An Instrumented Forceps for Quantifying Crush Forces in Optic Nerve Crush Murine Models

Injury to the optic nerve (ON) caused by trauma or diseases such as glaucoma can lead to degeneration of axons followed by progressive death of retinal ganglion cells (RGCs), ultimately resulting in irreversible vision loss. In order to find treatment methods for reversing the effects of neural degeneration triggered by injuries to ON, researchers perform controlled optic nerve crush (ONC) experiments on animal models where a blunt force mechanical insult is applied to the ON with forceps or tweezers, and the corresponding degree of degeneration in RGCs is studied. Currently, a major issue in establishing a reliable model for ONC is the lack of consistency in the tools used and the crush forces applied, making it difficult to quantify the degree of axonal damage and compare the ONC results obtained by different investigators across different labs. To rectify this situation, there is a need to develop a miniaturized tool that is tailored for ONC and enables reliable, quantifiable and repeatable ONC results. An important first step towards designing such a tool is the characterization of the crush forces and the corresponding axonal damage in a typical ONC experiment. Motivated by this need to measure the ONC forces, the broad objective of this thesis is, to develop an instrumented tool to deliver pressure insult and quantify ONC crush forces. Towards this goal, a pair of standard cross action forceps (Dumont N7) was instrumented with precision, miniature length, thin foil strain gauges. A pair of such commercially available strain gauges were mounted on the two arms of the forceps at optimal locations that maximize the force sensitivity of the forceps and packaged to minimize the bulkiness of the connecting wires. The voltage outputs of the strain gauges were carefully calibrated to correspond to the tip forces of the forceps using a load cell and a DAQ system. The performance of the instrumented forceps was tested by conducting ONC experiments on 8 eyes of 4 mice with varied forces and durations. The ONC forces applied over 3-25 s in these experiments were measured to range between 0.25 N and 0.38 N, and the corresponding force impulses were found to correlate linearly to the degree of axonal damage caused.