The distal musculotendinous unit of the flexor carpi radialis muscle is subjected to an amazing variation of stresses as it performs its function of flexing and abducting the hand. The relatively poor blood supply of the distal musculotendinous unit limits the ability of the muscle and tendon to heal when traumatized. Over time, muscle tears and tendinopathy develop, further weakening the musculotendinous unit and making it susceptible to additional damage and ultimately complete rupture.

The flexor carpi radialis tendon of the hand may develop tendonitis after overuse or misuse, especially when performing activities that require repeated flexion and abduction of the hand. Acute flexor carpi radialis tendonitis has been seen in clinical practice with increasing frequency due to the increasing popularity of sports such as tennis and golf. Improper stretching of flexor carpi radialis muscle and flexor carpi radialis tendon before exercise has also been implicated in the development of flexor carpi radialis tendonitis as well as acute tendon rupture. Injuries ranging from partial to complete tears of the tendon can occur when the distal tendon sustains direct trauma while it is fully flexed under load or when the wrist is forcibly flexed while the hand is full ulnar deviation.

The pain of flexor carpi radialis tendonitis is constant and severe and is localized to the dorsoradial aspect of the wrist. The patient suffering from flexor carpi radialis tendonitis often complains of sleep disturbance due to pain. Patients with flexor carpi radialis tendonitis exhibit pain with active resisted flexion of the hand and with ulnar deviation of the wrist. In an effort to decrease pain, patients suffering from flexor carpi radialis tendonitis often splint the inflamed tendon by limiting hand flexion and ulnar deviation of the wrist to remove tension from the inflamed tendon. If untreated, patients suffering from flexor carpi radialis tendonitis may experience difficulty in performing any task that requires flexion and abduction of the wrist and hand such as using a hammer. Over time, if the tendonitis is not treated, muscle atrophy and calcific tendonitis may result, or the distal musculotendinous unit may suddenly rupture. Patients who experience complete rupture of the flexor carpi radialis tendon will not be able to fully and forcefully flex the hand or fully abduct the wrist.

Plain radiographs are indicated in all patients who present with wrist and hand pain. Based on the patient’s clinical presentation, additional testing may be indicated, including complete blood cell count, sedimentation rate, and antinuclear antibody testing. Magnetic resonance imaging or ultrasound imaging of the wrist and hand is indicated if flexor carpi radialis tendinopathy or tear is suspected (Figs. 70.1 and 70.2). Magnetic resonance imaging or ultrasound evaluation of the affected area may also help delineate the presence of calcific tendonitis or other hand pathology.

Located in the forearm, the flexor carpi radialis muscle serves to flex and abduct (radially deviate) the hand (Fig. 70.3). The flexor carpi radialis muscle finds its origin on the medial epicondyle of the humerus and finds its insertion on the bases of the second and third metacarpals, with a secondary insertion on the base of the trapezium (see Fig. 70.3). The flexor carpi radialis muscle is innervated by the median nerve and receives its blood supply from the ulnar artery. It is at its points of insertion and at the point at which the distal flexor carpi radialis musculotendinous unit passes beneath the flexor retinaculum that it is susceptible to the development of tendonitis, tears, and rupture (Fig. 70.4).

The benefits, risks, and alternative treatments are explained to the patient, and informed consent is obtained. The patient is then placed in the sitting position with the elbow flexed to about 100 degrees and the forearm resting comfortably palm up on a padded bedside table with the fingers slightly flexed, which will relax the flexor tendons (Fig. 70.5). With the patient in the above position, the distal crease of the wrist is identified, and the patient is asked to forcibly flex his or her hand against resistance (Fig. 70.6). The tendon of the flexor carpi radialis tendon will be evident closest to the thumb (see Fig. 70.6). A high-frequency linear ultrasound transducer is placed in a transverse position over the tendon, and an ultrasound survey
scan is taken (Fig. 70.7). The tendon should appear just radial to the median nerve, which appears as a bundle of hyperechoic nerve fibers surrounded by a slightly more hyperechoic neural sheath lying beneath the flexor retinaculum (Fig. 70.8). The median nerve can be distinguished from the flexor tendons by simply having the patient flex and extend their fingers and observing the movement for the tendons. The flexor tendons will also exhibit the property of anisotropy with the tipping of the ultrasound transducer back and forth over the tendons. The flexor carpi radialis will be the most radial and superficial of the superficial flexor tendons. There may be significant effusion surrounding the tendon, which will appear on transverse ultrasound imaging as a hypoechoic ring around the tendon. If there is a question as to whether the tendon is the flexor carpi radialis tendon, the ultrasound transducer can be turned to the longitudinal plane, and the tendon can be followed distally to its insertion on the trapezium (Fig. 70.9). Color Doppler may identify hyperemia of the musculotendinous unit. After the musculotendinous unit is identified as it passes under the flexor retinaculum, the skin overlying the area beneath the ultrasound transducer is prepped with antiseptic solution. A sterile syringe containing 2.0 mL of 0.25% preservative-free bupivacaine and 40 mg of methylprednisolone is attached to a 1½-inch, 22-gauge needle using strict aseptic technique. The needle is placed through the skin just above the transverse ultrasound transducer and is advanced using an out-of-plane approach with the needle trajectory adjusted under real-time ultrasound guidance so that the needle tip ultimately rests in proximity to the tendon of the flexor carpi radialis as it lies within the carpal tunnel beneath the flexor retinaculum (Fig. 70.10). When the tip of the needle is thought to be in satisfactory position, after careful aspiration, a small amount of local anesthetic and steroid is injected under real-time ultrasound guidance to confirm that the needle tip is in the proper position. After proper needle tip placement is confirmed, the remainder of the contents of the syringe are slowly injected. There should be minimal resistance to injection, and no paresthesias should be elicited. If calcific tendonitis is present, a two-needle ultrasound-guided lavage and aspiration technique may be beneficial.