The pars interarticularis, or isthmus, is the portion of bone between the superior and inferior articular processes in the dorsal arch of the lumbar vertebrae. A number of studies have demonstrated that the pars interarticularis in the lumbar spine resists a significant amount of force and faces particularly high loads under extension and extension combined with rotation. Data suggest that the pars interarticularis is the weakest component of the neural arch. The combination of high load bearing and structural weakness is thought to predispose to fracture. While some fractures can heal spontaneously, an established nonunion is known as a pars defect.

A pars defect effectively disengages the facet joint from contributing to resistance to anterior subluxation. This leaves the disk to be the primary resistance to this force. While not all individuals with a pars defect will develop a slip, in those that do, it is thought that it is failure of the disk to resist anterior subluxation that eventually leads to spondylolisthesis. With progressive disk degeneration and subluxation, the neural foramina between the slipped vertebrae can be narrowed. While pars defects can be present at any level of the lumbar spine, they most commonly occur at L5–S1. The prevalence of pars defects in the general population is approximately 5%.

The pathogenesis of isthmic spondylolisthesis has long been controversial, with theories ranging from birth defects, genetic anomalies, to distant traumatic sequelae. Most commonly, it is believed to be a stress fracture that occurs due to repetitive loading. Gravity and upright posture place unique stresses on the pars interarticularis. Adding axial, rotational, cyclic flexion, and extension loading forces only further add to stress the isthmus may experience. Given these factors, microfractures of the pars can develop. Although these fractures may heal completely, fibrous union often occurs which can remain stable or allow spondylolisthesis.

While the incidence of isthmic spondylolisthesis in the general population is 4% to 8%, the observation of a higher incidence (25% to 30%) among first-degree relatives may indicate a genetic component. It has also been noted that anterior migration of the vertebral body progresses more significantly in isthmic spondylolisthesis during adolescence, perhaps due to growth potential or hormonal influences. It is believed that competitive female athletes, such as cheerleaders and gymnasts, are particularly at risk for developing spondylolisthesis due to hormonal changes associated with menarche coupled with the physical stress of their particular sport.

The incidence of spondylolysis is higher than that of spondylolisthesis with roughly 50% of pars defects giving rise to subluxation. Thus the reported incidence of spondylolysis ranges from 4.4% to 5.8% while that of isthmic spondylolisthesis ranges from 2.6% to 4.4%. The first peak of presentation occurs between ages 5 and 7, while the second occurs during the teenage years. While the incidence of isthmic spondylolisthesis is twice as great in men than women, the chance of slip progression is four times that in females than males. Congenital anomalies such as spina bifida occulta are frequent in persons with isthmic spondylolisthesis (24% to 70%). It has been postulated that spina bifida occulta might increase the force transmitted to the pars and weaken it.

A prospective radiographic study analyzed 500 patients for pars defects, noting a 4.4% prevalence
in 6-year-old children and 6% in adults. Ninety percent of abnormalities were discovered at the L5 level. Twenty-one of the 27 defects were noted to be bilateral, the majority of whom had a spondylolisthesis.

Isthmic spondylolisthesis can be divided into three recognized subtypes (Fig. 21.1). Subtype A describes the classic lytic lesion of the pars interarticularis indicative of a stress or fatigue fracture. Subtype B, which is often thought to represent a healed pars fracture, is referred to as an elongated pars. The last, subtype C, which is the rarest of the three subtypes, refers to an acute fracture of the pars interarticularis.

The most common method to radiographically classify the degree of spondylolisthesis is the Meyerding classification. It uses the percentage of subluxation of the upper vertebra in relation to the superior end plate of the lower vertebra. The categories include grade I (0% to 25% subluxation), grade II (25% to 50% subluxation), grade III (50% to 75% subluxation), and grade IV (75% subluxation). Complete or 100% spondylolisthesis is termed spondyloptosis.

Spinopelvic morphology and orientation are important factors that need to be considered in spondylolisthesis. One important factor to take into account other than translational subluxation is the angular displacement that occurs with spondylolisthesis. Sagittal rotation, also known as “slip angle,” allows one to measure the degree of local lumbosacral kyphosis (Fig. 21.2). By taking the intersection of two lines, the first being a line parallel to the superior end plate of L5 and the second being a perpendicular to a tangent line drawn along the
posterior aspect of the sacrum. This slip angle in the normal patient usually measures zero.

Imaging of spondylolisthesis can begin with standard plain radiographs that include lateral, anteroposterior, and oblique views. In an effort to produce radiographs that are more sensitive, some advocate using a 30-degree oblique cranial tilt to demonstrate spondylolysis. The abnormality is classically described as a fracture of “the Scottie dog’s neck” (Fig. 21.3). Flexion-extension radiographs may demonstrate dynamic mobility that may exist. CT scans are the most sensitive imaging modality in detecting the presence of a pars defect. That said, it is more common for an MRI to be the first advanced imaging study for a symptomatic patient. While pars defects can be seen on an MRI, sensitivity and specificity is not well established. MRI is useful in detecting stress reactions in the pars region, noted by intraosseous edema, prior to fracture. A SPECT scan is the most sensitive imaging modality at detecting such stress reactions, however. MRI scans can directly demonstrate foraminal stenosis at the level of spondylolisthesis which can be caused by a combination of uncovered disk, fibrous tissue from the pars defect, and portions of the articular processes. Whether by MRI, CT, or plain films, a characteristic finding of an isthmic spondylolisthesis is anterior subluxation of the vertebral body without anterior displacement of its associated spinous process or lamina. It should be noted that the central canal in low-grade slips is preserved because of this feature.