Biomechanical assessment and fatigue characteristics of an articulating nucleus implant

Abstract

BackgroundExtrusion is a known complication of lumbar nucleus replacement devices. Despite this fact, this complication has not been well studied in an in vitro cadaveric model under fatigue-loading conditions.MethodsLumbar constructs (with treated and control levels) were tested in intact, postdisectomy, and postnucleus implant conditions under compression, torsion, and bending for initial biomechanical assessment. Constructs were then tested for 100(k) cycles under fatigue loading to assess extrusion risk. Potential adverse effects to vertebral and endplate fractures were assessed using gross dissection and macroscopic and micro-computed tomography evaluation techniques.ResultsBased on the initial biomechanical assessment, implantation of the nucleus device significantly increased disc height compared with the discectomy condition, and there were no significant differences between the intact and implanted conditions for range of motion or stiffness. All constructs completed the 100(k) cycles with no extrusions. There was evidence of implant shift toward the right lateral annulus on postfatigue images. Postfatigue dissection and imaging showed no evidence of macroscopic endplate or trabecular fractures.ConclusionUsing a 2-level lumbar in vitro construct, the biomechanical function of the treated level with an articulating nucleus implant was similar to intact. In vitro fatigue testing showed no implant extrusion and macroscopic changes to the bony structure or cartilaginous endplates when comparing treated and intact levels.

Canine models of spine disorders

“…These biomechanical data can then be cor- ing can be used to assess the compressive modulus, elasticity, creep, stress-relaxation, and permeability of the IVD in order to characterize its functional composition, integrity, and viscoelasticity. 53,64,[190][191][192][193][194][195]…”

“…Incremental loading tests are designed to measure material responses to forces applied in an increasing or decreasing stepwise manner 190 . For IVD testing, incremental loading can be applied in compression to a single disc or FSU or in compression, bending, and/or rotation to a spinal segment.…”

“…Compression tests are commonly performed on IVDs, FSUs, and spinal segments. As IVD compression is essentially constant due to muscle forces and gravity, and resistance to compression is a key feature of disc health, various forms of compression testing can be used to assess the compressive modulus, elasticity, creep, stress‐relaxation, and permeability of the IVD in order to characterize its functional composition, integrity, and viscoelasticity 53,64,190‐195 . In theory, compressive, bending, rotational, biaxial, and multiaxial biomechanical tests can be incremental, single or cyclic or both, and nondestructive or destructive.…”

Comparison of four in vitro test methods to assess nucleus pulposus replacement device expulsion risk

“…Lin et al 36 adapted this hula‐hoop test by rotating bovine specimens at 15° increments between +135° and −135° with respect to the annular incision and subjecting them to 1 min of cyclic loading after each rotation; a complimentary approach has also been to bend specimens by 5° and apply compression at 2 mm/min. 34 , 36 Meanwhile, Bao et al 54 and Ordway et al 55 assessed the risk of NRD expulsion by subjecting human cadaveric lumbar specimens to 100 000 offset loading cycles (2.5–7.5 Nm), while Christiani et al 56 tested porcine specimens using lateral bending, ramping an offset load to failure. The relative usefulness of one protocol over another to assess NRD expulsion risk has not been comprehensively studied, while comparisons between NRDs across studies are challenging due to the methodological differences described.…”

Nucleus Disc Replacement: Design and Material Selection FEA Analysis

“…Introduced in the 60', there has been a steady search to fnd a material that reproduces intact disc biomechanics, creating a design that is easy to insert and with a minimal risk of subsidence or extrusion. Silicones, diverse polymers, hydrogels, and polyurethanes (PU) have been tested and ruled out because of cracking [25], subsidence [26], or extrusion [27]. In addition, researchers have improved designs continuously to ease insertion.…”