Indirect decompression using oblique lateral interbody fusion (OLIF) is used in patients with various lumbar degenerative diseases, and previous studies reported favorable clinical outcomes with few early complications following OLIF.1–6 Oblique lateral interbody fusion improves the patient's symptoms through intervertebral disk height restoration, spondylolisthesis reduction, and segmental stabilization. Improvements in the radiological parameters are proven in the literature, not only after OLIF,7,8 but also following other indirect decompression methods, such as anterior lumbar9–11 and extreme lateral interbody fusions.12–14 Dimensions of both central canal and inter-vertebral foramens improved following an indirect decompression in these studies.
However, clinical significance of these radiological improvement following indirect decompression was not fully evaluated in the literature. Few studies examined the association between the clinical outcomes and radiological effect of indirect decompression but failed to establish any relationships.7,9 Therefore, the objectives of this prospective case series study were (1) to assess the clinical and radiological outcomes at 1 year postoperatively and (2) to determine the radiological parameters associated with the clinical outcomes following single-level OLIF for lumbar degenerative diseases.
Materials and Methods
From June 2015 to August 2017, a total of 44 patients who underwent single-level OLIF and posterior percutaneous pedicle screw instrumentation on the L2-3, L3-4, and L4-5 levels for lumbar degenerative diseases were enrolled in this study. Preoperative diagnosis was stratified as follows: (a) degenerative spondylolisthesis with central and/or foraminal stenosis (DS group), (b) spondylolytic spondylolisthesis with central and/or foraminal stenosis (SS group), and (c) spinal stenosis without spondylolisthesis but with disk height loss (ST group). The severity of spinal stenosis and the grade of spondylolisthesis were not considered in patient enrollment.
Patients with relative contraindications for OLIF such as lateral recess stenosis, extruded or sequestered intervertebral disk, severe facet arthropathy, or facet cysts were excluded from the study. The L5-S1 level was not included in the study because of the difference in the morphology of the spinal canal and surgical techniques. Additionally, patients with a history of infection, trauma, and surgery on the lumbar spine were excluded from the study. Patients who underwent additional surgery on the lower extremities within 1 year after the OLIF procedure were also excluded from the analysis because this may have affected the clinical scores, especially walking ability and pain in the lower extremities. This study received institutional review board approval, and all patients provided informed consent.
Surgery was performed using a minimally invasive anterior retroperitoneal approach with the patient in the right decubitus position. For interbody fusion, a polyetheretherketone cage loaded with allogeneic demineralized bone matrix mixed with cancellous bone was inserted into the disk space in all cases. The height of the cage was determined to maximize the disk height restoration without causing an endplate fracture. After changing to the prone position, posterior percutaneous pedicle screw instrumentation was performed under intraoperative fluoroscopic guidance. No surgical drain was used, and patients started ward ambulation 1 day after the procedure without any orthosis or brace.
Clinical Outcome Assessment
The patients' demographic data and other factors such as body mass index and T-score of the lumbar spine from dual-energy x-ray absorptiometry scan were collected preoperatively. Clinical outcomes were assessed using multiple patient-reported outcome measures, including the Oswestry Disability Index (ODI),15 Japanese Orthopaedic Association Back Pain Evaluation Questionnaire (JOABPEQ),16 Short Form-36 Health Survey (SF-36),17 and visual analog scale (VAS) for back pain and lower-extremity radiating pain. These outcome measures were obtained before and 1 year after the operation.
Anterior and posterior disk height and lumbar and segmental lordosis angle were measured using standing neutral lateral radiographs obtained preoperatively and at 1 week and 1 year postoperatively. Disk height was defined as a perpendicular distance from an anterior or posterior tip of the endplate to the opposite endplate. Segmental angle was measured as a Cobb's angle between the upper and lower end-plates of the level of operation and lumbar lordosis as a Cobb's angle between the lower endplate of T12 and upper endplate of S1. Cage subsidence, which is defined as any breach of endplates by the interbody cage, was assessed using the standing neutral lateral radiographs at 1 week and 1 year postoperatively. The status of interbody fusion was evaluated using computed tomography (CT) at 1 year postoperatively. Complete fusion was defined as the presence of bridging trabecular bone with a remodeling of the endplates, and the presence of only continuous bone density was regarded as intermediate fusion.18 Additionally, loosening of the pedicle screws was assessed using CT scan.
Magnetic resonance imaging (MRI) was performed 1 year postoperatively to evaluate changes in the dimensions of the central canal and neural foramina. The same standardized protocol (3.0 Tesla, 2-mm slice thickness) as the preoperative MRI was used for the 1-year postoperative MRI, and images for the measurement were selected from the slices at the corresponding anatomical locations to improve comparability. For the central canal, the anterior-posterior distance and cross-sectional area (CSA) were measured from the T2-weighted axial image (Figure 1A–B) using software equipped in the picture archiving and communication system. For the intervertebral foramen, the foraminal height, width, and CSA were measured from T2-weighted sagittal images (Figure 1C–D). Magnetic resonance images were measured at 3 different times to minimize the measurement error, and the average value obtained was used for analysis. Moreover, the severity of the central and foraminal stenosis was evaluated using the grading system proposed in the literature.19,20 Measurement of radiological parameters was blinded from the clinical results.
Measurement of central canal (A, B) and foraminal (C, D) dimensions using T2-weighted magnetic resonance images. Central canal dimensions (cross-sectional area: white dashed line; anterior-posterior distance: yellow line) are measured from the axial image (A) that corresponds to the line bisecting the intervertebral disk in the sagittal view (B). Foraminal dimensions (cross-sectional area: white dashed line; foraminal height and width: yellow lines) are measured from the sagittal image (C) that corresponds to the perpendicular line that contacts with the lateral border of the facet joint in the axial view (D). Foraminal height is measured as the longest vertical distance possible, and foraminal width is the shortest distance that is perpendicular to the line for the foraminal height measurement.
A paired t test was performed to examine the improvement of clinical scores and radiological parameters following OLIF. One-way analysis of variance with the post hoc Tukey test, chi-square test, and Fisher's exact test were used to compare the clinical outcomes of groups with different preoperative diagnosis. The Student's t test was used to compare the clinical scores of groups with and without complete fusion or cage subsidence. Pearson correlation analysis was performed to identify the association between the clinical scores and radiological improvements. Statistical analysis was performed using SPSS Statistics version 25.0 software (IBM Corp, Armonk, New York). P<.05 was considered statistically significant.
Among 44 enrolled patients, 3 were lost to follow-up and were excluded from the study. A total of 41 patients (15 males, 26 females) with a mean age at surgery of 66.9 years (range, 56–81 years) were included in the analysis. Regarding preoperative diagnosis, 26 (63.4%) of 41 patients had degenerative spondylolisthesis, 8 (19.5%) patients had spondylolytic spondylolisthesis, and 7 (17.1%) patients had spinal stenosis with disk height loss. No significant difference was noted in patient factors between the diagnostic groups (Table 1). The L4-5 level was the most common surgical level (24 of 41, 58.5%), followed by L3-4 (14 of 41, 34.1%) and L2-3 (3 of 41, 7.3%).
Patient Factors Stratified by Preoperative Diagnosis
All scoring items from multiple questionnaires showed significant improvement following OLIF (Figure 2). No significant difference was observed in the diagnostic groups in preoperative and 1-year postoperative raw scores and net and percent improvement of all scoring items when examined using the one-way analysis of variance test (Figure 3). Regarding perioperative complications, 3 patients received an epidural steroid injection within 1 week postoperatively for residual lower-extremity radiating pain, which resolved after an injection. Otherwise, no perioperative complications, such as surgical infections and neurovascular structure injuries, occurred in the cohort. There were no revision surgeries on the lumbar spine for all patients within the study period.
Improvement of clinical score in all patients following oblique lateral interbody fusion. All scoring items on the Short Form-36 Health Survey (SF-36) (A), pain scales (B), and Japanese Orthopaedic Association Back Pain Evaluation Questionnaire (JOABPEQ) (C) are significantly improved at 1 year postoperatively (black bars) when compared with preoperative scores (white bars). Error bars indicate standard deviation, and asterisks indicate statistically significant improvement of scores (P<.05) when examined by the paired t test. Abbreviations: LBP, low back pain; LF, lumbar function; MCS, mental health composite summary; MH, mental health; ODI, Oswestry Disability Index; PCS, physical health composite summary; RP, radiating pain; SLF, social life function; WA, walking ability.
Comparison of clinical score improvement of the individual diagnostic groups. Oswestry Disability Index (A), radiating pain (B), walking ability in Japanese Orthopaedic Association (JOA) Back Pain Evaluation Questionnaire (C), and Short Form-36 Health Survey (SF-36) physical component (D) scores all improved in all 3 diagnostic groups at 1 year postoperatively. The white bars are preoperative scores, and the black bars are scores at 1 year postoperatively. Error bars indicate standard deviation, and asterisks indicate statistically significant improvement of scores (P<.05) when examined by the paired t test. Abbreviations: DS, degenerative spondylolisthesis group; SS, spondylolytic spondylolisthesis group; ST, spinal stenosis with disk height loss group; VAS, visual analog scale.
Disk Height and Angles
Anterior and posterior disk height and segmental and lumbar lordosis angle were significantly improved at 1 week following OLIF (Figure 4). However, a significant decrease in disk height and segmental angle at 1 year postoperatively was observed, although these parameters improved when compared with the preoperative status. Lumbar lordosis gradually improved until 1 year postoperatively.
Changes in radiologic parameters from plain radiographs. Anterior and posterior disk height (A) and angles of the segment and the lumbar spine (B) all showed improvement immediately after oblique lateral interbody fusion. Numbers next to the symbols represent mean value of a disk height or an angle. Asterisks indicate the interval with significant improvement (P<.05 in paired t test). Abbreviation: PO, postoperative.
Of 41 patients, 6 (14.6%) and 13 (31.7%) had cage subsidence in the postoperative radiograph at 1 week and 1 year postoperatively, respectively. No difference in the rate of subsidence was found in the diagnostic groups when examined using Fisher's exact test (P=.705). After stratification by the presence of the cage subsidence at 1 year postoperatively, the group of patients with cage subsidence showed higher ODI (P=.012) and lower physical (P=.007) and mental (P=.016) health composite summary scores on the SF-36 by Student's t test (Table 2). However, no significant difference was noted in patient and surgical factors between the groups with and without cage subsidence.
Comparison of Patients With and Without Cage Subsidence
Based on the CT scan at 1 year postoperatively, 28 (68.3%) of 41 patients had complete interbody fusion (bridging trabecular bone with endplate remodeling), whereas the other 13 (31.7%) patients had intermediate fusion. None of the patients had screw loosening on CT scan. No significant difference in the proportion of patients with complete fusion was noted between the diagnostic groups (P=.895, Fisher's exact test). There was no statistically significant difference in any of the clinical scoring items between the complete and intermediate fusion groups when examined by Student's t test.
Magnetic Resonance Imaging Measurements
All radiological parameters measured from the 1-year postoperative MRI showed statistically significant improvement following OLIF, except for foraminal width (Table 3). The severity of central canal and foraminal stenosis also improved at 1 year postoperatively (Figure 5). Among the radiological parameters evaluated from the MRI study, the improvement ratio of foraminal height on the side of predominant symptoms was associated with the percent improvement of lower-extremity radiating pain VAS of the corresponding side (Pearson coefficient=0.384; P=.013) and the JOABPEQ walking ability score (Pearson coefficient=0.319; P=.042) (Table 4).
Changes in Radiological Parameters From Magnetic Resonance Imaging Study
Distribution of the grades of the central canal (A) and foraminal (B) stenosis in the preoperative (Preop) and 1-year postoperative (PO) magnetic resonance images. Foraminal stenosis on the side of the predominant radiating pain is shown in this graph.
Correlation Analysis Between Clinical Scores and Parameters From Magnetic Resonance Imaging Study
Previous studies have reported radiological improvement, including central canal and foraminal dimensions, following indirect decompression.7–14 However, these studies did not examine the association of these radiological parameters with the clinical outcomes or failed to find any relationship after the investigation. Fujibayashi et al7 evaluated the correlation between the Japanese Orthopaedic Association score and the extension ratio of the CSA of the central canal following OLIF but found no significant associations. The authors stated that the lack of correlation could have resulted from the fact that the clinical results of OLIF are more related to the effect of the stabilization rather than that of indirect neural decompression.
In contrast, the current study found that foraminal height measured on MRI is associated with lower-extremity radiating pain and walking ability in the correlation analysis. This may be due to the currently used patient-reported outcome measurements and radiological parameters, which were not assessed in the previous studies. However, because this study only included patients who underwent OLIF combined with percutaneous pedicle screw fixation and did not compare the results with that of the standalone OLIF, the role that the posterior stabilization plays in the clinical and radiological improvements following OLIF could not be determined.
Another strength of this study is that the authors measured the radiological parameters using 1-year postoperative MRI. Most of the previous studies on radiological outcomes of indirect decompression evaluated postoperative images obtained shortly after the operation, the longest interval being 6 months.8 The interval from operation to radiological evaluation is important because remodeling of the spinal canal can occur gradually following indirect decompression with stabilization. Regarding the extension ratio of the spinal canal CSA, the current study showed 63.9% and 103.7% improvement of the central canal and foraminal areas, respectively, whereas other studies had an improvement of 19.0% to 36.5% of the central canal area7,8,13 and 43.3% to 69.6% of the foraminal area.8,9,13 Although this difference can be a result of spinal canal remodeling, objective quantification of spinal canal remodeling cannot be determined because there is no comparison between the images from the immediate postoperative period and long-term follow-up. In addition, evaluating the clinical significance of the spinal canal remodeling was beyond the scope of this study.
Restoring disk height is important in the OLIF procedure, as seen in the results of this study. However, a problem that may arise from the effort to maximize the elevation of the foraminal height is cage subsidence. For this reason, the current study showed a relatively higher subsidence rate (31.7% at 1 year) compared with other studies on OLIF.5,6,21 The occurrence of subsidence can be caused by multiple factors, including patient factors such as osteoporosis and obesity and surgical factors such as excessive reaming of the endplates.5 The authors compared multiple patient and surgical factors that may be related to cage subsidence but found no significant associations in this study cohort. Therefore, a conclusion on the adequate height of the cage and how much distraction is required for clinical improvement could not be reached from the current research, and further studies are needed. Nevertheless, because the patients with subsidence had poor clinical outcomes, efforts to reduce subsidence should be applied, such as avoiding excessive reaming of endplates, especially in osteoporotic, elderly patients.
The complete fusion rate of this study (68.3%) was lower than that of previous OLIF studies, which mostly showed fusion rates of more than 90% at 1 year postoperatively.2,4,6 This could be a result of different fusion materials used; bone morphogenetic protein or autogenous bone graft and bone marrow aspirates were not used in the current study. Another reason is that the definition of complete fusion in this study included endplate remodeling, which was not specifically discussed in other studies. All patients in the current cohort showed continuous bone density along the disk space and no screw loosening, and no difference in clinical scores were observed between the complete and intermediate fusion groups. Additional studies with long-term follow-up are required for definitive fusion rate in the current cohort.
A limitation of this study was measurement errors, especially MRI. Despite using various measures to minimize errors, including multiple measurements, using the standardized MRI protocol, and selecting images from corresponding anatomical locations, there were still errors. Metal artifacts in postoperative MRIs when measuring foraminal dimensions was an issue in some patients. Choosing images sliced at the same anatomical location was impossible in some cases due to the inevitable changes of the regional anatomy after the operation. In future studies, a specialized 3-dimensional imaging software can be used to improve accuracy.11
Additional limitations of this study included a small sample size and short follow-up. Furthermore, although various conditions such as the L5-S1 level and lateral recess stenosis were excluded from the study, the study cohort was still heterogeneous, with 3 different diagnostic groups, which can have different anatomical configurations. Other limitations were the lack of a control group and the fact that the clinical and radiological outcomes of indirect decompression in this study cannot be compared with those of direct decompression with or without posterior spinal instrumentations. Despite these limitations, to the authors' knowledge, this is the first study to report the association of clinical and radiological outcomes following the OLIF procedure.
A greater improvement in foraminal height measured on MRI was associated with less radiating lower-extremity pain and greater walking ability following OLIF, and cage subsidence resulted in poor clinical scores in the current study. Therefore, restoration of foraminal height while preserving the integrity of the end-plates is associated with favorable results following OLIF.
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Patient Factors Stratified by Preoperative Diagnosis
|Factor||Total (N=41)||DS (n=26)||SS (n=8)||ST (n=7)||P|
|Age, mean±SD, y||66.9±6.0||66.5±6.6||66.3±3.9||68.9±6.1||.712a|
|Sex, male:female, No.||15:26||10:16||1:7||4:3||.189b|
|Body mass index, mean±SD, kg/m2||25.1±3.3||25.3±3.1||25.6±4.4||23.9±2.8||.566a|
Comparison of Patients With and Without Cage Subsidence
|No Subsidence (n=28)||Subsidence (n=13)|
|Clinical scores at 1 year postoperatively|
| SF36 PCS||40.2±5.5||34.4±6.0||.007b|
| SF36 MCS||39.8±4.8||35.8±4.5||.016b|
| Back pain VAS||1.9±1.5||2.5±1.9||.347|
| Radiating pain VAS||1.6±1.5||2.1±2.0||.426|
| Age, y||66.3±6.3||68.2±3.6||.317|
| Body mass index, kg/m2||24.8±3.0||25.7±3.9||.410|
| Cage height, mm||11.9±1.5||12.0±2.0||.803|
| Cage height—preoperative disk height, mm||3.5±2.8||3.5±2.6||.923|
|Foraminal measurements on 1-year postoperative magnetic resonance image|
| Height, mm||18.6±2.6||15.9±2.5||.008b|
| Cross-sectional area, mm2||90.1±24.9||66.9±23.9||.023b|
Changes in Radiological Parameters From Magnetic Resonance Imaging Study
|Preoperative (A)||Postoperative (B)|
| Anterior-posterior distance, mm||11.8±3.5||14.8±3.3||1.25||<.001b|
| Cross-sectional area, mm2||89.8±62.8||147.2±62.1||1.64||<.001b|
| Height, mm||12.5±5.1||17.9±2.7||1.43||<.001b|
| Width, mm||2.4±1.3||3.2±1.0||1.33||.081|
| Cross-sectional area, mm2||40.9±19.5||83.3±25.6||2.04||<.001b|
Correlation Analysis Between Clinical Scores and Parameters From Magnetic Resonance Imaging Study
|Clinical Scorea||Magnetic Resonance Imaging Parameterb||Pearson Correlation Coefficient||Pc|
|Radiating pain visual analog scale||Foraminal height||0.384||.013d|
|Foraminal cross-sectional area||0.141||.346|
|Central canal cross-sectional area||0.132||.410|
|Oswestry Disability Index||Foraminal height||0.176||.272|
|Central canal cross-sectional area||0.141||.378|
|Japanese Orthopedic Association Back Pain Evaluation Questionnaire walking ability||Foraminal height||0.319||.042d|
|Central canal cross-sectional area||0.142||.376|