Optimum design of an interbody implant for lumbar spine fixation

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Abstract

A new minimally invasive surgical technique for lumbar spine fixation is currently in development. The procedure makes use of an interbody implant that is inserted between two vertebral bodies. The implant is packed with bone graft material that fuses the motion segment. The implant must be capable of retaining bone graft material and supporting spinal loads while fusion occurs. The different load conditions analyzed include: compression, flexion, extension, and lateral bending. The goal of this research is to obtain an optimum design of this interbody implant. Finite element-based optimization techniques are used to drive the design. The multiobjective optimization process is performed in two stages: topology optimization followed by shape optimization. As a result, the final design maximizes the volume allocated for the bone graft material and maintains von Mises stress levels in the implant below the stress limit. The finite element-based optimization software GENESIS is used in the design process.

Introduction

The spinal column consists of 24 separate bones, called vertebrae, plus the fused bones of the sacrum and the coccyx. Intervertebral discs are located between the vertebrae. An intervertebral disc is composed of a fiber-like outer lining called the annulus and a gelatin-like inner core called the nucleus. The discs serve as shock absorbers, load distributors and spacers. As the spine ages, the nucleus loses its ability to hold water, which results in decreased ability to absorb shock and a narrowing of the nerve openings.

Many disc disorders are difficult to treat through non-surgical methods. Lumbar fusion is the most common surgical procedure for alleviating pain associated with disc disorders. The objective of this technique is to stabilize the spine and, therefore, eliminate the relative motion of the vertebrae adjacent to the degenerated disc(s). Lumbar spinal fusion involves the use of bone the graft material along with medical instrumentation such as cages, hooks, plates, rods and screws. The bone graft grows in and around the instrumentation, forming a structure similar to reinforced concrete. There are several types of spinal fusion surgery options described in the literature. For a review, see [1].

The degenerative changes within the disc account for the majority of chronic lower back pain treated in spine clinics. Lower back pain is one of the most common and significant musculoskeletal problems in the world. In the US, for example, some studies have shown that 80% of Americans will experience lower back pain in their lifetime [2]. In the 1980s and 1990s, the rate of lumbar fusion surgery increased over 60% in this country [3]. Currently, it is estimated that more than 500,000 spine surgeries are performed each year in the US alone [4]. The failure rate after lumbar fusion has been reported to be as high as 37% [5], [6]. This has motivated the development of new surgical procedures.

The trend in spine surgery is the application of minimally invasive procedures. In contrast to existing procedures, minimally invasive techniques utilize tiny percutaneous incisions through which small and specialized instruments are inserted. The research presented in this paper relates to a new surgical procedure for lumbar spine fixation that is currently in development. The new minimally invasive surgical procedure involves the use of a novel interbody fusion implant. The function of this implant is to house the bone graft material and ensure the structural stability of the motion segment while the bone graft heals. The healing process can take several months. The design of this type of device is usually accomplished by trial and error using finite element analysis. These ad hoc approaches are time consuming and might not lead to an optimum design.

This work applies finite element-based structural optimization methods to design the interbody fusion implant. The goal is to obtain an optimum design that is constrained to support the mechanical stresses while maximizing the volume available for bone graft material. The optimization process is performed in two stages. The first stage seeks to minimize strain energy under mass fraction constraints using a topology optimization technique. The second stage seeks to minimize mass under stress constraints using a shape optimization technique. GENESIS, a finite element-based optimization software, is used to drive the design process.

Topology optimization is being used more often in recent studies to find preliminary, sometimes completely innovative, structural configurations that meet specific conditions, i.e. the objective function and constraints. Shape optimization is used to fine-tune the preliminary design using a more defined geometry. This work provides a short overview of these finite element-based optimization techniques.

Section snippets

Finite element-based optimization techniques

Finite element-based optimization techniques were first developed by Lucien Schmit, a UCLA Professor, in the 1960s [7]. He recognized the potential of combining optimization techniques with finite element analysis (FEA) for structural design. Today, three types of finite element-based optimization approaches are available within commercial FEA software: parameter optimization, shape optimization and topology optimization.

Parameter optimization uses physical properties as design variables. It

Design stage I: topology optimization

The first stage in the optimum design of the interbody fusion implant is to perform a topology optimization over the design domain. The goal is to obtain a first approximation of the optimum geometry, which will later be fine-tuned using shape optimization.

Design stage II: shape optimization

A spline approximation of the geometry generated by topology optimization is used as the initial design for the shape optimization stage. The grid perturbation approach drives design process of the optimum shape (see Section 2).

Discussion

The solution of a multiobjective function for topology optimization, Eq. (14), requires a complex finite element model (see Section 3.3). A simpler approach would be to obtain one topology optimization per load condition [26]. Then, these topologies would be superimposed to obtain the final topology design. In that case, the finite element models would be more simple; however, the resulting topology would not able to reproduce the required trade off between the different load conditions [27].

Conclusions

Finite element-based optimization is a technique that can be effectively used to design medical devices. In this application, an optimum geometry for a new interbody fusion implant is obtained using topology and shape optimization methods. The implant is designed to maximize the volume available for the bone graft material subject to flexion, extension and lateral bending loads. Topology optimization is used to generate candidate topologies for the implant design. The topology optimization

Acknowledgements

Support for this research has been provided by the Colombian Institute for Development of Science and Technology ‘Francisco Jose de Caldas’—COLCIENCIAS, the Fulbright Program, NSF Grant DMI01-14975, and the State of Indiana 21st Century Research and Technology Fund.

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