Pathophysiology

As understanding of spondylolisthesis progressed, classifications of common subtypes emerged. The most widely used classification system today was described by Wiltse.^8–11 This system represents a further development of the classification described by Newman and Stone,¹² who, in 1962, reported the long-term outcomes of 319 patients with spondylolisthesis. In their series, spondylolisthesis was classified in terms of radiographic appearance and proposed etiology.

Wiltse separated spondylolisthesis into five main groups (Table 27–1). Type I, also known as congenital or dysplastic spondylolisthesis, is secondary to a congenital defect of the superior sacral facet or the inferior L5 facet or both with gradual anterior translation of the L5 vertebra. Type II, also known as lytic or isthmic spondylolisthesis, involves a defect in the isthmus or pars interarticularis. Type II is classified further into three subtypes: Type IIA represents a spondylolysis or a stress fracture of the pars region. Type IIB represents an intact but elongated pars caused by repeated stress and bony remodeling. Type IIC represents an acute traumatic fracture of the pars leading to anterolisthesis; this is the rarest of the subtypes. It is not the pars defect itself but the anterior translation that allows the lesion to be termed spondylolisthesis. Type III is degenerative in origin and is a disease of older adults that develops as a result of facet arthritis and remodeling. Such long-standing intersegmental instability can lead to either anterolisthesis or posterolisthesis. As the disease progresses, the articular processes may become more horizontally shaped, creating the potential for rotational deformity as well. Type IV is a post-traumatic disruption of posterior elements other than the pars (as in type IIC). This disruption is a gradual event and not an acute fracture-dislocation as seen in type IIC. Type V involves the destruction of the posterior elements in the setting of a pathologic process, such as malignancy, Paget disease, tuberculosis, or giant cell tumors. Additionally, an iatrogenic spondylolisthesis may occur after facetectomy.

TABLE 27–1 Spondylolisthesis Classification by Wiltse¹⁸

Wiltse type I and type II constitute most cases, and these are the focus of this chapter. Although the classification schemes described allow for the systematic study of these disparate disease entities, they are of no proven prognostic value in the prediction of deformity progression.

The extent to which spondylosis depends on genetic or developmental factors is controversial. In 1982, Marchetti and Bartolozzi¹³ divided spondylolisthesis into developmental and acquired subtypes. Developmental etiologies included elongation of the pars, lytic lesions, and traumatic events, whereas acquired etiologies included iatrogenic, pathologic, and degenerative conditions. In 1994, a revised classification system subclassified the developmental group further into high or low dysplastic. In these two subgroups, the pars interarticularis was described as being either osteolytic or elongated. Traumatic lesions were incorporated into the acquired group, and the iatrogenic etiology was relabeled as postsurgical (Table 27–2). Although developmental abnormalities of the posterior arch are typically insufficient to cause spondylolysis in the absence of other inciting factors, they may play a significant role in the predisposition to spondylolysis and subsequent spondylolisthesis.

TABLE 27–2 Spondylolisthesis Classification by Marchetti and Bartolozzi¹³

A significant genetic predisposition is suggested by the observation that spondylolysis occurs in 15% to 70% of first-degree relatives of individuals with the disorder.^14–22 Spondylolisthesis also shows a strong familial association, with an incidence in first-degree or second-degree relatives of approximately 25% to 30%.^5,9,11,23 A radiographic study by Wynne-Davies and Scott¹⁵ showed that dysplastic spondylolisthesis has a familial incidence of 33%, whereas the isthmic variant has a familial incidence of 15%. Compared with the incidence in the general population, this represents a fourfold and twofold increased familial risk in patients with dysplastic and isthmic spondylolisthesis. Wynne-Davies and Scott¹⁵ suggested a multifactorial autosomal dominant pattern of inheritance with incomplete penetrance. Wiltse²² suggested, however, that a cartilaginous defect in the vertebrae may be of autosomal recessive inheritance with varying expressivity. Additionally, the correlation between spina bifida occulta and spondylolisthesis strengthens the suggestion of a hereditary contribution.

In combination with developmental susceptibilities, certain activities are risk factors for spondylolysis because of the nature of the biomechanical stresses imparted on the pars interarticularis. Biomechanical analyses have shown that hyperextension and persistent lordosis increase shear stresses at the neural arch.^24–27 Wiltse and colleagues²⁸ hypothesized that most cases of isthmic spondylolysis should be considered fatigue fractures caused by repetitive load and stress as opposed to a single traumatic event, although a traumatic event may lead to completion of a fracture already in development. Farfan and colleagues²⁹ hypothesized that a single event leads to the initial microfracture in the pars, with fractures occurring as a result of repetitive overload. As a result of these biomechanical data, activities that involve hyperextension of the lumbar spine, such as gymnastics, weightlifting, diving, football, and volleyball, have been implicated as causative factors in the development of spondylolysis.^27,30–32

Persistent lumbar lordosis may also increase susceptibility to spondylosis; Ogilvie and Sherman¹⁴ reported a 50% prevalence of asymptomatic spondylolysis in patients with Scheuermann kyphosis. The tendency toward progression of slippage during adolescence and the observation that girls are several times more likely to have an increase in deformity are also suggestive of a hormonal role in the development of spondylolisthesis.²³

Epidemiology

The exact prevalence of spondylolysis is uncertain because it is asymptomatic in a large proportion of patients. Reports regarding the prevalence of spondylolysis are based primarily on painful or symptomatic spondylolysis or cases associated with listhesis. The prevalence in whites has been reported as 3% to 6% with a male-to-female ratio of 2 : 1.^33–35 Roche and Rowe³⁵ examined 4200 cadaveric specimens and found an overall prevalence of spondylolysis of 4.2%. Considerable ethnic variability exists in the prevalence of spondylolysis, with a lower prevalence in African Americans (1.8% to 2.4%) than in whites (5.6%).^36–39 The highest prevalence has been reported in the Eskimo population, with rates of 13% in adolescent patients and 54% in adults.⁴⁰ Although this prevalence may suggest a genetic predisposition, it has also been posited that Eskimos, who carry their infants in a papoose, place undue stress on the pars interarticularis.³⁶

The reported incidence of isthmic spondylolisthesis ranges from 2.6% to 4.4%.^5,41–43 In the largest prospective radiographic study, Fredrickson and colleagues⁴¹ evaluated 500 patients at age 6 years with a 20-year follow-up. A pars defect was appreciated in 4.4% of 6-year-old children. By age 12, 5.2% of the cohort were noted to have the defect (85% participation rate). This increased to 6% by age 18; however, most of the nonaffected patients had dropped out of the study (34% participation rate). Back pain had developed in only four of the patients, and one patient required an operative procedure to decompress a herniated disc at a level cephalad to the spondylolisthesis. Pars defects at L5 were noted to be bilateral in 78% of cases, with most of these progressing to spondylolisthesis. As a corollary to this study, Fredrickson and colleagues⁴¹ also evaluated 500 newborns and found no evidence of spondylolysis or spondylolisthesis. The only reported case of a pars lesion in a newborn has been published by Borkow and Kleiger.⁴⁴ Isthmic spondylolisthesis is rare in children younger than 5 years old, with only a few reported cases.^43,45,46

In spondylolysis, the pars interarticularis defect may be unilateral or bilateral. If the defect is bilateral, the chance of progression to listhesis is greater. The most common location of a spondylitic defect is L5 (85%),⁴⁷ and the defect may be observed as high as L2; multilevel defects are seen infrequently. Rarely, multiple defects may be seen at the same level. Ariyoshi and colleagues⁴⁸ reported a case of spondylolysis at three sites in L5 involving the bilateral pars interarticularis and the center of the right lamina.

The most common site of isthmic spondylolisthesis is at the L5-S1 level secondary to osteolysis at L5. Estimates show that this lesion is located at the L5 pars interarticularis in 90% of type II cases, at L4 in 5%, and in more cephalad vertebrae in the remaining 5% of cases.¹¹ Additionally, authors reported spina bifida occulta at the same level in 30% of patients with pars lesions. The incidence of spina bifida associated with spondylolisthesis has been reported to range from 24% to 70%.^9,23,49,50 Age at presentation with isthmic spondylolisthesis follows a bimodal distribution. One peak occurs between the ages of 5 and 7 years, and the second occurs in the teenage years.^9,19,49,51 The incidence in athletes who subject themselves to excessive lumbar posturing, such as gymnasts, soccer players, pitchers, cricket bowlers, and divers, is higher than in the general population.^53–60

In pediatric patients, dysplastic and isthmic are the most commonly encountered subtypes, with the latter representing 85% of the cases. As with spondylolysis, isthmic (type II) spondylolisthesis is two times more frequent in boys than girls.³⁵ Dysplastic spondylolisthesis, similar to its isthmic counterpart, is also most commonly found at the L5-S1 junction. The incidence is two times higher in girls,^61,62 and based on more recent published reports, it accounts for 14% to 21% of total cases.^62,63

History and Physical Examination

Spondylolysis may be discovered incidentally or may manifest with low back pain typically in the teenage years.⁶⁴ In approximately half of cases, the onset of low back pain is associated with a history of trauma or an inciting event.^23,65,66 Usually these patients complain of focal low back pain, only rarely radiating to the buttocks or posterior thigh, which becomes worse with activity or on hyperextension of the spine.^31,67–70 Lifting and weight bearing can exacerbate the pain, and a forced lumbar extension often intensifies the symptoms. Neurologic involvement is rare in isolated spondylolysis. Medical professionals who have little experience with spondylolysis often assume the defect to be a sequela of trauma requiring immediate immobilization and surgical intervention. In these cases, it is the responsibility of the spine surgeon to offer reassurance that imminent neurologic compromise is highly unlikely.^71,72

Physical examination of the lumbar spine reveals focal tenderness in acute cases and mild discomfort in chronic cases. Patients maintain a full range of forward flexion (unless the hamstrings are tight) that is usually painless, but hyperextension movement leads to an exacerbation of symptoms as does lateral bending or rotation. Other associated physical signs are an antalgic gait, increased lumbar lordosis, and hamstring tightness. A single-leg hyperextension test is used for the diagnosis and differentiation of unilateral spondylolysis from bilateral lysis. This test is performed by the patient bearing weight on one leg with the hip and knee of the other leg flexed, while hyperextending the lumbar spine. This maneuver is performed on both sides; asymmetrical low back pain indicates unilateral spondylolysis. Bilateral lesions show symmetrical or asymmetrical pain with this maneuver.^73,74 The neurologic examination in isolated spondylolysis is generally normal, with radicular findings suggestive of foraminal stenosis owing to inflammation or instability.

Spondylolisthesis may manifest in a similar fashion but is also typically associated with hamstring tightness. This tightness manifests as a muscle spasm of the posterior thighs associated with a fixed flexion at the hip and knees. An increased popliteal angle is present on straight-leg raise. Increased popliteal angle is almost always observed universally, even in low-grade spondylolisthesis. Electromyographic and neurologic abnormalities are typically absent; this suggests that there is not a neurologic basis for the hamstring tightness, but that it likely results from the patient’s attempts to maintain global sagittal balance.^63,75 Other authors hypothesize that tightness results as a sequela of chronic nerve root irritation from the instability and micromotion of the involved segment.^75–78 Patients often ambulate and stand with increased flexion at the hips and knees, also known as the Phalen-Dickson sign.⁷⁶ This flexed posturing increases as the amount of slippage increases. The patient may also exhibit a shuffled or short-stepped gait.⁷⁸

Patients with spondylolisthesis may initially present with focal neurologic deficits or radiculopathy, although this is uncommon. Bilateral radicular symptoms are more commonly observed than unilateral radiculopathy. Typically, the L5 root is involved with pain radiating to the buttocks and posterior thigh or weakness of the extensor hallucis longus. Constant loading of the pars defects may hinder bony healing, resulting in a fibrous union that may be a persistent source of pain. Local expansion of fibrocartilaginous scar tissue within the area of the pars defect may cause nerve root compression. Tension on the nerve root also increases with progression of olisthesis, increasing further the likelihood of radicular symptoms with disease progression.⁷² In higher grade subluxations, traction of the cauda equina over the sacrum may exist. This traction may lead to signs and symptoms of cauda equina compression, such as perineal paresthesia, decreased sphincter tone, and urinary retention. Additionally, traction of the cauda is thought to create a reflex spasm of the hamstrings.^79,80

Higher grade spondylolisthesis results in a palpable step-off over the spinous processes. In isthmic spondylolisthesis, the step-off is typically found at the L4-5 junction, as the neural arch of the L5 vertebrae does not translate anteriorly with the body but remains within its geographic location in relation to the sacrum. In dysplastic spondylolisthesis, the neural arch is still attached to the vertebral body and slides anteriorly with the body, producing a palpable step-off that is typically appreciated at the lumbosacral junction. Lumbosacral kyphosis with a retroverted sacrum results in heart-shaped, flattened buttocks. In severe cases, the trunk appears grossly shortened, and the rib cage lies within close proximity to the iliac crests.

Scoliosis also may be associated with spondylolisthesis.^{12,25,81–85} The incidence has been reported to be 60%. Scoliosis may result because of a combination of hamstring and paraspinal muscle spasm, rotational deformity, or truncal asymmetry. If scoliosis is secondary to spondylolisthesis (nonstructural), it usually resolves after treatment of the olisthesis. The patient may also have an adolescent idiopathic curve with a low-grade spondylolisthesis that was detected incidentally on radiographic evaluation.

Radiographic Evaluation

Many imaging modalities may be useful in the diagnosis and evaluation of spondylolisthesis. Radiographic evaluation of spondylolisthesis begins with plain radiographs, including lateral, anteroposterior, and oblique views.⁸⁶ The anteroposterior view should be angled 15 degrees to the inclination of the L5-S1 disc (Ferguson view). This view not only allows for visualization of the presence of sacral spinal bifida, but also evaluates the size of the lumbar transverse processes and height of the disc.

The defect in isthmic spondylolysis is visualized as lucency in the region of the pars interarticularis. The lucency is commonly described as having the appearance of a collar or a “broken neck on the Scotty dog” seen in lateral oblique radiographs. A spot lateral view is able to identify only 19% of pars defects,^33,87 whereas oblique lateral views can detect the pars defect in 84% of cases.⁸⁸ It is important to take right and left oblique views because pars defects may be unilateral in some cases, and the collar may be visible in only one projection.

Although oblique views are most sensitive in diagnosing spondylolysis, the lateral view is optimal for appreciating the degree of olisthesis in spondylolisthesis. The lateral view should be performed with the patient standing. Flexion-extension views may assess for the presence of associated instability. This subtle movement may be an important pain generator and is essential for further treatment planning. Additionally, these views show the extent of postural reduction of the lumbosacral angulation and translation that may be obtained without formal release.

Because the sensitivity of plain radiographs is limited, radionuclide (technetium 99mm) bone imaging may be a good option in cases of suspected spondylolysis with negative plain radiographs. A bone scan identifies pars interarticularis stress fractures that can be missed in oblique radiographs because a stress reaction may be present without a bony defect. Patients who have had a recent trauma or performed strenuous activity and are symptomatic have a bone scan showing increased uptake in the spondylolytic area; however, patients with chronic low backache can have normal bone scans if the defect is chronic, is sclerotic, and has lost its blood supply. Single photon emission computed tomography (SPECT) is more sensitive and provides more details than plain x-rays and technetium bone scan.^89,90 A “hot” scan insinuates increased activity, and the patient may benefit from orthotic immobilization, whereas a “cold” scan suggests a chronic lesion that is not metabolically active and is unlikely to respond to immobilization alone.⁷²

Thin-cut axial computed tomography (CT) is highly accurate at visualizing osseous anatomy and is superior to plain radiography in its ability to show dysplastic facets and pars defects. CT may also be used after plain radiographs or bone scan to assess the healing potential of an identified pars defect.⁹¹ In addition to showing spondylolysis accurately, CT may identify changes in the apophyseal joints associated with degenerative and reverse spondylolisthesis and can show minimal degrees of spondylolisthesis by the presence of a pseudobulging disk.⁹²

Magnetic resonance imaging (MRI) is a highly sensitive imaging technique that allows for additional visualization of soft tissue and neural structures and is recommended in all cases associated with neurologic findings. MRI offers the distinct advantage of being able to image the spine in any plane without exposure to ionizing radiation. Sagittal thin slices (3-mm slice thickness for T1-weighted images and 4-mm slice thickness for T2-weighted images) are able to identify 95% of pars defects, with T1-weighted images being more sensitive than T2-weighted images.⁹³ In the early course of the disease, MRI helps in identifying the stress reaction at the pars interarticularis before the end-stage bony defect.^94,95 In more acute presentations in which plain radiographs may be negative, a fat saturation technique can be applied to minimize signal from fat and to bring out signal from fluid structures such as bone edema. MRI also allows for evaluation the spinal cord and its associated elements with greater anatomic detail and without the procedural risks associated with CT myelography. MRI may show the degree of impingement of neural elements by fibrous scar tissue at the spondylolytic defect. Additionally, involvement of adjacent discs should be evaluated because abnormal biomechanics can lead to early degenerative changes at adjacent levels.

The most commonly used radiographic grading system for spondylolisthesis is the one proposed by Meyerding in 1932.^95a The degree of slippage is measured as the percentage of distance the anteriorly translated vertebral body has moved forward. On the lateral radiograph, a line is drawn along the posterior sacral border. A line perpendicular to this is drawn at the superior part of the sacrum. The anterior translation or displacement of the inferior border of L5 as a proportion of the width of S1 is expressed as a percentage. The Meyerding classification grades increasing olistheses from I to IV (Table 27–3). Spondyloptosis, in which the fifth lumbar vertebra has slipped forward over 100% of the gliding plane past the sacral promontory, is referred to as grade V. Spondylolysis without olisthesis is referred to as grade 0.

TABLE 27–3 Meyerding Classification*

* Grades 0 and V were added later on.

Although the Meyerding classification system quantifies translational subluxation in the anteroposterior plane, it does not quantify the sagittal rotation of a vertebral body that may coexist in spondylolisthesis. This angular displacement is referred to as the slip angle, and as with the Meyerding grading system, the erect lateral radiograph is the basis for measurement. The slip angle is calculated by measuring the angle formed by the intersection of two lines: (1) a line perpendicular to the posterior cortex of the sacrum and (2) a line paralleling the inferior endplate of L5. In the normal spine, slip angle values should be close to zero. The slip angle quantifies the lumbosacral kyphosis and was shown by Boxall and colleagues⁶³ to be the most useful tool in determining the risk of the progression in a skeletally immature patient. A slip angle greater than 55 degrees is associated with a high probability and increased rate of progression.

Sacral inclination or pelvic tilt refers to the vertical position of the sacrum. It is the angle formed by the intersection of two lines: (1) a line perpendicular to the floor and (2) a line parallel to the posterior cortex of the sacrum. Normal values are greater than 30 degrees. With an increasing slip, lumbosacral kyphosis is increased, and the sacrum is forced into a more vertical orientation decreasing the pelvic tilt.

In 1983, Wiltse and Winter⁹⁶ proposed a classification that separated the tangential movement seen in low-grade slips (grade I and II) from the angular and tangential movement that was appreciated in high-grade slips (grade III or higher). The three measurements that were factored were degree of slip, vertebral wedging, and sacral rounding. These authors recommended the forward displacement of the fifth lumbar vertebra in relationship to the sacrum be measured as an actual percentage as first described by Taillard⁴² and later recommended by Laurent and Osterman.⁹⁷ It was stressed that even a small degree of progression should be measured, and this was not quantifiable on the Meyerding scale. Sacral tilt as described previously and sagittal rotation or slip angle were also used. The method for measuring slip angle, which Wiltse and Winter⁹⁶ termed sagittal rotation, was modified by measuring the angle formed by the intersection of two lines: (1) a line extending off the anterior cortex of the L5 vertebral body and (2) a line off the posterior border of the first sacral vertebrae. Wiltse and Winter⁹⁶ believed the endplates of the L5 and S1 bodies to be unreliable osseous structures secondary to osseous hyperplasia.

Conservative Management

Treatment of spondylolysis mainly focuses on pain relief, core muscle strengthening, and restoration of full lumbar range of motion. Achieving these goals enables the patient to return to normal activity without any restrictions. Management of spondylolysis depends on the severity of the symptoms and level of activity. Initial conservative management in the form of activity restriction and bracing (for pain relief) relieves symptoms in patients with spondylolysis. It is likely that most lesions do not heal with bone but become a stable fibrous union that remains relatively asymptomatic.

Conservative management of spondylolysis includes complete cessation of activity, rehabilitation with strengthening of the abdominal and paraspinal musculature, minimization of pelvic tilt, and perhaps antilordotic bracing.⁹⁸ Conservative management protocols also depend on several factors such as disease involvement (spondylolysis vs. spondylolisthesis), level and laterality of the defect (unilateral vs. bilateral pars defects), duration since injury (acute vs. chronic), and age of the patient.⁹⁹ Many authors prefer to use a total-contact, low-profile polyethylene orthosis, which is designed to maintain an antilordotic posture and extends from just below the nipples to 1 inch above the greater trochanter. The brace is worn for 23 hours/day for minimum of 3 to 6 months.¹⁰⁰ If clinical symptoms improve, the brace can be gradually weaned through a period of part-time wear.

Excellent clinical outcomes have been reported with a course of activity restriction and bracing that prevents repetitive hyperextension movements at the lumbar spine.^100–103 Good to excellent results with brace therapy have been shown in 80% of patients with grade 0 or I spondylolisthesis.^100,104,105 Bell and colleagues¹⁰⁴ showed prevention of increased slip angle and 100% reduction of pain in 28 patients with grade I or II spondylolisthesis after a mean brace treatment of 25 months. In a series of 82 symptomatic patients with various degrees of spondylolisthesis, Pizzutillo and Hummer¹⁰⁵ reported that nonoperative treatment of grade II or less was shown to relieve pain reliably in two thirds of patients. A study by Steiner and Micheli¹⁰⁰ showed radiographic evidence of healing pars defects in 12 of 67 patients with spondylolysis or grade I spondylolisthesis after treatment in a modified Boston brace. Excellent or good results were achieved in 78% with return to full activities. Patients with spondylolysis and grade I spondylolisthesis may return to full activity and sports with resolution of symptoms and documented lack of slip progression. Controversy exists regarding postbrace activity level for patients with grade II spondylolisthesis. The general consensus is that after successful brace treatment a child with grade II spondylolisthesis may return to sports that do not involve hyperlordotic posturing.^18,26,27

Patients with acute pars interarticularis fractures are best treated with immediate initiation of bracing for pain relief and restriction from athletic activity with continued mobilization for activities of daily living. Anderson and colleagues¹⁰⁷ used clinical evaluation and SPECT imaging to compare the rate of response to early versus late initiation of bracing. In this study, patients with early bracing showed rapid relief of symptoms, a short bracing time, and rapid reduction of SPECT ratio. Patients showing a spondylolytic defect on plain radiography but whose bone scans were negative were determined to have inactive (terminal) spondylolytic defects, pseudarthrosis, or old unhealed fractures.^73,108 Athletes with low back pain and increased uptake on SPECT scan at the pars interarticularis but no defect on radiographs typically respond to a period of rest and active rehabilitation; very few athletes develop defects or persistent back pain.¹⁰⁹

As the understanding of spinal biomechanics has progressed, Panjabi¹¹⁰ posited the concept of specific training of lumbar muscles in chronic low back pain. According to his concept, specific training of muscles around the lumbar spine improves the dynamic stability and controls segmental spinal motion. The local muscular system that controls the lumbar spine consists of lumbar multifidus, internal oblique, and transverse abdominis.¹¹⁰ A randomized trial by O’Sullivan of 44 patients who were treated with two different protocols showed that a specific strengthening program was more effective than generalized back strengthening exercises.⁹⁸ Along with exercises that target specific core muscle groups with the spine in neutral position, strengthening of hip flexors and hamstring stretching are important and recommended.^100,101,111

Patients with low-grade dysplastic spondylolisthesis are less likely than patients with isthmic spondylolisthesis to respond to conservative measures,⁵ but conservative therapy is still recommended as the initial modality. The importance of radiographic and neurologic follow-up should be stressed to these patients because they are at a higher risk for slip progression owing to facet hypoplasia. Radiographic follow-up is recommended at least annually until skeletal maturity and more frequently during peak height velocity before puberty. Documentation of slip percentage, angle, sacral inclination, wedging, and pelvic tilt is recommended as part of proper documentation of progression of the deformity.

Surgical Treatment

Surgical intervention is indicated for patients with persistent pain, progressive spondylolisthesis, or neurologic symptoms who fail conservative management. Treatment approach is influenced by the level of spinal maturity, degree of slippage, symptoms, the patient’s activity level, and expected progression. In contrast to a comparable adult, an asymptomatic adolescent may be a candidate for surgical intervention because of expected progression of deformity in a high-grade slip, which may lead further to mechanical and neurologic dysfunction. In a skeletally immature patient with slippage greater than 50% or a mature adolescent with a slip greater than 75%, operative intervention is recommended even if the patient is asymptomatic.^106,112,113 Surgical decompression is also indicated when a patient has neural compromise, with a radiculopathy or bowel or bladder dysfunction.^114–116

Surgical treatment options may be broadly divided into two categories: direct repair of the pars defects versus arthrodesis of the involved segments to prevent slip progression with or without decompression of affected neural structures. Procedures for direct fixation of pars defects include Buck technique,¹¹⁷ Scott wiring,¹¹⁸ and repair with an ipsilateral pedicle screw and hook.^119,120

Fusion of the involved level has been widely advocated as treatment of symptomatic spondylolysis.^106,121 The long-term effects of fusion in a young patient must be considered, however, owing to the potential for adjacent segment degeneration.^122,123 Based on their simulated lumbar fusion studies in cadavers, Weinhoffer and colleagues¹²⁴ concluded that increased intradiscal pressure at the level of fusion could lead to accelerated degeneration at the adjacent discs. Kinematic studies of adjacent vertebra after fusion have shown disc degeneration, increased stress at the facet joints, hypertrophy of the facets, and hypermobility at the adjacent level.^123,125,126 Based on these kinematic studies and the goal of preserving motion when possible, isolated repair of the pars interarticularis defect is the preferred treatment for symptomatic pars defects in patients with no slip or disc degeneration at that level and relief from the diagnostic injection. Fusion is an option if an attempt at pars repair is unsuccessful, the lamina is dysplastic, the defect is very large, or disc degeneration or listhesis is present. Some authors maintain that results for fusion are better at L5 because of the narrow lamina at L5 and the steep lordotic angle that may be present.¹²⁷

To increase the probability of response to surgical treatment, Wu and colleagues¹²⁸ reported on the use of preoperative diagnostic pars injection at the site of the defect. In their series of 100 patients who had failed conservative management, the pain generator was confirmed by injecting 1.5 mL of bupivacaine (Marcaine) into the lytic area. Reproduction of similar pain and pain relief of at least 70% of the usual pain quality for more than 6 hours were considered as a positive response, and these patients subsequently showed an excellent outcome after repair of the defect.¹²⁸

Buck fusion is an open technique in which the fibrous tissue at the pars defect is identified, thoroughly débrided, and stabilized with a 4.5-mm stainless steel cortical screw in compression.¹¹⁷ Buck¹¹⁷ concluded that this technique was indicated only in cases in which the gap was smaller than 3 to 4 mm. Various studies showed 88% to 100% defect healing and satisfactory results with his technique.^129–131 Direct repair using a screw is a demanding procedure, however; owing to the narrowness of the lamina, a minimal displacement or malposition of the screw can lead to implant failure or complications such as nerve root irritation, injury to the posterior arch or dura, or pseudarthrosis.^132,133

In the Scott technique, a stainless steel wire is looped from the transverse processes to the spinous process of the level involved and tightened, in conjunction with local iliac crest bone graft.¹¹⁸ This wire creates a tension band construct, placing the pars defect under compression, and holds the bone graft in place. Bradford and Iza¹³⁴ reported 80% good to excellent results and 90% radiographic healing of the defects. This technique requires greater surgical exposure, with extensive stripping of the muscles to expose the transverse process. Complications such as wire breakage are common with this technique. Salib and Pettine¹³⁵ modified this technique by passing a wire around the cortical screws introduced into both pedicles and tightening it beneath the spinous process. Biomechanical tests show that fixation of the wire to the pedicle screw does not increase the stiffness of the system.¹³⁶ Both cerclage techniques have good defect healing rates of 86% to 100%.^{118,135,137,138} Songer and Rovin¹³⁹ modified this construct by replacing the wire with a cable tied up to a pedicle screw and then passed and wrapped around the contralateral lamina. This system provides solid fixation, and the authors reported excellent outcome in five of seven patients and 100% solid union in all patients.

Morscher and colleagues¹⁴⁰ introduced a new technique to repair the pars defect with a laminar hook, which is loaded with compression by a spring placed against a screw threaded in the articular process. Healing rates with this technique range from 56% to 82%.^140–144 The major drawback of this procedure is screw penetration to the inferior articular process of the cephalad vertebra, which can lead to screw loosening or breakage.¹⁴⁵ Gillet and Petit¹⁴⁶ introduced the concept of the rod screw construct, in which the rod is firmly fixed to the spinous process, and published excellent outcomes in 6 of 10 patients.

Taddonio, using the Cotrel-Dubousset system, first introduced a repair using pedicle screw fixation.²²¹ Tokuhashi and Matsuzaki¹²⁷ reported excellent outcomes with the Isola pediculolaminar system. Kakiuchi¹⁴⁷ reported similar results using Texas Scottish Rite Hospital instrumentation system; with this technique, hooks are fixed at the lamina and connected with a rod to an ipsilateral pedicle screw after compression. Roca and colleagues¹⁴⁸ reported 92% excellent results with their new pedicle screw hook construct system in adolescents, but they have not recommended this technique for patients older than 20 years. Pellise and colleagues¹⁴⁹ advised 1-mm thin cuts to assess the pars anatomy, but 2.5-mm cuts help in assessing bone healing after direct repair in spondylolysis.

The authors’ preferred technique for pars repair is to use minimal access tubes or retractors to obtain exposure of the pars defect and débride the fibrous tissue and hypertrophic nonunion with a bur and curets to bleeding bone, but care must be taken not to enlarge the defect further and destabilize the segment. Iliac crest bone graft is placed into the defect, and a cannulated laminar screw is placed percutaneously over a predrilled guidewire from the ipsilateral inferior lamina across the defect to engage the cortical bone of the pedicle or superior endplate for compression, avoiding the facet joint. Additional graft (or bone graft replacement) is placed over the defect, extending from the lamina to the junction of the transverse process. The patient is immobilized in a low-profile thoracolumbosacral orthosis for 12 weeks (hip joint locked with a leg extension for the first 6 weeks) and then progressed to rehabilitation. Healing is checked at 6 months, and the patient is allowed to resume all sports.

For a pediatric patient with grade I or II spondylolisthesis, dysplastic spondylolisthesis at the lumbosacral junction, or a slip secondary to a defect of the L5 pars who has failed conservative treatment, posterior in situ fusion is recommended from L5 to S1. With the widespread use of pedicle screws and the myriad screw options that are available, numerous studies have been performed supporting the use of transpedicular fixation. Transpedicular fixation has been shown to increase the rate of fusion, and a positive correlation has been reported between successful fusion and clinical outcome.^150–156 Other series have not shown a statistically significant difference between instrumented and noninstrumented posterior fusions.^157,158 In one study of 10 patients in a cohort who had the working diagnosis of spondylolisthesis, 5 underwent instrumented fusion, 4 of whom achieved an excellent or good outcome, compared with 2 of 5 who underwent a noninstrumented fusion.¹⁵⁹

Lenke and colleagues¹⁶⁰ performed noninstrumented in situ fusions in 56 pediatric patients with isthmic spondylolisthesis. Based on radiographic evidence, only 50% showed a solid fusion mass, whereas 33% showed radiographic changes highly unlikely or with no evidence of a fusion mass. Despite poor fusion rates, overall clinical improvement was noted in greater than 80% of the cohort with preoperative symptoms of back or leg pain or hamstring tightness. A trend for improved clinical outcome with increased rigidity of fixation has been noted.¹⁵¹ Pedicle screw fixation systems have been shown to be mechanically superior to other fixation while allowing for the selective segmental force without extension to adjacent levels.¹⁶¹ Additionally, the use of instrumentation obviates the need for postoperative casting in a compliant patient. If exposure of midline structures and decompression is not warranted, the paraspinal approach described by Wiltse and colleagues^11,162 is recommended because it avoids neural arch defects, minimizes soft tissue trauma, and improves visualization of posterolateral structures. Additionally it helps maintain position of the bone graft and may promote fusion. During surgical dissection, care must be taken to protect facets at levels cephalad to the proposed fusion because this may create instability or degeneration later on. Minimally invasive techniques are available.

The method of immobilization after an in situ posterior fusion ranges from bed rest to bilateral pantaloon spica casts for 6 months. Literature can be found to support either end of the spectrum.^{112,163–169} Boxall and colleagues⁶³ and Sherman and colleagues¹⁷⁰ compared in situ patients who were immobilized in a cast or orthosis with patients who were treated with bed rest.^63,170 Each study showed no statistical difference in the fusion rate based on immobilization methods.

Decompression is warranted in patients with neurologic findings. Patients with low-grade spondylolisthesis generally do not have significant neurologic symptoms. In an adult patient with radiculopathy, it may be acceptable to perform only a decompressive procedure as described in 1955 by Gill and colleagues.¹¹⁴ The removal of loose posterior elements and cartilaginous tissue can increase vertebral column instability and further progression of deformity, however—an unacceptable risk in the pediatric spine.^171,172 Although a wide decompression may be warranted, it should be augmented with spinal fusion in a growing child.¹¹⁴ Studies have also shown an increased risk of progression of deformity in patients with L5 laminectomy and posterior fusion versus patients with posterior fusion alone.^172–174

Treatment of high-grade spondylolisthesis is a topic of great debate. Symptomatic patients with high-grade spondylolisthesis tend to fare poorer with nonoperative treatment compared with their counterparts with low-grade spondylolisthesis.¹⁷⁵ In high-grade spondylolisthesis, correction of the slip angle rather than the degree of anterior listhesis should be addressed. Although studies show that patients with greater than 50% of slippage may not have a poor nonoperative outcome,¹⁷⁶ fusion is the general treatment of choice among spinal surgeons. In determining the most appropriate procedure, one must take into account all presenting symptoms, neurologic function, radiographic findings, clinical deformity, patient’s age, and the surgeon’s experience.

As with low-grade spondylolisthesis, in situ fusion was a described treatment for pediatric patients with high-grade spondylolisthesis; however, cranial extension including L4 is recommended.¹⁶⁵ A Wiltse approach is suggested unless decompression is warranted. As reported by Pizzutillo and colleagues,¹⁷⁷ bone graft placement at the level of or anterior to the transverse processes extending to the sacral ala helps to ensure a large posterolateral fusion mass, which can effectively counteract shear forces at the lumbosacral junction. Allograft or autograft or both may be used, balancing the rate of successful fusion versus the potential for donor site pain and morbidity.^{173,178–181}

Postoperative progression of deformity has been appreciated in patients and has been attributed to pseudarthrosis, lack of postoperative immobilization, lack of graft consolidation or maturation, or deterioration of the solid fusion mass. Progression has been appreciated in patients with a solid fusion mass as evidenced by radiography. Patients with a greater preoperative deformity are at higher risk.^{165,168,172,182,183} The advance of slippage is usually minor in these cases, and studies have shown that radiographic evidence of pseudarthrosis does not always lead to pain.^63,160,184 Studies with long-term follow-up of patients with high-grade spondylolisthesis show in situ fusion to be a viable solution in maintenance of symptom relief and prevention of degenerative arthrosis of mobile cephalad spinal segments.^{163,185–187}

Grzegorzewski and Kumar¹⁶⁸ found no radiographic pseudarthrosis in 21 patients with high-grade spondylolisthesis treated with in situ fusion, postoperative immobilization in a pantaloon spica cast, and 4 months of bed rest. Although five patients showed evidence of slip progression, two of whom showed an increased slip angle within the 1st year, only four patients had symptoms of back pain after postoperative follow-up of almost 13 years. Overall reports show radiographic evidence of successful fusion to range from 71% to 100% and relief of back pain and neurologic symptoms to range from 74% to 100% in patients after in situ fusion.^{1,165,170,175} Patients with high-grade spondylolisthesis who are at risk of developing pseudarthrosis are patients who require a wide decompression secondary to L5 radiculopathy or sacral root symptoms and patients with excessive mobility at the L5-S1 junction. Patients with hypoplastic transverse processes, spina bifida, and sacral malformation are also at risk of pseudarthrosis.

Transsacral fusion using either fibular graft or mesh case has been shown more recently to be a viable treatment option. By providing an anterior column support and fusion bed, increased structural stability can be achieved. Smith and Bohlman¹⁸⁷ suggested a modification to posterolateral fusion to decrease the incidence of pseudarthrosis and progression of deformity. Eleven patients with high-grade spondylolisthesis were treated in a single-stage procedure involving spinal decompression, in situ posterolateral arthrodesis with autologous iliac crest graft, and anterior arthrodesis with a fibular graft inserted from the posterior approach. A cannulated drill was used to develop a transsacral osseous tunnel extending into the L5 vertebral body. A mid-diaphyseal fibular graft was harvested, trimmed, and inserted into this tunnel, acting as a dowel in the lumbosacral junction. Preoperative neurologic findings were sensory deficits in all but one patient and cauda equina syndrome in five patients. Six patients had prior spinal operations that had failed. The average duration of follow-up was 64 months showing a solid fusion mass with complete or major neurologic recovery in all patients. Average time to solid fusion was 12 weeks.

In a patient with sagittal balance and high-grade spondylolisthesis, an in situ procedure or partial reduction can be performed, and a cage or fibular dowel can be inserted anteriorly from L5 into S1 or posteriorly with a retrograde direction from S1 into L5. Posterior insertion of the transvertebral cage or fibular graft is advantageous because it obviates the need for an anterior approach to the lumbosacral region, which has its own drawbacks. There is less blood loss and less risk of injury to great vessels. Because the entire procedure can be done with the patient in one position and with one incision, operative time is also greatly reduced. A partial reduction can be performed by use of concave rods, and fusion should be augmented with posterior instrumentation.

Mahmood and colleagues¹⁸⁸ presented a case series in which a transsacral mesh cage was used in lieu of a fibular strut graft. Partial reduction was accomplished with a pedicle screw curved rod construct after which an osseous tunnel was established and a transsacral cage impregnated with bone graft was inserted from a posterior approach. A distinct benefit of using a cage is increased biomechanical stability, as studies have shown fibular strut resorption, deformation, and even fracture.^189–191 Additionally, the use of a cage avoids potential donor site morbidity.^192,193 Average radiographic and clinical follow-up of these patients was 38 months showing evidence of fusion and relief of symptoms.

There is no clear indication for when reduction of a high-grade spondylolisthesis is necessary, as opposed to performing a fusion with mild correction of the slip angle. Many authors suggest an in situ fusion or mild correction is indicated for patients who exhibit sagittal balance and acceptable slip angle. When considering reduction, improvement of slip angle should be the primary objective rather than improvement of grade of listhesis. In patients with a high-grade slip, a larger slip angle correlates with increased risk of progression of deformity.^75,77 Reduction of spondylolisthesis results in improved sagittal balance, improvement in cosmesis, and a biomechanically stable fusion mass. In addition, by reducing the deformity, canal stenosis is improved, and tension on nerve roots and the cauda equina is reduced. Improvement of overall sagittal alignment leads to improved posture, improved gait, and increased function. Reduction of spondylolisthesis in skeletally immature patients is recommended for patients with a high slip angle (>45 degrees), patients with severe sagittal imbalance, and patients who are at high risk of developing a pseudarthrosis with in situ fusion.^63,163,194

Numerous methods of reduction have been described. The earliest reported reduction was published in 1936 by Jenkins,^194a who used longitudinal traction followed by anterior fusion; however, the reduction could not be maintained. Since his initial report, variations of Jenkins’ described technique have been published.^{163,185,195–205} Reduction techniques may be as minimally invasive as external casting after bone graft placement or as complex as staged procedures involving multiple posterior and anterior approaches.

Reduction with external casting is particularly beneficial in young patients, in whom pedicle screw fixation is not feasible. After an open procedure in which posterior elements are decorticated and bone graft is placed around the proposed fusion site, the surgical wound is closed. The patient is placed on an antilordotic frame or spica table with extension of the spine to reduce the lumbosacral kyphosis. The patient should be awake for this part of the procedure to report any changes in neurologic function. If this is impossible, the use of neuromonitoring may help in the neurologic assessment during the reduction. To hold the reduction, the spica cast should be extended to the trunk and incorporate at least one thigh. Burkus and colleagues¹⁶⁶ showed that the use of pantaloon spica cast immobilization led to a decrease in sagittal translation of more than 5% in three quarters of patients treated with cast immobilization and a decrease in the slip angle of more than 5% in 58% of patients treated with cast immobilization. Of the patients who did not undergo cast immobilization, 45% had an increase in sagittal translation of more than 5%, and 56% had an increase in slip angle of more than 5 degrees.

In patients in whom instrumentation can be placed, reduction followed by instrumentation for stability is recommended. Published procedures include halo-femoral or halo-pelvic traction and anteroposterior fusion followed by application of a pantaloon spica cast to apply anteriorly directed pressure.^163,199 Other authors have described anterior release with partial reduction and anterior interbody fusion,²⁰⁶ intraoperative closed reduction followed by instrumented posterior fusion,²⁰⁷ and a two-stage procedure with a posterior decompression and halo-skeletal traction followed by interbody fusion.²⁰⁸ Drawbacks to these procedures included lengthy preoperative hospitalization for traction and lengthy postoperative immobilization in a cast. The study by Burkus and colleagues¹⁶⁶ compared patients treated with a pure in situ fusion with patients who underwent posterior fusion and were reduced postoperatively in a pantaloon spica cast. Reduction was found to be safe, and fusion rates were noted to higher, in addition to less chance of late slip and slip angle progression in patients who were treated with a reduction.

Mehdian and Arun²⁰⁹ published a three-stage procedure using a combined anterior and posterior approach performed in one operative sitting. In the first stage, a laminectomy of L5 is performed with wide decompression of the L4-S1 nerve roots. L5-S1 discectomy was performed next followed by an osteotomy of the posterosuperior aspect of S1. The second stage consisted of a transperitoneal approach to the L5-S1 level, allowing removal of the anterior disc protrusion and associated thickened anulus fibrosus, effectively allowing posterior translation of the superior body. In the final stage, the patient is repositioned prone and instrumented from L4-S1. Bilateral pedicle screws are initially placed at L4 and S1, and a reduction can be performed with the assistance of curved rods, after which bilateral L5 pedicle fixation points can be established. Cages may be inserted in the L5-S1 interspace to promote a solid arthrodesis.

The authors’ preferred method for treating high-grade slips with significant lumbosacral kyphosis is postural reduction with positioning under anesthesia and a wide decompression of the L5 and S1 nerve roots bilaterally. The dysplastic L5-S1 disc is removed with a transforaminal approach, and the dome of the sacrum is osteotomized (sacroplasty) to facilitate gentle reduction. Reduction pedicle screws are used to reduce the slip gently, an interbody graft is placed, and the construct is compressed posteriorly to obtain lordosis.

Treatment of severe deformity, including spondyloptosis, can be challenging to the most experienced spine surgeon. The natural history of spondyloptosis is unclear because of its rarity and because it is frequently not reported separately from high-grade spondylolisthesis (grade III and IV). Most authors agree that in a symptomatic patient, benign neglect is not a viable option. The surgical management of spondyloptosis in children is variably documented in the literature. Some authors propose that posterior fusion in situ with or without decompression is a safe and reliable procedure,¹⁶⁸ whereas others suggest that reduction of the slipped vertebra may prevent some of the adverse sequelae of in situ fusion, which include nonunion, bending of the fusion mass, and persistent or increasing lumbosacral deformity.^{191,210–212} Many investigators advocate a combined anterior and posterior fusion using instrumentation. An in situ circumferential fusion as described by Smith and Bohlman¹⁸⁷ has the lowest risk for iatrogenic nerve injury.

Resection of the L5 vertebra with reduction of L4 onto S1 was initially described by Gaines and Nichols in 1985.²¹³ The initial stage of the procedure involves an anterior L5 vertebrectomy in which the L5 body is removed to the base of the pedicles. The second stage is performed through a midline posterior approach involving resection of the now loose L5 posterior elements, decompression, and instrumented reduction through transpedicular instrumentation of L4 onto S1.^213,214 Lehmer and colleagues²¹⁴ performed a retrospective review evaluating indications, techniques, results, and patient satisfaction. Of patients, 25% were found to require reoperation secondary to delayed union or instrumentation failure. Three quarters were noted to have early postoperative neurologic deficits, more than half of which were present preoperatively, and most resolved. All three patients with preoperative cauda equina syndrome recovered postoperatively, and patient questionnaires revealed a high patient satisfaction rate.

As with other lumbar fusion surgery, the most common complication from an operative intervention is pseudarthrosis. Reported rates vary from 0% to 39%,^{83,164,166,169,215} and pseudarthrosis occurs more frequently in fusions performed for lytic (type IIA) spondylolisthesis.²¹⁶ X-rays often show lucency around pedicle screws, instrumentation failure, progression of slip angle, or increased vertebral displacement.

Reports exist of increase in spondylolisthesis even with an uninstrumented solid arthrodesis as shown radiographically.* In most of these reports, x-rays and not CT was used to assess fusion mass, and many of these cases may have been pseudarthroses. Increased slip was reported in noninstrumented fusions, providing a sound argument for instrumented fusion.

As per the 2003 Mortality and Morbidity report of the Scoliosis Research Society, the incidence of neurologic complications with lytic spondylolisthesis surgery is 3.1%.²¹⁷ Radiculopathy is the most common surgical complication after reduction. Intraoperative manipulation can cause direct dural trauma injuring multiple sacral and lumbar nerve roots and resulting in postoperative deficits. The L5 nerve roots are most commonly involved, and reports show variable rates of resolution, with the highest risk associated with aggressive reductions of high-grade listhesis.^218–220

Cauda equina syndrome is a potentially disastrous complication that can occur as a result of intraoperative technique, as a result of postoperative conditions, or with no apparent antecedent cause.^{208,219,221–223} Schoenecker and colleagues¹¹⁶ described 12 cases after in situ arthrodesis for grade III or IV L5-S1 spondylolisthesis. During the procedures, there was no evidence of compromise of the cauda equina. Of 12 patients, 5 showed complete recovery, and 7 had permanent residual deficits manifested by bowel and bladder dysfunction. Although the exact etiology is unknown, it may be related to vascular phenomena, transient anterior displacement of L5 during the surgical exposure causing laminar impingement on the sacral dome, or a period of hyperextension during patient positioning.²²⁴ With reduction of the deformity, the risk is far greater. If cauda equina syndrome is suspected, surgical decompression is imperative. Sacroplasty and resection of the adjacent disc or lamina of L5 or both is recommended because it is thought to facilitate neurologic recovery.¹¹⁶

With surgical advancements in technique and instrumentation, new biologic and mechanical fusion adjutants, neuromonitoring, and advanced imaging, it is hoped that further reductions in complication rates may be achieved despite the risks inherent to these highly invasive procedures.