Mohini Rawat, DPT, MS, ECS, OCS, RMSK and Mukund Patel, MD, FACS
Contents
Carpal Tunnel and Its Structures
Anatomy of the region is shown in Figure 2-1.
Figure 2-1. (A) Structures inside the carpal tunnel. The flexor retinaculum (grey) forms the roof of the carpal tunnel. The median nerve (yellow) is the most superficial structure just beneath the flexor retinaculum. Flexor tendons lie underneath the median nerve. The floor of the carpal tunnel is formed by carpal bones (brown). (B) Cross-sectional anatomy at the proximal carpal tunnel. (C) Cross-sectional anatomy at the distal carpal tunnel. (FCR = flexor carpi radialis; FPL = flexor pollicis longus; MN = median nerve; P = flexor digitorum profundus tendons; S = flexor digitorum superficialis tendons.)
- Patient position: Sitting or in supine with wrist in full supination and resting on the table
- Probe/transducer position:
a. Short axis (SX) view/transverse view: Use the pisiform as a bony landmark for the SX/transverse view of the carpal tunnel.
b. Long axis (LX) view/longitudinal view: Once you locate the median nerve in the SX view, keeping the nerve in focus, rotate the probe 90 degrees to see the LX view of the median nerve (Figures 2-2 through 2-4).
Figure 2-2. (A) Probe placement for the SX view of the median nerve. (B) SX view of the carpal tunnel at the level of the pisiform. (C) Labelled SX view of the carpal tunnel at the level of the pisiform. On the radial side: FCR outside the carpal tunnel, FPL radial-most in the carpal tunnel, flexor tendons (T) in the carpal tunnel underneath the median nerve (larger yellow circle). Outside the carpal tunnel on the ulnar side: ulnar nerve (smaller yellow circle) and ulnar artery (red circle).
Figure 2-3. (A) Probe placement for the LX view of the median nerve. (B) LX view of the carpal tunnel: The median nerve appears as a hypoechoic band overlying hyperechoic fibrillar flexor tendons.
Figure 2-4. (A) Proximal carpal tunnel anatomy and ultrasound image. (B) Distal carpal tunnel anatomy and ultrasound image. The FCR can be seen deep in the medial groove of the trapezium in its own compartment. (A = ulnar artery; MN = median nerve; P = flexor digitorum profundus tendons; PL = palmaris longus; S = flexor digitorum superficialis tendons.)
- Relevant anatomy: From superficial to deep, structures in the carpal tunnel are arranged in the order of skin, subcutaneous layer, flexor retinaculum (roof of the carpal tunnel), median nerve, flexor tendons, and carpal bones (floor of the carpal tunnel).
- Points to remember: A cross-sectional area of median nerve more than 10 mm2 at the level of the pisiform is considered abnormal.1
Median nerve mobility in the carpal tunnel can be assessed with dynamic examination as the patient flexes and extends the fingers and wrist. Median nerve mobility is negatively correlated with severity of the carpal tunnel syndrome.2,3
Subsynovial connective tissue, which appears as a hypoechoic nonmoving layer surrounding the flexor tendons under the flexor retinaculum, is thicker in patients with carpal tunnel syndrome than in normal healthy controls (Figure 2-5).4,5
Figure 2-5. Subsynovial connective tissue is a hypoechoic interface (between cursors) bound by the hyperechoic thin boundaries between the nerve on top and flexor tendons below.
Anomalies are common in the wrist. Some of the anomalies that may be present in wrist scans are bifid median nerve, persistent median artery, anomalous muscle of forearm in carpal tunnel (eg, flexor digitorum superficialis [FDS]), anomalous muscle of hand in carpal tunnel (eg, lumbrical muscle), and reverse palmaris longus (rare).6
Sonoelastography is a newer area in ultrasound where stiffness of the structure is assessed. It has been reported that stiffness of the intracarpal tunnel structures in carpal tunnel syndrome is higher than in the healthy controls.7
- Relevant anatomy: From superficial to deep, structures in the carpal tunnel are arranged in the order of skin, subcutaneous layer, flexor retinaculum (roof of the carpal tunnel), median nerve, flexor tendons, and carpal bones (floor of the carpal tunnel).
Structures Outside the Carpal Tunnel
- Patient position: Sitting or in supine with wrist in full supination and resting on the table
- Probe/transducer position:
a. SX view/transverse view: Use the pisiform as a bony landmark for the SX/transverse view of the carpal tunnel. Sweep the transducer proximal and distal to scan the structures as they enter the wrist.
b. LX view/longitudinal view: Once you locate a structure of interest in the SX view, keeping the structure in focus, rotate the probe 90 degrees to see the LX view of the structure of interest (Figure 2-6).
Figure 2-6. Structures outside the carpal tunnel. (A) Probe placement. (B) SX view showing the FCR outside the carpal tunnel in its own sheath, the ulnar artery (white A) and ulnar nerve outside the carpal tunnel, the median nerve (MN) inside the carpal tunnel and the palmaris longus (PL) outside the carpal tunnel as the most superficial structure. (C) LX view showing the palmaris longus tendon (white arrows) as the most superficial thin band and the median nerve (MN) as a hypoechoic band. The flexor tendons are a thicker hyperechoic band deeper to the median nerve.
- Relevant anatomy: The FCR is on the radial side of the carpal tunnel in its own sheath and runs distally through the groove on the medial side of the trapezium.8 The ulnar nerve and artery are on the ulnar side next to the pisiform bone. The palmaris longus tendon continues as the palmar aponeurosis in the palm and appears as a hyperechoic structure in the middle just superficial to the flexor retinaculum in the volar aspect of wrist.
- Points to remember: The palmaris longus may be absent in a small percentage of the population. There is an anomaly, reverse palmaris longus, that may be present in a very small percentage of the population where the muscle part of the palmaris longus is present at the level of the volar wrist and the tendon is located proximally.6
Volar-Radial Aspect of the Wrist
- Patient position: Sitting or in supine with wrist in full supination and resting on the table
- Probe/transducer position:
a. SX view/transverse view: Use the pisiform as a bony landmark for the SX/transverse view of the carpal tunnel and then slide the probe on the radial aspect to view the FCR, which lies outside the carpal tunnel in its own sheath.
b. LX view/longitudinal view: Once you locate the FCR in the SX view, keeping the FCR in focus, rotate the probe 90 degrees to get the LX view of the FCR. In the same view, the scaphotrapezial joint can be visualized (Figure 2-7).
Figure 2-7. FCR tendon. (A) Probe placement. (B) SX view of the FCR tendon on the radial side outside the carpal tunnel. The median nerve is visible as a hypoechoic structure ulnar to the FCR. (C) LX view of the FCR tendon (white arrows), which crosses the scaphotrapezial joint to insert on the anterior aspect of the base of the second metacarpal (MC).
- Relevant anatomy: The FCR inserts on the anterior aspect of the base of the second metacarpal and gives small slips to the trapezial tuberosity and third metacarpal base (Figure 2-8).8
Figure 2-8. Relevant anatomy: The FCR inserts at the base of the second metacarpal and gives off tendinous slips to the base of the third metacarpal and tuberosity of trapezium.
- Points to remember: Severe cortical irregularities at the scaphotrapeziotrapezoid joint are suggestive of arthritic changes and are frequently associated with FCR tenosynovitis or rupture.8
- Relevant anatomy: The FCR inserts on the anterior aspect of the base of the second metacarpal and gives small slips to the trapezial tuberosity and third metacarpal base (Figure 2-8).8
Volar-Ulnar Aspect of the Wrist
- Patient position: Sitting or in supine with wrist in full supination and resting on the table
- Probe/transducer position:
a. SX view/transverse view: Use the pisiform as a bony landmark for the SX/transverse view. Move the probe slightly proximal to visualize the flexor carpi ulnaris (FCU) tendon.
b. LX view/longitudinal view: Using the pisiform as a bony landmark, keep the probe along the FCU tendon, with the distal end of the probe on the pisiform (Figure 2-9).
Figure 2-9. FCU tendon. (A) Probe placement. (B) SX view of the FCU tendon (between the white arrows) and median nerve (MN) radial to the FCU. (C) LX view of the FCU tendon (white arrows) attaching to the pisiform and its distal tendinous slips to the hamate and fifth metacarpal base (not shown).
For ulnar nerve imaging, locate the ulnar nerve in SX view and, keeping the nerve in focus, rotate the probe 90 degrees to get the LX view of the ulnar nerve. The ulnar nerve is just radial to the pisiform bone, followed by the ulnar artery, which is radial to the ulnar nerve (see Figures 2-2 and 2-4).
- Relevant anatomy: The FCU is the ulnar-most tendon in the volar wrist and inserts into the pisiform bone and via ligaments to the hamate and fifth metacarpal base (Figure 2-10).
Figure 2-10. Relevant anatomy: The FCU inserts into the pisiform and via ligaments to the hamate and fifth metacarpal base.
- Points to remember: FCU tenosynovitis may be considered as a differential diagnosis in ulnar-sided wrist pain. For patients presenting with ulnar nerve symptoms, evaluate this area for any space-occupying lesions like ganglion cysts, synovial masses, or soft tissue growths.
Six Dorsal Compartments
(Tendons From Radial to Ulnar Aspect)
Anatomy of the region is shown in Figure 2-11.
Figure 2-11. The 6 dorsal compartments starting from the radial aspect: (1) abductor pollicis longus (APL) and extensor pollicis brevis (EPB); (2) extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB); (3) extensor pollicis longus (EPL); (4) extensor digitorum communis (EDC) and extensor indicis proprius (EIP); (5) extensor digiti minimi (EDM); (6) extensor carpi ulnaris (ECU).
- Patient position: Sitting or in supine with wrist in full pronation and resting on the table
- Probe/transducer position: The wrist has 6 dorsal compartments of tendons (see Figure 2-11). The first dorsal compartment tendons are the most radial tendons of the wrist. Starting from radial to ulnar, the 6 dorsal compartment tendons can be scanned. The SX view is the best view to locate the tendon of interest, and then the transducer can be rotated 90 degrees to get the longitudinal view of each tendon.
a. SX view/transverse view: All the tendons of the 6 dorsal compartments can be scanned in the SX view. It is important to align the transducer in the SX to the tendon and not to the wrist to get the best view of the tendon. Remember that each tendon has a slightly oblique course, and it is important to align the transducer perpendicular to the tendon to get the best visualization.
b. LX view/longitudinal view: Once the tendon is located in the SX view, the transducer can be rotated 90 degrees to get the LX view of each tendon.
- Relevant anatomy: The wrist has 6 dorsal compartments:
a. The first dorsal compartment consists of the abductor pollicis longus and extensor pollicis brevis. In a small part of the population, both tendons are separated by a septum. Knowledge of the presence and absence of a septum is important if local steroid injection is indicated in De Quervain’s tenosynovitis. In the presence of a septum, the involved tendon can be selectively injected to maximize the benefit of the injection (Figure 2-12).9
Figure 2-12. First dorsal compartment. (A) Probe placement. Note that the transducer is in the SX to the tendons of the first dorsal compartment, which is not a true SX view of the wrist. (B) Ultrasound image of the first dorsal compartment tendons. The extensor pollicis brevis (white arrow) is dorsal to the abductor pollicis longus (white triangle). (C) Relevant anatomy of the first dorsal compartment, which is the radial-most compartment of the 6 dorsal compartments of the wrist, with the transducer position.
b. The second dorsal compartment consists of the extensor carpi radialis longus and extensor carpi radialis brevis. This compartment has the fewest anatomical anomalies (Figure 2-13).
Figure 2-13. Second dorsal compartment. (A) Probe placement. (B) Ultrasound image of the second dorsal compartment tendons: extensor carpi radialis brevis (white triangle) and extensor carpi radialis longus (white arrow). (C) Relevant anatomy of the second dorsal compartment, with transducer position.
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