Abstract
Introduction
Usually, the analysis of the spinal sagittal shape is based on a vertebral semantic, focusing on the curves (lordosis, kyphosis) and the spinopelvic parameters (pelvic incidence, sacral slope, pelvic tilt, sagittal balance). This analysis failed in defining a normal shaped spine, and in relating strictly the incidence to the lordosis. The author proposes a very different analysis of the sagittal spinal shape, named “ the Pendulum Law.” It enables to define an ideal biomechanical shaped spine and classify the sagittal shape precisely.
Material and Methods
300 sagittal shapes have been studied. A vertical analysis (according to gravity) is performed, reintegrating the spine into an analysis of the muscular body's scheme and relating it to the respiration. The biomechanical spine is assimilated to three intricated Mongol bows. Different apex (contact between the vertebral body and the plumb line) are defined. They are related to muscular tension lines or attaches. The lungs apex are paramount to explain the spine. The trunk is always swaying but lungs pressure (when breathing in) stabilizes it either in an anteroposterior plan or in the sagittal plane. The repartition of the apex has been assessed in all the spine drawing specific lines.
Results
More than 95% of the studied spine abide by the “Pendulum law” which states that: “ whatever the spine is (including the common variation or abnormalities), the standing position consists in placing the Superior Lung apex according to the following rules: In the anteroposterior view: the plumb line equidistant from the lung apex is maintained above the sacral apex. In the sagittal view: the plumb line from the superior lung apex is equidistant of the thoracic apex and the lumbar apex. The two Pelvico-thoracic lines (the line linking the sacrum apex to the antero-inferior lung apex and the line linking the center of the femoral head to the postero-inferior lung apex) cross each other in the point “E,” exactly above the plumb line dropping from the superior lung apex and in front of the lumbar apex. Therefore, the Pendulum Law relates the sagittal shape to the gravity, the muscular body's scheme and the breathing. An ideal biomechanical sagittal shape lines up the thoracic apex and the sacral apex vertically. The alignment of the symphysis, the lumbar apex and the thoracic apex cross the vertebral bodies obliquely and harmoniously. The lumbar apex (in front of the upper plate of L4) is placed above the center of the femoral head.
Conclusion
The Pendulum Law relates the sagittal shape to gravity, the muscular body's scheme and respiration. Most of the commons Abnormalities are integrated according to this rule. The Pendulum is the “vertical tension” of the body's scheme that enables us to move harmoniously. (As the length of the femur is strictly related to the ideal flexion/extension of the Knee). Lordosis is related to pneumatic stabilization of the trunk and definitively not to the pelvic incidence.