The LSJ is an area of transition from the relatively mobile lumbar spine with its normal lordotic curve, to the rigid kyphotic sacrococcygeal spine. Because the sacroiliac joints are relatively fixed with little motion, the pelvis and the hip joints play an important role in the overall orientation of the lumbosacral spine and its contribution to overall spinal balance in the sagittal plane. Several authors have described radiographic parameters that can be used to quantify sacropelvic and spinopelvic alignment [7–11]. Further, pelvic and sacral morphology has been cited as an important factor contributing to the development of spondylolisthesis. Some of the important anatomic factors that can be quantified on radiographs include pelvic tilt (PT), sacral slope (SS), pelvic incidence (PI), and lumbar lordosis (LL).

Pelvic tilt describes the orientation of the pelvis with respect to a vertical reference line. Anterior pelvic tilt is also referred to as pelvic flexion, anteversion, and inclination while posterior pelvic tilt is also referred to as pelvic retroversion, extension, or reclination [12]. Pelvic tilt is the angle subtended by a vertical line and a line connecting the center of the superior S1 endplate to a point that bisects the center of rotation of the hip joints (Fig. 2.2a). Pelvic tilt is not only a critical parameter in the evaluation of patients with spondylolisthesis, but also plays an important role in patients undergoing total hip replacement, re-orienting acetabular osteotomies, or treatment for femoro-acetabular impingement. Sacral slope describes the orientation of the superior endplate of the sacrum with respect to a horizontal reference line (Fig. 2.2b). Importantly, both PT and SS are measures of sacropelvic orientation as they are dependent on the position of the individual in space.

Unlike PT and SS, pelvic incidence is a measure of sacropelvic morphology as it is a measurement that is unique and fixed amongst individuals and does not change with positioning (assuming insignificant motion at the sacroiliac joints). Pelvic incidence was first introduced by Duval-Beaupère and colleagues in the early 1990s, and is measured by the angle subtended by a line perpendicular to the endplate of the superior endplate of S1, and a line connecting the middle of the superior endplate of S1 and the hip axis [13] (Fig. 2.2c). Conveniently, PI is the arithmetic sum of PT and SS. Because of this, the morphology of the sacropelvis (as defined by the PI) is closely correlated with the orientation of the pelvis in space. For example, if a patient has a very high PI, then the PT, SS, or both must also be high. Other measurements that have been described to quantify sacropelvic morphology are the pelvic radius angle [14] and pelvic-sacral angle [15]; however, these lack the geometric correlation with SS and PT. Pelvic incidence plays an extremely important role in overall sagittal balance, with higher PI requiring increased lumbar lordosis to maintain an upright balanced posture [9].

There have been several studies examining these measurements in both adult [16] and pediatric [17] populations defining their normal values (Table 2.1). The inter-observer and intra-observer reliability for measurement of PI is excellent [18]. PI has been shown to increase slightly and constantly over the course of childhood before stabilizing in adulthood [19]. Compared to normal patients, several investigators have shown that PI is significantly higher in those patients with spondylolisthesis [20–23]. Interestingly, there is a linear correlation of higher PI with worsening spondylolisthesis (Table 2.1), which makes sense biomechanically. A low PI means low values for pelvic tilt and sacral slope, which results in a relatively flat lumbar lordosis and a horizontal sacral endplate creating low shear stresses at the LSJ. Alternatively, a high PI means high values for pelvic tilt and sacral slope, increased lumbar lordosis, and a more vertical sacral endplate resulting in higher shear stress across the LSJ. However, no study has yet been able to demonstrate a causative relationship between sacropelvic alignment and the development of spondylolisthesis.

An important point to make is that two individuals with the same sacropelvic morphology (same PI) can have different sacropelvic orientation (Fig. 2.3). The Spine Deformity Study Group has investigated sacropelvic balance in patients with either high vs. low-grade spondylolisthesis. Patients with low-grade spondylolisthesis can be subcategorized into two groups: those with low PI (<45°) and those with high PI (>60°). It is thought that patients with high PI suffer tension failure of the pars causing spondylolysis, while patients with low PI undergo pars failure via a “nutcracker” effect, by impingement of the L5 posterior elements between L4 and S1 during extension [24]. Interestingly, patients with high-grade spondylolisthesis almost invariably have high PI, suggesting that low-grade slips with low PI never progress to a high-grade slip. High-grade slips can be subcategorized into those with a balanced pelvis versus those with an unbalanced pelvis. Patients with a balanced pelvis have a low PT and a high SS, with a posture similar to normal individuals with high PI but without spondylolisthesis, whereas those patients with an unbalanced pelvis have a retroverted pelvis with a high PT and low SS.

Significant remodeling of the lumbosacral spine can occur in the setting of spondylolisthesis. This remodeling can involve the posterior elements and/or the anterior elements. Dysplasia in the pars interarticularis is causative in dysplastic spondylolisthesis, but can also be secondary from a stress reaction or a malunion after repeated fractures. Dysplasia can also involve the lumbosacral kyphosis (LSK), trapezoidal L5, sacral dysplasia and kyphosis, bifid posterior arch, hypoplastic or aplastic facet joints, dysplasia of the pedicles, lamina or facets, as well as small transverse processes. In fact, Curylo and colleagues showed that 62 % of patients with spondyloptosis had evidence of posterior element dysplasia [20].