a 55-year - old woman presented with painful limitation of motion of her right wrist . it was planned to treat the fracture by open reduction and internal fixation with volar plating . the surgical dissection began with identification of the flexor carpi radialis ( fcr ) tendon . after the tendon sheath was released , the fcr tendon was retracted , and the flexor pollicis longus ( fpl ) tendon was identified . usually , the pronator quadratus ( pq ) and the fracture can be seen through the interval between the fpl and the brachioradialis ( br ) . but in this case , there was an anomalous musculotendinous structure between the fpl and the br ( fig . the functions of the fpl and the anomalous muscle were checked by tensioning them , respectively . tension on the anomalous muscle flexed the wrist without thumb motion . based on the anatomic location and function of this anomalous muscle , because the patient had consented only to the procedures related to the distal radius fracture , we did not perform further anatomical dissection beyond that required for internal fixation of the distal radius fracture . the flexor tendons and the median nerve were ulnarly swept , and the fcrb was retracted radially , to expose the pq . the pq was released from its radial insertion , and ulnarly reflected to expose the fracture ; and then volar plate fixation was performed , as usual . because the fcrb did not act as a hindrance to the volar plate fixation , additional procedure for the fcrb was not needed . a 49-year - old woman injured her left forearm , while making up her room , when a box fell on her left forearm . for the radius shaft fracture , the standard volar approach was used . as in case 1 , the fcrb was found between the fpl and the br ( fig . 2 ) . because the surgical approach was more proximal and longer , the origin of the fcrb was exposed . the fcrb originated from the volo - radial border of the radius between the radial insertion of the pq and the origin of the fpl . the functions of the fcr , fpl , and fcrb were checked by tensioning them , respectively . after stripping the radial insertion of the pq , the plate fixation was performed without difficulty . because it was not within the scope of the surgery a 55-year - old woman presented with painful limitation of motion of her right wrist . it was planned to treat the fracture by open reduction and internal fixation with volar plating . the surgical dissection began with identification of the flexor carpi radialis ( fcr ) tendon . after the tendon sheath was released , the fcr tendon was retracted , and the flexor pollicis longus ( fpl ) tendon was identified . usually , the pronator quadratus ( pq ) and the fracture can be seen through the interval between the fpl and the brachioradialis ( br ) . but in this case , there was an anomalous musculotendinous structure between the fpl and the br ( fig . the functions of the fpl and the anomalous muscle were checked by tensioning them , respectively . tension on the anomalous muscle flexed the wrist without thumb motion . based on the anatomic location and function of this anomalous muscle , because the patient had consented only to the procedures related to the distal radius fracture , we did not perform further anatomical dissection beyond that required for internal fixation of the distal radius fracture . the flexor tendons and the median nerve were ulnarly swept , and the fcrb was retracted radially , to expose the pq . the pq was released from its radial insertion , and ulnarly reflected to expose the fracture ; and then volar plate fixation was performed , as usual . because the fcrb did not act as a hindrance to the volar plate fixation , additional procedure for the fcrb was not needed . a 49-year - old woman injured her left forearm , while making up her room , when a box fell on her left forearm . for the radius shaft fracture , the standard volar approach was used . as in case 1 2 ) . because the surgical approach was more proximal and longer , the origin of the fcrb was exposed . the fcrb originated from the volo - radial border of the radius between the radial insertion of the pq and the origin of the fpl . the functions of the fcr , fpl , and fcrb were checked by tensioning them , respectively . after stripping the radial insertion of the pq , the plate fixation was performed without difficulty . because it was not within the scope of the surgery the fcrb is a rare anomalous muscle of the wrist . from the 19th century , it has been described in several cadaveric dissection studies , especially in japanese literature.3,4 ) it was first described by fano in 1851 , and the name of flexor carpi radialis brevis vel profundus was given by woods in 1867.3 ) the incidence of the fcrb has been reported to be 2.6% to 7.5%.4 ) the fcrb originates from the lower one - third of the radius on its volar surface , or volo - radial border between the origin of the fpl and the insertion of the pq , and it inserts into the base of the second or third metacarpal . but , its insertion is subject to frequent variations , and may be into any metacarpal base except the first or fifth , and radial side carpal bones , such as the scaphoid , trapezium , trapezoid , and capitate.3 ) the fcrb is innervated by the anterior interosseous nerve.1,3 ) while the fcr is superficial to the deep fascia in the same plane with the palmaris longus , the fcrb is located in the deep compartment , and occupies the space between the fpl and the br , superficial to the pq . the muscle belly of the fcrb is noticeable by its presence in the distal forearm , where tendinous and neurovascular structures are predominant . when it enters the hand , it passes the special osteofibrous tunnel of the fcr , which is separated from the carpal tunnel , and formed by the attachment of the flexor retinaculum to the two borders of the groove on the trapezium.2,5,6 ) 
 fig . 3 is an anatomical illustration showing the relationship between the fcrb , fpl , fcr , and pq . the fcr may have origins from the tendon of biceps , lacertus fibrosus , brachialis , the coronoid process of the ulna , and the anterior oblique line of the radius.6 ) but , such a case is extremely rare ; and even if it arises from the anterior oblique line of the radius , it should be superficial to the flexor digitorum superficialis.3 ) so , the fcrb can be distinguished from a split - off accessory head of the fcr . dodds1 ) reported a case in which the fcrb originated from the distal radial metaphysis , occupying the position of the radial insertion of the pq , and causing hypoplasia of the pq . nakahashi and izumi4 ) described the fcrb that passed the wrist through the carpal tunnel , and interconnected to the extensor carpi radialis brevis between the bases of the second and third metacarpals . recently , as volar plating of distal radius fractures is becoming popularized , surgeons have more likelihood of meeting the fcrb . in a review of the english literature , we found only ten clinical cases ; and all the cases were reported after 2006.2,7,8,9,10 ) the published case reports , including our two cases , are summarized in table 1 . among them , seven cases were related to the distal radius fracture.7,9 ) one case was discovered during surgical treatment of a wrist laceration,8 ) and the remaining two cases were presented as a painful swelling or mass caused by tenosynovitis of the fcrb.2,10 ) anomalous muscles in the hand and wrist often arouse compression neuropathy . the fcrb may also cause anterior interosseous nerve compression.5 ) but because the compression site is far distal to its branches supplying the fpl and the flexor digitorum profundus , it rarely - if ever - evokes clinically significant symptom.1 ) moreover , because the fcrb is usually located in the osteofibrous tunnel of the fcr , which is outside of the carpal tunnel , it is less likely to cause carpal tunnel syndrome.2 ) in contrast , the fcrb can cause tenosynovitis . in a case report of the fcrb that presented as a painful mass , peers and kaplan2 ) described extensive tendinosis and tenosynovitis of the fcrb and the fcr , and performed complete excision of the fcrb in its entirety . kosiyatrakul et al.10 ) also reported a case of tenosynovitis of the fcrb . in any event , , there has been no case except the two cases mentioned above.2,10 ) the clinical significance of the fcrb is its proximity to the standard volar approach for the distal radius fracture.7,9 ) as the trend toward treating distal radius fractures with volar plating is growing , it is important for surgeons to be aware of this anomalous muscle . in the author 's experience , kang et al.7 ) and mantovani et al.9 ) also mentioned that the presence of the fcrb neither required additional procedures , nor induced intraoperative or postoperative complications . but , even if a volar plate fixation will not be affected by the presence of the fcrb , knowledge about this anomalous muscle will be helpful to prevent surgeon 's perplexity , when faced with this muscle . in that case , tensioning test of the suspected tendon is informative . mantovani et al.9 ) reported six cases of the fcrb in 172 distal radius fractures ( 3.5% ) . we performed volar plating for 71 cases of the distal radius fracture between 2008 and 2010 . as mantovani et al.9 ) pointed out , further clinical reports and anatomical studies will be needed , to determine the exact incidence within the wider population .
during imaging studies or surgical procedures , anomalous forearm and wrist muscles are occasionally encountered . among them , the flexor carpi radialis brevis is very rare . because the trend is growing toward treating distal radius fractures with volar plating , the flexor carpi radialis brevis is worth knowing . here , we report two cases with a review of the literature .