Etiology and Types of Spondylolisthesis

Biomechanically, the motion segment is stabilized by the presence of the intervertebral disc and facet joints. Disruption of this three-joint complex through anatomic variation, either congenital or acquired, results in malalignment. The lordotic lower lumbar spine is continually subjected to gravitational forces that pull the vertebral bodies ventrally.

Spondylolisthesis is also graded in severity from 1 to 5 according to the Meyerding system.¹ Grades 1 and 2 are termed low-grade, while the remainder are considered high-grade (Fig. 84-1).

FIGURE 84-1 Meyerding grading system.

Several classification systems have been developed, but Wiltse’s classification from 1957 remains useful and focuses on the etiology of the slip² (Box 84-1). It focuses on the dorsal elements, which counteract the forces discussed previously. The most commonly encountered types are the isthmic and degenerative types, and discussion of these will occupy the bulk of this chapter.

Congenital Spondylolisthesis

Congenital spondylolisthesis is more common in females, with a 2:1 ratio. This type is thought to account for 14% to 21% of all cases of spondylolisthesis.

A congenitally dysplastic dorsal arch, which includes the pars interarticularis and facet joints, allows for misalignment across a vertebral segment (Fig. 84-2). These defects are commonly seen at the L5 level (in a case of L5-S1 spondylolisthesis) but may include abnormalities of the sacral ala or superior articular facet.³

FIGURE 84-2 A and B, Congenital spondylolisthesis.

Three subtypes have been described. In Type A, the facets are horizontally oriented and therefore are unable to act as a buttress to prevent slippage. In Type B, the facets are asymmetrical and sagittally oriented. Type C cases involve other malformations that do not fit into the first two categories.

Neurologic symptoms may occur with relatively little displacement of the vertebral body if the dorsal arch is intact. This is because the arch becomes docked on the ventral vertebral body, compressing the cauda equina as a result. Cauda equina syndrome has been noted.

Isthmic Spondylolisthesis

Isthmic spondylolisthesis (IS) represents the most common form of spondylolisthesis (Fig. 84-3). In contrast to the congenital form, it is more common in males. Therefore, when found in females, it tends to represent a more significant condition, with more severe symptoms and a higher rate of progression.

FIGURE 84-3 A–C, Isthmic spondylolisthesis.

IS is most commonly seen at the L5-S1 level. Some consideration has been given to anatomic factors such as pelvic incidence^4–7 and lumbosacral transitional vertebrae.⁸

A modification of the Wiltse classification includes subgrouping of the isthmic category. Type 2A is the commonly seen lytic fatigue fracture of the pars interarticularis. Type 2B spondylolisthesis is seen in the case of an elongated pars, which may result from pars fracture with subsequent union in the distracted position. This should not be confused with a congenitally dysplastic pars. Type 2C is seen in an acute traumatic fracture of the pars.

There is a familial and genetic predisposition to IS. Relatives of IS patients have a 30% or more increased risk of having the disorder.^9–11 Inuit Eskimos have up to a 50% incidence of IS in their population, compared to 6% quoted for the general population and 2.8% in people of African descent.

While genetics plays a role, there is strong evidence for environmental factors in the development of IS. Factors that place increased force across the vertebral column, especially the lower lumbar spine, may result in fatigue fracture of the pars interarticularis, with resultant ventrolisthesis. The bipedal, erect gait of humans places greater stress across the lordotic lower lumbar spine than is seen in animals that have a quadruped gait. Activities that further accentuate the lordosis, such as hyperextension, exacerbate this picture. Therefore, adolescents who are involved in sports such as gymnastics, weight lifting, swimming, and diving have been known to display a higher incidence of symptomatic spondylolysis.^10–12

Wiltse demonstrated that most cases of spondylolisthesis present before the end of the first decade. Fredrickson demonstrated a 6% incidence of spondylolisthesis in the general population.¹⁰ Saraste noted earlier disc degeneration at the level of the slip, and risk factors for back pain included spondylolysis at the L4 level and greater than 25% slip.¹¹

In Frederickson’s 45-year follow-up study, only 5% of patients demonstrated progression. However, when the most symptomatic patients are followed, the incidence appears higher, at 20%. When progression occurs in the adult years, it usually results in no worse than a grade 2 slip.¹⁰

Risk factors associated with progression include skeletal immaturity associated with a high-grade slip. A high slip angle (>50 degrees) may predict progression.^5,13,14 The slip angle is measured between a line drawn along the superior end plate of L5 and the perpendicular to another line drawn along the dorsal vertebral border of S1 (Fig. 84-4). A high angle signifies kyphosis.

FIGURE 84-4 Horizontal or sagittal facet joint alignment predisposes to degenerative spondylolisthesis.

Most authors contend that progression of the slip after skeletal maturity occurs as a result of disc degeneration below the level of the slip. Patients may present early or late in life. During adolescence, symptoms relate to the pars fracture and include axial back pain with or without leg pain. In later adult life (after age 50 years), discogenic back pain and radicular leg pain related to worsening foraminal stenosis become a problem. Patients who present early in life are felt to represent a different group than the 6% of the general population with pars defects (who may or may not be symptomatic and have an incidence of back pain that follows that of the general population).¹³

While the pars fracture may or may not heal, once a slip has occurred, it is thought to persist, if not progress, with time. Only a single case report exists documenting spontaneous resolution of a slip in an adolescent patient.¹⁵

Iatrogenic Spondylolisthesis

An overly aggressive surgical decompression that does not respect the need for preservation of at least half the facet joint and 1 cm of the pars interarticularis places the patient at risk for iatrogenic intraoperative or postoperative fracture and spondylolisthesis (Fig. 84-5). This condition is poorly tolerated and almost uniformly involves revision surgery, which has a higher rate of complications.²⁰

FIGURE 84-5 Iatrogenic spondylolisthesis in a patient with severe chronic back and leg pain who underwent multiple surgeries for lumbar decompression and fusion. Note the pars fracture at the cephalad-most level of the decompression (adjacent to a solid L5-S1 noninstrumented fusion). An interbody construct was chosen for anterior support, given the significant instability noted with complete reduction of the slip simply with prone positioning under anesthesia.

Presentation

Patients typically present with a complaint of back and leg pain. The pain is typically mechanical, positional, and activity-related. Leg pain may be radicular and dermatomal in nature or be associated with neurogenic claudication. Such claudication symptoms are seen in DS patients with central stenosis (Fig. 84-6) and include cramping bilateral buttock and thigh pain, “discomfort,” or “fatigue.” This improves with postural changes, including flexion and rest. Patients tend to lean on a cart at the supermarket, on a bench at the park, or on furniture and countertops at home. They describe less difficulty going up hills (in a relatively flexed position) than down. They may also be able to ride a bicycle (again placing the lumbar spine in a flexed position) for far longer than they are able to walk. IS patients, on the other hand, commonly suffer from radicular symptoms related to foraminal stenosis (Fig. 84-7). “Pseudoradicular” leg pain has been described in IS patients who demonstrate more of a referred type of leg pain pattern that does not fit a specific dermatome.

FIGURE 84-6 Stenosis pattern in degenerative spondylolisthesis (DS). Note the central, lateral recess and foraminal stenosis all present in this patient with L4-5 DS with stenosis. The facet joints show severe degenerative changes.

FIGURE 84-7 Foraminal stenosis in isthmic spondylolisthesis.

On physical examination, IS patients may demonstrate hamstring tightness as the pelvis retroverts to compensate for the lumbosacral kyphosis. As a result of this pelvic malalignment, patients develop flattening of the buttocks. A stepoff may be noted above the LS junction, followed by a proximal compensatory hyperlordosis. Hamstring tightness can result in a waddling gait, as the patient is unable to fully extend the hip to take a long stride. In severe cases, a crouched gait is seen, in which the hamstring tightness is so severe as to necessitate walking with the knees flexed. Signs of neurologic impairment, including numbness and focal weakness, can be seen.

DS patients have been noted to have a higher body mass index.²¹ They are often limited in their mobility and may demonstrate difficulty in the physician’s office when transitioning between sitting and standing, owing to development of proximal gluteal and quadriceps weakness. Extension is limited and painful, some patients being unable to stand erect during an acute exacerbation.

Imaging

Plain lateral radiographs will show a slip, and this may be more evident on flexion-extension radiographs in cases of dynamic instability. A pars fracture may be visible on anteroposterior or lateral images. Oblique radiographs show the pars en face, and the fracture can be seen as a collar on the “scotty dog.” In cases of high-grade slips with compensatory pelvic verticalization, a “heart-shaped” pelvis is seen. Radiographs should be obtained in the upright position, as the slip may reduce in the supine position. Lateral radiographs should be obtained in the true lateral position, as even slight rotation may result in an underappreciation for the degree of slip.²²

While patients with spondylolysis alone (without listhesis) do not demonstrate radiographic abnormalities in spine morphology, those with IS do have a high lordosis angle, L5 vertebral body wedging, and L4-5 disc wedging. In DS, spine morphology shows wedging of the L5 vertebral body but less wedging of intervertebral discs.^7,23–26

Adolescents presenting with low back pain commonly have pars fractures that may or may not show on plain radiograph, or even on bone scan or MRI. Single-photon emission computed tomography (SPECT) scans have an increased sensitivity for detection of spondylolysis.²⁷

Radiographic predictors of instability include spondylolisthesis, facet widening, end-plate degenerative changes, sagittal facet orientation, and facet sclerosis, widening of the facet being more associated with dynamic instability.^28–30 Lumbosacral transitional anatomy may be a contributor as well.⁸

CT scans can be helpful in showing a pars fracture. To the untrained eye, bilateral pars fractures may appear similar to facet joints. Axial images should be scrutinized carefully in the area of the pars and correlated with sagittally reconstructed images. Three-dimensional reconstructed images are beneficial in demonstrating pathology related to congenital malformations.

MRI scanning is considered for patients with complaints of leg pain or those who are to undergo surgical intervention. Neural compression can be detected in this manner, as can synovial cysts and facet joint effusions, which have a correlation with spondylolisthesis.^28–31 Symptoms can be correlated with degree of intervertebral disc degeneration associated with the spondylolisthesis.¹⁶ In cases of pathologic spondylolisthesis, MRI assists the surgeon in delineation of an associated soft tissue mass and the extent of metastatic spread. The degree of slip cannot be assessed reliably on such scans, and some slips are missed secondary to spontaneous postural reduction in the supine position in the MRI scanner. Newer technology, allowing for functional MRI scans in the upright position, have on occasion detected greater pathology.^14,32

Nonsurgical Treatment

Nonoperative management of IS patients includes observation with activity restriction and physical therapy for instruction in a flexion exercise program. Bracing with a soft corset, a hard clamshell lumbosacral orthosis, and formal casting have all been employed with success in adolescent patients with a “hot” spondylolysis (i.e., one that is active on bone scan, with physiologic potential for healing, whether fibrous or bony).^33–35

In the skeletally immature patient, a low-grade slip should be observed with serial radiographs every 6 months until skeletal maturity.³⁶ Symptomatic patients should have activities restricted, and consideration may be given to bracing. Adolescent patients with high-grade slips or a high slip angle are at risk for progression and are offered surgery.

In DS cases, the first line of treatment includes judicious use of over-the-counter medications with food. The importance of a discussion with patients regarding gastrointestinal side effects of commonly used nonsteroidal anti-inflammatory agents cannot be overstated.

A formal regimen of exercises performed under the dutiful eye of a good physical therapist can be of tremendous benefit, not only in assuaging a patient’s acute symptoms but also in training for proper “back hygiene.” Patients receive instruction on how to avoid activities or injuries that would contribute to future episodes. Exercises focus on flexion, which limits forces across a painful spondylolysis or a painful facet and can increase neural canal and foraminal dimensions, resulting in improvement of radicular symptoms. Lumbar traction is of variable benefit and may provide a counter to associated muscle spasm.³⁷ Patients benefit most in acutely painful (<6 weeks) situations. Chiropractic care often employs modalities similar to those of physical therapy but may focus on passive interventions such as mobilization and manipulation, ultrasound, and electrical stimulation.^35,38

Epidural steroid injections are of significant benefit to the patient with radicular leg pain, with the potential for significant symptomatic relief in an expedited fashion. A series of up to four injections in concert with other treatments can allow patients who would otherwise be considered surgical candidates to avoid surgery.³⁹