Rigidity of the cores ("nuclei") in Prestige LP and ProDisc-C prostheses guarantee initial maintenance of disc height, but high contact stress takes place at the bone-end plate interface if they are improperly placed or undersized. Anchorage designs add an additional factor that may increase propensity of subsidence, indicated by the high contact stress occurring at the end plate flanges of Prestige LP, and at midline keel fixation on the end plate of ProDisc-C. Although Bryan disc differs in these 2 concerns, it also creates much larger displacement during motion with more variation in disc height that may theoretically increase the load sharing of facet and/or uncovertebral joints compared to more rigid artificial discs.
ObjectThe goal of this study was to evaluate and compare load sharing of the facet and uncovertebral joints after total cervical disc arthroplasty using 3 different implant designs.MethodsThree-dimensional voxel finite element models were built for the C5–6 spine unit based on CT images acquired from a candidate patient for cervical disc arthroplasty. Models of facet and uncovertebral joints were added and artificial discs were placed in the intervertebral disc space. Finite element analyses were conducted under normal physiological loads for flexion, extension, and lateral bending to evaluate von Mises stresses and strain energy density (SED) levels at the joints.ResultsThe Bryan disc imposed the greatest average stress and SED levels at facet and uncovertebral joints with flexion-extension and lateral bending, while the ProDisc-C and Prestige LP discs transferred less load due to their rigid cores. However, all artificial discs showed increased loads at the joints in lateral bending, which may be attributed to direct impinging contact force.ConclusionsIn unconstrained/semiconstrained prostheses with different core rigidity, the shared loads at the joints differ, and greater flexibility may result in greater joint loads. With respect to the 3 artificial discs studied, load sharing of the Bryan disc was highest and was closest to normal load sharing with the facet and uncovertebral joints. The Prestige LP and ProDisc-C carried more load through their rigid core, resulting in decreased load transmission to the facet and uncovertebral joints.
Biodegradable cages have received increasing attention for their use in spinal procedures involving interbody fusion to resolve complications associated with the use of nondegradable cages, such as stress shielding and long-term foreign body reaction. However, the relatively weak initial material strength compared to permanent materials and subsequent reduction due to degradation may be problematic. To design a porous biodegradable interbody fusion cage for a preclinical large animal study that can withstand physiological loads while possessing sufficient interconnected porosity for bony bridging and fusion, we developed a multiscale topology optimization technique. Topology optimization at the macroscopic scale provides optimal structural layout that ensures mechanical strength, while optimally designed microstructures, which replace the macroscopic material layout, ensure maximum permeability. Optimally designed cages were fabricated using solid, freeform fabrication of poly(e-caprolactone) mixed with hydroxyapatite. Compression tests revealed that the yield strength of optimized fusion cages was two times that of typical human lumbar spine loads. Computational analysis further confirmed the mechanical integrity within the human lumbar spine, although the pore structure locally underwent higher stress than yield stress. This optimization technique may be utilized to balance the complex requirements of load-bearing, stress shielding, and interconnected porosity when using biodegradable materials for fusion cages.
The present study utilizes image-based computational methods and indirect solid freeform fabrication (SFF) technique to design and fabricate porous scaffolds, and then computationally estimates their elastic modulus and yield stress with experimental validation. 50:50 Poly (lactide-co-glycolide acid) (50:50 PLGA) porous scaffolds were designed using an image-based design technique, fabricated using indirect SFF technique, and characterized using micro-computed tomography (micro-CT) and mechanical testing. Micro-CT data was further used to non-destructively predict the scaffold elastic moduli and yield stress using a voxel-based finite element (FE) method, a technique that could find application in eventual scaffold quality control. Micro-CT data analysis confirmed that the fabricated scaffolds had controlled pore sizes, orthogonally interconnected pores and porosities which were identical to those of the designed files. Mechanical tests revealed that the compressive modulus and yield stresses were in the range of human trabecular bone. The results of FE analysis showed potential stress concentrations inside of the fabricated scaffold due to fabrication defects. Furthermore, the predicted moduli and yield stresses of the FE analysis showed strong correlations with those of the experiments. In the present study, we successfully fabricated scaffolds with designed architectures as well as predicted their mechanical properties in a nondestructive manner.
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