Spondylolisthesis, Spondylolysis, and Spondylosis

The structural arrangement of the spine facilitates a wide range of movement and distribution of forces from the lower to the upper extremities. Extension of the spine places more stress on the facet joints of the spine, whereas flexion exerts more strain on the intervertebral disk. When movements involve both spine extension and rotation, the stress is intensified and can lead to stress fractures. ^[20]

Ligament attachments also play a role in stabilizing the spine. The vertebral body is surrounded by the anterior and posterior longitudinal ligaments, which attach to the ligamentous anulus of the intervertebral disk. The interspinous and interspinous ligaments and the facet joints maintain posterior stability by preventing excessive forward slippage of vertebrae. The inferior facet of one vertebra interlocks with the superior articular process of the above facet. This allows for conservation of stability by the ligaments if a spondylolytic fracture occurs. ^[20]  The pars interarticularis (isthmus) is the bone between the lamina, pedicle, articular facets, and the transverse process; it can resist significant forces during normal motion.

In individuals with congenital-type spondylolisthesis, dysplastic articular facets predispose the spinal segment to olisthesis as a consequence of their inability to resist anterior shear stress. The pars may be intact, or it may undergo microfractures. Thus, it may not be the initiator of olisthesis in dysplastic types. The risk of slip progression is high. 

Degenerative spondylolisthesis results from intersegmental instability. The pathophysiology of disk degeneration and facet arthropathy has been investigated extensively; however, the nature and etiology of pain generation in the absence of canal or lateral recess stenosis are still debated.

Degeneration of the anulus fibrosus results in radial tears through which a posteriorly migrated nucleus pulposus can herniate. Degeneration of the disk may also lead to changes affecting the stability of the spinal motion segment, thus affecting the articular facets. Disk desiccation places greater stress on the facets, which are then subjected to shear forces. The subluxation occurs as a result of progressive facet incompetence. This type most commonly occurs at L4-5 and L3-4.

The pars interarticularis may be congenitally defective (eg, in the spondylolytic subtype of isthmic spondylolisthesis) or undergo repeated stress under hyperextension and rotation, resulting in microfractures. If a fibrous nonunion forms from ongoing insult, elongation of the pars and progressive listhesis results. This occurs in the second and third subtypes of type 2 (isthmic) spondylolisthesis. These typically present in the teenage or early adulthood years and are most common at L5-S1.

A unilateral pars defect (spondylolysis) may not demonstrate any degree of slippage; thus, a patient may have spondylolysis without spondylolisthesis. The reverse is also true as in the degenerative-type slips described below.

Biomechanical factors are significant in the development of spondylolysis leading to spondylolisthesis. Gravitational and postural forces cause the greatest stress at the pars interarticularis. Both lumbar lordosis and rotational forces are also believed to play a role in the development of lytic pars defects and the fatigue of the pars in the young. An association exists between high levels of activity during childhood and the development of pars defects. Genetic factors also play a role.

In degenerative spondylolisthesis, intersegmental instability is present as a result of degenerative disk disease and facet arthropathy. These processes are collectively known as spondylosis (ie, acquired age-related degeneration). The slip occurs from progressive spondylosis within this three-joint motion complex. This typically occurs at L4-5, and elderly females are most commonly affected. The L5 nerve root is usually compressed from lateral recess stenosis as a result of facet and/or ligamentous hypertrophy.

In traumatic spondylolisthesis, any part of the neural arch (usually not the pars) can be fractured, leading to the unstable vertebral subluxation.

Pathologic spondylolisthesis results from generalized bone disease, which causes abnormal mineralization, remodeling, and attenuation of the posterior elements leading to the slip.

The etiology of spondylolisthesis can be classified into one of five types: dysplastic, isthmic, degenerative, traumatic, and pathologic. The following classification of spondylolisthesis into five types on the basis of etiology is adapted from Wiltse et al ^[4] :

• Type I - The dysplastic (congenital) type originates from a neural arch defect in the upper sacrum or L5; 94% of these cases are associated with spina bifida occulta, and there is a high rate of nerve root involvement
• Type II - The isthmic (early in life) type results from a defect in the pars interarticularis, which permits forward slippage of the superior vertebra, usually L5; there are three recognized subcategories—namely, (a) lytic (ie, spondylolysis) or stress fracture of the pars, (b) elongated yet intact pars, and (c) acutely fractured pars
• Type III - The degenerative type is the most common form of spondylolisthesis and is an acquired condition resulting from chronic disk degeneration and facet incompetence, leading to long-standing segmental instability and gradual slippage, usually at L4-5; spondylosis is a general term reserved for acquired age-related degenerative changes of the spine (ie, diskopathy or facet arthropathy) that can lead to this type of spondylolisthesis
• Type IV - The traumatic (any age) type results from fracture of any part of the neural arch or pars that leads to olisthesis; this category is subdivided into acute and stress-related subtypes ^[21]
• Type V - The pathologic type results from localized or generalized bone disease, such as Paget disease, infectious process, osteogenesis imperfecta, or metastatic disease.

There is an increased risk of developing spondylolisthesis if an individual has a first-degree relative with spondylolisthesis, scoliosis, or occult spina bifida at the S1 level. ^[22]

Spondylolysis is seen in approximately 6% of the adult population but can be more frequent among adolescent athletes. ^[1]  The incidence is higher among athletes playing sports that require increased spine motion and lumbar extension, such as gymnastics, dance, American football (the lineman position in particular), wrestling, and diving. ^[23]  Spondylolysis is two to three times more common in males than females. ^[2]

Radiographically visualized spondylolysis is associated with spondylolisthesis in approximately 25% of cases. ^[2]

Dysplastic spondylolisthesis is more common in the pediatric population, with an estimated prevalence of 6-7% by the age of 18 years. Dysplastic spondylolisthesis is more common in females. ^[24]  Ninety-four percent of these cases are associated with spina bifida occulta.

The incidence of the isthmic type (see Etiology) of spondylolisthesis is higher in the young adult population, with males more commonly affected than females. However, symptoms of isthmic spondylolisthesis may develop in adulthood.

Degenerative spondylolisthesis occurs mainly in adults and is more common in females than males. ^[24]  As many as 5.8% of men and 9.1% of women are believed to have this type of olisthesis. It occurs most frequently at the L4-L5 level. 

Grade I spondylolisthesis accounts for 75% of all cases (see Staging). ^[24]

An estimated 80% of spondylolysis cases are asymptomatic, and a majority of these cases do not worsen to a progressive lesion. Between 75% and 95% of symptomatic isthmic spondylolysis cases will improve with conservative management, and 9-15% will require surgical intervention. ^[2]

Lumbar fusion is being performed with more frequency across the United States, albeit with considerable regional variations. These variations have been attributed to a multitude of factors, from advances in instrumentation to better understanding of bone healing. Lack of clearly defined indications for fusion has been another contributing factor. The evidence supporting fusion for spondylolistheses types I, II, IV, and V and iatrogenic spondylolisthesis is strong. Controversy exists regarding persons with degenerative-type slips (type III), degenerative scoliosis, and mechanical back pain.

Very few prospective randomized trials have assessed the long-term outcome of lumbar fusion in these patients. Variables used to evaluate the effectiveness of this procedure have included patient level of function, pain, satisfaction, return to work, and quality of life. Radiographic confirmation of fusion, complications, and cost are other important criteria in the evaluation of the overall outcome.

A prospective randomized study performed by Zdeblick et al confirmed that the addition of rigid posterior instrumentation increases the rate of fusion and correlates with less pain and a greater rate of returning to work. ^{[25, 26]}

In contrast, Franklin et al retrospectively evaluated the outcome of lumbar fusion in patients receiving workers' compensation in the state of Washington and found that 68% of patients experienced worsening of back and leg pain, and 56% experienced a quality of life that had not improved or was worse. ^[27] The authors concluded that the use of instrumentation doubled the risk of a second surgical procedure. Ironically, 62% of patients stated that they would undergo the surgery again.

The influence of psychosocial factors must be considered in any outcome study, and this retrospective study demonstrated that it is indeed difficult to ascertain whether a poor result is due to inappropriate patient selection process, to the surgical procedure, or to failure of outcome measurement. ^[27] Prospective studies with clearly defined diagnostic categories would probably produce the greatest improvement in the outcome of lumbar fusions.

In a prospective study of degenerative slips, Herkowitz et al found that an attempted fusion gave better clinical outcomes than decompression alone. ^[28]

The results on isthmic-type spondylolisthesis have been the most promising. Most investigators have noted a 75-95% rate of good-to-excellent outcome. Most patients undergoing surgery have reported improvements in quality of life and level of pain. Surprisingly, the outcome in most studies has not correlated with the degree of spondylolisthesis or the slip angle.

Some long-term follow-up studies have supported conservative treatment of asymptomatic children and teenagers with spondylolisthesis (type I or II), regardless of grade; however, most investigators advocate fusion when the slip is symptomatic, unresponsive to conservative measures, or high-grade.

Data from the Spine Outcomes Research Trial (SPORT) study were analyzed to determine if duration of symptoms affects outcomes after treatment of spinal stenosis or degenerative spondylolisthesis. ^[29] In spinal stenosis patients who had had symptoms for more than 12 months, outcomes were worse than in spinal stenosis patients who had had symptoms for less than 12 months; the latter experienced significantly better surgical and nonsurgical treatment outcomes. On the same basis of symptom duration before treatment, no differences were noted in outcomes for degenerative spondylolisthesis patients.