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American Association for Hand Surgery

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Calculation of Strain in the Median Nerve with Increasing Distal Radius Fracture Displacement
David M Brogan, MD1, Chuka Obiofuma, BS2, Spencer Lake, PhD2, Leanne Iannucci, BS, M.Eng.2 and Christopher J Dy, MD2, (1)Washington University School of Medicine, St. Louis, MO, (2)Washington University in St. Louis, St. Louis, MO

Hypothesis
A link between distal radius fractures and carpal tunnel has been established in the literature, however the etiology of this link is still unclear1. We hypothesize that increasing dorsal displacement in distal radius fractures increases the strain on the median nerve, increasing the risk of median nerve pathology such as carpal tunnel syndrome.
Methods
We began with validation of our methods using ex-vivo measurement of median nerve strain in 4 cadaveric forearms placed in a custom designed jig with the nerve fixed at each end. The nerves were stretched in increments of 0.635 cm, as measured with a digital caliper. Strain was measured using a camera with optical markers glued to the epineurium . Custom MATLAB software was then used to perform segmentation analysis of the images taken by the camera to calculate the absolute distance between optical markers, and compare this to the known change in nerve length. After validation of the system, we then it used to measure the strain in 10 cadaver arms with a simulated distal radius fracture (DRF) and increasing dorsal angulation. The median nerve of each arm was exposed and optical markers applied. A custom designed jig rigidly fixed each cadaver arm to ensure reproducible measurements. Increasing dorsal angulation was applied to the epiphyseal fragment, and the resultant strain in the median nerve calculated. The measured strain at each angle was then analyzed using a linear regression model.
Results
The linear regression model in the validation experiment demonstrated a regression coefficient of 1.00067 (p<0.0001) of change in nerve length on measured nerve strain with r2=0.899, suggesting a high correlation between the known applied strain and the measured strain. This validates our methodology measuring these changes with optical tracking. Linear regression analysis of the effect of increasing dorsal angulation on strain in the osteotomy model yielded a regression coefficient of -0.000048 (p=0.714), r2=0.00129 (Figure 2).
Summary

  • Increases in median nerve strain at the wrist are negligible with increasing dorsal tilt in a distal radius fracture model, suggesting that increased pressure within the carpal tunnel is the primary cause of median neuropathy in distal radius malunion
  • Prior work has demonstrated 19 mm excursion of the median nerve with wrist motion2, which likely accommodates for increased dorsal angulation
  • The results validate our clinical experience that transverse carpal ligament release can effectively resolve carpal tunnel syndrome even in the setting of significant malunion.



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