Original ArticleBiomechanical Analysis of Different Lumbar Interspinous Process Devices: A Finite Element Study
Introduction
Rigid fixation using a metal plate or pedicle screw system has been a widely used surgical intervention for treating lumbar spinal stenosis (LSS).1 However, rigid fixation may result in adverse changes at adjacent levels, including increased range of motion (ROM) and intradiscal pressure (IDP).2 Recently, nonfusion devices such as the interspinous process device (IPD) have been developed to mitigate adjacent segment degeneration (ASD) through preserving spinal movement and improving load transmission of the lumbar spine.3 Previous studies have indicated that the IPD is an efficient alternative to rigid fixation because it provides satisfactory clinical and functional outcomes.4 In addition, a meta-analysis including 27 clinical studies and 2241 patients also reported that compared with rigid fusion, the IPD showed the same postoperative outcome, but may lower the incidence of ASD.5
A number of IPDs, such as Coflex (Paradigm Spine, Wurmlingen, Germany), DIAM (Medtronic Sofamor Danek, Paris, France), Wallis (Abbott Laboratories, Bordeaux, France), and X-Stop (Medtronic, Inc., Sunnyvale, California, USA) have been developed to treat LSS with different biomechanical designs. Some studies have investigated the biomechanics of different IPDs. Wilke et al.6 reported that the IPDs strongly stabilized and reduced the IDP only in extension, but had little effect in other motion modes in their in vitro study. Lo et al.7 in their finite element (FE) study found that Coflex-F (addition of 2 rivets to the Coflex device) can stabilize the surgical level in all motion modes, and their results were consistent with a previous in vitro study.8 Park et al.9 researched the effects of the ligature's pretension on the spinous process fracture by FE method. Some scholars also optimized the structure of IPDs by topology optimization.10, 11 However, the influence of the different biomechanical designs of IPDs on lumbar stability, IDP, and facet joint force (FJF) has not been fully understood.
Hence, the aim of this study was to investigate the biomechanics of different IPDs using FE method. Three IPDs (Coflex-F, DIAM, and Wallis) and a pedicle screw system were selected in this study. The main variables include ROM and IDP at the surgical and adjacent levels, and FJF at adjacent levels.
Section snippets
FE Modeling of the Intact Lumbar Spine and IPDs
A 3-dimensional (3D) FE model of the L1-5 segments was developed as shown in Figure 1. The geometry of the lumbosacral spine was obtained from 0.7-mm-thick computed tomography scans of a healthy adult female (age, 36 years; height, 158 cm; weight, 52 kg). The computed tomography scan images were transformed into a 3D geometric model in Mimics (Materialise Inc., Leuven, Belgium). Then the geometric model was meshed in Hypermesh (Altair Technologies, Inc., Fremont, California, USA). The final FE
Model Validation
Under 4 pure moments, the results of ROM were compared with the previous experimental results by Renner et al.,20 as shown in Figure 3A. For the L4-5 IDP and axial displacement, the results were compared with those in the studies by Berkson et al.21 and Brinckmann et al.22 (Figures 3B and 3C). A mesh convergence test was performed based on the ROM of the intact model. The final mesh density was chosen since the difference was within 5%.
ROM
At the surgical level (L3-4), the results of the ROM were
Discussion
IPDs are designed based on the concept of restoring foraminal and disk heights and unloading the facet joints while allowing ROM to some extent.25 Encouraged by its minimally invasive characteristics, IPDs have become popular as an alternative treatment for LSS, and various products are currently available in the market. However, comparative biomechanical studies among the different systems are rare. It is necessary to evaluate how different design features of the IPDs influence lumbar
Conclusions
In this study, we used a L1-5 FE model to investigate the biomechanics of different IPDs. Each implant exhibited advantages and disadvantages. Compared with rigid fixation, the IPDs demonstrated less compensation at adjacent levels in terms of ROM, IDP, and FJF, which may lower the incidence of ASD in the long term. Among the 3 IPDs, the DIAM device exhibited the most comparable ROM, IDP, and FJF at adjacent levels compared with the intact model. However, the DIAM device cannot provide
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2021, Clinical BiomechanicsCitation Excerpt :The model included cortical bone, cancellous bone, posterior elements, intervertebral discs, and 7 types of ligaments. Detailed modeling procedures and validation results were described elsewhere (Shen et al., 2019). To simulate the ALIF procedure, with the patient in the supine position, the L3–4 segment of the lumbar spine underwent partial discectomy and total nuclectomy by the anterior approach, which included removal of the anterior longitudinal ligaments, entire nucleus pulposus, and partial annulus (Fan and Guo, 2019).
Conflict of interest statement: This study was supported by the National Key Research and Development Plan (2016YFC1102002) and the Application Demonstration Project of Shenzhen (KJYY20170405161248988).