Real Color Sectioned Images and Correspondence with Ultrasound Images of the Palmar Wrist

Abstract

The aim of this study was to identify sequential changes in the actual sites and shapes of wrist structures on the high-resolution sectioned images and to compare these with those on ultrasound (US) images. These images of the wrist may yield guidelines for diagnosing various pathological lesions in the wrist. In the sectioned images of a whole cadaver body, and the images including the wrist were selected, and part of the hand was cropped. A wrist of one healthy Korean subject was scanned by ultrasonography to obtain US images of the wrist. Wrist structures were identified and labeled in detail on the sectioned images and the corresponding US images. Serial changes of the sites and shapes of the wrist structures mainly occurred in the bones, median nerve, ulnar artery, ulnar nerve, and the flexor pollicis longus tendon, along with thickening of the thenar and hypothenar muscles. The present results could provide a new guide for anatomy mapping, diagnosing, and detecting various lesions of the wrist. They can also be useful and effective for educational purposes related to interpreting US images.

1. Introduction

The wrist joint contains not only a series of complex articulations between two forearm bones, the eight carpal bones, and five metacarpals [1] but also various muscles, vessels, and nerves for the movement of five fingers. Since many structures pass in small spaces of the wrist and most of them are affected by contraction and relaxation of muscles, several diseases, such as carpal tunnel syndrome (CTS), Guyon’s canal syndrome, and direct trauma, commonly occur in the wrist, which can be diagnosed using ultrasonography [2,3,4].

Diagnosis and proper initial management of acute fractures of the carpal bones are critical due to the relatively low blood supply to many wrist bones, and injuries might not be identified by a cursory examination or in routine wrist radiographs [5]. The complex shape and articulations of the carpal bones make it difficult to distinguish them in ultrasound (US) images. The presence of several structures in the wrist also complicates the interpretation of these images.

Surgical techniques involving short incisions and endoscopic procedures demand a thorough knowledge of the anatomy and variations of wrist structures [6]. Since the internal course, depth, and sites of these wrist structures vary according to the level of the wrist, these can significantly affect the understanding of the three-dimensional anatomy and when detecting lesions for accurate diagnoses. It is, therefore, necessary to trace the internal courses and sites of wrist structures using serial images.

Comparisons between patients with CTS and normal subjects reveal that the median nerve (MN) exhibits different movements after wrist and finger motions, including sliding and transverse movements in the carpal tunnel [7,8]. Bolles et al. (2020) [2] described flexor carpi radialis (FCR) tenosynovitis as mimicking CTS when diagnosed and monitored using US images. Due to the anatomical proximity, FCR tendon disorders (e.g., tenosynovitis) may mimic CTS. Serial morphological and positional changes of structures can therefore be important for performing various diagnoses and when evaluating biomechanics and pathophysiological processes.

In January 2021, we made the sectioned images of whole male body. In the images, each structure in the wrist could be shown in real color (48 bits color) and high resolution (0.06 mm × 0.06 mm sized pixels) [9]. US is also widely used for evaluating hand and wrist lesions [10], and it represents a rapid, cheap, noninvasive, and dynamic method for examining the soft-tissue structures of the wrist and hand [11]. Therefore, the sectioned images and corresponding serial US images with labeling can provide a useful anatomical map for accurate wrist diagnoses.

The aim of this study was to identify sequential changes in the actual sites and shapes of wrist structures on the sectioned images and to compare these with those on serial US images. These images of the wrist may yield guidelines for diagnosing various pathological lesions in the wrist.

2. Materials and Methods

2.1. Axial Serial Sectioning

In January 2021, we made the sectioned images of the whole body of 70-year-old male cadaver. Color depth, intervals, resolution, and pixel size of the sectioned images were 48 bits color, 0.5 mm (trunk), 8688 × 5792 pixels, and 0.06 mm × 0.06 mm, respectively. Most gross structures in the sectioned images could be identified [9], and therefore we selected the images for this study.

In the sectioned images of the whole body, images of bones, muscles, arteries, and nerves of the wrist were selected at 1 mm intervals. In the selected images (resolution, 8688 × 5792), a part of the right hand was cropped to make the wrist sectioned images (resolution 2092 × 1205). All sectioned images were rotated horizontally using Adobe Photoshop CC (Adobe System Inc., San Jose, CA, USA).

2.2. Serial US Images

Signed written informed consent was obtained from the single subject (a healthy female aged 31 years, right-hand dominant). The exclusion criteria were previous trauma, arthritis, pathological condition, surgeries for a fracture or laceration, and a history of wrist diseases or disorders. The subject was asked to sit in a chair and put the forearm of her left hand on the table in a supine position. A real-time two-dimensional B-mode US device (ECUBE 15, ALPINION Medical Systems, Seoul, South Korea) with a 40 mm linear-array transducer (12 MHz; L3-12X, ALPINION Medical Systems) was used to obtain US images of the wrist from the distal forearm adjacent to the wrist joint to the second through fifth metacarpal bases. For the US images of the palmar wrist, transverse reference lines (red and green in Figure 1) were serially defined at approximately 3-mm intervals from the distal forearm adjacent to the wrist joint to the proximal bases of the metacarpals. A large mound of gel was used to optimize the visualization of the structures. Transverse-view images were also obtained.

2.3. Comparison between the Sectioned and US Images

Wrist structures were identified and labeled in detail on the high-resolution sectioned images and then also on the corresponding US images (

Table 1. Labeled structures in sectioned images and US images.

System	Structures
Skeletal	Radius, Ulna, Scaphoid (Sca), Lunate (Lun), Triquetrum (Tri), Pisiform (Pis), Trapezium (Trm), Trapezoid (Trd), Capitate (Cap), Hamate (Ham), First metacarpal bone (1st M), Second Metacarpal bone (2nd M), Third Metacarpal bone (3rd M), Fourth Metacarpal bone (4th M), Fifth Metacarpal bone (5th M)
Muscular	Flexor carpi radialis muscle (FCR), Palmaris longus muscle (PL), Flexor carpi ulnaris muscle (FCU), Flexor digitorum superficialis muscle (FDS), Flexor digitorum profundus muscle (FDP), Flexor pollicis longus muscle (FPL), Pronator quadratus (PQ), Abductor pollicis longus muscle (APL), Palmaris brevis muscle (PB), Abductor pollicis brevis muscle (APB), Flexor pollicis brevis muscle (FPB), Opponens pollicis muscle (OP), Adductor pollicis muscle-oblique head (AP-o), Adductor pollicis muscle-Transverse head (AP-t), Abductor digiti minimi muscle (ADM), Flexor digiti minimi brevis muscle (FDMD), Opponens digiti minimi muscle (ODM), First Palmar interossei muscles (PI 1), Second Palmar interossei muscles (PI 2), Third Palmar interossei muscles (PI 3), Fouth Palmar interossei muscles (PI 4), Flexor retinaculum (FR)
Vascular	Ulnar artery (UA)
Nervous	Median nerve (MN), Ulnar nerve (UN)

). Since skeletal structures had occupied a higher proportion and had distinct and larger changes in shape compared with the other structures in the wrist sectioned images, a correlation between the sectioned and ultrasound images was implemented focusing on the skeletal and muscular structures in the wrist.

2.4. Ethics Statement

The cadaver used to obtain the sectioned images had been legally donated to the Dongkuk University College of Medicine. This study was conducted in accordance with the Declaration of Helsinki from the World Medical Association (version from October 2013). The donor was not from a vulnerable population, and his next of kin voluntarily provided written informed consent. This study was approved by the Institutional Review Board of the Catholic Kwandong University (IRB no. CKU-21-01-0109). All study procedures were fully explained to a volunteer.

3. Results

Sectioned images and US images of the wrist were compared and labeled to analyze serial changes of the sites and shapes of the wrist structures. Serial changes of the sites and shapes of the structures from the distal forearm to the proximal metacarpal distinctively occurred in the bones, including their articulating parts, and in the MN, ulnar artery (UA), ulnar nerve (UN), and the flexor pollicis longus (FPL) tendon, along with thickening of the thenar and hypothenar muscles. In the sectioned images and US images, wrist structures were identified as follows.

3.1. At the Level of the Distal Radius and Ulna

The distal radius and ulna were observed at the distal forearm (Figure 2A,B). The pronator quadratus (PQ) was located anterior to the distal radius. The FCR and flexor carpi ulnaris (FCU) tendons were located beneath the skin at the radial and ulnar sites of the palmar wrist, respectively. The flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP) tendons appeared in two layers in the anterior forearm. The FPL tendon was radial to the FDP tendons. The palmaris longus (PL) tendon was superficial to and on the ulnar side of the MN. The MN was just anterior to the FPL tendon on the radial side of the FDS tendons. Both UA and UN were passed between the fourth and fifth FDS and FCU tendons. The distal radioulnar joint was at a more-distal level of the radius and ulna (Figure 2C,D).

3.2. At the Level of the Radiocarpal Joint

At the radiocarpal joint, the radius was in contact with the scaphoid and lunate (Figure 3A,B). The MN slightly moved to the ulnar side. The MN was on the FPL tendon and the second and third FDS tendons. The UA and UN were just radial to and deep to the FCU tendon. At a more-distal level of the radiocarpal joint (Figure 3C,D), the scaphoid and lunate became larger than at the more-proximal level. The shape of the anterior border of the lunate became sharp and triangular. The UA and UN gradually moved to the radial side. At an even more-distal level (Figure 3E,F), the anterior border of the lunate was smooth and convex, and the amounts of muscle fibers in the FDS and FDP decreased. The PL, FCR, and FCU tendons had constant sites and sizes.

3.3. At the Level of the Proximal Part of the Carpal Bones

In the proximal part of the carpal bones, the scaphoid, lunate, and triquetrum articulated each other (Figure 4A,B). The PL, FCR, and FCU tendons had constant sites and sizes. The MN was between the PL and FDS second tendons, and the UA and UN were radial to the FCU tendon. At a more-distal level (Figure 4C,D), the scaphoid became longer in the ventrodorsal direction. The pisiform appeared, and the triquetrum became larger. The capitate appeared between the scaphoid and lunate. The FCU tendon was just anterior to the pisiform and attached to it. The MN was on the FPL and the second and third FDS tendons. The UA and UN were radial to the FCU tendon and the pisiform and were located anterior to the third and fourth FDS tendons. At an even more-distal level (Figure 4E,F), the round tubercle of the scaphoid was close to the FCR tendon. The capitate became larger, and the hamate appeared, while the PL tendon became flat. The abductor digiti minimi (ADM) appeared on the ulnar side of the FCU tendon. The MN was on the second and third FDS tendons, the FPL tendon, and deep to the PL tendon. The UA and UN were radial to the FCU tendon and the pisiform and were located anterior to the fourth and fifth FDS tendons.

3.4. At the Level of the Distal Part of the Carpal Bones

In the distal part of the proximal carpal bones (Figure 5A,B), the scaphoid became smaller, and the capitate became larger. The PL tendon became flat and was located between the abductor pollicis brevis (APB) and palmaris brevis (PB). The FCR tendon was deep to the APB. The ADM became thicker and was located on the ulnar side of the pisiform and FCU tendon. The APB muscle and abductor pollicis longus tendon were observed in the thenar eminence. The FPL tendon was oval in shape and was located just radial to the FDS and FDP tendons. The MN was on the second and third FDS tendons and was located deep to the PL tendon. The UN and UA were radial to the pisiform and were located anterior to the fourth and fifth FDS tendons and deep to the PB. At a more-distal level (Figure 5C,D), the pisiform and trapezium became smaller. The trapezium appeared to articulate with the scaphoid, and the ADM became thicker. The FPL tendon was just on the ulnar side of the trapezium. The MN lay on the second and third tendons of the FDS, deep to the PL tendon. The UN and UA were radial to the pisiform, on the fourth and fifth tendons of the FDS, and deep to the PB.

3.5. At the Proximal Part of the Distal Carpal Bones

In the proximal part of the distal carpal bones (Figure 6A,B), the distal scaphoid was observed. The trapezoid appeared, and the groove and tubercle of the trapezium were distinctive. The FCR tendon passed through the groove of the trapezium. The FPL tendon was oval-shaped and was just radial to the FDS and FDP tendons. The opponens pollicis muscle (OP) appeared deep to the APB. The MN was on the second and third FDS tendons and was located deep to the PL tendon. The UN and UA were radial to the pisiform, anterior to the fourth and fifth FDS tendons, and deep to the PB. At a more-distal level (Figure 6C,D), the hook of hamate was rising, located on the ulnar side of the FDP tendons, and the trapezoid became larger. The APB and OP became thicker and extended toward the FDS second tendon. Some PB fibers were observed, while the MN was between the second and third FDS tendons. The UN and UA were located anterior to the hook of hamate, deep to the PB, and radial to the ADM.

3.6. At the Distal Part of the Distal Carpal Bones

In the distal part of the distal carpal bones (Figure 7A,B), the first metacarpal appeared radial to the trapezium. The proximal bases of the second and third metacarpals appeared between the trapezoid and capitate. The hook of hamate was distinctive on the ulnar side of the FDS and FDP tendons. The APB and OP became thicker and extended to the flexor retinaculum (FR). The MN was between the second and third FDS tendons. The UA and UN were between the hook of hamate and the PB, radial to the ADM, and the UA was divided into branches. At a more-distal level (Figure 7C,D), the distal carpals are articulated with the metacarpal bases. In the hypothenar eminence, the flexor digiti minimi brevis (FDMB) appeared radial to the ADM. The MN was between the second and third FDS tendons. The UA and UN were superficial to the FDMB, located radial to the ADM.

3.7. At the Level of the Proximal Part of the Metacarpal Bases

In the proximal part of the metacarpal bases (Figure 8A,B), the distal hook of hamate and metacarpals were observed. The FR was attached to the apex of the hook of the hamate. The FDS and FDP tendons were anterior to the second, third, and fourth metacarpals and radial to the hook of hamate. The FPL tendon was oval-shaped and was just radial to the second FDS and FDP tendons. The APB and OP became thicker and extended to the FR. The MN was between the second and third FDS tendons. The UA and UN were anterior to the hook of hamate, superficial and radial to the FDMB, and deep to the PB. At a more-distal level (Figure 8C,D), the proximal bases of the second, third, fourth, and fifth metacarpals are articulated with the adjacent other metacarpals. The thenar (APB and OP) and hypothenar (FDMB and ADM) muscles became thicker. The FCR tendon was on the anterior surface of the base of the second metacarpal. The MN was between the second and third FDS tendons. The UA and UN were between the PB and FDMB.

3.8. At the Level of the Distal Part of the Metacarpal Bases

In the distal part of the metacarpal bases (Figure 9A,B), the thenar (APB and OP) and hypothenar (FDMB and ADM) muscles became thicker. The APB and OP extended to the FR. The oblique head of the adductor pollicis (AP-o) was anterior to the second metacarpal. The FPL tendon was round in shape, and it was slightly prominent toward the radial side. The MN was between the second and third FDS tendons. The UA was just radial to the FDMB, and the UN was superficial to the FDMB. The first palmar interosseous muscle was between the first and second metacarpals, and the fourth palmar interosseous muscle was anterior to the fifth metacarpal. At a more-distal level (Figure 9C,D), the metacarpals decreased in size, and the palmar interosseous muscles were between the metacarpals. The hypothenar muscles (FDMB and ADM) became thicker. The second and third tendons of both the FDS and FDP were slightly separated from their corresponding fourth and fifth tendons. The FPL tendon was slightly separated from the FDS and FDP tendons, moving towards the radial side. The MN was between the second and third FDS tendons, and the OP became thicker. The AP-o became thicker and extended to the third metacarpal. The flexor pollicis brevis appeared on the ulnar side of the APB. The MN was between the second and third FDS tendons. The UA was radial to the FDMB and superficial to the opponens digiti minimi.

3.9. Serial Changes of the Main Structures in the Palmar Wrist

Serial changes of the main structures in the palmar wrist were analyzed by observing the serially sectioned and US images. The MN exhibited distinctive positional changes relative to the FDS and FPL tendons according to the level at which the wrist was viewed. At the distal forearm, the MN was radial to the second FDS tendon and anterior to the FPL tendon. At the proximal carpal bones, the MN slightly articulated with the second FDS tendon and the FPL tendon. At the distal carpal bones, the MN was on the second and third FDS tendons and was deep to the PL tendon. At the distal part of the distal carpal bones and the metacarpal bases, the MN was between the second and third FDS tendons.

At the distal forearm, the UA and UN were deep to the FCU tendon. The UA and UN gradually moved toward the radial side. At the proximal carpal bones, the UA was radial to the FCU tendon and anterior to the fourth FDS tendon. The UN was deep to the FCU tendon. At the pisiform level, the UA and UN were radial to the pisiform and FCU tendon, anterior to the fourth and fifth FDS tendons, and deep to the PB. At the hook of hamate level, the UA and UN were superficial to the FDMB.

The FCR and FCU tendons appeared at constant locations at the distal forearm and proximal carpal bone levels. At the distal carpal bones, the FCR tendon was deep to APB and coursed through the groove of the trapezium. At the metacarpal bases, the FCR was on the anterior surface of the second metacarpal. At the pisiform level, the FCU tendon was attached to the pisiform.

At the distal forearm and proximal carpal bones, both the tendons and muscle fibers of the FDS and FDP were observed. The second and third tendons were slightly separated from the corresponding fourth and fifth tendons of the FDS and FDP, respectively, at the proximal carpal bones and proximal part of the distal carpal bone levels. At the level of the metacarpal bases with interosseous muscles, the second FDS and FDP tendons were separated from their other corresponding tendons, moving toward the radial side.

At the distal forearm, the FPL tendon was separated from and on the radial side of the FDP tendons. At the radiocarpal joint and proximal carpal bones, the FPL tendon was just radial to the FDP tendons, deep to the MN, and had a round shape. At the distal carpal bones, the FPL was radial to the FDS and FDP tendons and had an oval shape. At the metacarpal base, the FDP tendon became prominent toward the radial side and then moved to the radial side.

The PL tendon was on the second FDS tendon and on the ulnar side of the MN at the distal radius and ulna. At the levels of the radiocarpal joint and the proximal and distal carpal bones, the PL tendon was on the MN as that nerve moved to the ulnar side on the surfaces of the FDS tendons. As the PL tendon was between the APB and PB, the PL tendon became flat. The PL tendon became smaller at the level of the metacarpal bases.

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 coursed 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 radioulnar joint 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. Hung 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. Manfield 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. When the UN is compressed by ganglions or due to fracture of the hook of hamate, serial images of the internal course of the UN can be helpful for detecting the lesion sites and predicting the symptoms.

The PL is highly variable. Its agenesis may appear unilaterally or bilaterally and reportedly occurs in 4% to 25% of individuals. The PL variants mostly include a duplicated, digastric, entirely muscular, and reversed PL, a structure that is tendinous proximally and muscular distally (in contrast to the normal PL) [18,19]. Since the PL muscle is a very useful graft source in tendon surgery, every surgeon should be aware of the variations in the PL tendon insertion [20]. In the present study, the PL tendon was superficial and on the ulnar side of the MN at the distal forearm; it was superficial to the MN at the proximal and distal carpal bones. When the PL tendon is harvested, these positional changes of the PL tendon relative to the MN at several levels of viewing the wrist must be noted to graft in wrist tendon surgery.

Limitations of this study were that the wrists were different in the sectioned and US images and thus had slight variations. Although subjects of the sectioned and US images were different, having different ages and sexes, large differences were not observed between the sectioned and US images. There were minor variations of the structures (

Table 2. Difference of the structures between the sectioned and ultrasound images.

Structure	Level	Difference
Median nerve	At the distal radius and ulna (Figure 2)	The median nerve was radial to the flexor digitorum superficialis tendons in the sectioned image and was on the second and third flexor digitorum superficialis tendons in the ultrasound image.
Palmaris brevis	From the distal part of the proximal carpal bones to the proximal part of the metacarpal bases (Figure 5, Figure 6, Figure 7 and Figure 8)	Extension and amount of the palmaris brevis were different between the sectioned and ultrasound images.
Abductor digiti minimi	At the distal part of the proximal carpal bones (Figure 5)	The abductor digiti minimi was on the ulnar side of the pisiform on the sectioned image and was anterior to and on the ulnar side of the pisiform on the ultrasound image.
Thenar muscles (abductor pollicis brevis, opponens pollicis)	From the proximal part of the distal carpal bones to the distal part of the metacarpal bases (Figure 6, Figure 7, Figure 8 and Figure 9)	Amount and extension of the thenar muscles (abductor pollicis brevis, opponens pollicis) were different between the sectioned and ultrasound images.

). However, this limitation had little impact on the validity of results, as observations were focused on the basic anatomy of the wrist.

5. Conclusions

The present results could provide a new guide for anatomy mapping, diagnosing, and detecting the various lesions of the wrist. They can also be useful for educational purposes related to interpreting US images.