# Real Color Sectioned Images and Correspondence with Ultrasound Images of the Palmar Wrist
## 4 Discussion  
This study is the first to produce and then compare serially sectioned and US images of the wrist. On the serial images, the sites, shapes, courses, and positional relationships of wrist structures, such as the bones, nerves, vessels, and muscles, varied according to the level at which the wrist was viewed. The serially sectioned images of the wrist revealed details on the internal sites and courses of structures, thus enabling clear correlations and distinguishing of structures on the serial US images. On the sectioned images and US images, distinctive changes in the carpal bones and their articulations could be identified. The shape and size of the same bone changed between the serial images due to its irregular shape and articulations with its adjacent bones. In particular, the scaphoid and trapezium, which make up nearly 90% of carpal bone fractures [5], exhibited various changes in their shapes and sizes on the serial images due to their long axes and tubercles. When the carpal bones are fractured, comparison of serially sectioned images with serial US images can therefore facilitate the ability to distinguish between fractured bones and the adjacent lesions at the various levels of wrist. Entrapment of the MN in the carpal tunnel accounts for 90% of nerve entrapment neuropathies [12]. Causes of this syndrome include repetitive strain, wrist fracture, rheumatoid arthritis, space-occupying lesions, dialysis-related amyloidosis, and diabetes mellitus, while some cases have no apparent cause [13]. In the present study, the locations of the MN and its positional relationships with adjacent flexor tendons differed between the serial images. As the MN covered distally from the distal forearm to the wrist, this nerve moved from the radial side of the FDS and FDP tendons at the distal forearm and distal radioulnarjoint to the surfaces of the second and third FDS tendons at the distal carpal bones, then between the second and third FDS tendons distal to the level of the hook of hamate. [PERSON] et al. (2021) [14] reported a case of CTS caused by the MN being squeezed between the FPL and flexor digitorum tendons, which they diagnosed using US. Therefore, serial positional changes of the MN site should be considered when performing assessments of CTS and its mechanisms. The FPL tendon, which is the tendon closest to the radius in the carpal tunnel, also exhibited positional changes. At the level of the distal forearm, this tendon was slightly separated from the second FDS and FDP tendons. At the level of the carpal bones, the FPL tendon was just radial to the FDS and FDP tendons. At the level of the metacarpal bases, the FPL tendon moved to the radial side, separating from the second FDS and FDP tendons. [PERSON] et al. (2014) [15] reported two cases of CTS concomitant with FPL tenosynovitis using US. When evaluating the tenosynovitis location within the carpal tunnel and its contribution to CTS in the presence of increasing intracarpal tunnel pressure, serial positional changes of the FPL tendon can be important. The UN and UA enter Guyon's canal, or the ulnar tunnel of the wrist, which is bordered by the transverse carpal ligament (deep), volar carpal ligament (superficial), pisiform (ulnar side), and hook of hamate (radial side). Guyon's canal is further subdivided into three zones. Zone 1 is proximal to the bifurcation of the UN into the deep motor branch and the superficial sensory branch. Zone 2 is distal to zone 1, surrounding only the motor branch. Zone 3 is distal to zone 2, surrounding only the superficial sensory branch of the UN [16]. Most cases of Guyon's canal syndrome are attributed to a ganglion cyst and repetitive trauma. Signs and symptoms can be purely motor, purely sensory, or mixed depending on the zone of the distal UN lesion [17]. The serially sectioned and US images in the present study demonstrated the internal course and positional change of the UN in the ulnar tunnel according to the surrounding structures.