How can the shape/morphology of the pelvis be quantified in a simple but useful way for clinicians? Different parameters have been used to describe pelvic morphology based on standing lateral radiographs [1], but our preference goes to pelvic incidence (PI), a simple measurement introduced by Duval-Beaupère et al. [2, 3] PI is a fundamental pelvic anatomic parameter that is specific and constant for each individual and determines pelvic orientation as well as the size of lumbar lordosis (LL). PI remains relatively constant before walking age and thereafter, it increases significantly during childhood and adolescence until reaching its maximum and thereafter constant value in adulthood [4, 5]. PI is defined as the angle between a line perpendicular to the sacral plate and a line joining the sacral plate to the center of the axis of the femoral heads (Fig. 8.3). It is important to understand that PI is a descriptor of pelvic morphology and not of pelvic orientation: therefore, its angular value is unaffected by changes in human posture and will remain the same whether a subject is standing, sitting, or lying down, with the assumption that there is no significant motion occurring at the sacroiliac joints. In contrast, the pelvic tilt (PT) and the sacral slope (SS) are position-dependent variables, and are very useful to characterize the spatial orientation of the pelvis. SS is defined as the angle between the sacral endplate and the horizontal line (Fig. 8.3), while PT is defined as the angle between the vertical line and the line joining the middle of the sacral endplate and the axis of the femoral heads (Fig. 8.3). Because they are measured with respect to the horizontal and to the vertical, respectively, SS and PT describe the orientation of the pelvis in the sagittal plane and not its morphology. PI, SS, and PT are particularly useful because it can be demonstrated that PI is the arithmetic sum of the sacral slope (SS) + pelvic tilt (PT), the two position-dependent variables that determine pelvic orientation in the sagittal plane (Fig. 8.3). Because of this mathematical association between PI, SS, and PT, the morphology of the pelvis, as quantified by PI, is therefore a strong determinant of the spatial orientation of the pelvis in the standing position: the greater PI, the greater has to be SS, PT, or both. PI, PT, and SS are best measured from a standing lateral radiograph of the entire spine including the pelvis when evaluating the global sagittal balance.

Vaz et al. [6] have studied and reported the association and ranges of PI, PT, SS, Lumbar Lordosis (LL), and Thoracic Kyphosis (TK) in 100 young normal adult volunteers and have shown that all these parameters are closely linked and balance themselves, by muscular activity, to maintain the global axis of gravity over the femoral heads. The pelvic shape, best quantified by the PI angle, determines the position of the sacral end. The spine reacts to this position by adapting through LL, the amount of lordosis increasing as the SS increases to balance the trunk in the upright position (Fig. 8.4). Berthonnaud et al. [7], in a review of 160 normal adult volunteers, have shown that the pelvis and spine in the sagittal plane can be considered as a linear chain linking the head to the pelvis where the shape and orientation of each anatomic segment are closely related and influence the adjacent segment, to maintain a stable posture with a minimum of energy expenditure. Changes in shape or orientation at one level will have a direct influence on the adjacent segment. Knowledge of these normal relationships is of prime importance for the comprehension of sagittal balance in normal and pathologic conditions of the spine and pelvis. Mac-Thiong et al. [8, 9] have reported similar results in a reference pediatric and adolescent population of 180 subjects aged 4–18 years.

A balanced pelvis and spine results in a global spino-pelvic balance that is typically maintained within a narrow range of values in normal individuals. As shown by Mac-Thiong et al. [9, 10], the C7-plumbline of 85 % of normal adults and children aged >10 years stands behind the hip axis. In order to maintain an adequate global spino-pelvic balance (with minimum energy expenditure) throughout life despite ongoing degenerative changes, adjusting pelvic orientation is the key. Accordingly, a small increase in PT and a reciprocally small decrease in SS can occur with aging, thereby causing retroversion of the pelvis in an effort to prevent forward displacement of C7-plumbline [11]. Similarly in conditions that will tend to move the C7-plumbline forward, such as in spondylolisthesis or post-traumatic kyphosis, retroversion of the pelvis can also decrease the incidence of global sagittal imbalance. In addition, young patients with healthy discs and muscles also can increase their lordosis in an attempt to prevent the forward displacement of the C7-plumbline.

Sagittal sacro-pelvic morphology and orientation modulate the geometry of the lumbar spine and consequently, the mechanical stresses at the lumbo-sacral junction. In L5-S1 spondylolisthesis, it has been clearly demonstrated over the past decade that sacro-pelvic morphology is frequently abnormal and that combined with the presence of a local lumbo-sacral deformity and dysplasia, it can result in an abnormal sacro-pelvic orientation as well as in a disturbed global sagittal balance of the spine. These findings have important implications for the evaluation and treatment of patients with spondylolisthesis, and especially for those with a high-grade slip.

When compared with normal populations, PI is significantly higher [12–14] in spondylolisthesis and the difference in PI tends to increase in a direct linear fashion as severity of the spondylolisthesis increases [13]. The cause–effect relationship between pelvic morphology and spondylolisthesis remains to be clarified. Other measures of spino-pelvic balance are also significantly different in control populations compared to subjects with L5-S1 spondylolisthesis (Table 8.1).

In static standing position, the way SS and PT balance refers to the concept of sacro–pelvic balance. Members of the Spinal Deformity Study Group (SDSG) have specifically investigated sacro-pelvic balance in low-grade and high-grade spondylolisthesis (HGS). Roussouly et al. [15] proposed two different subgroups of sacro-pelvic balance observed in subjects with low-grade spondylolisthesis, that could be related to the etiology. In their opinion, patients with high PI and SS have increased shear stresses at the lumbo-sacral junction, causing more tension on the pars interarticularis at L5, and ultimately a pars defect (Fig. 8.5). On the opposite, patients with a low PI and a smaller SS have impingement of the posterior elements of L5 between L4 and S1 during extension, thereby causing a “nutcracker” effect on the pars interarticularis at L5 until lysis occurs (Fig. 8.5). On the basis of K-means cluster analysis, Labelle et al. [16] have confirmed the existence of these two distinct subgroups of sacro-pelvic balance in a larger SDSG cohort of low-grade isthmic spondylolisthesis: a subgroup with normal PI (between 45° and 60°) or low PI (<45°), and a subgroup with high PI (>60°). The clinical relevance of these findings is that since PI is always much greater than normal in HGS [13], it is assumed that the risk of progression in the low-grade subgroup with a normal PI is much lower than in the subgroup with an abnormally high PI value. It is hypothesized that the subgroup with normal PI corresponds to acquired traumatic cases with an acute or stress fracture (Marchetti and Bartolozzi [17] classification) in subjects with a normal sacro-pelvic morphology, whereas the other subgroup with high PI is associated with more dysplastic cases, but this assumption remains to be v1erified. As for HGS, Hresko et al. [18] have identified two subgroups of patients: balanced versus unbalanced pelvis (Fig. 8.6). The “balanced” group includes patients standing with a high SS and a low PT, a posture similar to normal individuals with high PI, whereas the “unbalanced” group includes patients standing with a retroverted pelvis and a vertical sacrum, corresponding to a low SS and a high PT. Each new subject with HGS can be easily classified by simply using the raw SS and PT values or, in borderline cases, by using the nomogram provided by Hresko et al. [18].