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The Spine Blog

Friday, February 7, 2020

Ideal Sagittal Alignment: Taking Age and Pelvic Morphology Into Account

Spine surgeons have been focused on achieving appropriate sagittal balance in their adult deformity patients for at least the last 10 or 15 years.1 When the topic was initially introduced, it was felt that the goal for all patients was to achieve "optimal" sagittal balance, namely a sagittal vertical axis (SVA) < 5 cm, pelvic tilt (PT) < 20⁰, and pelvic incidence (PI) minus lumbar lordosis (LL) < 9⁰.2 This approach resulted in a period of very aggressive correction, even in elderly patients, and was associated with a high rate of proximal junctional failure and revision surgery. More recently, spine surgeons have recognized that correcting elderly patients to these sagittal alignment "norms" derived from a younger population is not ideal. To better individualize alignment goals, Dr. Protopsaltis and the International Spine Study Group performed an analysis of over 900 adult deformity patients and over 100 asymptomatic volunteers. In addition to considering age, they also considered the role of pelvic incidence in establishing alignment goals. They also evaluated the T1 pelvic angle (TPA), an overall measure of sagittal alignment that is independent of lower extremity alignment. They created linear regression equations predicting sagittal alignment parameters (TPA, SVA, PT, and PI – LL) based on PI and SF-36 PCS score. They used these equations in their adult deformity cohort to determine SVA values that corresponded to normative age-specific PCS scores for different PIs. They reported that the ideal SVA based on the deformity cohort increased with age and increasing PI. For example, a patient under 45 with a PI < 40⁰ achieved a typical age-matched PCS score when SVA was -4 mm. On the other end of the spectrum, for patients over 65 with a PI > 75⁰, the optimal SVA was +57 mm. A similar pattern was observed for TPA using the asymptomatic cohort, with ideal TPA increasing from -3⁰ for younger patients with low PI to +25⁰ for elderly patients with high PI.

This group continues to publish insightful papers based on their very large adult deformity database, and they have done a nice job adjusting their beliefs about ideal correction as they have gained more data. The current paper supports the concept that less aggressive correction is desirable in elderly patients and also addresses the role that pelvic morphology plays. It is intuitive that patients with high pelvic incidence tend to tolerate higher degrees of positive sagittal "imbalance". For a fixed sacral slope (SS), pelvic tilt increases directly with pelvic incidence (i.e. PI = SS + PT). High PI patients have the femoral heads relatively anterior in relation to the sacrum, so a more forward alignment of the head is tolerated well as the C7 plumb-line can remain over the pelvic base of support. The strength of the correlations in this paper for the linear regression equations predicting sagittal alignment parameters based on PI and patient reported outcomes (PROs) are relatively modest (i.e. r2 < 0.3), indicating that these equations explain less than 30% of the observed variance. In other words, many other factors beyond sagittal balance parameters are driving outcomes in this population. Additionally, this paper looks only at baseline PROs and does not validate the concept that correction to the norms generated by these equations results in better outcomes and fewer re-operations. Nonetheless, the authors have made a compelling case that a "one size fits all" approach is inappropriate for adult deformity surgery, and factors such as age and pelvic morphology need to be considered while planning the magnitude of correction.

Please read Dr. Protopsaltis's paper on this topic in the February 15 issue. Does this change how you view the role of age and pelvic morphology while planning adult deformity surgery?

Adam Pearson, MD, MS

Associate Web Editor

REFERENCES

1.            Glassman SD, Bridwell K, Dimar JR, Horton W, Berven S, Schwab F. The impact of positive sagittal balance in adult spinal deformity. Spine 2005;30:2024-9.

2.            Schwab F, Patel A, Ungar B, Farcy JP, Lafage V. Adult spinal deformity-postoperative standing imbalance: how much can you tolerate? An overview of key parameters in assessing alignment and planning corrective surgery. Spine 2010;35:2224-31.