Improvement of Numerical Ankle/Foot Model: Modeling of Deformable Bone 973331

Since many years, the vehicle industry is interested in occupant safety. The dummy use in crash tests allowed to create protective means like the belt and the airbag that diminished the injuries of the head and the thorax, which are often lethal for the car occupant. An other objective appears now: to improve the car safety to avoid the injuries which are not fatal but which can cause disability and which cause great cost in hospitalization and rehabilitation. The lower extremity protection, in particular the one of the ankle and the foot region, has become the subject of diverse research efforts by its high percentage of injuries in car crashes. But the dummy mechanics cannot reproduce the accurate ankle and the foot kinematics during an impact loading like in vehicle crash. Therefore, ankle/foot complex numerical models are an essential tool for the car safety improvement.

In previous papers ([1], [2]), the response of a numerical ankle/foot model with rigid bones during impact loading was validated. The tests used for these validations correspond to the principal movements in car crashes: dorsiflexion, inversion and eversion. The influence of a few parameters of the biological component modeling was studied in another paper ([3]).

The present paper describes a new modeling approach of the principal bones of the ankle/foot model. The most often injured ankle/foot bones in vehicle accidents are the tarsal bones (particularly the calcaneum, the talus, the navicular and the cuboid) and the fibular and tibial malleoli. These bones are therefore modeled as deformable bodies. The cortical bone is modeled by shell elements while the trabecular bone is modeled independently by solid elements. Both meshes are connected via a tied contact interface that permits to tie arbitrarily meshed solid to shell surfaces, including finite normal gaps. Both types of elements use linear elastic materials. Nonlinear materials, including damage, have been provided for in the used material models but are not used in this study for lack of calibration data. This new deformable bone finite element modeling technique is validated for the dorsiflexion impact loading as was done previously for the model with rigid bones.

The influence of the modeling with deformable bones is studied, in particular, concerning the kinetic response. The interest of using a deformable model is, for instance, the possibility to simulate the bone and soft tissue injuries. The deformability of the bone model ultimately permits to assess the damage behavior of the main parts of the ankle/foot complex during an impact loading. The localization of the maximum stress allows to identify the regions where injury can occur.

In future work, nonlinear bone material behavior and explicit fracture and damage criteria will be applied for the bone fracture and for the ligament tear. The behavior of the ankle/foot model with rigid and some deformable bones, respectively, will be studied for other impact and static loading cases.

Equally important is the addition of the influence of the soft tissues in future models. Some first order effects, such as soft contact padding and attenuation through energy dissipation have been identified in this study and modeled via equivalent springs, as well as internal material and external relative motion damping.