh-Nevin sign), and instead produces a flat, pinched, triangular pinch because the terminal joints will not flex. Pronation is weak with the elbow fully flexed, which isolates pronator quadratus.

The prodrome. The palsy is often preceded by a period of severe deep forearm or arm pain lasting days to weeks, followed by painless weakness. That sequence should raise neuralgic amyotrophy (Parsonage-Turner syndrome).

Causes. The clue in the history points to the cause:

The critical differential. Attritional rupture of flexor pollicis longus and the index flexor digitorum profundus, in rheumatoid arthritis or after a distal radius volar plate, produces an identical inability to make the OK sign. The tenodesis test separates them: passively extend the wrist. If the tendons are intact, the thumb and index terminal joints flex passively through the tenodesis effect and the problem is the nerve; if they are ruptured, they do not.

Management. The sequence for a palsy:

• Observation is the default for a spontaneous palsy, for at least 3 to 6 months and often longer, because neuralgic amyotrophy accounts for many cases and recovers spontaneously.
• Electrodiagnostics at 3 to 4 weeks to confirm, and again at 3 months to look for reinnervation.
• Exploration and decompression if there is no recovery by around 6 months, or earlier if there is a mass or a clear compressive history.
• Late reconstruction by flexor digitorum superficialis of the ring finger to flexor pollicis longus, and side-to-side tenodesis of the index profundus to the intact ulnar-innervated profundus tendons.

The common flexor origin. The humeral head of pronator teres arises from the common flexor origin, along with flexor carpi radialis, palmaris longus, flexor digitorum superficialis and the humeral head of flexor carpi ulnaris. Medial epicondylitis (golfer's elbow) is a tendinosis at this origin, most consistently involving the interface between pronator teres and flexor carpi radialis. It is provoked by resisted wrist flexion and resisted pronation, and by passive wrist extension with the elbow extended.

Where it hurts. Tenderness is at or within 1 cm of the medial epicondyle. In pronator syndrome the maximal tenderness is further distal, over the muscle belly.

The ulnar nerve overlap. Ulnar neuropathy at the elbow coexists with medial epicondylitis in a substantial proportion of patients, reported in up to a quarter to a half of surgical series. Failure to identify it is a common cause of a disappointing result after medial epicondylitis surgery, so any patient with medial elbow pain must have the ulnar nerve examined:

• Tinel sign at the cubital tunnel
• Elbow flexion test
• Sensation in the little finger and ulnar half of the ring finger
• First dorsal interosseous power

The medial elbow differential. Four conditions to separate:

Pronator teres as a deforming force. The muscular deforming forces in radial shaft fractures are a standard exam topic, and pronator teres is central to them.

The rule to state in a viva. The rotation of the proximal fragment is determined by which muscles remain attached to it, and the distal fragment must be brought to match. A fracture above the pronator teres insertion leaves the proximal fragment supinated; a fracture below it leaves the proximal fragment neutral.

The radial bow. Pronator teres inserts at the apex of the radial bow, the point of maximum lateral convexity of the radius. The bow is essential to forearm rotation because it provides the space for the radius to rotate around the ulna, so the muscle's unopposed pull, or a malreduction that straightens the bow, directly reduces pronation and supination.

Restoring the bow to within a few millimetres and a few degrees of the contralateral side correlates with recovery of forearm rotation, and is an explicit goal of fixation.

The standard wrist extension transfer. Pronator teres to extensor carpi radialis brevis is the first transfer named in every radial nerve palsy reconstruction, and it appears in essentially every published set.

Why pronator teres is the right donor. Its expendability is weighed separately below; its other qualities are these:

• It is median-innervated, and therefore unaffected by the radial nerve lesion.
• It is synergistic. Pronation and wrist extension occur together in normal grasp, so motor re-education is straightforward: the patient is taught to think about pronating to extend the wrist, then progressively dissociates the two.
• Its excursion and power are adequate. It comfortably provides the excursion wrist extension needs, and although it loses roughly one MRC grade on transfer, the resulting wrist extension is functional.
• It has a straight line of pull once the tendon is passed around the radius.

Expendable, but not free. Pronator quadratus continues to pronate, patients function well and rarely complain, and so the loss of pronator teres is usually described as costing little. It has been measured, and the cost is larger than that. Skie tested six cadavers and six patients and found a statistically significant loss of pronation strength of 24 to 44 per cent, depending on the method, with the cadaveric arm confirming reduced force and range after simulating the transfer.

Patients still function, the transfer remains the standard, and the loss of power and endurance is the accepted trade-off. "Expendable" should mean that the loss is acceptable, and there is still a loss. Consent for it, and weigh it where pronation is load-bearing (manual work, crutch or wheelchair use) and where the transfer is being done as an internal splint while awaiting nerve recovery, which is the situation the authors specifically flag.

Do not harvest it in an anterior interosseous nerve lesion. Pronator quadratus would then also be out, and the patient would lose active pronation entirely.

Why ECRB rather than ECRL. Extensor carpi radialis brevis inserts on the base of the third metacarpal, in line with the axis of the forearm, and produces central, balanced wrist extension. Extensor carpi radialis longus inserts on the base of the second metacarpal and produces radially deviated extension. Transferring into brevis alone gives a balanced wrist; some surgeons weave through both tendons, and then the tension must be balanced or longus dominates and the wrist deviates radially.

Technique. The steps, in order:

1. Harvest the pronator teres tendon with a long strip of periosteum from its radial insertion, to lengthen it and provide robust tissue for the weave.
2. Pass it around the radius subcutaneously, from volar to dorsal, superficial to brachioradialis and extensor carpi radialis longus, to reach extensor carpi radialis brevis.
3. Make a Pulvertaft weave into extensor carpi radialis brevis, tensioned with the wrist in about 30 to 45 degrees of extension and the elbow flexed.
4. Check the tension before closing: with the wrist in the set position the transfer should be taut, and the tenodesis effect should be preserved.

Over-tensioning produces a fixed wrist extension deformity that impairs the tenodesis grip and finger flexion.

The standard radial nerve palsy transfer set. One transfer for each lost function:

As an internal splint alongside nerve transfer. Patterson and colleagues reported that 15 of 16 patients undergoing nerve transfer for radial nerve palsy also received a pronator teres to extensor carpi radialis brevis tendon transfer at the same time. The tendon transfer provides immediate wrist extension while the nerve transfer reinnervates the finger extensors over months, acting as an internal splint. In that series the nerve transfer group achieved significantly higher grip strength, with no difference in pinch strength or patient-reported outcome.

Reading that comparison. Read it carefully before quoting it as nerve-beats-tendon. Because all but one of the nerve transfer patients also had the tendon transfer, the study compares tendon transfer alone against tendon transfer plus nerve transfer, which is an addition, not an alternative. The groups also differed significantly in the three variables that most affect a strength result: the nerve transfer patients were younger, operated on sooner after injury, and followed for longer.

The defensible reading is the authors' own. Both approaches improve pain, function and satisfaction, and the choice turns on presentation timing and on whether the patient can wait months for reinnervation.

Pronator teres in the volar Henry approach. The Henry approach exposes the whole volar radius, and each third requires a different muscle to be dealt with:

The pattern. Supinate to release the supinator, pronate to release pronator teres. In each case you rotate the forearm to bring the muscle's radial insertion anteriorly into view, which is easy to state under pressure.

The rest of the approach. The key steps and structures:

• The interval is between brachioradialis (radial nerve) and flexor carpi radialis (median nerve), a true internervous plane.
• The superficial radial nerve stays with brachioradialis and is retracted laterally.
• The radial artery is retracted medially with flexor carpi radialis.
• The leash of Henry, the radial recurrent vessels, is ligated proximally.

Pronator teres as a donor beyond radial nerve palsy. The muscle is used for more than wrist extension:

Selection principles for any tendon transfer. The criteria every donor is judged against:

1. Expendability. The donor's own function must be dispensable or duplicated. Pronator quadratus duplicates pronation, so pronator teres qualifies.
2. Adequate power. The transfer loses approximately one MRC grade, so a grade 5 donor becomes a grade 4 recipient.
3. Excursion match. Wrist extensors need about 3 cm, finger extensors about 5 cm and finger flexors about 7 cm. Pronator teres provides enough for the wrist but is marginal for the fingers.
4. A straight line of pull and a smooth gliding bed.
5. Synergy. A synergistic donor is easier to re-educate, and pronation and wrist extension are synergistic.
6. Supple joints and a stable skeleton. A transfer cannot move a stiff joint.
7. One tendon, one function. Do not ask a single transfer to do two jobs.

Pronator syndrome decompression. The steps:

• Position supine, with the arm on a hand table and a tourniquet.
• Make a lazy-S incision across the antecubital fossa, beginning on the medial aspect of the distal arm, crossing the flexion crease obliquely and continuing distally over the volar-radial forearm. Never cross the crease at a right angle.
• Protect the medial antebrachial cutaneous nerve in the subcutaneous plane.
• Release all four sites systematically:
  1. Ligament of Struthers. Palpate the distal medial humerus for a supracondylar process (obtain a pre-operative radiograph if one is suspected), and excise the process and the ligament.
  2. Lacertus fibrosus. Divide it.
  3. Pronator teres. Release the tendinous bands within the muscle. The deep (ulnar) head may be released or the tendinous portion divided, though a formal tenotomy is not routinely required and risks weakening pronation.
  4. Flexor digitorum superficialis proximal arch. Divide the fibrous arch and follow the median nerve distally beneath it.
• Follow the nerve throughout, and do not release individual sites blind.
• Close without tension, and begin early active motion.

Counselling. Results are less predictable than for carpal tunnel release. In the Hartz series failure was caused by inadequate decompression or by the diagnosis being wrong, which is why the systematic release of all four sites and careful pre-operative exclusion of carpal tunnel syndrome and cervical radiculopathy both matter.