High-frequency Ultrasound Study Of The Carpal Tunnel:Normal Measurements And Clinic Implications

[Objective] To study the ultrasonic appearances and measure the dimensions of the internal structures of normal carpal tunnel with high-resolution ultrasonography, in order to establish the normal reference ranges.[Materials and Methods] High-frequency ultrasonography examinations were performed on 236 normal carpal tunnels of 118 cases, focusing on the ultrasonic appearance, shape, size and anatomic relationship of the median nerve and flexor tendons. Following measurements were done: (1) the anterpposterior and mediolateral diameters and the cross-sectional areas of median nervewere measured at three points: the distal radius, the pisiform bone, and the hook of the hamate bone. (2) the sectional areas of carpal tunnel and the palmar displacement of the flexor retinaculum at the levels of the pisiform bone and the hamate bone, (3) the thickness of the flexor retinaculum and the radiopalm ligament at sagittal plane, and (4) calculation of the flattening rate and swelling rate of the median nerve. Statistical analysis was carried out to address the correlation between the ultrasonic measurements and other variables like sex, age, body height and weight, and wrist circumference. [Result] High-frequency ultrasound clearly depicted themedian nerve and the flexor tendons on both transverse and sagittal planes. The median nerve was shown to be hypoechoic round or oval dots on cross-sectional images, surrounded by a zone of slightly hyperechogenicity. On the other hand, the flexor tendons were slightly hyperechoic round or oval structures with a narrow hypoechogenic rim. Both appeared as linear structures at longitudinal planes with similar echo features as the transversal planes. The anteroposterior and mediolateral diameters and the sectional areas of the median nerve were: 4.3+0.5mm, 1.9+0.2mm, 6.0+1.0 mm2 at the distal radius, 5.0+0.6mm, 1.9+0.2mm, 7.2+1.2mm at the pisiform bone, and 5.7+0.5mm, 1.6+0.2mm, 6.6+1.2 mm2 at the hamate hook respectively. The mean flattening rate of the median nerve and the PD of the flexor retinaculum were 2.2+0.4 at the distal radius, 2.7+0.5 and 1.9+0.8mm at the pisiform bone, 3.1 +0.5 and 1.8+0.6mm at the level of the hamate hook respectively. The swelling rate of the median nerve was 1.2+0.2. The sectional areas of the carpal tunnel with its internal structures (Median nerve, flexor tendons and tendon sheath) were 142.7+ 21.1 mm2 at the pisiform and 111.8 +15.5 mm2 at the hamate bone respectively. The sectional area of the carpal tunnel waspositively correlated with sex, age, body height, weight, and circumference of wrist. The radiopalm ligament had correlation with sex, body height, weight and wrist circumference, and correlation was also found between the internal diameter of carpal tunnel and the body weight and wrist circumference.[Conclusion] High-frequency ultrasonography depicts clearly the configuration, dimension and anatomical relationship of the median nerve and flexor tendons of the carpal tunnel, and can enable accurate measurement of these structures. Such information will be very helpful in the diagnosis and management of patients with carpal tunnel syndrome.