Introduction

Compared with traditional posterior approaches, anterior or lateral approaches, such as anterior lumbar interbody fusion (ALIF) and oblique lumbar interbody fusion (OLIF), can provide a similar fusion rate while avoiding violating posterior muscles and nerve structures [1, 2], thus reducing operation time and blood loss, and have relatively fewer postoperative complications [3]. These techniques are widely used in patients based on the above advantages. ALIF has been used to treat degenerative lumbar diseases in the past few decades [4], with the method leading to satisfying clinical relief for patients. However, the need to move abdominal contents and the aorta/vena cava during the operation to obtain a more satisfactory working corridor [5, 6] is likely to cause vascular injury, deep vein thrombosis, and ileus [5]. OLIF is a newly introduced surgical approach first conducted in 2012. Unlike traditional LLIF, OLIF naturally protects the lumbar plexus because it avoided incision into the psoas muscle [7]. Consequently, there is no need for interoperating nerve monitoring, and OLIF leads to fewer complications compared with LLIF [8].

OLIF shares the same retroperitoneal working corridor as ALIF, but as a newly established surgical approach, few existing studies have revealed the direct comparison of OLIF and ALIF. Zhuo et al. [6] compared ALIF and OLIF in patients with degenerative lumbar diseases and found that OLIF can achieve similar clinical outcomes compared with those of ALIF, with shorter operation times and less blood loss. However, the ALIF approach allowed a larger cage to be implanted; therefore, patients in the ALIF group achieved a larger increase in postoperative disc height. Hee et al. [9] found that patients in the OLIF group had larger disc heights and greater sagittal disc angles than those of the ALIF group, in contrast to Zhuo et al., who concluded OLIF to be more effective in restoring sagittal balance. As it is still controversial whether ALIF or OLIF is a better option for degenerative lumbar diseases, we conducted this systematic review to further discuss the issue. The aim of this study was to compare the pre- and post-operative clinical effectiveness (visual analogue scale [VAS] score, Oswestry disability index [ODI]), radiographical parameters (disc height [DH], segmental lordosis angle [SLA], lumbar lordosis [LL], and fusion rate), intraoperative parameters (operation time [OT], estimated blood loss [EBL], and length of hospital stay [LOS]) and complication rate of ALIF and OLIF to comprehensively evaluate the two surgical approaches for degenerative lumbar diseases.

Methods

This study was performed following the guidelines of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses. (PRISMA 2020 Checklist) [10]

Search strategy

Based on the PRISMA 2020 Checklist, a systematic literature search was performed in PubMed, Embase, Web of Science, and Cochrane Library without time limitations in April 2022. The terms to find relevant studies were as follows: “oblique lateral interbody fusion,” “oblique lumbar interbody fusion,” “oblique lateral lumbar interbody fusion,” “retroperitoneal lumbar interbody fusion,” “pre-psoas lateral interbody fusion,” “anterior to psoas lumbar interbody fusion,” “anterolateral approach to lumbar,” “OLLIF,” “OLIF,” “ATP approach,” “anterior lumbar interbody fusion,” “ALIF,” “degenerative disc disease,” “degenerative lumbar disease,” “lumbar degenerative spondylolisthesis,” degenerative kyphoscoliosis,” and “spinal stenosis.” The search results were exported into NoteExpress V3.0 for further processing. Meanwhile, we checked the reference lists of the included studies to access more relevant studies.

Selection criteria

The inclusion criteria for the present study were as follows:

  1. 1.

    Population: patients treated by OLIF or ALIF with a diagnosis of lumbar degenerative disease, degenerative disc disease, degenerative kyphoscoliosis, or degenerative spondylolisthesis using computed tomography and magnetic resonance imaging.

  2. 2.

    Interventions: the patients treated by oblique lateral interbody fusion were in the OLIF group, while patients treated by anterior lumbar interbody fusion were designated as the ALIF group.

  3. 3.

    Outcomes: the primary outcomes of the study were to compare clinical (including VAS score, ODI score, and complication rate) and radiographical (including DH, SLA, LL, and fusion rate) parameters after OLIF or ALIF surgical strategies. The secondary outcomes included the measurements of mean OT, EBL, and LOS between the two surgery strategies.

  4. 4.

    Article type: published clinical research articles, excluding case reports and review articles.

We excluded duplicated, nonclinical studies, studies without primary outcomes, and studies with unavailable data.

Data extraction

Two reviewers (D.S. and W.S.L) independently extracted the following data from the included studies using a predesigned data extraction form: first author, publication year, study design, demographic information (number, sex, and mean age), inclusion disease, DH, SLA, LL, VAS score, ODI, fusion rate, complication rate, OT, EBL, and LOS. Any discrepancies in data extraction between the two reviewers were resolved through mutual agreement or consulting the third reviewer (Y.H.).

Statistical analysis

Statistical analyses were conducted using SPSS version 20.0 for Windows. Continuous variables were reported as mean ± standard deviation (SD) and were compared with the unpaired t test or nonparametric Mann–Whitney test. Two-sided P values were reported, and significance was established at a level of P < 0.05.

Results

Thirty-six articles were eventually included in the study (Fig. 1): 1 was of level II evidence [11], 32 were of level III evidence [6, 9, 12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41] and 3 were of level IV of evidence [42,43,44]. Overall, 1057 patients were included in the OLIF group (mean age 63.64 ± 3.51 years; mean body mass index 23.96 ± 1.25 kg/m2; women accounted for 61.97% of cases), and the mean follow-up (FU) period was 15.96 ± 8.36 months. Approximately, 984 patients who underwent ALIF were included in the study (mean age 53.14 ± 8.20 years; mean body mass index 26.31 ± 1.63 kg/m2; women accounted for 57.93% of cases), and the mean FU period was 25.68 ± 10.35 months. The general information of included patients is summarized in Table 1.

Fig. 1
figure 1

PRISMA flow chart for the search and inclusion strategy

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Table 1 Patients’ demographics and studies levels of evidence
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Twenty-nine articles included the treated levels: the surgeries were performed at a total of 2103 segment levels (1006 in the OLIF group, 1097 in the ALIF group), and 77.6% of the surgeries were performed at L4–S1 segments. There were 904 cases (43.13%) at L5–S1, 725 cases (34.37%) at L4–5, 347 cases (16.50%) at L3–4, 110 cases (5.24%) at L2–3, 14 cases (0.67%) at L1–2. The main indication for surgery was degenerative disc disease, spondylolisthesis, spinal stenosis, and adult degenerative scoliosis.

In the OLIF group, six articles included the size of the cage used in the surgery, with the cage height ranged from 8 to 14 mm, and an average height of 10.85 ± 1.09 mm [6, 9, 15, 19, 23, 26]. In the ALIF group, one article included a significantly higher cage [37], ranging from 12 to 19 mm, with the average cage height in five articles of the ALIF group being 13.61 ± 2.00 mm [6, 9, 30, 37, 41]. The cage used in the ALIF group is significantly higher than that in the OLIF group (P < 0.05).

In the OLIF group, 11 articles included the use of posterior percutaneous pedicles screw fixation for stabilization of all included patients [9, 12, 14,15,16, 18, 20, 21, 23, 25, 43], three articles included the use of lateral screw after OLIF [13, 26, 27], and four articles included posterior depression and screw fixation only for the patient with persistent radiculopathy [6, 22, 24, 42]. Only two articles used stand-alone OLIF with an anterior locking plate [17, 19]. In the ALIF group, six articles reported percutaneous pedicles screw fixation for all patients [9, 33, 34, 36, 40, 44], 10 articles included stand-alone ALIF [6, 28,29,30,31,32, 35, 37, 38, 41], and the stabilization was achieved by anterior plate fixation or using other self-locking devices along with the cage. A total of two articles used posterior fixation only for patients with Grade I spondylolisthesis, interarticularis fractures, or a history of laminectomy/facetectomy [11, 39]. A total of 361 ALIF procedures (36.38%) were accomplished with the assistance of vascular surgeons [6, 28, 31, 35, 37, 40].

For clinical effectiveness, pain relief was commonly measured by calculating the VAS scores of back and leg as well as ODI. VAS leg pain scored improved by a mean of 5.01 ± 1.16 in the OLIF group [9, 13, 17, 18, 20,21,22, 25, 26] and 3.81 ± 2.14 in the ALIF group [9, 11, 28, 30, 34, 36, 39], with no significate difference found between the groups (P = 0.17). VAS back pain scores improved by a mean of 3.59 ± 1.86 in the OLIF group [9, 13, 16, 18,19,20,21,22, 25] and 4.04 ± 1.28 in the ALIF group [9, 11, 28, 30, 32, 34, 36, 39], with no significate difference was found between groups (p > 0.05). In terms of ODI, the OLIF group [9, 12, 13, 16, 17, 19,20,21, 24,25,26,27, 42, 43] showed a mean improvement of 32.61 ± 13.55, while the ALIF group [9, 11, 28,29,30,31,32, 34, 35, 39, 44] showed a mean improvement of 28.50 ± 12.44. These data were higher than the reported minimum clinically important difference of ODI (11%) after adult spinal deformity surgery [45]. There was no significant difference in the ODI between groups (P > 0.05). The clinical results are summarized in Table 2.

Table 2 Clinical outcomes reporting VAS leg and back scores and ODI
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As for radiographic parameters, we compared the change of DH, SLA, and LL between the OLIF and ALIF groups. A total of 13 authors in the OLIF group [6, 9, 13,14,15, 17,18,19,20, 23, 26, 42, 43] reported a mean increase of 3.48 ± 1.26 mm DH (53.9% improvement), while 9 authors in the ALIF group [6, 9, 28, 30, 32,33,34, 37, 40] reported a mean increase of 4.70 ± 2.52 mm DH (81.2% improvement); however, the difference between groups was not significant.

A total of 10 authors reported the change in SLA in the OLIF group [6, 9, 13,14,15, 18, 19, 23, 26, 43], with an average of 4.40 ± 2.05°increase. Approximately, nine authors in the ALIF group [6, 9, 28,29,30, 32, 34, 39, 41] reported an average of 6.72 ± 2.82°increase in SLA, with no significant difference found between the groups.(P > 0.05).

As for LL, five authors in the OLIF group [14, 16, 17, 19, 43] reported mean of 8.35 ± 3.29°alteration, while eight authors in the ALIF group [29, 30, 32, 34, 39,40,41, 44] reported a mean of 3.64 ± 1.94°alteration. The change of LL was significantly larger in the OLIF group than in the ALIF group. The result of radiographical changes is listed in Table 3.

Table 3 Radiographical parameters in included articles
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In the evaluation of surgical parameters, we compared operation time, estimated blood loss, and the length of hospital stay between the two surgical strategies. Operation time was recorded by 16 authors in the OLIF group and 8 authors in the ALIF group. The mean operation time in the OLIF surgery was 134.12 min and in the ALIF surgery was 126.09 min, with no significant difference between the two groups (P > 0.05). Estimated blood loss was reported in 16 OLIF articles and 9 ALIF articles, with the average blood loss being 105.18 ml in OLIF surgery, and 131.71 ml in ALIF surgery. No significant difference was found between the groups. For the length of hospital stay, patients in the OLIF group generally experienced 6.08 ± 2.14 days of hospital care, while those in the ALIF group stayed in the hospital for 4.06 ± 2.56 days. There was no significant difference between the groups (all P > 0.05). The results of operation-related parameters are summarized in Table 4.

Table 4 Operation parameters in included articles
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Over 90% of the total fusion rate was achieved using both methods (90.83% in the OLIF group, 93.66% in the ALIF group). In terms of complications, the overall complication rate was 18.83% (199 in 1057 patients) in the OLIF group and 7.32% (72 in 984 patients) in the ALIF group. The most common complications in the OLIF group were cage subsidence (11.35%), pain/numbness/weakness in the thigh or psoas (2.37%), sympathetic chain symptoms (1.7%), and lumbar plexopathy (1.04%). Postoperative ileus (1.52%) was the most common complication in the ALIF group, followed by cage subsidence (1.32%), and pain/numbness/weakness in the thigh or psoas (1.32%), and other complications are listed in Table 5. Compared with the OLIF group, the ALIF group rarely presented cage subsidence (1.32%) and pain/numbness/weakness in the thigh or psoas (1.32%).

Table 5 Summary of the principal complications extracted from the included articles
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Discussion

We systematically reviewed the difference in radiographical and clinical outcomes between OLIF and ALIF for degenerative lumbar diseases. To the best of our knowledge, there has been no systematic review of the literature directly comparing the two techniques. For the radiographical results, obtaining larger DH and SLA restoration indicates that the surgery is more beneficial for the patients. We found that both OLIF and ALIF can significantly increase the DH and SLA compared with the preoperative data. DH restoration is closely related to the size of the cage implanted in the surgery, and higher cages were used for the ALIF group compared with the OLIF group (10.85 ± 1.09 mm vs 13.61 ± 2.00 mm, P < 0.05). The change of DH in the ALIF group was relatively larger (4.70 ± 2.52 mm vs 3.48 ± 1.26 mm); however, no statistical difference was found. The size of cage chosen would depend on the patient’s individual condition, and patients inserted with a larger cage gain more DH [6, 9, 26]. We believe the variation of cage size in the two groups is not approach-related. Although an oversized interbody cage would more effectively increase the intervertebral height, stress between the cage and the osseous endplate would increase, readily causing the sinking of the cage [17]. This mechanism might be the possible explanation for the similar change of DH between the OLIF and the ALIF group.

In terms of SLA restoration, a study [46] showed larger preoperative disc angle, more considerable lift up of the disc space, and a more anterior cage position associated with larger SLA restoration in LLIF. Studies have shown that the LLIF operation may cause approximately 34% of the cage to be implanted in the anterior position [47]; thus, LLIF or OLIF should be more beneficial for postoperative SLA restoration. Our result showed no significant difference between the two groups (P > 0.05), with the average change of SLA was more remarkable in the ALIF group (4.40 ± 2.05°vs 6.72 ± 2.82°). This result was consistent with that of Watkins et al.[48], who found that compared with LLIF, ALIF can significantly increase the SLA, but they did not further explore the reasons. We further compared preoperative disc angles between the two groups and found the ALIF group showed larger preoperative SLA. In conclusion, the slight difference in SLA restoration might be caused by preoperative SLA variation and difference in DH restoration.

A well-balanced spine is the goal of all spine surgeries. We evaluated the postoperative sagittal balance of patients by restoration of general LL. The OLIF group gained more LL restoration than the ALIF group (8.35 ± 3.29°vs 3.64 ± 1.94°, p < 0.05). We found that the majority of patients in the OLIF group (11 of 20 studies) underwent percutaneous pedicles screw fixation after cage implantation, while most ALIF procedures were strengthened by self-locking plate or intrinsic screws. Additional posterior fixation could help maintain the correction of the LL previously achieved by the cage implantation [41]; this larger LL restoration in the OLIF group might be a selective bias. The effective restoration of LL could strongly prevent flat back syndrome upon long-term follow-up after lumbar fusion surgeries [49]; we believe additional posterior fixation can be beneficial for patient.

Clinical relief of pain is critical to patients’ quality of life. We evaluated the clinical effectiveness of OLIF and ALIF by VAS leg and back pain scores, as well as ODI. The postoperative change of VAS score and ODI revealed a notable clinical relief of pain among patients in both groups, and no significant statistical difference was found. The change in ODI was higher than the reported minimum clinically important difference of ODI (11%) after adult spinal deformity surgery [45] in both groups. ALIF has been performed for decades and may have the best outcomes in indirect decompression methods [50]; many studies showed significant indirect foraminal decompression using ALIF [51, 52]. As a newly introduced anterolateral indirect decompression method, OLIF could reach a comparable clinical and radiographical effectiveness to that of the ALIF technique; we believe OLIF can emerge as a new indirect decompression method that can be utilized in treating degenerative lumbar diseases.

We found that fusion rates in OLIF and ALIF were both satisfying (> 90%). The complication rate of OLIF was significantly higher than that of ALIF (18.83% vs 7.32%). Cage subsidence (11.35%, total of 120 levels) is the most common complication in the OLIF group; however, this complication only happened in 1.32% of the ALIF group. Kotheeranurak et. al [53] found age > 60 years, bone mineral density < -2.5, higher cage height, and severe multifidus muscle fatty degeneration were the top risk factors of cage subsidence after OLIF. Patients included in the OLIF group were characterized a mean age over 60 years old, while patients’ mean age in the ALIF group was younger (around 53 years old); this might be an explanation for the difference. Meanwhile, the cage used in ALIF surgeries is often accompanied by a self-locking anterior plate or screw; this design further prevented cage subsidence after stand-alone ALIF surgeries. The most common complication in the ALIF group was postoperative ileus; this is an approach-related complication. Traditional ALIF was performed using a supine position; bowel mobilization was required to get access to the spine. OLIF surgeries were operated on right lateral decubitus, abdominal contents would automatically shift aside because of gravity, so less bowel mobilization is needed in OLIF surgeries, and postoperative ileus was rarely seen (less than 1%). Other complication includes thigh or psoas pain/numbness/weakness, sympathetic chain symptoms, lumbar plexopathy, vascular injury, deep venous thromboses, pseudarthrosis, wound infection, and incisional pain. Most of the complications were relieved within 3 months after the surgeries and with no need for special treatment, only a few cases needed reoperation.

Surgical expenditure is critical for patient when making surgery decisions. However, none of the included articles reported total cost of the surgeries. We assume the OLIF technique can be cheaper for patients, for about 36.68% ALIF surgeries were accomplished by assistance of vascular surgeons, and that would surely increase the expected surgical expenditure.

This study has some limitations. First, there is no published RCT study on OLIF. Many included studies were designed as prospective or retrospective cohort studies. Secondly, surgical indications of the included population were highly variable, so the result could just establish a general impression between the two surgical approaches; more multicentre prospective randomized controlled trials directly comparing OLIF and ALIF are required to provide the possibility of further evaluation.

Conclusion

In conclusion, for the surgical treatment of lumbar degenerative diseases, both ALIF and OLIF could provide satisfactory outcomes with an overall fusion rate of over 90% in both groups. OLIF showed comparable radiographic and clinical results to the ALIF, with lower expected surgical expenditure. However, the complication rate in the OLIF group was significantly higher than in the ALIF group; the most concerning complication in the OLIF group was cage subsidence. More prospective randomized controlled trials directly comparing OLIF and ALIF are needed, to promote the development of precision therapy for lumbar degenerative diseases in geriatric spine surgery.