On the development of an osseo-ligamentous finite element model of the human ankle joint

Abstract

A three-dimensional finite element model of the human ankle joint has been developed to study the mechanisms of impact injury to the major bones of the foot. The model is based on anatomically realistic bone geometry obtained from medical imaging and includes the major ligaments of the ankle. Careful consideration was given to model the soft tissues of the plantar surface of the foot. The model was used to simulate axial impulsive loading applied at the plantar surface of the foot. The stability of the ankle joint was achieved in the model strictly by the intrinsic anatomical geometry and the ligamenteous structure. The time history response of input/output accelerations and forces compared reasonably well with experimental data. Results indicate that the calcaneus experiences the highest stresses followed by the tibia and talus. This is in agreement with several experimental data on calcaneal fracture in axial dynamic loading. Also, the model gives stress localization in the lateral–collateral ligaments that agrees with injury observations for that region. The significance of the model lies in its potential uses as a research tool for understanding the mechanical response of the ankle related to injury and degenerative disease.

Introduction

Lower extremity trauma is common among athletes involved in high-energy sports, and among car crash occupants. According to Thomas et al. (1995), there are more than 100,000 ankle fractures that occur each year in the United States. Full recovery from these injuries is not always possible, resulting in great disruptions in the lives of the victims. They come at a great cost to society due to temporary and permanent impairment and disability (Mackenzie, 1986). It is estimated that the medical cost of ankle fracture repair is around $10,000 on average (Miller et al., 1995). The understanding of the mechanical behaviour of the ankle joint under injurious situations can enhance the delivery of treatment and rehabilitation services. It can also play a major role in the prevention of injury.

Epidemiological, clinical and biomechanical studies have implicated axial impact to plantar surface of the foot as a main cause of lower extremity trauma States, 1986, Pattimore et al., 1991, Kallina et al., 1995. Studies conducted by Radin et al. (1973) and Simon et al. (1972) have indicated a relationship between excessive levels of axial loads generated from gait and the damage of the articular cartilage in the lower extremity. Wosk and Voloshin (1981) and Voloshin et al. (1981) studied the capability of the human shock absorber system to attenuate input dynamic loading during gait. Using accelerometers mounted to specified points at the knee, pelvis, and head, the authors determined the percentage of peak acceleration attenuation offered by each body region. They observed a 30% reduction of the peak of the impulses at the knee and 70% attenuation at the head. Voloshin and Wosk (1981) also showed that healthy subjects possess more efficient shock-absorbing capability than do subjects with joint disorders. Recently, Yoganandan et al., 1995, Yoganandan et al., 1996 evaluated the biomechanics of the human-ankle complex under axial impact. The applied dynamic axial loads to the heel of cadaveric specimens and observed fracture. Values of 15.1 kN (SD=2.7) and 10.2 kN (SD=1.5) were determined for the mean dynamic forces at the plantar surface of the foot and the proximal end of the tibia, respectively. Both of these values were of sufficient magnitude to cause ankle fracture.

Over the past 20 years, the finite element method (FE) has evolved into a well-established computational tool in biomechanics Huiskes and Chao, 1981, King, 1984. Once validated, a FE model can be used as a tool for parametric investigation of stress distribution in biological structures. Recently, three-dimensional FE models for investigating the response of lower extremity to dynamic loads have been developed and reported Beaugonin et al., 1995, Tannous et al., 1996. The rationale for this research is that a FE model of the human lower leg can facilitate the understanding of lower leg injury mechanisms and can be used in a variety of applications in the medical and safety arenas.