Correlation between retinal ganglion cell loss and nerve crush force-impulse established with instrumented tweezers in mice.

Objectives: Rodent models of optic nerve crush (ONC) have often been used to study degeneration and regeneration of retinal ganglion cells (RGCs) and their axons as well as the underlying molecular mechanisms. However, ONC results from different laboratories exhibit a range of RGC injury with varying degree of axonal damage. We developed instrumented tweezers to measure optic nerve (ON) crush forces in real time and studied the correlation between RGC axon loss and force-impulse, the product of force and duration, applied through the instrumented tweezers in mice.Methods: A pair of standard self-closing #N7 tweezers were instrumented with miniature foil strain gauges at optimal locations on both tweezers' arms. The instrumented tweezers were capable of recording the tip closure forces in the form of voltages, which were calibrated through load cells to corresponding tip closure forces over the operating range. Using the instrumented tweezers, the ONs of multiple mice were crushed with varied forces and durations and the axons in the immunostained sections of the crushed ONs were counted.Results: We found that the surviving axon density correlated with crush force, with longer duration and stronger crush forces producing consistently more axon damage.Discussion: The instrumented tweezers enable a simple technique for measurement of ONC forces in real-time for the first time. Using the instrumented tweezers, experimenters can quantify crush forces during ONC to produce consistent and predictable post-crush cell death. This should permit future studies a way to produce nerve damage more consistently than is available now.