IntroductionThe role of the pelvic area in sagittal balance is evident for spinal surgeons, but the influence of the coxofemoral joint is underestimated and inadequately explained by conventional imagery. Comprehensive analysis of the pelvic and subpelvic sectors as part of the sagittal, frontal and cross-sectional balance of the trunk sheds new light on some spinal diseases and their relation to the pelvis.MethodsThis analysis, based on innovative radiologic methods as the EOS® technology but also on a new look at conventional imaging makes it possible to better analyze standing lateral images and seated images.ResultsDisturbances can come from atypical morphotypes or from unusual postures as in aging spine. The measurement of available extension and the concept of available flexion provide new information regarding individual’s adaptation to the imbalance induced by disorders of the spine or lower limbs.ConclusionA comprehensive assessment of each patient and in particular of the complex comprising the spine and the pelvis, is essential for understanding each individual’s adaptation to the imbalance induced by disorders of the spine or lower limbs.

Level III. Prospective diagnostic study.

Trunk balance in upright stance expresses an individual postural strategy found on anatomic and functional parameters. The "pelvic vertebra" is an essential transitional region for the coherence of spinal parameters (notably, lumbar lordosis and thoracic kyphosis) and pelvic parameters (sacral slope, pelvic tilt and incidence). Deterioration of this postural harmony is often associated with spinal aging, maladjusted spinal arthrodeses, or mechanical abnormalities of the hip joints. Spinal surgeons are aware of the importance of detecting and analyzing sagittal imbalance, whether compensated or not. The influence of the hip joint, however, is underestimated and poorly objectified on conventional imaging, as are its interrelations with overall lower-limb posture. Currently, hip surgeons focus basically on the pelvis as bone reference in planning implantation, peroperative adjustment and failure analysis. The antero-posterior (AP) pelvic view is the gold standard, with lateral views being little used. Influenced by the classic anatomic attitude in favor of transverse slices in dorsal decubitus, CT is considered the reference method for "horizontal" assessment of the hip joint. The present study draws attention to a more global vision of the pelvic and subpelvic regions in the sagittal balance of the trunk, relying on the sitting as well as the standing posture, as both involve subtle mechanisms of adaptation governed by the pelvic incidence angle.