Ultrasound Assessment of the Transverse Carpal Ligament Thickness: A Reliability and Validity Study

Shen, ZL; Gordon, JL; Cheng, J; +Li, ZM +Departments of Biomedical Engineering, Orthopaedic Surgery, and Physical Medicine and Rehabilitation, Cleveland Clinic, Cleveland, OH [email protected] INTRODUCTION: The transverse carpal ligament (TCL) forms the palmar boundary of the carpal tunnel, prevents volar migration of the underlying median nerve and flexor tendons, and plays a critical role in regulating carpal tunnel mechanics. Knowledge of the TCL is necessary to facilitate further investigation into the TCL morphology related etiology of carpal tunnel syndrome (CTS). TCL morphology has been studied with varying techniques, including caliper, micrometer, histology, silicon casing with digitization, computed tomography (CT), and magnetic resonance imaging (MRI). Though these studies provide information regarding the TCL thickness, they are either expensive, time consuming, or require dissection. A relatively cheap, non-invasive methodology proper for clinical assessment of the TCL morphology is needed, such as ultrasound imaging. The purposes of this study were (1) to develop a methodology to measure the TCL thickness using ultrasound imaging and (2) to test the reliability and validity of this methodology since ultrasound is considered the most operator-dependent imaging modality. METHODS: Ultrasound Examination Eight fresh frozen human cadaveric hands were thawed overnight. The hands were stabilized in a custom-made thermoplast splint using Velcro straps in a supine posture with the thumb and fingers fully extended. A Siemens Acuson S2000 ultrasound system with a 18L6 HD wide band linear array transducer was used for this study. The machine settings were kept identical across all examinations with a frequency of 12 MHz. The main challenge of ultrasound examination was to standardize the location where TCL thickness would be measured. First, we located a desired plane of view, in which the trapezium, the hook of hamate, the TCL, and the thenar muscles were easily identified. Second, the transducer was held in place while the outline of the transducer was traced on the cadaver with a permanent marker. Third, using clear Transpore breathable surgical tape, we affixed a metal rectangularshaped reference marker on the radial aspect of the hand. The short sides were ~16 mm apart, about the width of the transducer. The long sides were ~5 mm apart and oriented crossing the transducer, which created interference patterns visible on the ultrasound image and allowed tracing the transducer location in the radial-ulnar direction. Finally, we replaced the transducer on the previously delineated carpal tunnel location. Three operators conducted ultrasound examinations of each specimen in two sessions. Each operator collected three images during each session. Measuring TCL Thickness from the Ultrasound Images Each operator performed image analysis on his/her own ultrasound images. The surface projection of the thenar muscle attachment site (TMAS) and the interference pattern created by the reference marker were marked on the top of each image (crosses in Fig. 1). The distance between these two locations was measured by on-screen calipers and used for later identification of the location where the TCL thickness should be measured on the dissected TCL. Each operator manually outlined the volar and dorsal boundaries of the TCL using ImageJ. All images were processed using a customized MATLAB function to measure the TCL thicknesses at the TMAS (arrow in Fig. 1).