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ORIGINAL ARTICLES

Minimally Invasive Lateral Lumbar Interbody Fusion

Clinical and Radiographic Outcome at a Minimum 2-year Follow-up

Kotwal, Suhel MD*; Kawaguchi, Satoshi MD, PhD; Lebl, Darren MD; Hughes, Alexander MD; Huang, Russel MD; Sama, Andrew MD; Cammisa, Frank MD; Girardi, Federico MD

Author Information
doi: 10.1097/BSD.0b013e3182706ce7
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Abstract

Spinal fusion remains a major surgical intervention for degenerative conditions, especially for those with instability or deformity of the lumbar spine. Among the various techniques for achieving spinal fusion, interbody fusion techniques offer distinct advantages, including restoration of disk space height, correction of the deformity, and removal of the intervertebral disk as a possible pain generator.1–4

After establishment of the traditional open interbody fusion techniques such as anterior lumbar interbody fusion1 and posterior lumbar interbody fusion,3 attempts at minimizing surgical morbidities associated with these open procedures have drawn more attention.5–8 Minimally invasive lateral lumbar interbody fusion (LLIF), also termed as extreme lateral interbody fusion, is a recent addition to such attempts.9,10 This procedure is performed through a lateral, retroperitoneal, transpsoas approach, and is characterized by the use of (1) a tubular retractor to minimize the surgical approach and (2) real-time neuromonitoring to ensure safe passage through the psoas muscle. To date, proof of the conceptual advantages of minimal invasive LLIF has been limited to early postoperative outcomes and complications,9–19 with the longest mean follow-up duration of 22 months.20 Only 3 studies16,20,21 have addressed both pain and functions of the patients.

The present study was designed to determine the clinical and radiographic outcomes of patients undergoing minimally invasive LLIF, focusing on the results of a minimum 2-year follow-up.

MATERIALS AND METHODS

Patients

The present study was approved by the Institutional Review Board of the authors’ institution. Between March 2006 and December 2008, 141 patients underwent LLIF using minimally invasive transpsoatic approach (minimally invasive LLIF). Operations were performed by 5 senior authors. Of these 141 patients, 118 patients (84%) were followed up for a minimal of 2 years and were subjected to the analysis. Clinical pictures of these patients are summarized in Table 1. The mean follow-up duration was 27.5 months (range, 24–38 mo).

TABLE 1
TABLE 1:
Demographic Information of 118 Patients

Surgical Technique

In the absence of coronal deformity, the decision of approaching the lumbar spine through left-sided or right-sided approach was based on the accessibility of the L4–5 disk space in relation to the iliac crest. In cases of degenerative scoliosis, the operated levels were approached through side of the concavity, in an effort to reach multiple levels through a single small incision. Fibers of the psoas muscle were separated under direct vision, carefully protecting and avoiding the traversing nerves, a self-retaining tubular retractor with illumination was placed after radiographic confirmation of the disk space.

Electromyography probes (NeuroVision monitor; Nuvasive Inc., San Diego, CA and regular active-run EMG) were used as an additional modality to prevent neural injury besides direct visualization in all cases during psoas muscle dissection. After discectomy of the desired level, the disk spaces were sized with trial components of sequentially increasing height in increments of 2 mm. The antero-posterior size was determined by the width of the end plates at that level based on intraoperative fluoroscopic guidance and preoperative templating. All disk levels were implanted with 7.5-degree lordotic LLIF cages. The maximum distraction achieved after discectomy using the trial inserts was approximated to the height of the implanted cage. The cages were packed with bone graft materials according to the surgeon’s preference.

In case where posterior instrumented fusion had been planned, the patient was then placed in the prone position. Using a midline subperiosteal approach, laminectomy was performed. Subsequently, bones were grafted over the facet joints and transverse processes using the standard posterolateral fusion technique. The segments were fixed with unilateral or bilateral pedicle screws and rods.

Postoperatively, patients were allowed to ambulate as tolerated, with a nonelastic lumbar corset and walking aid under the physical therapist’s supervision.

Evaluation

Data on operation time, estimated blood loss, length of hospital stay, and operation-related complications including deep venous thrombosis, pulmonary embolism, cardiac events, and the necessity for revisions were gathered.

Clinical and radiographic evaluations were made preoperatively and at the final follow-up. Clinical outcomes were determined by using Visual Analog Score (VAS) for the degree of pain (trunk or lower extremity), and Oswestry Disability Index (ODI) and Short Form-12 (SF-12) scoring methods for patient function.

Radiographic evaluations included: (i) disk height; (ii) segmental coronal angulation; (iii) segmental lordotic angulation; (iv) Cobb angle; (v) cage subsidence; and (vi) fusion status. Disk height, segmental coronal angulations, and segmental lordosis were measured on plain radiographs electronically using picture archiving and communication system. In patients with scoliosis of 10 degrees or more, standing 36-inch antero-posterior radiographs were taken to measure Cobb angle. Successful fusion was determined radiographically with the following criteria: (i) presence of trabecular bone formation through the cage; (ii) absence of mobility (2 degrees or less) of the fusion segment on flexion/extension lateral radiographs; and (iii) absence of radiolucencies around the pedicle screws. The radiographs were reviewed by an author and group of 20 patients were remeasured after 1 month by the same observer. After measurements for the study were completed, differences of >2 mm or 2 degrees were adjudicated by 1 of the senior authors.

Statistical tests were performed using a commercial statistical software package (PRISM Version 5.01 for Windows; Graphpad). Data were statistically tested for normal distribution using the D’Agostino and Pearson omnibus normality test before applying either 2-tailed paired Students t test or Wilcoxon matched-pair test for normally and not-normally distributed data for intrasample comparison. Values are reported as mean and SDs. Significance level was set at P<0.05.

RESULTS

Surgical Procedure

Fifty patients underwent 1-level, 28 underwent 2-level, 29 underwent 3-level, and 11 underwent 4-level procedures (Fig. 1). A total of 102 patients received LLIF with posterior instrumented fusion and the remaining 16 patients had LLIF as a stand-alone procedure with no posterior fusion. A total of 112 patients were implanted with Poly-ether-ether-ketone cage (Nuvasive Inc.) and 6 patients received Carbon fiber cages (Depuy Spine Inc., Raynham, MA).

FIGURE 1
FIGURE 1:
Lateral lumbar interbody fusion: location, length, and number of the patients.

The operation time averaged 184 minutes (range, 88–256 min) in 16 patients with stand-alone LLIF and 324 minutes (range, 75–690 min) in 102 patients with posterior instrumented fusion. Estimated blood loss averaged 200 mL (range, 10–500 mL) in patients with stand-alone LLIF and 537 mL (range, 25–3000 mL) in those with posterior fusion. Length of stay in the hospital was 7.7 days on an average (range, 2–22 d).

Clinical and Radiographic Outcomes

Clinical outcomes were determined by using VAS, ODI, and SF-12. As shown in Figure 2, VAS (Fig. 2A), ODI (Fig. 2B), and physical components summary of SF-12 (Fig. 2C) improved significantly at the follow-up. In contrast, mental components summary (MCS) of SF-12 (Fig. 2D) failed to show a significant improvement.

FIGURE 2
FIGURE 2:
Clinical outcomes evaluated by Visual Analog Score (VAS), Oswestry Disability Index (ODI), and Short Form (SF)-12. A, The VAS scores were 8.7±1.3 cm preoperatively and 4.1±2.8 cm at the follow-up. B, ODI were 30.0±10.1 preoperatively and 17.1±12.8 at the follow-up. C, Physical components summary of SF-12 were 27.0±1.3 preoperatively and 38.1±15.0 at the follow-up. D, Mental components summary of SF-12 were 43.0±11.4 preoperatively and 42.4±11.9 at the follow-up. * indicatesP<0.01.

Figure 3 shows radiographic outcomes evaluated by restoration of disk height (Fig. 3A), correction of coronal angulation (Fig. 3B), and correction of lordotic angulation (Fig. 3C). All but the lordotic angle at T12–L1 level improved significantly (P<0.01). Cobb angle also improved significantly in 31 patients with degenerative scoliosis (Figs. 4, 5).

FIGURE 3
FIGURE 3:
Radiographic outcomes evaluated by disk height (A), coronal angulations (B), and lordotic angulation (C).
FIGURE 4
FIGURE 4:
Radiographic outcomes evaluated by Cobb angle. The Cobb angle was 24.8±9.8 degrees preoperatively and 13.6±10.3 cm at the follow-up. * indicatesP<0.01.
FIGURE 5
FIGURE 5:
Preoperative antero-posterior and lateral radiograph of a patient with degenerative disk disease and scoliosis. Standing 36-inch antero-posterior radiographs were taken. A, Preoperative antero-posterior radiographs; B, Preoperative lateral radiographs; C, Postoperative antero-posterior radiographs; D, Postoperative lateral radiographs.

Of a total of 237 levels operated on, subsidence of the cage was evident in 34 levels. Successful fusion was achieved in 209 levels (88%) in 104 patients (Figs. 6A–C), including 6 levels with cage subsidence.

FIGURE 6
FIGURE 6:
A, A representative radiograph showing successful fusion. B and C, Coronal and axial computed tomographic scan images for a successfully fused level.

Complications

The most frequent complication was anterior thigh pain seen in 43 patients (36%). Of these, hip flexor weakness and numbness of anterior thigh were also noted in 20 and 13 patients, respectively. At the final follow-up, none of them but 1 had continuous pain in the thigh. Additional surgery was performed in 3 of 14 patients who failed to obtain successful fusion, and 1 patient with adjacent level degeneration.

Other complications included ileus in 4 patients, pulmonary insufficiency that required delayed extubation in 2, arrhythmia in 2, gastric ulcer in 1, urinary retention in 1, and delayed wound healing in 1. They were managed by conservative treatment.

DISCUSSION

In the present study, we analyzed pain, function, and radiographic features of 118 patients who underwent minimally invasive LLIF and were followed up for a minimum of 2 years. We found that (i) the VAS scores, ODI, and physical components summary, but not MCS, of SF-12 improved significantly at the follow-up; (ii) disk height, coronal angulation, and lordotic angulation at each level and the Cobb angle were restored at the statistically significant extent; (iii) successful fusion was achieved in 209 levels (88%); and (iv) anterior thigh pain was the most frequent complications seen in 36% of the patients.

In literature, 5 studies12,16,18,20,21 have analyzed pain status using the VAS score, including 3 studies16,20,21 addressing the patient function as well using ODI or SF-36. They demonstrated significant improvements of the VAS scores and ODI and SF-36. In the present study, the MCS of SF-12 did not improve significantly. As all of the 3 previous studies16,20,21 with functional analysis, have focused on adult scoliosis cases, we reanalyzed the scores of SF-12 in the 31 patients with scoliosis. However, no significant improvement of the MCS was demonstrated in this population either. A lack of significant change in the SF-12 MCS was also reported in patients with osteoporotic vertebral fracture22 and those who had undergone revision surgery for lumbar pseudarthrosis.23

The efficacy of LLIF in correction of the scoliosis curvature has well been demonstrated in the previous studies.10,13,16,20,21 In contrast, Acosta et al21 failed to demonstrate the improvement in the sagittal balance. Because LLIF does not involve sectioning the anterior longitudinal ligament, hinged distraction of the adjacent bodies tends to be unsatisfactory. In addition, LLIF allows the implantation of a cage with a relatively larger “footprint,” to a cage utilized in posterior-alone surgery, including posterior lumbar interbody fusion or trans-foramenal interbody fusion. This may be advantageous in a wider vertical stress distribution, thereby reducing the incidence of cage subsidence, especially in osteoporotic group of patients (Figs. 7A–E). It is worthwhile to note that, the authors electively plan for a second-stage posterior segmental instrumentation with LLIF to prevent subsidence, as its ability to do so as a stand-alone procedure, have not been confirmed as of yet.

FIGURE 7
FIGURE 7:
A 79-year-old man with L2–5 degenerative disk disease presenting with axial back pain and neurogenic claudication underwent 2-stage L2–5 lateral lumbar interbody fusion and posterior segmental pedicular screw fixation. A, Preoperative plain antero-posterior (AP) radiograph; preoperative coronal and sagittal computed tomographic scan images (B, C); postoperative AP and lateral radiographs (D, E).

The most frequent complication in our series was postoperative thigh pain (36%), followed by weakness of hip flexion in 17% of the patients. In the literature, thigh pain was reported to occur in 23%19 and 39%15 of patients. Weakness of hip flexion was reported from 1% to 36%.10,12,13,15 Blunt dissection through the psoas muscle fibers, retraction during procedure and hematoma are suggested to be the causative factors of these symptoms. There have been no clear technical recommendations for prevention. Our variation of the commonly used technique is direct visualization of the psoas muscle. Thereafter, navigational real-time directional neuromonitoring is used.

There are several drawbacks of our study. Varied indications for which LLIF was performed may confound the interpretation of clinical outcomes. Stand-alone LLIF and LLIF with posterior instrumentation were analyzed without separation. Data were not serially collected during the follow-up. Computed tomographic scan images were not available so that subjective radiographic criteria were utilized to determine the fusion status. The present study represents the third report of our series with minimally invasive LLIF.24,25 The drawbacks of a single study will be resolved step by step with accumulation of a series of studies with different foci.

In conclusion, this is the first report with a minimum 2-year follow-up, demonstrating the pain, function, and radiographic outcomes of patients who had undergone minimally invasive LLIF. Our results support the efficacy of this surgical procedure in improvements of clinical and radiographic features.

REFERENCES

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Keywords:

lateral lumbar interbody fusion; minimally invasive surgery; 2-year follow-up; clinical outcomes; radiographic outcomes

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