Sagittal spinal pelvic alignment.

The goal of any ambulatory patient is to maintain a horizontal gaze with the least amount of energy expenditure. With progressive deformity, and in particular sagittal malalignment, significant compensatory mechanisms must be used to achieve this goal. Each pelvis dictates the amount of lumbar lordosis required through its morphometric parameter pelvic incidence. The pelvis may compensate for decreasing lumbar lordosis (eg, age, flat back deformity) by retroverting and increasing pelvic tilt and decreasing the sacral slope. Underappreciation for these spinopelvic compensatory mechanisms leads to surgical under-correction, iatrogenic flat back and poor clinical outcomes.