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In MR images, the median nerve of carpal tunnel syndrome (CTS) patients frequently appears flatter than in healthy subjects. The purpose of this work was to develop a metric to quantify localized median nerve deformation rather than global nerve flattening, the hypothesis being that localized median nerve deformation would be elevated in CTS patients. Twelve patients with CTS and 12 matched normals underwent MRI scanning in eight isometrically loaded hand conditions. 2D cross sections of the proximal and distal tunnel were analyzed for nerve cross sectional area, flattening ratio, and a position shift to the dorsal side of the tunnel. Additionally, new metrics based on the angulation of the nerve perimeter in 0.5-mm lengths around the boundary were calculated. The localized deformation metrics were able to detect differences between CTS patients and healthy subjects that could not be appreciated from the flattening ratio. During most hand activities, normal subjects had a higher average percentage of locally deformed nerve boundary than did CTS patients, despite having a rounder overall shape. Less local nerve deformation in the CTS patient group resulting from its interaction with flexor tendons suggests that the nerve may be less compliant in CTS patients.

Carpal tunnel syndrome is a frequently encountered peripheral nerve disorder caused by mechanical insult to the median nerve, which may in part be a result of impingement by the adjacent digital flexor tendons. Realistic finite element (FE) analysis to determine contact stresses between the flexor tendons and median nerve depends upon the use of physiologically accurate material properties. To assess the transverse compressive properties of the digital flexor tendons and median nerve, these tissues from ten cadaveric forearm specimens were compressed transversely while under axial load. The experimental compression data were used in conjunction with an FE-based optimization routine to determine apparent hyperelastic coefficients (μ and α) for a first-order Ogden material property definition. The mean coefficient pairs were μ=35.3 kPa, α=8.5 for the superficial tendons, μ=39.4 kPa, α=9.2 for the deep tendons, μ=24.9 kPa, α=10.9 for the flexor pollicis longus (FPL) tendon, and μ=12.9 kPa, α=6.5 for the median nerve. These mean Ogden coefficients indicate that the FPL tendon was more compliant at low strains than either the deep or superficial flexor tendons, and that there was no significant difference between superficial and deep flexor tendon compressive behavior. The median nerve was significantly more compliant than any of the flexor tendons. The material properties determined in this study can be used to better understand the functional mechanics of the carpal tunnel soft tissues and possible mechanisms of median nerve compressive insult, which may lead to the onset of carpal tunnel syndrome.