Finite element analysis of the lumbar spine with a new cage using a topology optimization method

Abstract

In recent years, degenerative spinal instability has been effectively treated with a cage. However, little attention is focused on the design concept of the cage. The purpose of this study was to develop a new cage and evaluate its biomechanical function using a finite element method (FEM).

This study employed topology optimization to design a new cage and analyze stress distribution of the lumbar spine from L1 to L3 with a new cage by using the commercial software ANSYS 6.0. A total of three finite element models, namely the intact lumbar spine, the spine with double RF cages, and with double new cages, were established. The loading conditions were that 10 N m flexion, extension, lateral bending, and torsion, respectively, were imposed on the superior surface of the L1 vertebral body. The bottom of the L3 vertebral body was constrained completely.

The FEM estimated that the new cage not only could be reduced to 36% of the volume of the present RF cage but was also similar in biomechanical performance such as range of motion, stress of adjacent disc, and lower subsidence to the RF cage. The advantage of the new cage was that the increased space allowed more bone graft to be placed and the cage saved material. The disadvantage was that stress of the new cage was greater than that of the RF cage.

Introduction

Posterior lumbar interbody fusion is an effective technique for treating degenerative spinal instability, and the final goal of the procedure is to restore disc height, enlarge the stenotic foramen, and support the anterior spinal column. The procedure often obtains the bone grafts from the iliac crests. However, this is associated with donor site morbidity, postoperative discomfort, and infection. Therefore, Bagby [1] developed the lumbar interbody fusion cage during the 1980s.

The spinal interbody fusion cage is a small, porous, hollow implant, either cylindrical or nearly cuboid in shape. It can replace the degenerative disc and distract the intervertebral body, thus restoring physiological disc height. The bone grafts can be inserted into the hollow and porous cage allowing the growth of bone through the cage, resulting in bony fusion. Furthermore, it can increase the mechanical strength and fusion rate.

Since 1994, more than 80,000 lumbar interbody fusion cages have been implanted for the treatment of degenerative discs [2], and the excellent fusion rates have been reported in some clinical experimental data [3], [4], [5], [6]. Although the initial clinical reports were positive and the cage is used more widely, severe complications and poor outcomes such as subsidence, dislodgement, or adjacent disc degeneration can happen when using implants [2], [7], [8], [9].

Currently, many kinds of spinal cage designs are available (BAK, Sulzer-Spinetech, Minneapolis, Minn. Ray Cage, Surgical Dynamics, Norwalk Conn.; Brantigan I/F, Depuy-Acromed Corp., Cleveland, OH; Contact Fusion Cage, Stratec, Oberdorf, Switzerland; Harms mesh cage, Depuy-Acromed Corp., Cleveland, OH; SynCage, Mathys Medical Ltd., Bettlach, Switzerland; and others) and widely utilized, but little scientific or technical literature has reported on their design concepts.

The finite element model (FEM) has the advantage of easily modifying cage geometry without the need for cadaveric or animal specimens. Therefore, the finite element method has been used widely for analyzing biomechanical problems and has been successfully used in many other studies on the lumbar spine [10], [11], [12], [13], [14], [15], [16]. On the other hand, topological optimization is a form of shape optimization aimed at finding the best use of material for a body. The best use of material, in the case of topological optimization, represents the “maximum-stiffness” design. Consequently, the study took advantage of the topological optimization in the finite element analysis to design a new cage and evaluate its biomechanical behavior.

The main clinical parameters that were considered were the range of motion (ROM), the maximum subsidence of the cage, the maximum dislodgement of the cage, and the stress on the adjacent disc.