Flexor carpi radialis brevis muscle, a rare anomalous muscle of the forearm; a case report and literature review

Introduction

Flexor carpi radialis brevis (FCRB), among the anomalous muscles of the forearm and wrist, is one of the extremely rare anomalous muscles. In 1851, Fano [1] first reported the flexor carpi radialis as an anomalous muscle of the forearm, and called it “Radiocarpien”. It was further described by Woods [2] in 1867 and the term “flexor radial” was created. In 1897, Le Double [3] classified FCRB variants into four types according to different morphologies of the muscle belly and tendon configuration, including the Radio-Palmaire type, the Radio-Carpien type, the Radio-Metacarpien type, and the Mixed type, of which the second was the most common. According to related literature reports, the occurrence rate of FCRB appears to be 2.6% in the Japanese population [4], 4-7.5% in European white subjects [3], and 3.4% in African American subjects [5]. As the anomalous muscle rarely causes clinical symptoms, it is often overlooked in imaging examinations of the forearm and wrist. Because of this, it is only occasionally reported in autopsies and surgical exploration. In this report, we provided a case study of a dual-forearm FCRB and a retrospective analysis of the existing published literature on FCRB.

Case report

During the dissection of the anterior group muscles in a left forearm specimen, we found a spindle-shaped penniform muscle in the deep side of the flexor digitorum superficialis (FDS) and flexor pollicis longus (FPL). The muscle started from the facies volaris distal radii and moved along the radialis of the flexor pollicis longus. The anterolateral side of the radius, which is at the level of the processus styloideus radii plane, is the muscle-belly of the muscle, and it occupies the terminus of pronator quadratus. According to the examination, the pronator quadrates were found with dysplasia, and was moved and changed to the tendons around the level of the radiocarpal joint. The tendon traveled along the ulnar side of the flexor carpi radialis (FCR) tendon, passing through the carpal canal together, and ending at the base of the palmar side on the second metacarpal bone (Figure 1). When the tendon was stretched, wrist flexion occurred. The blood supply came from a branch of the radial artery, and was innervated by the anterior interosseous nerve. No other abnormal muscles or bone structures were found in the dissection of the specimen. Besides, the same situation was observed during dissection of the subject’s right forearm (Figure 2). However, the insertion of FCRB was at the base of the first metacarpal base.

In addition, we searched the literature published in PubMed, EMBASE, and Ovid databases up to December 2018 by using “Flexor carpi radialis brevis” and “FCRB” as keywords. A total of 17 full-text articles were retrieved, including 27 cases of patients with FCRB (Table 1).

Discussion

FCRB is a rare abnormal muscle of the distal forearm, and its incidence varies in different reports, which fluctuates between 2-8%. Among the total incidence of FCRB, the rate of FCRB was 4-7.5% in Europeans [2], 1.8-2.6% in Japanese, and 1.9% in Japanese autopsies [8], and 3.4% in African Americans [21]. FCRB often originates from the volar side of the lower third of the shaft of the radius, and the area between the origin of the flexor pollicis longus and the end of the pronator quadratus at the radius [6,7,16]. It travels on the ulnar side of the flexor carpi radialis, stretching across the surface of the pronator quadratus, and changes into the tendon at the level of the radiocarpal joint surface. It passes through the radial carpal canal parallel to the tendon of the radial flexor carpal muscle and enters the metacarpus. However, there are also reports [20] where the FCRB tendon crosses the FCR tendon from the ulnar to the radial side. There is a variety of termini in FCRB tendon, including the trapezial, trapezoid and capitate bones, and the base of the second, third, and fourth metacarpal bones [6,7,11,13-15,18,19]. The FCRB is innervated by a branch of the median nerve, the anterior interosseous nerve [6,8,14]; whereas the blood supply is provided by the anterior interosseous artery [8], mediating the functions of wrist flexion and finger flexion [6,14,20-22]. Mimura et al. [14] described the occurrence of FCRB contraction when stimulating the anterior interosseous nerve with an electrical nerve stimulator during surgery. In this study, the autopsy of the forearm muscle groups revealed that the flexion of the wrist can be observed when FCRB tendon is extended by the anterior interosseous nerve innervation which was similar to the reports by Kosiyatrakul et al. [20] and Kang et al. [21]. In previous literature reports, FCRB was often accompanied with dysplasia of the pronator quadratus. Nagata J et al. [7] reported 5 patients with FCRB, in which 2 of 3 patients with muscle belly-type FCRB had pronator quadrates dysplasia, and the remaining 2 patients with tendon-type FCRBs were observed without PQ dysplasia. In this autopsy, the bilateral FCRB of the subject belonged to muscle-belly-type FCRBs and had the pronator quadratus dysplasia. Dodds et al. [6] believed that the muscle-belly of the muscle-belly type FCRB occupied the terminus of the pronator quadrates at the distal end of radius, resulting in the dysplasia of pronator quadratus.

With the increasing incidence of distal radius fractures in recent years, distal radius locking plate internal fixation with the Volar or Henry approach has been widely used by orthopedic surgeons, which has resulted in increased report rates of FCRB. After reviewing the relevant literature, we found 11 cases of FCRB were reported in the treatment of distal radius fractures, and 1 case was found in the treatment of wrist laceration [10]. Among them, Nagata et al. [7] reported 123 cases of distal radius fractures, and 5 of which was found with FCRB, with an incidence rate of 4.1%. Mantovani et al. [9] used locking plates with the Volar approach to treat 172 cases of distal radius fractures, and found 6 cases of FCRB, with an incidence of 3.5% (3 males and 3 females). Lee et al. [16] reported an incidence of 2.8% in 71 cases of patients who underwent the same procedures and approaches. According to the experience of the above three authors, the placement of the locking plate would not be affected by the intraoperative detection of FCRB, and postoperative complications were seldom occurred. Only one case [7] reported pain at the radial side of the wrist joint 6 months after surgery, and the surgery was performed again. The synovial tissues around the FCRB tendon were observed with significant hyperplasia. Histological examination revealed that the synovial tissues were surrounded by small blood vessels with fibroblast hyperplasia and extensive inflammatory cell infiltration. This result indicated that FCRB tenosynovitis could possibly occur in FCRB patients with locking plate internal fixation.

At present, there are few reports on clinical symptoms caused by FCRB. Because the FCRB tendons are usually parallel to FCR tendons and pass through the bone fibers of the carpal canal to enter the wrist, it is generally unlikely to compress the median nerve to cause carpal tunnel syndrome. However, the FCRB can compress the anterior interosseous nerve, thus causing clinical symptoms [23]. A retrospective analysis of preoperative MRIs by Mimura et al. [14] in 379 patients with carpal tunnel syndrome revealed 6 cases of FCRB, with an incidence of approximately 1.6%. However, the causal relationship between FCRB and carpal tunnel syndrome remains unclear. Peers et al. [13], Hongsmatip et al. [15] and Kosiyatrakul et al. [20] have respectively reported the symptoms of cases caused by FCRB. All three reported patients had the same clinical symptoms: unexplained wrist joint pain and swelling at the radial side, which was aggravated during flexion of the wrist joint. Physical examination showed obvious pain upon pressure at the radial side of the wrist joint, a local mass, and negative Tinel’s sign. Preoperative MRI and intraoperative examination found that the FCRB tendon crossed the FCR tendon from the ulnar to the radial side. Hongsmatip et al. [15] believed this crossing could cause “intersection syndrome”. In 1841, Velpeau described this disease for the first time. In 1987, Dobyns coined the term “intersection syndrome”, which is also known as “crossover syndrome”. The syndrome mainly occurred in the abductor pollicis longus, extensor pollicis brevis, and between the tendons of the extensor carpi radialis longus and brevis muscles, and occasionally occurred in other abnormal crossing tendons [24]. A conservative approach has been recommended for the treatment of this disease, including physical therapy and non-steroidal anti-inflammatory drugs (NSAIDs). For a very small number of stubborn cases or patients who needed quick relief from the painful symptoms, surgical incision should be conducted to the deep fascial region of the intersection region, and the possible bursae mucosae should be removed to limit the vicious cycle of muscle-tendon friction and swelling [25]. Two patients [13,18] underwent intraoperative resection of the FCRB, achieving pain relief and normalization of wrist activity during follow-up. One patient [15] received physical therapy and NSAIDs for treatment, and the pain was ameliorated after 3 weeks. During operation, one patient [20] was found to have deformed FCRB tendons, and the gap between FCR tendons were filled with a large amount of synovial tissues. The gap was separated and resected during the operation, and the pain disappeared after 2 months of follow-up.

In conclusion, knowledge of the anomalous muscles of the forearm and wrist can highly improve clinician’s understanding of the disease during surgery and imaging examination, and reduce unnecessary trauma caused by misdiagnosis. In addition to tenosynovitis, ganglion cyst and lipoma, anomalous muscle FCRB and intersection syndrome should be considered when pain, swelling and palpable mass were encountered in the radial side of wrist joint.

Disclosure of conflict of interest

None.