In Vivo Biomechanical Testing of Nerve: A Procedure for Biomechanical Analysis of Brachial Plexus Nerve Injury in a Neonatal Pig Model

Brachial plexus is a network of nerves in the shoulder that carry signals from the spinal cord to the upper limbs. Traumatic conditions that stretch the nerves beyond their limit may cause rupture leading to a loss of muscular function.

To quantify the stretch bearing limit or failure strain of the brachial plexus, set up a load cell and clamp with a mobilizing device called an actuator. The actuator enables clamping of the target specimen with a specific force measured using the load cell. Connect this biomechanical assembly to a computer and data acquisition system.

Once the setup is ready, take an anesthetized neonatal pig in a supine position with exposed brachial plexus. Cut the nerve and anchor the incised end to the clamp. Now, mark various nerve segments to observe the marker's displacement during elongation. Place a reference ruler close to the clamped nerve to set the scale for imaging purposes.

Now set a stretch rate on the computer system and start the test. The clamp begins pulling the nerve longitudinally. Record the nerve elongation pattern to capture the nerve's breaking point. Use the data acquisition system to calculate the failure strain. Repeat the experiment to determine the biomechanical properties of individual nerve segments.

To set up the biomechanical testing device, attach the base of the device to a cart, and use large C-clamps to attach the electromechanical actuator to the base. Attach a 200-Newton load cell to the actuator, and screw in a clamp with padded plexiglass, to prevent stress concentration at the clamping site.

Using a tripod, attach a camera that can record up to 100 frames per second at a 658 x 492 pixel resolution, and attach USB cables from the camera, actuator, and load cell to the computer, to integrate and synchronize all of the components of the setup. Then, plug the computer, actuator, and load cell into a power source.

To calibrate the load cell before recording the applied loads, use the adjustable handle to set the actuator at a 90-degree angle, such that it is aligned vertically, and check the angle with a protractor. Open the load cell software and click Start to show a live readout of the voltage.

Next, hang 0 to 1,000-gram weights from the clamp at 100-gram increments, recording the measured voltages at each load. When the voltages have been recorded for all 10 weights, calculate the slope, and intercept, to determine the linear equation of the voltages and weights.

For biomechanical testing of the isolated brachial plexus nerve, use fine scissors to cut the nerve, and use a custom clamp to clamp the cut side of the nerve. Label the clamped nerve segment with black acrylic paint, and place a 1-centimeter ruler flat within the animal to set the scale for the data analysis. In the camera software, place the camera field of view directly over the tested segments to allow monitoring of the motion and/or displacement of the markers, and to determine the actual tissue strain at a specific time point.

Record the baseline measurements, such as the height at which the nerve inserts into the body from the table, the height of the clamp from the table, the angle of the actuator, and the full length of the tissue. Open the programming software and click Run. Enter the file name and displacement and click Initialize and TARE. Click Start to stretch the brachial plexus segment.

The tissue will be pulled at an assigned rate of 500 millimeters per minute until complete failure occurs in any segment of the nerve tissue. Then, save a video file, the applied tensile load, the displacement of the tissue, and the duration of the test, and record the failure site as the segment at which the tissue ruptures.