Photoelastic Stress Analysis

Photoelastic stress analysis is a long-established experimental technique which takes advantage of a curious property of many translucent polymeric materials, as well as glass: birefringence, or temporary double refraction. If these materials are subjected to an applied strain, the refractive index of the material changes, and the magnitude of the change of refractive index is different in the two in-plane principal strain directions. This results in light passing through the strained material at different speeds according to the applied strain. If a birefringent component, or opaque component with an applied birefringent surface coating, is illuminated with polarised light, and an observer with polarising sheet inspects the material when under strain, an interference pattern is visible which contains information on both the difference between the in-plane strains and their directions.

As an example, presented below is a case study to illustrate the method. Pedicle screw fixation is commonly used in treatment of spinal disorders - understanding the induced stress resulting from the insertion of a pedicle screw can help with optimization of screw thread design. Current finite elemental methods are challenging to use due to the complexity of the dynamic interaction between the implant and the bone. Our goal was to develop a repeatable technique based on photoelastic stress analysis to characterise and quantify the strain distribution during pedicle screw insertion in to a standard material.

Photoelastic4

Methods

An optically homogeneous and isotropic birefringent material was developed to study the induced insertion strain of a pedicle screw. Best results were obtained from food grade gelatin in desiccated granular form, which has been used recently with considerable success in the dental research field. Gelatin specimen blocks were carefully molded and pre-drilled to the inside diameter of the pedicle screw, these specimen blocks were then placed inside a circular transmission polariscope. The sample was illuminated with white light and the pedicle screw inserted with images captured every 1/8th of a turn.

Photoelastic5

Photoelastic Data Interpretation

By analysing the interference fringes, the difference in magnitude of the in-plane principle strains can be calculated. With the known properties of the gelatin the magnitude of the principle stresses can be determined. Two localised effects were observed with pedicle screw insertion into a gelatin specimen block: contact stresses immediately above/below the crest of the screw threads (left hand image, above); and compression between threads (right hand image, above).

Quantitative Data

As an example, the fringes produced at the tip of a thread crest are assigned a number – known as fringe order – increasing radially towards the screw. As the pitch of the spacing between fringes decreases towards the thread crest, the differences in principal stresses are increasing at a greater rate with respect to distance, which is indicative of a stress concentration.

Photoelastic6

Analysis of the localised fringe distribution, along with the optical and material properties of the gelatin, allows a quantitative assessment of the stress concentration magnitude to be made. Incremental insertion of a pedicle screw into the birefringent medium permits a detailed stress distribution sequence to be evaluated.

Photoelastic7

Discussion

A repeatable experimental photoelastic model was developed to characterize the stress distribution generated by the insertion of a pedicle screw into a standard material. This technique is a valuable tool to compare the performance of pedicle screws of different designs and determine the influence of various implant features. It is important to note that the model developed in this study was not intended to replicate the anatomy and mechanical properties of the human pedicle.

Strain Solutions is registered in England and Wales, company number 05305042.

Please contact webmaster@strainsolutions.com with comments on this site.