Spondylolisthesis, Spondylolysis, and Spondylosis

Background

Spondylolisthesis refers to the forward slippage of one vertebral body with respect to the one beneath it. This most commonly occurs at the lumbosacral junction with L5 slipping over S1, but it can occur at higher levels as well. It is classified on the basis of etiology into the following five types^[1] :

• Congenital or dysplastic
• Isthmic
• Degenerative
• Traumatic
• Pathologic

The term spondylolisthesis was coined by Killian in 1854 to describe gradual slippage of the L5 vertebra due to gravity and posture. In 1858, Lambi demonstrated the neural arch defect (absence or elongation of the pars interarticularis) in isthmic spondylolisthesis. Albee and Hibbs separately published their initial work on spinal fusion. Their methods were applied quickly to cases involving trauma, tumors, and, later, scoliosis. In the latter half of the 20th century, spinal fusion was increasingly used to treat degenerative disorders of the spine, including degenerative spondylolisthesis and degenerative scoliosis.

Spondylolisthesis may or may not be associated with gross instability of the spine. Some individuals remain asymptomatic even with high-grade slips, but most complain of some discomfort. It may cause any degree of symptoms, from minimal symptoms of occasional low back pain to incapacitating mechanical pain, radiculopathy from nerve root compression, and neurogenic claudication.

Many cases can be managed conservatively. However, in persons with incapacitating symptoms, radiculopathy, neurogenic claudication, postural or gait abnormality resistant to nonoperative measures, and significant slip progression, surgery is indicated. The goal of surgery is to stabilize the spinal segment and decompress the neural elements if necessary.^{[2, 3, 4, 5, 6, 7, 8]}

For more information on this topic, see Spondylolisthesis Imaging, Spondylolysis Imaging, Lumbar Spondylosis, Diagnosis and Management of Cervical Spondylosis, and Lumbosacral Spondylolysis.

For patient education resources, see Back Pain.

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Anatomy

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.

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Pathophysiology

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.

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Etiology

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

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Epidemiology

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.

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Prognosis

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.

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History and Physical Examination

The clinical presentation differs, depending on the type of slip and the age of the patient.

During the early years of life, the presentation is one of mild low back pain that occasionally radiates into the buttocks and posterior thighs, especially during high levels of activity. The symptoms rarely correlate with the degree of slippage, though they are attributable to segmental instability.

Neurologic signs often correlate with the degree of slippage and involve motor, sensory, and reflex changes corresponding to nerve-root impingement (usually S1). Progression of listhesis in these young adults usually occurs in the setting of bilateral pars defects and can be associated with the following physical findings:

• Palpable stepoff in higher-grade slips
• Restricted spinal motion
• Hamstring tightness
• Inability to flex the hips with fully extended knees
• Hyperlordosis of the lumbar and thoracolumbar regions
• Hyperkyphosis at the lumbosacral junction (as the center of gravity shifts to compensate for slip progression)
• Trunk shortening when a complete slip is present (spondyloptosis)
• Gait difficulty (worse with high-grade slips)

The patient with degenerative spondylolisthesis is typically older and presents with back pain, radiculopathy, neurogenic claudication, or a combination of these symptoms. The slip is most common at L4-5 and less common at L3-4. The radicular symptoms often result from lateral recess stenosis from facet and ligamentous hypertrophy, disk herniation, or both. The L5 nerve root is affected most commonly and causes weakness of the extensor hallucis longus. Concomitant central stenosis and neurogenic claudication may or may not exist.

The cause of claudication symptoms during ambulation is multifactorial. The pain is relieved when the patient flexes the spine by sitting or by leaning on shopping carts. Flexion increases canal size by stretching the protruding ligamentum flavum, reducing the overriding laminae and facets, and enlarging the foramina. This relieves the pressure on the exiting nerve roots and thus decreases the pain.

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Laboratory Studies

Obtain routine preoperative laboratory tests for patients undergoing surgery.

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Imaging Studies

Isthmic defects are best observed on oblique lumbar radiographs. Lateral plain radiographs with flexion and extension views are the studies most commonly used to demonstrate segmental instability. Some practitioners advocate the use of lateral bending films as well, especially in persons with degenerative listhesis and scoliosis. (See the images below.)

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Spondylolisthesis, spondylolysis, and spondylosis. Isthmic spondylolisthesis (type IIa) with grade 2 slippage of L5 over S1 and spondylolysis (lytic p....

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Spondylolisthesis, spondylolysis, and spondylosis. Although interbody devices afford immediate stability to anterior column, their use as standalone d....

Although computed tomography (CT) is poor for demonstrating spondylolisthesis, it is useful in demonstrating pars interarticularis (isthmus) defects, facet arthropathy, canal diameter, foraminal stenosis, and disk herniation. When combined with myelography (static or dynamic flexion and extension views), CT may demonstrate evidence of nerve-root compression and concomitant instability. Myelography generally is not indicated unless neurologic signs or pain unexplained by findings other imaging methods exists.

Magnetic resonance imaging (MRI) is most sensitive in demonstrating soft tissues and ascertaining the presence of central and foraminal stenosis. It also can demonstrate endplate reactive changes (Modic types I and II) observed in individuals with degenerative spondylolistheses. Use of MRI in isthmic and dysplastic types is limited.

Bone scanning can be very useful in demonstrating acute fracture of the pars interarticularis in persons with isthmic-type spondylolisthesis. It is also used in degenerative-type slips to reveal any acute reaction, though it has low specificity in this application.

The use of diskography is advocated by some in individuals with degenerative disk disease with low back pain due to intradiskal pathology. Patients with multilevel disk degeneration spanning long segments of the spinal column may benefit from provocative diskography in order to limit the levels fused to the symptomatic levels.

Myelography is usually performed through a transcutaneous subarachnoid injection of radiopaque dye. When combined with CT, myelography is highly specific for central, lateral recess, and foraminal stenosis. Dynamic imaging (with flexion and extension lateral radiographs) also can be obtained, in which the dye column characterizes the position of the neural elements during motion.

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Approach Considerations

The goal of surgery is to stabilize the segment with listhesis and decompress any of the neural elements under pressure. Restoration of normal sagittal alignment must also be achieved. In evaluating a patient, many factors must be considered, including age, degree of slip, and risk of slip progression. Thus, each patient's treatment algorithm should be individualized to achieve optimal outcome.

The indications for spinal fusion clearly differ in the pediatric and adult populations. For the younger population, the following factors are known to correlate with a higher risk of slip progression:

• Younger age (< 15 years)
• High-grade listhesis (>30%)
• Female sex
• Ligamentous laxity
• Type 1 (dysplastic) slip
• Lumbosacral hypermobility

However, many young patients are treated by means of immobilization or activity modification alone, with a significant success rate. In the absence of high-grade slips, minimal symptomatology, and lack of slip progression, fusion is generally not indicated in this population.

Before surgery is considered for adult patients presenting with degenerative spondylolisthesis, minimal neurologic signs, or mechanical back pain alone, conservative measures should be exhausted, and a thorough evaluation of social and psychological factors should be undertaken.^[15]

Indications for surgical intervention (fusion) are as follows:

• Neurologic signs - Radiculopathy (unresponsive to conservative measures), myelopathy, neurogenic claudication
• Any high-grade slip (>50%)
• Type 1 and type 2 slips, with evidence of instability, progression of listhesis, or lack of response to conservative measures
• Traumatic spondylolisthesis
• Iatrogenic spondylolisthesis
• Type 3 (degenerative) listhesis with gross instability and incapacitating pain
• Postural deformity and gait abnormality

Surgery is contraindicated if the patient is in poor medical health and if the operative risk is not outweighed by the potential benefits.

Anticoagulation with warfarin, or antiplatelet therapy, can make the risk of hemorrhage much higher than routinely expected. Antiplatelet therapy should be discontinued 3-5 days before the procedure. Warfarin should be stopped 5-7 days before the procedure, and a prothrombin time (PT) within the reference range should be achieved before surgery.

Smoking significantly decreases the chance for a successful fusion. Some surgeons prefer that a patient commit to smoking cessation up to one month before the surgical procedure.

Correction of the listhesis is associated with risk of neurologic injury, both transient and permanent. Some surgeons prefer to fuse the spine in place rather than to reduce the subluxation. In persons with higher-grade spondylolisthesis, use of interbody grafts is associated with a high rate of complications. However, the use of these devices adds to the stability of the spinal segment, helps with the reduction of the deformity, and helps achieve sagittal balance, thus ensuring better outcome.

As the understanding of spinal instability and biology of bone healing increases, we will be able to better define the population of patients with spondylolisthesis who would benefit most from lumbar fusion or particular methods of fusion and fixation.

Production of bone morphogenic protein (BMP) is a promising venture that undoubtedly will affect the outcome of lumbar fusion. Advances in technology have led to better instrumentation, and further advances are anticipated. Artificial disks and lordotic tapered cage devices are under investigation; they clearly will affect the technical aspects of the operation. The use of bone-growth stimulators is a potentially useful tool for higher fusion rates, though there is a need for long-term data. Osteoinductive pastes and other semisolid mixtures have been introduced to the market; they also promise to enhance the success of this operation.

Although technology continues to improve the performance of surgical treatment, the most challenging task is simply optimal patient selection. As stated previously, clear indications for fusion must be present in order to optimize outcome, and controversies still exist, especially in the treatment of degenerative spondylolisthesis, that must be resolved in a methodic and scientific manner. Prospective randomized studies with independent evaluators probably will produce the greatest improvement to the outcome of lumbar fusions.

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Medical Therapy

Conservative measures are aimed at symptomatic relief and include the following:

• Activity modification, bedrest during acute severe exacerbations
• Analgesics (ie, nonsteroidal anti-inflammatory drugs [NSAIDs])
• Bracing
• Therapeutic strengthening and stretching exercises

The likelihood of success with nonoperative treatment is high, especially in younger patients. In older patients who have low-grade slips resulting from disk degeneration, traction has been used with some success. The authors recommend that any manipulation or traction be performed under the care of a clinician and a physical therapist.

One of the challenging tasks is to treat patients with severe back pain and marginally abnormal radiographs. Such patients may have degenerative disk disease (eg, multilevel disk desiccation observed on magnetic resonance imaging [MRI]) or even low-grade (typically < 25%) slips, and they typically experience pain that is out of proportion to the physical or radiographic findings.

Back pain in general is a major public health problem and remains a primary cause of disability in the United States. Its causes are numerous, and no simple diagnostic method exists for excluding structural causes. It is important for any clinician who cares for patients with spinal problems to address behavioral and psychosocial factors that may contribute to a patient's disability.

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Principles of Operative Management

The goal of surgical treatment in this setting is to decompress the neural elements and immobilize the unstable segment or segments of the spinal column. This is usually performed with elimination of motion across the facet joint and the intervertebral disk through arthrodesis (fusion).^{[4, 8]}  (See the images below.)

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Spondylolisthesis, spondylolysis, and spondylosis. Use of direct electrical current for stimulation of fusion has been advocated by some to enhance fu....

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Spondylolisthesis, spondylolysis, and spondylosis. Spontaneous reduction of slip (either partial or complete) has been reported by surgeons using inte....

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Spondylolisthesis, spondylolysis, and spondylosis. Carbon fiber interbody cage used in reduction of slip.

The SPORT (Spine Patient Outcomes Research Trial) study analyzed the cost-effectiveness of surgery versus nonoperative care in patients with spinal stenosis, degenerative spondylolisthesis, and intervertebral disk herniation.^[16]  In this trial, surgical therapy overall improved health and provided better value over 4 years as compared with nonoperative care.

Aspects of surgical treatment

Fusion

Multiple methods exist for achieving intersegmental fusion in the lumbosacral spine.^[17]  The authors concentrate on the three most widely used methods, as follows:

• Posterolateral (intertransverse) fusion
• Lumbar interbody fusion
• Pars repair

Most surgeons use the intertransverse or transverse process/sacral ala arthrodesis with the use of iliac crest autograft alone or in conjunction with allograft. This may be performed over one or multiple levels with high success rates (up to 90%) of fusion. Some surgeons prefer a two-level fusion (ie, L4>S1) for treating high-grade (>50%) listheses. Segmental spinal instrumentation allows rigid fixation of the fused segments and the possibility of performing reduction of the segment with listhesis. There is some evidence to suggest that recombinant human BMP (rhBMP) is a safe and effective grafting material for the treatment of lumbar spondylolisthesis.^[18]

Biomechanically, lumbar interbody fusion increases the stability of the spinal segment by placing structural bone graft in compression in the anterior and middle columns and increases the overall surface area of the bony fusion.^[19] It can be done with posterior (ie, posterior lumbar interbody fusion [PLIF]) or anterior (ie, anterior lumbar interbody fusion [ALIF]) approaches. A growing number of surgeons use interbody grafts to augment their posterolateral fusion techniques to achieve higher rates (>95%) of arthrodesis. It should be noted that grade 2 or higher slips are predisposed to higher rates of graft complications.

In low-grade lytic slips, the pars can be directly repaired with a Scott wiring technique or the Van Dam modification. This preserves segmental motion and has successfully been used to fuse the pseudarthrosis at the pars in selected patients.

Dean et al studied 58 patients who underwent anterior cervical decompression and fusion, with an iliac crest structural graft, for degenerative spondylolisthesis from 1974 to 2003; they were evaluated for neurologic improvement and osseous fusion. The investigators found that the average neurologic improvement was 1.5 Nurick grades and that the overall fusion rate was 92%.^{[20, 21]}

Fixation

Although the use of spinal instrumentation in skeletally immature patients is considered optional by some surgeons for some patients with isthmic-type spondylolisthesis, most spinal surgeons believe that rigid fixation is needed to achieve a solid fusion reliably. For degenerative-type slips, fixation has been shown to achieve higher rates of solid arthrodesis.

Decompression

Usually in degenerative or traumatic spondylolisthesis, decompression of the neural elements, both centrally and laterally, over the nerve roots is indicated. Optimal decompression is usually achieved through a posterior laminectomy and total facetectomy with radical decompression of the nerve root (ie, Gill procedure).

In a study by Schaeren et al, decompression and dynamic stabilization showed excellent results, after a follow-up of at least 4 years, in elderly patients with spinal stenosis and degenerative spondylolisthesis. Patient satisfaction was high, with 95% stating they would undergo the procedure again.^[3]

Reduction

Some surgeons attempt to reduce the spondylolisthesis in order improve the overall sagittal alignment and spinal biomechanics. This has the benefit of improving standing posture and placing less strain on the posterior fusion mass and spinal hardware, thus reducing the incidence of nonunion and spondylolisthesis progression. The quoted rate of transient or permanent nerve-root injury associated with reduction is 5-30%.

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Surgical Therapy

Preparation for surgery

The surgeon should plan the approach (anterior vs posterior); determine the methods of fusion (ie, iliac crest autograft) and fixation (ie, transpedicular screws); and discuss the risks, benefits, and alternatives of each decision with the patient. Patients can require blood transfusions after spinal fusion and should be given the option of predonating their blood for an autologous transfusion. Some surgeons use blood salvage systems that collect the patient's blood lost during surgery for return to the patient in order to try to minimize the need for transfusion.

Recent plain radiographs with flexion and extension views help define the grade of spondylolisthesis and help with the operative approach. Although most spine surgeons are familiar with pedicle screw placement in the lumbosacral region, computed tomography (CT) helps determine the diameter and trajectory of each pedicle and can be a useful adjunct to preoperative imaging. This is especially useful in correction of listhesis in thoracic and upper lumbar vertebrae (ie, in traumatic spondylolisthesis).

The use of perioperative antibiotics is mandatory. Studies have demonstrated a lower rate of infection with a single dose of cefazolin given within 30 minutes of the incision. For patients with true allergy to beta-lactams, alternative coverage with macrolides or aminoglycosides can be achieved.

Smoking is associated with a high (up to 50%) nonunion rate, and the cessation of smoking is an essential part of the patient's commitment to the success of the operation.

Antiplatelet therapy should be discontinued 3-5 days prior to the procedure. Perioperative and postoperative use of anti-inflammatory medication is not recommended, because they can inhibit fusion.

Operative details

Depending on the symptoms (pure nerve-root compression vs mechanical pain due to segmental instability), different operative techniques are available. Simple minimally invasive microdecompression via a very small incision is often successful in low-grade slips with only single-root involvement. This operation can be performed through a miniopen microscopic or endoscopic approach; this releases the pressure on the traversing and exiting roots through subarticular decompression in the lateral recess. The more classical approach for a definitive posterior fusion and instrumentation is described below.

Via a posterior midline approach, the lumbodorsal fascia is divided, and a subperiosteal dissection of erector spinae muscles is performed over the posterior elements of the involved vertebrae (typically L5 and S1).

Some surgeons prefer the harvesting of iliac crest autograft prior to the fascial opening. This can be performed through the same incision on one or both iliac crests in lumbosacral fusion operations. The fascia overlying the crest is opened. Care is taken to preserve the integrity of the sacroiliac joints. The thickest area for obtaining cancellous bone is decorticated, and multiple gouges are used to retrieve the autograft. Hemostasis is obtained, and the fascia is closed over a drain.

In type IIa (lytic) slips, the spondylolysis can often be observed on palpation with the hypermobility of the L5 posterior elements and the incompetent pars. The lateral exposure is extended past the lateral facets and to the transverse processes. Self-retaining retractor systems hold the entire exposure accessible to the surgeon. The intertransverse plane is cleaned, and a fusion bed for the bone graft is prepared. In fusions involving the sacrum (most lytic types), the sacral ala should be exposed, and the alo-transverse plane is used for the posterolateral fusion.

More minimally invasive solutions would involve approaching each facet via a muscle-splitting approach (sparing the midline structures) and directly decompressing the lateral recess directly. This can be combined with minimally invasive fusion and instrumentation of the affected segment. Percutaneous or miniopen instrumentation has also been used with great success.

Decompressive laminectomies and facetectomies are typically initiated with rongeurs (eg, the Leksell rongeur) and completed with high-speed drills. The Gill laminectomy involves complete removal of the posterior elements of L5 and both articular facets. Because of the incompetent pars in type IIa slips, this can be performed with relative ease using large rongeurs.

Frequently, the entire loose posterior arch need not be removed, but the ever-present remnant that is attached to the pedicle must be removed to ensure adequate decompression of the L5 root. Preserving the posterior arch and grafting across the pars defect resulted in better fusion rates in the Nachemson series. All bone is typically saved and mixed with the cancellous autograft.

After the nerve roots are identified, decompressive foraminotomies are performed, following the course of the nerve roots through their respective foramina. Depending on the grade of the slip, the exiting nerve root (L5 root in most slips) takes a sharp angle during its course and may be kinked as it exits in the L5-S1 foramen. Distraction and partial reduction of the slip can lessen the amount of stretch that the slip places on the nerve root. However, reports quote an up to 30% rate of nerve-root injury resulting from attempted reduction.

Some advocate radical excision of the intervertebral disk to help with the reduction as well as placement of an interbody graft. The risk of transient nerve-root injury is slightly higher with this maneuver (reduction); however, the immediate support afforded by the anterior column support increases the rate of fusion, helps with distraction and reduction, and relieves the acute course of the exiting root. In low-grade slips, especially those of the degenerative type, the restoration of foraminal height provided by the interbody graft helps with the exiting nerve-root symptoms.

After adequate decompression of the involved nerve roots, the lateral recesses are inspected, and the medial walls of the pedicles are palpated. Introductory holes are drilled in each of the four pedicles, while a probe ensures that the medial wall remains intact. Depending on the instrumentation used, the holes are enlarged and probed; under fluoroscopic guidance, the holes are tapped, and transpedicular screws are placed. The interconnecting rods or plates, depending on the system, are then attached.

At this point, the final distraction and reduction can be achieved before the entire fixation is tightened. The wound is irrigated copiously. Some advocate the use of antibiotics in the irrigation; no studies have suggested a lower infection rate as a result of this practice.

The high-speed drill is then used to decorticate the surfaces used for fusion, which are typically the lateral part of the lateral facets, the transverse processes, and the sacral ala. The bone graft is then laid along the prepared fusion bed and compressed. Some surgeons use a variety of pastes with osteoinductive properties to hold the fusion graft and enhance the success of fusion. Long-term results on these materials appear promising. The wound is then closed in multiple layers with watertight closure of the lumbodorsal fascia over one or two drains.

A number of minimally invasive and robotic-assisted approaches have been described as well.^{[22, 23, 24]}

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Postoperative Care

Routine postoperative laboratory tests should include assessment of the hematocrit. Plain anteroposterior and lateral radiographs of the operated segment(s) are recommended. The use of postoperative braces is dependent on surgeon preference.

The patient is mobilized within 24 hours. Adequate pain relief is essential for deep breathing and early ambulation. Involvement of therapists in the patient's initial activities helps encourage and reassure the patient.

Anti-inflammatory medications (eg, steroids or ketorolac) are to be avoided, in that they may interfere with the fusion effort. If the nerve root is injured from traction or manipulation, a short course of tapered steroid therapy is warranted.

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Complications

Specific complications of lumbar fusion performed via a posterolateral approach include the following:

• Injury to the nerve root - The risk is low (< 1%) but increases with more radical facetectomy and PLIF application; more commonly, transient neuropraxia from excessive retraction results in PLIF correction of high-grade spondylolisthesis
• Cerebrospinal fluid leak - The risk is reported at 2-10%, depending on the series; the highest risks are in revisions and in elderly persons with severe stenosis and friable dura
• Failure or lack of fusion and/or pseudoarthrosis - This complication occurs in 5-25% of cases; the risk is highest when interbody grafts are placed as standalone devices and lowest with the addition of posterolateral fixation with pedicle screws; smoking increases the failure rate by as much as 50%
• Failure of fixation - This complication is rare (0.5-3%) and includes interbody graft expulsion or fracture, pedicle screw pullout, fracture, or migration (usually out of the lateral pedicular wall)

General surgical complications, such as hemorrhage and infection, occur in 1-5% of patients. A risk exists of injuring retroperitoneal structures such as the iliac vessels, the sympathetic chain, or the hypogastric nerves. This risk is obviously higher with anterior approaches (ie, ALIF) but also has been observed during radical excision of the anterior annulus fibrosis and PLIF procedures.

• References

Long-Term Monitoring

After the routine postoperative check in 4-6 weeks, plain radiographs should be performed to evaluate the fusion and fixation if used. The patient is expected to have mild discomfort during normal motion for the first few weeks. High-level athletic activity should be avoided for up to 3 months for the fusion to heal completely.

• References