Compression Modeling of Biomedical Polyurethanes

Abstract

Lower back pain is one of the most common medical problems, and in the 80% of cases it is associated to lumbar disc degeneration. Nucleus replacement appears as a good alternative to current surgical procedures since it is less invasive and can restore normal biomechanics. In the present work, the compressive mechanical behavior of specially designed polyurethane elastomeric foams was studied. A constitutive model capable of describing highly compressible elastomer mechanics was adopted. In order to characterize the implant response in an elastic confinement configuration, the following extreme cases were studied: uniaxial and confined compression. Previous studies revealed that the constitutive model had to predict the material response in different stress states simultaneously. In this case the problem becomes incompletely defined and is of an inverse type, so the computational implementation became mandatory. Consequently, an inverse program was developed. By successive finite element simulations, the program adjusts the model parameters using a trust-region algorithm. Finally, a simple finite element model of an intervertebral disc was constructed to simulate disc biomechanics in intact, denucleated and implanted conditions.