The Flexor Carpi Radialis Brevis: A Description of an Anomalous Wrist Flexor and Surgical Exposure Technique
Authors: Bates Taylor , Nuelle Julia , Pierrie Sarah 
Published: June 2021

The flexor carpi radialis brevis (FCRB) is a rare anomalous flexor of the wrist. Surgeons occasionally encounter it intraoperatively during exposure of the volar radius.MethodsIn two patients, this muscle was identified by its longitudinal fibers during the approach to distal radius fractures. To adequately expose the fracture and preserve the innervation and arterial supply, the FCRB was elevated alongside the pronator quadratus and was retracted ulnarly to protect neurovascular structures.DiscussionThe flexor carpi radialis brevis is an anomalous muscle originating from the volar distal radius. It was first described in 1867 by John Wood, an anatomist and surgeon at King's College Hospital in London (Wood, 1867). It can be identified by its distinct longitudinal fibers on the volar aspect of the distal radius. The longitudinal muscle fibers help differentiate it from the transverse fibers of the PQ. Its presence is relevant to surgeons who may encounter it during operative management of distal radius fractures.The estimated prevalence of the FCRB is 2–8% (Kordahi, Sarrel, Shah, & Chang, 2018). It has been described in several case reports, either found incidentally during cadaver dissections or during surgical approaches. In addition, it has been implicated as a cause of radial sided wrist pain from symptomatic tears, intersection with the FCR, and carpal tunnel syndrome (Hongsmatip, Smitaman, Delgado, & Resnick, 2018; Kordahi et al., 2018; Mimura et al., 2017).The FCRB insertion has been observed on the second metacarpal base, on the base of the third metacarpal, or in between the second and third metacarpals (Laugharne & Power, 2010; Mimura et al., 2017; Nakahashi & Izumi, 1987). It is commonly associated with an atrophic PQ (Dodds, 2006; Kang, 2006; Laugharne & Power, 2010). Unlike the FCR, which is innervated by the median nerve, the FCRB is innervated by the anterior interosseous nerve (Dodds, 2006; Mimura et al., 2017; Nakahashi & Izumi, 1987). Knowledge of pertinent anatomy has implications for surgical exposure so that surgeons do not dennervate or devascularize the muscle.FCRB management during surgical exposures has been described in two cases. In one case, the muscle had to be fully released before fracture reduction and internal fixation could be completed (Laugharne & Power, 2010). In another case, treatment of a malunion of the distal radius, the FCRB was elevated radially and the flexor tendons and median nerve were retracted ulnarly (Kang, 2006).ConclusionAfter identifying the FCRB intraoperatively in two patients, we were able to safely expose and reduce the distal radius fractures by elevating the muscle with PQ in a radial to ulnar fashion. This approach protects the median nerve and preserves its innervation and blood supply from the anterior interosseous neurovascular bundle. This case series illustrates that awareness of the anomalous FCRB can guide surgeons and anatomists during dissection of the volar forearm.Support or Funding InformationNone to discloseThis abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.