Analysis of balance is emerging as an important parameter in spinal deformity. Force plate technology permits a quantitative study of balance through centre of pressure (COP) measurement. COP measurements obtained from the force plate approximate the projected centre of gravity. In a standing subject the COP reflects the projected centre of gravity however repeatability and reliability of such analysis is lacking.

COP measurements were obtained from eight asymptomatic volunteers (mean age 32) with no history of back pain or previous spinal surgery. Each subject stood on a Zebris force plate platform for 30 seconds daily. 15 sets of data were acquired for each subject. For one subject, an additional 15 sets of data were collected on one day for comparison to the longitudinal data.

Intra- versus inter-subject reliability analysis revealed a Cronbach’s alpha value > 0.9 for the following COP movement parameters: distance travelled over 30 seconds, distance travelled in the first and last five seconds, and average speed. Comparison of the mean intra- versus inter-subject coefficients of variation revealed significant differences for all parameters (p< 0.004).

COP movement parameters are reliable in terms of intra-subject repeatability and can detect significant individual subject movement patterns. This suggests that COP movement patterns over time are idiosyncratic for each individual. While the repeatability of COP measurement has been established, the sensitivity to change with pathology and in response to treatment for spinal pathology remains to be evaluated.

Précis: Using full length x-rays and force plate technology, the purpose was first to investigate the relationship between the gravity line and spino-pelvic parameters on asymptomatic adult volunteers and then to analyse age related changes. Trunk inclination and pelvic parameters appears as the two key-factors of the GL location; with age the GL location regarding the heels does not change but trunk global inclination shifts forward, pelvic tilt increases, and the pelvis shifts toward the heels.

Introduction: Although work by several authors has placed emphasis on global balance in the setting of spinal deformity, the relationship of spino-pelvic parameters related to this concept remains poorly defined. Using the force plate device and radiographic measurement, this study aimed to define the relationship between these parameters and the location of the gravity line (GL) in asymptomatic adult population.

Materials and Methods: 75 asymptomatic adult volunteers were recruited and subdivided by age (18–40, 41–60, > 61). Full-length free-standing AP and lateral radiographs were obtained with simultaneous assessment of the force plate gravity line (GL) location. The latter was projected on each x-ray to compute distance between anatomical components and GL and correlate its location with radiological parameters. Age related changes were investigated using ANOVA with Bonfer-roni-Dunn Post-Hoc test.

Results: Radiographic measurements revealed strong correlations between trunk global inclination and distance from S1 to the GL (r=0.7), sacral slope and pelvic incidence (r=0.78), distance from the bi-femoral head axis to the GL and S1 to the GL (r=0.73), and sacral slope and lordosis (r=0.89). With advancing age, the GL location with respect to the heels does not change and a global spino-pelvic regulatory mechanism appears to maintain this posture: trunk global inclination shifts forward, pelvic tilt increases, and the pelvis shifts toward the heels, increasing its distance from the GL.

Discussion: his study demonstrates the importance of pelvic parameters and trunk inclination in the regulation of the GL location. The relationship between the gravity line, pelvic parameters, and overall spinal alignment may emerge as essential in the evaluation of spinal deformity. Further investigation in this field may lead to a formula of balance that can assist in optimal planning of corrective procedures for spinal deformity.

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.