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Illustrated Encyclopedia of Human Anatomic Variation: Opus I: Muscular System: Alphabetical Listing of Muscles: P

Pronator Teres

Ronald A. Bergman, PhD
Adel K. Afifi, MD, MS
Ryosuke Miyauchi, MD

Peer Review Status: Internally Peer Reviewed

Macalister has found the following variations for this muscle:
  1. It may be split into separate coronoid and condyloid muscles for its entire length (Albinus);
  2. The coronoid head may be absent;
  3. The two heads may be separated for their entire length but insert in common;
  4. A doubled coronoid head, the second lying posterior to the artery (Macalister, 1868). Kelly found a similar muscle but both heads were posterior to the median nerve;
  5. The coronoid head was twice found attached to brachialis anticus (brachialis);
  6. Or going, not to the pronator, but to palmaris longus;
  7. A single slip from the coronoid process giving origin to two slips, one of which goes into the pronator, the other to flexor carpi radialis;
  8. An accessory medioulnar origin was found by Brugnone;
  9. The condyloid origin has been found with a sesamoid bone in it;
  10. With an accessory origin from the semilunar fascia of the biceps;
  11. With an accessory slip from brachialis anticus (brachialis);
  12. Macalister (1866) found a treble origin once: one head from the biceps tendon, between its usual tendon and its semilunar fascia; one from the humerus from about three inches and a half (1 inch = 2.54 cm) above the the condyle to about an inch and a half above it; and one ( a short narrow tendon) from the tip of the medial condyle;
  13. A high origin, like the second head of the preceding, was reported by Struthers (1854), and it was nearly separate for its whole length from the rest of the muscle;
  14. A case of this humeral head not detached was described by Nuhn;
  15. A more complex case of high origin is described by Hyrtl, in which there existed one origin from the intermuscular ligament, one from the medial condyle, and one from the humerus, at the intersection of coracobrachialis, lying over brachialis-anticus;
  16. A high humeral origin has been found co-existing with a supra-condyloid process of the humerus, by Gruber, Struthers, Schwegl, and by Voss of Christiania (Oslo); in the latter case an additional humeral head lay above the supracondylar process;
  17. The insertion of pronator teres has been found by Macalister sending a slip into the radial origin of the flexor sublimis (flexor digitorum superficialis), and the same is described by Otto;
  18. Koster (1856) in a case of congenital deformity, found this muscle inserted into the lower end of the radius.

The most interesting variation of this muscle is the extension of its origin, in a proximal direction, to a supracondylar process or to the ligament (of Struthers) that connects it to the medial epicondyle. In these cases, the course of the brachial artery is usually changed: accompanied by the median nerve, it passes beneath the process , deep to the accessory portion of the muscle, and reaches the antecubital space. In cases of high bifurcation of the brachial artery it is usually the ulnar artery that passes deep to the process or ligament. The ligament of Struthers has been implicated as one of the causes of the pronator syndrome. The pronator syndrome, the most proximal of the median nerve entrapment neuropathies, presents with pain located on the volar surface of the distal arm and proximal forearm, which is usually associated with increased arm/forearm activity. It may also be associated with generally reduced sensibility or at a minimum, reduced sensibility of the the radial three and one half digits of the hand.

Mori reported on the relationship between the median nerve and pronator teres as follows: The median nerve passes between the the humeral and ulnar heads in 95% of cases, it passes between flexor digitorum profundus and pronator teres in 0.25%, and passes through the humeral head in 0.25% of cases.

Even in the absence of a supracondylar process, an accessory bundle or head may arise from the median intermuscular septum, the humerus, fascia of the arm, or adjacent muscle. Additional heads may arise from the biceps, brachialis, or humerus near the insertion of coracobrachialis. The coronoid head is often absent or rudimentary. Pronator teres has also been seen reinforced by muscle fibers arising the anterior surface of the ulna. Accessory fasciculi may also connect it with flexor carpi radialis, flexor digitorum superficialis, or brachialis.

The two heads of the muscle may be completely separated, and the heads, coronoid and humeral, may be doubled. The radial insertion may be more extensive than usual and fasciculi may extend to the long flexor of the thumb. A sesamoid bone may be found in the thick humeral tendon of pronator teres.

Syn.: m. pronator rotundus, Länglicher or runder Vorwälrts- or Einwärtswender or Dreher, Long ou rond pronateur.

Image 153

Pronator Teres Third Head.
from Barrett.

Image 109

Pronator Teres, Accessory Head from Brachialis.
A dissected specimen from the University of Iowa College of Medicine.

Image 176A

Union of Biceps Brachii and Pronator Teres
source unknown to authors.

Image 73.

High Origin of Pronator Teres from Ligament of Struthers and Supracondylar Process of Humerus.
These illustrate potential entrappment sites for the brachial artery and the median nerve.
from Calori.


Anson, B.J., Ed. (1966) Morris' Human Anatomy, 12th ed., The Blakiston Division, McGraw-Hill Book Company, New York.

Bianchi, A. (1906) Faisceau accessorie du m. pronator teres du nerf médian et de l'artère humérale àla région du coude. Arch. Ital. Biol. 45:283-284.

Barrett, J.H. (1935-36) An additional (third and separate) head of the pronator teres muscle. J. Anat. 70:577-578.

Calori, L. (1880) Intorno al canale sopracondiloideo dell'omero nell'uomo. Mem. R. Accad. Sci. Istituto di Bologna S.4. 2:37-46.

Giannelli, L. (1899) Sur une anomalie peu commune du muscle rond pronateur. Arch. Ital. Biol. 31:186.

Henle, J. (1871) Handbuch der Muskellehre des Menschen, in Handbuch der systematischen Anatomie des Menschen. Verlag von Friedrich Vieweg und Sohn, Braunschweig.

Latarjet, A. (1948) Testut's Traité d' Anatomie Humaine, 9th ed. G. Doin & Cie., Paris.

LeDouble, A.F. (1897) Traité des Variations du Système Musculaire de l'Homme et leur Significtion au Point de Vue de l'Anthropologie Zoologique. Libraire C. Reinwald, Schleicher Freres, Paris.

Macalister, A. (1875) On the nature of the coronoid portion of pronator radii teres. J. Anat. Physiol. 2:8-12.

Macalister, A. (1875) Observations on muscular anomalies in the human anatomy. Third series with a catalogue of the principal muscular variations hitherto published. Trans. Roy. Irish Acad. Sci. 25:1-130.

Mori, M. (1964) Statistics on the musculature of the Japanese. Okajimas Fol. Anat. Jap. 40:195-300.

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