Morphological and functional assessment of the flexor carpi radialis brevis using conventional ultrasound and elastography.

The flexor carpi radialis brevis (FCRB) is a supernumerary musculotendinous structure of the wrist that has been the focus of some interest in the last decade. While its anatomy is well known, its in vivo function remains unknown as it has never been studied. Eleven cases of FCRB underwent a multimodal ultrasound consisting of B-mode, color Doppler and shear wave elastography. A pennate shape was observed in all cases and the mean value of the cross-sectional area was 0.8 cm <sup>2</sup> (SD 0.3 cm <sup>2</sup> ). Young's modulus was significantly (p < 0.01) different between the resting position and active flexion or passive extension. Our study demonstrates that the FCRB shows biomechanics of a typical skeletal muscle and is voluntarily controlled by flexing the wrist. Absent in other vertebrate taxa, the FCRB probably plays a role in active stability of the wrist in Human