Adding-on is a term used by spine surgeons to describe a specific pattern of scoliosis progression after surgery: new curve development below the lowest fused vertebra. A patient who had a thoracic scoliosis corrected with fusion to L1, for example, may develop a progressive lumbar curve below L1 in the months to years after their surgery. The fused segment holds; the correction is intact; but below the lowest instrumented level, the spine is progressing in a way that was not expected.
Adding-on is not rare. It is recognized in published series as one of the more common indications for reoperation after AIS surgery, yet it receives very little attention in patient-facing resources. Patients who experience it often have difficulty understanding what is happening — they were told their surgery was successful, and technically the fusion is holding — and are sometimes told to watch and wait without clear information about at what point intervention is appropriate.
What Causes Adding-On?
The most common cause of adding-on is selection of a lower instrumented vertebra (LIV) that did not adequately encompass the structural extent of the scoliosis curve. In AIS surgery, one of the most important and most technically demanding decisions is where to stop the fusion — the lower instrumented vertebra. Stopping too high leaves unfused levels that are still part of the structural deformity; the residual structural curve at and below the LIV continues to exert rotational and translational forces on the unfused lumbar spine, and the unfused segment progressively develops a new curve.
Additional contributing factors include:
- Residual rotation at the LIV — if the lowest fused vertebra is not adequately derotated and horizontalized, its tilt exerts a progressive deforming force on the mobile segment below
- Unfused disc wedging at the LIV — a disc that is still wedged at the last fused level perpetuates the mechanical imbalance into the unfused segment below
- Continued skeletal growth — in patients who had surgery with significant growth remaining, the unfused segments below the LIV continue to grow asymmetrically under the influence of the residual curve pattern
How It Presents
Adding-on typically presents as a progressive shift or lean that was not present immediately after surgery but develops over months to years. Patients notice that their waistline asymmetry is returning, that their trunk is drifting to one side, or that their shoulder balance — which was corrected at surgery — is deteriorating as the lower spine progressively curves. Back pain may accompany the curve progression, particularly as the unfused lumbar levels rotate and degenerate under asymmetric loading.
Radiographically, adding-on is defined as a progressive increase in the Cobb angle of the lumbar curve below the LIV, combined with increasing lateral deviation of L5 or the sacrum relative to the center sacral vertical line. Serial standing X-rays — not a single set — are required to document the trajectory. A single postoperative image showing a lumbar curve below the fusion is insufficient to diagnose adding-on; the pattern must be demonstrated to be progressive over time.
When Is Observation Appropriate vs. When Is Revision Needed?
Not every patient with some adding-on below their AIS fusion requires revision. The clinical decision depends on several factors:
- Rate of progression. A curve that is progressing consistently across multiple imaging timepoints argues for intervention earlier rather than later. A stable or very slowly progressing curve may be observed.
- Degree of coronal imbalance. Adding-on that is producing significant trunk decompensation — a trunk shift greater than 3 centimeters — is more compelling for revision than adding-on that remains radiographic without clinical decompensation.
- Skeletal maturity. Patients who are still growing at the time adding-on is detected have a different risk profile from skeletally mature patients; growth modulation in the unfused segment can drive rapid progression in immature patients.
- Symptoms. Pain, progressive deformity, or functional limitation that is clearly related to the adding-on pattern supports intervention.
Revision Surgery for Adding-On
Revision surgery for adding-on after AIS typically involves extension of the fusion distally to include the structural curve that was incompletely captured by the original fusion. The revision adds one or more levels below the original LIV, incorporating the levels that are driving the progressive decompensation and stabilizing the construct to prevent further adding-on below the new LIV. This is generally less complex than revision surgery for pseudarthrosis or osteotomy-based correction, but it does require working around the prior instrumentation at the lower end of the existing construct and careful planning of the new fusion endpoint.
Published evidence shows that early revision — before significant deformity has accumulated — produces better outcomes and is technically simpler than delayed revision of a more advanced deformity. This is one of the clearest arguments for not taking a pure watch-and-wait approach when adding-on is documented to be progressive.
For the comprehensive overview of scoliosis surgical failure patterns and revision options, see the main When Scoliosis Surgery Doesn’t Go As Planned page. If you are also experiencing trunk imbalance that you believe is related to your adding-on, see Still Off-Balance After Scoliosis Surgery.
About Dr. Zeeshan Sardar
Dr. Sardar, MD, MSc, F.R.C.S.C, is Co-Chief of Spinal Deformity Surgery at NewYork-Presbyterian / Columbia University, specializing in AIS revision surgery including extension of fusion for adding-on and correction of postoperative coronal imbalance. To schedule a consultation, call 212-932-5187 or visit the contact page.
This post is for educational purposes only and does not constitute individualized medical advice.