Sagittal Spinal Alignment: An Evolutionary Perspective

For many decades the primary focus of attention in patients with deformity of the spine has been exclusively addressing the coronal alignment of the spine, while ignoring the importance of global sagittal alignment and balance. However, over the past several decades there has been intense scrutiny of these alignment parameters of the spine, with many studies identifying the importance of sagittal balance. ^, Researchers have developed a greater appreciation of the fact that, although spinal alignment is intimately associated with global spinal balance, these two aspects are in fact fundamentally distinct. In fact, global spinal balance is inclusive of all the regional spinal alignment parameters that must be assessed when evaluating the normal human standing alignment, because humans are biomechanically unique with respect to their upright posture and method of locomotion. ^, Spinal alignment is variable between humans, changes with aging, and refers only to the spine alignment within a set range of normal parameters that are determined by standing posterior-anterior (PA) and lateral 36-inch radiographs. It differs from global spinal balance, which includes the entire skeleton’s contribution to maintaining an ergonomically balanced upright posture. By and large, spinal balance is the ability of the human body to maintain its center of gravity over a base of support, mainly the pelvis. The contribution of the pelvis, hips, knees, and ankles is critical to this balance. Pelvic dynamics and the intrinsic range of motion of these joints are the primary compensators of maintaining an upright posture with aging and following iatrogenic flatback spinal fusion surgery. Although not as commonly described, but arguably important, alterations in muscular tension, such as hip flexion contractures, may similarly affect the body’s ability to compensate for spinal deformity. In fact, changes in two of the three major spinopelvic indices (pelvic tilt [PT] and sacral slope [SS]) are measured to attempt to quantify the compensatory mechanisms that maintain the head’s center of gravity over the pelvis. However, it is important to understand that spinopelvic indices are measurements aimed at evaluating a dynamic process, which is spinal balance. Spinal balance is the intricate and complicated coordination that exists between the musculoskeletal system and the nervous system’s sensory and motor pathways, and their effect on the alignment of the spinopelvic axis.

Excessive imbalance of the spine in any direction that cannot be easily compensated for by the lower extremities’ range of motion and is outside of the normal Dubosset “cone of economy” of the human trunk will inevitably lead to increased energy expenditure and significant patient dissatisfaction. This includes excessive sagittal, coronal, and/or rotational imbalance of the cervical, thoracic, and lumbar spine, alone or in combination, that results in a profound increase in the muscle force contraction demands of the paraspinal muscles and hip extensors, which struggle to restore normal upright balance. The increased muscle force contraction exerted to maintain compensation for an imbalanced spine will result in a substantial increase in oxygen consumption by the muscles and ultimate fatigue as the patient attempts to maintain an upright and ergonomic position. The purpose of this chapter is to discuss the intricacies of the human upright bipedal development, anatomy, and alignment along with the effects of aging and surgery in the context of global sagittal balance.