Abstract
Background Prone transpsoas lateral lumbar interbody fusion (PTP-LLIF) enables simultaneous lateral interbody access and posterior instrumentation from a single prone position. While expandable interbody cages have demonstrated improved radiographic restoration in other lumbar fusion approaches, their outcomes in PTP-LLIF remain poorly defined. This study compares radiographic and perioperative outcomes between static and expandable cages in single-level PTP-LLIF.
Methods A retrospective cohort study was conducted at a single academic center evaluating patients who underwent single-level PTP-LLIF with posterior instrumentation. Patients were stratified by cage type (static vs expandable). Standing full-length radiographs were analyzed preoperatively, early postoperatively, and at latest follow-up. Outcomes included disc height, foraminal height, disc angle, segmental lordosis, lumbar lordosis, spinopelvic alignment, and perioperative variables. An additional 1:1 matched cohort analysis was performed based on age, body mass index, American Society of Anesthesiologists classification, operative level, preoperative foraminal height, and preoperative segmental lordosis.
Results A total of 45 patients met inclusion criteria, including 38 (84%) with static cages and 7 (16%) with expandable cages. Expandable cages demonstrated significantly greater early postoperative anterior disc height (7.1 ± 1.8 vs 5.0 ± 2.2 mm, P = 0.026) and posterior disc height (4.3 ± 1.8 vs 2.6 ± 2.0 mm, P = 0.05). Posterior disc height remained significantly greater at late follow-up (P = 0.032). Expandable cages were also associated with greater increases in lumbar lordosis (11.4° ± 4.9° vs 4.0° ± 4.8°, P = 0.006 early; 11.6° ± 4.2° vs 3.0° ± 4.8°, P < 0.001 late) and segmental lordosis (7.9° ± 3.8° vs 3.8° ± 3.4°, P = 0.028 late). Postoperative pelvic incidence–lumbar lordosis mismatch trended lower in the expandable cohort (P = 0.092 early, 0.068 late). Perioperative outcomes, including estimated blood loss, operative time, and length of stay, were comparable between groups. Similar findings were observed in the matched cohort analysis.
Conclusions Expandable cages in PTP-LLIF were associated with greater improvements in disc height and sagittal alignment parameters compared with static cages. Findings were consistent with literature on other lumbar fusion approaches. Given the small sample size and potential for confounding by indication, these findings should be interpreted cautiously.
Clinical Relevance Expandable cage technology may enhance sagittal alignment correction in PTP-LLIF, though further study is required to determine independent contribution and clinical impact.
Level of Evidence 3.
- prone transpsoas
- lateral lumbar interbody fusion
- expandable cage
- segmental lordosis
- sagittal alignment
- minimally invasive spine surgery
Introduction
Over the past 2 decades, spine surgery has shifted toward minimally invasive surgical (MIS) techniques aimed at reducing surgical footprint while preserving the biomechanical objectives of fusion.
Among these techniques, the lateral lumbar interbody fusion (LLIF), which accesses the intervertebral disc space through a retroperitoneal transpsoas corridor, has become increasingly prevalent. LLIF can facilitate restoration of disc height, achieving indirect neural decompression while avoiding many of the morbidity risks associated with anterior and posterior fusions.1
Prone transpsoas (PTP) LLIF represents a recent technical evolution. By allowing lateral interbody access and posterior instrumentation from a single prone position, PTP may improve operative efficiency while preserving radiographic objectives of lateral fusion.2 The natural lordosis created by prone positioning may also facilitate sagittal alignment correction, and gravity additionally allows for abdominal contents to be drawn away from the spine, creating unobstructed corridors.2,3
Restoration of sagittal alignment, particularly segmental and lumbar lordosis (LL), remains an important objective in lumbar fusion owing to their association with improved pain, functional, and clinical outcomes.4–7 Additionally, radiographic restoration is critical in preventing postoperative deformity and adjacent segment disease.8,9 Several radiographic parameters, including anterior disc height (ADH), posterior disc height (PDH), foraminal height (FH), segmental lordosis (SL), LL, and spinopelvic alignment, are commonly used to evaluate the effectiveness of interbody fusion procedures.
Interbody cage design may influence these outcomes. Traditionally, fusions have used static cages; these cages are fixed in size and lordotic angle, necessitating repetitive trialing. Therefore, customization to a patient’s anatomy is limited by incision windows and stepwise size constraints in trials. These insertion mechanics may also increase the risk of endplate violation and nerve root injury.10
Expandable cages have been developed to address these challenges. These implants can be inserted in a collapsed configuration and expanded across height and/or lordosis after placement within the disc space, allowing insertion through narrow surgical corridors with subsequent in situ adjustment to patient-specific disc-space anatomy. Such cages have seen utilization across transforaminal lumbar interbody fusion (TLIF), anterior lumbar interbody fusion, and LLIF to improve disc height and correct lordosis.11
Several systematic reviews and meta-analyses have examined the radiographic advantages of expandable cages in lumbar fusion, finding greater ADH, SL, and FH restoration in expandable cages.12–14 However, despite the increasing prevalence of both expandable cages and PTP, the outcomes of expandable cages in PTP remain undefined in the literature.
Therefore, we performed a retrospective cohort study comparing radiographic and perioperative outcomes between static and expandable cages in single-level PTP-LLIF with posterior instrumentation. We hypothesized that expandable cages in PTP would be associated with greater restoration of ADH, PDH, LL, and SL while maintaining similar perioperative outcomes.
Methods
Study Design and Ethics Approval
A retrospective cohort analysis was performed. Two surgeons’ case logs at a single academic institution were reviewed for patients who underwent single-level, single-stage PTP-LLIF with supplemental posterior instrumentation. The study was conducted under an existing protocol approved by the Duke Health Institutional Review Board and determined to be exempt from review (Protocol #Pro00071025). Due to the retrospective nature of the study, the requirement for informed consent was waived.
Patient Selection
We reviewed the case logs of 2 fellowship-trained spine surgeons at a single institution to identify all patients who met the following criteria:
Age ≥18 years.
Single-level PTP performed in a single operative stage.
Supplemental posterior pedicle screw fixation performed during the same anesthetic event.
Availability of preoperative standing full-length radiograph, immediate/early postoperative standing full-length radiograph, and later available standing full-length radiograph.
Patients were excluded from the study if they did not meet the inclusion criteria (Figure 1). Commonly, this included patients who had standalone PTP without posterior instrumentation (n = 10), multilevel PTPs (n = 31), staged procedures (n = 32), and prior fusion at the index level (n = 4).
Cohort derivation flow diagram. Patients were excluded for multilevel procedures, staged operations, standalone PTP without posterior instrumentation, or prior index-level fusion. The final cohort was stratified by cage type, and a 1:1 matched cohort was subsequently derived. LLIF, lateral lumbar interbody fusion; PTP, prone transpsoas.
Collected baseline variables included age, body mass index (BMI), American Society of Anesthesiologists classification, sex, surgical history, operative level, primary surgical indication, and radiographic follow-up intervals. Primary surgical indication was abstracted from operative reports and preoperative clinical documentation. When multiple pathologies were present, a single primary indication was assigned based on the principal pathology motivating the index fusion.
Cage Type
Patients were stratified into static and expandable cage cohorts. Static cage models included the IdentiTi-LIF (Alphatec Spine, Carlsbad, CA), Modulus XLW (NuVasive, San Diego, CA), EIT-LLIF (DePuy Synthes, Raynham, MA), Nano Titan (Medtronic, Minneapolis, MN), Cohere XLW (NuVasive, San Diego, CA), Modulus XL (NuVasive, San Diego, CA), and Coroent XL Wide (NuVasive, San Diego, CA). Expandable cage models used included the Duo Ti (Spineology Inc., St. Paul, MN), ELSA (Globus Medical, Audubon, PA), and RISE-L cages (Globus Medical, Audubon, PA).
Intraoperative cage selection was performed at the discretion of the operating surgeon based on intraoperative assessment of patient-specific anatomy, including disc collapse, desired lordotic correction, and endplate quality. All cages were placed via a PTP-LLIF approach.
Radiographic Assessment
Radiographic analysis was performed using standing full-length lateral radiographs obtained at the latest preoperative baseline, earliest postoperative interval, and latest postoperative follow-up (Figure 2). For patients with any subsequent lumbar fusion surgery, the latest postoperative radiograph was collected prior to downstream operation. All measurements were performed using digital Picture Archiving and Communication System (PACS) software by 2 independent reviewers trained in spinal radiographic analysis using standardized anatomical landmarks. Measurements were performed independently, and discrepancies were resolved by consensus prior to final analysis. Investigators were blinded to clinical treatment groups and outcomes during measurement.
Preoperative (A and D), immediate postoperative (B and E), and late postoperative (C and F) sagittal radiographs for patients having undergone expandable (A, B, and C) and static (D, E, and F) cage placement.
Radiographic parameters collected included ADH, PDH, FH, disc angle, SL, LL, pelvic incidence (PI), pelvic tilt, sacral slope, and PI-LL mismatch (Figure 3).
Representative standing lateral radiograph demonstrating measurement of anterior disc height (green), posterior disc height (blue), segmental lordosis (yellow), and lumbar lordosis (purple).
SL was defined as the Cobb angle between the superior endplate of the cranial vertebra and inferior endplate of the caudal vertebra at the index level. LL was defined as the Cobb angle measured from the superior endplate of L1 to the superior endplate of S1.
For each parameter, we calculated the preoperative value, immediate change from preoperative baseline at the earliest available postoperative radiograph, and change from preoperative baseline at the latest available radiograph.
Secondary Operative Outcomes
Perioperative outcomes collected included estimated blood loss (EBL), operative time, total length of hospital stay, and intraoperative complications. Intraoperative complications included nerve root injury, dural tear, vascular injury, or endplate violation and were collected through retrospective review of operative reports and inpatient perioperative documentation. Broader postoperative complications, postdischarge 30-day events, readmissions, reoperations, and long-term adverse events were not systematically captured and were therefore not analyzed as comparative outcomes.
Statistical Analysis
Continuous variables were assessed for normality using Shapiro-Wilk testing. Data are presented as mean ± SD. Between-group comparisons were performed using independent-sample Student’s t tests. Within-group pre- and postoperative comparisons were performed using paired Student’s t tests. Mean differences were calculated with an accompanying 95% confidence interval. A nonparametric approach through Mann-Whitney U testing was conducted for sensitivity analysis. Effect sizes for continuous between-group comparisons were calculated using Cohen’s d.
Significance testing for categorical variables was done using Fisher’s exact testing or Fisher-Freeman-Halton testing as appropriate. All statistical tests were 2-sided, with significance set at α = 0.05.
Matched Cohort Analysis
A matched cohort analysis was performed to address the potential for confounding by indication, as cage selection was not randomized, and to address cohort imbalance from the limited sample size of expandable cages. Each patient in the expandable cage cohort (n = 7) was matched 1:1 with a patient from the static cage cohort using nearest-neighbor matching without replacement. Matching variables were selected based on potential influence on intraoperative cage selection and postoperative radiographic outcomes and included preoperative SL, preoperative FH, operative level, age, BMI, and ASA classification.
Following matching, baseline demographics and operative variables were reassessed between groups to confirm balance. Standardized mean differences in the form of Cohen’s d were calculated for matched covariates to assess for adequacy of matching, with values <0.20 considered acceptable balance. Radiographic and perioperative outcomes were compared using paired t tests; this approach was chosen to evaluate mean within-pair differences between each expandable-cage patient and their matched static-cage comparator to preserve the matched-pair structure and account for correlation induced by matching. As a sensitivity analysis, nonparametric paired testing was also conducted using Wilcoxon signed-rank testing.
Results
Demographics and Baseline Comparison
A total of 45 patients met inclusion criteria, including 38 static cage patients (84%) and 7 expandable cage patients (16%). Mean age was 63.0 ± 10.4 years, mean BMI was 30.5 ± 5.4, 18 patients were men (40%), and 20 patients (44%) had prior fusion. There were no statistically significant demographic differences between cohorts (Table 1).
Demographics of patients undergoing single-level prone transpsoas lateral lumbar interbody fusion.
Primary surgical indication was most commonly spondylolisthesis (27/45, 60%), followed by degenerative disc disease/spondylosis (11/45, 24%), adjacent segment disease (3/45, 7%), lumbar/foraminal stenosis (2/45, 4%), and facet arthropathy (2/45, 4%); the distribution did not differ significantly between cohorts (P = 0.291).
Mean radiographic intervals were 107.1 ± 72.2 days preoperatively, 16.4 ± 25.1 days early postoperatively, and 569.5 ± 410.0 days at latest follow-up.
Cage Characteristics
A detailed comparison of implant characteristics is presented in Table 2. There were no significant differences in cage footprint (969.2 ± 182.5 vs 900.0 ± 326.6 mm2, P = 0.556) or lordotic angle (10.1° ± 0.6° vs 8.7° ± 6.4°, P = 0.595) between static and expandable cohorts. However, expandable cages achieved significantly greater final implant height despite comparable footprint and lordotic angle (13.5 ± 2.4 mm vs 9.2 ± 3.2 mm, P = 0.005). Expandable cages were inserted in a collapsed configuration and expanded in situ, with 71.4% allowing simultaneous height and lordotic expansion. While all patients received posterior instrumentation, specific fixation techniques, including rod contouring and compression, were not standardized and were applied at surgeon discretion in both groups.
Cage characteristics.
Perioperative Outcomes
Perioperative outcomes did not differ significantly between cohorts (Table 3). Static and expandable cohorts had similar EBL (85.9 ± 92.6 vs 72.1 ± 26.1 mL, P = 0.448) and operative time (187.7 ± 101.3 vs 187.9 ± 22.0 min, P = 0.991). Length of stay was 2.5 ± 1.6 days in the static cohort and 3.9 ± 2.0 days in the expandable cohort (P = 0.12).
Operative metrics of patients undergoing single-level prone transpsoas lateral lumbar interbody fusion.
Radiographic Comparisons
Expandable cages were associated with significantly greater early postoperative ADH (7.1 ± 1.8 vs 5.0 ± 2.2 mm, P = 0.026), although late ADH was not significantly different (P = 0.16). Early and late postoperative PDH was greater in the expandable cohort as well (4.3 ± 1.8 vs 2.6 ± 2.0 mm, P = 0.05; 3.7 ± 1.1 vs 2.5 ± 1.9 mm, P = 0.032), though these findings did not remain significant on sensitivity analysis (P = 0.062 and 0.076, respectively). Expandable cages were also associated with greater LL change at early and late follow-up (11.4° ± 4.9° vs 4.0° ± 4.8°, P = 0.006; 11.6° ± 4.2° vs 3.0° ± 4.8°, P < 0.001) and greater SL change at late follow-up (7.9° ± 3.8° vs 3.8° ± 3.4°, P = 0.028), with early SL change trending toward significance (P = 0.062). Early and late PI-LL mismatch favored the expandable cohort directionally on t testing (P = 0.092 and P = 0.068, respectively), while Mann-Whitney U sensitivity analysis reached statistical significance (P = 0.024 and P = 0.010, respectively). Given this discordance, these findings were interpreted cautiously. Postoperative FH, disc angle, PI, pelvic tilt, and sacral slope were not significantly different. Preoperative FH was significantly larger in the static cohort (16.2 ± 3.1 vs 11.9 ± 3.1 mm, P = 0.01); no other preoperative radiographic measures differed significantly (Table 4).
Radiographic outcomes of patients undergoing single-level prone transpsoas lateral lumbar interbody fusion.
Matched Cohort Analysis
A total of 7 patients from the expandable cage cohort were matched to 7 patients from the static cage cohort. Baseline demographics were well balanced between groups, with age, BMI, ASA classification, operative level, preoperative FH, and preoperative SL all having standardized mean differences <0.2 (Table 5). All 14 patients had spondylolisthesis as the primary surgical indication.
Matched demographics.
EBL, operative time, and length of stay remained similar between cohorts, though length of stay trended higher in the expandable cage cohort (P = 0.072).
In the matched cohort, expandable cages continued to be associated with significantly greater improvements in LL (Δ early: 11.4° vs 3.0°, P = 0.010; Δ late: 11.6° vs 0.7°, P = 0.006) and SL (Δ late: 7.9° vs 2.4°, P = 0.024). Improvements in PI-LL mismatch were borderline in the expandable group at late follow-up (P = 0.05). Anterior disc height at early follow-up was greater in the expandable cohort (P = 0.044), but otherwise, differences in disc height and FH did not reach statistical significance in the matched analysis (Table 6).
Matched outcomes.
Discussion
This study represents, to our knowledge, one of the first direct comparisons of static and expandable interbody cages in PTP-LLIF. In this retrospective cohort study, patients who received expandable cages were found to have significantly greater postoperative improvements in ADH, PDH, LL, SL, and trended toward lower PI-LL mismatch.
A matched cohort analysis was performed to specifically address the marked cohort size imbalance and risk of confounding by indication inherent to this nonrandomized study. Matching prioritized preoperative SL, preoperative FH, and operative level, as these variables may reflect the primary intraoperative drivers of cage selection, namely the need for sagittal correction and indirect decompression. As such, these parameters serve as partial radiographic surrogates for the surgical decision to utilize an expandable vs static cage. After matching on these factors, the observed differences in LL and SL persisted, with late PI-LL mismatch reaching borderline significance, suggesting that the baseline differences in deformity severity or correction requirements may not fully account for the observed radiographic associations.
These findings suggest that expandable cage technology may be associated with greater sagittal alignment correction when used in the PTP approach.
Restoration of Disc Height and Indirect Decompression
Restoration of disc height and FH is central to indirect neural decompression in LLIF. In the overall cohort, expandable cages were associated with significantly greater early postoperative ADH and PDH, while only PDH remained significant at late follow-up. The loss of significance for late ADH may reflect graft settling, disc space remodeling, or limited power, whereas PDH stability may be greater in our study due to the axial load distribution conferred by the posterior instrumentation. Importantly, disc-height differences outside early ADH did not remain significant in matched analysis, suggesting that baseline anatomy, cage selection, or other technical factors may have contributed to overall cohort findings.
Despite the greater disc height restoration in overall analysis, postoperative FH did not differ significantly between cohorts. This may reflect the multifactorial determinants of foraminal dimensions, including disc height, facet joint orientation, and posterior column alignment.
Notably, preoperative FH was significantly larger in the static cage cohort. This may suggest preferential use of expandable cages in patients with more severe disc collapse or foraminal narrowing, reflecting surgeon selection based on intraoperative assessment. However, controlling for preoperative FH in the matched cohort analysis yielded similar radiographic outcomes nonetheless.
Our data are consistent with the limited findings available in the literature. Huo et al conducted a prospective cohort study comparing expandable cages to static cages in MIS-LLIF and similarly reported significantly greater postoperative ADH in expandable constructs. Though all surgeries were done with posterior instrumentation, postoperative PDH was not significantly different between the cohorts.15 Our finding of nonsignificant FH restoration differences was also shared by Hiyama et al, who examined early postoperative radiographs following MIS-LLIF.16 However, the study also found no difference in ADH or PDH between expandable and static cohorts, diverging from our results.
In the context of TLIFs, numerous studies find greater disc height restoration with expandable cages. Daher et al, a meta-analysis, supports significantly greater ADH, PDH, and FH increases with expandable cages.14 Multiple single-center retrospective cohorts correspondingly report significantly greater ADH and PDH increases in expandable cohorts.17,18
Sagittal Alignment and Lordotic Restoration
Restoration of sagittal alignment, particularly LL and SL, is among the most critical objectives in contemporary spine surgery given its strong correlation with clinical outcomes in the literature.6,7
In this study, expandable cages were associated with greater increases in SL and LL at late postoperative intervals and LL at early intervals, with similar findings persisting in matched analysis. These results may reflect biomechanical advantages of expandable cage design; since hardware lordosis is customizable for patient-specific anatomy, sagittal correction can be intraoperatively tailored. The significantly greater implant height observed in the expandable cohort suggests that the observed improvements in SL and LL may partially reflect differences in achieved disc space distraction. However, since footprint and intrinsic lordotic angles did not differ significantly between cohorts, this suggests that the mechanism of correction may be related to controlled in situ height restoration rather than the selection of larger-area implants.
Additionally, the persistence of lordotic restoration may suggest short- to intermediate-term maintenance of correction in this cohort, possibly due to limited endplate compromise or hardware migration; both of these have been noted in the literature to have decreased incidence with expandable cages.10 However, longer follow-up and fusion/subsidence assessment are needed to determine durability.
Multiple articles across LLIF and TLIF have highlighted the efficacy of expandable cages in achieving greater SL.7,10,19–21 Some of these studies have found similar degrees of SL increase with expandable cages as our findings of 7.9° ± 4.0° at early follow-up and 7.9° ± 3.8° at late follow-up. Bättig et al, who looked at LLIFs, report SL increases from 1.3° to 13.3° at 12-month follow-up. Jha et al similarly report an increase of 5.3° ± 7.5° at early follow-up and 5.7° ± 7.4° at late follow-up in TLIFs.
Theoretical synergy exists between expandable cages and the PTP approach to LLIF. Rohde et al, a systematic review and meta-analysis on PTP, identifies that PTP improves LL more effectively than traditional dual-position LLIF.22 A review by Jacome et al also finds superior SL restoration in PTP compared with traditional LLIF.2 Since PTP allows for posterior instrumentation to be installed in the same position as the intervertebral cage, cage-induced lordotic correction may be “locked in” without the worry of a positional change reducing cage-optimized lordosis. With our findings suggesting greater lordotic restoration with expandable cages, it is possible that a workflow combining PTP with expandable cages could facilitate more precise and durable control of final sagittal alignment; however, further randomized investigation would be required to definitively assess.
PI-LL Mismatch
Expandable cages in overall analysis trended toward lower early and late postoperative PI-LL. Conversely, expandable cages in our matched cohort analysis were associated with lower PI-LL mismatch at late follow-up, though significance was borderline in paired analysis and did not remain significant on sensitivity testing.
PI-LL mismatch is widely recognized as one of the most clinically relevant parameters in sagittal alignment and is associated with improved functional outcomes and lower rates of postoperative mechanical complications when restored toward physiological targets.6
The trend toward lower PI-LL mismatch observed in the expandable cage cohort may reflect the greater segmental and overall LL changes observed in this group. Because changes in SL contribute to the cumulative lumbar arc, even modest improvements at the operative index level may have meaningful impacts on global alignment. Expandable cages have been noted in TLIFs to confer a larger decrease in PI-LL mismatch compared with static cages.7
However, the inconsistent significant findings between overall and matched analyses warrant caution; PI-LL mismatch may be influenced by factors beyond cage expansion alone, including rod contouring, posterior compression, cage position, endplate remodeling, and hardware subsidence. The lack of consistent significance across timepoints may therefore not indicate the absence of effect, but rather the multifactorial nature of the parameter with the limited power of a small expandable cohort. Taken together, our observed trends suggest that expandable cages may contribute to improved PI-LL correction in PTP-LLIF, though larger future studies with standardized assessment of aforementioned confounders are needed to clarify direct attribution to expansion itself.
Perioperative Metrics
EBL, operative time, and length of stay did not differ between cohorts in either overall analysis or matched analysis.
In particular, EBL and operative time are notable in their lack of significance. As expandable cages do not require sizing trials, we expected significantly lower operative time and therefore EBL in the expandable cohort. However, prior literature similarly reports no significant differences in these perioperative outcomes between static and expandable LLIF cohorts. A recent meta-analysis by Beyer et al found comparable EBL, operative time, and length of hospital stay among static and expandable cage cohorts in LLIFs.23
No specified intraoperative complications were identified in either cohort. However, because the study was underpowered to compare adverse events and did not systematically evaluate broader postoperative or postdischarge complications, these findings should not be interpreted as evidence of equivalent safety between cage types.
Limitations and Future Directions
There were several limitations in this study.
Its retrospective, 2-surgeon, single-institution design introduces selection bias and may limit generalizability.
Cage selection was performed at surgeon discretion based on intraoperative assessment, introducing potential confounding by indication. Expandable cages may have been preferentially used in cases requiring greater restoration of disc height or lordosis. Although matched cohort analysis partially mitigates this concern, nonstandardized technical factors, including disc release, cage positioning, rod contouring, and posterior compression, remain unaccounted for and may independently influence sagittal correction.
Despite concerns regarding potential undersizing or oversizing of implants, there were no significant differences in cage footprint or intrinsic lordotic angle between cohorts, suggesting that differences in footprint or intrinsic lordotic angle alone are unlikely to fully explain the observed radiographic differences. However, expandable cages achieved greater final implant height. This may reflect expansion technology, implant sizing strategy, or both. Therefore, residual technical confounding remains possible.
Clinical outcomes, including patient-reported pain and functional scores, were not collected. Therefore, while restoration of sagittal alignment is associated with improved clinical outcomes, this study cannot determine whether observed radiographic differences translate into meaningful clinical benefit. Additionally, although early and late radiographs were available, there was no predefined minimum follow-up duration, introducing heterogeneity in long-term assessment.
Complication ascertainment was limited to predefined intraoperative events identified through operative reports and inpatient perioperative documentation. Therefore, this study cannot compare broader postoperative, postdischarge, or long-term adverse-event profiles between cage types.
Furthermore, interobserver reliability statistics, including intraclass correlation coefficients, were not available for this retrospective analysis. To mitigate measurement variability, radiographic measurements were performed independently by 2 trained reviewers using standardized anatomic landmarks, with discrepancies resolved by consensus before final analysis; however, the absence of formal ICCs remains a limitation.
Finally, the expandable cohort was small, with only 7 patients; this may limit analyses and reduce power. Although significant radiographic associations were observed and persisted for lordotic parameters after matching, these findings should be considered as a promising basis for additional investigation.
Future research should use large multicenter prospective cohorts to evaluate expandable cages specifically within PTP-LLIF. Prospective studies incorporating fusion rates and subsidence analysis would provide a more comprehensive assessment of long-term durability of constructs. Additional incorporation of patient-reported outcomes, standardized follow-up, cage positioning, posterior rod contouring/compression, and other technical considerations would better define whether expandable cages independently contribute to radiographic correction and whether these differences translate into meaningful clinical benefit.
Conclusion
Expandable cages in PTP were associated with significantly greater improvements in disc height and sagittal alignment parameters compared with static cages. These findings are consistent with literature on other lumbar approaches and persisted most consistently for lordotic parameters following matched cohort analysis, suggesting a potential association with greater radiographic correction. However, these findings must be interpreted in the context of potential confounding by indication, implant sizing differences, and small sample size. Further prospective studies incorporating clinical outcomes are needed to clarify the clinical significance of these radiographic improvements.
Footnotes
Funding The authors received no financial support for the research, authorship, and/or publication of this article.
Declaration of Conflicting Interests M.M.A. is a consultant for a Globus company that manufactures one of the devices examined in this study.
Disclosures M.M.A.: Consultant for Globus, TrackX, BrainLab, and Arthrex. K.D.T.: Consultant for SI-Bone, Globus, LifeNet Health, and Cerapedics, and leadership role with Congress of Neurological Surgeons. All other authors: no disclosures.
IRB Approval Approved by Duke University Health System Institutional Review Board under an existing protocol approved by the Duke Health Institutional Review Board and determined to be exempt from review (Protocol #Pro00071025).
- This manuscript is generously published free of charge by ISASS, the International Society for the Advancement of Spine Surgery. Copyright © 2026 ISASS. To see more or order reprints or permissions, see http://ijssurgery.com.
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