The proximal boundary of the flexor retinaculum is not readily demarcated, and previous reports of three distinct regions of the flexor retinaculum were not consistent with the authors' experience. This study was undertaken to clarify the proximal boundary and the constituent parts of the flexor retinaculum. A total of 56 cadaveric wrists were used in the study. The proximal boundary of the flexor retinaculum was identified by a change in thickness and colour of the longitudinally sectioned surface of the continuous membranous sheet of the flexor retinaculum and antebrachial fascia. Steel wires were placed on the proximal and distal boundaries, and anteroposterior radiographic images were taken. MRI was carried out before dissection or serial section. The locations of the proximal and distal boundaries of the flexor retinaculum varied. The flexor retinaculum was comprised of two parts, which were distinguishable by thickness and transparency. These two parts were also identified on MR images and by light microscopy.
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