**1. Introduction**

170 Polyurethane

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Biomechanical experimentation and computer simulation have been the major tool for orthopaedic biomechanics research community for the past few decades. In validation experimentations of computer models as well as *in vitro* experimentations for joint biomechanics and implant testing, human cadaver bones have been the material of choice due to their close resemblance to the *in vivo* characteristics of bones. However, the challenges in using cadaveric bones such as availability, storage requirements, high cost and possibility of infection have made synthetic bone analogs an attractive alternative.

There are a variety of synthetic bone materials available but polyurethane foam has been used more extensively in orthopaedic experiments, especially in fracture fixation testing. The foams are produced by a polymerization reaction with a simultaneous generation of carbon dioxide by the reaction of water and isocyanate. The resultant product is a closed cell structure, which is different from the open porosity of cancellous bone. However the uniformity and consistency in their material properties make rigid polyurethane ideal for comparative testing of various medical devices and implants.

Therefore we have extensively used synthetic bones made of polyurethane foam in various orthopaedic biomechanical researches from optimization of bone graft harvester design to acetabular fractures and the stability of osteosynthesis. We identified important design parameters in developing bone graft harvester by performing orthogonal cutting experiment with polyurethane foam materials. We also validated the fracture prediction capability of our finite element (FE) model of the pelvis with a validation experiment with polyurethane foam pelvis. We also performed in vitro experimentation to compare the stability of different types of osteosynthesis in acetabular fractures and used this result again to validate our fracture fixed pelvis model. These results as well as reports from others that

© 2012 Shim et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2012 Shim et al., licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

highlight the use of polyurethane in orthopaedic biomechanical experiment will be included in this chapter.

Use of Polyurethane Foam in Orthopaedic Biomechanical Experimentation and Simulation 173

The stress-strain curve of polyurethane foam exhibits similar pattern as cancellous bone. Figure 2 shows a schematic compressive stress-strain curve for polyurethane foams which shows three regions; firstly they show linear elasticity at low stresses followed by a long plateau, truncated by a regime of densification at which the stress rises steeply (Gibson and Ashby, 1988). Linear elasticity is controlled by cell wall bending while the plateau is associated with collapse of the cells by either elastic buckling or brittle crushing. When the cells have almost completely collapsed opposing cell walls touch and further strain

compresses the solid itself, giving the final region of rapidly increasing stress.

**Figure 2.** Typical compressive stress-strain curve of polyurethane foam

the stress rise steeply.

The compressive stress-strain curve of cancellous bone has the similar three regimes of behaviour (Figure 3). Firstly the small strain, linear elastic region appears which is mainly from the elastic bending of the cell walls. Then the linear-elastic region ends when the cells begin to collapse and progressive compressive collapse gives the long horizontal plateau of the stress-strain curve which continues until opposing cell walls meat and touch, causing

Such similarities have made polyurethane (PU) foams as popular testing substitutes for human cancellous bones and many researchers have quantitatively characterized material properties of polyurethane foam to investigate the suitability and usefulness of PU foams as bone analog. Szivek, Thomas and Benjamin (Szivek et al., 1993)conducted the first study on mechanical properties of PU foams with different microstructures. Compression testing was done to identify elastic modulus and compressive strength. The same group conducted further studies with three compositions of PU foams and evaluated their properties as well(Szivek et al., 1995). Thompson and co-workers(Thompson et al., 2003) analyzed compressive and shear properties of commercially available PU foams. They tested samples of four grades of rigid cellular foam materials and found out that elastic

Specifically, there will be three sections in this chapter. The first section will describe the basic material properties of rigid polyurethane foam. We will especially highlight the similarities and differences between the foam and human bone. The second section will then present the review of the literature focusing on the use of polyurethane foam in biomechanical experimentations. We will conclude the chapter with our use of polyurethane foam in bone grafting harvester design, fracture predictions and stability testing of osteosynthesis.
