Measurement of flexor retinaculum bowing (arrow heads). After drawing a line connecting the hook of the hamate and tubercle of the trapezium where the flexor retinaculum is attached (transverse dotted line), the distance from the line to the top of the flexor retinaculum is measured (vertical dotted line). The radial and ulnar sides branch of the bifid median nerve (arrows) passes beneath the flexor retinaculum.