COMPRESSION MODELLING OF BIOMEDICAL POLYURETHANES

M. MACHADO; P FRONTINI

Linz

Conferencia; Polymer Science and Technology (APST); 2011

Lower back pain is one of the most common medical problems, and in the 80% of cases it is associated to lumbar disc degeneration. Polyurethane based nucleus replacement devices are rapidly emerging to address specific conditions of degenerative disc disease [1]. The central core, the nucleus pulpous, is surrounded by the outer annulus and when a compressive force is applied on the disc, the nucleus transfers load to the annulus through the creation of an intradiscal pressure. In the present work, the compressive behaviour of specifically designed biomedical polyurethane elastomeric foam is investigated. The constitutive model adopted was the Ogden-Storakers model [2]. In order to describe the implant response in an elastic confinement configuration, extreme confinement cases were studied: uniaxial and confined compression. Confined tests exhibited non-linear behaviour. On the contrary non-linearity was almost imperceptible in uniaxial tests. Therefore, simultaneously fit of both stress states was necessary to obtain a representative functional form of confined state. Consequently, an inverse program was developed in Matlab. By successive simulations on Abaqus, the program adjusts the model parameters using a trust-region algorithm. Model calibration was performed in three different ways by employing either experimental data from uniaxial compression test, experimental data from confined compression test and experimental data from both tests. The relevance of each of these three methods is discussed. The inverse modelling from experimental data using a sole test configuration was not satisfactory. Only when both test configurations data were consider at the same time, the inverse program achieved a parameters set capable of capture the actual material mechanical response with an acceptable error for both stress states. Finally, once the constitutive model was calibrated, a simple FEM model of an intervertebral disc was constructed [3]. It was concluded that the implant presence restores the Von Mises values and partially recovers the radial displacements corresponding to an intact annulus fibrous. Since the similar mechanical response observed, the nucleus pulpous replacement by a polyurethane foam implant would be reliable.