Why Some Patients Stop Improving After Minimally Invasive Spine Surgery — And What Early X-Rays May Reveal
A MIS-ISSG study finds that coronal malalignment detectable as early as six weeks after minimally invasive spine deformity surgery predicts which patients will experience a decline in functional outcomes between years one and two.
OPENING SUMMARY
Most patients who undergo minimally invasive surgery for adult spinal deformity improve significantly — and the expectation is that recovery continues to build over time. But for some patients, the trajectory reverses: they feel better at one year than they did before surgery, but worse at two years than they did at one. This study asks what separates those patients from the ones who keep improving — and finds that the answer may be visible on a routine postoperative X-ray as early as six weeks after surgery. The culprit is not surgical complexity, patient age, or preoperative health status. It is coronal malalignment — a side-to-side imbalance of the spine that, when left uncorrected, appears to undermine the durability of functional recovery.
STUDY SNAPSHOT
Study type – Retrospective analysis of prospectively collected MIS-ISSG database
Number of patients – 67
Surgical approach – Circumferential minimally invasive surgery (MIS) for ASD
Follow-up – 1 and 2 years postoperatively
"Somewhat improved" group – 31 patients (higher ODI at 2 years than at 1 year, but still improved vs. baseline)
"Very improved" group – 36 patients (stable or improved ODI from 1 to 2 years)
Key radiographic measure – Central sacral vertical line (CSVL) — a measure of coronal spinal balance
Critical early timepoint – 6-week postoperative X-ray
Published in – Spine
DOI – 10.1097/BRS.0000000000005767
WHY THIS MATTERS
Minimally invasive surgical techniques for adult spinal deformity have expanded significantly over the past decade, offering patients meaningful reductions in blood loss, operative time, and hospital stay compared to traditional open surgery. As MIS becomes more widely used for increasingly complex deformity cases, understanding its long-term functional trajectory — not just short-term recovery — becomes critically important.
The question this study addresses is one that patients and surgeons both care about deeply: after a successful minimally invasive surgery, what predicts whether the benefit holds or erodes over time? Previous research has established that coronal malalignment — a failure to restore side-to-side spinal balance — is associated with disability in spinal deformity patients. What has not been established, until now, is whether coronal malalignment after circumferential MIS specifically predicts delayed functional deterioration between the first and second postoperative year.
The answer has implications not just for how MIS is performed, but for how patients are monitored after surgery and what early radiographic findings should prompt closer follow-up or intervention.
KEY FINDINGS
1. Nearly half of patients experienced some functional decline between years one and two. Of the 67 patients studied, 31 — just under half — reported higher disability scores at two years than at one year. This does not mean surgery failed for these patients: all 31 continued to show meaningful improvement compared to their preoperative baseline. But the reversal in trajectory between years one and two is clinically significant and points to a modifiable factor worth identifying.
2. The two groups were indistinguishable before and during surgery. Statistical analyses found no significant differences between the somewhat improved and very improved groups in baseline demographics, surgical variables, or preoperative characteristics. Whatever was driving the divergence in functional trajectory did not exist — or was not measurable — before the patients left the operating room. The differentiating factor emerged postoperatively.
3. Coronal malalignment was significantly greater in the somewhat improved group at every follow-up interval. The central sacral vertical line — a radiographic measure of coronal spinal balance — was significantly higher in the somewhat improved group at all postoperative timepoints. At the six-week postoperative visit, the mean CSVL was 36.26 mm in the somewhat improved group versus 22.8 mm in the very improved group, a statistically significant difference (p=0.01). This early gap in coronal alignment persisted across follow-up intervals.
4. The coronal alignment signal was present as early as six weeks after surgery. The six-week timepoint is particularly clinically significant because it is early enough that intervention — whether through rehabilitation, bracing, or reoperation — might still be possible before functional decline becomes entrenched. The fact that the difference in CSVL was already apparent and statistically significant at six weeks suggests a potential window for identifying at-risk patients and acting on that information.
5. Coronal alignment deserves parallel attention alongside sagittal alignment in MIS planning. Surgical planning and outcome assessment in ASD has traditionally emphasized sagittal (front-to-back) alignment restoration. This study contributes to a growing body of evidence that coronal (side-to-side) alignment is an equally important determinant of functional outcomes — and may be particularly relevant in MIS cases, where achieving full coronal correction can be technically more challenging than in open surgery.
PRACTICAL IMPLICATIONS
Postoperative coronal alignment should be assessed early and systematically. This study suggests that the six-week postoperative X-ray is not merely a confirmatory step — it may be a prognostically important data point. Measuring and recording CSVL at this early timepoint could help identify patients at risk for delayed functional decline before that decline is clinically apparent.
Patients with elevated early postoperative CSVL may warrant closer surveillance. While the study does not establish a specific intervention for patients with high early CSVL, identifying them early creates the opportunity for more frequent follow-up, earlier discussion of concerns, and — where appropriate — consideration of whether additional management is warranted. Defining the threshold CSVL value that should trigger heightened surveillance is a logical next step for future research.
Coronal correction should be a primary surgical goal in circumferential MIS, not a secondary one. The findings reinforce that MIS techniques for ASD must be evaluated and planned not only for their ability to restore sagittal balance, but for their ability to achieve adequate coronal correction as well. For surgeons planning circumferential MIS cases, this study supports giving coronal alignment targets equal standing in the surgical planning process.
The finding that the groups were indistinguishable preoperatively has important implications for patient counseling. Because no preoperative factor separated the two groups, surgeons cannot identify at-risk patients before surgery using standard baseline data. This means postoperative monitoring — rather than preoperative selection — is the critical opportunity for intervention, and reinforces the value of systematic early radiographic follow-up in all MIS-ASD patients.
CONCLUSION
This study offers a clear and clinically actionable finding: in patients who undergo circumferential minimally invasive surgery for adult spinal deformity, early postoperative coronal malalignment — measurable as soon as six weeks after surgery — predicts which patients will experience a decline in functional outcomes between one and two years. Because the two groups were indistinguishable before surgery, the postoperative X-ray becomes a critical monitoring tool rather than a formality. For a field that has rightly celebrated the benefits of minimally invasive approaches, this study is a timely reminder that achieving durable outcomes requires not just a successful operation, but sustained attention to alignment in the months that follow.
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