Basic ScienceSubsidence and fusion performance of a 3D-printed porous interbody cage with stress-optimized body lattice and microporous endplates - a comprehensive mechanical and biological analysis
Introduction
Interbody fusion cage subsidence has been identified as a serious complication associated with spinal fusion surgery [1], which can lead to poor fusion rates and subsequently poor patient outcomes [2], [3], [4], and even increased risk of revision [5]. Significant attention has been focused on the selection of materials and their architecture in the design of interbody cages to create a mechanical and biological environment favorable to clinical fusion outcomes. Compared to solid polyetheretherketone (PEEK) cages, solid titanium alloy (Ti-6V-4Al) cages, particularly those manufactured with widely used subtractive manufacturing and surface texturing techniques (eg, blasting, etching, etc.), are known for enhanced osseointegration performance but higher stiffness [6] and excessive radiographic artifacts [7].
Porous titanium devices, benefiting from the rapid advance of additive manufacturing technology and related computational design tools, offer an attractive solution to reduce cage stiffness to mitigate the risk of subsidence, while still maintaining the desirable osseointegration performance for spinal surgery. Several porous titanium interbody fusion cages have been introduced with varying design rationales that typically fall mainly under two domains - architecture optimization of body region and modification of surface textures [[8], [9], [10],11]. Though the evidence landscape is quickly evolving, studies on the topics of subsidence and osseointegration of porous titanium cages are generally scarce, particularly within the clinical setting of spinal fusion [12]. A systematic review (2017) that evaluated the fusion rates of titanium and PEEK cages only identified one study that included a porous titanium cage design [13]. More recently, several biomechanical studies utilizing computer simulation or physical testing have shown promising results to support the claimed advantages associated with the use of porous titanium cages [6,14,15]. However, these studies are often limited by the nature of the methods employed. For example, computer simulations based on finite element analysis often have to substitute solid materials with reduced elastic moduli to simulate an porous material, due to the difficulty in modeling the complex architecture of porous materials [6,[14], [15], [16]]. Biomechanical testing using surrogate polyurethane test blocks or cadaveric specimens has been widely used in the literature to assess an interbody fusion cage's resistance to subsidence [17], [18], [19]. However, large variations in the test block geometries (or specimen conditions for cadaveric studies) and testing protocols make the data comparison between studies difficult or impossible. Moreover, unlike in vivo studies using established animal models, biomechanical testing lacks the effects of dynamic biological processes, which could otherwise strengthen the conclusions. We are only aware of a recent study that comprehensively evaluated the osseointegration performance between solid PEEK, solid titanium and 3D-printed porous titanium cages [20]. However, no mechanical testing data is reported, which could provide additional insights into the biomechanics of cage subsidence.
Recently, a novel 3D-printed porous titanium cage with new porous designs including a stress-optimized body lattice that replaces a traditional solid cage body and microporous endplates in place of smooth cage endplates has been introduced. This cage has demonstrated excellent early clinical subsidence and fusion performance, according to a clinical study based on 29 patients at a maximum follow-up of 1 year [21]. However, how these two porous design features influence subsidence and osseointegration performance remains unclear. Therefore, the main objective of this study was to elucidate the relative contribution of a porous design in each of the two major domains (body and endplates) to cage stiffness and subsidence performance, using standardized testing methods. Additionally, an analysis of fusion progression via an established ovine interbody fusion model was performed to support the biomechanical testing findings and evaluate the role of these porous domains in bony healing.
Section snippets
Designing principles of the porous titanium cage
A 3D-printed porous titanium (Ti-6Al-4V) lateral interbody fusion cage (Modulus XLIF, NuVasive) with microporous endplates and stress-optimized body lattice was evaluated for mechanical testing performances. The cage body was designed by using a stress optimization process (Figs. 1 and 2). Clinically relevant loads are applied to each interbody to optimize the internal body lattice (Fig. 1A). The lattice unit cell size is specified, and the strut thickness is set as an optimization variable in
Mechanical testing for cage stiffness and subsidence performance
For the static axial compression testing, the original porous titanium cage (Modulus XLIF, NuVasive) with stress-optimized body lattice and microporous endplates showed the lowest stiffness with a value of 40.4±0.3 kN/mm (Mean±SEM), while the modified cage with solid body and smooth endplate showed the highest cage stiffness of 58.4±0.2 kN/mm (Fig. 3). On two-way ANOVA analysis, both porous designing variables (body lattice and microporous endplates) showed significant impacts on the implant
Discussion
Subsidence and nonunion are serious complications after an interbody fusion surgery, which may warrant reoperation. Recent studies suggest that 3D-printed porous titanium cages can effectively reduce the risk of postoperative subsidence, improve the segmental stability and increase bone ingrowth profile [10,21,24,29]. However, studies investigating the underlying mechanism of these mechanical and biological benefits associated with the use of porous titanium cages are generally lacking. In this
Declarations of Competing Interests
NM, KL, GMW, JM, YP and MJ are employees of NuVasive. MHP DW TW WRM received Research grants from NuVasive.
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FDA device/drug status: Approved (Modulus XLIF).
Author disclosures: GF: Nothing to disclose. NM: Nothing to disclose. KL: Nothing to disclose. MHP: Grant: NuVasive (F, Paid directly to institution). DW: Grant: NuVasive (F, Paid directly to institution). TW: Grant: NuVasive (F, Paid directly to institution). WRW: Grant: NuVasive (F, Paid directly to institution). GMW: Nothing to disclose. JM: Nothing to disclose. YP: Nothing to disclose. MJ: Nothing to disclose.