SPINE Volume 33, Number 14, pp 1572–1578 ©2008, Lippincott Williams & Wilkins Standing Balance and Sagittal Plane Spinal Deformity Analysis of Spinopelvic and Gravity Line Parameters Virginie Lafage, PhD,* Frank Schwab, MD,* Wafa Skalli, PhD,† Nicola Hawkinson, NP,* Pierre-Marie Gagey, MD,‡ Stephen Ondra, MD,§ and Jean-Pierre Farcy, MD* Study Design. Prospective study of 131 patients and volunteers recruited for an analysis of spinal alignment and gravity line (GL) assessment by force plate analysis. Objective. To determine relationships between GL, foot position, and spinopelvic landmarks in subjects with varying sagittal alignment. Additionally, the study sought to analyze the role of the pelvis in the maintenance of GL position. Summary of Background Data. Force plate technology permits analysis of foot position and GL in relation to radiographically obtained landmarks. Previous investiga- tion noted fixed GL-heel relationship across a wide age range despite changes in thoracic kyphosis. The pelvis as balance regulator has not been studied in the setting of sagittal spinal deformity. Methods. The 131 subjects were grouped by sagittal vertical axis (SVA) offset from the sacrum: sagittal for- ward (2.5 cm), neutral (-2.5 cm  SVA  2.5 cm), and sagittal backward (SVA -2.5 cm). Simultaneous spi- nopelvic radiographs and GL measure were obtained. Offsets between spinopelvic landmarks, heel position, and GL were calculated. Group comparisons were made for all offsets to determine significance. Results. Aside from the offset T9-GL and GL-heels, all other offsets between spinopelvic landmarks and GL re- vealed significant differences (P  0.001) across the 3 subject groups. However, with increasing SVA, the GL kept a rather fixed location relative to the feet. A correla- tion between posterior pelvic shift in relation to the heels with increasing SVA in this study population was con- firmed (r = 0.6, P  0.001). Conclusion. Increasing SVA in standing subjects leads to a posterior pelvic shift in relation to the feet. However, no significant difference in GL-heel offset is noted with increasing SVA. It thus appears that pelvic shift (in rela- tion to the feet) is an important component in maintaining a rather fixed GL-Heels offset even in the setting of vari- able SVA and trunk inclination. Key words: spinal alignment, force plate, gravity line, sagittal balance. Spine 2008;33:1572–1578 The aging process and deformity of the spinal column can lead to altered sagittal plane alignment. To date, optimal spinal balance remains poorly defined. How- ever, a number of radiographic parameters have been developed as guides of sagittal alignment. Values of tho- racic kyphosis by Cobb measurements, and lumbar lor- dosis are commonly used to assess regional alignment. Global spinal alignment is commonly assessed by a plumbline method 1–4 drawn onto full-length radio- graphs. More recently, pelvic parameters 5–7 have also been examined to improve evaluation of sagittal plane balance. Although ranges of normal values 7–11 and series of patients with specific pathology such as spondylolis- thesis 12–14 have been published, the correlation between these radiographic parameters and standing balance is not yet fully understood. Although radiographs permit an evaluation of verte- bral alignment and pelvic landmarks, current standard techniques using 36 cassettes do not capture foot posi- tion. Analysis of spinal and pelvic offsets from standing foot position can be captured through forceplate analy- sis: a forceplate device is made of pressure sensors dis- tributed across a level surface and permit detailed anal- ysis of foot position and pressures of a subject that stands on such a platform. Given that a freestanding person must be balanced over a small area between the feet to avoid requiring support (or falling over), 10,15 it would appear that spinal imbalance or misalignment must lead to compensatory mechanisms to maintain a center of force [gravity line (GL)] in close relation to the feet. 10 It has been reported that normal global balance in adults by radiographic plumbline falls within a narrow range from the pelvis (sacrum, or S1). Jackson 3 has re- ported values in asymptomatic adults with a mean sag- ittal vertical axis [SVA (distance form a plumbline drawn form C7 and the posterosuperior corner of S1)] offset of 0.5 cm (SD  2.5 cm). According to these data, offset greater than 2.5 cm anteriorly or posteriorly are consid- ered beyond the normal range. The purpose of this study was to evaluate the impact of SVA offset on pelvic parameters and spinopelvic posi- tion relative to the feet in standing subjects. This study also sought to analyze the effects of SVA offset on GL position. Enrolment was limited to adults without signif- icant coronal deformity or imbalance. A hypothesis of this study was that healthy volunteers and patients (even in the setting of marked sagittal plane deformity) main- tain a GL within a fixed area relative to the feet. Further- more, changes in pelvic location (translation and orien- tation) associated with SVA offset were hypothesized to occur. From the *NYU Hospital for Joint Diseases, New York, NY; †Labo- ratoire de BioMecanique, ENSAM/CNRS URM8005, Paris; ‡Associ- ation Franc ¸aise de Posturologie, l’Hay-les-roses, France; and §North- wester Medical Center, Chicago, IL. Acknowledgment date: March 27, 2007. First revision date: January 8, 2008. Acceptance date: January 8, 2008. The manuscript submitted does not contain information about medical device(s)/drug(s). Corporate/Industry funds were received in support of this work. No benefits in any form have been or will be received from a commer- cial party related directly or indirectly to the subject of this manuscript. Supported by Medtronic Sofamor Danek. Address correspondence and reprint requests to Virginie Lafage, PhD, Research Scientist, NYU Hospital for Joint Diseases, 380 2nd Ave., Suite 1001, New York, NY 10010; E-mail: virginie.lafage@gmail.com 1572