Detailed 3D Muscle Approach for Computing Dynamic Loads on the Lumbar Spine for Implant Design

Abstract

Nowadays low back pain is beneath the population one of the most common and significant musculoskeletal problems. The causes as e.g. reduced body motion or loads out of the daily work life are versatile. The medical handling ranges form physiotherapies up to spine surgeries replacing intervertebral discs with implants. Through the fact, that more and more young people are affected and that the lifestyle of older people is changing, the requirements of the implants are steadily increasing.

For new implant designs the application of numerical simulations has a decisive role as it enables in combination with optimization algorithms a target-oriented development. While the final optimization is carried out on sub-models, the computation of adequate loads is done by applying human body models, representing the static and dynamic properties in detail. The human body model CASIMIR, presented here, was used in different investigations where a good correlation to measurements of static and dynamic quantities was found.

In order to increase the accuracy of these occupant results, CASIMIR was enhanced by a detailed buttock and thigh model, to improve the force transmission to the lumbar spine. In the first model setup the muscle tissue properties have been separated into two parts: a continuum model (3D), representing the passive tissue behaviour, and a discrete model (1D) with springs and dampers for the filamentary force transmission of muscle contraction.

In the second model setup a combination of the active and passive tissue behaviours was investigated. Thereby it was shown by a simulation with an idealized muscle that an interaction of both parts is possible, i.e. an active contraction goes along with an increased stiffness of the compressive behaviour.

The paper, presented here, describes the different parts of the model setup, the simulation and the results of both setups.