Lytic spondylolisthesis/lysis and its treatment continue to be problematic. Surgeons and patients want the latest techniques, instrumentation, and grafting options combined with the best outcomes, yet with the least exposure to problems and complications. The Scoliosis Research Society Focus Edition of Spine presents the current expression of this goal. Complications may occur in the best of surgical environments, a fact that both patient and surgeon must acknowledge. An awareness of this is critical to minimize the occurrence of adverse events in the treatment of spondylolisthesis.
Pseudarthrosis is the most common complication of lumbar fusion surgery, and it more occurs frequently in fusions done for lytic spondylolisthesis than for those done in lumbar disc degeneration.1
When there is lucency around pedicle screws, broken or uncoupled implants, progressive vertebral displacement or increasing deformity, the diagnosis of failed fusion can be made. High resolution spiral computed tomographic scans may indicate a pseudarthrosis when there is persistent pain but no other radiographic signs indicating failure to heal. In children, an increase in spondylolisthesis even with uninstrumented solid arthrodesis has been reported; however, the responsibility is with the surgeon to ensure the integrity of the fusion mass and differentiate between this phenomenon and pseudarthrosis.2
In general, it is incorrect to regard a failed fusion as an acceptable outcome. While it is true that not all fusions produce good results and not all failed fusions are symptomatic, there is a strong statistical correlation between pseudarthrosis and a poor result.3,4 To minimize the occurrence of pseudarthrosis, one must adhere to meticulous technique and respect the principles of osteosynthesis. Innovative technologies are an adjunct to fusion surgery but do not supersede basic principles.
The three basic principles of fusion surgery are as follows:
The uninstrumented fusion rate in children and adults has been reported as satisfactory; however, pseudarthroses do occur.2,5–8 Posterolateral fusion with the addition of pedicle instrumentation can decrease the pseudarthrosis rate in adults.9,10
Posterior and posterior lateral fusions do not entirely immobilize the disc space.11 The addition of an interbody fusion with pedicle instrumentation places the graft in compression and provides maximum immobilization of the motion segment.12
The indications for use of an orthosis following low lumbar surgery are not well defined. Conventional orthoses without a leg or thigh extension do not predictably immobilize the lumbosacral joint, and they have little effectiveness in reducing the load on internal fixation devices.13,14
2. Fusion bed
Bone graft cannot form a contiguous fusion mass if there is nonviable bone at the interface of the structures being fused or if there is fibrous tissue interposition. Technique-related issues such as marginal osteonecrosis caused by heat buildup from a bone saw can delay incorporation of an interbody fusion mass.
The size and geometric association of the adjacent vertebrae can influence whether an additional segment should be included in the fusion or whether an interbody fusion is appropriate. An interbody fusion is usually needed for a spondylolisthesis pseudarthrosis repair or if the transverse processes are small.12 For slips that do not reduce to less than 50%, it is advisable to go one additional segment higher or perform an interbody fusion.15 It is at the discretion of the surgeon as to whether the interbody fusion is accomplished through the transforaminal or anterior approach.
3. Bone graft
Autogenous bone graft, which is osteoconductive, osteoinductive, and possibly osteogenetic due to surviving osteocytes and mesenchymal stem cells, remains the best source of grafting material. Because of the operative time and morbidity associated with separate-site autograft harvesting, various bone grafting substitutes have been developed, including recombinant human bone morphogenic protein, demineralized bone matrix, and concentrated platelets or stem cells. Enhancement of osteosynthesis with these products follows a dose-related response. The amount of cytokine, whether it is human-derived or recombinant, determines its effectiveness.16,17 Surgeons using these products should expect them to be validated with regard to the concentration achieved with the active molecule.
Fibular struts from L5 to S1 may be appropriate for high-grade spondylolisthesis and serve a dual purpose of internal fixation and bone graft, although there is no demonstrated difference between allograft and autograft.18
The 2003 Mortality and Morbidity report of the Scoliosis Research Society indicated that the incidence of neurologic deficits in most spinal surgeries has remained constant over the past several years with one notable exception. During the interval 1996–2002, there has been an increase in the percentage of degenerative and lytic spondylolisthesis undergoing reduction: degenerative, 15% to 33%; and lytic, 22% to 44%. The incidence of neurologic problems associated with degenerative spondylolisthesis reduction surgery has not changed over the reporting period and has remained at 0.3% to 1.0%. The incidence of neurologic complications with lytic spondylolisthesis surgery has increased from 1.3% to 3.1%. There has been no change in the incidence of deaths, dural tears, deep infections, or implant-related complications.19 The Morbidity and Mortality reporting system of the Scoliosis Research Society may have imperfections, and the implications of this statistic are not clear. The Mortality and Morbidity report does not differentiate between attempts at greater reduction of spondylolisthesis, variances in reporting, differences in the use of emerging techniques, e.g., posterior lumbar interbody fusion, transforaminal lumbar interbody fusion, anterior lumbar interbody fusion, or changes in instrumentation preferences.
Cauda equina syndrome can result from several intraoperative and postoperative conditions. This can occur without reduction and with no apparent explanation but may be related to vascular phenomenon, transient anterior displacement of L5 during the surgical exposure, or a period of hyperextension during patient positioning.20
Intraoperative direct trauma resulting from dural manipulations can injure sensitive lumbar and sacral nerve roots resulting in a multidermatomal deficit.
In the process of reducing L5 on S1, it is possible to displace the posterior section of the dumbbell-shaped L5–S1 disc. The preoperative sagittal MRI raises the awareness level of the surgeon, and exploration of the disc space during and after the reduction should minimize this occurrence.
A postoperative epidural hematoma may produce compression resulting in a multidermatomal deficit. The hematoma may accumulate several hours or days following surgery, and the patient and medical professionals should be aware of the possibility of a delayed-onset neurologic deficit.
When a cauda equina syndrome is suspected, timely advanced imaging in the form of an MRI or myelogram/CT and surgical decompression, if appropriate, is imperative. Controversy exists regarding the timing of decompression for cauda equina syndrome.21
When no compression of the neural structures can be demonstrated, the surgeon and patient are faced with the dilemma of whether to remove any spinal instrumentation. If the deficit is minor and the reduction is small, a wait-and-see attitude is justifiable. Large reductions and significant neurologic loss would tend to bias the decision toward removing or adjusting the instrumentation and allowing some loss of reduction. These are difficult decisions and must be individualized on the basis of thorough radiographic evaluation, the timeline of neurologic loss, and the extent of reduction.
L5 root deficits can occur with either direct trauma during instrumentation of the vertebrae or decompression of the nerve. More difficult to define are those lesions produced by traction of the root during the reduction process. Early detection with evoked EMG monitoring may reduce the incidence of this problem, but delayed deficits can occur presumably on a vascular basis. In laboratory studies, the strain on the L5 nerve root is nonlinear during reduction with a small amount occurring in the first 50% of reduction and the remaining 50% reduction resulting in 71% of the strain.22 This is reflected in clinical practice with the L5 nerve function being the most commonly involved level.23
When the spine is repositioned over the sacrum, the upper lumbar roots may also experience a deficit because of traction. When such a deficit occurs intraoperatively, an appropriate decrease in the reduction or revision of instrumentation is done.
Early postoperative deficits should be evaluated by an MRI or CT/myelogram in an attempt to differentiate traction deficits from nerve compression deficits.
Electromyography and nerve conduction studies are not helpful in diagnosing acute nerve root lesions and usually do not reflect changes during the first 21 days after injury.
Autonomic nerve dysfunction manifesting as retrograde ejaculation can be a complication of the anterior surgical approach to the lower lumbar spine.24 This condition may be transient or permanent. The occurrence rate may be lessened by blunt dissection and the avoidance of electrocautery in the retroperitoneum. An endoscopic surgical approach does not lessen the occurrence.25 No surgical treatment is effective in reversing the condition. Preoperative discussion and postoperative counseling to inform the patient that it does not produce impotence may alleviate the psychologic effects.
Neuromonitoring during lumbar surgery may lessen the incidence of lumbar and sacral nerve root deficits during lumbar surgery.26 It is particularly important when spondylolisthesis reduction is involved. Somatosensory-evoked potentials alone are not reliable in detecting intraoperative lumbar or sacral nerve root injuries.27 Evoked electromyography can be used to monitor lumbar and sacral nerve roots, including the rectal sphincter during pedicle screw placement, dural manipulation, and spondylolisthesis reduction.28
Chronic pain at a significant level is a most disturbing development for both the patient and surgeon. While fusions for lytic spondylolisthesis have a higher clinical success rate than fusions for discogenic disease, unsatisfactory results do occur.9,29 With the patient's consent, it is appropriate for the surgeon to rule out pseudarthrosis, symptomatic adjacent segment degeneration, persistent nerve root compression, instrumentation irritation, flat back syndrome, low-grade infection, and discogenic pain if an interbody fusion has not been performed. Other causes of unexplained pain such as endometriosis, systemic inflammatory disease, or complex regional pain syndrome require a multidisciplinary approach.
Spondylolisthesis acquisita occurs when the superadjacent motion segment undergoes changes through one of three processes. In the process of instrumenting a pedicle of the cephalad most vertebra, the adjacent facet capsule and joint can be compromised. Experimental studies demonstrate that excision of the lumbar facet capsule and articular cartilage increase segmental motion.30 This instability may lead to premature degenerative spondylolisthesis at the segment above the fusion. Second, if a laminectomy is performed at the level above the instrumented level and the pars interarticularis is thinned excessively, a pars stress fracture may occur leading to an acquired superadjacent isthmic spondylolisthesis. The third possibility is adjacent segment degeneration.
Adjacent segment degeneration is an enigma. Experimentally and intuitively lumbar fusions place an increased stress on the superadjacent level; and the longer the fusion, the greater the stress.31 However, reports vary in the incidence and time interval before this stress manifests itself as a radiographic or clinically significant disease.32,33 One series reported the incidence of adjacent segment degeneration as high as 35%, while others indicate that the clinical significance is minimal.34,35 If the adjacent segment degenerates, does this reflect the natural history of lumbar disc disease or is it primarily due to the influence of a one- or two-level subjacent fusion? While both are probably a factor, the goal should be to leave as many lumbar levels unfused, consistent with the goals of surgery. When a normal L5–S1 is the subjacent level of an L4–L5 fusion, its inherent stability seems to shield it from significant premature degeneration. Physiologic sagittal contour plays a role in minimizing adjacent level degeneration.36 If there is a flat lumbar spine, the adjacent level may assume a compensatory hyperextension, thus accelerating the process of facet degeneration (Figures 1 and 2).37
S1–S2 deformity can occur when a fusion is instrumented to S1 and a stress fracture is sustained at S2 resulting in an acquired intrasacral kyphosis. A transitional L5–S1 unfused joint may also become symptomatic following fusion to the adjacent level. In both cases, an extension of the fusion to the pelvis may be indicated. An extension of the lumbar instrumentation by long screws into the iliac wings is used for fixation, bone graft is extended to the S3 lamina, and an iliolumbar fusion is done.
Operative treatment of spondylolisthesis requires consideration of proper surgical indications, fusion principles, appropriate instrumentation and reduction techniques, neurophysiology, sagittal contour, and preoperative patient counseling. This can minimize both the occurrence and psychologic impact of complications in the surgical treatment of spondylolisthesis.
- Meticulous technique is the basis for successful arthrodesis in spondylolisthesis surgery.
- Neurologic complications should be investigated on a timely basis.
- Transition syndromes can be minimized with proper planning.
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