Faculty Bibliography

Hypothesis: Mechanisms of relaxation after sagittal correction may differ from the compensation mechanisms of sagittal deformity development. Design: Retrospective cohort. Introduction: Adult spinal deformity patients recruit compensatory mechanisms to maintain alignment with increasing deformity. While attention has been paid to pre-operative compensatory adaptations, little is known regarding the sequence of relaxation of these parameters based upon the amount of residual PI-LL mismatch. This study details the progression of compensatory mechanism relaxation as PILL mismatch improves. Methods: Single site review of post-operative full-body x-rays of patients, at least 9 months after surgery. Radiographic measurements were obtained with dedicated spine software and included PT, knee flexion (KA), ankle dorsiflexion (AD), pelvic shift (PShift), T1 spinopelvic inclination (T1SPi), SVA and T1 pelvic angle (TPA). Patients were stratified by their remaining need for lordosis based on ageadjusted normative published values. Group comparisons were carried out via ANOVA analysis. Results: 262 patients were included, mean age 61.5, mean BMI 28, and males 31 %. PI-LL groups were significantly (p<0.05) different in terms of PT, KA, PShift, TPA, SVA and AD (Fig). Analysis of the sequence of correction revealed that the majority of age-adjusted offset in terms of SVA and PT was corrected (59 %, 57 %) during the first stage of correction ([>30degree] to [20degree-30degree] of remaining need for lordosis). This stage was associated with significant changes in PShift and T1SPi (p<0.05). Additional stages of corrections were associated with gradual decreases in SVA, while no additional decreases in PT were observed until the post-operative lordosis was near ideal. Conclusions: When improving from severe to moderate age-adjusted PI-LL mismatch, there is a correction in age-adjusted SVA to normal range. Nevertheless, these patients still exhibit high degree of compensation with respect to the pelvis and lower limbs to maintain alignment. PT and SVA can be corrected to an age-adjusted ideal when PI-LL is also corrected

Hypothesis: A single T1 Spino Pelvic Angle (TPA) can be associated with drastically different HRQOL but it can easily be supplemented to convey both global alignment and meaningful clinical outcome. Design: Retrospective cohort. Introduction: TPA is a valuable perioperative planning tool that accounts for both pelvic tilt (PT) and trunk inclination. However, it is limited as a standalone parameter because it does not distinguish patients' ability to compensate with pelvic retroversion. For a given TPA, patients who are unable to recruit compensatory mechanisms may have significantly worse HRQOL scores than those who can. Can TPA be augmented to better describe a patient's global alignment and more accurately predict HRQOL? Methods: Single-center study of patients with full body X-ray, HRQOL and TPA >10degree. Proportions of PT to TPA (PTp = PT/TPA) and T1SPi to TPA (T1SPip = T1SPi/TPA) were calculated and investigated against increased values of TPA. Then, 2 sub-groups were created (HighPT and LowPT) based on mean (PTp) +/- 0.5 standard deviation. HighPT and LowPT were compared across the entire cohort using an unpaired T-test. Results: 230 patients were included (58.7 +/- 15.5 y, 60 %F). Mean sagittal parameters included: PI-LL 12.3 +/- 16.3degree, SVA 41 +/- 49 mm, TPA 21.9 +/- 10.1degree and PT 24.4 +/- 8.6degree. The analysis of PTp distribution revealed a decrease in PT recruitment as TPA increases (137 +/- 39 % for patients with TPA <15degree, 87 +/- 15 % for patients with TPA>40degree). Comparing LowPT (n = 57) with HighPT (n = 69) revealed that for a similar TPA (24.1 vs. 22.1degree, p = 0.308), patients with LowPT (and therefore little compensatory PT) had significantly worse HRQL scores in terms of ODI (45 vs. 32 in HighPT; p = 0.002) and EQ-5D (9.7 vs. 8.5 in HighPT, p = 0.003). Conclusions: While TPA captures the severity of deformity, disability is a product of deformity severity and the inability to recruit compensatory mechanisms. TPA measures the severity of the thoracolumbar deformity separate from pelvic compensation. Therefore for a complete picture of standing sagittal alignment, TPA should be considered in conjunction with PT to convey the full radiological and clinical picture. Failing to do so potentially masks a patient's disability

Hypothesis: Global sagittal angle is clinically relevant. Design: Single center retrospective review. Introduction: According to Dubousset's "conus of economy" theory, deterioration of sagittal alignment requires higher energy expenditure to maintain erect posture. Since the clinical impact of sagittal alignment is affected both by the severity of the deformity and a patient's inability to recruit compensatory mechanisms, it is important to investigate new parameters that reflect both disability level and compensatory mechanisms for all patients. This study investigates the clinical relevance of the GSA. Methods: Retrospective review of patients who underwent full body X-rays and completed ODI and SRS-22. GSA was defined as the angle subtended by a line from the midpoint of the femoral condyles to the center of >7, and a line from the midpoint between the femoral condyles to the posterior superior corner of the S1 sacral endplate. After evaluating the correlation of GSA/ODI with classic sagittal parameters, linear regression models were generated to investigate how ODI related to radiographic parameters (TPA, PT). TPA and PT's relation to GSA was then investigated. Results: 143 patients (mean 44y) were identified. GSA correlated significantly with ODI, PT, SVA, TPA and lower limb alignment. Regression between ODI and classic parameters only retained GSA as independent predictor (r = 0.517, r2 = 0.267, p<0.001). Analysis of standardized coefficients revealed that GSA increases when TPA increases (beta: 1.991) with concurrent decrease in PT (-1.323). These findings echoed those of ODI, which increased with increased TPA (beta: 1.038) and decreased PT (-0.696). Conclusions: GSA goes further than classic parameters and quantifies the clinical impact of all patients' compensation mechanisms in addition to their malalignment. Analyzing the relationship between GSA and PT revealed that compensation is the body's defense against malalignment, and patients who lack compensation have both higher GSA and higher ODI scores. GSA is a significant and clinically relevant metric that quantifies both spinal deformity and disability even among those with alternative compensation mechanisms