Discussion
The brachioradialis, extensor carpi radialis longus, and extensor carpi radialis brevis originate from the anterolateral aspect of the lateral column of the distal humeral metaphysis and form the dorsal mobile wad. The origin of the brachioradialis is about 7 cm in length and begins about 10 to 11 cm above the elbow.5 The origin and insertions of the mobile wad, specifically the brachioradialis, provide a tremendous mechanical advantage with respect to elbow flexion against resistance, particularly with the forearm in the pronated and semipronated positions.6 With the elbow in 30° of flexion, a force 3 times the body weight can be encountered during strenuous lifting.6,7 We hypothesized these large forces likely led to this injury pattern in the patient we have described.
The literature includes 2 case reports of avulsion fracture of the brachioradialis muscle from its origin on the lateral supracondylar humeral ridge.1,2 To our knowledge, however, there have been no reports of pure avulsion. In our patient’s case, there was no bony fracture, but rather avulsion of the extensor carpi radialis longus and brachioradialis at their origin, with the underlying fibers of the extensor carpi radialis brevis remaining in continuity. Because of the rarity of this injury pattern, there was a significant delay in diagnosis. On initial presentation, the differential diagnosis for lateral elbow pain and tenderness included occult fracture, intracapsular plica, osteochondritis dissecans lesion, radial tunnel syndrome, lateral or posterolateral instability, and lateral epicondylitis. Given the absence of antecedent elbow symptoms before the injury, the dynamic soft-tissue asymmetry of the mobile wad with wrist extension, and the palpable soft-tissue defect, we thought the presentation was inconsistent with a simple inflammatory or overuse syndrome, such as lateral epicondylitis. In addition, the physical examination findings were inconsistent with radial tunnel syndrome or disruption of the lateral collateral ligament complex. Elbow MRI did not show an occult fracture, plica, or osteochondritis dissecans lesion but did reveal joint effusion and signal change in the common extensor tendon origin. Interestingly, MRI did not definitively show a tear of the mobile wad. This may be explained by the fact that the fibers of the underlying extensor carpi radialis brevis remained intact. Also potentially involved are the static nature of MRI and potentially suboptimal sequencing and axis of acquisition resulting from the relative infrequency of imaging this joint at certain health care institutions. Our case demonstrates the limitations of MRI in this setting and highlights the need for a detailed history and thorough physical examination for diagnosis.
Funk and colleagues8 used electromyography (EMG) to study the activity of the elbow musculature in uninjured subjects. EMG data were obtained with the elbow joint subjected to resisted flexion, extension, abduction, and adduction. During resisted elbow flexion, there was an increasing amount of activity in the extensor carpi radialis with larger angles of elbow flexion. In addition, the brachioradialis demonstrated the most muscle activity of any of the elbow flexors with 90° or more of elbow flexion and forearm pronation, as opposed to other positions in which the brachialis was the primary flexor. For this reason, we hypothesized that our patient’s forearm was pronated and his elbow flexed to 90° or more when he braced for impact. The ensuing injury resulted from a violent eccentric contraction that caused extensive rupture of the lateral elbow musculature from its broad origin. With the forearm in supination or neutral position, we would have expected a possible injury to the distal biceps as opposed to the brachioradialis and extensor carpi radialis.
In our patient, this injury caused much functional disability, especially with elbow flexion and wrist extension. We hypothesized that, for the muscles to function properly, anatomical restoration would have to be achieved at their known footprint to maintain their mechanical advantage. Therefore, surgical intervention was indicated in our patient, an active laborer. Given the absence of an osseous fracture fragment in this injury pattern, healing must occur at the bone–tendon interface. As tendinous healing is more tenuous and protracted than osseous healing, we preferred transosseous repair. We believed that better tendon-to-bone healing would be possible with drilled osseous tunnels rather than with suture anchors. New studies describing alternative successful methods of treatment would add to our limited body of knowledge regarding this rare injury.
Conclusion
This is the first report of avulsion of the extensor carpi radialis longus and brachioradialis from their origins. Given the biomechanics and anatomy of the dorsal mobile wad, we posit that our patient’s injury occurred when he fell onto his outstretched hand secondary to overwhelming eccentric muscle contracture at time of impact. This injury caused significant upper extremity dysfunction, and surgical intervention was required.