The term, spondylolisthesis. is used to describe forward displacement of one vertebral body on another. Such a displacement frequently occurs at the lumbosacral junction in association with a defect, called spondylolysis. in the fifth lumbar (L5) isthmus (pars interarticularis) on each side. Spondylolisthesis may also develop secondary to degenerative changes of the facet joints and intervertebral disc between adjacent spinal segments; this frequently occurs between the L4 and L5 vertebral segments. Most patients with spondylolisthesis present with low back pain, and plain radiographs are usually sufficient for diagnosis. Although the majority of patients will obtain pain relief from conservative measures, some will require neural decompression, bony fusion, or spinal instrumentation.

History & Classification

You must refer to several neurosurgical textbooks , such as Neurosurgery by Robert H. Wilkins  & Setti S. Rengachary,

PATHOGENESIS

Isthmic Spondylolisthesis

Earlier theories that isthmic defects were the result of separate ossification centers have been largely discredited. These theories were based primarily on irreproducible work that claimed the existence of two ossification centers for each side of the posterior vertebral ring. More recent findings support the theory that spondylolysis and isthmic spondylolisthesis probably result from a combination of genetic and mechanical factors. The increased prevalence of spondylolysis among  first-degree relatives of patients with isthmic spondylolysis or spondylolisthesis  strongly supports an inherited predisposition, although a definite pattern of transmission has not been identified. Similarly, the observation that spondylolysis is found more frequently among female gymnasts and college football players suggests that mechanical factors also play a significant role. The impact of mechanical factors is further supported by the failure of a review of non ambulatory patients to find cases of spondylolysis. Furthermore, photoelectric modeling experiments demonstrate that the peak mechanical stress of the lumbar spine is centred at the isthmus. Therefore, it is now generally accepted that these isthmic defects are the result of successive fatigue fractures that occur more easily in patients with a genetic predisposition.

These fatigue fractures lead to pathologic changes of the isthmus, facet joints, and intervertebral disc. The isthmic defect is usually surrounded by an acellular and avascular band of coarse fibrous tissue that resembles normal ligamentum but may incorporate zones of fibrocartilage, hyaline cartilage, or endochondral bone formation. Hypertrophy of the facet joints and alterations in the chemical composition of the intervertebral disc, specifically a reduction in proteoglycans and collagen types I and III occur secondarily. These factors operating between the L5 and SI vertebral segments contribute to symptomatology by encroaching on the intervertebral foramen on one or both sides and directly compressing the L5 nerve root or tethering the L5 nerve root by anchoring it to the anteriorly displaced vertebra.

Degenerative Spondylolisthesis

Degenerative spondylolisthesis occurs in patients with chronic intersegmental spinal instability and underlying degenerative joint disease and is most frequently observed between the L4 and L5 vertebral segments. A number of anatomic variants thought to predispose to this instability have been identified. These variants include hypolordosis, sacralization of the L5 vertebral body, a rectangular L5 vertebral body, narrow L4 inferior articular processes, a low position of the iliac crests relative to the spine, and sagittally oriented facet articulations.These variants are thought to secure the lumbosacral junction while amplifying the forces of body weight at the junction between the L4 and L5 vertebrae. Such forces result in degenerative changes at the articular facet joints that include joint capsular laxity, synovitis, cartilage fibrillation and degeneration, osteophyte growth, and fractures of the articular processes. Similarly, the intervertebral disc becomes dehydrated and fibrotic. Such changes lead to disc space narrowing and hypertrophic changes at the facet joints and vertebral endplates.

As with isthmic spondylolisthesis the L5 nerve root is most frequently injured. The pathologic process is now operating between the L4 and L5 vertebral segments, however. It may be compromised by a herniation of the L4- L5 intervertebral disc, by compression within the narrowed lateral recess, or by tethering across the posterior border of the L5 vertebral body with progressive anterolisthesis. With progressive anterolisthesis, the intact isthmus of the superior vertebra eventually comes to rest on the superior articular process of the lower vertebra, however, and slippage is generally limited to 30 percent.

CLINICAL FEATURES

Most patients with spondylolysis or spondylolisthesis are asymptomatic. Symptomatic patients usually present with low back pain, and this is usually attributed to isthmic or facet pseudoarthrosis or disc disintegration. Rarely, patients will also have radicular symptoms signifying involvement of the spinal nerve roots. Neurological findings are unusual, however, and generally limited to L5 dermatomal sensory changes in both isthmic and degenerative spondylolisthesis. Severe anterolisthesis, however, stretches the remainder of the cauda equina over the border of the lower vertebral body and can result in additional radiculopathies, neurogenic claudication, perineal pain or numbness, or bowel and bladder dysfunction. Any neurological deficit mandates sufficient radiographic evaluation to exclude other pathologic processes.

Additional imaging modalities are always needed to evaluate patients with radicular pain or neurological deficits.  Magnetic resonance images are essential by providing better soft tissue definition and multiplanar images that track the nerve root well beyond the foramen. Scintigraphy, which is rarely in use, can be performed to exclude acute fractures and malignant disease. Finally, discography and facet arthrography may be helpful in some instances but have not been adopted universally

The following indications must meet criteria for surgical treatment:

1. Presence of mobile spondylolisthesis  usually, higher than grade I by Meyerding and isthmolysis, unilateral or bilateral.
2. Progressive deterioration within months  with failure of conservative measures.
3. Presence of neurological deficit, which usually manifested by weak dorsiflexion of all toes either foot with LBP.
4. The dynamic X-rays showing some grade of mobility in flexion and extension.
5. MRI of the lumbar spine must be performed, to plan preoperatively, eliminate all the causes of pain or neurological deficit.
6. Detailed discussion with the patient, about the pros  & cons and possible complications, after such procedures.
7. Psychological and litigational conditions, must be overweighed, before attempting such procedures.

TREATMENT

Conservative Treatment

Most patients without neurological symptoms or signs should initially be managed conservatively. However, studies have shown that up to one-third of patients with isthmic or degenerative spondylolisthesis are at risk for progressive listhesis, which may lead to neurological deficiencies. Unfortunately, this high-risk population has been difficult to define. Factors reported to be associated with progressive slippage include skeletal immaturity, female sex, dysplasia of the posterior elements, spina bifida occulta, increased slip at diagnosis, a high sagittal rotation angle, and rounding of the superior aspect of the sacrum. Individually, though, none of these factors has been shown to have a consistent effect on slip progression. Still, patients with a combination of these risk factors should be considered for early surgery.

Both flexion exercises and braces designed to reduce lumbar lordosis have demonstrated some efficacy in symptomatic patients with Meyerding grade I or II spondylolisthesis. Other nonoperative techniques such as bedrest, restriction of sports activities, anti-inflammatory analgesics, or injection of the disc space or facet joints with steroids or topical anesthetics may be of diagnostic assistance, but are of uncertain long-term therapeutic benefit

Operative Treatment

Indications for operative treatment include failure of conservative therapy, disturbing postural abnormality, neurological deficit, observed slip progression, or the presence of several of the risk factors for slip progression as described above. Pathologic processes that may lead to similar symptoms and signs, however, must first be excluded. Operative treatment for spondylolysis and spondylolisthesis employs variable combinations of neural decompression, bony fusion, and instrumentation. Careful patient selection can lead to excellent results with each of these methods.

Decompression

Bony Fusion

Fusion of the lumbar spine with bone grafts is useful in patients with slip progression or persistent pain despite conservative measures. This can be achieved by a variety of methods including anterior interbody, posterior interbody, posterior, or intertransverse fusion. No prospective randomized study has compared these different methods. In an extensive literature review, however, Turner et al. assessed patients who underwent spinal fusion, usually for a herniated disc or spondylolisthesis, and found no statistically significant difference in the clinical outcome attained by each of these methods. Although Turner et al. did find anterior interbody fusions to have a significantly higher rate of pseudoarthrosis, this probably represents a patient selection bias because many surgeons use this approach for more difficult cases. Still, many spinal surgeons may be unfamiliar with the approach required for anterior interbody fusion. In addition, such an approach does not permit simultaneous decompression, and, therefore, anterior interbody fusions are generally avoided. Similarly, posterior fusions make subsequent decompression difficult and need to be extended above the loose L5 posterior element in isthmic spondylolisthesis. Therefore, posterior fusions are also used infrequently. Finally, although excellent results have been reported by some authors using posterior interbody fusion, experience with this technique is limited, and it necessitates removal of the posterior elements and retraction of the cauda equina. Consequently intertransverse fusion, also known as ala-transverse and posterolateral fusion, had gained the widest clinical acceptance in the past.

The main disadvantage of pedicle screw systems is the possibility of direct injury to the nerve root in the intervertebral foramen. Universal transpedicular instrumentation systems are the most versatile because they employ a series of devices connected with rods that can be molded in three dimensions. Systems that fit these criteria are many in the market.

Spinal instrumentation for direct repair of spondylolysis should be considered in symptomatic young patients with Meyerding grade I isthmic spondylolisthesis. Stainless steel 18-gauge wire looped around each transverse process and tied below the spinous process of the same vertebral segment, or screws placed through the lamina, across the isthmic defect, and into the pedicle have been used for this purpose in the past. The fibrous tissue within the spondylolytic defect which may be compressing the nerve root is removed prior to instrumentation, Bone graft can be harvested from the spinous processes.  Clinical results are similar for each of these methods, and more than 80 percent of properly selected patients will have an acceptable result and a permanent fusion at the defect. Results tend to be worse in older patients and in patients with greater displacements. This procedure should not be expected to relieve symptoms derived from other ethiologies, and, therefore, is contraindicated in patients with evidence of degenerative disc disease. Success with these procedures may be predicted by pain relief after lidocaine infiltration of the spondylolytic defect. The main advantage of this approach in eligible patients is the maintenance of all mobile disc segments.

Instrumentation can also be used to reduce and to maintain a reduction of a high grade spondylolisthesis. The goal of reduction is to normalize the biomechanical alignment of the spine. This is designed to make stabilization easier, to decrease the pseudoarthrosis rate, to reduce progression of the deformity, and to correct the cosmetic deformity. Although reduction procedures are the only way that meaningful correction of the severe deformity associated with high degree slips can occur, most of these patients when treated with intertransverse fusion alone will have resolution of pain and neurological symptoms. In addition, the risk of permanent motor weakness secondary to a reduction procedure, which usually was involving the L5 nerve root,  approached 20 %. Therefore, the increased risk of reduction, in old fashion treatment modalities, was often difficult to justify and was not advised without careful attention to the relatively high risk of neurological injury.

Surgical Standards:

1. Skeletonization of L4, L5 laminae and the upper border of the sacrum, performed, trying during that , to check for movable segments, for further intraoperative confirmation. It happened several times, that the patient was planned for such a procedure, and this trick , changed the plan of surgery, before attempting to go far, to such areas as the transverse processes. Using towel clips is a useful tool for such task.

2. Check for recent fractures at the isthmus, since malpractice of others especially in the third world, by using a baton for LBP patients, which causing fractures of the spinous process, lamina and isthmus. To these special cases, which the patient try to escape these events before surgery, I modified a new technique by using miniscrews, after drilling the surface of the bone , to accept  three or four screws from the orthopeadic hand set screws. It usually works excellent and the construct becoming stable.

3. Remove the whole lamina of L5  with all the cartilage, performing during that foraminotomy of both S1 roots for not less than 10 mm distance.  Expose both L5 roots  and remove all the compressing elements. By doing this, you avoid any injury to these roots , and nibbling with drilling  down to the pedicle, you actually not in need for fluoroscopy. This also facilitate later the process of reduction. Don't forget to gather the bone for subsequent grafting.

4. Try all the time to preserve the epidural fat, since it   prevents subsequent fibrosis and in case, of its poverty, you can transfer the most near local fat tissue. These fatty tissues are more similar to the epidural fat , than the subcutaneous or the fat over the dorsal sheet, or other artificial materials, with which I had in the past a bad experience, such as the adcon-gel, which to my knowledge is out of production.

5. Perform bilateral discectomy L5-S1, trying to make the holes very small, to preserve the PLL and the posterior aspect of the annulus fibrosis, to prevent recurrent disc . Try meticulously clean the disc space and try during that to reduce the slippage by curved blunt metallic devices, they are many. Observe the adjacent  vertebral edges to notice the difference in the level. You can use the flexible neuroendoscope to watch the emptiness of the disc space. Some times, you need to perform tiny drilling at the level of the disc space to insert the reducing device, as seen in the present video.

6. Using the polyaxial transpedicular screws, facilitate the easiness of rods placement and check fluoroscopy must be done, in spite of the previously mentioned exposed pedicles. Do not be confident, all the time double check your actions. After inserting the four screws, the rods must be a little bit longer, to perform distraction and subsequent reduction.

7. After your device in place and the reduction is acceptable, and you are satisfied with the images, check the roots and the pedicles, which could have small cracks at the insertion canal and remove them, to prevent any unnecessary  malformed structures in the roads. Check the disc space from both sides and perform further discectomy, if needed.

8. The gathered bone, at the start of the operation is chipped in small pieces and realigned lateral to the rods  with refreshment of the transverse processes  for future bony fusion. You must take in consideration, that in some cases the construct becoming loose and this graft can help in this situation, in case of removing the device after several months, so whatever the reason was.

9. During closure and fat tissue transfer, observe the chips of bone so as to prevent them from slipping to the neural elements. Water-tight closure with subcuticular stitching, avoid Ready-vac drains.