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Research Papers

Computational Model of the Lower Leg and Foot/Ankle Complex: Application to Arch Stability

[+] Author and Article Information
Joseph M. Iaquinto

Departments of Biomedical Engineering and Orthopaedic Surgery, Orthopaedic Research Laboratory, Virginia Commonwealth University, Richmond, VA 23284

Jennifer S. Wayne1

Departments of Biomedical Engineering and Orthopaedic Surgery, Orthopaedic Research Laboratory, Virginia Commonwealth University, Richmond, VA 23284jwayne@vcu.edu

1

Corresponding author.

J Biomech Eng 132(2), 021009 (Jan 29, 2010) (6 pages) doi:10.1115/1.4000939 History: Received September 13, 2009; Revised September 27, 2009; Posted January 04, 2010; Published January 29, 2010; Online January 29, 2010

Abstract

The aim of this work was the design and evaluation of a computational model to predict the functional behavior of the lower leg and foot/ankle complex whereby joint behavior was dictated by three-dimensional articular contact, ligamentous constraints, muscle loading, and external perturbation. Three-dimensional bony anatomy was generated from stacked CT images after which ligament mimicking elements were attached and muscle/body loading added to recreate the experimental conditions of selected cadaveric studies. Comparisons of model predictions to results from two different experimental studies were performed for the function of the medial arch in weight bearing stance and the contributions of soft tissue structures to arch stability. Sensitivity simulations evaluated selected in situ strain and stiffness values for ligament tissue. The greatest contributor to arch stability was the plantar fascia, which provided 79.5% of the resistance to arch collapse, followed by the plantar ligaments (12.5%), and finally the spring ligament (8.0%). Strains measured after plantar fasciotomy increased in the remaining plantar ligament by $∼300%$ and spring ligament by $∼200%$. Sensitivity tests varying both in situ strain and stiffness across reported standard deviations showed that functional trends remained the same and true to experimental data, although absolute magnitudes changed. While not measured experimentally, the model also predicted that load can increase dramatically in the remaining plantar tissues when one of such tissues is removed. Overall, computational predictions of stability and soft tissue load sharing compared well with experimental findings. The strength of this simulation approach lies in its capacity to predict biomechanical behavior of modeled structures and to capture physical parameters of interest not measurable in experimental simulations or in vivo.

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Figures

Figure 1

Scanned and reconstructed 3D model of the bony anatomy of the foot. Model is scanned and reconstructed in a neutral anatomical orientation. Added to the model after reconstruction are the plantar ground plate and the proximal load fixture (A) through which the compressive load was applied. Inferior-superior view of the foot with all ligaments (B).

Figure 2

Arch height changes in a loaded intact specimen (upper left), with suppression of the spring ligament (upper right), plantar fascia (lower left), and all three plantar structures (lower right). Note measurement of midtalar neck to ground (superimposed over darker intact distance) and orientation of calcaneus during successively weaker arch simulations.

Figure 3

Comparison between computational model and experimental cadaver determination of relative contribution of each of the three plantar structures to arch stiffness. This was calculated as the ratio of that deficient state’s displacement to the total displacement created in the absence of all three plantar structures (14).

Figure 4

Strain response of the spring ligament and plantar ligament in the intact state and after plantar fascia was removed under 920 N axial compressive loading. In both the computational model and experimental study, the increase in strain in the plantar ligament was double the increase seen in the spring ligament (11).

Figure 5

Load present in the three plantar structures at different simulation states. Load magnitude appears as zero when the structure is in its transected simulation state.

Figure 6

Arch height contribution of single structures (spring ligament, plantar ligament, and plantar fascia) for sensitivity tests of 2%, 4%, and 6% global in situ strain

Figure 7

Arch height contribution of single structures (spring ligament, plantar ligament, and plantar fascia) for sensitivity tests of global stiffness where stiffness values are a standard deviation below and above the average for the plantar fascia (±35%) and remaining ligaments (±43%).

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