Abstract
Study design: Biomechanical study of a nucleus replacement with a finite element model. Objective: To validate a Bionate 80A ring-shaped nucleus replacement. Methods: The ANSYS lumbar spine model made from lumbar spine X-rays and magnetic resonance images obtained from cadaveric spine specimens were used. All materials were assumed homogeneous, isotropic, and linearly elastic. We studied three options: intact spine, nucleotomy, and nucleus implant. Two loading conditions were evaluated at L3-L4, L4-L5, and L5-S1 discs: a 1000 N axial compression load and this load after the addition of 8 Nm flexion moment in the sagittal plane plus 8 Nm axial rotation torque. Results: Maximum nucleus implant axial compression stresses in the range of 16–34 MPa and tensile stress in the range of 5–16 MPa, below Bionate 80A resistance were obtained. Therefore, there is little risk of permanent implant deformation or severe damage under normal loading conditions. Nucleotomy increased segment mobility, zygapophyseal joint and end plate pressures, and annulus stresses and strains. All these parameters were restored satisfactorily by nucleus replacement but never reached the intact status. In addition, annulus stresses and strains were lower with the nucleus implant than in the intact spine under axial compression and higher under complex loading conditions. Conclusions: Under normal loading conditions, there is a negligible risk of nucleus replacement, permanent deformation or severe damage. Nucleotomy increased segmental mobility, zygapophyseal joint pressures, and annulus stresses and strains. Nucleus replacement restored segmental mobility and zygapophyseal joint pressures close to the intact spine. End plate pressures were similar for the intact and nucleus implant conditions under both loading modes. Manufacturing customized nucleus implants is considered feasible, as satisfactory biomechanical performance is confirmed.
Highlights
Lumbar back pain is one of the most common diseases in modern sedentary society.[1]
We aimed to assess with a lumbar spine parametric finite element model (FEM) a new ring-shaped nucleus implant made of a polymeric material
The maximum stresses on the nucleus implant were 25 MPa for the compression stress and 10 MPa for the tensile stress. As both values were below the strength limits of the nucleus implant material, under normal loading conditions, there was no risk of permanent deformation or severe damage
Summary
Lumbar back pain is one of the most common diseases in modern sedentary society.[1] its etiology is ample,[2] degenerative disc disease and disc herniation are leading causes.[3] Surgical treatments for these entities can be divided into fusion and motion preservation.[4] Among the latter, we found total disc prosthesis[5] and nucleus replacement to be suitable.[6] The second is mainly indicated for disc herniation and early disc degeneration with a preserved annulus fibrosus,[6] while total disc prosthesis is recommended in severe disc disease.[7]. Many nucleus disc replacements have been designed in the past, with only a few reaching the market[8] and even less still in clinical use. The problems are varied and include material degradation,[9] design flaws, extrusion,[10] and subsidence[11] the search for the ideal nucleus replacement material and design continues.[12] we decided to create a new nucleus implant based on past issues and failures
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