The impact of patient-specific spine rods on spinopelvic parameters after short segment degenerative lumbar fusions

Introduction

Degenerative lumbar conditions, such as spinal stenosis, degenerative disc disease, facet arthropathy, and spondylolisthesis, are prevalent sources of low back pain, radiculopathy, and claudication (1-5). Despite conservative treatments, lumbar fusion emerges as an effective intervention for managing symptoms and instability associated with degenerative diseases (1,6). However, suboptimal outcomes, characterized by persistent pain or recurrent symptoms due to residual stenosis, adjacent segment degeneration, or post-operative malalignment, remain a concern (7-10).

In recent years, advancements in spinal imaging systems, pre-operative planning software, and a deeper understanding of spinal alignment have underscored the importance of achieving harmonious spinopelvic balance (11-14). Parameters like sagittal vertical axis (SVA), pelvic tilt (PT), and pelvic incidence to lumbar lordosis (PI-LL) mismatch have gained traction for predicting and enhancing outcomes in degenerative lumbar pathologies. PI-LL, particularly, serves as a marker of spinopelvic alignment, with a PI-LL ≤10° indicative of a well-aligned spine (13,15). Yet, while routinely assessed in adult deformity surgery, their significance in short-segment constructs for degenerative spine surgery remains unclear (16,17).

A seminal study by Leveque et al. retrospectively assessed spinopelvic malalignment following short-segment degenerative lumbar fusion procedures (18). Their findings, derived from a multi-institutional cohort of 578 patients at 18 institutions across the United States, underscored the prevalence of malalignment both pre- and post-operatively, with 30% of patients malaligned pre-operatively and 28% malaligned post-operatively. Alignment was preserved in 63%, restored in 9%, and not corrected in 21% of cases. Notably, 7% of patients had a PI-LL ≤±10° prior to surgery but were malaligned in post-operative imaging, placing them in the worsened category. Over 25% of patients were malaligned post-operatively, highlighting the importance of considering spinopelvic alignment in decision-making and operative planning (18).

While the study by Leveque et al. sheds light on the challenges of achieving spinopelvic alignment in short-segment lumbar fusion, it also raises questions about the optimal strategies for addressing these challenges. Conventional spinal rods, while widely used, present several notable limitations in the context of degenerative lumbar fusion. Primarily, these rods are designed with a one-size-fits-all approach, meaning they are pre-curved based on general anatomical averages rather than tailored to individual patient needs. This can result in a mismatch between the rod’s curvature and the patient’s specific spinal alignment, leading to suboptimal correction of deformities and persistent post-operative misalignment. Additionally, the static design of conventional rods does not account for the dynamic changes in spinal alignment that occur as patients heal. As the spine adapts and shifts during the recovery process, conventional rods may not adequately accommodate these changes, potentially resulting in worsening deformity or misalignment over time. Moreover, this mismatch and lack of adaptability can contribute to a higher risk of complications such as rod breakage, screw pull-out, and adjacent segment degeneration.

In response to these limitations, patient-specific spine rods (PSSRs) have been developed to offer a more personalized approach to spinal surgery. By tailoring the rods to the individual’s spinal anatomy and alignment needs, PSSRs aim to enhance the accuracy of deformity correction, improve post-operative alignment stability, and reduce the risk of complications. The customization of these rods promises a more precise fit, better accommodating the dynamic nature of spinal healing and potentially leading to improved clinical outcomes. PSSRs have emerged as a promising intervention for correcting spinal deformities (19,20). These custom rods, tailored to individualized surgical plans and manufactured based on pre-operative spinopelvic parameters, have demonstrated significant improvement in maintaining and improving spinopelvic alignment in adult spinal deformity surgery (19,20). Recent studies have shown that PSSRs can provide excellent correction in adult spinal deformity surgery, with benefits including shorter operating times, fewer mechanical complications such as rod failure, and improved execution of the pre-operative surgical plan, potentially leading to improved spinopelvic alignment and health-related quality of life (HRQOL) scores (13,21).

However, the application of PSSRs in short-segment lumbar fusions for degenerative conditions remains uncertain. This study aims to bridge this gap by comparing radiographic parameters of patients who underwent lumbar fusion using PSSRs for degenerative lumbar conditions to patients who underwent fusion with conventional rods. Leveraging the data from the Leveque et al. study, which predominantly employed standard rods, allows for comparison and assessment of the implications of PSSRs in degenerative lumbar fusion surgery. Through this investigation, we seek to elucidate the potential benefits of PSSRs in optimizing spinopelvic alignment and ultimately improving patient outcomes in this clinical context. We present this article in accordance with the TREND reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-24-75/rc).

Methods

After obtaining institutional review board approval, a retrospective analysis was conducted on patients undergoing short-segment lumbar fusions. The study was conducted in accordance with the Declaration of Helsinki (as revised in 2013). The study was approved by the Colorado Multiple Institutional Review Board (COMIRB) (No. 20-2245) and informed consent was obtained from all individual participants. To mitigate potential confounding variables associated with historical controls and variations in surgical techniques, this study focused exclusively on cases performed by the same team of surgeons at our institution. To address potential confounders related to variations in surgical technique and surgeon experience, we adhered to a standardized surgical protocol developed by our institution. By conducting the study at a single institution, we ensured consistent guidelines and practices were followed for all procedures.

The PSSR comparison group comprised surgeries performed by one of three surgeons between September 2016 and August 2018, including either anterior lumbar interbody fusion (ALIF) or transforaminal lumbar interbody fusion (TLIF) combined with posterolateral fusion. The goal of this study was to compare radiographic outcomes of lumbar fusions performed using PSSRs with those reported in historical cohorts, particularly focusing on the differences in spinopelvic alignment. To achieve this, radiographic outcomes were systematically analyzed up to a follow-up period of 2 years.

Follow-up evaluations were scheduled up to 2 years post-operatively. During each follow-up visit, standardized radiographic assessments were conducted to measure key parameters, including PI and LL. The changes in PI-LL mismatch were specifically analyzed to assess the impact of PSSRs on spinopelvic alignment over time. This longitudinal approach aimed to provide a comprehensive evaluation of the effectiveness of PSSRs in maintaining or improving radiographic outcomes compared to historical data.

Inclusion criteria encompassed patients aged 18 years and above who underwent posterior instrumented lumbar fusion involving four levels or fewer with interbody fusion. Patients who underwent posterolateral fusion without interbody support, those with fusions extending into the thoracic spine, or those with fusions spanning more than four levels were excluded. Additionally, patients lacking visible femoral heads on pre-operative imaging were excluded from the analysis.

PI and LL measurements were obtained pre- and post-operatively using imaging software, based on either standing lumbar biplanar films or full body biplanar films. Patients were categorized into four groups (preserved, restored, not corrected, and worsened) based on pre- and post-operative PI-LL measurements. The PSSR cohort was compared to the historical cohort published by Leveque et al., which included 578 patients at 18 institutions across the United States.

Statistical analysis

Statistical analysis was conducted using one-way analysis of variance (ANOVA) tests and one- and two-sample t-tests, performed with R programming language software. P values were calculated using a two-sample t-test for equality of proportions with continuity correction, with significance set at P<0.05.

This approach facilitated a comprehensive comparison of radiographic parameters between patients undergoing lumbar fusion with PSSRs and those with conventional rods, providing valuable insights into the efficacy of PSSRs in optimizing spinopelvic alignment.

Results

Out of sixty patients who underwent lumbar fusion with PSSRs, fifty met the inclusion criteria for analysis. Among them, 37 underwent TLIF with posterolateral instrumented fusion, while 13 underwent ALIF with posterolateral instrumented fusion. Pre-operative measurements for the PSSR cohort indicated an average LL of 49.4°, PI of 58.1°, and PI-LL of 8.6° (Table 1). Post-operatively, the averages were LL of 54.3°, PI of 58.7°, and PI-LL of 4.4°. Statistically significant changes were observed in average LL and average PI-LL from pre- to post-operative time points.

Comparison of pre-operative parameters between the PSSR cohort and the historical cohort from Leveque et al. revealed significant differences, indicating a baseline distinction between the two groups (Table 2). Notably, while the pre-operative PI-LL for the PSSR group (8.6°) exceeded the threshold for spinopelvic malalignment, it was significantly higher than the average for the Leveque cohort (3.7°, P=0.005). Moreover, the PI-LL change post-operatively by an average of −4.2° in the PSSR group, compared to minimal change (−0.1°) in the comparison cohort (P=0.001).

Figure 1 illustrates alignment changes in the PSSR cohort, with 74% (37/50) of patients demonstrating preservation of spinopelvic parameters post-operatively. Additionally, nine patients (18%) with pre-operative spinopelvic mismatch experienced restored PI-LL alignment post-operatively. Only four out of fifty patients exhibited spinopelvic mismatch post-operatively, with two each in the worsened and not corrected categories. Thus, 92%, 46 out of 50 patients achieved radiographically appropriate spinopelvic alignment after lumbar fusion with PSSRs.

Figure 1 Division of the PSSR cohort into four categories, based on change in spinopelvic alignment from pre- to post-operative radiographs (n=50). Patients were considered malaligned if PI-LL ≥10. PSSR, patient-specific spine rod; PI-LL, pelvic incidence to lumbar lordosis; pre-op, before surgery; post-op, after surgery.

Significant differences were observed in average pre-operative LL and PI-LL among the four categories (Table 3). While post-operative radiographic parameters did not differ significantly between categories, the degree of change from baseline in average LL and PI-LL varied significantly (P<0.001). Categorical distribution analysis between the PSSR and non-PSSR cohorts (Table 4) revealed a higher proportion of preserved parameters (74% vs. 63.5%, P=0.18) and fewer patients with not corrected parameters (4% vs. 21.3%, P<0.01) in the PSSR group. Although not statistically significant, a greater proportion of patients in the PSSR group exhibited restored parameters (18% vs. 8.7%, P=0.05), while fewer were in the worsened category (4% vs. 6.6%, P=0.67).

Figure 2 illustrates post-operative alignment changes between the two cohorts, showing a trend towards improved alignment with steeper downward sloping lines indicating greater correction. Despite not meeting the technical definition for acceptable alignment, patients in the custom rod cohort exhibited substantial improvement in spinopelvic alignment. Overall, the proportion of malaligned patients pre-operatively decreased from 22% to 8% in the PSSR cohort, compared to a change of 30% to 27.9% in the Leveque cohort (Figure 3).

Figure 2 Comparison of changes in spinopelvic alignment pre- and post-operatively between the Leveque cohort and the PSSR cohort. (A) Changes in spinopelvic alignment in the Leveque cohort, categorized as preserved, restored, not corrected, and worsened. The graph depicts shifts in PI minus LL degrees pre- and post-operatively (18). (B) Changes in spinopelvic alignment in the PSSR cohort, similarly categorized into preserved, restored, not corrected, and worsened, showing pre- and post-operative PI minus LL degrees (18). PI, pelvic incidence; LL, lumbar lordosis; pre-op, before surgery; post-op, after surgery; PSSR, patient-specific spine rod.

Figure 3 Comparison of alignment distributions pre- and post-operatively with standard rods versus patient-specific rods, demonstrating greater changes in alignment with PSSRs (18). UCH, ubiquitin C-terminal hydrolase; PSSR, patient-specific spine rod; pre-op, before surgery; post-op, after surgery.

Discussion

Following lumbar fusion surgery, residual pain or future disability remains a concern for many patients (7). This can be attributed to factors such as adjacent segment degeneration, occurring in 31% to 83% of cases post-lumbar arthrodesis, and iatrogenic flatback resulting from inadequate lordosis, leading to pain, disability, and sagittal balance issues (8,17,22,23).

This analysis aimed to assess the impact of PSSRs on radiographic parameters following lumbar fusion for degenerative conditions. Leveraging the multicenter retrospective study conducted by Leveque et al., we compared our patient cohort to their historical cohort (18). Our findings revealed a significant difference in the average pre- and post-operative PI-LL (P≤0.001) with the use of PSSRs. Notably, while Leveque et al. reported a lack of statistically significant difference in PI-LL from pre- to post-operative stages (P=0.79), our results suggest the beneficial impact of custom rods on post-operative spinopelvic parameters, with a substantial reduction in PI-LL discrepancy.

However, it is important to acknowledge the limitations inherent in this comparison. One notable limitation is the inability to perform a propensity score matching analysis between our cohort and the historical cohort from Leveque et al., as we did not have access to individual patient data from their study. This lack of direct data access prevented us from matching patients based on baseline characteristics, such as preoperative PI-LL, which could influence the outcomes. Consequently, baseline differences between the cohorts may affect the comparability of our results. The cohort from Leveque et al. [2018] included patients without well-defined exclusion criteria such as prior spinal fusions or the severity of preoperative deformities. This lack of stringent criteria could result in differences in baseline patient characteristics between their cohort and our PSSR cohort, potentially impacting the comparability of outcomes. For instance, patients in the Leveque et al. cohort may have had more severe preoperative conditions or prior spinal interventions, which were not accounted for in their study but were rigorously considered in our patient selection.

These discrepancies in patient selection criteria may introduce variability in the outcomes, affecting the interpretation of our results. To address this, future studies should aim to include more detailed patient selection criteria and consider using propensity score matching or other statistical techniques to control for baseline differences when comparing outcomes between different cohorts. This would help in reducing potential biases and improving the robustness of comparisons across studies.

Examining general outcomes, we observed an overall improvement in alignment with the use of PSSRs. A greater proportion of patients in the preserved and restored categories and fewer in the not corrected and worsened categories demonstrated improved alignment post-operatively. Specifically, 74% of patients in our PSSR cohort fell into the preserved category, indicating maintenance of spinopelvic alignment, while 18% were restored to appropriate alignment. In contrast, only 4% of patients were categorized as not corrected, suggesting minimal post-operative misalignment, and 4% were classified as worsened, indicating a slight deterioration in alignment. This suggests that PSSRs may offer better maintenance or correction of spinopelvic alignment compared to conventional rods.

Only two of our patients went from the aligned to the malaligned category with the use of PSSRs, placing them in the “worsened” category. The first of these cases was a patient who underwent a three-level ALIF with interbody fusions and instrumented posterior spinal fusion from L3–S1. Pre-operatively, LL was −48.4°, sacral slope (SS) was 27°, PT was 26.9°, and PI was 53.9°. Post-operatively, LL was −43.1°, SS was 29.9°, PT was 24.1°, and PI was 54°. This resulted in a pre-operative PI-LL mismatch of 5.5° and a post-operative PI-LL of 10.9°, exceeding the threshold of 10° used to define malalignment and technically placing this patient in the “worsened” category. In the second case, a patient underwent two-level ALIF and instrumented posterior spinal fusion from L4 to S1. The pre-operative LL was −50.5°, SS was 35.5°, PT was 11.1°, and PI was 46.6°. Post-operatively, LL was −56.1°, SS was 34.2°, PT was 11.5°, and PI was 45.7°. This resulted in a pre-operative PI-LL of −3.9°, which post-operatively worsened to −10.4°, crossing the malalignment threshold by just 0.4°. While the clinical relevance of values so close to the threshold of 10° can be debated, they technically place patients in the “malaligned” category even with use of a PSSR. This suggests that even with advanced PSSR technology, their use does not guarantee appropriate correction in alignment.

Even patients categorized as not corrected showed substantial improvement in pre- to post-operative PI-LL parameters. Despite residual PI-LL exceeding 10°, these patients exhibited significant improvement compared to the non-custom rod cohort, with an average improvement of over 20° compared to only 2° in the non-custom rod cohort. Furthermore, patients categorized as worsened in the Leveque cohort experienced nearly double the increase in PI-LL compared to those in the custom rod cohort.

While the radiographic improvements observed with PSSRs are promising, their clinical significance must be considered. Improved PI-LL alignment can lead to better distribution of spinal loads and reduce the risk of adjacent segment degeneration, a common complication in lumbar fusion surgeries. Additionally, effective sagittal alignment correction is associated with reductions in back pain and disability. Studies have shown that optimal alignment correlates with better HRQOL scores, indicating improved patient outcomes (13,21). Although our study primarily focuses on radiographic changes, future research should integrate patient-reported outcomes to fully understand how these improvements translate into clinical benefits.

Our study included lumbar fusions performed via ALIF and TLIF, with no significant differences in radiographic parameters were noted between the two procedures (24). This suggests that both techniques can effectively contribute to spinopelvic alignment improvement.

This study represents the first analysis, to our knowledge, of the use of custom PSSRs for degenerative lumbar fusions. Despite promising results, additional limitations warrant consideration. We were constrained by the retrospective nature of the study, necessitating comparison with a historical cohort, which may introduce biases. However, it’s noteworthy that this historical cohort spanned 18 institutions with over 500 patients, potentially accounting for variances in surgical preferences, technologies, and techniques across the country. Furthermore, our study did not evaluate global spinopelvic alignment parameters, specific patient-reported outcomes, or complication rates. Despite these limitations, our findings provide valuable insights into the potential of custom spine rods to enhance spinopelvic alignment and patient outcomes, laying the groundwork for future research in this area.

Conclusions

In summary, our study emphasizes the significant impact of PSSRs in enhancing spinopelvic parameters in short segment lumbar fusions for degenerative conditions. Patients who underwent fusion with PSSRs experienced a notable improvement in PI-LL and demonstrated a heightened capacity to achieve pre-operative alignment objectives. These findings highlight the efficacy of custom rods in facilitating the restoration or preservation of spinopelvic alignment during lumbar fusion procedures. Considering these results, the integration of PSSRs should be carefully considered to optimize surgical outcomes and enhance patient care in this context.

Acknowledgments

Special thanks to Christina Khat, BS, from School of Medicine, University of Colorado Orthopedics, Aurora, CO for her assistance in manuscript preparation and submission.

Footnote

Reporting Checklist: The authors have completed the TREND reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-24-75/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-24-75/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-24-75/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-24-75/coif). The 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 the Colorado Multiple Institutional Review Board (COMIRB) (No. 20-2245) 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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