Section on innovative spine research and novel technologies: fusion cage design, materials, and coatings: Science versus hype

Abstract

Background

Interbody fusion has been performed for many decades with evolving strategies and approaches to improve safety, fusion rates, and clinical outcomes. There have been ongoing advancements in the understanding of spinal biomechanics driving improved implant designs. The advent of 3D printing has allowed new concepts of cage designs to become a reality. The purpose of this narrative review is to provide an overview of current and developing technologies in the area of fusion cage design as well as concerns of the impact of marketing versus science in this area.

Methods

A literature search was conducted on PubMed and OVID to identify articles related to interbody fusion cage design. The focus of the review was on current and future interbody fusion cage design.

Results

The area of fusion cage design has been rapidly growing. The greatest advancements have been in the area of additive manufacturing which has allowed for much more intricate implant design. Significant advancements have also been made in improving the surfaces of implants to promote osseointegration. Advancements in materials, manufacturing, and sensor technologies are paving the way for developing intelligent interbody fusion cages capable of monitoring fusion progression and potentially stimulating bone growth.

Conclusion

There have been many exciting developments in the area of fusion cages, primarily related to materials, design, and surface topology, much of which have been driven by the advent of 3D printing. While many of these technologies may hold promise, there is relatively little data for the particular purported benefits of these implants. Much work is needed to understand the mechanobiological function of these devices, their clinical outcomes, indications, and cost-effectiveness.

Introduction

Interbody fusion is a commonly performed procedure to treat a wide variety of spinal problems. Through the decades, it has progressed from using only allograft or autograft placed into the evacuated disc space, to incorporating a vast array of fusion cages combined with many options for graft materials. Striving for improved fusion constructs was driven primarily to address pseudoarthrosis and related problems. The 1980′s and 1990′s saw the beginning of clinical trials evaluating interbody fusion cages. These were primarily hollow threaded dowels made of titanium alloy and perforated to allow bone to grow into graft on the inside of the cages. This design imparted strong structural support but allowed motion in flexion/extension. At the same time, rectangular carbon fiber fusion cages were being used.

Advancements in biomechanical knowledge, with a greater understanding of cellular proliferation, mechanotransduction and cellular responses, materials science, additive manufacturing, and miniaturized electronics have significantly expanded possibilities for optimizing fusion cage design in recent years, a trend expected to continue. The purpose of this article is to provide an overview of current and developing technologies in the area of fusion cage design, as well as concerns of the influence of marketing versus scientific evidence in this field.

Materials

Basic properties of any medical implant material focus on safety and include durability and biocompatibility. Any material used in fusion cages should be bioinert within bodily tissues and minimize adverse reactions within the host. The material should be structural sound to withstand loads imparted onto it without fracturing and encourage bone apposition. Transitioning from traditional metallic materials to innovative synthetic and composite materials reflects a deeper understanding of biomechanics and materials science, enabling development of implants that more closely match the anatomical and pathological conditions of individual patients [1].

Currently, the most commonly used materials are titanium, including titanium alloys, and polyetheretherketone (PEEK), and to a lesser extent, silicon nitride, and tantalum. Titanium alloy emerged as an early favorite for interbody fusion cages. It resists corrosion, is strong enough to bear loads encountered in the interbody space, and is osteoconductive. The disadvantages of titanium included having a high elastic modulus, making it more likely to produce stress shielding, introducing a greater risk of subsidence and pseudoarthrosis.

Early titanium cages tended to be block-like devices machined through subtractive manufacturing methods that were very stiff with increased risk of subsidence. The advent of additive manufacturing (ie, 3D printing) allowed designs with open architectures, porous structures, and surface topographies that improved cellular adhesion and osseointegration.

There was much excitement with the introduction of PEEK fusion cages in the 1990s. The primary benefit was the modulus of elasticity of PEEK closely resembled that of bone [2,3]. This property should reduce problems of stress shielding and subsidence. PEEK also has the favorable characteristic of radiolucency. For fusion cages, this allows monitoring of bony incorporation of and produces no artifact on future MRI or CT. The disadvantage of PEEK is its hydrophobic property. This leads to the lack of bony attachment, with fibrous tissue formation around the implant, compromising the ability to attain a solid fusion [2,4].

Silicon nitride, a ceramic biomaterial, is used in fusion cages, though to a much lesser extent than PEEK or titanium. The primary potential advantage of silicon nitride is that it has been reported to have reduced bacterial colonization and biofilm formation compared with PEEK or titanium [5,6]. In a rat model, it was also found to be associated with greater levels of new bone formation compared to these materials [6]. Similar to PEEK, silicon nitride is also radiolucent.

Tantalum has not frequently been used in fusion cages, but the material possesses favorable properties such as resistance to corrosion and biocompatibility. Porous tantalum has been reported to have structural and mechanical properties similar to those of human bone, allowing trabecular bone to grow into the pores of a cage implant to achieve better osseointegration [7]. However, it is very expensive and has generally been replaced by titanium for use in spine surgery.

Fusion cage design

Cage design elements

There are several factors to consider in fusion cage design. Ensuring the safety of new devices and materials is paramount. Cages must be structurally robust to withstand postoperative forces, prevent subsidence and migration, restore disc space height and lordosis, and promote rapid bone incorporation to achieve solid fusion. Scott-Young et al [8]. emphasized the importance of considering mechanobiology in the design and material selection of fusion cages to optimally stimulate bone formation. They noted the importance of understanding cellular response to mechanical signaling and how these concepts need to be incorporated into cage design.

Early fusion cage designs were limited by manufacturing and machining capabilities and essentially were static spacers used to distract the disc space and hold bone graft. These were generally rectangular or cylindrical shapes with openings to allow bone to grow from the vertebral bodies into the graft inside the cage.

Development of additive manufacturing allowed fusion cages of much more intricate designs incorporating mechanobiologic principles. This transformed cages from structurally supportive spacers into active participants in the fusion process. These cages often feature lattice structures and porous architectures, intended to mimic natural bone microenvironment, reducing the risk of stress shielding while promoting a more favorable biological response [9,10].

Open architectures and roughened titanium surfaces enhance interactions at the implant-bone interface. This also allows early osseointegration over a greater surface area with ingrowth into the implant from multiple planes of entry. Open lattice designs can be fabricated to provide greater surface area for bone ingrowth for attachment throughout the device. This results in better distribution of axial stresses across the fusion site with reduced stress transfer to vertebral endplates, allowing for some of the sections to be in compression, while others are in tension.

One of the first 3D printed interbody fusion cages was comprised of truss structures forming an open network of titanium struts with roughened surfaces (Fig. 1). This design was found to transfer stresses along the struts and within the implant [11]. This created a biomechanical environment that provided microstrains at the bone graft interfaces deep within the cage, as well as along the surfaces, which based on Wolff’s Law, can be optimized at the bone interface to potentially produce earlier arthrodesis. This was confirmed in an ovine study comparing these cages to PEEK cages. Titanium cages with the truss design exhibited better remodeled bone across the fusion site versus PEEK cages. This study provided initial evidence suggesting that microstrain as a result of axial loading from the spine, is transferred within the implant to the bone graft interface. It is very important to note that not all 3D printed lattice cages will perform the same way. Specifically, it was the unique structural design of this device that provided loading of the bone graft material within and throughout the interbody cage [12]. The loading, stress distribution, and biological response of each specific cage design must be evaluated separately to determine its performance.

Fig. 1.

3D printed titanium cage incorporating porous truss design with hierarchical surface roughness (figure courtesy of 4WEB Medical with permission).

A biomechanical study compared titanium versus PEEK solid cages to porous 3D printed titanium cages for lateral interbody fusion [13]. The results suggested that the porous lattice titanium cage with microporous endplates may reduce subsidence risk and promote fusion, as it was the only device demonstrating boney ingrowth at12 weeks. It also demonstrated the greatest degree of osseointegration.

Another strategy for 3D printed lattice cages was to mimic the porosity of natural bone [14,15]. The rationale is that using a more porous cage to match compromised bone quality may reduce subsidence risk. Cages have been produced to match low, mid, and high density bone mineral density scores.

The next step in 3D printed cages was the advent of custom cages (Fig. 2) made for individual patients, with the goals of fitting a cage to each operated disc level with respect to size, endplate shape, and desired lordosis. Pear-shaped vertebral endplates have been reported to be associated with a significantly higher rate of cage related adverse events [16], particularly posterior retropulsion [17]. Such endplates shapes are not likely well-fitted using noncustom cages.

Fig. 2.

3D printed titanium patient specific implant designed from the patient’s CT scan (figure courtesy of Degen Medical with permission).

A different concept for a cervical fusion cage has been described as dynamic “S-shaped” or “Z-shape” cages made of titanium or PEEK [18,19]. The concept was to reduce the mismatch of the modulus of elasticity between the cage and vertebral bodies to reduce subsidence risk by allowing compression of the cage while boney incorporation with the graft occurred.

Enhancing fusion and reducing adverse events

One design goal is to maximize the total contact area between vertebral bodies and the surface of the fusion cage to maximize osseointegration and decrease risk of greater or focal stresses imposed on the vertebral endplate, in particular, at the vertebral endplate centrum. This is addressed through the shape of the device surfaces including a larger footprint that captures or is in closer proximity to the cortical margin of vertebral body endplates. This strategy distributes force over a greater contact area, lowering stress across the endplates and decreasing subsidence risk. A systematic review analyzing endplate geometry with respect to interbody fusion failures found decreased contact area between a fusion cage and the endplates was related to adverse events after interbody fusion [16].

Using finite element analysis, the relationship between elastic modulus of fusion cages incorporated with parameters such as endplate thickness, bone quality, and pedicle screw fixation was investigated to assess how differences in these parameters could alter endplate stress profiles possibly related to increased risk of implant subsidence or pistoning through the vertebral endplates [20]. Endplate stress was greater in osteoporotic bone. While this stress could be lessened by reducing the elastic modulus of the interbody cage; this produced a resultant increase in stress on posterior fixation.

Restoring disc space height and lordosis

In the treatment of symptomatic disc degeneration, the disc may be narrowed or collapsed. One of the goals of interbody fusion is to restore the height of the disc space. Along with height restoration, increased segmental lordosis may also be needed. Many manufacturers offer cages of various angulations to provide lordosis at the operated level.

Surgical approach considerations

The number of approaches for interbody fusion complicates fusion cage design. The anterior approach allows placement for one, large lordotic cage that covers a large proportion of the vertebral body endplates. This cage design is different from that required for TLIF or PLIF, where cage size is significantly limited by the anatomy of the approach to protect neural tissue and other shapes or multiple cages are used. Lateral approach interbody fusion allows using larger cages than the posterior approach, but the cages are smaller used than for ALIF, and do not offer as much lordosis. With this in mind, while the essentials of cage design are the same for all approaches to the interbody space, cages much be adapted for each specific technique and serve different anatomical needs. Consideration of the biomechanical environment and the implant and bone interface are vital to achieving successful fusion.

Expandable cages

LLIF, TLIF, and PLIF may have benefits over traditional ALIF including, primarily decreased risk of injury to major vascular structures and an access surgeon is not needed. There are disadvantages as well, including risk of direct neural injury and the limitation of having to use smaller cages. Cage size and shape are limited by the surgical approach and the patient’s anatomy. This is particularly important in patients with significant disc space collapse and/or need for an oblique cage to improve segmental lordosis.

A wide variety of expandable cages were introduced to address these shortcomings of static cages. Expandable cages are inserted through a relatively narrow channel, then mechanically manipulated to expand within the disc space to increase the height, and with some devices, increase lordosis. A systematic review found that expandable cages made of titanium were related to significantly greater improvements in patient-reported outcomes compared with PEEK cages or PEEK/titanium combinations [21]. Unfortunately, the authors were unable to perform any meaningful analysis of subsidence rates due to the differences in how this was recorded between studies.

The FDA Manufacturer and User Facility Device Experience (MAUDE) database can be used to identify the types of complications reported for specific implants. However, it cannot be used to determine the complication rate (as there is no denominator for the number of implants). Among the 821 expandable lumbar cage complication reports, the most common complications were cage breakage during insertion (25.7% of reports), postoperative migration without collapse (16.0%), postoperative collapse (15.6%), and inserter breakage (11.1%) [22].

A biomechanical study found that a cage designed with independently adjustable anterior and posterior heights significantly reduced pressure distribution at the implant vertebral endplate interface, thereby reducing stresses along the edges of the implant [23].

One study found that expandable cages were related to increased disc space height at 2 weeks and 6 months after surgery compared with static cages [24]. However, these differences no longer existed at 12-month follow-up. There was no statistically significant differences between cages with respect to lordotic measurements at any follow-up point. Subsidence was significantly more common with expandable cages (14.1% vs. 6.6%).

Though expandable cages are generally thought of for lateral and posterior approaches to the spine, there have also been expandable cages intended for anterior approach surgery. The first modular cage was constructed in the operating room and used struts of select sizes to create the desired height and obliquity of the cage that was then implanted as a single unit [25]. Another modular cage has been introduced more recently designed with the concept to protect the vertebral body endplates by inserting relatively flat device endplates between the vertebral bodies (Fig. 3). A core of the desired height and lordosis, with a cavity for graft material, is then positioned between the endplates [[26], [27], [28]]. Protecting endplates may reduce subsidence and maintain lordosis at the operated segment.

Fig. 3.

A modular cage in which device endplates are implanted, then a core of desired lordosis is placed between the endplates (figure courtesy of Axis Spine Technologies with permission).

Stand-alone cages

Combining interbody fusion with pedicle screws and rods came into favor to provide stability and reduce implant migration risk. However, pedicle screws increase costs, operative time, blood loss, complication risks, and tissue morbidity. With enhancements in fusion cage designs, routine use of supplemental pedicle may fall into question. A biomechanical study found that an expandable lateral cage with integrated lateral fixation and using longer screws provided comparable stability as the same device supplemented with posterior pedicle screws and rods [29].

A meta-analysis comparing interbody procedures of stand-alone ALIF, TLIF, and PLIF found ALIF produced outcomes similar or superior to the posterior approaches and had a similar adverse event profile [30]. This could most likely be attributed to the location of the integrated fixation accompanying the stand-alone interbody fusion device being closer to the center of rotation than that of a pedicle screw or alternative posterior column fixation system. Results of a clinical study investigating stand-alone ALIF, found significant improvement in back pain, leg pain, and ODI scores with no cases of cage failure or vertebral body fracture [31]. The authors concluded that stand-alone ALIF is a viable procedure for the treatment of symptomatic disc degeneration who do not have specific indications for supplemental posterior instrumentation. Studies such as this suggest that with innovations in interbody fusion cage design, the need for routine supplemental posterior fixation may be questioned and used only when there is a specific indication for its need.

New design concepts

Combining computer design, 3D printing capabilities, and finite modelling for analyses, has led to new design concepts. One such study describes variation from lattice designs for porous 3D printing to twist-based metamaterial cages [32]. While any such new design must undergo testing of the design concepts, works such as this demonstrates that cage design is still producing new ideas for improving fusion rates while reducing risks of problems such as pseudarthrosis and subsidence.

“Smart” fusion cages

The concept of using cages with electronic components is not new. As early as 2007 load sensors, telemetry unit, and a power supply mechanism were incorporated into a vertical cage used for vertebral body replacement [33]. Per protocol, the implant was used in only 10 patients for research purposes to assess the load on the cage during activities.

Fusion cage design has not escaped the rapidly expanding field of electronic technology. “Smart” cages encapsulate electronics within the cage to monitor fusion incorporation, and in some cases, stimulate bone growth [[34], [35], [36]]. None are currently available for use in the United States, but one such device has been classified as a breakthrough technology by the FDA. With respect to monitoring fusion incorporation, the premise is that as bone begins to incorporate at the vertebral endplates into the fusion cage the load on the implant is reduced, being redistributed to the newly formed boney fusion. Current smart devices are designed with load cells, gyroscopes, or other technologies that can communicate via the cloud to the patient’s cell phone or physician’s office to monitor load and movement [37]. Some devices may also be designed to emit an electronic signal to stimulate bone growth.

Other considerations

Fusion cages, and related placement instruments, should be designed to cause minimal tissue trauma during implantation. One characteristic that may be overlooked until the need arises, is that of cage removal. Removal may be needed in cases of cage migration, infection, fracture, spinal trauma, or other reasons.

Implant surfaces and coatings

The success of spinal implants can be greatly enhanced by a favorable biologic interface between the implant and surrounding bone. Implant surfaces, be it the original device surface or a coating put on the original surface, play a crucial role in osseointegration. One of the benefits of 3D printed fusion cages has been the enhanced surface topography of implants to support osseointegration. Surface topographies, such as added roughness to an implant surface can increase surface energy, resulting in increased cellular adhesion. Furthermore, added roughness to the surface will increase the coefficient of friction and lower the potential risk of implant migration.

Implant surface/coating

One important property for lumbar fusion cages is bioactivity. The surface or coating should promote osseous attachment, cell proliferation, and differentiation at the bone-implant interface [38]. This has been a key focus of recent design improvements by enhancing surface structure and roughness.

There are several methods of preparing surfaces on fusion cages. With the goal of promoting as much bony attachment to the cage as fast as possible. Surface topology is important in this capacity as roughened and porous surfaces increase the surface area for cellular attachment to the cage. Strategies for enhancing implant surfaces include manufacturing devices with the desired surface roughness or porosity, acid etching, grit blasting, plasma coatings, and others.

One uniquely designed study compared fusion rates of titanium coated PEEK versus uncoated PEEK in PLIF patients by putting a cage of each type into each operated disc space [39]. Follow-up CT found significantly greater indications of bone attachment to the coated cages. In a randomized, multicenter study, more robust fusion of the cage with the endplates was found at 6 months with titanium coated PEEK PLIF cages compared to uncoated cages [40]. The development of porous PEEK, may reduce the need for coatings on PEEK cages as this material was found to increase osseointegration [41].

Hydroxyapatite (HA) is a Calcium phosphate (CaP) ceramic material which has mineral composition similar to the inorganic portion of natural bone. The excellent biocompatibility and osseoconductivity of HA make it a commonly used coating for spinal implants to facilitate formation of direct bone apposition and improve the implant-bone interface. The chemical and crystallographic structure of HA is similar to that of the mineral content of bone, thus making it possible to obtain a favorable biological response leading to direct integration of the implant with bone tissue. HA has been found to support bone cell attachment, proliferation, and differentiation, which should result in better osseointegration and implant stability [[42], [43], [44], [45]].

CaP coatings, including different kinds of calcium phosphates, such as tricalcium phosphate and biphasic calcium phosphate, are similar to HA in terms of biocompatibility and osseointegration. These CaP coatings have been extensively studied for their potential to induce direct bone-to-implant contact and thus to improve the implant-bone interface. The chemical and structural similarities between CaP and the mineral part of the bone seem to induce a favorable biological response, leading to bone attachment to the implant. Like HA, CaP coatings have been found to support bone cell adhesion, proliferation and differentiation, resulting in better osseointegration and implant stability [46,47].

The effect of titanium cage surface area, porosity, and surface topography on implant subsidence was investigated in a biomechanical study [48]. Reduced subsidence was significantly related to increased surface contact area as well as a nonporous surface topography. Subsidence was not related to implant porosity.

While PEEK has favorable mechanical properties for use as an interbody cage, the PEEK surface is not conducive to bone attachment. This gave rise to the concept of coating the superior and inferior surfaces of PEEK cages with titanium to enhance osseointegration. The use of titanium coatings has been documented to enhance the bone-implant interface and improve the integration of PEEK implants with bone [49,50].

Extracellular matrix coatings (ECM) are applied to mimic the application friendly environment of the implant by replicating the natural scaffold of the bone ECM. These coatings are based on the ECM components such as collagen, proteoglycans, and growth factors to provide a better microenvironment for the attachment, proliferation and differentiation of bone cells. Therefore, ECM coatings provide a more biologically relevant surface that may improve the tissue regeneration and the implant-bone interface. The ECM coating acts as scaffolding and provides signaling cues for the integration of the implant with the surrounding bone for better osseointegration and implant stability.

Tantalum is one of the most corrosion resistant and biocompatible metals with a very porous structure that can improve the ingrowth and integration of bone tissue into the implant surface. The standard porous structure of the tantalum coating provides a way for migration, proliferation and differentiation of bone cells leading to the formation of a strong and long lasting bone-implant interface [1,51].

There are some studies supporting that with the advances in fusion cage and surface design, the need for expensive bone graft materials may be reduced or potentially eliminated. A clinical study investigated the use of an acid-etched, nanosurface-modulated, titanium ALIF cage combined with allograft chips and local blood, rather than biological graft material [52]. The fusion rate was high at 12-month follow-up with significant improvement in pain and functional scores.

Another study investigated using a 3D printed tantalum cage with no graft window in an ovine model, reporting excellent osseointegration [53]. The authors further reported good clinical outcome in a small pilot series of patients using the cage without graft. Another ovine study found that intricate titanium cages without any graft material fused faster than PEEK cages with autograft [54].

One coating characteristic that has gained attention in recent years is antibacterial properties to minimize bacterial adhesion and biofilm formation on the implant surface, ultimately reducing the risk of implant associated infections. Antibacterial coatings are currently being explored as an important approach to reducing the incidence of implant-associated infections that frequently lead to the need for revision surgery. These coatings are designed to stop bacterial colonization and biofilm formation on the implant surface. Antibiotic-releasing coatings can assist in delivering the drug to the specific region, thus preventing infection and implant failure [55,56]. These coatings enable the controlled and prolonged delivery of antibiotics at the site of application, favoring use in the prophylaxis and treatment of postoperative infections.

Silver nanoparticles have a high antibacterial activity and are, therefore, a potential coating material for spinal implants [57]. The integration of silver nanoparticles into implant coatings reduces the risk of bacterial colonization of the implant surface and the formation of biofilms.

Chitosan/HA Composite Coatings: Chitosan, a natural polymer, and hydroxyapatite can give spinal implant coatings bioactivity and antibacterial features [58,59]. The chitosan-hydroxyapatite composite coatings may also help to improve osseointegration and prevent bacterial growth, which may lead to implant-related infections.

Several basic science studies have described various coatings to titanium to reduce biofilm formation, reducing infection risk [60]. One study described the concept of improving the antibacterial and osseointegration characteristics of PEEK [61]. They incorporated a combination of polydopamine coating and modifying the implant surface with manganese and silver ions. While a unique concept, clinical efficacy has yet to be investigated.

One of the sometimes overlooked properties of cage coatings is adherence to the fusion cage. Problems such as wear debris, delamination, and/or abrasion may occur with a coated device. A biomechanical study investigating the effects of impaction on cage surfaces in a PLIF model found that surface etched titanium had no signs of surface damage [62]. In contrast, titanium coated PEEK cages produced wear debris of various sizes and in 26% of implants, the coating was abraded. A similar result was found when comparing the impaction durability of smooth PEEK, porous PEEK, and plasma-sprayed titanium coated PEEK [63]. Smooth and porous PEEK showed minimal macroscopic signs of surface damage while the titanium coated PEEK had substantial coating loss.

Another biomechanical study found that titanium plasma-sprayed coated PEEK was the only coating that had abrasion levels above the FDA guidance level set for wear particles [64]. The other cages that were uncoated PEEK, nanocoated with CaP, or nanocoated with titanium were all well below the levels of spray-coated PEEK. The authors theorized that the difference may be due to the plasma spray coating being significantly thicker than the nanocoatings.

Science versus hype

There is no doubt that there have been, and will continue to be, numerous developments in fusion cage materials, manufacturing methods, and designs (including surface topography and coatings). Most come with a rationale for why a particular new design concept is superior to existing options. It is difficult to differentiate which design concepts have been driven by clinical needs, that is, designed to address shortcomings of existing fusion cage options, or which may be driven by a very competitive market.

Scott’s parabola, describing the rise and fall of enthusiasm for new techniques, should not be forgotten [65]. In this continuum, initially, something seems to be a promising idea, there are favorable reports and press releases with enthusiasm from noted surgeons, and use of the technology increases. Then there are a few random case reports noting complications, poor clinical outcomes, and/or device failures. Discussion between individual providers and/or members of industry confirm negative experiences with the device and it eventually falls into disuse or only very minimal use. Part of the process may be related to designing a device to address some of the shortcomings of current options, while replacing them with new unanticipated problems. Survey results from surgeons found approximately 68% indicated that scientific literature and conference presentations demonstrating strong clinical evidence as the primary factor in decision making toward adopting new technology [66].

In the context of fusion cage innovations, the rationale for many new designs may appear viable and sometimes designed to address a specific problem. There is generally a lack of strong clinical science to support the claims. In an age of ever-increasing demands for evidence and cost-effectiveness, these are frequently missing for many new innovations. Fusion cages are now generally approved through the 510(k) process with the Food and Drug Administration (FDA) without the rigorously executed trials required for an Investigational Device Exemption (IDE) which produce high quality safety and outcome data. However, the IDE process is very expensive and lengthy and therefore not practical for each device design.

However, considering the numerous new cages introduced in recent years, with more on the way, there is a paucity of research available assessing safety and effectiveness for many of these devices. Surgeons should employee a high level of caution when considering which cages to use.

With respect to the benefits of surface modifications and/or coatings to provide antibacterial properties for interbody fusion, there is little clinical data, particularly from studies with high levels of evidence, to support their use. While surgery-related deep infection is clearly a significant problem, trying to evaluated the benefit of any intervention is challenging. Fortunately, the infection rate is low following interbody fusion, requiring an extremely large number of patients for meaningful analysis of the impact of proposed device modifications.

Discussion

Although the authors did not conduct a formal systematic review to identify all of the literature on fusion cage design, it is apparent that there is a paucity of literature to support the clinical benefit of many of the fusion cage designs, particularly in combination with specific graft materials.

Care must be exercised with interpreting the literature, particularly reviews of fusion cages. For example, if trying to compare titanium and PEEK lumbar interbody cages, care should be taken with respect to the cage design. Early cages were often rectangular with little attention paid to the surface topography. Later publications represent more current designs incorporating intricate 3D printing and surfaces designed for osseointegration.

There will continue to be great innovations in spine surgery. However, technology can only do so much. No material or cage design can make up for poor patient selection or poor surgical technique. Some problems such as improper implant placement within the disc space, use of undersized or oversized cages, cannot be overcome by intricate implant designs or the best of materials.

There needs to be a healthy level of skepticism for particular cages in the face of the lack of high-quality evidence. There are multiple tiers of research needed in this area. First, biomechanical, basic science, and clinical studies are needed to determine if cage designs fulfill the theoretical benefits such as less subsidence, faster fusion due to favorable osseointegration properties, lower infection rate, etc. If this can be demonstrated, the next step is addressing if there is any demonstrable clinical benefit of certain cage designs evidence by safety and clinical outcomes. Cost-effectiveness also needs to be addressed. The final step of maximizing technology in spinal fusion cages should focus on matching which cage is safest, produces the most favorable clinical outcomes, and is most cost-effective within specific patient subgroups.

Authors contribution

DDO: developed initial concept for the content, participated in writing/editing; DCS: participated in writing/editing; ZB: participated in review/editing; LAF: participated in content development, writing/editing. All authors reviewed and edited the manuscript.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.