One Alignment Target That Could Cut Reoperation Risk in Half: L1 Pelvic Angle in Minimally Invasive Spine Deformity Surgery
A 13-center study of 1,104 patients finds that achieving the ideal L1 pelvic angle during minimally invasive ASD correction is associated with a reoperation rate of 15% versus 33% — and that every one-degree deviation from the target increases reoperation risk by 1.8%.
OPENING SUMMARY
In adult spinal deformity surgery, the difference between a durable outcome and a return to the operating room may come down to a single alignment measurement made before the patient leaves the table. The L1 pelvic angle — a radiographic parameter that reflects the relationship between the first lumbar vertebra and the pelvis — has previously been shown to predict reoperation risk in open spinal deformity surgery. But whether it carries the same predictive weight in minimally invasive surgery, and whether MIS techniques can reliably achieve the ideal target, has been an open question. This study answers both — and the results make a compelling case for L1PA as a central surgical target in MIS-based ASD correction.
STUDY SNAPSHOT
Study type – Retrospective cohort study using a multicenter, prospectively collected ASD database
Number of patients – 1,104
Number of centers – 13
Minimum follow-up – 2 years
Inclusion criteria – Scoliosis ≥20°, SVA ≥5 cm, PT ≥25°, or TK ≥60°
MIS definition – Circumferential MIS (LLIF/ALIF with percutaneous posterior fixation)
Ideal L1PA definition – Within ±5° of (0.5 × pelvic incidence) − 21
Outcome collection timepoints – Preoperative, 6 weeks, and final follow-up
Primary outcomes – Reoperation rate, SRS-22 total score, SF-36 PCS score
Ideal L1PA achieved — MIS – 63%
Ideal L1PA achieved — Open – 61% (P = 0.342)
Published in – Spine, Volume 51, Issue 5, March 1, 2026, pages E118–E122
DOI – 10.1097/BRS.0000000000005511
Corresponding author – Ryan Khanna
WHY THIS MATTERS
Minimally invasive surgical techniques for adult spinal deformity have expanded significantly over the past decade, offering meaningful advantages in blood loss, recovery time, and hospital stay compared to traditional open surgery. But as MIS has been applied to increasingly complex deformity cases, a persistent concern has emerged: can MIS techniques achieve the same precision of alignment correction as open surgery? And when they do — or don't — does it matter to the same degree?
The L1 pelvic angle addresses these questions through a specific, calculable alignment target. Unlike global alignment parameters that describe the overall balance of the spine and pelvis, L1PA captures alignment at the first lumbar vertebra relative to the pelvis — a level that is particularly relevant to the mechanical loading of the lumbar fusion construct. Prior research established that achieving ideal L1PA reduces reoperation risk in open ASD surgery. This study extends that finding to MIS, demonstrating that the target is equally achievable and carries even greater predictive significance in the MIS context.
For surgeons performing minimally invasive ASD correction, L1PA provides something rare and valuable: a specific, measurable, intraoperatively actionable target whose achievement is directly linked to whether the patient will need another surgery.
UNDERSTANDING L1 PELVIC ANGLE
The L1 pelvic angle is a radiographic measurement that describes the sagittal orientation of the L1 vertebra relative to the pelvis. It is calculated from standing lateral radiographs and reflects how the lumbar spine is positioned relative to the pelvic foundation on which it rests.
The ideal L1PA target is individualized to each patient's anatomy using the formula: (0.5 × pelvic incidence) − 21. Pelvic incidence is a fixed anatomical parameter — it does not change with posture — and it fundamentally determines how much lumbar lordosis a patient needs. By incorporating pelvic incidence into the L1PA target formula, the ideal target is calibrated to each patient's specific anatomy rather than applied as a one-size-fits-all number. In this study, a patient was considered to have achieved ideal L1PA if their postoperative L1PA fell within 5 degrees of their calculated individual target.
KEY FINDINGS
1. Ideal L1PA is achievable with MIS at rates equivalent to open surgery.
Ideal L1PA was achieved in 63% of MIS cases and 61% of open cases — a difference that was not statistically significant (p = 0.342). This finding is foundational to the study's clinical message: MIS techniques are not inferior to open surgery in their ability to hit this alignment target. The concern that less invasive approaches necessarily sacrifice alignment precision is not borne out by this data.
2. Among MIS patients, ideal L1PA was associated with dramatically lower reoperation rates.
MIS patients who achieved ideal L1PA had a reoperation rate of 15%, compared to 33% in those who did not — a more than twofold difference that was highly statistically significant (p <0.01). This is the study's most clinically impactful finding: achieving the alignment target was associated with cutting reoperation risk by more than half in the MIS population.
3. The association between L1PA and reoperation was specific to MIS — not observed in the open group.
In contrast to the MIS group, ideal L1PA was not associated with reduced reoperation in the open surgery group. This differential finding is clinically interesting and raises questions about why L1PA predicts outcomes more strongly in MIS. One hypothesis is that MIS constructs, which use percutaneous fixation rather than the direct exposure and correction possible with open techniques, are more sensitive to the precision of alignment achieved — making the margin between ideal and subideal L1PA more consequential for mechanical durability.
4. MIS patients who achieved ideal L1PA reported meaningfully better patient-reported outcomes.
Beyond reoperation rates, achieving ideal L1PA in MIS patients was associated with significantly greater improvement in both disease-specific and general health quality-of-life measures. SRS-22 total scores improved by 0.85 points in the ideal L1PA group versus 0.40 in the non-ideal group (p <0.01) — more than double the improvement. SF-36 physical component scores improved by 9.1 versus 4.6 points (p = 0.04). Patients didn't just avoid reoperation — they felt measurably better.
5. Every degree of deviation from ideal L1PA increased reoperation risk by 1.8%.
Across all patients, a continuous dose-response relationship was identified between deviation from ideal L1PA and reoperation risk: each one-degree deviation from the target was associated with a 1.8% increase in reoperation probability (p = 0.04). This finding establishes that L1PA precision matters on a continuous scale — not just as a binary achieved-or-not metric — and reinforces the importance of surgical technique and intraoperative planning in minimizing deviation from the individualized target.
PRACTICAL IMPLICATIONS
L1PA should be calculated preoperatively and targeted explicitly during MIS planning.
The individualized nature of the L1PA target — calculated from each patient's pelvic incidence — means that achieving it requires knowing the target before surgery begins. Surgeons performing MIS for ASD should calculate the ideal L1PA as a standard component of preoperative planning and use it as an explicit intraoperative target alongside global alignment goals.
The dose-response relationship argues for maximizing precision, not just achieving approximate correction.
The finding that each one-degree deviation from ideal L1PA increases reoperation risk by 1.8% suggests that alignment precision within the MIS context is not binary — it is graduated. Surgical techniques, implant selection, and intraoperative assessment tools that minimize deviation from the L1PA target may provide compounding benefits in reoperation risk reduction. This finding supports investment in intraoperative navigation, image guidance, and planning software that can improve alignment precision in MIS procedures.
The differential effect between MIS and open surgery warrants further investigation.
The finding that ideal L1PA predicted outcomes in MIS but not in open surgery is intriguing and not fully explained by the current data. Future research should investigate whether this reflects differences in construct mechanics, correction technique, patient selection, or some other factor. Understanding this differential may lead to refinements in how alignment targets are applied and prioritized differently between MIS and open approaches.
Achieving 63% ideal L1PA rates means 37% of patients are not hitting the target — a meaningful quality improvement opportunity.
While the 63% achievement rate in MIS is comparable to open surgery, it also means that more than one in three MIS patients is not achieving the ideal L1PA target — and those patients face more than twice the reoperation risk. Identifying the surgical, anatomical, and technical factors that predict failure to achieve ideal L1PA, and developing strategies to address them, represents a significant opportunity to improve outcomes at the population level.
Patients can be counseled using the L1PA framework.
The clarity of the L1PA metric — a specific, calculable, individualized target with a direct link to reoperation risk — makes it a useful tool for preoperative patient counseling. Explaining to patients that there is a specific alignment goal for their surgery, that achieving it is associated with substantially lower reoperation risk and better functional outcomes, and that their surgical team is planning explicitly to hit that target, may improve patient understanding and engagement with the surgical process.
CONCLUSION
This study establishes the L1 pelvic angle as a key surgical target in minimally invasive adult spinal deformity correction — one that is achievable at rates equivalent to open surgery and whose achievement is directly linked to both reoperation risk and patient-reported quality of life. The finding that MIS patients who achieved ideal L1PA had a reoperation rate of 15% versus 33% for those who did not, combined with the continuous relationship between L1PA deviation and reoperation risk, provides a compelling and actionable evidence base for incorporating L1PA into preoperative planning and intraoperative decision-making in MIS-based ASD surgery. For a field seeking to optimize the durability of minimally invasive correction, L1PA may be one of the most important numbers in the room.
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