Bilateral Variations of Pronator Teres in the Upper Limb

During clinical practice, physicians need to have a sound knowledge of vascular and nerve variations in the body. Patients presenting with various clinical signs and symptoms need to be thoroughly investigated with anatomic variations in mind to prevent misdiagnosis. Most nerve variations are related to their formation or their course and are frequently associated with the variability of structures that surround them. These structures most commonly include blood vessels, ligaments, and muscles. Such variations should be foremost in a physician's mind when analyzing clinical symptoms. This will aid in accurate diagnosis, and if surgical intervention is warranted, such awareness would minimize intraoperative errors. This article discusses a variation in the pronator teres muscle, the branching pattern of the brachial artery, and the median nerve.

During the dissection of 11 cadaveric specimens within the Geisinger Commonwealth School of Medicine, an elderly female cadaver exhibited bilateral variations in the pronator teres muscle, which originated from the mid-humerus, instead of the medial epicondyle. Careful dissection revealed associated neurovascular variations in the arm, elbow, and forearm in relation to the muscle. The pronator teres muscles in the remaining 10 cadavers in the lab were examined for variations and their lengths were measured and compared with the cadaver under study.

Unlike the normal origin at the medial epicondyle as described in textbooks, it was observed that the humeral heads of the pronator teres muscle originated at mid-humerus level bilaterally, associated with the passage of the median nerve and ulnar artery posterior to it. This muscle was 19 cm in length bilaterally, approximately 5.5 cm longer than the average lengths of pronator teres measured bilaterally in the other cadavers. The abnormally high origin of this muscle was associated with the finding of a median nerve coursing posterior to it to the forearm, failing to appear in the antecubital fossa. Although the ulnar head appeared normal, there were bilateral variations in the median nerve during its passage between the two heads of the pronator teres at the proximal forearm as it proceeded to the deeper compartment of the forearm. The brachial artery was observed to divide into radial and ulnar arteries at the mid-humerus level. The radial artery replaced the brachial artery in the antecubital fossa and the ulnar artery accompanied the median nerve posterior to pronator teres into the forearm.

Such variations observed bilaterally have not yet been reported in the literature. Knowledge of these variations in the origin of pronator teres muscle, the absence of specific neurovascular structures as expected within the cubital fossa, and the awareness of early bifurcation and variation in their course can be very profound for physicians, as this region is often involved in the creation of arterio-venous fistulas for medical procedures, surgical treatment options for supracondylar and radial head fractures, and to differentiate median nerve compression in pronator teres syndrome versus carpal tunnel syndrome.