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Ultrasound Measurement of the Cross-sectional Area of the Median Nerve: The Effect of Teaching on Measurement Accuracy
Jared A. Crasto, MD; Michael E. Scott, MD; John R. Fowler, MD
University of Pittsburgh Medical Center, Pittsburgh, PA
Introduction: Ultrasound (US) is an accurate, efficient, and cost-effective tool utilized in the diagnosis of carpal tunnel syndrome (CTS). The main checkrein to its ubiquitous adoption is the user-dependent variability discouraging those unfamiliar with the technique. While many studies call for the adoption of US as a point-of-care (POC) test for the timely, facile diagnosis of CTS, few have examined its learning curve. The present study intended to demonstrate that with concise instruction and guidance, orthopaedic surgeons at various levels of training could perform US measurements of the median nerve with an acceptable degree accuracy.
Materials & Methods: 7 orthopaedic residents and 5 hand fellows all with variable experience in musculoskeletal ultrasound took serial measurements of the median nerve at the carpal tunnel inlet using a 15-6MHz US probe utilizing an ellipse technique. After a 5-minute teaching session, all participants repeated measurements. A single cadaveric specimen was used throughout the study. Measurements were compared with the measurement of a fellowship-trained hand surgeon with extensive experience in US diagnosis of CTS. This was considered the accepted measurement. Descriptive statistics characterized each group. Paired t-tests compared averages of both the percentage of correct measurements and their values. A Bland-Altman plot was constructed to insure there was no bias.
Results: The rate of participants selecting the correct structure to measure on US was 33% before teaching and 97% after (p < 0.01). The average measurement before teaching was 10.0mm and after teaching was 5.3mm (p < 0.05). The measurement made by a fellowship-trained hand surgeon with extensive experience in US diagnosis of CTS was 6mm. The average participant deviation from this accepted measurement was +4.0mm before teaching, and -0.7mm after (p < 0.05). A Bland-Altman plot depicting this relationship is included below.
Conclusion: The present study demonstrates that the technique of sonographic measurement of the median nerve at the carpal tunnel can be quickly taught to orthopaedic trainees with an acceptable degree of accuracy. Since the method is valid for confirmatory testing in CTS, available on a POC-basis, cost-effective, and well-tolerated by the patient, the authors feel it should be more fervently utilized in orthopaedic practice, and should become part of residency curricula in the future.
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