The FCR originates from the medial epicondyle of the humerus and inserts into the trapezius, the second metacarpal, and the third metacarpal bones, and functionally contributes to the motion in flexion and the radial deviation of the wrist joint [6]. Although there have been reports of anatomical variations in its origin, insertion, and the presence of additional slips, complete absence of the FCR is an extremely rare anomaly that has been reported only twice in English-language reports [7, 8]. Rumball et al. [7] reported the absence of the FCR in a young boy undergoing tendon transfer for the reconstruction of postero interosseous nerve palsy, and Sofos et al. [8] reported the anomaly in a case of ligament reconstruction and tendon interposition arthroplasty for thumb carpometacarpal joint arthritis. Both reports focused on the need to avoid using a donor tendon in cases of absent FCR. However, the FCR also plays a very important role as a landmark in the distal forearm, and most surgery on the distal volar forearm is likely to be performed based on the location of the FCR [3, 9]. To our knowledge, this is the first report of an absent FCR identified during the commonly performed volar approach for fixation of a distal radius fracture.
Because important neurovascular structures exist close together in the distal volar forearm, there have been many complications reported for volar plating of distal radius fractures, including injuries to the median nerve, the PCB of the median nerve, and the radial artery [4, 10]. The FCR is designated as a favorable landmark because of its superficially palpable location, strong and thick structure, and rare anatomical variations. To avoid iatrogenic neurovascular injuries, detailed anatomical studies of the distal volar forearm have been based on the FCR [3, 9]. However, in rare cases where the FCR is absent, there is the possibility of misidentifying the PL as the FCR based on its most radial location in the distal volar forearm, especially under conditions of soft-tissue swelling around the distal forearm resulting from high-energy force. We suggest that the surgical approach should be changed to the classic Henry’s approach when the surgeon observes serious anatomical anomalies in the distal volar forearm [4].
In the present case, in addition to the absence of the FCR, we also identified an anomalous FCRB muscle. In general, the FCRB is considered to be an accessory muscle of the FCR that arises from the volar surface of the radius and inserts at various sites, including the base of the metacarpal bone, trapezium, and capitate [11]. Although this muscle functions to allow weak flexion of the wrist, it seems to be less important in normal function [5]. Although the previous two reports concerning an absent FCR did not describe the existence of the FCRB, it seems to play a role in wrist flexion as an alternative to the FCR. Surgeons should have a detailed knowledge of the range of possible anomalies to complete the fixation of a distal radius fracture safely [5].
Although the FCR approach is commonly used for fixation of distal radius fractures because of its simplicity and safety, various types of anatomical variations and anomalies of the distal volar forearm structures including the median nerve, the PCB of the median nerve, and the flexor tendon have been reported. Because the FCR is designated as a favorable landmark because of its superficially palpable location, strong and thick structure, and rare anatomical variations, there is the possibility of iatrogenic complications in cases where the FCR is absent. We suggest that surgeons should have a detailed knowledge about the range of possible anomalies to complete the fixation of a distal radius fracture safely.
