International Journal of CAD/CAM Vol. 9, No. 1, pp. 121~128 (2009) Development of Computer Aided 3D Model From Computed Tomography Images and its Finite Element Analysis for Lumbar Interbody Fusion with Instrumentation Ashish Deoghare* and Pramod Padole Department of Mechanical Engineering, Visvesvaraya National Institute of Technology, Nagpur, India Abstract - The purpose of this study is to clarify the mechanical behavior of human lumbar vertebrae (L3/L4) with and without fusion bone under physiological axial compression. The author has developed the program code to build the patient specific three-dimensional geometric model from the computed tomography (CT) images. The developed three-dimensional model provides the necessary information to the physicians and surgeons to visually interact with the model and if needed, plan the way of surgery in advance. The processed data of the model is versatile and compatible with the commercial computer aided design (CAD), finite element analysis (FEA) software and rapid prototyping technology. The actual physical model is manufactured using rapid prototyping technique to confirm the executable competence of the processed data from the developed program code. The patient specific model of L3/L4 vertebrae is analyzed under compressive loading condition by the FEA approach. By varying the spacer position and fusion bone with and without pedicle instrumentation, simulations were carried out to find the increasing axial stiffness so as to ensure the success of fusion technique. The finding was helpful in positioning the fusion bone graft and to predict the mechanical stress and deformation of body organ indicating the critical section. Keywords: Intervertebral disc, Threshold, Segmentation, Stiffness 1. Introduction Inter-body fusion is a major surgical treatment for inter-vertebral disc degeneration and instability. A degenerative change of the lumbar motion segment often leads to the decrease in disc height that accompanies motion segment [6]. Artificial disc prostheses have been used as one of the treatment to protect the vertebrae from the excessive changes in mechanical stress. To minimize contact stress, researchers tend to design the device to cover the entire cross-sectional area of the vertebrae so that the load can be spread over a large surface area. Although this makes mechanical sense, it renders surgical insertion more difficult and requires almost every artificial disc to be implanted via an open anterior approach with a large incision, and also entails prolonged operative time. The method preserves the mobility between the two vertebrae but due to the complexity of the structure and functions of the disc, it has proven difficult to design an artificial total disc that mimics all the mechanical properties of a natural disc while retaining the required durability [1]. Inter-vertebral fusion is a surgical technique to restore the height of the degenerated disc space by inserting a spacer and bone graft or artificial fusion element between vertebrae. The advantages of using inter- vertebral fusion include the mechanical stability and maintained the original disc height. However, clinical observations have shown that, using fusion method confines the mobility between the motion segment and the restored disc height immediately after surgery tends to return even below the preoperative level [3,11]. To avoid this effect, pedicle fixation is used. But the early failure of instrumentation fixation could result into bone graft collapse, recurrence of spinal deformity and accelerating deterioration of the neighboring disc. Hence, for the successful fusion process, the parameters need to be thoroughly analyzed are, bone mineral density (BMD), contact areas between devices / grafts and its location, the extent to which the endplates are removed, material properties and position of instrumentation and spacer, and loading conditions [23]. There are many published reports describing the cage technique and effectiveness. However, little research information is available for the biomechanical behavior of inter-body fusion coupled with inter-body spacer for the patient specific model. Most of the researchers capture the anatomical data from the cadaver specimen by using the digitizing technique or else, the information was used from the previous literature to generate the FE model [10]. Such a simplified model reduces the computational cost and time, but the results so obtained are erroneous and do not match with the actual and may contribute to failure of the fusion technique. The objective of the present study is to investigate the biomechanical *Corresponding author: Tel: +91-(0)-0712- 2801307 Fax: +91-(0) 0712-2223230 E-mail: ashish_deoghare@yahoo.co.uk