Sagittal equilibrium of the pelvis: a study of ischio-pubic ramus orientation

Purpose of the study The sagittal equilibrium of the spine and pelvis has been examined in numer ous studies looking for the origin of certain posture disorders of the spin e and the cause of lower back pain. Sagittal x-rays of the pelvis provide a n analysis of the degree of inclination of the pelvis from the horizontal a nd the bi-coxo-femoral axis and information on the form of the sacrum. Ther e is no radiographic parameter however which analyzes the transition betwee n the pelvis-sacrum component and the femoral component, i.e, the periaceta bular region. in the sagittal plane, a line tangent to the ischio-pubic ram us would appear to best reflect the orientation of the periacetabular regio n and the muscular forces applied to this region. The purpose of this work was to analyze the inclination of the ischio-pubic ramus from the horizonta l and its relations with other sagittal radiographic parameters in a popula tion of growing children with spinal disorders. Materials and method The study population included 100 children with spinal disorders who underw ent a teleradiographic series with lateral view in the upright position. Mo st of the children had scoliosis (80 cases), 7 had kyphoscoliosis, 4 isthmi c spondylolysis with spondylolisthesis, 1 spondylodiscitis and 4 lower back pain, Mean age was 13 years (range 2.5-22 years). We measured 7 radiograph ic parameters: lumbo-sacral angle, slope of the sacrum, pelvic version, inc idence, thickness, overhang, and inclination of the ischio-pubic ramus from the horizontal. Data were analyzed to search for correlations between radi ographic parameters and between radiographic parameters and clinical featur es. Results Mean inclination of the ischio-pubic ramus from the horizontal was 33.9 deg rees (SD = 5.9 degrees). The only positive statistical correlation between the inclination of the ischio-pubic ramus and the clinical data was a relat ionship with the position of the arms compared with the horizontal (p = 0.0 4). There was no correlation with age, sex, ethnic background, etiology. Th ere was no correlation between the inclination of the ischio-pubic ramus an d the other radiographic parameters (coefficient r ranging from 0.06 to 0.4 3). Discussion The interdependence of sagittal radiographic parameters of the pelvis and t he spine have been largely demonstrated. Certain pathological situations (i sthmic spondylolysis with spondylolisthesis, lower back pain, etc.) can be explained by the value of these parameters, particularly incidence. The inc lination of the ischio-pubic ramus from the horizontal reflects the periace tabular region. When analyzed in the sagittal plane, it was found to be a m ore stable parameter, independent of most clinical criteria (particularly a ge, and etiology) and of the other radiographic parameters studied. The onl y determining factor appears to be acquisition of the upright position. The consistency of this parameter constrasts with the variability of the other radiographic parameters of the pelvis and the spine, particularly incidenc e, although the inclination of the ischio-pubis ramus is an expression of a region different than the pelvis. This study suggests that the periacetabu lar region plays a key role in acquisition of the upright position in human s. The periacetabular region would be a fixed point around which the lower limbs and spine describe varying orientations.