Gilbert Chan and John P. Dormans
DEFINITION
Spondylolisthesis is the forward displacement of a vertebra over the next adjacent segment.
In children and adolescents, this most commonly occurs in the presence of a spondylolytic defect or a nonunion of the pars interarticularis. It also may occur in the presence of inherent spinal anomalies such as deficient or maloriented lumbar and lumbosacral facets.
Spondylolisthesis has been grouped into five different types under the Wiltse-Newman classification: dysplastic, isthmic, degenerative, traumatic, and pathologic.25
Ordinarily, the dysplastic and isthmic types apply to children (FIG 1A).
The Meyerding classification is used to grade the degree of slippage (FIG 1B). The classification is divided into five grades:
Grade I: 0 to 25% slip
Grade II: 26% to 50% slip
Grade III: 51% to 75% slip
Grade IV: 76% to 100%
Grade V: corresponds to spondyloptosis
Anterior slippage of 50% or more (Meyerding grade III or IV) of the transverse width of the caudal segment is termed a high-grade slip.
ANATOMY
The vertebral bodies have a tendency to increase in size with progression caudally. This increase is believed to be related to the demands of increased stress and weight bearing placed on the lumbosacral spine.
The lumbar vertebrae are widest in transverse diameter than in the anterior-posterior plane. The lumbar foramina appear trefoil-like. The spinous processes are large and have an oblong appearance. Its transverse processes are long and slender and project directly lateral. The facet joints of the lumbar spine are oriented more toward the sagittal plane, allowing for more flexion and extension motion.
The neurovascular structures in the lumbar spine run a similar course when compared to the thoracic spine. The segmental vasculature arises directly from the aorta and run dorsally around the lateral aspect of each vertebral body. Branching occurs near the pedicles, where one branch supplies the spinal canal and the other supplies the paraspinal musculature. These vessels run between the transverse processes, where they may be susceptible to injury in more lateral exposures.
FIG 1 • A. The dysplastic and isthmic types of spondylolisthesis. B. The Meyerding classification is based on degrees of slippage: grade I, 0 to 25%; grade II, 26% to 50%; grade III, 51% to 75%; grade IV, 76% to 100%.
The spinal cord ends most commonly at the level of L1–2. The conus medullaris extends from the most distal portion and goes on to innervate the bowel and bladder. Beneath the conus, the lumbar and sacral nerve roots are arranged to form the cauda equina. Each of these roots exits segmentally below the pedicle of the corresponding vertebrae.
The pedicles are cylindrical structures that bridge the posterior elements of the spine with the vertebral body. The height and diameter of the pedicles increases from the thoracic to the lumbar spine. The transverse diameter of the pedicle gradually increases from L1 to L5. The transverse angulation of the pedicles is directed medially, increasing gradually from L1 to L5. The sagittal plane orientation of the lumbar spine pedicles is neutral.4,18,20,26,27
The spinal cord and the dural sac lie medial to the pedicles. Corresponding nerve roots are found both superior and inferior to each pedicle, with the inferior nerve root in closer proximity to the pedicle.5,20
The orientation of the facet joints in the lumbar and lumbosacral spine is related to function. In the upper part of the lumbar spine, the orientation of the joints allows for multidirectional stabilization. This is in contrast to the lumbosacral facet joint, which is flat and more coronally oriented and acts to resist shearing forces through the joint.7
PATHOGENESIS
Spondylolisthesis is a disorder related to upright posture, increasing the forces acting upon the lower segments of the spine, and is never seen in the nonambulatory individual.
The lumbar spine is subject to high shear forces and compressive loads. The “bony hook,” consisting of the pedicle, the pars interarticularis, and the inferior facets, provides stability by resisting these shear forces and preventing forward slippage or sliding over the inferior endplates.
In the setting of congenital or dysplastic spondylolisthesis, the spine begins to slip even if the posterior elements are intact. This is brought about by the structural abnormality's inability to resist the load and shear forces seen in the lumbosacral spine.
In the isthmic type of spondylolisthesis, secondary to a pars defect, the high shear and compressive forces occurring through the lumbar spine and lumbosacral joint are less well resisted. This is due to the loss of posterior restraint, allowing forward displacement of one vertebral segment over the next more caudal level.9
NATURAL HISTORY
Harris and Weinstein10 reviewed 38 cases with high-grade spondylolisthesis treated nonoperatively and with in situ fusion with a mean follow-up of 24 years and showed that 36% of patients treated nonoperatively were asymptomatic, 55% had back pain, and 45% had neurologic symptoms.
Beutler et al,1 in a 45-year follow-up study of 30 patients diagnosed with spondylolysis, screened in the 1950s from a pool of 500 first-grade children, showed that no patients with unilateral pars defects developed spondylolisthesis. They also showed that cases with bilateral pars defects and low-grade slips follow a course similar to that seen in the general populace. Slowing of slip progression was observed with each decade.
FIG 2 • A 14-year-old boy diagnosed with spondylolisthesis with flattening of the lumbar lordosis.
In a comparison of the progression of the slip between isthmic and dysplastic types of spondylolisthesis, dysplastic types showed increased progression.15
PATIENT HISTORY AND PHYSICAL FINDINGS
In symptomatic patients, the most common clinical manifestation is low back pain, with or without radicular pain radiating through the L5 or S1 dermatome. Onset of pain is usually chronic and insidious, but acute episodes do occur.
In patients with radicular symptoms, unilateral involvement is more common.
Flattening of the lumbar lordosis is commonly seen on physical examination (FIG 2).
Abnormal gait exemplified by a hip-flexed, knee-flexed gait pattern may be present.
Hamstring tightness, which also may be present, is tested by measuring the popliteal angle. Many patients with high-grade slips will have a tendency to develop tight hamstrings owing to the development of abnormal biomechanics in the lumbar spine.
Straight leg raise should be done to test for nerve root compression or hamstring tightness. A positive examination with radicular pain denotes either an L5 or S1 nerve root compression. Radicular pain elicited before 70 degrees is indicative of root compression, whereas that elicited above 70 degrees might denote extraspinal compression of the sciatic nerve. Pain in the posterior thigh denotes hamstring tightness.
A rectal examination should be done in the presence of bladder and bowel dysfunction.
Examination should also include the Lasgues test. If the pain is exacerbated, that finding supports the diagnosis of a nerve root compression. Posterior thigh pain secondary to tight hamstrings will not be aggravated.
IMAGING AND OTHER DIAGNOSTIC STUDIES
Initial imaging includes standing posteroanterior and lateral radiographs of the spine as well as oblique views (FIG 3A–C).
Plain radiographs are used to establish the overall alignment of the spine in both the coronal and sagittal planes. The sagittal alignment should be noted, particularly the degree of lumbar lordosis above the lumbosacral kyphosis. Any structural abnormalities in the spine aside from the slip should be noted. These abnormalities include the presence of spina bifida occulta, scoliosis, or sagittal plane abnormalities. Other spinal problems should be treated as separate entities.
FIG 3 • PA (A), lateral (B), and left and right oblique (C) radiographs demonstrating high-grade spondylolisthesis. Axial (D) and sagittal (E) CT scan sections demonstrating bony deformity. F. MRI demonstrating high-grade spondylolisthesis.
CT scans are valuable in defining the exact bony abnormality and will help in preoperative planning (FIG 3D,E).
MRI studies are indicated when there is evidence of neurologic compromise. MRI provides good visualization of nerve root or cauda equina compression (FIG 3F).
DIFFERENTIAL DIAGNOSIS
Mechanical disorders: trauma, overuse syndromes, herniated disc, slipped vertebral apophysis
Developmental disorders: Scheuermann's kyphosis
Inflammatory disorders: discitis, vertebral osteomyelitis, calcific discitis, rheumatologic conditions
Neoplastic disorders
NONOPERATIVE MANAGEMENT
The treatment of high-grade spondylolisthesis is surgical. Even in asymptomatic cases, the risk of progression or the development of cauda equina syndrome warrants surgical intervention.
SURGICAL MANAGEMENT
The first goal of surgical management is to avoid complications (“first do no harm”).
Surgical management is indicated in high-grade slips, with or without the presence of neurologic compromise, or in refractory symptomatic patients.
Selecting the appropriate surgical intervention for each patient requires:
A thorough evaluation of the deformity
An in-depth understanding of the nature of the pathology
An understanding of the indications for treatment
Awareness of the limitations of each procedure and its possible complications
Preoperative Planning
A detailed assessment of the history and physical and neurologic examinations should be performed.
All imaging studies must be carefully reviewed and analyzed with attention to trying to correlate physical and neurologic findings with those found in special examinations.
The degree of the slip as seen on the lateral standing spine radiographs is assessed and graded according to the Meyerding classification.
Slippage of 50% or more is considered a high-grade slip.
The slip angle measures the degree of lumbosacral kyphosis.
A slip angle greater than 50 degrees is associated with progression, instability, and pseudoarthrosis (FIG 4A).
Pelvic incidence (PI) is a fixed anatomic parameter that estimates the position of the sacral endplates and overall pelvic morphology. It helps determine the overall sagittal profile of the spine.3,12
The PI increases with age and stabilizes in adulthood.14
The mean PI is 47 degrees in children and 57 degrees in adults.
Increased PI is indicative of increased lumbar lordosis and increased shear forces (FIG 4B). Increased PI may predispose to the development of spondylolisthesis.3,11,12
In the presence of spondylolisthesis, an increased PI may be indicative of an unbalanced pelvis and is a risk factor for slip progression. Slip reduction is required in these cases to restore proper spinopelvic biomechanics and stabilize the spine. In those cases, where the spine is balanced and PI is low, a fusion in situ may be all that is required for treatment.
Positioning
The patient is positioned prone on the operative frame.
Two operative positions are commonly used for posterior approaches to the spine.
The first is the knee-chest position, where both the hips and knees are flexed.
The second position is with the use of a four-poster frame, where the lower extremities are fairly parallel to the trunk. In this position, the patient is supported under the anterior superior iliac spines and pectoral muscles bilaterally.
FIG 4 • A. Slip angle is a measure of lumbosacral kyphosis. A slip angle greater than 50% is associated with progression, instability, and pseudoarthrosis. B. Pelvic incidence is the angle formed between a line perpendicular to the center of the sacral endplate and a line connecting this point to the center of the femoral heads. It is a good indicator of pelvic shape and morphology. Increased pelvic incidence has been associated with increased shear forces and the development of spondylolisthesis.
Our preference is to place the patient in the Jackson spinal table with the hips and knee in the flexed position, allowing for easier access to the lumbar spine.
The position of the face and arms is important. The face should be adequately supported, making sure that no excessive pressure is applied, especially around the orbits. The neck should be in neutral position. The upper extremities should also be in 90–90 position, in which the arms are in 90 degrees of abduction and the elbows are in 90 degrees of flexion. The upper extremities should be adequately padded to allow for venous and arterial access. Adequate padding, support, positioning, and monitoring of the upper extremities likewise prevents undue neurologic injury due to stretch or excessive pressure.19
TECHNIQUES
ROUTINE REVISION WITHOUT DIAPHYSEAL DEFECT
Bohlman Technique
In the Bohlman technique,22 the procedure starts with a standard posterior midline approach extending from the second lumbar vertebra to the level of S2.
The spine is exposed subperiosteally, revealing all the posterior elements.
The posterior elements of L5 and S1 are removed (the posterior elements of L4 are removed if needed).
A wide foraminotomy is performed to decompress the L5 and S1 nerve roots.
The dura is gently freed and retracted.
A curved osteotome is used to make a ventral trough through the sacrum to remove pressure from the dura (TECH FIG 1A).
The dura is retracted, and a ⅛-inch guide pin is placed in the midline of the sacrum toward the body of L5. This is done under fluoroscopic guidance to ensure proper placement.
A ½-inch cannulated drill bit is used to drill over the guide pin, taking extra care not to violate the anterior cortex of L5. The depth is approximately 5 cm.
A mid-diaphyseal fibular graft is trimmed to fit into the drill hole and inserted (TECH FIG 1B). The position of the graft is confirmed fluoroscopically.
Alternatively, a split fibular graft can be inserted as described by Bohlman in 1982.2
This is performed by inserting a guide pin through the posterior prominence of the sacrum approximately one cm from the midline to the body of L5, avoiding the first sacral nerve root. The position of the guide pin is confirmed radiographically. This process is done bilaterally.
TECH FIG 1 • A. A wide laminectomy is performed removing the posterior elements of L5 and S1. The dura is retracted gently, and a curved osteotome is inserted to perform a sacroplasty to take pressure off the dura. B. A fibular strut graft is fashioned and inserted into the sacrum to the body of L5. The procedure is then completed by performing bilateral posterolateral fusion.
A ⅜-inch cannulated drill bit is used to drill over each guide pin, taking extra care not to violate the anterior cortex of L5. The depth is approximately 5 cm on both sides.
A fibular graft is split in half and trimmed. It is then inserted and countersunk 2 mm into each hole.
A standard posterolateral transverse process fusion is done, extending from the sacral alae to L4, to complete the procedure.
Children's Hospital of Philadelphia (CHOP) Technique
Incision and Dissection
The lumbar spine is approached posteriorly through a direct midline incision extending from L2 to S2 (TECH FIG 2A).
The dissection is done using loupes for magnification and head lamps for illumination.
The midline incision is carried down to the fascia through sharp dissection of the skin and subcutaneous tissue.
The midline dissection is carried down subperiosteally, exposing the posterior elements of the spine, with care taken to protect the most proximal intact facets (TECH FIG 2B,C).
TECH FIG 2 • A. A direct midline posterior skin incision along the spine is made, extending from L4 to S2. B,C. The fascia is incised along with the skin incision, and the paraspinal muscles are dissected off of the posterior elements subperiosteally.
TECH FIG 3 • A. A wide laminectomy and adequate decompression of the nerve roots are performed. B. A sacroplasty is then performed to take pressure off of the dura.
In isthmic spondylolisthesis, removal of loose bodies and the posterior elements of L5 is done.
Decompression
The nerve roots of L5 and S1 are identified, and a wide decompression of the L5/S1 roots is carried out bilaterally (TECH FIG 3A).
The dura and neural elements over the sacrum are gently retracted, and a sacroplasty is done using an osteotome or a high-speed diamond burr (TECH FIG 3B).
Reduction and Fusion
Pedicle screws are placed into the L4, L5, S1, and S2 pedicles (TECH FIG 4A).
The anesthetic and spinal monitoring team is informed before any corrective maneuvers are performed.
The reduction is performed under fluoroscopic guidance, avoiding overcorrection. With the use of reduction tools attached to the pedicle screws, the slip angle is gradually reduced under fluoroscopic guidance by applying a dorsal extension maneuver to the lumbar pedicle screws (TECH FIG 4B).
TECH FIG 4 • A. Pedicle screws are placed from L4 to S2. Once all of the pedicle screws have been placed, a gentle reduction is performed, aimed at correcting the slip angle. B. Using reduction tools attached to the pedicle screws, a dorsal extension force is applied to the lumbar spine while a counterforce is applied to the sacrum. This maneuver gently corrects the slip angle and restores lumbar lordosis. Attention to spinal cord monitoring is crucial at this point in the operation to avoid undue neurologic injury. C. Following correction of the slip angle, gradual correction of the slip is performed by applying pressure on the sacrum while the lumbar spine is held and a gentle force is applied in the opposite direction, affording reduction. Overcorrection of the slip should be avoided. D. The entire construct is checked after reduction under fluoroscopy to ensure proper implant placement and adequate correction of the spondylolisthesis, and final tightening is done. E. The dura is gently retracted, and a cage is placed to add anterior column support. F. Alternatively, fibular strut grafting may be used to provide anterior column support. This is done by inserting a guide pin through the sacrum to the body of L5. G. A. cannulated drill bit is used to ream through to the body of L5. H. A split fibular graft is fashioned and countersunk into each drill hole bilaterally.
The reduction is performed slowly and maintained over time to allow for stretch of the soft tissue. Once a satisfactory correction of the slip angle has been achieved, gradual reduction of the slip is performed by applying force to the sacrum, while a counterforce is applied to the lumbar spine (TECH FIG 4C).
Reduction of the slip angle is much more important than reduction of the slip itself. Close attention to spinal cord monitoring is crucial during the entire reduction maneuver.
The rods are templated, cut, and contoured, and then attached to the construct while reduction is maintained.
Final tightening of the entire construct is done and checked with fluoroscopy or plain radiographs (TECH FIG 4D).
The L5–S1 disc is identified and removed, a fusion is performed, and the L5–S1 disc space is filled with cancellous autograft or allograft.
An anterior cage is placed to provide adequate anterior column support (TECH FIG 4E).
As an alternative, a double split fibular strut graft (modified Bohlman technique) can be inserted from the sacrum to the body of L5 to add anterior column support. The dura is gently retracted to one side, and a guide pin is inserted from the sacrum to the body of L5 (TECH FIG 4F).
A 6-mm cannulated reamer is then used to ream this channel (TECH FIG 4G).
A fibular auto- or allograft is then placed through the channel and countersunk (TECH FIG 4H).
The same procedure is repeated on the contralateral side.
The procedure is completed by placing bone graft lateral to the implants along the transverse processes from L4 to the sacrum.
Meticulous hemostasis is carried out, and a layer-by-layer closure of the operative site is performed.
FIG 5 • Radiographs from a 17-year-old girl with high-grade isthmic spondylolisthesis who underwent decompression, reduction, and instrumented fusion. A,B. Initial PA and lateral radiographs showing the preoperative deformity. C,D. PA and lateral films showing postoperative correction using the CHOP technique.
POSTOPERATIVE CARE
High-quality radiographs are taken immediately postoperatively to ensure proper graft and instrumentation placement before the patient is taken out of the operative suite (FIG 5).
In the immediate postoperative period, the hips and knees are flexed and elevated using pillows to alleviate pain.
Pain control is instituted (eg, intrathecal analgesia and IV patient-controlled analgesia), and the patient is fitted with a thoracolumbosacral orthosis (TLSO) for comfort. The patient is then encouraged to stand and ambulate as tolerated. Postoperative anteroposterior and lateral standing spine radiographs are taken before discharge.
Activity restriction (ie, avoidance of bending and rotational motion) is carried out until fusion has occurred.
The patient may return to sports and strenuous physical activity after 1 year as long as spinal fusion has been confirmed. Adequate precautionary measures should be taken before engaging in any contact sport.
Full-contact sports, which may entail collision, should still be avoided.
OUTCOMES
In high-grade spondylolisthesis treated with in situ fusion techniques, clinical improvement in back pain symptoms has been reported in 74% to 100% of cases. Solid fusion rates have also been reported to be 71% to 100%.6,8,13,23,24
A study on 18 adolescents with high-grade spondylolisthesis treated with instrumented reduction and fusion reports complete resolution of preoperative neurologic symptoms with 100% fusion rates. No loss of fixation or instrument-related failures were reported at a minimum of 2 years' follow-up.21
Another series16 comparing in situ fusion, decompression, reduction and instrumented posterior fusion, and circumferential fusion techniques in treating high-grade spondylolisthesis reports a 45% (5 of 11 patients) pseudoarthrosis rate in patients treated with in situ fusion and a 29% (2 of 7 patients) pseudoarthrosis rate in cases treated with posterior decompression, instrumentation, and fusion. All of these cases had small transverse processes (less than 2 cm2). Circumferential techniques achieved the highest fusion rates. Excellent functional outcomes were observed in those cases where a solid fusion was achieved. Final outcomes, however, did not differ among the three groups.
Another study comparing posterior fusion and reduction with posterior fusion and reduction augmented by anterior column support reported a 39% pseudoarthrosis rate in posterior fusion alone. In the cases augmented with anterior column support, 100% fusion rates were achieved.16
COMPLICATIONS
Pseudoarthrosis
Pseudoarthrosis is the most common complication.
Signs include lucency around implants, implant breakage, and slip progression.
Pseudoarthrosis may be minimized by using meticulous technique and proper preparation of the graft site.
Neurologic complications
Root lesions (L5 root)
From direct trauma, manipulation of nerve roots, epidural hematoma formation (compression)
Cauda equina syndrome
Autonomic dysfunction
Chronic pain
Immediate release of the correction should be done when necessary.
Must be thoroughly evaluated with proper imaging techniques
May be minimized by good preoperative planning and meticulous surgical technique and by using multimodality spinal cord monitoring
Transition syndromes
Spondylolisthesis acquisita
Adjacent segment degeneration
S1–S2 deformity
Instrument-related complications
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