Development of a method for quantitative stress analysis in bones by three‐dimensional photoelasticity

Abstract

Comparative critical examination of methods suitable for studying stress in bones have shown that the three-dimensional photoelastic method is one of the most reliable. Described herein is the method for obtaining, by fusion, full-scale models in epoxy resin, that are exactly equivalent to external shape of the prototypes.This technique offers the advantages of being applicable without variation to any bone segment and of enabling a large number of additional resin castings to be made from the same mould. Hence it is possible to produce a very large number of copies of the same bone segment that will be suitable for comparative studies of different load situations.As an example, quantitative data expressing both surface and internal tension trends in the proximal third of a normal human femur are given.It is well known that a bone exposed to mechanical stress becomes elastically deformed and develops corresponding internal tensions which, influencing its biodynamics, improve the bone structure, making it mechanically better suited for the stresses applied. The internal structure of bones thus is in direct relation to the mechanical stresses exerted on them.The correlation between mechanical requirements and bone structure, with particular reference to the arrangement and orientation of cancellous bone trabecules, has been known since early in the last century. Ward (1838) was the first to provide detailed description of cancellous bone in the femoral neck, by identifying the three main trabecular systems; furthermore, comparing the construction of femur epiphysis to that of a crane, he identified a zone subjected to compression along the medial cortical bone and a zone of tension along the lateral cortical bone. These results were later confirmed by Wyman (1857). Humphry (1858) observed that in the femoral neck the main trabecular systems cross each other at right angles, and that the trabeculae are arranged perpendicularly in relation to the articular surface of the femur head. In 1867 von Meyer published, according to data of Culmann (1866), his theory regarding cancellous bone architecture, with a definition of trabecular trajectories arranged along the principal stress lines.On the basis of Meyer's and Culmann's data, the close relationship between the function and architecture of cancellous bone was demonstrated by Wolff (1892), and developed and confirmed by Roux (1893) in his theory of functional adaptation.More recently the now undisputed link between shape, structure, and mechanical function of bone has been verified in a series of studies which have used more sophisticated and accurate methods of investigation. Among these, the most reliable are: finite elements (theoretical calculation method), electric strain gauges, brittle coatings, holographic interferometry, and photoelasticity methods (all experimental methods).The finite elements method applied to bones (Zinkiewicz,

Pattern of collagen fiber orientation in the ovine calcaneal shaft and its relation to locomotor‐induced strain

“…The finite element method predicts that, in the case of long bones, the direction of principal strain will be similar on the bone surface and deep to the surface. This finding also gains support from data obtained using the photoelastic method2 of stress analysis (see Bianchi et al, 1983) 'Certain plastic materials become birefringent when loaded and observed in polarized light. By constructing and loading a plastic model of a bone, it is possible to see lines (or fringes) of differing optical intensity which reveal the trajectories of stress that occurred in the element during loading.…”

Quantitative and comparative stress analysis in human femur under two different static situations by three‐dimensional photoelasticity

“…For the femur model construction the technique develbped in the previous study (Bianchi et al, 1983) was :mployed. A full-size model of a normal human femur was obained by placing the bone in a suitable aluminum conainer int0 which was poured an elastomer formed from wo liquid components (RTV-M 533 with T 35 Wacker iardener), which, when mixed in appropriate proporions, solidify.…”