Porous Biodegradable Lumbar Interbody Fusion Cage Design and Fabrication Using Integrated Global-Local Topology Optimization With Laser Sintering

Abstract

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.

Ceramics and ceramic coatings in orthopaedics

“…However, with the advent of higher precision equipment, building of micron and submicron features is on the horizon [643]. Continued development of these technologies will pave the way for their use in many reconstructive or arthroplastic procedures, including trauma [644], oral [645] , cranio-maxillofacial [646], and spinal [647] surgeries, and ultimately total joint replacement [648].…”

Biomechanical investigation into the structural design of porous additive manufactured cages using numerical and experimental approaches

“…Lee et al developed porous cage and lumbar L3-L4 models to investigate the biomechanics of the bone-implant constructs [16]. Additionally, Kang et al investigated porous biodegradable fusion cages using ligamentous finite element models of mini-pig L2-L5 lumbar spine [17]. Although the bone-implant constructs developed by the previous studies could provide referable biomechanical outcomes, their numerical models may be oversimplified.…”

Static and dynamic fatigue behavior of topology designed and conventional 3D printed bioresorbable PCL cervical interbody fusion devices

“…Improved 3D printing technologies, which enable fabrication of topology optimized designs, allow for improvements in the mechanical design of scaffolds to withstand typical loads by placing material in critical load bearing paths, but leaving sufficient porosity and permeability for bone ingrowth and biologic delivery [14, 15]. Combining topology design and optimization with 3D printing technologies such as laser sintering, allows for the realization of complex topology designed cages fabricated for resorbable polymers [15, 16] However, despite the sophisticated 3D printing and optimization capabilities, the effects of these designs and resorbable polymer properties on the structural strength of the implants is not well known.…”

“…Combining topology design and optimization with 3D printing technologies such as laser sintering, allows for the realization of complex topology designed cages fabricated for resorbable polymers [15, 16] However, despite the sophisticated 3D printing and optimization capabilities, the effects of these designs and resorbable polymer properties on the structural strength of the implants is not well known. As bioresorbable cages tend to have weaker material properties than their nonresorbable metal and permanents polymer (PEEK and PEKK) counterparts, optimum designs must be utilized to maximize their mechanical and structural strength.…”

“…Thus, the 5 million cycles recommended in ASTM F2077 would go well beyond that required for successful spinal fusion. Moreover, while previous studies have measured the properties of bioresorbable materials under various static loading setups[15, 19-22], there has been limited investigation into dynamic and fatigue properties of these implants[23]. In fact, to our knowledge, no previous dynamic fatigue testing under ASTM F2077 of any 3D printed bioresorbable cages has been previously reported.…”