3D Numerical Analysis of an ACL Reconstructed
M. Chizari, B. Wang
School of Engineering, University of Aberdeen, Aberdeen AB24 7QW, UK Abstract: Numerical methods applicable to the tibia bone and soft tissue biomechanics of an ACL reconstructed knee are presented in this paper. The aim is to achieve a better understanding of the mechanics of an ACL reconstructed knee. The paper describes the methodology applied in the development of an anatomically detailed three-dimensional ACL reconstructed knee model for finite element analysis from medical image data obtained from a CT scan. Density segmentation techniques are used to geometrically define the knee bone structure and the encapsulated soft tissues configuration. Linear and non-linear elastic constitutive material models are implemented to mechanically characterize the behaviour of the biological materials. Preliminary numerical results for the model qualitative evaluation are presented.
Keywords: Finite element, modelling, ACL reconstruction, 3D knee Model, CT scan 1. Introduction
Geometric complexity and non linearity of the materials of the knee make the analytical solutions of the mechanical behaviour of the knee joint difficult. The knee is the most complex joint within the human body. Proper motion of the joint relies significantly on the function of the soft tissue constituents including the four ligaments of the tibiofemoral joint. These ligaments allow primarily flexion/extension and rotation of the joint by enabling the bony constituents (femur and tibia) to translate and rotate relative to each other. In addition to the ligaments, soft cartilage in the joint space permits nearly frictionless contact between the bones. Computational modeling of the knee provides a way for better understanding the interplay between the hard and soft tissue constituents of the knee during normal and pathologic function (Bischoff et al., 2008). Additionally, properly validated models can be used in the design of knee implant systems by understanding the mechanics of the restored knee and guiding optimization of the design in order to more closely replicate the healthy knee. This paper presents a numerical method applicable to the bone and soft tissue biomechanics of the ACL reconstructed knee. The paper uses a finite element model to analyse an anatomically detailed three-dimensional ACL reconstructed knee joint using medical image data obtained from a CT scan.
A geometrical model and mesh design for the component parts (pre-processing) were created using the software package program Mimics 10 (Mimics, 2007). The data analysis (post2 2009 SIMULIA Customer Conference
processing) and numerical resolutions (solver) were performed with the Abaqus 6.7 (Abaqus, 2007).
The Mimics software is an image-processing package with 3D visualization functions that interfaces with common scanner formats. It is an interactive tool for the visualization and segmentation of CT images as well as MRI images and 3D rendering of objects. The software enables the user to control and correct the segmentation of CT-scans and MRI-scans. For instance, image artifacts coming from metal implants can be removed. It is also a generalpurpose segmentation program for gray value images. It can process any number of 2D image slices (rectangular images are allowed). The interface created to process the images provides several segmentation and visualization tools.
Abaqus is a finite element code that is fully vectorised for use on computers. It uses an interactive post-processing protocol that provides displays and output lists from restart and results files written by the code. A basic concept in the code is the division of the problem history into steps, a step being any convenient phase of the history, such as thermal transient, a creep load, a dynamic transient, etc.
In this code the time incrementation scheme is fully controlled by the stability limit of a central...