Ambispective study of fusion and subsidence in degenerated L4–5/L5–S1 discs treated with oblique lumbar interbody fusion/anterior lumbar interbody fusion using 3D-printed titanium open arch cages

Introduction

Lumbar interbody fusion (LIF) cages have been utilized for decades to facilitate interbody vertebral fusion and aim to restore intervertebral height and lordosis and regain biomechanical stability of the anterior column (1-3). Advancements in surgical techniques and medical technologies provide surgeons with many different options of surgical approaches and LIF cages to treat degenerative disc disease (DDD) of the spine. Anterior LIF (ALIF) is a common surgical procedure providing the benefit of preserving the posterior and psoas muscles while improving postoperative stability (4). Previous research has reported that the anterior approach resulted in greater improvement in radiographic and clinical measurements compared with posterior approaches (1,5,6). The popularity of ALIF continues to rise, which may be attributed to the favorable findings in published studies. Medicare utilization data reports a 592.81% increase in ALIF procedures in the United States between the years 2000 and 2019 (7). Recently, performing ALIF surgery from an oblique approach was introduced combining elements of both ALIF and direct lateral LIF (LLIF) (8,9). Oblique LIF (OLIF) has been reported to offer advantages such as reduced rates of procedure-related injuries, including fewer visceral injuries, lower incidence of retrograde ejaculation, and lower incidence of incisional hernias compared to ALIF; and fewer lumbar plexus injuries compared to LLIF (10,11).

As the popularity of the procedure grows, the designs of cages are also evolving to improve fusion rates and optimize clinical outcomes. Current key engineering considerations include cage dimensions, composite surface materials, integrated fixation, and expandable technologies (10,11). SPIRA^® (Camber Spine Technologies, King of Prussia, PA, USA), a novel three-dimensional (3D)-printed titanium cage featuring a patented open architecture, is designed for improved clinical outcomes. The novel implant has been engineered to minimize interfacial stress for improved biomechanics and biological response, contributing to the potential for greater fusion success through an enhanced mechanobiological environment. Moreover, this design optimizes the load distribution at the vertebral endplate interfaces for a more physiologic vertebrae-cage-vertebrae load construct, with the potential for reducing the risk of subsidence.

It is well established that the highest mechanical loading of the spine occurs at L4–5 and L5–S1 due to their weight-bearing role and the change in spinal curvature. Moreover, these lumbar levels have the lowest fusion rates owing most probably to these high mechanical loads (10,11). The objectives of this study are to evaluate the performance and safety of the open-arch cage in the treatment of degenerative discs of the lumbar spine at these challenging levels, L4–5 and L5–S1. The primary objectives of the study were assessing both radiographic fusion and incidence of implant subsidence at a minimum of 12 months postoperative. The secondary objectives of the study included the safety evaluation of the device. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-183/rc).

Methods

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. This study was approved by the Western Institutional Copernicus Group (WCG) Institutional Review Board (IRB) (No. #1356354) and informed consent was obtained from all individual participants. This was an ambispective, single center study of consecutive patients previously treated with ALIF or OLIF fusion at L4–5 and/or L5–S1 with the open-arch cage between May 2020 and June 2022. See Figure 1 for illustration of device.

Figure 1 Overview of Spira^®, 3D-printed titanium interbody device, highlighting its open-architecture arched design and micro-textured surface (Camber Spine Technologies, King of Prussia, PA, USA).

Study inclusion criteria selected patients with intractable back and/or leg pain having clinical and radiological evidence of lumbar DDD at L4–5 and/or L5–S1 who failed 6 months of conservative treatment. These patients were at least 18 years of age and were willing and able to provide informed consent to participate per study protocol. Subjects were excluded if they were currently imprisoned, experiencing major mental illness, or contraindicated for a computed tomography (CT) scan.

Surgical technique

Each procedure was performed by a single surgeon, without the involvement of an access surgeon. At the L5–S1 level, the disc space was accessed between the vascular bifurcation from an oblique approach. The L4–5 surgery was performed using an anterior to psoas approach, and the OLIF technique has been previously described in published literature (10,12). Briefly, subjects are positioned in the lateral decubitus position, and incisions are made anterior to the disk space. Blunt, manual dissection was carried out over the abdominal wall down to the anterior spine. The surgical level is then confirmed by fluoroscopy before proceeding with the interbody fusion procedure.

Study protocol

All subjects were prospectively scheduled to discuss the research study in detail, and receive a lumbar CT scan. Research data, including demographics, medical history, surgical history, complications, and previous radiographic imaging, were collected retrospectively.

The secondary objectives of the study were obtained from the research data, including a safety evaluation of the device, length of hospital stay, incidences of device-related and/or procedure-related complications, and both surgical revisions and reoperations.

Primary radiographic assessment

The primary objectives of the study were assessing radiographic fusion and incidence of implant subsidence at a minimum of 12 months postoperatively. Radiographic fusion was defined as a CT fusion grade of 3 (Figure 2). Subsidence was measured in millimeters (mm) of endplate penetration and defined as greater than 2 mm subsidence of any part of the implant measured on sagittal and/or coronal reconstructions from the natural endplate at each individual interface. All radiographic outcomes were assessed by a single, independent, board-certified orthopaedic spine surgeon, who was blinded to the subjects’ care and clinical status.

Figure 2 Visual CT classification of fusion grading. This figure was reused from an Open Access article (13) under the terms of the Creative Commons Licensing Agreement CC BY-NC-N (https://creativecommons.org/licenses/by-nc-nd/4.0/). CT, computer tomography.

Statistics

Statistical analysis included descriptive statistics, including mean, median, standard deviation, minimum, and maximum values for continuous variables, as well as frequencies and percentages for categorical variables. Logistic regression was applied to identify independent factors associated with subsidence and fusion. Statistical significance was determined at an alpha level of 0.05. All analyses were performed using the SPSS Version 29 (IBM Corp, Armonk, NY, USA).

Results

A total of 33 patients with 39 levels treated met the inclusion criteria for the study and were included for analysis. All patients had supplemental anterior and/or posterior fixation except one, and no patient was treated with rhBMP-2. The mean follow-up was 30 months postoperative, with a range of 24 to 45 months. Demographics and medical history for this study cohort are reported in Table 1. The cohort had a mean age of 57 years, a mean body mass index (BMI) of 32 kg/m², and they were 60.6% female. All of the subjects had at least one comorbidity, and 78.8% (26/33) of subjects presented with two or more. Most common comorbidities were high BMI (90.9% above 25 kg/m²), heart disease (51.5%), smoking (24.2%), osteoarthritis (21.2%), other (21.2%), and thyroid disease (15.1%). Most of the subjects, 81.8% (27/33), underwent a single level procedure. Considering levels treated, L4–5 and L5–S1 were 43.6% (17/39) and 56.4% (22/39) of the treated levels respectively. The mean length of stay was 1.3 days, with a range of 1 to 3 days.

Fusion, as defined by a grade of 3 in the grading system described in the methods section, was 96.97% (32/33) considering patients treated, and 97.44% (38/39) considering levels treated. The single level that did not fuse was grade 2 at an L5–S1 level. Subsidence defined by greater than 2 mm of endplate penetration was 15.15% (5/33), considering patients treated, and 12.82% (5/39), considering levels treated. A breakdown of fusion rate and subsidence by patient and level treated is presented in Table 2.

In this study, 11 of 33 patients (33.3%) had a total of 23 adverse events (AEs). None of the AE’s were device-related, 8 were directly procedure-related, and 11 were possibly related to the index procedures. Details of AEs are reported in Table 3. Among the 23 AEs, 6 were classified as serious AEs (SAEs) and underwent a second surgery after the index surgery (Table 4). Therefore, the total operation rate after the index surgery was 18.2% (6/33), and half of these patients who required a second operation had that surgery at the index level yielding a 9% reoperation rate at the index surgery disc level. Notably, none of these three reoperation surgeries at the index level involved the ALIF or OLIF operative sites. The other three patients requiring operation after the index operation had indications unrelated to the device or index procedure. The remaining procedure-related AEs were treated with conservative management.

Among the subjects who met the definition of subsidence in this study, only one experienced an SAE. The limited subsidence detected in this study did not result in instability, pseudoarthrosis, or neural compression.

Univariate logistic regression models did not detect significant associations between demographic variables including age and BMI with fusion success nor incidence of subsidence, all P>0.05.

Discussion

Interbody fusion remains the most performed spinal surgery in the US, with more than 450,000 cases reported per year (13). Surgical advancements in technique and material science have provided surgeons and patients with numerous interbody fusion options in the treatment of DDD, including transforaminal LIF (TLIF), posterior LIF (PLIF), LLIF, ALIF, and OLIF. The total number of ALIF procedures performed has reported an average annual increase of 24.1%, due to low complications, mortality, and readmissions (14). Fusion results for ALIF and OLIF found in this study with the open-arch design cages at L4–5 and L5–S1, where the spine mechanical loads are greatest, compare very favorably with the results of interbody fusion studied with large-scale meta-analysis. Previous meta-analysis found an ALIF fusion rate of 91.4% at L5–S1 (15). Similarly, Schroeder et al. found an ALIF fusion rate at L5–S1 of 90.5% in their meta-analysis of 1,507 patients analyzing 42 articles (16). Interestingly, Schroeder et al. results were no different than the fusion results for TLIF by meta-analysis. Moreover, in the setting of OLIF, Chung et al. evaluated 124 consecutive subjects and reported a fusion rate of 92.0% (17). The results here in this limited study show a better L5–S1 fusion rate of 95.4% with an L4–5 fusion rate of 100% and an overall level fusion rate of 97.44%. These results support the validity of the open-arch design concepts for using a 3D-printed titanium cage with an arched open architecture.

Besides fusion, which is the primary goal of LIF surgery, another primary concern is an implant design that minimizes subsidence. While interbody cage subsidence may be asymptomatic and not require treatment, it remains a potential complication of both ALIF and OLIF. These complications may lead to significant morbidity such as disc height collapse, neural compression, segmental kyphosis, loss of stability and vertebral body fracture (18-20). In addition, subsidence may lead to persistent motion between the cage device and bone interface increasing the risk of pseudarthrosis (21). Factors that have been found to be associated with subsidence were older age, higher BMI, severe American Society of Anesthesiologists (ASA) grade, and osteoporosis (22). A previous biomechanical study demonstrated the critical role of cage design in reducing the risk of subsidence and promoting bone growth (23). The definition of subsidence, a radiographic finding, varies across studies. The subsidence definition of less than 2 mm of cage endplate penetration used in this study is rigorous. Despite this, the subsidence results of this study compare quite favorably with others presented in the literature and support the open-arch design concept for distributed load. Beutler et al. and Zavras et al. both published single center subsidence rates of 32.6% and 22.9%, respectively (19,22). The favorable result for the subsidence rate in this study was 12.82% for all treated levels. Remarkably, in this study, when evaluating just single-level ALIF and OLIF surgical applications, the interbody cages had an even better subsidence rate of 7.41% possibly owing to lower induced biomechanical loads in single level fusions when compared to multiple level fusions (18,24). Somewhat surprisingly, no significant association between BMI and subsidence, seen in other studies, was observed in this study. This lack of association between BMI and subsidence may be due to a combination of the low observed subsidence rate and the homogeneity of the patients sampled, with over 90% with elevated BMI above 25 kg/m².

SAEs observed in this study are relatively low compared to published literature. Mobbs et al. conducted a prospective study evaluating approach-related complications of ALIF, and reported postoperative retroperitoneal hematoma (0.9%), acute arterial thrombosis (0.4%), acute bowel obstruction (0.4%), retrograde ejaculation (1.8%), incisional hernia requiring revision (1.3%), and vascular injury (6.6%) (25). None of these reported complications were observed in the current study. The reoperation rate at the index level in this study was 9.0%, which is slightly higher than the 7.7% reported previously in a prior ALIF study (26). However, the complication rate reported in the Schroeder et al. ALIF meta-analysis was 18.7% (16). The OLIF approach shares a similar complication profile with ALIF, with similar reported complications of vascular injuries and postoperative ileus (27,28). It must be pointed out however, that all these patients had at least one comorbidity, and 80% had 2 or more, which obviously adds to the morbidity and mortality complication risks. Elevated BMI was the most prevalent comorbidity, with 90% of patients having a BMI above 25 kg/m² and 60% having a BMI above 30 kg/m². Increased body weight results in higher spine mechanical loading, which may contribute to complications of subsidence, instability, and pseudoarthrosis in patients undergoing lumbar spine surgery (16,29,30).

Limitations of this study include a relatively small sample size from a single institution and the ambispective approach. Obviously, a large randomized, prospective, multi-center study with a control arm for comparison would be ideal. It is well known that these ideal studies are expensive and logistically challenging, generally only conducted as part of a Food and Drug Administration (FDA) Parenteral Drug Association (PDA) protocol, and rarely conducted on FDA approved devices because of the forementioned limitations.

Moreover, it is quite true that larger studies with diverse patient populations are needed to delineate demographic factors associated with the incidence of subsidence and the success of fusion, and that this information could lead to cage design improvements. This study did not show an effect of age or BMI on either fusion and subsidence possibly owing to both the small study size and narrow distribution of outcomes with a both a high fusion rate and a low subsidence rate. There are known effects of age on both bone mineral density and ASA grade, which have been found to be associated with subsidence (18,22,24). Retrospectively, neither of these factors was evaluated suggesting another possible limitation of this study.

Conclusions

Despite the study’s inherent limitations, the results of this study do provide support for the success of the open-arch design in providing a cage with mechanical characteristics supporting fusion and limiting subsidence. An interbody level fusion rate of 97.44% and a subsidence rate of 12.82% in the most mechanically demanding areas of the lumbar spine strongly support the efficacy of the open-arch design from an historical comparison perspective. Moreover, the open-arch design has an even better subsidence rate of 7.41% in single-level applications. This study also supports the safety of the current interbody cage with no device related complications and procedural complications in line with other studies.

Acknowledgments

The authors would like to acknowledge Lisa Heffelfinger for her support of the study.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-183/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-183/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-183/prf

Funding: This clinical research study was financially supported by Camber Spine Tech.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-2025-aw-183/coif). All authors report that this study was supported by Camber Spine Tech. J.I.W. is a current employee of Orthopaedics Northeast. P.N. is a current employee of the Spine Institute of Louisiana and The Spine Network. S.T. and M.S. are current employees of The Spine Network. 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. The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the Western Institutional Copernicus Group (WCG) Institutional Review Board (IRB) (No. #1356354) and informed consent was obtained from all individual participants.

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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