The level of the spondylolisthesis is important, as it will help with the clinical presentation as well as the future progression. Neurologic symptoms, when present, often occur due to compression of the exiting nerve root, and it is important to correlate the symptoms with the imaging. While L5/S1 is the most common location for an isthmic spondylolisthesis, an L4/5 isthmic spondylolisthesis is a significant risk factor for slip progression and symptoms [19, 27]. Additionally the percent of slip at initial diagnosis is one of the most important factors in predicting future progression. In a 14.3-year follow-up of 272 patients, the only predictor of slip progression was the amount of slip at initial diagnosis; patients with an initial slip greater than 20 % have a significantly increased risk of progression [28].

The lumbosacral angle, or the slip angle, is a measurement of the sagittal alignment of L5 and S1. There are multiple descriptions of how to calculate this value. It can be calculated by measuring the angle produced by a line parallel to the inferior end plate of L5 and one perpendicular to the posterior aspect of the S1 body; [26] or it can be calculated using the angle produced by lines along the inferior end plate of L5 and the sacral end plate [29]. However, caution is warranted when measuring the inferior L5 end plate, as it can be dysplastic and, therefore, distorted, in spondylolisthesis [30]. Unfortunately, while much of the literature emphasizes the slip angle, it has not been found to be of prognostic value for clinical symptoms or slip progression [15, 28] (Fig. 5.2).

The lumbar index is a measurement designed to quantify the change in the shape (i.e., dysplasia) of the L5 vertebral body from a square to a trapezoid in patients with isthmic spondylolisthesis. The lumbar index is calculated by dividing the height of the posterior vertebral body of L5 by the height of the anterior vertebral body of L5 [26, 31]. The average lumbar index in patients with isthmic spondylolisthesis ranges between 0.70 and 0.76; patients without an isthmic spondylolisthesis have a lumbar index around 0.90 [1, 30–32]. In patients in adolescence and early adulthood, the lumbar index has not been found to have any prognostic value for slip progression, but at a 45-year follow-up, Beutler et al. found a lower lumbar index was associated with an increased slip progression [15, 28] (Fig. 5.3).

Sacral inclination is a measure of the vertical orientation of the sacrum. It is calculated by measuring the angle formed by a line parallel to the posterior S1 vertebral body, and a line perpendicular to the floor [26]. While often reported, there is no evidence that sacral inclination affects slip progression or clinical symptoms. It is, however, related to pelvic incidence and pelvic tilt. With that relationship, sacral inclination (also called sacral slope) may have a role in the development of isthmic spondylolisthesis as well as sagittal balance (Fig. 5.4).

Sacral rounding is a change to the sacral morphology (i.e., dysplasia) leading to a more rounded or dome shape. It is graded zero to three, with zero having no sacral rounding, one having less than 33 % sacral rounding, two having between 33 and 66 %, and three being more than 66 % [26]. While there has been no definitive link to the amount of sacral rounding and slip progression [28], in a case series of 27 patients with spondyloloptosis, all patients had significant sacral rounding [33] (Fig. 5.5).

Thirty percent of adult patients with isthmic spondylolisthesis have spina bifida occulta [13], and while it is more common in severe slips, it does not increase the risk of slip progression [28].

Duval-Beaupère et al. [39] initially described pelvic incidence in 1992 as an angle formed by a line drawn from the center of the sacral end plate to the center of the femoral head, and a line drawn through the center of the sacral end plate that is perpendicular to the end plate. The average pelvic incidence in adults is 52°, with a normal range for men of 53.2° ± 7.0° and 48.7° ± 7.0° for women [40].

Sacral slope is the angle created by a horizontal line and a line drawn down the sacral end plate [41]. The average sacral slope is 39.4° ± 9.3° [35]. The pelvic tilt is the angle formed by a line that runs from the center of the sacral end plate to the center of the femoral head and a vertical line through the center of the femoral head. The average pelvic tilt is 12.3° ± 5.9° [35], and an increase in pelvic tilt correlates to a retroverted pelvis.

Due to the geometric relationships of these measures, the pelvic incidence is the sum of the sacral slope and the pelvic tilt, and while the overall pelvic incidence remains constant in adulthood, the sacral slope and pelvic tilt change based on the position of the pelvis [41, 42] (Fig. 5.6). The pelvic incidence is therefore a measure of pelvic morphology, while pelvic tilt and sacral slope are measures of pelvic orientation.

Pelvic incidence is correlated strongly to isthmic spondylolisthesis and is significantly higher in patients with both low-grade (68.5°) and high-grade (79.0°) slips compared to controls. Additionally, patients with a high-grade slip have a significantly higher pelvic incidence than those with a low-grade slip [40]. With this relationship being better understood, multiple authors have discussed the possibility of two different mechanisms for isthmic spondylolisthesis depending on the pelvic incidence. Patients with an increased pelvic incidence have more shear stress across the pars interarticularis resulting in the primary mechanism of isthmic spondylolisthesis in this population. Comparatively, patients with a low pelvic incidence may be predisposed to impingement of the posterior elements leading to repeated microtrauma [6, 7]. However there has been no literature to date that can definitively show any causality between pelvic incidence and isthmic spondylolisthesis [7]. Additionally, while there is a correlation between high-grade slips and increased pelvic incidence, there is no evidence that an increased pelvic incidence increases the risk of slip progression after initial diagnosis [43].

An abundance of recent literature has established the importance of the global sagittal balance in adult spinal deformity patients. These patients have significantly better health-related quality of life outcomes if normal sagittal balance is maintained [36, 37, 44, 45]. Patients with low-grade isthmic spondylolisthesis are typically not at an increased risk of sagittal malalignment [46, 47], however, patients with a high-grade slip are at an increased risk of global sagittal malalignment [48, 49].

Pelvic incidence correlates the plane of the sacral endplate to the axis of rotation of the femoral head, and since the pelvic incidence is fixed, the pelvis and spine adapt to balance the trunk in the upright position [7]. In patients with a high-grade slip and sagittal imbalance, balance is restored initially through an increase in lumbar lordosis with a corresponding linear increase in sacral slope. Additional balance is achieved through an increase in pelvic tilt leading to a retroverted pelvis [35, 38, 41, 48, 50] (Fig. 5.7). When attempting to evaluate the global sagittal balance, multiple metrics must be used: the sagittal vertical axis (SVA) should be less than 50 mm; the pelvic tilt should be less than 20°; and a patient’s lumbar lordosis should be within 9° of the pelvic incidence [36, 38] (Fig. 5.8).

It is important not to focus only on one of these values in isolation. While Harroud et al. [45] found a significant decrease in health-related quality of life measurements in patients with a high-grade spondylolisthesis whose SVA fell anterior to the center of the hip, some patients may achieve a balanced SVA by severely retroverting the pelvis. Lafage et al. [36] established that patients with an SVA less than 50 mm and a pelvic tilt less than 25° have better health-related quality of life measurements, compared to patients with either an SVA less than 50 mm or patients with a pelvic tilt less than 25°.

In an effort to incorporate a patient’s spinopelvic parameters and global balance into the spondylolisthesis classification system, the Spinal Deformity Study Group has developed a new classification based on the degree of slip, the pelvic incidence, and the sagittal balance [7] (Fig. 5.9).