**2.3. Historical perspective on understanding of sagittal alignment**

The evolution of our understanding of sagittal alignment has been developing for over 150 years. It is von Meyer who is credited with the discovery of the weight center of the human body at the level of the second sacral vertebra [6]. Although highly variable, most clinicians and researchers reporting on alignment agree that the line of gravity should pass near the mastoid process of the temporal bone, just anterior to the second sacral vertebrae, just posterior to the hip joint, and just anterior to the knee and ankle joint [6, 7, 9, 10]. Thus, balanced about this line of gravity, man is able to remain upright with mild anterior/posterior sway and minimal energy expenditure.

The German orthopedist, Franz Staffell, in 1889, is credited with further sub-classification of ideal posture into categories (i.e., round, flat, lordotic) [11].

Statements such as that made by Schulthess in 1905 are indicative of the openness of clinicians to the variation in the sagittal plane versus the assumption of one ideal posture and all else faulty [12].

Kendall, Kendall, and Boynton in 1952 described an ideal postural type and three faulty postural types (kyphotic-lordotic, flat back, and swayback) [7]. Rex McMorris in 1961 described what he termed faulty postural types in children [13]. Roussouly in 2005 identified lordotic types, which will be discussed in further detail later in this chapter [14]. Mac-Thiong in 2010 described six postural types [15].

Our evolution in understanding the relationship of the pelvis to the spine can be traced to the early 1960s. Joanne Bullock-Saxton, in her narrative review in 1988, citied work by Hollinshead in 1962 observing a relationship between the position of the pelvis and the amount of lumbar lordosis [16]. Indeed, others also discussed the interaction between pelvic obliquity or pelvic inclination and its role in determining the degree of lumbar lordosis. Additionally, the obliquity of the sacrum was determined to be related to the degree of lumbar lordosis [16]. During et al. explained the relationship between the position of the sacrum and the depth of lumbar lordosis as functional. The steeper the slope of the upper portion of the sacrum, the deeper the lumbar lordosis needs to be in order to maintain optimal position of the upper part of the body over the lower along the line of gravity [17].

The advent of instrumentation with spinal fusion in the surgical management of scoliosis and spinal deformity has led to a greater push to understand sagittal alignment. Although early surgical instrumentation was effective in addressing the frontal plane aspect of the scoliotic alignment, follow-up revealed often deleterious effects on the sagittal plane [18]. Doherty, in 1973, described what was later coined by Moe and Denis as "flatback syndrome," characterized by a fixed forward inclination of the trunk due to the loss of normal lumbar lordosis [18]. The early instrumentation combined the use of a straight rod with distractive forces and, when intervention extended to lower lumbar levels, the combination of these forces led to a loss of lumbar lordosis [18]. Recognition of this postoperative outcome led to advancements in surgical technique, which is beyond the scope of this text, as well as the understanding of the need to not only preserve but also enhance lumbar lordosis in order to minimize the risk of postoperative flatback syndrome. The identification of pelvic incidence, a morphological parameter that describes the relationship of the sacrum to the femur, represented a turning point in the movement to better address the sagittal plane from an operative perspective [19]. These parameters and their clinical implications may also be useful, as we will see, for the physiotherapist working with the older adult with spine deformity, as it gives us parameters within which we can better prognosticate the type of client we may be able to work with successfully.
