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Précis: Gravity Line (GL) measurement by forceplate offers key information on standing balance. In this study x-ray measurements and GL offsets were calculated in two adult: volunteer controls, sagittal plane deformity patients. The deformity group revealed significant pelvic retroversion and posterior sacral displacement regarding GL and heels. However, GL-heel and GL-femoral head offsets were similar indicating that sagittal plane deformity may induce posterior pelvic translation and retroversion in order to maintain an inherent ideal/fixed GL-heel relationship.

Introduction: Sagittal spinal imbalance in the adult remains poorly understood and challenging. Limitations of radiographic analysis have lead researchers to apply forceplate technology to enhance the study of spinal balance through evaluation of the gravity line (GL). The aim of this study was to investigate differences between asymptomatic adults and patients with sagittal spinal deformities, with a hypothesis that imbalance would lead to changes in the GL – spinal relationship.

Material and Method: This prospective study included 44 asymptomatic subjects (mean 57yo) and 40 patients with sagittal deformities (mean 65yo, inclusion criteria: L1-S1 lordosis< 258, Pelvic Tilt> 208, C7 plumbline> 5 cm). Coronal plane deformities were excluded. Full-length free-standing sagittal radiographs were obtained with simultaneous acquisition of the GL and heel position (by forceplate). Spino-pelvic radiographic parameters were calculated and distances (offsets) from the GL analysed. Group differences were evaluated by independent sample t-tests.

Results: Groups did not differ in age, thoracic kyphosis, offsets from femoral heads to heels, femoral heads to GL, and GL to heels. As per inclusion criteria the sagittal deformity group had greater mean C7 plumbline (8cm vs 0cm), increased pelvic tilt (27° vs 13°) and loss of lordosis (46° vs 58°). The sagittal deformity group also had greater pelvic incidence (60° vs 51°), anterior trunk inclination (−3° vs −11°), S1 displacement toward the heels (distance decreased, 87 vs. 46mm). All differences p< 0.001.

Discussion: The sagittal spinal deformity group revealed marked differences; the sacrum has a more posterior position in relation to the GL and heels. However, the GL to femoral head offset was not markedly influenced. The additional finding of no change in the GL to heel offset and rather fixed GL-femoral head offset appears to indicate that sagittal spinal deformity induces a posterior sacral translation and pelvic retroversion in order to maintain a fixed GL-heel relationship.

Correspondence should be addressed to Jeremy C T Fairbank at The Nuffield Orthopaedic Centre, Windmill Road, Headington, Oxford OX7 7LD, UK