Objective. To propose and validate a dimensional parameter, the sagittal pelvic thickness (SPT) (distance between the middle point of the upper sacral plate and the femoral heads axis, expressed as a ratio with the length of the upper plate of S1: (SPT/S1) for the analysis of the sagittal balance of the pelvispinal unit. Methods. The parameters were analysed on standing radiographic imaging and compared for normal, low back pain, children, and spondylolysis cases. Results. Values of SPT/S1 were observed significantly higher in high grade spondylolysis populations and in children (3,5 and 3,7) than in normal population (3,3). A geometrical connection with the classical angular parameters validated SPT/S1. Conclusion. SPT/S1 was considered reflecting the lever arm of action of spinopelvic muscles and ligaments and describing the ability of a subject to compensate a sagittal unbalance. It was proposed as an anatomical and functional pelvic parameter. 1. Introduction A strict relation was described between the sagittal pelvic anatomy and the sagittal shape of the spine, particularly the amount of lordosis needed for each individual. Therefore, angular parameters were recommended because they are usable disregarding the size of the subjects [1–5]. In the same way, the distinction was established a long time ago by morphologists and paleontologists between the “pelvis in tension” of the quadrupeds and the “pelvis in pressure” characterizing the bipedalism [6]. They were distinguished according to their more or less lengthened form, defined by the distance between the upper sacral plate and the coxofemoral joints: the sagittal pelvic thickness (SPT). In spite of characterizing the sagittal pelvic anatomy as well as, angular parameters, SPT was poorly studied. By a radiographic study, we investigated here its significance on the spinopelvic sagittal balance and its clinical relevance. 2. Material and Methods Angular and dimensional parameters were measured on 272 lateral radiographies including the pelvis, the femoral heads and the lumbar column, in standardized standing position [7]. For each, a scaling was incorporated allowing correction of the radiographic distortion. Data of four population groups were analyzed (Table 1). Table 1 The first group comprised 61 healthy voluntaries (column A). Data were obtained several years ago from for original orthopaedic studies [5]. At this time, these subjects provided their consent for the use of their radiographic and clinical data. The second group comprised 147 subjects suffering of low back pain from common chronic
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