Direct Linear Transformation for the Measurement of In-Situ Peripheral Nerve Strain During Stretching

Summary

This protocol implements a stereo-imaging camera system calibrated using direct linear transformation to capture three-dimensional in-situ displacements of stretched peripheral nerves. By capturing these displacements, strain induced at varying degrees of stretch can be determined informing the stretch injury thresholds that can advance the science of stretch-dependent nerve repair.

Abstract

Peripheral nerves undergo physiological and non-physiological stretch during development, normal joint movement, injury, and more recently while undergoing surgical repair. Understanding the biomechanical response of peripheral nerves to stretch is critical to the understanding of their response to different loading conditions and thus, to optimizing treatment strategies and surgical interventions. This protocol describes in detail the calibration process of the stereo-imaging camera system via direct linear transformation and the tracking of the three-dimensional in-situ tissue displacement of peripheral nerves during stretch, obtained from three-dimensional coordinates of the video files captured by the calibrated stereo-imaging camera system.

From the obtained three-dimensional coordinates, the nerve length, change in the nerve length, and percent strain with respect to time can be calculated for a stretched peripheral nerve. Using a stereo-imaging camera system provides a non-invasive method for capturing three-dimensional displacements of peripheral nerves when stretched. Direct linear transformation enables three-dimensional reconstructions of peripheral nerve length during stretch to measure strain. Currently, no methodology exists to study the in-situ strain of stretched peripheral nerves using a stereo-imaging camera system calibrated via direct linear transformation. Capturing the in-situ strain of peripheral nerves when stretched can not only aid clinicians in understanding underlying injury mechanisms of nerve damage when overstretched but also help optimize treatment strategies that rely on stretch-induced interventions. The methodology described in the paper has the potential to enhance our understanding of peripheral nerve biomechanics in response to stretch to improve patient outcomes in the field of nerve injury management and rehabilitation.

Introduction

Peripheral nerves (PNs) undergo stretch during development, growth, normal joint movement, injury, and surgery¹. PNs display viscoelastic properties to protect the nerve during regular movements^2,3 and maintain the structural health of its nerve fibers². Because PN response to mechanical stretch has been shown to depend on the type of nerve fiber damage⁴, injuries to adjacent connective tissues^2,4, and testing approaches (i.e., loading rate or direction)⁵

Protocol

All procedures described were approved by the Drexel University Institutional Animal Care and Use Committee (IACUC). The neonatal piglet was acquired from a United States Department of Agriculture (USDA)-approved farm located in Pennsylvania, USA.

1. Stereo-imaging system setup

1. Attach a stereo-imaging camera system that captures up to 100 frames/s (FPS) to a utility stand. The stereo-imaging camera system used in this study is a passive stereo camera with two horizontally aligned cameras (referred to as the left and right cameras) separated by a baseline of 63 mm (Figure 1).

Results

Using the described methodology, various output files are obtained. The DLTdv7.m *_xyzpts.csv (Supplemental File 12) contains the (x, y, z) coordinates in millimeters of each tracked point at each time frame that is further used to calculate the length, change in length, and strain of the stretched PN. Representative length-time, change in length-time, and strain-time plots of a stretched PN are shown in Figure 10. The stretched PN had an insertion marker, four markers along.......

Discussion

Studies reporting biomechanical properties of peripheral nerves (PNs) because of stretch injury vary, and that variation can be attributed to testing methodologies such as testing equipment and elongation analysis^{5,6,7,8,9,10,11,12,}

Materials

Name	Company	Catalog Number	Comments
Clear Acrylic Plexiglass Square Sheet	W W Grainger Inc	BULKPSACR9	Construct three-dimensional control volume
Stereo-imaging camera system - ZED Mini Stereo Camera	StereoLabs Inc.	N/A	N/A
Imaging Software - ZED SDK	StereoLabs Inc.	N/A	N/A
Maintence Software - CUDA 12	StereoLabs Inc.	N/A	Download to run ZED SDK
Camera stand - Cast Iron Triangular Support Stand with Rod	Telrose VWR Choice	76293-346	N/A
MicroSribe G2 Digitizer with Immersion Foot Pedal	SUMMIT Technology Group	N/A	N/A
Proramming Software - MATLAB	Mathworks	N/A	version 2019A or newer
DLTcal5.m	Hedrick lab	N/A	Open Source
DLTdv7.m	Hedrick lab	N/A	Open Source