Outcomes comparison between novel hydroxyapatite enhanced polyetheretherketone and titanium implants after anterior lumbar interbody fusion

Introduction

Anterior lumbar interbody fusion (ALIF) has become an increasingly common approach to lumbar spine fusion over the past 2 decades, with first concepts dating back to the 1930’s, technique and implant modifications occurring throughout the 1980’s and 1990’s, and the introduction of the Current Procedural Terminology (CPT) code in 1996 (1-3). The ALIF approach has been extensively evaluated, and reports demonstrate the potential for better access to the disc space for disc removal and endplate preparation, larger implant footprint for stabilization, greater space for bone graft placement and surface area for fusion, the ability to maintain or restore proper alignment and achieve indirect decompression, less muscle dissection, and reduced rates of complications (1,3-7). Yet, concerns for subsidence and pseudoarthrosis continue to plague all spinal fusion procedures, particularly as the number of spinal fusions continues to increase, the age of surgical patients advances, and the bone quality of spine patients continues to decline (3,8-14).

ALIF implant materials most utilized are titanium and polyetheretherketone (PEEK), which are recognized for their excellent biocompatibility but have been linked as contributors to implant subsidence and pseudoarthrosis, respectively (7,15,16). There are two widely studied differences between titanium and PEEK materials. The first is the mismatch found between the modulus of elasticity, or stiffness, of titanium alloys and that of cortical bone, which is better matched to PEEK (cortical bone: 10–20 GPa, titanium alloys: 110 GPa, PEEK: 3–4 GPa). This mismatch increases the risk of subsidence with unaltered titanium alloy implants. The second material difference extensively studied is the ability to achieve osseointegration for fusion. Unlike titanium, PEEK is bioinert, and its hydrophobic surface lacks the ability to bond with bone naturally, increasing the risk of pseudoarthrosis (13,15,17-19).

Considerable research and development have gone into the improvement of titanium and PEEK interbody implants to reduce the risk of subsidence and pseudoarthrosis. Increasing the porosity and surface roughness of titanium implants, such as with 3-dimensional (3D) printing, have the potential to alter the mechanical properties, reduce the elastic modulus mismatch, and mitigate the risk of subsidence (18,20-23). Surface-coating technology for PEEK implants, such as with titanium or hydroxyapatite (HA), has the potential to alter the biologic properties, improve the osseointegrative potential, and promote fusion (17,21,24-30). Two novel ALIF implants using high performance materials: (I) HA infused PEEK (HA PEEK) implant with a titanium alloy faceplate and integral screws; and (II) 3D printed titanium alloy implant with microtextured surface integrated with HA [titanium hydroxyapatite (TiHA)] and integral screws, were designed to mitigate the historic issues associated with each untreated material.

While numerous studies have compared outcomes between untreated titanium and PEEK interbody implants, and others have evaluated newer surface-treated materials against their untreated counterparts, few have directly compared the performance of modern surface-enhanced titanium and PEEK implants. Additionally, patient-reported outcomes (PROs) are underrepresented in past comparative studies. Therefore, head-to-head analyses are needed to address this knowledge gap and evaluate the relative clinical performance of these novel ALIF implant technologies. The present study compares HA PEEK and TiHA implants with respect to fusion, subsidence, reoperation, and PROs using data collected from a real-world clinical database. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-25-115/rc).

Methods

Data collection and patient selection

This is a multicenter, retrospective cohort study of prospectively collected data from the BioBase^® Registry. The BioBase^® Registry is a multicenter, observational, quality-assessment repository. The database tracks fusion cases from over 50 sites, both private and academic, to achieve real-world radiographic and PROs. Radiographic outcomes are assessed by an independent third party on a real-time basis to avoid surgeon bias. Postoperative computed tomography (CT) scans are not collected or part of the registry’s protocol. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved independently by the Institutional Review Board (IRB) at each participating site, with each site maintaining its own IRB approval, including Advarra IRB/CR00460400. Informed consent was taken from all the patients.

All adult ALIF cases were initially extracted from the database between 2018 and 2024, then cases were excluded if they were not circumferential fusions, if the number of treated levels was greater than 3, and if follow-up was less than 1 year. Cases were excluded from analysis if 12-month radiographic images were unavailable or of insufficient quality for subsidence and fusion analysis. The remaining cases were divided into two groups based on the interbody implant used, HA PEEK or TiHA (Figure 1). Baseline and surgical characteristics extracted from the registry included those associated with the outcomes of interest, including body mass index (BMI), Charleson Comorbidity Index (CCI), nicotine use, levels treated, and graft material. Graft materials included iliac crest autograft, local autograft, allograft, synthetic graft, and bone morphogenetic protein-2 (BMP, INFUSE, Medtronic, Memphis, TN, USA). Data obtained for graft material was categorized into two groups: cases with BMP and cases without BMP.

Figure 1 ALIF implants: HA PEEK (left) and TiHA (right), Innovasis, Inc., Salt Lake City, Utah. Permission has been obtained from Innovasis, Inc., available at: https://www.innovasis.com/thoracolumbar. ALIF, anterior lumbar interbody fusion; HA, hydroxyapatite; PEEK, polyetheretherketone; TiHA, titanium hydroxyapatite.

Outcome measures

The primary outcomes were subsidence at 12 months and fusion at 6 and 12 months. To measure these radiographic outcomes, available radiographs collected as part of the registry were analyzed by a third party. The secondary outcomes were reoperation rates and change in PROs at 12 months, both of which were extracted from the database. Reoperation within 12 months was defined as any subsequent lumbar surgery performed for any indication, including implant failure or migration, nonunion, adjacent segment pathology, malalignment, neural compression, or infection. Patient reported outcome measures (PROMs) used in this study were the Oswestry Disability Index (ODI) and the visual analog scale (VAS, 0–100) for back pain. Comparisons were made between preoperative and 1-year postoperative scores for both PROMs. Established minimum clinical important change (MCIC) for ODI is >10 points and for VAS back pain is >20 points for subacute or chronic low back pain (31).

Radiographic analysis

Each interbody fusion level was independently assessed for subsidence and fusion by an external Imaging Core Lab using validated, automated computer vision-based analysis tools (32). Subsidence was defined as a loss of disc space height ≥2 mm, determined by comparing immediate postoperative standing lateral lumbar radiographs with those obtained at the 1-year postoperative follow up. Fusion was evaluated using validated motion-detection software and defined as <3° of angular motion on dynamic imaging. Each interbody level was analyzed at both 6 and 12 months postoperatively using standing flexion-extension radiographs. Representative examples of 1- and 2-level fusions in each implant group are shown in Figures 2,3.

Figure 2 Postoperative lateral lumbar radiographs depicting single level ALIF with HA PEEK implant (left) and TiHA implant (right). ALIF, anterior lumbar interbody fusion; HA, hydroxyapatite; PEEK, polyetheretherketone; TiHA, titanium hydroxyapatite.

Figure 3 Postoperative lateral lumbar radiographs depicting 2-level ALIF with HA PEEK implant (left) and TiHA implant (right). ALIF, anterior lumbar interbody fusion; HA, hydroxyapatite; PEEK, polyetheretherketone; TiHA, titanium hydroxyapatite.

Statistical analysis

The statistical analysis was conducted using Stata (StataCorp. 2023. Stata Statistical Software: Release 18. College Station, TX, USA: StataCorp LLC) to estimate sample size, summarize the data, and examine relationships between variables. Descriptive statistics included mean and standard deviation (SD) for normally distributed continuous variables, median and interquartile range (IQR) for non-normally distributed variables, and frequencies and percentages for categorical variables. Statistics were calculated based on available data. Frequencies and percentages were computed using the number of non-missing responses as the denominator. Variables with missing data were not imputed. Normality was evaluated using Shapiro-Wilk tests, and the appropriate parametric or non-parametric tests were selected based on data distribution. All analyses were performed at a significance level of P<0.05.

For bivariate analysis, an Independent Samples t-test was used to compare means for normally distributed data, the Wilcoxon Rank Sum Test for non-normally distributed data, and the likelihood ratio Chi-squared test or Fisher’s Exact Test to assess associations between categorical variables and method of treatment. Firth logistic regression was employed to address issues of data sparsity in certain covariate combinations, where limited cell counts could otherwise lead to unstable or infinite parameter estimates in standard logistic regression. Crude and adjusted regression were performed to identify how predictor variables influence the outcome of fusion. Regression results are reported as odds ratios (ORs) with 95% confidence intervals (CIs).

Candidate variables for inclusion in the multivariable regression model were selected a priori based on clinical relevance and having known prognostic factors for the outcome of interest. Variants of the full model were then tested to evaluate model fit, including consideration of interaction terms and non-linear effects. Model performance and selection were guided by a combination of clinical interpretability and statistical criteria such as variance inflation factor (VIF, multicollinearity check), area under the receiver operator curve (AUC, model discrimination), and Pseudo R² (model explanatory power).

Results

Background and surgical characteristics

Across 13 sites, 181 patients (48.1% HA PEEK, 51.9% TiHA) met the criteria for inclusion and were analyzed (Figure 4). The median age was 55 [43–63] years, 47.8% were males, and 24.2% were nicotine users. The BMI category most represented was overweight-obese (30–35+ kg/m²) at 75.8% and 96.1% had a CCI <2. Hospital length of stay (LOS) was a median of 3 [2–4] days. Surgical indications were not mutually exclusive, and stenosis (59.0%) had the highest representation, followed by degenerative pathologies (49.1%), spondylolisthesis (45.3%), and others (24.2%). The proportion of levels treated were 47.5% 1-level, 44.8% 2-level, and 7.7% 3-level. Between group comparisons revealed no significant differences for age, sex, BMI, CCI, nicotine use, and hospital LOS. Differences were found with respect to the number of levels treated (59.8% HA PEEK 1-level, 36.2% TiHA 1-level, P=0.003) and BMP use (28.7% HA PEEK, 86.2% TiHA, P<0.001) (Table 1).

Figure 4 Study flow diagram. HA, hydroxyapatite; I/E, inclusion/exclusion; PEEK, polyetheretherketone; TiHA, titanium hydroxyapatite.

Primary outcomes

For the primary outcomes of subsidence and fusion, the overall cohort demonstrated favorable results at 12 months. Across 273 treated levels, 97.7% did not exhibit subsidence, 97.1% achieved fusion at 6 months, and 95.2% were fused at 12 months. Between-group comparisons revealed no significant difference in subsidence at 12 months (4.1% HA PEEK, 0.7% TiHA, P=0.10). Fusion rates were high in both groups at 6 months (94.2% HA PEEK, 99.3% TiHA, P=0.04) and 12 months (92.0% HA PEEK, 98.0% TiHA, P=0.02), modestly favoring the TiHA group (Table 2).

To further explore predictors of fusion at 12 months, patient and surgical characteristics were compared between fused and non-fused levels. No significant differences were observed for age, BMI, CCI, nicotine use, number of levels treated, or changes in ODI and VAS back pain scores. A trend toward significance was noted for sex, with a higher proportion of females achieving fusion (46.7% males, 53.3% females, P=0.045). Significant differences were observed for implant type (44.2% HA PEEK, 55.8% TiHA fused, P=0.02) and BMP use (36.9% fused without BMP, 63.1% fused with BMP, P=0.04) (Table 3).

Given the observed between-group differences and the known influence of certain predictors on fusion, such as BMP use, both crude and adjusted regression analyses were performed to assess associations with the fusion outcome. In the crude analysis, titanium implant type (OR 3.78, 95% CI: 1.1–13.0, P=0.04) and BMP use (OR 3.60, 95% CI: 1.14–11.36, P=0.03) were significantly associated with higher odds of fusion. However, in the adjusted model, no predictors remained statistically significant after controlling for covariates. An interaction term (TiHA × BMP) was evaluated but found to be non-significant and subsequently removed, indicating no evidence of effect modification and suggesting that the crude associations were driven by confounding (Table 4).

Secondary outcomes

For the secondary outcomes of reoperation rate and change in ODI and VAS back pain scores at 12 months, the overall cohort demonstrated positive results. The reoperation rate at 12 months was 9.9%. ODI scores improved by a median of 26.0 (12.0–38.7) points and VAS back pain scores improved by a median of 31.0 (10.0–60.0) points at 12 months, both beyond MCIC. Between group comparisons showed no significant difference in the reoperation rate at 12 months (9.2% HA PEEK, 10.6% TiHA, P=0.81). The improvement in ODI and VAS back pain scores at 12 months met established MCIC values for both groups but improvement favored the HA PEEK group (ODI: −29.0 HA PEEK, −22.7 TiHA, P=0.049 and VAS: −40.0 HA PEEK, −25.0 TiHA, P=0.02) (Table 2).

Discussion

This study compared subsidence and fusion rates between surface-modified interbody implants for ALIF to assess whether the known limitations of untreated PEEK and titanium implants have been overcome, creating better outcomes and a level playing field for implant selection in fusion procedures. For the HA PEEK group with HA infused PEEK plus a titanium alloy faceplate and integral screws, 95.9% of cases did not develop subsidence and 92.0% were fused at 12 months. For the TiHA group with the 3D printed titanium alloy implant with microtextured surface integrated with HA and integral screws, 99.3% of cases did not develop subsidence and 98.0% were fused at 12 months. These rates exceed values for untreated PEEK and titanium implants in the literature (13,16,33).

Subsidence can be a devastating complication following any lumbar fusion, including after ALIF. The intervertebral height restoration gained from the ALIF approach can be reversed leading to poor spinal alignment, pain, and dysfunction. The indirect decompression achieved during ALIF can collapse, causing foraminal stenosis and neurologic symptoms. Subsidence can also compromise the fusion potential. These poor outcomes attributed to subsidence make it a common reason for revision surgery (13). Of 181 patients in this cohort, 6 (2.3%) developed subsidence up to 12 months.

In a systematic review by Parisien et al. comparing subsidence outcomes between lumbar fusion approaches, the subsidence occurrence rate was 12.8% for ALIF. All ALIF studies included in the review (n=390) used untreated PEEK interbody implants (34). In the current study, the HA PEEK group had a much lower (4.1%) subsidence rate at 12 months. In a systematic review with meta-analysis by Tan et al., comparisons were made between untreated PEEK (n=231) and titanium (n=259) implants to assess subsidence and fusion rates. A higher rate of subsidence was found in the titanium group [risk ratio (RR): 2.17, 95% CI: 1.13–4.16, P=0.02] (33). In the current study of surface-enhanced implants, no significant difference in subsidence rates was found between implant groups at 12 months (P=0.95).

In the same systematic review, titanium implants were more likely to achieve final fusion compared to PEEK (OR 2.12, 95% CI: 1.05–4.28, P=0.04) (33). In the present study, fusion rates were high in both groups, with the TiHA group demonstrating higher rates at both 6 and 12 months; however, regression analysis provided additional context. Although crude analyses suggested higher odds of fusion with TiHA implants and with BMP use, these associations did not persist after adjustment for confounding variables. The non-significant interaction between TiHA and BMP further supports the absence of a synergistic effect between implant type and graft material. Collectively, these findings indicate that the apparent advantage of TiHA observed in unadjusted analyses was largely attributable to the higher frequency of BMP use in that group rather than inherent differences between implant materials. This interpretation aligns with the findings of Galimberti et al., who reported fusion rates of 98% in ALIF procedures using BMP versus 89% without BMP at final follow up, underscoring the strong independent effect of BMP on fusion outcomes (35).

Most studies evaluating fusion outcomes report results at 12 months; however, time to fusion is an increasingly relevant concept gaining attention in the literature. Zhang et al. reported that 51% of levels were fused at 6 months and 85% at 12 months following LLIF with untreated PEEK and BMP, while Galimberti et al. found ALIF fusion rates of 97% with BMP and 87% without BMP at 6 months (35,36). In the current study, 97.1% of levels were fused at 6 months, and fusion status at this time point was highly concordant with 12-month outcomes, with only a few cases demonstrating delayed union. These findings suggest that, with modern surface-enhanced interbody implants, radiographic evidence of fusion at 6 months is a strong predictor of definitive arthrodesis. Early consolidation may allow for earlier return to physical activity and work and is particularly relevant for patients with risk factors for poor bone quality (37-39). Thus, a 6-month postoperative assessment may serve as a practical and clinically meaningful benchmark for determining fusion success in contemporary ALIF practice.

Radiographic comparisons between implant materials rarely include PROs. In a meta-analysis by Seaman et al., only one lumbar study reported PROs and found no differences between untreated PEEK and titanium implants for VAS back pain at 24 months (16). Similarly, a more recent meta-analysis comparing titanium-coated and uncoated PEEK implants reported no significant differences in VAS back pain or ODI across four randomized trials involving PLIF and TLIF procedures (29). In the present ALIF study, both implant groups achieved clinically meaningful improvements in VAS back pain and ODI, with a trend favoring the HA PEEK group. This difference may relate to the higher proportion of single-level cases in the HA PEEK group, consistent with prior reports, and is unlikely to be due to implant type (40).

Limitations

The limitations of this study include those inherent to the study design of retrospective analysis of prospectively collected registry data, including missing data, patient selection bias, and the lack of randomization between groups. However, registry data are valuable for studying real-world outcomes and trends in large populations and many endpoints were clearly defined in advance of data collection in the BioBase^® Registry, including the outcomes assessed in this study.

It was not unexpected that BMP played a role in fusion (36,41,42). Confounding in the assessment of fusion limits the interpretability of the results. However, both HA PEEK and TiHA implants demonstrated similarly low rates of subsidence and high rates of fusion. Although BMP use and implant selection were interrelated with TiHA implants being more common in multilevel fusions and used more frequently with BMP, the regression analysis controlled for these factors and explained the stronger unadjusted associations. After controlling for surgical complexity and graft use, both implant types achieved comparably favorable outcomes, reinforcing the effectiveness of surface enhancement across substrates in mitigating historical concerns of PEEK nonunion and titanium subsidence.

Fusion evaluation using functional radiographs without CT evaluation is limited in that motion is the only surrogate for fusion rather than assessing bridging bone as an additional data point. However, the risks associated with the radiation emitted from a CT scan at 6 months and again at 12 months to each patient is not justifiable as part of this study and is not standard practice of the contributing sites. The automated computerized algorithm used for functional radiographic analysis by a third party is a validated method that allows for the opportunistic evaluation of radiographs obtained as part of the postoperative routine by the contributing sites (32).

Conclusions

Surface-enhanced PEEK and titanium ALIF implants demonstrated high fusion rates, low rates of subsidence and reoperation, and clinically meaningful improvements in disability and back pain at 1 year. Historical concerns of nonunion with untreated PEEK and subsidence with untreated titanium were not observed, suggesting that surface-enhanced implant selection can be guided by clinical judgment and procedural considerations rather than differences in clinical outcomes. Additionally, high 6-month fusion rates in both groups suggest that modern surface-enhanced designs may facilitate earlier fusion, further supporting their clinical utility.

Acknowledgments

The abstract of this article has been presented at several meetings, including LSRS 2024, ISASS 2024, NASS 2024, EUROSPINE 2024, and has been presented in https://thejns.org/spine/view/journals/j-neurosurg-spine/40/5/article-p1.xml. Special thanks to Raylytic’s Imaging Core Lab (RAYLYTIC GmbH, Leipzig, Germany) for the high-quality automated method for subsidence and fusion analysis.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-25-115/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-115/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-25-115/prf

Funding: This work was supported by Innovasis, Inc. in the form of an educational research grant to the study sponsor, National Spine Health Foundation.

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-25-115/coif). L.D.O. and R.T.R. report research grants from Medtronic, Innovasis, Amgen, and UCB, which paid to the institution-National Spine Health Foundation. P.G.P. reports research support from CSRS; consulting fees from Medtronic, Royal Biologics, SpineWave, and Terumo; paid presenter for Globus Medical and Zimmer; other material support from Allosource; and Editorial Board for Spine. E.J. reports consulting fees from Medtronic, Innovasis, and Stryker. W.T.L. reports consulting fees from the National Spine Health Foundation. C.M.H. reports consulting fees from Medtronic, Globus Medical, Innovasis, and Spineart. The other author has no 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. This study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. Ethical approval was obtained by each contributing center separately as part of the BioBase^® Registry requirement, including Advarra IRB/CR00460400. Informed consent was taken from all the patients.

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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