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Surgery

A Radiographic Assessment of the Ability of the Extreme Lateral Interbody Fusion Procedure to Indirectly Decompress the Neural Elements

Oliveira, Leonardo BSc*; Marchi, Luis MSc*; Coutinho, Etevaldo MD*; Pimenta, Luiz MD, PhD

Author Information
doi: 10.1097/BRS.0b013e3182022db0
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The anatomic changes of hypertrophic articular processes causing spinal canal stenosis were first described by Verbiest in 1954.1 At present, stenosis is often defined as occurring in the central, lateral recess, or foraminal areas, and may also be categorized as due to soft tissue or hard tissue encroachment and/or spinal malalignment. When disc height is significantly reduced due to degeneration or malalignment such as spondylolisthesis, the interlaminar space and intervertebral foramina are also reduced. Bony encroachment through facet hypertrophy may also contribute.

The primary indication and goal of surgery in patients with symptomatic degenerative lumbar stenosis is neural decompression. This can be and often is accomplished by direct posterior resection of bone and/or soft tissue such as in laminectomy/laminotomy, facetectomy, and foraminotomy procedures. Complications associated with direct lumbar decompressive surgery include bleeding, epidural hematoma, deep venous thrombosis, dural tear, cerebrospinal fluid leak, infection, nerve root injury, epidural fibrosis, iatrogenic instability, and recurrence of symptoms.2–5 Wide bony decompression may necessitate stabilization by instrumentation and fusion due to the resection of elements responsible for natural spinal stability. Interbody fusion may also be indicated when there is a loss of disc height and alignment. Decompression alone in cases of malalignment has been shown to be less effective than anterior interbody fusion at alleviating pain and function,6,7 as interbody fusion provides restoration of disc height and correction of coronal and sagittal alignment.

The extreme lateral interbody fusion (XLIF) procedure has been described as an alternative approach to anterior column stabilization in degenerative, deformity, and traumatic conditions of the thoracolumbar spine.8–13 The XLIF procedure has advantages over the early stand-alone direct anterior lumbar interbody fusion techniques, in that the anterior longitudinal ligament is preserved, and the larger XLIF implant spans the dense ring apophysis rather than threading into the central weaker portions of the endplate. Restoration of disc height and correction of alignment can be better achieved through the ligamentotaxis created with the anterior longitudinal ligament and PLL intact, since these ligaments exert great function on spinal alignment and stabilization.14,15 Relief of both back and leg pain has been demonstrated using XLIF technique.9,11,12,16,17 However, the degree of indirect decompression has not yet been formally shown, nor have the indications when it is feasible to avoid direct posterior decompression been clearly identified. The goal of this study was to quantify the indirect decompression achieved in XLIF surgery, and to evaluate the circumstances under which it is likely or unlikely to achieve resolution of stenotic symptoms.

Patients and Methods

This study was a nonrandomized, single-center, ethics committee-approved evaluation of patients with symptomatic lumbar degenerative conditions with central and/or lateral stenosis. The immediate postoperative radiographic results of these surgeries were compared with preoperative baseline measures to evaluate the effect of indirect decompression provided by interbody distraction via XLIF surgery.

The indications for surgery were symptomatic single- or multilevel lumbosacral degenerative disease from L1–L2–L4–L5 with central and/or foraminal stenosis of at least 6 months duration that had not responded to conservative care. Patients with prior direct decompressive or fusion surgery at the operative level(s) were excluded. Additional exclusion criteria included autoimmune disease, malignancy, and pregnancy.

In all, 21 consecutive patients meeting inclusion/exclusion criteria consented to participate and enrolled in this prospective study from March 2008 through June 2009. All of the patients were treated with standalone XLIF using 18-mm wide polyetheretherketone cages (CoRoent XL, NuVasive, Inc., San Diego, CA) and a tricalcium silicate bone graft substitute (Actifuse, ApaTech, Foxborough, MA), without additional direct posterior decompression or internal fixation. Interbody device sizing was performed intraoperatively for each patient and each level, and implant height was chosen based on a snug distractive fit and correction to a disc height similar to adjacent normal levels. All surgeries were performed by the same surgeon.

Plain anteroposterior and lateral radiographs as well as T2-weighted sagittal and axial magnetic resonance imaging (MRI) views were obtained at a single radiographic facility (URP Diagnósticos, São Paulo, Brazil) before surgery and immediately after surgery (within 2 weeks of surgery). From the lateral radiographs, anterior and posterior disc height, foraminal height, and foraminal area were measured. In MRI, a single sagittal slice through the anatomic center of the spine (i.e., midsagittal) was used as the comparative measurement location for determining anteroposterior canal diameter. A single axial slice through the center of the disc was used as the comparative measure location in axial views for midsagittal canal diameter, canal area, and left and right subarticular diameter measurements. Custom software (VOXAR 3-dimensional) developed to analyze MRI was used to digitally measure linear dimensions and areas. All distance measures were made at the narrowest point in the view.

T tests and χ2 tests (Analyze-It Software, Ltd., Leeds, England) were used to determine statistically significant changes from pre- to postoperative, with a level of significance of 0.05.

Results

Forty-three (43) levels in 21 patients were treated with stand-alone XLIF: 14 females, 7 males; average age, 67.6 years (range, 40–83 years); average body mass index, 25.6. All patients had a primary diagnosis of lumbar stenosis, accompanied by degenerative disc disease with degenerative spondylolisthesis Grade I or II (n = 8) and/or degenerative scoliosis (n = 19). There were 4 single-level, 13 two-level, 3 three-level, and 1 four-level procedure including 3 at L1–L2, 6 at L2–L3, 17 at L3–L4, and 17 at L4–L5. The surgeries were performed in an average of 86.2 minutes per patient (47.0 minutes per level), and with 44.3 mL blood loss per patient (23.4 mL per level); no case needed blood transfusion. In 5 procedures, the vertebrae adjacent to the operative disc space were prophylactically supplemented with bone cement due to presumed poor bone quality (not confirmed by DEXA).

There were no intraoperative complications, and there were no major postoperative changes in sensory or motor lower extremity function. The average hospital discharge was after 29.5 hours, when patients could move with few restrictions.

In the immediate postoperative period, 14.3% presented with psoas weakness, a condition that resolved to normal muscle strength within a few days, with iliopsoas-directed exercises accelerating the recovery. One case (4.8%) involved a transient psoas hematoma, which resolved without treatment.

Central and foraminal decompression was significant, with an average 41.9% increase in disc height (P < 0.0001), 13.5% increase in foraminal height (P = 0.0027), 24.7% increase in foraminal area, and 33.1% increase in central canal diameter (P < 0.0001) (Table 1; Figures 1–3).

Table 1
Table 1:
Average Radiographic Measures
Figure 1
Figure 1:
Measures from plain lateral radiographs. Note that area measurements are relative to the left axis scale (in mm2), while height measurements are relative to the right axis scale (in mm). All postoperative measures were statistically significantly increased compared with preoperative (P < 0.05).
Figure 2
Figure 2:
Measures from axial MR images. Note that area measurements are relative to the left axis scale (in mm2), while distance measurements are relative to the right axis scale (in mm). All postoperative measures were statistically significantly increased compared with preoperative (P < 0.05).
Figure 3
Figure 3:
Measures from sagittal MR images. Postoperative measures were statistically significantly increased compared with preoperative (P < 0.05).

Two patients (9.5%) needed additional direct posterior decompression and/or instrumentation with pedicle screws due to inadequate resolution of stenotic symptoms. One of the 2 patients showed early postoperative subsidence of the standalone implant after intraoperative restoration of disc height and alignment (mentioned in case example 3 described later in the text); in the other, disc and foraminal height were not adequately restored, and so supplemental direct decompression (laminectomy) and pedicle screw fixation were performed (mentioned in case example 4 described later in the text).

Case Examples

Case 1.

A 75-year-old man with degenerative scoliosis with neurogenic claudication, complaining of axial and radicular pain (VAS of the back = 70; VAS of the left leg = 90; VAS of the right leg = 80; ODI = 88) underwent a 2-level L3–L5 procedure (Figure 4). One week after surgery, the patient reported healthy improvement, with ODI = 29, VAS of the back = 0, and VAS of the legs = 20.

Figure 4
Figure 4:
Case example 1. Pre- and postoperative sagittal and axial MRIs revealing indirect decompression of the central canal at L3–4 and L4–5.

Case 2.

A 74-year-old woman presented with degenerative scoliosis with axial and radicular pain (VAS of the back and right leg = 50; VAS of the left leg = 100; ODI = 67) and underwent an L3–L5 procedure (Figure 5). Immediately after operation, the patient reported psoas weakness, but relief of preoperative pain, with ODI = 54, VAS of the back = 40, and VAS of the legs = 0. At her 6-week follow-up, psoas weakness was resolved and ODI had improved to 12, VAS back to 20, VAS legs still = 0.

Figure 5
Figure 5:
Case example 2. Pre- and postoperative sagittal and axial MRIs revealing indirect decompression of the central canal at L3–4 and L4–5.

Case 3.

A 63-year-old woman presented with significant axial and radicular pain (VAS of the left leg = 100; VAS of the back = 80) due to single-level asymmetrical disc collapse and L4–L5 Grade II spondylolisthesis. She had no prior surgeries and was 1 of the 5 patients in this cohort who was prophylactically augmented with vertebral body cement before interbody fusion. She underwent a single-level L4–L5 stand-alone XLIF procedure, which restored significant disc height and partial reduction of the slip. Early postoperative lateral radiographs showed evidence of slight subsidence inferiorly with loss of sagittal correction (Figure 6). Revision with supplemental pedicle screw fixation was attempted due to the lack of improvement in pain and function scores.

Figure 6
Figure 6:
Preoperative lateral radiograph and sagittal MRI showing degenerative spondylolisthesis, intraoperative lateral fluororadiograph showing restoration of disc height and reduction of slip, and postoperative lateral radiograph and sagittal MRI showing loss of correction following subsidence.

Case 4.

An 83-year-old man presented with axial and radicular pain (VAS of the back and left leg = 70; VAS of the right leg = 100; ODI = 75). Preoperative examinations revealed a prior interspinous process spacing device at the L4–L5 level and anatomically short pedicles. After an L4–L5 XLIF procedure, the patient did not report clinical improvement, and radiographs revealed persistent central and foraminal stenosis (Figure 7). In this unique case, disc and foraminal height were not adequately restored. This patient's congenital stenosis and prior interspinous process spacing device, which was not removed at the time of XLIF surgery, may have limited the ability to adequately distract the anterior column and neural foramens. A hemilaminectomy was performed to achieve the necessary decompression, and the construct was supplemented with bilateral pedicle screws.

Figure 7
Figure 7:
Case example 4. Preoperative lateral radiograph showing congenitally short pedicles; and pre- and postoperative sagittal and axial MRIs revealing inadequate indirect decompression at L4–5.

Discussion

The published literature supports the practice of fusion with indirect decompression for spinal pathologies presenting with neurologic symptoms, particularly in cases of instability and malalignment.6,7,18–28 Consistent with these prior reports, the current study on anterior lumbar interbody fusion with XLIF showed radiographically its effectiveness in decompressing stenosis associated with common lumbar degenerative disorders.

Unlike direct anterior or posterior approaches, lateral access is ligament-sparing and allows for the placement of a large intrinsically stable implant to be placed completely across the interspace, resulting in a very stable interbody construct. Proper surgical technique includes aggressive disc removal and release of the contralateral anulus. This process not only assures a proper graft bed but also allows for interbody distraction that restores disc height and facilitates reduction of deformity through ligamentotaxis. Moreover, XLIF avoids many of the potential complications of traditional surgery, including those associated with direct decompression and posterior access.

Regarding when indirect decompression will be sufficient or not, a staged approach must be recommended, counseling patients about the fact that an additional microdecompression might be required if symptoms persist. Severe central spinal stenosis is a relative contraindication if the patient is unwilling to accept the possibility of a second operation (direct decompression) if neurologic symptoms persist after surgery. In the author's experience, and as was shown in this work, the second procedure is unnecessary in the majority of cases and spares the patient undue morbidity and risk of neural injury or scarring from direct posterior surgery.

In patients further along the degeneration disease process, lateral recess and foraminal stenosis are exacerbated by osteophyte formation. Low back pain is usually diminished or absent, and leg pain is persistent at rest. Interbody distraction and fusion may not adequately decompress stenosis in this stage because correction of alignment is blocked by osteophytes on the posterior surface of the articular processes, and anterior osteophytes still impinge on the neural elements.

In the authors' experience (although not necessarily statistically supported by the results in this small cohort), the requirement of direct decompression is determined based on the defining preoperative characteristics that include congenital stenosis/congenitally short pedicles; uncontained disc herniation; significant facet arthropathy/osteophyte formation (locked facets) coupled with calcified disc; PLL or osteophytes arising from the posterior endplates, with complete or near complete compromise of the lateral recess; synovial cysts; and/or radicular symptoms unimproved with flexion.

Conversely, XLIF appears to provide sufficient indirect decompression for disc bulge, such as in recurrent disc herniation; collapsed disc with loss of foraminal height and/or soft tissue encroachment (e.g., posterior anulus, PLL, ligamentum flavum) of the canal; lateral, retro-, or spondylolisthesis, with narrowing of the central and intervertebral foramens due to the malalignment; degenerative scoliosis with unilateral central or foraminal stenosis due to the malalignment.

The limitations of this study include small sample size and short radiographic follow-up. Maintenance of correction and decompression was not evaluated. However, 1 patient demonstrated subsidence and loss of correction in the short-term that limited the effect of decompression and resolution of symptoms. Wider interbody devices (cages) may reduce the incidence of subsidence and complications related to its occurrence. Larger cages have larger contact area and exert less pressure on the vertebral endplate. Supplemental internal fixation is not mandatory to achieve indirect decompression, but posterior or lateral fixation increases construct stiffness and may protect the indirect decompression, particularly in cases where bone quality and potential subsidence are a concern.

Although subsidence was an incidental finding in this study and its incidence has a clear effect on the maintenance of indirect decompression, conclusions relative to this question were not within the scope of the design of this study, which was to quantify the ability of interbody distraction through XLIF to increase the neural foramina dimensionally through interbody distraction. The question of subsidence and/or maintenance of correction and progression to fusion, along with lasting clinical benefit, is dependent on several variables, including grafting and internal fixation techniques, preferences for which vary among XLIF surgeons. Until further follow-up data are available, the long-term clinical and radiographic effects of standalone XLIF for stenosis are uncertain. However limited, the immediate postoperative quantifiable increases in central and lateral radiographic measures may be more generalizable to multiple applications of the XLIF procedure.

Conclusion

Interbody distraction through XLIF is a reproducible and effective mechanism for achieving indirect decompression in stenotic degenerative conditions.

Key Points

  • Disc height, foraminal height, and central canal measures are significantly increased following interbody fusion via stand-alone XLIF in most cases.
  • Congenital stenosis and locked facets may limit the extent of decompression achievable indirectly.
  • Maintenance of correction, although not studied in detail here, may be dependent on subsidence, which in turn is dependent on grafting and internal fixation techniques for eventual fusion in the corrected position.

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Keywords:

XLIF; claudication; MRI; foraminal height; foraminal area

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