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American Association for Hand Surgery

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Creating Space: Can the Transverse Safe Zone Be Increased for Carpal Tunnel Release with Ultrasound Guidance?
Jay Smith, MD and John P Beckman, BA, Institute of Advanced US Guided Procedures - Sonex Health, Inc., Eagan, MN

Introduction
During carpal tunnel release with ultrasound guidance (CTR-US) the transverse carpal ligament is transected within the transverse safe zone (TSZ), defined as the space between the median nerve (MN) and either the hook of the hamate or ulnar artery (UA), whichever is closer. Small TSZs and UA encroachment (i.e., UA closer to the MN) may increase the technical difficulty and risks of CTR-US. The SX-One MicroKnife® is a commercially available device used for CTR-US that creates space by using expandable balloons. The purpose of this study was to determine the effect of the SX-One MicroKnife® on the TSZ. A secondary objective was to validate the ability to sonographically detect anatomic variations in the carpal tunnel region.
Materials & Methods
A total of 52 unembalmed cadaveric specimens were utilized for this investigation. Specimens were obtained from 52 donors ages 34-99 years with BMIs of 14-36 kg/m2. A single experienced examiner scanned each specimen to measure the MN cross sectional area (CSA), determine the ulnar border and size of the TSZ, and identify anatomic variations. Following baseline measurements, the device was inserted into the tunnel, the balloons inflated, and the TSZ re-evaluated. Specimens were subsequently dissected to confirm anatomic variations.
Results
The mean MN CSA was 10.8 mm2 (range 6.0-28.4 mm2). At baseline, the mean TSZ size was 5.8 mm with a lower limit of 0 mm, a wide range of 0-13 mm, and an UA border in 54% of specimens. Following balloon inflation, the mean TSZ increased to 9.9 mm with a lower limit of 7.2 mm, a narrower range of 7.2-13.0 mm, and an UA border in 13% of specimens (Figure 1). There was a statistically significant increase in TSZ size (mean 4.0 mm, p<0.001) with similar effects for specimens with larger (CSA >10 mm2) and smaller (CSA < 10 mm2) nerves (4.2 mm vs. 3.9 mm, p=0.67). Several anatomic variations were identified and confirmed via dissection, including an ulnar origin of the thenar motor branch. In this specimen the TSZ increased from 3 mm to 9 mm following balloon inflation.
Conclusion
The SX-One MicroKnife significantly increased the size of the TSZ, increased the lower limit of the TSZ from 0 mm to 7.2 mm, and reduced the frequency of ulnar artery encroachment.


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