Anterior lumbar interbody fusion implants: a narrative review of current trends and future directions

Introduction

Patients with pathological lumbar spine conditions presenting with symptoms of pain and disability, documented mechanical compromise on imaging, and inadequate response to conservative treatments may be candidates for lumbar interbody fusion (LIF) to achieve decompression and improve spinal stability.

Background

LIF entails placing an implant and bone graft within the intervertebral space after discectomy and endplate preparation to stimulate bony fusion between adjacent vertebrae. It facilitates neural decompression and pain relief by restoring foraminal volume, anterior column height, vertebral alignment, and sagittal balance (1). The implant can be placed in a standalone manner, augmented with anterior plating, or paired with posterior pedicle fixation (2). LIF includes several approaches to access the lumbar disc space, allowing for discectomy, implant placement, and bony fusion with adjacent vertebrae. These techniques, posterior, anterior, lateral, or anterolateral, present unique anatomical challenges and spatial constraints. Surgeons must navigate these limitations carefully to ensure a safe and thorough discectomy and implant insertion. Consequently, implants are specifically designed to accommodate the structural demands and anatomical limitations of each approach. Anterior lumbar interbody fusion (ALIF), accesses the disc space through an anterior retroperitoneal or transperitoneal approach, offering distinct advantages over posterior approaches like posterior lumbar interbody fusion (PLIF) and transforaminal lumbar interbody fusion (TLIF). Notably, ALIF preserves posterior spinal elements, minimizing tissue disruption and avoiding manipulation of sensitive neural structures like the dural sac and nerve roots (3). Additionally, when used in circumferential fusion, ALIF has demonstrated improved fusion rates and superior restoration of segmental alignment compared to dorsal based techniques (3,4). Notably, ALIF implants possess a larger footprint compared to those utilized in posterior-based techniques and is purported as one of the reasons for its superior fusion rates.

Despite its numerous advantages, ALIF carries significant intraoperative and perioperative risks. A 2021 systematic review and meta-analysis by Feely et al., which analyzed 5,728 patients across 31 studies utilizing both retroperitoneal [open and minimally invasive (MIS)] approaches, identified several key complications, including retrograde ejaculation, vascular injury, visceral injury, postoperative ileus, and neurological compromise (5).

In the open retroperitoneal cohort (n=5,106), retrograde ejaculation was observed in 2% (n=102), vessel damage in 3.03% (n=155), and deep vein thrombosis (DVT) in 1.4% (n=71). Hematoma formation was noted in 0.6% (n=31), visceral injury in 0.37% (n=19), and hernia in 1.3% (n=66). Wound infections were reported in 1.9% (n=97), while postoperative ileus occurred in 5% (n=255). Dural tears had an incidence of 0.04% (n=2) (5).

In the open transperitoneal cohort (n=154), retrograde ejaculation was the most frequent complication, occurring in 12.2% (n=19). Vessel damage, wound infections, and dural tears each had an incidence of 0.7% (n=1), while ileus was noted in 3.7% (n=6). No cases of DVT, hematoma, visceral injury, or hernia were reported in this cohort (5).

Though these complications are certainly noteworthy, with appropriate patient selection and adherence to proper surgical technique, these complications are often mitigated.

Rationale and knowledge gap

ALIF implants are predominantly made of titanium and polyetheretherketone (PEEK), materials selected for their capacity to meet stringent implant acceptance criteria. While these materials have demonstrated favorable outcomes in traditional manufacturing, the integration of 3D printing technology offers the potential to enhance specific material properties. However, a knowledge gap remains regarding the effects of modifying these intrinsic properties through 3D printing. Understanding the impact of such changes on implant performance and functionality is essential for advancing ALIF technology.

Objective

This review examines current literature on ALIF implants, outlines the qualities of an ideal implant, and explores how 3D printing technology enhances these implants using traditional materials. By applying 3D printing advancements, the aim is to develop a new generation of ALIF implants that offer greater durability, facilitate indirect decompression, support arthrodesis, achieve alignment goals, and ultimately improve clinical outcomes. We present this article in accordance with the Narrative Review reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-24-114/rc).

Methods

A comprehensive search was conducted using electronic databases such as PubMed and Google Scholar to identify relevant articles. The search strategy included keywords and combinations such as “Anterior Lumbar Interbody Fusion (ALIF)”, “ALIF Implants”, “3D-Printed Implants”, “Medical Applications of 3D Printing”, “Off-the-Shelf (OTS) Implants”, “Patient-Specific Implants”. Publications were included if they discussed advancements in implant materials, properties, or the integration of 3D printing technology for ALIF. Articles not in English, non-peer-reviewed sources were excluded (Table 1).

The ideal implant and the ALIF footprint

A key factor influencing high rates of arthrodesis in ALIF is the implant footprint, encompassing the dimensions in the axial, coronal, and sagittal planes. A well-matched footprint ensures optimal fit, load distribution, and stability, minimizing subsidence risk (6). ALIF implants, capitalizing on the anatomical corridor afforded by the anterior approach, tend to have larger footprints (436 mm²) compared to posterior implants (185 mm²) (7). Additionally, the larger footprint of the implant enhances immediate post-operative stability by promoting uniform stress distribution across the vertebral endplate, an essential factor considering that approximately 80% of the spine’s compressive load is transmitted through the vertebral bodies (6). This stability is particularly crucial during the initial 6 weeks post-implantation, a critical period for arthrodesis and bony bridge formation, when the implant must support a substantial portion of the axial load (8). Insufficient osteointegration during this phase increases the risk of early implant failure prior to bony healing, which can result in unfavorable clinical outcomes.

The ideal implant, however, transcends just size and dimensions. It requires a multifaceted set of properties to ensure ideal biocompatibility and functionality to achieve favorable patient outcomes. Mechanical strength is vital, as the implant must support the significant physiological loads placed on the lumbar spine. Yet, this strength must not be accompanied by excessive rigidity, which could lead to detrimental complications such as bone-graft interface fractures or implant subsidence (9,10). Biocompatibility is crucial to minimize the risk of infection and adverse reactions. Osteoconductivity is another essential attribute, actively promoting bone growth and facilitating vertebral fusion. Lastly, radiolucency is important for clear visualization of the implant and surrounding structures during imaging (9,10).

Titanium

The use of titanium in ALIF implants dates back to the 1970s. However, the initial reports on bony fusion rates were inconsistent, ranging from 1% to 95%. These variations were likely attributed to disparities in surgical techniques and differing criteria for defining fusion success (9). Despite this, the biocompatibility of titanium remained largely unquestioned due to the formation of a TiO₂ layer that rendered it inert.

While titanium boasts biocompatibility and sufficient strength to handle physiological loads, its application in spinal implants posed two significant challenges. Firstly, its high elastic modulus (110 GPa) created a substantial mismatch with the lower moduli of vertebral trabecular bone (2.1 GPa) and cortical bone (2.4 GPa) (9). This mismatch results in stress shielding, where the implant’s rigidity causes force to be disproportionately distributed, bypassing the implant and reducing mechanical stimulation of the surrounding bone. This reduction in load bearing, coupled with local inflammation, contributes to bone-graft interface fractures and pseudoarthrosis (4,11).

Secondly, titanium’s inherent radiopacity poses a challenge in assessing fusion progress. Its high density makes it extremely bright on X-rays, often obscuring the surrounding bone and making it difficult to evaluate the formation of a bony bridge. While this didn’t necessarily indicate structural issues, the lack of visibility hinders accurate monitoring of fusion and identification of potential complications (12).

PEEK

To address the limitations of solid titanium implants, PEEK, a hard synthetic plastic material, emerged as a potential alternative. PEEK’s modulus of elasticity, closely approximating that of cortical bone, offered a potential advantage in promoting more uniform load sharing and stress distribution, potentially mitigating stress shielding (9). Additionally, PEEK’s radiolucency facilitated unobstructed visualization of the surrounding bone on X-rays, enabling a more concise assessment of fusion progress compared to radiopaque titanium (12).

However, while PEEK addressed certain drawbacks of titanium, its inherent bioinertness presented a distinct challenge. This characteristic hinders bone integration, as evidenced by reduced adherence and differentiation of osteoprogenitor cells on PEEK surfaces compared to titanium. Lack of cellular interaction significantly contributes to PEEK’s suboptimal osseointegration (13).

Limitations of traditional manufacturing

The pursuit of the ideal ALIF implant has led to a continuous evolution in design and materials. Titanium and PEEK, with their favorable properties, have long served as the gold standard in spinal fusion. However, limitations persist in their unaltered forms when produced using traditional subtractive manufacturing, such as stress shielding with titanium and poor osseointegration with PEEK, which have prompted further innovation. Promising innovations in design and fabrication, particularly with 3D printing, are emerging to address these limitations and further enhance implant performance in ALIF procedures. The “race between bone healing and construct failure” remains a central challenge in ALIF and spinal fusions.

ALIF implant fabrication with 3D printing

3D printing, also known as additive manufacturing, is revolutionizing the fabrication of orthopedic implants, including those for ALIF procedures. By translating digital 3D models into physical objects, this process offers unprecedented control over implant design and functionality (14).

The 3D printing journey begins with a computer-aided design (CAD) file of the implant, which is sliced into thin cross-sections that guide the 3D printer. Unlike traditional subtractive manufacturing methods, 3D printing adds material layer-by-layer, enabling the creation of intricate geometric and complex internal structures that were previously unattainable (14) (Figure 1).

Figure 1 A series of images showing the fabrication process of 3D-printed porous titanium ALIF and TLIF implants. (A) Laser sintering in progress: the image shows the SLS process, where a laser beam is selectively scanning and sintering the metal powder layer by layer to create the desired implant shape. This process occurs in a controlled environment to prevent oxidation and ensure precise melting of the material. (B) Layered structure formation: this image depicts the layered structure as it forms during the SLS process. The distinctive ridges indicate the incremental layer-by-layer addition of material, which eventually creates a solid implant. (C) Post-processing of implants: after the sintering or melting process, the implants undergo further finishing processes. Here, the image shows the removal of excess powder and smoothing of the implants to prepare them for clinical use. (D) Finalized implants ready for use: the final image shows the completed implants, with their intricate designs and porous structures optimized for osseointegration and mechanical compatibility with human bone. Original image produced using CarlsMed pre-operative planning software (CarlsMed, Inc., Carlsbad, California, USA). ALIF, anterior lumbar interbody fusion; SLS, selective laser sintering; TLIF, transforaminal lumbar interbody fusion.

This precise layer-by-layer deposition provides exceptional control over implant design at both the macro and microstructural levels. At the microscopic level, properties like porosity, stiffness, and even radiodensity can be meticulously controlled to optimize the implant’s interaction with bone. Surface modifications, such as altering texture or applying osteoinductive coatings like hydroxyapatite (HA), can further enhance bone integration and promote fusion (12).

At the macroscopic level, the implant shape can be customized to perfectly match a patient’s unique anatomy, ensuring optimal fit and function within the intervertebral space. These “patient-specific” (PS) implants are tailored to the individual using anatomical data from imaging studies like computed tomography or magnetic resonance imaging scans (12) (Figure 2).

Figure 2 Illustration of the CT-based planning incorporating the patient’s unique anatomical dimensions. Aprevo^® Intervertebral Body Fusion Device. Original image produced using CarlsMed pre-operative planning software (CarlsMed, Inc., Carlsbad, California, USA). ANG, angle; ANT, anterior; COR, correction; CT, computed tomography; LOR, lordosis; LVL, level; PI-LL, pelvic incidence-lumbar lordosis; POST, posterior.

Alternatively, “off-the-shelf” (OTS) 3D-printed implants offer optimized microscopic properties without the need for customization, providing a more accessible and cost-effective solution for surgeons and patients.

Porosity and bone integration

The concept of porosity in spinal implants, particularly in ALIF cages, draws inspiration from the success of porous metal implants in hip arthroplasty. The use of porous metals, such as titanium, in hip replacements has shown remarkable improvements in longevity and patient outcomes due to enhanced bone ingrowth and long-term stability (15). The interconnected pores within these implants act as a scaffold, facilitating tissue ingrowth and anchoring the implant securely to the surrounding bone. By promoting osseointegration and enabling bone bridging across the implant, the risk of implant failure and subsidence is reduced (11).

The efficacy of porous titanium metal in spinal fusion has been demonstrated in various studies. Fujibayashi et al. highlighted the importance of high porosity, large pore size, and high interconnectivity for successful bone ingrowth and tissue differentiation, leading to long-term implant stability (10). The study demonstrated that a porous titanium metal with 60% porosity, a 250 µm average pore size, and over 99% pore interconnectivity, combined with surface modifications, effectively prevented implant subsidence after a mean follow-up of 15.2 months (10). The creation of such intricate porous structures on a consistent, large scale was previously unattainable with traditional subtractive manufacturing.

Subsidence and elastic modulus

The issue of subsidence, or the sinking of an implant into the vertebral body, is a significant concern in ALIF procedures as it can lead to a reduction in disk height and a decrease in foraminal volume, potentially causing pain and neurological complications (8). A key factor contributing to subsidence is the implant’s elastic modulus, which measures its stiffness. When there is a substantial difference in stiffness between the implant and the surrounding cortical and trabecular bone, stress shielding can occur. This phenomenon arises when the rigid implant bears a disproportionate amount of the load, depriving the adjacent bone of the necessary mechanical stimulation needed for bone remodeling and fusion (9). Consequently, the weakened bone-implant interface heightens the risk of subsidence, compromising the stability and success of the surgical outcome.

3D printing technology offers an innovative solution to this challenge by enabling the fabrication of porous titanium implants. The strategic introduction of pores within the titanium matrix also effectively reduces the overall metal content of the implant, effectively lowering its elastic modulus, aligning it more closely with that of bone. Fujibayashi et al. demonstrated this concept, showcasing a 3D-printed porous titanium implant with 60% porosity exhibiting an elastic modulus of 4.2 GPa (10). This marked reduction in stiffness, while still maintaining adequate mechanical strength, fosters a more harmonious load-sharing dynamic between the implant and the bone, thereby mitigating stress shielding and the associated risk of subsidence (11).

Introducing pores into 3D-printed titanium implants, despite reducing overall metal content, does not compromise their mechanical strength. Donaldson et al. highlight that the implant’s rigid frame structure, even with decreased elastic modulus due to porosity, retains sufficient strength to meet the biomechanical demands of the lumbar spine (16). The study further references Brantigan et al., who suggested a minimum postoperative load-bearing capacity of 2,400 N for LIF constructs (17). The 3D-printed porous titanium implants, bolstered by their integrated outer frame, surpassed this benchmark, demonstrating a fatigue strength exceeding 10,000 N under repetitive compressive loading (10,17). This evidence reinforces that despite their porous nature, these implants can effectively maintain structural integrity and support the necessary lumbar spine loads, making them a viable and promising option for ALIF procedures.

3D printing and radiographic clarity

The inherent radiopacity of titanium implants enhances their visualization but poses challenges in evaluating surrounding vertebrae and key fusion indicators due to the metal’s high density, which produces a bright signal on X-rays and CT scans. However, the introduction of porosity through 3D printing techniques and a reduction in overall metal content significantly reduces the density, thereby decreasing radiographic interference. This adjustment enhances the visualization of adjacent bone structures and facilitates a more precise assessment of fusion progress. Notably, studies such as those conducted by Kabra et al. emphasize the benefits of 3D-printed porous metal implants in providing clearer visualization of fusion, despite being composed of titanium (18).

Surface modification and the role of 3D printing

The surface of an implant is critical in its interaction with the surrounding biological environment, heavily influencing its integration with bone and promotion of fusion (19,20). Traditional surface modifications, like plasma-sprayed HA coatings on titanium implants, have limitations, including potential degradation and challenges in achieving uniform coatings on intricate geometries (19).

3D printing revolutionizes this aspect by enabling the direct incorporation of surface modifications into the implant’s structure during fabrication. This eliminates the need for additional coating steps and ensures a consistent and durable modification across the entire surface, even in complex shapes. The surface texture can be precisely tailored to enhance cell adhesion and differentiation. Specific roughness patterns can stimulate osteoblast activity and bone formation, crucial for successful fusion (19). Osteoinductive coatings, such as HA, can be seamlessly integrated into the implant’s design, directly stimulating bone growth and accelerating fusion (19,20). 3D printing ensures precise and uniform application of these coatings to ALIF implants, maximizing their effectiveness.

Impact of 3D printing on implant customization and cost considerations

3D printing technology has fundamentally disrupted the conventional manufacturing model. Unlike traditional methods, where costs are closely tied to production complexity, 3D printing shifts one of the primary cost factors to the volume of material used, making the production of custom, one-off implants economically feasible (21,22). However, the design phase remains a significant cost driver and the main reason why 3D-printed custom devices currently remain more expensive than generic OTS implants. Creating PS models requires skilled labor, which significantly increases expenses (21,22). Nevertheless, rapid advancements in design technologies are poised to alleviate this burden. Emerging tools are streamlining the design process, with the potential to reduce the cost, coordination, time, and effort to creating PS devices. As these technologies continue to mature, the accessibility and affordability of custom implants are likely to increase, broadening their application in ALIF cases and the wider field of spinal surgery.

Limitations of OTS implants

OTS implants have traditionally served as the cornerstone of ALIF procedures and LIF, being mass-produced using standardized molds to offer a limited range of footprint sizes and dimensions. These implant sizes are specifically engineered to accommodate the anatomical norms of the general population, making them suitable for most routine cases. However, this standardized approach comes with significant limitations. Establishing production lines for OTS implants necessitates substantial upfront investments in both time and resources (21). Additionally, producing limited quantities or custom designs through traditional methods, such as injection molding and subtractive manufacturing, remains prohibitively expensive and impractical. As a result, this constraint has historically restricted the widespread availability of personalized implants (22).

Clinical benefits of PS implants

PS implants are meticulously engineered to conform to the individualized anatomical contours of each patient, addressing the complex geometric irregularities often present in pathological anatomy. This custom-tailored approach allows surgeons to design implants with precise dimensions and asymmetric sides as needed (12,16,18) (Figure 3). Custom design features include corrective angulation to restore lumbar lordosis, screw holes with preplanned trajectories that account for specific screw lengths, and end plate interface geometry matched to the patient’s unique anatomy (Figure 4). These enhancements ensure uniform loading across the end plates and implant, facilitating optimal integration with surrounding bone structures and significantly increasing the likelihood of achieving the desired preoperative alignment and biomechanical goals (23).

Figure 3 Illustration of an ALIF patient specific implant with custom asymmetric dimensions tailored to the patient’s anatomy. Original image produced using CarlsMed pre-operative planning software (CarlsMed, Inc., Carlsbad, California, USA). ALIF, anterior lumbar interbody fusion.

Figure 4 Illustration of the CT-based 3D planning used to design an ALIF PS implant contoured to a patient’s unique anatomy. Original image produced using CarlsMed pre-operative planning software (CarlsMed, Inc., Carlsbad, California, USA). ALIF, anterior lumbar interbody fusion; CT, computed tomography; PS, patient-specific.

Comparative analysis, OTS vs. PS implants in ALIF

OTS ALIF implants have also benefited from advancements in 3D printing technology. While PS implants are tailored to individual anatomical contours, OTS implants now incorporate micro modifications, such as increased porosity and altered surface textures, specifically designed to enhance arthrodesis and improve implant longevity. Although OTS implants maintain a standardized structure to accommodate a broader population, they share the same advanced material composition and surface technologies as PS implants. The primary difference lies in customization; OTS implants offer a uniform design, while PS implants are uniquely crafted to meet the specific anatomical needs of individual patients (Figure 5A,5B). Both types of implants leverage 3D printing advancements to optimize clinical outcomes and enhance patient care.

Figure 5 Pre-operative standing lumbar X-rays showing the fit of the proposed L5–S1 implant, paired with a 3D reconstruction of the previously fused adjacent L4–5 segment. (A) Pre-operative lateral/sagittal radiograph and 3D rendered CT image used for surgical planning. Original image produced using CarlsMed pre-operative planning software (CarlsMed, Inc., Carlsbad, California, USA). (B) Pre-operative AP/coronal radiograph and 3D rendered CT image used for surgical planning. Original image produced using CarlsMed pre-operative planning software (CarlsMed, Inc., Carlsbad, California, USA). CT, computed tomography.

Importance of preoperative sagittal alignment

The critical importance of achieving preoperative sagittal alignment, a key determinant of patient quality of life and postoperative success, has garnered increasing recognition in degenerative cases of spinal surgery. Restoring adequate lumbar lordosis is particularly crucial; failure to do so can result in chronic pain, imbalance, and progressive degeneration (24,25). PS implants have demonstrated superior performance in attaining these alignment goals, particularly in adult spinal deformity (ASD) corrections (23-25). While evidence supporting the use of PS implants in degenerative cases remains limited, the underlying biomechanical principles that confer benefits in ASD corrections suggest that similar advantages could be realized in degenerative conditions as well. However, the current body of literature tends to focus on PS implants in complex cases, leaving a gap in understanding their efficacy relative to OTS implants in more routine degenerative cases (12).

Future directions in ALIF implant technology

We share the sentiments of Mobbs et al. (22) in recognizing the significant potential of PS ALIF implants, particularly in cases of complex anatomy where the costs of revision surgery and associated complications could be substantial. However, for patients with less severe anatomical challenges, OTS implants may offer a more economical alternative that provides non-inferior or equivalent outcomes.

Looking ahead, advancements in 3D printing technology, workflow optimization, and CAD/computer-aided engineering (CAE) software are expected to streamline these processes, making the benefits of a PS approach more accessible and economically viable for a broader patient population. We envision a future where the choice between PS and OTS ALIF implants becomes systematic and straightforward, decisions guided by the individual anatomical and clinical needs of each patient.

Conclusions

ALIF is an effective surgical option for patients with lumbar pathologies, particularly at L4–S1 segments, who have failed conservative treatment and meet appropriate surgical indications. The efficacy of ALIF is due to the surgical approach and the implants used with it. 3D printing has enabled and will continue to produce implants with design features that maximize fusion, stability, and longevity.

Acknowledgments

None.

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Prashanth J. Rao and Andrew Lennox) for the series “Anterior Lumbar Interbody Fusion—A Definitive Guide for Surgeons” published in Journal of Spine Surgery. The article has undergone external peer review.

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-24-114/rc

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-24-114/prf

Funding: None.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-24-114/coif). The series “Anterior Lumbar Interbody Fusion—A Definitive Guide for Surgeons” was commissioned by the editorial office without any funding or sponsorship. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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