Additionally and more commonly, the median nerve can be compressed by an injury or thickening of a nearly imperceptible band of tissue called the lacertus fibrosis or bicipital aponeurosis. This tissue originates from the biceps brachii muscle and joins the fascia of the pronator-flexor mass spaning superficial to the underlying structures from lateral from the biceps brachii attachment and terminating medially at the insertion crossing the elbow joint. Most often, the only sign of this impingement is an abnormality of the nerve in this distribution. Rarely does the lacertus fibrosis have soft tissue thickening or possibly surrounding fluid or hematoma in the case of acute injury.
Moving distally to the antecubital fossa, the median nerve crosses the joint of the elbow and passes between the superficial and deep heads of the pronator teres muscle. This is another point of possible entrapment/impingement. Pronator tunnel syndrome occurs when the median nerve becomes compressed by the heads of the pronator teres muscle. This is easily identified using ultrasound. Understanding the anatomy of the pronator teres muscle is essential in identifying an impingement of the nerve in this location. The pronator teres muscle is commonly injured in throwing athletes and can mimic median nerve problems at this location. This can also be evaluated during the dynamic ultrasound exam.
The superficial head of the pronator teres muscle also called the humeral head, is the larger of the two portions of the pronator teres muscle and can be traced back to its attachment at the humerus (supracondylar location) as a landmark. The deep or ulnar head, which arises from the coronoid process of the humerus, can be seen deep to the median nerve. The muscle is bordered medially and slightly posteriorly by the common flexor tendon at the humeral condyle. Keep in mind that common flexor tendinitis/medial epicondylitis can mimic pronator teres syndrome or a pronator teres strain, especially in the throwing athlete. It can also be a comorbidity. When performing the exam, it is standard to evaluate the pronator teres muscle in the plane of the muscle, in the short axis, scanning from the humerus to the radial attachment, which is just distal to the radial tuberosity. Imaging the muscle on the long axis is also included in the exam. The starting position is with the patient’s hand in the supinated position.
Evaluating the nerve as it courses through the pronator tunnel includes qualitative evaluation of the nerve, measurements of diameter, as well as dynamic imaging, placing the hand in supination and pronation while visualizing the nerve (2).
