What is Spondylolisthesis, Listhesis etc

[Alastair]

Background: The word spondylolisthesis is derived from the Greek words spondylo meaning spine and listhesis meaning to slip or slide. Spondylolisthesis is a descriptive term referring to slippage (usually forward) of a vertebra and the spine above it relative to the vertebra below.

A grading system of spondylolisthesis is shown in Image 1.

Spondylolisthesis has many etiologies, all of which ultimately lead to a loss of the stability offered by the locking mechanism of the articular processes of the vertebrae that allow the superior vertebrae to slide forward over the inferior vertebrae (see Anatomy). The etiologies can be classified as congenital (dysplastic), spondylolytic (isthmic), degenerative, traumatic, pathologic, or iatrogenic (eg, postoperative).

The focus of this chapter is spondylolytic spondylolisthesis.



Pathophysiology: A working knowledge of normal anatomy and the locking mechanism is helpful in understanding how and where the mechanism can fail.

In the healthy person, the facet joint in the lumbar spine is, for the most part, in the coronal plane, with the inferior articular process of the superior vertebra located posteriorly and the superior articular process of the inferior vertebra located anteriorly. This configuration prevents forward movement of the vertebrae relative to each other. It locks in the superior vertebra relative to the inferior vertebra. This mechanism is important because the center of gravity of the human body is located anterior to the spine and exerts a forward slipping force on the spine, especially at the L5-S1 level. Furthermore, the anteriorly located center of gravity causes a rotating movement, with the axis of rotation oriented transversely at the L5-S1 level. Thus, in severe spondylolisthesis, a kyphotic deformity also develops.

Simplistically, a vertebra (and the spine above it) may slip if abnormalities are present within the facets and facet joints, articular processes, or neural arch (ie, connection between the facet joints and the vertebral body). Various etiologies affect one or more locations.


Spondylolytic spondylolisthesis

The spondylolytic (isthmic) type is the most common cause of spondylolisthesis. It affects the region of the pars interarticularis, which is roughly the region of the junction of the pedicle and lamina, where the articular and transverse processes of the vertebrae arise. A defect at this point functionally separates the vertebral body, pedicle, and superior articular process from the inferior articular process and remainder of the vertebrae. Thus, the defect cleaves the vertebra into 2 parts (see Image 4). The portion of the vertebra posterior to the defect remains fixed, and the anterior portions are free to potentially slip forward relative to the posterior structures and spine below. Note that bilateral pars defects are needed to allow slippage.

Established observations and factors relating to the development of this disorder are as follows: Apart from 1 reported case (Newell, 1995), humans are not born with a pars interarticularis defect. A congenitally dysplastic pars interarticularis, coupled with the stresses placed on the lumbar spine by upright (bipedal) posture with extension loading, appear to cause spondylolysis (Wiltse, 1976). A family history of spondylolysis and/or spondylolisthesis is commonly found. Upright posture places a continual downward and forward thrust on the lower lumbar spine, with the forces concentrating on the pars interarticularis. High-risk activities include gymnastics, rowing, tennis, wrestling, weightlifting, and football; all of these create mechanical stresses that play an important role in development of spondylolysis.

Placing such stresses on a weak pars results in a pars fracture (Dietrich, 1985). These microfractures heal with a false joint, a bony bridge across this portion of the arch, or fibrous bridging of the fracture.

Most defects are believed to begin as a stress fracture that most likely persists because of continued motion (especially extension movements of gymnasts, which have been implicated in causing the fractures in the first place), which usually impairs bone healing. Some defects heal and may do so with elongation of the pars, representing healing of repeated microfractures. No diastatic defect is seen on radiographic imaging in this case, but a spondylolisthesis does occur.

A study by Fredrickson and colleagues showed a 4.4% incidence of spondylolysis and a 2.6% incidence of spondylolisthesis at age 6 years and a 5.4% and 4.0% prevalence, respectively, in adulthood. Note that once the pars defect has occurred, not all patients necessarily develop a spondylolisthesis. The largest degree of slippage occurred during the adolescent growth spurt. After skeletal maturity, slip progression usually is minimal and often is related to disk pathology, which more commonly occurs in patients with at least a grade 2 spondylolisthesis.

The affected population shows a 2:1 male-to-female predominance (Wiltse, 1976). White men are affected more commonly than black men, and white women are affected more often than black women (Rowe, 1953). A near 50% prevalence is found in Eskimos. Although males more commonly have the pars defect, females are more likely to progress to higher degrees of spondylolisthesis.

Degenerative spondylolisthesis

The pars interarticularis is not affected by degeneration. As such, a pars interarticularis defect is not present.

Long-standing intersegmental instability leads to degenerative spondylolisthesis. This may arise from other problems, such as disk degeneration or spondylolytic spondylolisthesis. Surgical laminectomy is another cause.

Osteoarthritic changes develop in the facet joints. Eburnation and erosive changes occur, which may lead to abnormal alignment of the articular surfaces. Other factors include abnormalities of the ligamentous structures and intervertebral disk, such as loss of disk height. All of these combine to cause spondylolisthesis. Usually, the degree of spondylolisthesis is not great in this group. The L4-L5 vertebral space is affected 6-10 times more commonly than at other levels.

Note that retrolisthesis may result from the degenerative factors in some individuals. This is a slippage in the posterior direction (opposite to an anterolisthesis).

Black women are affected 3 times more commonly than white women. Degenerative spondylolisthesis usually occurs in patients older than 40 years.

Congenital spondylolisthesis

In the congenital (dysplastic) type, congenital anomalies of the vertebral arch and/or facets occur at the lumbosacral junction. Spondylolysis is excluded, as patients are not born with that lesion. Lesions in this category include dysplastic facets that may have an axial (horizontal) orientation, sagittal orientation, or some other cause, such as failure of vertebral body formation. The end result is that the facets do not lock in, and forward slippage is allowed. The pars may remain intact, develop poorly, elongate, or even lyse. Note that when an intact posterior arch accompanies forward slippage, potential exists for the arch to impinge on the cauda equina. Alternatively, spondylolisthesis of the L5 vertebra may cause S1 nerve root compression by the L5 vertebra inferior articular process.

The female-to-male predominance is 2:1 (Wiltse, 1976). This type accounts for approximately 15-20% of cases of spondylolisthesis. Symptoms usually develop during the adolescent growth period.

The facet joint is a synovial joint and is subject to the same osteoarthritic changes that affect other synovial joints in the body. Thus, loss of hyaline cartilage (which is eburnation of the joint surface) and reactive hypertrophic bony changes can lead to altered alignment of the facet joint allowing spondylolisthesis.

Traumatic spondylolisthesis

Trauma in the development of a spondylolysis is discussed in Spondylolytic spondylolisthesis above. Also, trauma can cause an acute fracture through a normal pars interarticularis and result in a diastatic defect that may lead to a spondylolisthesis. If immobilized, these should heal. Although fractures are also involved in the pathogenesis of the spondylolytic type, it is assumed that the traumatic variety has a normal pars and fractures due to excessive forces applied upon the pars, whereas in the spondylolytic variety, the pars fails under normal stresses (sustains stress fractures).

Traumatic forces may affect other parts of the spine to result in a spondylolisthesis. For instance, fractures may be seen in the articular processes or through the facet joints. Subluxation or dislocation (eg, jumped facet) of the facet joint may occur. Always consider associated ligamentous injury, which can occur if the traumatic force involves the disk, anterior and posterior longitudinal ligaments, interspinous ligament, supraspinous ligament, and the capsule and ligaments of the facet joints, causing facet-joint instability. Any 1 or more of these mechanisms may result in a spondylolisthesis.

Pathologic spondylolisthesis

Neoplasm or infection may involve the pars interarticularis, facets or pedicles. Malignancy, such as metastasis from primary breast, prostate, lung carcinoma and myeloma, do occur in the posterior elements. Infections, such as blood-borne staphylococcal osteomyelitis, also occur.


Frequency:


In the US: With the exception of 1 reported case, spondylolysis is not present at birth (Newell, 1995). One study reported a prevalence of 4.4% in children aged 6 years, with prevalence increasing to 5.4% by adulthood. It is presumed that the increase is related to the adolescent growth spurt. In the same study, prevalence of spondylolisthesis was reported at 2.6% and 4.0%, respectively.
Dysplastic spondylolisthesis represents approximately 14-21% of all cases of spondylolisthesis.

Internationally: Eskimos have a reported incidence of spondylolysis in adults of almost 50%, which is presumed to result from a combination of genetic and environmental factors.
Mortality/Morbidity:

Mortality has never been reported in spondylolisthesis.
Morbidity is not uncommon, since back pain and neural involvement, if severe enough, can affect activities of daily living.
Race: Studies from the United States demonstrate that the white population is affected more frequently with spondylolysis than is the black population. Eskimos also have a high incidence. The degenerative form has a higher prevalence in the black population.

Sex: Spondylolysis has a 2:1 male-to-female predominance compared to the congenital and degenerative forms of spondylolisthesis, which show a female-to-male predominance of 2:1 and 5:1, respectively.

Age: Spondylolysis, with the exception of 1 reported case, is not present at birth. Its appearance develops with increasing age, in keeping with the presumed pathogenesis relating to increasing activity and spinal loading. At age 6 years, a 4.4% prevalence is reported, and a 5.4% prevalence is reported in adulthood. Consequent spondylolisthesis has a reported prevalence of 2.4% and 4.0%, respectively. Congenital spondylolisthesis may be seen in early life, but patients usually present with symptoms during the adolescent growth spurt.

Anatomy: In a healthy person, the facet joint in the lumbar spine is, for the most part, in the coronal plane, with the inferior articular process (of the superior vertebra) located posteriorly and the superior articular process (of the inferior vertebra) located anteriorly. This configuration prevents forward movement of the vertebrae relative to each other. It locks in the superior vertebra relative to the inferior vertebra, which is important, since the center of gravity of the human body is located anterior to the spine. This mechanism exerts a forward slipping force on the spine especially at the L5-S1 level. Furthermore, the anteriorly located center of gravity causes a rotating movement, with the axis of rotation oriented transversely at the L5-S1 level. Thus, in severe spondylolisthesis, a kyphotic deformity also develops.

Clinical Details: Symptoms and signs in patients depend on the severity of the condition.

Pain is the most common symptom of spondylolysis and spondylolisthesis. Pain may originate in the area of lysis or may arise from other structures that have been affected by secondary changes of lysis or spondylolisthesis, such as degenerative change in the disk, facet-joint arthropathy, and ligamentous sprain or strain.

In addition, pain may arise from neural involvement, which may be from the spinal canal stenosis that can occur in high grades of spondylolisthesis. In this situation, an intact neural arch slides forward, narrowing the spinal canal and compressing the cauda equina. Clinical features of spinal claudication may ensue.

Alternatively, as the neural arch slides forward, the inferior articular process of the slipping vertebra can impinge on the nerve roots in the lateral recess of the spinal canal and cause clinical findings of radiculopathy. Typically, this may involve the L5 or S1 nerve roots.

These pain mechanisms are common to all etiologies of spondylolisthesis. However, studies show that patients with spondylolysis are relatively asymptomatic (Fredrickson, 1984). When a symptomatic adolescent is evaluated, other causes of pain should be considered; these include infection, neoplasm, osteoid osteoma, and disk herniation. Indeed, the same findings may hold true for mild grades of spondylolisthesis.

Preferred Examination: Lateral and anteroposterior plain radiographs of the lumbar spine should be obtained in patients with complaints of back pain. The lateral view is useful in detecting spondylolisthesis. The lateral view may demonstrate the pars defect, and bilateral oblique views are especially useful to visualize the pars interarticularis defect, which has the appearance of a Scottie dog with a collar (see Images 3-6). An elongated pars also may be seen.

Plain radiographs also may demonstrate congenital types and the changes of spondylosis. In the trauma setting, fractures may be apparent. Note that other causes of the patient's symptoms may be demonstrated, such as an osteoid osteoma, Paget disease, and osteolytic lesions. The grade of spondylolisthesis can be measured by using the lateral view (see Image 2).

Cross-sectional imaging should be considered next.

In patients with back pain and no clinical findings of nerve root involvement, CT scan of the lumbar spine yields information regarding spondylolisthesis and its cause along with other possible conditions, such as disk disease, disk herniations, spondylosis, and spinal canal stenosis. Other associations, such as spina bifida, may be seen. In patients with radiculopathy, CT myelography can yield information regarding nerve-root impingement and its etiology, such as disk herniation, abscess, or neoplasm.

CT of the spine can be performed with or without intrathecal contrast enhancement. Axial images are obtained in a plane parallel to the disk spaces at each level imaged. Sagittal reconstruction images are also obtained by using post-acquisition processing software. Bone window (eg, 1500/300 HU) and soft-tissue window (eg, 300/30 HU) settings are used.

Section-thickness selection is important to avoid problems, such as volume averaging, and thin sections should be used. Contiguous images also reduce such problems. Indeed, if the sections are too thick and if a gap is present between sections, spondylolysis can be missed. In such cases, sagittal reconstructions may be of help.

With spondylolysis, CT is performed as close as possible to 90° to the level of interest. CT scans typically demonstrate a horizontally oriented defect in the pars, which interrupts the normally complete bony ring of the posterior elements (see Images 7-8). Sagittal reconstruction images also show the spondylolysis (see Images 9-11).

Spondylolisthesis is evaluated best on lateral topogram but can be suggested in patients with spinal stenosis in the absence of disk pathology, posterior hypertrophic changes, or a congenitally narrow spinal canal. An elongated spinal canal is what one typically looks for (see Images 7-8).

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