Abstract
Nucleus replacement technologies are a minimally invasive alternative to spinal fusion and total disc replacement that have the potential to reduce pain and restore motion for patients with degenerative disc disease. Finite element modeling can be used to determine the biomechanics associated with nucleus replacement technologies. The current study focuses on a new nucleus replacement device designed as a conforming silicone implant with an internal void. A validated finite element model of the human lumbar L3–L4 motion segment was developed and used to investigate the influence of the nucleus replacement device on spine biomechanics. In addition, the effect of device design changes on biomechanics was determined. A 3D, L3–L4 finite element model was constructed from medical imaging data. Models were created with the normal intact nucleus, the nucleus replacement device, and a solid silicone implant. Probabilistic analysis was performed on the normal model to provide quantitative validation metrics. Sensitivity analysis was performed on the silicone Shore A durometer of the device. Models were loaded under axial compression followed by flexion/extension, lateral bending, or axial rotation. Compressive displacement, endplate stresses, reaction moment, and annulus stresses were determined and compared between the different models. The novel nucleus replacement device resulted in similar compressive displacement, endplate stress, and annulus stress and slightly higher reaction moment compared with the normal nucleus. The solid implant resulted in decreased displacement, increased endplate stress, decreased annulus stress, and decreased reaction moment compared with the novel device. With increasing silicone durometer, compressive displacement decreased, endplate stress increased, reaction moment increased, and annulus stress decreased. Finite element analysis was used to show that the novel nucleus replacement device results in similar biomechanics compared with the normal intact nucleus.
Highlights
Surgical treatments for symptomatic degenerative disc disease (DDD) refractory to conservative measures include spinal fusion, discectomy, and total disc replacement, with spinal fusion being the current standard of care
Probability that the benchmark errors was not exceeded was highest for flexion and extension and lowest for right and left lateral bending, with higher probabilities indicating a greater degree of agreement between the experimental and simulated responses (Table 4)
An optimally designed nucleus replacement device would result in an intervertebral disc (IVD) that has similar stiffness to that of an intact IVD with a normal nucleus while at the same time creating similar endplate and annulus stresses
Summary
Surgical treatments for symptomatic degenerative disc disease (DDD) refractory to conservative measures include spinal fusion, discectomy, and total disc replacement, with spinal fusion being the current standard of care. Fusion fundamentally alters the biomechanics of the spine, often resulting in reduced range of motion that can lead to degenerative effects in the motion segments. Nuclear replacement devices have been developed as a more minimally invasive treatment that could potentially correct the effects of DDD without negatively altering the biomechanics of the vertebral segment. Previous nucleus replacement technologies have resulted in adverse outcomes such as extrusion through the annulus fibrosus and subsidence into the vertebral bodies (Bertagnoli and Schonmayr, 2002; Klara and Ray, 2002; Allen et al, 2004; Bono and Garfin, 2004). New nucleus replacement devices must be evaluated for the ability to restore the natural biomechanics of the spine without adverse outcomes
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