The Standing Position: Its Principles and Spinopelvic Relations

Abstract

Permanent erect standing position is the main anatomical feature that differentiates humans from all animals. To achieve this situation, a specific combination between shape and positioning of both pelvis and spine is necessary. The pelvis is generally in a retroverted position and the spine presents successive curvatures: lordosis in the lumbar area and kyphosis in the thoracic area. This underlines the importance of the reciprocal anatomy in the sagittal plan of the sacral endplate and the bi-femoral axis. Described by Duval-Beaupère, the Pelvic Incidence (PI) is the main pelvic shape parameter that measures this relation. PI is directly linked with two positional parameters, Pelvis Tilt (PT) and Sacral Slope (SS) by the relation: PI = PT + SS. In a normal population PI has a great variability from 35° to 85° and beyond these limits, the functional anatomy is always pathological.

Linked with the sacral endplate, there is a direct relation between the spine and the pelvis, and due to the strong relation between PI and SS, the pelvis shape plays a big role in spinal curves’ orientation. Based on a functional segmentation limiting the lordosis from the sacral endplate to the inflection point where lordosis curve transitions in kyphosis, Roussouly has described four types of lordosis according to SS: types 1 and 2 for small SS (<35°), type 3 for average values (35° < SS < 45°), and type 4 for high values (SS > 45°). Due to the relation between PI and SS, one may encounter Types 1 and 2 with small PI and 3 and 4 with high PI.

The global analysis of the sagittal balance may be evaluated by C7 positioning. Different angles and distance were proposed to assess C7 versus pelvis position. Barrey proposed a ratio of distances that allows positioning the C7 plumb line regarding the femoral heads and the sacrum.

When the system is no longer balanced, there are two main compensation mechanisms to maintain an unbalanced but possible standing position: pelvis retroversion and spinal extension. Due to the variable spinal shapes associated with PI variability, the forces induced by the gravity on the functional vertebral unit may have variable degenerative expressions. This specific degenerative evolution associated with variable compensation mechanisms will generate the pathological shape that one may expect based on the original normal shape. That may help to understand the strategy of treatment targeting restoration of a balanced shape according to PI.