Spondylolisthesis, Spondylolysis, and Spondylosis

In persons with congenital-type spondylolisthesis, dysplastic articular facets predispose the spinal segment to listhesis 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 listhesis in dysplastic types. The risk of slip progression is high.

The pars interarticularis (isthmus) resists significant forces during normal motion. The pars may be congenitally defective (isthmic spondylolisthesis as spondylolysis) or may undergo repeated stress under hyperflexion and rotation that results in microfractures. Lumbar lordosis, gravity, posture, high-intensity activities (eg, gymnastics), and genetic factors all play a role in slip development. If a fibrous nonunion forms from an ongoing insult, elongation of the pars and progressive listhesis results; this is observed in another subtype of type 2 (isthmic) spondylolisthesis. In persons with spondylolysis, 30-50% are believed to progress to spondylolisthesis. The most common location is at L5-S1.

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 annulus fibrosis 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.

Spondylolisthesis can be graded according to the amount of vertebral subluxation in the sagittal plane, as adapted from Meyerding (1932):

• Grade 1 - Less than 25% of vertebral diameter
• Grade 2 - 25-50%
• Grade 3 - 50-75%
• Grade 4 - 75-100%
• Spondyloptosis - Greater than 100%

The dysplastic type occurs from a neural arch defect in the upper sacrum or L5. In this type, 94% of cases are associated with spina bifida occulta. A high rate of nerve root compression at the S1 foramen exists, though the slip may be minimal (ie, grade 1).

The pars interarticularis (isthmus) is the bone between the lamina, pedicle, articular facets, and the transverse process. This portion of the vertebra can resist significant forces during normal motion. The pars 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 2014, the French Society for Spine Surgery proposed a classification of degenerative spondylolisthesis that comprised the following five types ^[9] :

• Type 1 - Segmental lordosis (SL) >5°; lumbar lordosis (LL) > pelvic incidence (PI) – 10°
• Type 2 - SL < 5°; LL > PI – 10°
• Type 3 - LL < PI – 10°
• Type 4 - LL < PI – 10°; compensated sagittal balance with pelvic tilt (PT)  25°
• Type 5 - Sagittal imbalance with sagittal vertical axis (SVA) >4 cm

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 is multifactorial. A congenital predisposition exists in types 1 and 2, and posture, gravity, rotational forces, and high concentration of stress loading all play parts in the development of the slip.

The following scheme of spondylolisthesis types, based on etiology, is adapted from Wiltse et al ^[1] :

• Type 1 - The dysplastic (congenital) type represents a defect in the upper sacrum or arch of L5; a high rate of associated spina bifida occulta and a high rate of nerve root involvement exist (see Pathophysiology)
• Type 2 - The isthmic (early in life) type results from a defect in pars interarticularis, which permits forward slippage of the superior vertebra, usually L5; there are three recognized subcategories—namely, (1) lytic (ie, spondylolysis) or stress fracture of the pars, (2) elongated yet intact pars, and (3) acutely fractured pars
• Type 3 - The degenerative (late in life) type 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 4 - The traumatic (any age) type results from fracture of any part of the neural arch or pars that leads to listhesis
• Type 5 - The pathologic type results from a generalized bone disease, such as Paget disease or osteogenesis imperfecta

The incidence of isthmic type (see Etiology) of spondylolisthesis is believed to be approximately 5% on the basis of autopsy studies.

Degenerative spondylolisthesis is observed more frequently as the population ages and occurs most frequently at the L4-L5 level. As many as 5.8% of men and 9.1% of women are believed to have this type of listhesis.

Lumbar fusion is being performed with more frequency across the United States, with considerable regional variation. 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 are assessing 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. ^{[10, 11]}

In contrast, Franklin retrospectively evaluated the outcome of lumbar fusion in patients receiving Workers' Compensation in Washington state and found that 68% of patients experienced worsening of back and leg pain, and 56% reported their quality of life had not improved or was worse. ^[12] They concluded that the use of instrumentation doubled the risk of a second surgical procedure. Ironically, 62% reported they would undergo the surgery again.

The influence of psychosocial factors must be considered in any outcome study, and this retrospective study demonstrates that indeed it is difficult to ascertain whether a poor result is due to inappropriate patient selection process, to the surgical procedure, or to failure of outcome measurement. Prospective studies with clearly defined diagnostic categories would probably produce the greatest improvement to the outcome of lumbar fusions. ^[12]

In a prospective study of degenerative slips, Herkowitz showed that an attempted fusion gave better clinical outcomes than decompression alone. ^[13]

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 report an improvement in the quality of life and level of pain. Surprisingly, the outcome in most studies does not correlate with the degree of spondylolisthesis or the slip angle.

Some long-term follow-up studies support conservative treatment of asymptomatic children and teenagers with spondylolisthesis (type I or II), regardless of the 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. ^[14] In spinal stenosis patients with symptoms for more than 12 months, outcomes were worse than in spinal stenosis patients with symptoms for less than 12 months, who 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.