Amplification of Regional Human Ulnar Nerve Deformation by Tethering
Sameer Shah, PhD1, Justin R. Papreck2, Mark A. Mahan, MD3, David Weingarten, MD2 and Justin M. Brown, MD4
1Orthopedics, University of California, San Diego, San Diego, CA, 2 University of California, San Diego, San Diego, CA, 3Department of Neurosurgery, Barrow Neurological Institute, Phoenix, AZ, 4Division of Neurosurgery, University of California, San Diego, La Jolla, CA
The ulnar nerve responds to joint movement by sliding within its bed (translation) and deformation (strain). Deformation varies regionally, and higher strains have been reported near the elbow during its flexion. Ulnar nerve entrapment, such as that occurring during cubital tunnel syndrome, is likely to alter nerve kinematics. Such alterations may profoundly influence nerve conduction and the progression of neuropathy, but remain to be characterized. We tested the hypothesis that nerve entrapment increases nerve deformation at the region of tethering during joint movement, thereby amplifying nerve strain. Ulnar nerves were exposed in cadaveric arms with intact shoulders. Sutures were used as surface markers along a ~20 cm length of the ulnar nerve, centered at the elbow. A custom image analysis program determined the position of each suture relative to a stationary reference line connecting the medial and lateral epicondyles, orthogonal to the humeral axis. Changes in suture positions were used to calculate nerve translation and regional distributions of tensile strains following increased nerve deformation. Deformation was imposed at four joint configurations, each corresponding to progressive increases in wrist extension and elbow flexion. Kinematics were assessed under two models: native nerve position and entrapment. Entrapment was simulated by suturing the nerve to Osborne's ligament. Untethered nerves translated distally with elbow flexion, presumably due to nerve rotation away from the epicondyle axis. Tethering necessarily suppressed translation. ANOVA revealed significant effects of region and tethering on nerve strain following elbow flexion and wrist extension. Post hoc analysis (Tukey's HSD) indicated differences in regional strain in both native and entrapped conditions. Regions of the nerve near the elbow joint experienced higher strains (~20-25%) than distal or proximal regions (~8-10%). Tethering induced additional increases in regional strain. Strains distal to the tether were significantly increased, with strains in the vicinity of tethering especially pronounced. Strains proximal to the tether were unaffected. Our results suggest that ulnar nerve function may be impacted not only by compression at the site of entrapment, but also by excess tensile deformation imposed by suppressed nerve translation. Also, structural differences that accommodate regional differences in nerve strain are assumed to exist, though they remain to be determined. As a consequence, if regions of the nerve designed to accommodate low strains are forced by entrapment to bear high levels of deformation, this may amplify not only acute strain-associated deficits in conduction, but also the progression of degenerative changes associated with excess deformation.
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