Accuracy of patient-specific rods in long segment multilevel fusions: a single-institution study

Introduction

Adult spinal deformity (ASD) is associated with substantial pain, disability, and reductions in health-related quality of life (HRQoL) (1,2). Correcting sagittal malalignment is a principal driver of clinical improvement in ASD, and contemporary treatment paradigms emphasize restoration of spinopelvic harmony (3). The Scoliosis Research Society-Schwab (SRS-Schwab) framework operationalizes sagittal alignment with three modifiers, pelvic incidence-lumbar lordosis (PI-LL), sagittal vertical axis (SVA), and pelvic tilt (PT) that correlate with function and HRQoL and provide practical postoperative targets (PI-LL ≤10°, SVA ≤40 mm, PT ≤20°) (4). In parallel, the T1 pelvic angle (TPA) (target ≤20°) has been introduced as a global metric that incorporates both spinal inclination and pelvic retroversion and is less sensitive to radiographic magnification and patient positioning (5). Taken together, these parameters furnish a common language for preoperative planning, intraoperative decision making, and postoperative assessment in ASD (4,5).

Achieving these goals in multilevel reconstruction remains challenging. Soft tissue constraints, osteotomy mechanics, and postoperative settling can produce undercorrection or overcorrection, with the latter implicated in mechanical complications such as proximal junctional kyphosis (PJK) (3). While these concerns are central in deformity correction, long segment fusions performed for non-deformity indications also require careful consideration of sagittal balance. Even in patients with acceptable preoperative spinopelvic parameters, maintaining LL across extended constructs is critical to preserving function and preventing loss of alignment over time.

As a response, patient-specific rods (PSRs) have been adopted to translate digital plans into pre-bent constructs, with multiple series reporting high plan to execution fidelity across key sagittal parameters (6-8). PSRs may therefore serve a dual purpose: improving correction in misaligned patients and preserving favorable baseline alignment in non-deformity cases. They have also been hypothesized to reduce manual contouring related stress and may lower rod failure rates over time; however, early postoperative intervals are typically too short to observe rod fractures.

Despite advances in planning frameworks and alignment targets, postoperative sagittal outcomes after multilevel fusion using traditional intraoperative rod contouring remain variable, even in contemporary cohorts. Recent large series demonstrate that early postoperative global and regional alignment often falls short of intended targets, underscoring an execution gap between surgical planning and achieved correction (9). Against this background, contextual comparison to modern traditional-rod benchmarks provides a useful reference for evaluating the accuracy and consistency of newer alignment-executing technologies.

In this study, we evaluate the real-world accuracy of PSRs in achieving planned alignment across four sagittal parameters, PI-LL, SVA, TPA, and PT in patients undergoing multilevel reconstructive surgery, including both deformity and non-deformity cases. We hypothesize that PSRs facilitate close adherence to preoperative targets, improve attainment of established radiographic thresholds, and help prevent loss of lordosis in long segment fusions. We present this article in accordance with the STROBE reporting checklist (available at https://jss.amegroups.com/article/view/10.21037/jss-2025-1-249/rc).

Methods

Patient selection

We conducted a retrospective review of adult patients who underwent multilevel (≥3 levels) thoracolumbar fusion for spinal deformity using PSRs at Indiana University School of Medicine between January 2022 and December 2023. Patients were included if they had complete radiographic imaging at three timepoints: preoperative, planned, and postoperative (obtained between 6 and 12 weeks after surgery). This early postoperative interval was selected to evaluate plan-to-execution accuracy of sagittal correction, whereas longer clinical follow-up was used to assess early mechanical complications and revision events. Exclusion criteria included prior long segment fusion that overlapped with the planned construct, inadequate imaging, or surgery performed without PSR integration. The final cohort consisted of 18 patients.

Surgical technique and rod design

All procedures were performed by experienced spine surgeons using a posterior approach, with some cases including interbody fusion at select levels [e.g., oblique lumbar interbody fusion (OLIF) or transforaminal lumbar interbody fusion (TLIF)]. Preoperative planning was completed using dedicated software (UNiD^TM Adaptive Spine Intelligence, Medicrea), which generated target LL and PI-LL based on the patient’s PI, sagittal alignment, and age. Once the alignment plan was finalized, custom-contoured titanium rods were manufactured and delivered for use in surgery. The rods were designed to match sagittal alignment targets only; coronal plane deformities were addressed intraoperatively by conventional techniques, as the current iteration of the UNiD system does not permit three-dimensional rod customization.

Radiographic analysis

Measured parameters were LL, PI, PI-LL, SVA, PT, and TPA from standardized lateral full spine radiographs at the three time points. “Fixed” severity categories followed widely used thresholds: PI-LL ≤10°, SVA ≤40 mm; PT ≤20° (SRS-Schwab modifiers). TPA categorization used a target ≤20° from the original TPA report. These fixed cutoffs are displayed in Figure 1 (4,5). In addition, we performed a complementary, age-adjusted analysis (Figure 2) based on Lafage and colleagues’ age-stratified normative models that link Oswestry Disability Index (ODI)/physical component score (PCS) to radiographic alignment. In this framework, PT, PI-LL, SVA, and TPA targets vary by age band (<35, 35–44, 45–54, 55–64, 65–74, ≥75 years). Each patient’s value was classified as meeting or not meeting the age-appropriate target for their band, providing a second, clinically relevant lens on postoperative goal attainment (9).

Figure 1 Target measurements (fixed thresholds). This displays normal/moderate/severe distributions pre- and post-operatively for PI-LL, PT, SVA, and TPA using conventional cut-offs, PI-LL ≤10°, PT ≤20° and SVA ≤40 mm from the SRS-Schwab adult spinal deformity modifiers (Schwab et al., Spine 2013), and TPA ≤20° from the original description of T1 pelvic angle (Protopsaltis et al., Spine 2014). The pie charts illustrate substantial migration from moderate/severe to normal after surgery, especially for SVA) and PI-LL. PT shows smaller categorical movement, and TPA improves but remains more evenly distributed across categories, mirroring clinical experience that pelvic measures often lag behind global realignment. LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt; SRS-Schwab, Scoliosis Research Society-Schwab; SVA, sagittal vertical axis; TPA, T1 pelvic angle.

Figure 2 Target measurements (age adjusted). This figure classifies each parameter as abnormal (white) or target (gray) using age-adjusted thresholds from Lafage et al. (Spine 2016). Specifically, PT, PI-LL, SVA, and TPA cut-points vary by age band (<35, 35–44, 45–54, 55–64, 65–74, ≥75 years) derived from regression models that map age and HRQoL (ODI/PCS) to radiographic alignment. The pie charts show a clear shift toward green after surgery, most prominently for global parameters (SVA, TPA), with PI-LL and PT also moving toward age-appropriate targets but to a lesser extent, consistent with the notion that pelvic compensation normalizes more gradually than global alignment. HRQoL, health-related quality of life; LL, lumbar lordosis; ODI, Oswestry Disability Index; PCS, physical component score; PI, pelvic incidence; PT, pelvic tilt; SVA, sagittal vertical axis; TPA, T1 pelvic angle.

Overcorrection analysis

To evaluate the frequency of excessive correction, postoperative radiographs were screened for values exceeding physiologic alignment ranges based on SRS-Schwab modifiers (4). PI-LL <0°, SVA <0 mm, and PT <10° were defined as overcorrection thresholds, reflecting reversal beyond neutral alignment. PI was excluded as an anatomic constant, and TPA was not analyzed for overcorrection because current literature defines only undercorrection thresholds (≤20°). The count and proportion of patients meeting these criteria were summarized in Table 1.

Statistical analysis

Paired t-tests assessed change from preoperative to postoperative and from planned to postoperative for each radiographic parameter (α=0.05). Target attainment rates were summarized under two frameworks: fixed thresholds (Figure 1) and age-adjusted targets (Figure 2). This was an exploratory, single-institution series; we did not perform a formal sample size or power calculation, and no adjustments for multiple comparisons were applied. P values are descriptive and should be interpreted alongside effect sizes and target attainment rates.

To contextualize postoperative alignment outcomes, group-level radiographic data from a contemporary cohort treated using traditional intraoperative rod contouring were used as an external historic reference. Specifically, early postoperative (0–3 months) alignment values reported by Hiltunen et al. were extracted for LL, PI-LL mismatch, SVA, PT, and TPA. Because patient level data were unavailable and cohorts were not matched, between-group comparisons were limited to descriptive contrasts using standardized mean differences (Cohen’s d), which quantify effect magnitude without formal hypothesis testing (10).

Ethical consideration

The study was conducted in accordance with the Declaration of Helsinki and its subsequent amendments. The study was approved by the institutional review board of Indiana University School of Medicine (No. 27470), and informed consent was waived due to the retrospective design.

Results

Demographics and surgical characteristics

Eighteen patients met inclusion criteria (Table 2). Case-level surgical characteristics and early mechanical outcomes, including fusion length, revision status, follow-up duration, and occurrence of PJK, are summarized in Table 3. The cohort had a mean age of 61.1±16.0 years, with 72% female. The mean body mass index (BMI) was 29.2±5.1 kg/m², and 88% identified as White. Common comorbidities included diabetes (24%), coronary artery disease or heart failure (24%), autoimmune disease (17%), and osteoporosis (19%). Half of the cases were revision surgeries, and one third involved interbody procedures (OLIF/TLIF). The mean planned fusion length was 5.78±2.65 levels, with 2.7±1.4 planned osteotomies. Mean estimated blood loss was 1,080.6±475.6 mL, and average length of stay was 8.7±5.7 days. PJK occurred in 11% of patients.

Radiographic outcomes

Significant improvements were observed from preoperative to postoperative timepoints for LL, PI-LL, SVA, and TPA, while PT changes did not reach statistical significance (Table 4). Specifically, LL increased from 37.6° to 50.4° (P=0.002), PI-LL decreased from 19.9° to 6.8° (P=0.003), SVA improved from 66.3 to 26.6 mm (P=0.007), and TPA decreased from 23.8° to 17.5° (P=0.03). PI remained stable, as expected for a fixed anatomical parameter. Comparison of planned and postoperative measurements revealed no statistically significant differences for LL, PI-LL, SVA, TPA, or PT, indicating high execution fidelity of the surgical plan (Table 4). The largest mean baseline-to-postoperative improvements were seen in SVA (−39.7 mm) and PI-LL (−13.1°) (Table 5). Additionally, postoperative overcorrection frequencies are summarized in Table 1. Overall, 16.7% of patients showed PI-LL <0°, and 16.7% had negative SVA values, while only 5.6% demonstrated PT <10°, suggesting that true radiographic overcorrection was uncommon.

To contextualize the magnitude of postoperative alignment achieved with PSRs, early postoperative radiographic outcomes were compared with a contemporary historic cohort treated using traditional intraoperative rod contouring (Table 6). The external control cohort reported by Hiltunen et al. included early postoperative (0–3 months) group-level alignment values for LL, PI-LL mismatch, SVA, PT, and TPA and reflects modern deformity correction using conventional rod bending techniques (10). Relative to this benchmark, the PSR cohort demonstrated a moderate standardized effect favoring improved global alignment as measured by SVA (Cohen’s d=−0.47), while differences in PI-LL, PT, and TPA were negligible. LL values were modestly lower in the PSR cohort (Cohen’s d=−0.46), without evidence of excessive correction or adverse pelvic compensation.

Individual alignment trajectories

Figure 3A-3F illustrates alignment changes across preoperative, planned, and postoperative timepoints. LL, PI-LL, SVA and TPA demonstrated substantial planned improvements that were largely maintained postoperatively. PT changes were modest and more variable. These visual trends underscore the high degree of alignment retention relative to the surgical plan, particularly for SVA and PI-LL.

Figure 3 Temporal changes in sagittal alignment parameters from preoperative to planned and postoperative states. (A-F) depict the temporal changes in key sagittal alignment parameters from preoperative to planned and postoperative states. (A) shows a significant increase in LL from preoperative to planned values, with a slight decrease postoperatively while maintaining improvement from baseline. (B) demonstrates PI remained stable across all timepoints, consistent with its nature as a fixed anatomical parameter. (C) highlights a marked reduction in PI-LL from preoperative to planned correction, with partial rebound postoperatively but still improved relative to baseline. (D) reveals a substantial decrease in SVA following surgical planning, with maintenance of near-target alignment postoperatively. (E) shows TPA decreased significantly from preoperative to planned alignment, with a small postoperative increase yet remaining improved. (F) illustrates PT reduction from preoperative to planned values, followed by a modest postoperative increase, maintaining gains over preoperative status. LL, lumbar lordosis; PI, pelvic incidence; PT, pelvic tilt; SVA, sagittal vertical axis; TPA, T1 pelvic angle.

Target-based alignment outcomes

Categorical distributions improved postoperatively using conventional cutoffs. Normal SVA (≤40 mm) increased from 33% to 83%; PI-LL ≤10° from 39% to 67%; TPA ≤20° from 44% to 67%; PT ≤20° remained 44% (with a shift from severe to moderate). Using the age-stratified framework described in Methods, the proportion meeting age-appropriate targets also increased after surgery. Normal classifications improved from 11.1% to 44.4% for PI-LL, 38.9% to 72.2% for SVA, 33.3% to 44.4% for PT, and 33.3% to 66.7% for TPA. These pies demonstrate a complementary perspective to Figure 2, with a larger share of patients achieving alignment appropriate for their age group postoperatively.

Discussion

In this single institution cohort, PSRs were associated with favorable changes in LL, PI-LL, SVA, and TPA, with high plan to execution concordance, and higher rates of meeting spinopelvic targets under both frameworks, fixed thresholds (Figure 1) and age-adjusted targets (Figure 2). These patterns are consistent with prior syntheses and clinical series demonstrating PSR feasibility and strong fidelity between planned and achieved sagittal alignment (5-8).

When contextualized against contemporary benchmarks for traditional intraoperative rod contouring, early postoperative alignment achieved with PSRs compared favorably for global balance while remaining similar for spinopelvic parameters (Table 6). Relative to the historic cohort reported by Hiltunen et al., PSR use demonstrated a moderate standardized effect favoring lower postoperative SVA, whereas differences in PI-LL, PT, and TPA were negligible. LL values were modestly lower in the PSR cohort without evidence of excessive correction, suggesting that PSRs may enhance execution consistency rather than promote overcorrection (10).

Radiographic changes were most pronounced for PI-LL and SVA, with most patients shifting from moderate or severe malalignment into the normal range postoperatively. Importantly, as shown in Table 1, overcorrection below physiologic targets occurred in only a minority of cases (<17% in all measured variables), reinforcing that PSRs achieve precise but not excessive correction. Based on Figure 1, 83% achieved SVA ≤40 mm, 67% met PI-LL ≤10°, 44% met PT ≤20°, and 67% reached TPA ≤20°. Figure 3 shows these gains were largely preserved from plan to postoperative measurement, especially for SVA, PI-LL and TPA, whereas PT displayed greater variability, aligning with known resistance of pelvic parameters to full normalization.

Diebo et al. described “modern iatrogenic flatback”, showing persistent postoperative malalignment despite greater awareness (11). Similarly, Barrey and Darnis [2015] emphasized that failure to restore lordosis leads to poor outcomes, including adjacent segment degeneration and disability (12). Despite advances in preoperative planning and intraoperative execution, iatrogenic flatback syndrome remains a significant concern after multilevel fusion. Inadequate restoration or loss of LL contributes to sagittal imbalance, chronic pain, and need for revision. Symptomatic loss of lordosis, a precursor to iatrogenic flatback syndrome, has been reported in about 5% of fusion patients at mid-term follow-up (1). More recently, up to 38% of short-segment fusions show hypo-lordotic alignment, a finding associated with higher revision risk and progression to sagittal imbalance (4). These findings highlight that PSRs may help preserve lordosis and reduce the risk of alignment loss in long fusions for non-deformity indications.

Our plan fidelity mirrors the systematic review and single-center experiences reporting accurate execution with pre-bent constructs, and aligns with broader reports that PSR use can influence sagittal parameters beyond classic long segment ASD, including shorter degenerative constructs, suggesting generalizability as the technology matures (5-7,13-15). Because PSRs execute the plan, preoperative templating and its predictive validity are critical: standardized digital planning workflows and data on plan accuracy support the notion that precise plans are a prerequisite for precise rods (16,17).

Not all parameters corrected equally. PT improved yet showed greater variance, consistent with pelvic compensation normalizing more slowly than global balance. Clinically, both under and over correction have been linked to mechanical complications such as PJK (18-20). Age-adjusted alignment concepts further suggest that overshooting age ideal targets, rather than merely exceeding fixed cutoffs, may elevate PJK risk, supporting our dual framework (fixed and age-adjusted) for interpreting outcomes (21,22).

Cost and resource utilization are important considerations when adopting PSR technology. Although PSRs incur higher upfront implant costs compared with conventionally contoured rods, their potential value may lie in improved execution consistency, reduced intraoperative rod manipulation, and mitigation of alignment variability rather than immediate perioperative efficiency gains. In this series, operative time, blood loss, and length of stay were not directly compared with a non-PSR cohort, and no conclusions regarding cost-effectiveness can be drawn. Further studies incorporating operative efficiency metrics, longer-term mechanical outcomes, and revision rates will be necessary to determine whether the added cost of PSRs is offset by downstream clinical or economic benefits.

This study has several limitations. Its retrospective design, small sample size, and single-institution experience limit generalizability and preclude definitive conclusions regarding comparative effectiveness. In addition, comparison with traditional rod contouring was performed using a contemporary historic cohort rather than a matched internal control. Because patient-level data from the external cohort were unavailable, between group differences were assessed using standardized effect sizes rather than formal hypothesis testing, and these comparisons are intended to provide contextual benchmarking of alignment magnitude and consistency rather than establish comparative efficacy. Finally, follow-up was limited to early postoperative radiographs (6–12 weeks) for assessment of alignment accuracy, and longer-term radiographic durability and clinical outcomes were not assessed. Additionally, patient reported outcomes (PRO) were not collected in this series, as the primary objective was to evaluate radiographic plan to execution accuracy rather than short term clinical response; incorporation of validated PRO measures will be important in future studies assessing the clinical impact of this technology.

Conclusions

In this single-institution series, PSRs were associated with close adherence to planned sagittal alignment and favorable early postoperative radiographic profiles. Most patients achieved established spinopelvic targets with low rates of radiographic overcorrection. When viewed against contemporary traditional rod contouring benchmarks, PSR-assisted constructs demonstrated comparable spinopelvic alignment and favorable global balance. Although limited by sample size and follow-up duration, these findings illustrate the potential of PSRs to support accurate alignment execution in long-segment thoracolumbar fusion.

Acknowledgments

None.

Footnote

Reporting Checklist: The authors have completed the STROBE reporting checklist. Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-1-249/rc

Data Sharing Statement: Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-1-249/dss

Peer Review File: Available at https://jss.amegroups.com/article/view/10.21037/jss-2025-1-249/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-2025-1-249/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 and its subsequent amendments. The study was approved by the institutional review board of Indiana University School of Medicine (No. 27470), and informed consent was waived due to the retrospective design.

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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