Abstract
While spinal fusion using rigid rods remains the gold standard treatment modality for various lumbar degenerative conditions, its adverse effects, including accelerated adjacent segment disease (ASD), are well known. In order to better understand the performance of semirigid constructs using polyetheretherketone (PEEK) in fixation surgeries, the objective of this study was to analyze the biomechanical performance of PEEK versus Ti rods using a geometrically patient-specific poroelastic finite element (FE) analyses. Ten subject-specific preoperative models were developed, and the validity of the models was evaluated with previous studies. Furthermore, FE models of those lumbar spines were regenerated based on postoperation images for posterolateral fixation at the L4–L5 level. Biomechanical responses for instrumented and adjacent intervertebral discs (IVDs) were analyzed and compared subjected to static and cyclic loading. The preoperative model results were well comparable with previous FE studies. The PEEK construct demonstrated a slightly increased range of motion (ROM) at the instrumented level, but decreased ROM at adjacent levels, as compared with the Ti. However, no significant changes were detected during axial rotation. During cyclic loading, disc height loss, fluid loss, axial stress, and collagen fiber strain in the adjacent IVDs were higher for the Ti construct when compared with the intact and PEEK models. Increased ROM, experienced stress in AF, and fiber strain at adjacent levels were observed for the Ti rod group compared with the intact and PEEK rod group, which can indicate the risk of ASD for rigid fixation. Similar to the aforementioned pattern, disc height loss and fluid loss were significantly higher at adjacent levels in the Ti rod group after cycling loading which alter the fluid–solid interaction of the adjacent IVDs. This phenomenon debilitates the damping quality, which results in disc disability in absorbing stress. Such finding may suggest the advantage of using a semirigid fixation system to decrease the chance of ASD.
Highlights
Degenerative lumbar diseases such as the spinal stenosis, lumbar instability, degenerative spondylolisthesis, and spondylolytic spondylolisthesis can cause clinical symptoms such as the low back pain (Serhan et al, 2011)
Similar to the aforementioned pattern, disc height loss and fluid loss were significantly higher at adjacent levels in the Ti rod group after 16 h of cycling loading during daily activities, which alter the fluid–solid interaction of the adjacent intervertebral disc (IVD)
This study presents a validated geometrically patient-specific poroelastic finite element (FE) modeling technique, which has the potential to be utilized for clinical applications to analyze lumbar spine biomechanics
Summary
Degenerative lumbar diseases such as the spinal stenosis, lumbar instability, degenerative spondylolisthesis, and spondylolytic spondylolisthesis can cause clinical symptoms such as the low back pain (Serhan et al, 2011). Posterolateral fusion (PLF) and posterior lumbar interbody fusion (PLIF) techniques using rigid rods [i.e., pure titanium (Ti), Ti alloy, or cobalt-chrome (CoCr) rods] have been widely used in the treatment of degenerative lumbar disease (Schwab et al, 1995; De Iure et al, 2012; Campbell et al, 2017). The persistence of symptoms and the progression of degenerative disease were reported in some cases after PLF/PLIF, which is recognized as adjacent segment disease (ASD) (Rahm and Hall, 1996; Wang et al, 2017). Semirigid rods using polyetheretherketone (PEEK) were successfully used in fixation surgeries and good outcomes were reported (Highsmith et al, 2007). Some conflicting outcomes have been reported in the literature when comparing PEEK rods against rigid ones after spinal fixation (Ormond et al, 2016)
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