OUTCOME MEASURES: Sagittal full body radiographic parameters, and demographics information. METHODS: Patients older than 18 years old were drawn from a database of full body images. After evaluation of the spino-pelvic and lower-limb radiographic parameters, correlations analysis was carried out between SVA and PShift as well as between T1 spino-pelvic inclination (T1SPi) and PAngle (the angular version of PShift). Multivariate regression analy- sis was then carried out based on spinal global alignment (SVA, T1SPi) and demographics to predict the global alignment of the lower limbs (Pshift or PAngle). RESULTS: A total of 853 patients (mean age=59.3 years old, mean BMI=25.3 kg/m2, 59.4% female) were included in the analysis. Mean sag- ittal spino-pelvic parameters were PI-LL=10°±19, PT=21°±11, SVA=44 mm±59, and T1SPi=-1.36°±6.2. The lower limb alignment was: hip extension=201.4±10.7, knee flexion=5.8±8.1, PShift=40 mm±43, and PAngle=2.6°±2.8. There was as strong correlation between SVA and PShift (r=0.728 p<.001), and T1SPi with PAngle (r=0.600 p<.001). Larger BMI and age were associated with a larger PShift and PAngle for the same SVA. Multivariate regression analysis (R-square=0.555) demonstrated that SVA was the strongest predictor of PShift (standardized coefficient 0.667), fol- lowed by BMI (0.129), and advanced age (0.100). Analysis of the beta coefficients demonstrated that for a given age and BMI, an increase of 5 cm in SVA was associated with an increase of 2.5 cm in PShift (49.2%). Similar results were established between PAngle (R-square=0.434) and T1SPi (stan- dardize beta: 0.528), Age (0.217) and BMI (0.157). CONCLUSIONS: Lower limb compensation is directly associated with tho- racolumbar spinal alignment and counteracts anterior malalignment by repositioning the gravity line over the feet. Age and BMI also impact lower limb compensation required to maintain a free-standing posture. Patient- specific demographics and radiographic parameters influence lower limb compensation. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. https://doi.org/10.1016/j.spinee.2017.07.049 36. Recruitment of Lower Limb Compensation in the Setting of Anterior Global Malalignment is Affected by Patient Characteristics: a Study of 2498 Patients Renaud Lafage, MSc 1 , Jeffrey Varghese 1 , Sebastien Pesenti, MD 1 , Tejbir Pannu, MD, MS 1 , John Y. Moon, BS 2 , Peter L. Zhou, BA 2 , Jared C. Tishelman, BA 2 , Bryan M. Beaubrun, BS 2 , Dennis Vasquez-Montes, MS, BA 2 , Themistocles S. Protopsaltis, MD 2 , Thomas J. Errico, MD 2 , Aaron J. Buckland, MBBS, FRACS 2 , Frank J. Schwab, MD 1 , Virginie Lafage, PhD 1 ; 1 Hospital for Special Surgery, New York, NY, US; 2 Hospital for Joint Diseases at NYU Langone Medical Center, New York, NY, US BACKGROUND CONTEXT: Recent data on sagittal alignment high- lighted the importance of lower limb compensatory mechanisms. A posterior translation of the pelvis versus the feet (PShift) is necessary to maintain an erect posture in the presence of sagittal deformity. PURPOSE: This study aims to investigate the effect of pelvic incidence (PI), age and BMI on lower-limb chain of compensation associated with PShift. Hypothesis for this study was: lower limbs compensation is affected by patient characteristics (age, BMI) and morphology (pelvic incidence). STUDY DESIGN/SETTING: Single-center retrospective review. PATIENT SAMPLE: A total of 2,498 adult patients from a single center. OUTCOME MEASURES: Full body radiographic parameters (lower limbs alignments, including knee flexion, ankle dorsiflexion and pelvic translation). METHODS: Adult patients were drawn from a database of full body images. After measurements, patients were stratified into 7 groups of increased PShift. lower limb parameters (pelvic tilt [PT], hip extension [SFA], knee angle [KA] and ankle angle [AA]) were compared across the PShift groups while taking into account patient characteristics: pelvic incidence (low<40, mid:40–50, high>65), age (young<40yo, mid:40-65yo, old>65yo), and BMI (<or>30 kg/m2). RESULTS: A total of 4574 sagittal X-ray images from 2498 patients (57 yo, BMI=27.5 kg/m2, 62.9% female) were analyzed and revealed mean spinopelvic parameters of PI=55±17, PT=20±11, PI-LL=6±17 and SVA=29 mm±51, as well as mean lower-limb parameters of PShift=30 mm±38, SFA=203°±11, KA=4°±9, and AA=6°±5. Lower limb parameters were all significantly dif- ferent across PShift groups (p<.001). Across the entire spectrum of PShift, stratification by PI demonstrated an increase in PT and SFA recruitment as PI increased (mean PT or SFA difference across PI groups: 7°, p<.001), but no significant difference in KA or AA. The analysis by age revealed that younger patients had significantly smaller values of PT, SFA, KA, and AA for PShift <70 mm; but similar lower-limb recruitments when PShift reached 80 mm. The analysis by BMI showed that for PShift<0, high BMI pts had a larger KA and AA; for PShift>0 they had a smaller recruitment of PT and SFA. CONCLUSIONS: This study described how the lower-limbs are recruited to regulate the translation of the pelvis but also how this chain of compen- sation is affected by patient characteristics. Refining the understanding of the spine/lower-limb relationship is a step toward closing the gap between spinopelvic and full standing axis analyses. FDA DEVICE/DRUG STATUS: This abstract does not discuss or include any applicable devices or drugs. https://doi.org/10.1016/j.spinee.2017.07.050 37. The Ankle-Pelvic Angle (APA): A Summary Measurement of Pelvic and Lower Extremity Compensation Max Vaynrub, MD 1 , Jared C. Tishelman, BA 1 , Samantha R. Horn, BA 1 , John Y. Moon, BS 1 , Peter L. Zhou, BA 1 , Bryan M. Beaubrun, BS 1 , Peter G. Passias, MD 2 , Aaron J. Buckland, MBBS, FRACS 1 , Thomas J. Errico, MD 1 , Themistocles S. Protopsaltis, MD 1 ; 1 Hospital for Joint Diseases at NYU Langone Medical Center, New York, NY, US; 2 NYU Medical Center Hospital for Joint Diseases, NY Spine Institute, New York, NY, US BACKGROUND CONTEXT: Adult sagittal spinal deformity (SSD) leads to recruitment of compensatory mechanisms to maintain upright posture as per Dubousset’s Conus of Economy. After regional spinal compensation and pelvic tilt (PT) are exhausted, lower extremity compensation is recruited. Knee flexion (KA) and ankle flexion (AA) increase to drive pelvic shift (PSh) posteriorly. PURPOSE: We aim to describe a summary angle, ankle-pelvic angle (APA), which incorporates all aspects of lower extremity compensation in a single measurement, to demonstrate its correlation with SSD, and to identify a cutoff value that indicates the presence of compensation. STUDY DESIGN/SETTING: Retrospective review of single-center full- body imaging database. PATIENT SAMPLE: Patients with spine complaints who underwent full- body stereoradiographic imaging from a single center. OUTCOME MEASURES: Radiographic measurements including T1- pelvic angle (TPA), PT, PSh, KA,AA, and APA. METHODS: Full-body sagittal stereoradiographic images were analyzed preoperatively and postoperatively. Spinal and lower extremity alignment was quantified with existing measures and the APA, the angle between a line drawn from the bicoxofemoral axis to the midpoint of the sacral endplate, and a line from there to the ankle malleoli. Lower extremity compensation was defined as KA>6.9° based on analysis of normal TPA patients. Regres- sion analysis was used to represent the predictive relationship between TPA and APA. RESULTS: A total of 861 patients (mean age 55.1 years, 60.4% female) were analyzed. 37.3% had SSD (TPA>20°). Patients with lower extremity compensation had higher APA than those without compensation (21.6° vs 17.7°, p<.001). APA demonstrated strong correlation with TPA (r=.81, p<.001), as well as PT, PSh, knee flexion and AA (r=.98 to .24, all p<.001). Corrected postop TPA correlated with postop APA (R=.87 p<.001). Using linear regression analysis, a TPA of 18.3° and an APA of 19.7° corresponded to the threshold value of lower extremity compensation (R2=.660, p<.001). S58 NASS 32 nd Annual Meeting Proceedings / The Spine Journal 17 (2017) S41–S88 Refer to onsite annual meeting presentations and postmeeting proceedings for possible referenced figures and tables.Authors are responsible for accurately reporting disclosure and FDA device/drug status at time of abstract submission.