[Sagittal equilibrium of the pelvis: analysis of the inclination of the ischio-pubic ramus from the horizontal].

PURPOSE OF THE STUDY The sagittal equilibrium of the spine and pelvis has been examined in numerous studies looking for the origin of certain posture disorders of the spine and the cause of lower back pain. Sagittal x-rays of the pelvis provide an analysis of the degree of inclination of the pelvis from the horizontal and the bi-coxo-femoral axis and information on the form of the sacrum. There is no radiographic parameter however which analyzes the transition between the pelvis-sacrum component and the femoral component, i.e. the periacetabular region. In the sagittal plane, a line tangent to the ischio-pubic ramus would appear to best reflect the orientation of the periacetabular region 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 horizontal and its relations with other sagittal radiographic parameters in a population of growing children with spinal disorders. MATERIALS AND METHOD The study population included 100 children with spinal disorders who underwent a teleradiographic series with lateral view in the upright position. Most of the children had scoliosis (80 cases), 7 had kyphoscoliosis, 4 isthmic spondylolysis with spondylolisthesis, 1 spondylodiscitis and 4 lower back pain. Mean age was 13 years (range 2.5-22 years). We measured 7 radiographic parameters: lumbo-sacral angle, slope of the sacrum, pelvic version, incidence, thickness, overhang, and inclination of the ischio-pubic ramus from the horizontal. Data were analyzed to search for correlations between radiographic parameters and between radiographic parameters and clinical features. RESULTS Mean inclination of the ischio-pubic ramus from the horizontal was 33.9 degrees (SD =5.9 degrees ). The only positive statistical correlation between the inclination of the ischio-pubic ramus and the clinical data was a relationship with the position of the arms compared with the horizontal (p =0.04). There was no correlation with age, sex, ethnic background, etiology. There was no correlation between the inclination of the ischio-pubic ramus and the other radiographic parameters (coefficient r ranging from 0.06 to 0.43). DISCUSSION The interdependence of sagittal radiographic parameters of the pelvis and the spine have been largely demonstrated. Certain pathological situations (isthmic spondylolysis with spondylolisthesis, lower back pain, etc.) can be explained by the value of these parameters, particularly incidence. The inclination of the ischio-pubic ramus from the horizontal reflects the periacetabular region. When analyzed in the sagittal plane, it was found to be a more stable parameter, independent of most clinical criteria (particularly age, and etiology) and of the other radiographic parameters studied. The only 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 incidence, although the inclination of the ischio-pubis ramus is an expression of a region different than the pelvis. This study suggests that the periacetabular region plays a key role in acquisition of the upright position in humans. The periacetabular region would be a fixed point around which the lower limbs and spine describe varying orientations.