Anterior cervical hybrid constructs reduce superior adjacent segment burden compared to multilevel anterior cervical discectomy and fusion

Introduction

A staggering 3.2 million people suffer from cervical degenerative disc disease in the United States annually (1). The standard surgical treatment is anterior cervical discectomy and fusion (ACDF) for cases refractory to nonoperative measures. Patients with multilevel pathology undergoing an all-fusion procedure have an increased risk of developing adjacent segment degeneration (ASD), which can result in a domino effect requiring additional surgery and limit long-term surgical success (2,3). Anterior cervical disc replacement (ACDR) has become an alternative treatment of cervical disc disease that has been shown to provide the same therapeutic benefit as ACDF while maintaining or restoring physiologic range of motion (ROM) at the treated level(s), thereby decreasing the burden on adjacent segments. As a result, the incidence of ASD and the need for additional surgery is reduced (4-7).

Combining ACDF and ACDR in a “hybrid” surgery [anterior cervical hybrid (ACH)] is a relatively novel technique for multilevel disc degeneration. This allows the surgeon to customize treatment on a per-level basis. ACH has been shown to be a safe and effective surgical treatment in early case reports and studies (8-10). An expected benefit of this partial motion-preserving surgery is reduced stress transfer to the superior adjacent segment as compared to a motion-sacrificing fusion-only construct (11,12). There are early studies that compare radiographic parameters between ACDF and ACH constructs, yet further information is needed to demonstrate consistency (13-17). The primary purpose of this study was to compare postoperative superior adjacent segment motion between ACH and multilevel ACDF surgeries. Secondary aims included comparison of construct motion, global motion, inferior adjacent segment motion, and cervical sagittal alignment parameters between ACDF and ACH. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-23-135/rc).

Methods

Study design and patient selection

This is a retrospective cohort study of patients undergoing 2- or 3-level ACDF or hybrid surgery for degenerative pathologies at a multi-surgeon, single center between 2013 and 2021. Hybrid surgery was defined as any combination of ACDF and ACDR. Implant selection for fusion and arthroplasty was based on clinical judgment. The preoperative plan for all patients and all surgical levels included consideration for motion preservation. Arthroplasty implant-specific Food and Drug Administration (FDA) guidelines and insurance authorizations played the greatest role in which patients underwent a partial motion-preserving procedure versus a motion-sacrificing procedure. Adult patients (18 years or older) with a minimum of 6 months of follow-up and appropriate pre- and post-operative imaging (upright neutral lateral, flexion, and extension X-rays) were eligible for inclusion. Patients were excluded if the primary diagnosis was trauma, infection, or tumor, if imaging was insufficient to make appropriate X-ray measurements, if radiographs were taken outside of the institution, or if radiographs were flagged for patients who were unable to comply with the institution’s standard ROM requirements during flexion and extension imaging. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by institutional board of Advarra (No. Pro00045821). Informed consent was taken from all the patients.

Data collection

Patient demographics including age, sex, body mass index (BMI), Charlson Comorbidity Index (CCI), nicotine use, and lowest T-score were collected from medical and operative records. Preoperative radiographs and postoperative radiographs at the most recent follow-up were reviewed and digitally measured by two fellowship-trained spine surgeons. Figure 1 represents an example of these measurements. Cobb angles were measured on lateral flexion/extension cervical X-rays to determine lordosis globally (C2–C7), across the operative construct, and at the superior and inferior adjacent segmental levels. ROM was calculated as the sum of the absolute values of the Cobb angles in flexion and extension. Changes in motion after surgery were determined by calculating the difference between postoperative and preoperative motion parameters. Neutral lateral cervical X-rays were used for alignment measurements of C2–C7 global lordosis, cervical sagittal vertical axis (cSVA), and T1 slope (T1S). The formula used for calculating ideal cervical lordosis (CL) was ideal CL = T1S – 16.5°±2° as presented by Staub et al. in 2018 as a method of predicted normative CL (18). Sagittal alignment restoration was evaluated by calculating the difference between the actual postoperative CL and the preoperatively calculated ideal CL.

Figure 1 Example of measurements made on lateral X-rays, including CL, cSVA, T1S, and Cobb angles to measure global (C2–C7) lordosis, and adjacent segment lordosis (superior and inferior). CL, cervical lordosis; cSVA, cervical sagittal vertical axis; T1S, T1 slope.

Statistical analysis

All analyses were performed using IBM SPSSv28.0 (Armonk, NY, USA). Values are represented as number and percentage or mean ± standard deviation. Differences between the ACH and ACDF cohorts were determined using the independent t-test for continuous variables and Fisher’s exact test for categorical variables. Statistically significant P value was set at P<0.05.

Results

Demographics and surgical details

Of 147 total patients, 100 patients remained after exclusion criteria were applied, with 50 patients in each cohort. Figure 2 represents a flow diagram of the cohort. Mean age in years was within the same decade, but younger in the ACH cohort (50.6±8.7 years in ACH, 58.6±11.1 years in ACDF, P<0.001). The ACH cohort had a lower CCI (0.82±0.87 ACH, 1.73±1.40 ACDF, P<0.001). Mean follow-up time was 20.5±19.0 and 16.3±15.6 months in ACH and ACDF cohorts, respectively (P=0.23). The proportion of levels treated (2-level cases: 36% ACH, 40% ACDF; 3-level cases: 64% ACH, 60% ACDF) was similar between the groups, P=0.83 (Table 1).

Figure 2 Flow diagram representing the cohort. ACDF, anterior cervical discectomy and fusion; ACH, anterior cervical hybrid; A, arthroplasty; F, fusion.

Within the 2-level ACH cohort, hybrid constructs were as follows superiorly to inferiorly: 89% arthroplasty (A)/fusion (F) and 11% F/A. Within the 3-level ACH cohort, hybrid constructs were: 56% A/F/F, 22% A/A/F, and 22% A/F/A (Figure 2). Examples of these five different hybrid constructs are depicted in Figures 3,4. Within all 2- and 3-level hybrid constructs, the majority had arthroplasty superiorly (96%) and fusion inferiorly (82%).

Figure 3 Lateral X-rays showing examples of 2-level hybrid constructs in this cohort: (A) fusion/arthroplasty and (B) arthroplasty/fusion.

Figure 4 Lateral X-rays showing examples of 3-level hybrid constructs in this cohort: (A) arthroplasty/fusion/fusion, (B) arthroplasty/fusion/arthroplasty, and (C) arthroplasty/arthroplasty/fusion.

Radiographic parameters: motion

The postoperative superior adjacent segment motion increased in the ACDF cohort and decreased in the ACH cohort (−1.3°±5.3° ACH, 1.6°±4.6° ACDF, P=0.005). The postoperative inferior adjacent segment motion was similarly increased in both groups (0.5°±5.2° ACH, 0.7°±4.2° ACDF, P=0.79). Total construct ROM was significantly greater in the ACH cohort (16.3°±8.7° ACH, 4.7°±3.3° ACDF, P<0.001), and the resultant loss of motion at the construct level was significantly less in the ACH cohort (−10.0°±11.7° ACH, −18.1°±10.8° ACDF, P<0.001). Total preoperative global motion (C2–C7) was higher in the ACH cohort (48.2°±12.7° ACH, 40.6°±12.7° ACDF, P=0.003), which remained higher postoperatively (38.0°±12.8° ACH, 28.0°±11.1° ACDF, P<0.001), and the loss of global motion was similar between groups (−10.2°±12.5° ACH, −12.6°±15.2° ACDF, P=0.40). Pre and postoperative ROM parameters are listed in Table 2.

Radiographic parameters: alignment

Pre- and post-operative cSVA did not differ significantly between the groups, therefore no significant change in cSVA postoperatively was found between the groups (1.3 mm ACH, −0.1 mm ACDF, P=0.34). Preoperative global lordosis and T1S were significantly lower in the ACH cohort compared to ACDFs (4.3° ACH, 10.5° ACDF, P=0.03 and 28.7° ACH, 32.8° ACDF, P=0.03, respectively). Postoperative global lordosis and T1S remained lower in the ACH cohort (9.6° ACH, 16.3° ACDF, P=0.004 and 29.2° ACH, 33.5 ACDF, P=0.009, respectively). The two groups were similar in postoperative alignment restoration (−2.6°±8.4° ACH, 0.04°±12.2° ACDF, P=0.21). Pre and postoperative alignment parameters are listed in Table 3.

Discussion

The results show partial motion-preservation of the construct and globally without increased motion of the superior adjacent segment for ACH surgeries when compared to multilevel ACDFs in this study. Total construct motion was 16.3° on average in the hybrid cohort compared to 4.7° in the ACDF cohort contributing to a greater postoperative global ROM of 38.0° in the ACH cohort compared to 28.0° in the ACDF cohort. Clinical efficacy and safety of ACH surgery have been well established in the current literature (9,19-24). However, several reviews and meta-analyses have found that data specifically analyzing motion kinematics in ACH surgery as compared to matched ACDF counterparts is limited by small sample sizes (13-17). This study provides additional clinical data supporting the use of ACH surgery for patients with multilevel cervical pathology.

While ACDF is the standard treatment for symptomatic cervical degenerative disc disease, there is a risk of ASD. A recent meta-analysis of 19 studies by Toci et al. found a significant cumulative ASD rate of 19.7% following ACDF compared to 14.4% from ACDR (25). Hypermobility at adjacent segments to fusion-only constructs contributes to ASD, and this effect is reproducible when comparing ACDF to hybrid surgery. An early study by Lee et al. reported significant increases in ASD when comparing 2-level fusion-only constructs to 2-level hybrid constructs (26). Similarly, in Xiong et al.’s retrospective study comparing ACDF to ACH, superior adjacent segment mobility was significantly increased at six years in their ACDF group (10.07°±4.82° preoperative and 13.26°±5.95° at follow-up, P<0.05) compared to a decrease in their hybrid group (11.34°±6.49° preoperative and 9.26°±6.07° at follow-up, P<0.05) (27). Huang et al. compared 3-level ACH with ACDF and found that the superior adjacent segment ROM was significantly higher in the ACDF group (28). Similar findings have been reported in other early studies comparing hybrid surgery to matched ACDF counterparts in patients with 2- and 3-level cervical degenerative disc disease (DDD) (29-31).

In this study, superior adjacent segment motion increased postoperatively in the ACDF cohort while hybrids had a decrease in this parameter (P=0.005). This suggests that the combination of arthroplasty with fusion may reduce the potential risk of ASD and subsequent re-operation. The decrease in superior adjacent segment mobility after ACH might represent the return of that level’s physiologic motion; or in other words, after the pathology is treated and motion is partially preserved, the superior adjacent level burden lessens from its previous state of overcompensation. There was no significant difference between pre- and post-operative total inferior adjacent segment motion for either the ACH or ACDF cohort in our study. This finding was expected given the majority (82%) of our ACH cohort were fused inferiorly, adjacent to the inferior segment.

An advantage of hybrid surgery over the traditional fusion-only approach is the partial preservation of physiologic motion. Patients undergoing multilevel cervical fusion have a restricted ROM, and this can be more problematic for younger or more active patients. While hybrid constructs always contain at least one level of fusion, combining ACDF with a motion-preserving arthroplasty has been shown to effectively partially preserve physiologic global ROM (31-33). ACH patients in our study had an average of 48.2° of global motion preoperatively and 38.0° postoperatively. This outperformed our ACDF cohort clinically, which had a mean preoperative global ROM of 40.6° and 28.0° postoperatively, though the overall change did not reach statistical significance between groups.

Ji et al. conducted a study comparing 2-level fusions with 2-level hybrid surgeries and reported that preoperative global ROM was 51.4° compared to 44.9° at 2 years after ACDF and preoperative global ROM was 54.0° and 54.1° at 2 years after ACH. This difference was statistically significant at the 2- and 3-year follow-up points but not at 5 years postoperatively (34). A recent meta-analysis by Zhang et al. found statistically significant preservation of C2–C7 ROM in ACH constructs compared to ACDF constructs (17).

In our study, although the postoperative global motion was significantly greater in the ACH cohort, the total loss of global motion after surgery was not significantly different between groups. This may be explained by the increase in the superior adjacent segment motion seen in the ACDF group countering the decrease in the superior adjacent segment motion in the ACH group. The clinical benefit of partially preserving the construct ROM coupled with a return (decrease) to a more physiologic superior adjacent segment motion is expected to be greater than sacrificing motion at the level of the construct and developing compensatory hypermobility at the superior adjacent segment.

Both ACH and ACDF improved cervical alignment parameters. Preoperative global lordosis improved from 4.3° to 9.6° in the ACH cohort and 10.5° to 16.3° in the ACDF cohort. To better understand alignment, ideal CL was calculated based on preoperative parameters, which differed significantly between cohorts (P=0.03). The global lordosis achieved after surgery was compared to the ideal lordosis calculated before surgery to ascertain the degree of alignment restoration, which was not different between the groups, suggesting that ACH and ACDF similarly restore alignment. In addition, the change in cSVA was small in each cohort and the comparison between cohorts did not reach significance, demonstrating that both constructs produce similar sagittal alignment results. Figures 5,6 are ACH case examples where preoperative cervical kyphosis is present.

Figure 5 Case example demonstrating 3-level degenerative changes and kyphosis treated with ACH (arthroplasty/fusion/fusion). MRI, magnetic resonance imaging; CT, computed tomography; ACH, anterior cervical hybrid.

Figure 6 Case example demonstrating 3-level degenerative changes and kyphosis treated with ACH (arthroplasty/fusion/arthroplasty). MRI, magnetic resonance imaging; CT, computed tomography; ACH, anterior cervical hybrid.

A study by Wang et al. found increases in CL following hybrid surgery, ACDF, and ACDR, however, these results were not significantly different between the groups (35). These are similar to the findings reported by Ding et al.’s study, which reported no statistical difference in CL between ACH and ACDF (32). A recent radiographic study by Chen et al. comparing 3-level ACH and ACDF demonstrated that the fusion group had a significant decrease in CL and segmental angles at 1 year compared to the hybrid groups (36). These and other studies suggest that ACH surgery is at least as effective as ACDF surgery in restoring CL (37).

This study has several limitations. A relatively small sample size of patients undergoing elective surgery at a single institution may limit generalizability to a more heterogeneous population and may introduce bias. Procedural selection bias is a consideration in this retrospective study. The determination between ACH and ACDF in this study remained uniform. The preoperative plan for all patients and all surgical levels included consideration for motion preservation. Arthroplasty implant-specific FDA guidelines and insurance authorizations played the greatest role in which patients underwent a partial motion-preserving procedure versus a motion-sacrificing procedure. While our overall mean patient age was 55 years, the ACH cohort was somewhat younger than the ACDF cohort. This could potentially act as a confounder, as younger patients may naturally restore motion better than older patients, however, mean ages within each surgical group remained in the fifth decade. The arthroplasty implant-specific FDA guidelines used to determine eligibility for motion-preservation more often aligned with the anatomy of younger patients, which is indicative of real-world cases. This retrospective study focused solely on kinematic parameters using radiographs and does not specifically examine complication rates or patient-reported outcomes. Future studies comparing surgical outcomes between ACH and ACDF could include the evaluation of any revision surgery secondary to complications such as heterotopic ossification or osteolysis in disc replacement levels, pain from motion preservation in disc replacement levels, pseudoarthrosis in fused levels, or adjacent segment surgery.

Conclusions

ACH surgery is an effective technique for the treatment of 2- and 3-level cervical degenerative disc disease when at least one level of fusion is necessary. This partial motion-preserving surgery has the potential to restore superior adjacent segment motion to function more physiologically compared to the motion-sacrificing all-fusion approach without limiting the alignment restoration potential. The reduction in superior adjacent segment motion after hybrid surgery in this study favors the potential for long-term success of partial motion preservation with regards to adjacent level burden.

Acknowledgments

We would like to thank Dr. Leah Carreon at Norton Leatherman Spine Center for providing statistical support. The abstract has been presented at several scientific meetings, including the Global Spine Congress, the American Association of Neurological Surgeons, the American Academy of Orthopedic Surgery and the Canadian Spine Society.

Funding: None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-23-135/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-23-135/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-23-135/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://jss.amegroups.com/article/view/10.21037/jss-23-135/coif). L.D.O. and R.T.R. received institutional research support from Medtronic and Innovasis. C.R.G. serves on an advisory board for Stryker/K2M, Medtronic, and Augmedics. C.R.G. reports royalties and consulting fees from Stryker/K2M and Medtronic; institutional educational research support from Medtronic; owns stock in Audmedics and NSite. E.J. reports consulting fees from Stryker, Medtronic, and Innovasis. C.M.H. reports personal fees from Medtronic, Globus Medical, Spineart, and Innovasis. The other authors have 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. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by institutional board of Advarra (No. Pro00045821). 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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