Commentary on “Three-Dimensional-Printed Titanium Versus Polyetheretherketone Cages for Lumbar Interbody Fusion: A Systematic Review of Comparative <i>In Vitro</i>, Animal, and Human Studies”

Yanting Liu; Siravich Suvithayasiri; Jin-Sung Kim

doi:10.14245/ns.2346488.244

Lumbar interbody fusion (LIF) surgery is a valuable approach to treating conditions such as degenerative disc disease, spinal instability, and various deformities. This surgical intervention employs multiple techniques, including anterior LIF (ALIF), posterior LIF (PLIF), transforaminal LIF (TLIF), oblique LIF, and lateral LIF, each tailored to address specific spinal issues [1-3]. Central to these techniques is the utilization of interbody devices, which are crafted from different materials designed to offer stability and encourage bone fusion. Among them, titanium (Ti) alloy and polyetheretherketone (PEEK) have been the dominant materials in the market since the late 1990s [4].

Ti alloy and PEEK are the 2 dominant materials used in interbody devices [5]. Ti alloys offer superior strength, biocompatibility, and osseointegration capabilities, making them ideal for promoting spinal fusion and ensuring long-term stability. On the other hand, PEEK offers radiolucency, a modulus of elasticity closer to that of human bone, and reduced stress shielding, which contributes to better patient outcomes and minimizes complications. However, there are drawbacks associated with each material as well. Ti alloys may lead to stress shielding effects. It has an elastic modulus far exceeding that of cancellous bone, which might increase the risk of implant subsidence. Moreover, it can pose challenges with imaging due to artifacts. On the other hand, although PEEK has excellent radiolucency, it is chemically inert. It demonstrates poor osseointegration due to its proclivity to form biofilms, potentially affecting the fusion process and long-term stability. These issues have led to ongoing research and the development of new materials and technologies.

Ti-coated PEEK implants represent a cutting-edge approach to combine the advantages of both Ti and PEEK materials, aiming to strike an optimal balance between their respective strengths [6]. These hybrid implants seek to harness the biocompatibility and favorable mechanical properties of PEEK, along with the enhanced osseointegration potential of Ti. However, despite their innovative design, Ti-coated PEEK implants do not appear to significantly reduce subsidence risk compared to traditional Ti implants. Additionally, they carry the risk of surface coating delamination during the implantation process, which could compromise their effectiveness and lead to potential complications. Another promising area of research concentrates on developing composite materials that incorporate the benefits of multiple components [7]. One such example is the investigation of a composite material composed of PEEK and hydroxyapatite, a naturally occurring mineral found in bone tissue. By combining these 2 materials, researchers hope to improve osseointegration—the direct structural and functional connection between living bone and the implant—while maintaining the desirable mechanical properties associated with PEEK, such as lower stiffness and better load distribution. This composite material could potentially offer better outcomes in spinal fusion surgery by enhancing implant-bone integration and reducing complications related to stress shielding and subsidence.

Recently, 3-dimensional-printed Ti (3D-pTi) interbody devices have gained attention for their porous structure [5]. In this review [8], 2 retrospective human cohort studies with 299 human subjects showed that 3D-pTi devices demonstrated reduced subsidence rates and minimal device-related reoperation risk compared to PEEK devices. In 3 prospective animal model studies, 3D-pTi devices displayed better osseointegration, bone/cartilage content, and bone apposition ratios. Two nonclinical laboratory examination studies revealed that 3D-pTi devices had increased load distribution and better cell proliferation and differentiation than PEEK implants. Overall, the reviewed studies favored 3D-pTi over PEEK devices.

As described in the review, 3D-pTi interbody devices not only promotes bone growth (osteoinduction) but also provide mechanical properties closely resembling native bone [9]. This porous architecture is designed to facilitate osseointegration and reduce stress shielding, potentially enhancing long-term outcomes. Moreover, 3D-printing technology enables the customization of implant shapes and sizes to fit individual patients’ anatomies better. These devices are engineered to mimic trabecular bone with highly porous surfaces that encourage bony ingrowth and lower the elastic modulus to closely emulate cancellous bone [10]. Furthermore, the 3D-pTi implants also exhibit greater radiolucency compared to conventional Ti implants. Thus, this could potentially enable a more accurate assessment of the bony union status. However, 3D-printed cages can differ significantly due to variations in the porosity of the printed Ti. While highly porous devices allow for greater load sharing, porosities above 70% may compromise structural integrity. High porosity increases the load-sharing capacity of 3D-pTi implants until it nearly approximates that of PEEK; however, below a critical threshold in the implant mass-to-volume ratio, intrapore struts become thinner and more vulnerable to degradation and buckling. This is due to the elastic modulus of 3D-pTi cages, which directly correlates with the degree of porosity. One study demonstrated that reducing the elastic modulus (E) using porous 3D modeling yielded a modulus similar to human bone.

From that point, another potential is that the 3D-pTi may also offer benefits in terms of reducing the risk of implant migration. The porous structure of these devices forms a rough surface, which promotes friction between the implant and adjacent endplates. This increased friction creates a more secure connection, potentially lowering the risk of migration. Implant migration is a concern in spinal fusion surgery, as it can lead to complications, such as nonunion, subsidence, or nerve impingement, which may require revision surgery. Therefore, reducing the risk of migration is essential for better patient outcomes.

Theoretically, modern 3D-printing technologies offer the potential to customize the elastic modulus of implants to match a patient’s underlying bone density, enabling optimal implant-patient compatibility. However, the cost-effectiveness of such a solution may currently be impractical. Additionally, potential challenges, including the high cost of 3D-printing technology and potential difficulties in maintaining manufacturing consistency, need to be addressed and overcome to fully realize the benefits of these promising 3D-pTi interbody devices.

3D-pTi interbody devices boast potential benefits beyond their osseointegration capabilities as they enhance radiographic properties. Radiolucency is a critical attribute for spinal implants, as it enables a more accurate assessment of fusion progress during follow-up appointments. Traditional Ti intrabodies, due to their high radiopacity, can hinder the radiographic evaluation of fusion by obscuring the visualization of the fusion mass. Devices with higher porosity levels generally exhibit greater radiolucency, making them even more advantageous for monitoring the fusion process and ensuring successful patient outcomes. However, it is essential to acknowledge that the degree of radiolucency may vary based on the specific design and porosity of the 3D-pTi implant.

The current study acknowledges a number of limitations that should be taken into consideration. First, there is a limited number of studies directly comparing 3D-pTi and PEEK interbody devices for LIF, with most being animal or nonclinical studies. This restricts the ability to draw broad conclusions, although preliminary evidence supports 3D-pTi use over PEEK implants. The review does not compare variations like ALIF, TLIF, and PLIF devices, which may have clinical applications. Further investigation is needed, though previous reviews suggest device choice depends on anatomy rather than material. The studies used different metrics for device characterization and efficacy, making comparisons challenging. Therefore, future research should develop standardized comparison tools for this. Additionally, demographic and surgery-related factors were not compared across the studies. Different 3D-pTi implants were used in each study, with varying porosity and structure, which may impact clinical effectiveness. None of the included studies investigated the influence of unique lattice structures on device efficacy or had more than 1-year follow-up. Fusion rates at later time points might not show a difference between 3D-pTi and PEEK implants. The studies reviewed used 3D-pTi devices and commonly available PEEK materials, but advancements in PEEK technology have since occurred. Future efforts should compare 3D-pTi technology to these modern, modified PEEK materials. Finally, as previously described, the cost-effectiveness analysis and the potential impact of osteobiologics elimination on successful fusion surgery are also warranted.

In conclusion, 3D-printed Ti interbody devices represent a promising advancement in spinal fusion surgery. These implants offer several advantages over conventional Ti and PEEK devices, including improved osseointegration, reduced risk of implant subsidence, and better radiographic assessment of fusion. However, more research is needed to fully understand the long-term clinical outcomes and potential complications associated with these devices. Large, randomized controlled trials with extended follow-up periods are recommended to determine the true efficacy of 3D-pTi implants in spinal fusion surgery and to establish their role firmly in the treatment of degenerative spinal conditions.