A systematic review of anterior lumbar interbody fusion (ALIF) versus posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), posterolateral lumbar fusion (PLF)

The rate of elective lumbar fusion has continued to increase over the past two decades. However, there remains to be a consensus on the optimal fusion technique. This study aims to compare stand-alone anterior lumbar interbody fusion (ALIF) with posterior fusion techniques in patients with spondylolisthesis and degenerative disc disease through a systematic review and meta-analysis of the available literature. A systematic review was performed by searching the Cochrane Register of Trials, MEDLINE, and EMBASE from inception to 2022. In the two-stage screening process, three reviewers independently reviewed titles and abstracts. The full-text reports of the remaining studies were then inspected for eligibility. Conflicts were resolved through consensus discussion. Two reviewers then extracted study data, assessed it for quality, and analysed it. After the initial search and removal of duplicate records, 16,435 studies were screened. Twenty-one eligible studies (3686 patients) were ultimately included, which compared stand-alone ALIF with posterior approaches such as posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), and posterolateral lumbar fusion (PLF). A meta-analysis showed surgical time and blood loss was significantly lower in ALIF than in TLIF/PLIF, but not in those who underwent PLF (p = 0.08). The length of hospital stay was significantly shorter in ALIF than in TLIF, but not in PLIF or PLF. Fusion rates were similar between the ALIF and posterior approaches. The Visual Analogue Scale (VAS) scores for back and leg pain were not significantly different between the ALIF and PLIF/TLIF groups. However, VAS back pain favoured ALIF over PLF at one year (n = 21, MD − 1.00, CI − 1.47, − 0.53), and at two years (2 studies, n = 67, MD − 1.39, CI − 1.67, − 1.11). The VAS leg pain scores (n = 46, MD 0.50, CI 0.12 to 0.88) at two years significantly favoured PLF. The Oswestry Disability Index (ODI) scores at one year were not significantly different between ALIF and the posterior approaches. At two years, ODI scores were also similar between the ALIF and the TLIF/PLIF. However, the ODI scores at two years (2 studies, n = 67, MD − 7.59, CI − 13.33, − 1.85) significantly favoured ALIF over PLF (I2 = 70%). The Japanese Orthopaedic Association Score (JOAS) for low back pain at one year (n = 21, MD − 0.50, CI − 0.78) and two years (two studies, n = 67, MD − 0.36, CI − 0.65, − 0.07) significantly favoured ALIF over PLF. No significant differences were found in leg pain at the 2-year follow-up. Adverse events displayed no significant differences between the ALIF and posterior approaches. Stand-alone-ALIF demonstrated a shorter operative time and less blood loss than the PLIF/TLIF approach. Hospitalisation time is reduced with ALIF compared with TLIF. Patient-reported outcome measures were equivocal with PLIF or TLIF. VAS and JOAS, back pain, and ODI scores mainly favoured ALIF over PLF. Adverse events were equivocal between the ALIF and posterior fusion approaches.


Introduction
Lumbar spondylosis can lead to back pain, radiculopathy, reduced quality of life, and work absenteeism. The rate of elective lumbar fusion for degenerative diagnoses has been increasing over the past two decades, with patients presenting with spondylolisthesis accounting for most of these operations in the USA [1]. Lumbar fusion is performed in adults with chronic and debilitating symptoms resulting from degenerative disc disease (DDD) and lumbar spondylolisthesis. Several fusion options exist, including anterior lumbar interbody fusion (ALIF), posterior lumbar interbody fusion (PLIF), transforaminal lumbar interbody fusion (TLIF), and posterolateral lumbar fusion (PLF). There is no consensus on the optimal technique for patients with DDD or spondylolisthesis. Clinical decision-making may be guided by patient history, physical examination, radiology, surgeon specialty, training, and preference [2]. ALIF provides some advantages over posterior fusion, including excision of the entire disc with restoration of disc height and lordosis, direct and indirect neural decompression, a larger cross-sectional area of graft material, insertion of a cage and graft under compression, and avoidance of damage to the erector spinae and the posterior ligamentous structures. The disadvantages include the risk of injury to vessels, viscera, parasympathetic and sympathetic nerves, and muscular atony of the abdominal wall [3]. ALIF procedures may be combined with posterior instrumentation (360-degree fusion). With the appropriate patient selection, stand-alone ALIF may be a more effective treatment method for symptomatic DDD and spondylolisthesis. It also avoids additional surgical and hospital recovery times, blood loss [4], and potential costs and morbidity associated with further posterior surgery. The advantages of stand-alone ALIF concerning patient-reported outcome measures (PROMs) still need to be determined.
Previous studies have included complex populations, including patients with spinal deformity, long constructs, prior spinal fusion, and advanced morbidities. Similarly, different anterior or posterior fusion approaches have been aggregated (e.g. stand-alone ALIF with 360-degree fusion). To improve our understanding of lumbar fusion procedures, we performed a systematic review and meta-analysis comparing stand-alone ALIF in patients with DDD and spondylolisthesis with posterior lumbar fusion techniques (PLIF/ TLIF/PLF).

Methods
This systematic review compared stand-alone ALIF with posterior lumbar spinal fusion in adults with back or leg pain, with a minimum of 6 weeks of nonoperative management. Randomised trials, cohort studies, and case series were included. Case reports and studies evaluating uninstrumented posterior fusion, long constructs, kyphosis/scoliosis, or circumferential fusion were excluded. The details are reported in the Prospero register [5].

Identification of relevant trials
Studies were sought by searching MEDLINE, EMBASE, and the Cochrane Register of Trials from inception to February 2022 without language restriction (see Additional File 1) according to the general principles of PRISMA-S [6]. The reference lists of the included studies and the literature reviews were searched for further studies. In addition, unpublished studies were sought by searching 'grey' literature using OpenGrey [7] and ClinicalTrials.gov [8] databases.

Study selection, data extraction, and analysis
Three reviewers independently screened titles and abstracts (JR, MR, and DN). The full-text reports of potentially relevant studies were inspected. Where uncertainty arose, a resolution was achieved through discussion. Two reviewers (JR, SL) extracted study data, and quality was assessed using the Cochrane Back Review Group's Risk of Bias criteria [9]. For those studies that only reported the median, interquartile range, or minimum and maximum values instead of means and standard deviations (SD), these were derived according to Hozo [10], Wan [11] and Walter [12].
The studies were analysed using Review Manager (Rev-Man [Computer program]. Version 5.4.1, The Cochrane Collaboration, 2020). Relative risk and 95% confidence interval (CI) were calculated for dichotomous data. For continuous data, the mean difference and 95% CI were calculated. A random-effects model was chosen to account for studies that estimated other related intervention effects. Thus, CIs for the average intervention effect will be more expansive, and the statistical significance will correspondingly be more conservative. Heterogeneity among the studies was tested using the chi-square test and I 2 statistics.

Results
A total of 16,435 records were identified from database searches, and the included studies' reference lists were inspected (Fig. 1). Following deduplication and exclusion of irrelevant studies, 94 full-text articles were examined for relevance. After further scrutiny, 21 studies were included. The search within OpenGrey and ClinicalTrials.gov databases did not identify additional studies. An updated search performed in February 2022 (n = 2147 records) did not identify any new studies.
Studies were conducted in eight countries: the USA (n = 10), Germany (n = 2), China (n = 2), Japan (n = 2), South Korea (n = 2), Denmark, Italy, and the UK. One study randomised patients to stand-alone ALIF or PLF, and the remaining 20 used non-randomised cohort designs. Spondylolisthesis and DDD were the primary diagnoses. The average age of the participants was 46 years, and the study size ranged from 21 to 969 (Table 1).
Seventy-three full-text articles were excluded. Seven were excluded because posterior fusion arms (e.g. PLIF and TLIF) were combined and reported as a single comparator; 13 due to patients not satisfying the inclusion criteria (e.g. spinal deformity, revision surgery); 19 were excluded because ALIF used 360-degree fusion rather than standalone fusion; nine, because ALIF was compared with other surgical approaches (e.g. Extreme Lateral Lumbar Interbody Fusion), or non-contemporary techniques (uninstrumented

Risk of bias assessment
The methodological quality of the included studies is shown in (Fig. 2).
The quality of the studies could have been more evident because of inconsistent reporting. Most studies did not report attrition or whether assessors were blinded to the treatment groups. Baseline prognostic risk factors (PRF) were not reported in 11 studies; four studies stated that baseline PRF was significantly different due to imbalances in age [14], PROMs [24], bone mineral density and spondylolisthesis [31], and smoking status [29]; six studies reported no significant difference in PRF across surgical groups, and one study used a randomised design with an overall low risk of bias [26]. The remaining 20 studies were prospective or retrospective observational studies in which the overall risk of bias was high or unclear. All non-randomised studies were at increased risk of allocation bias. Surgeons could not be blinded to the intervention, and most studies did not report how the fusion method was chosen. Four studies stated how the fusion approach was chosen. In one study [31], patients with primary lower back pain received ALIF, patients with single-leg pain received TLIF, and patients with neurogenic claudication received PLIF. Other single studies selected the fusion method for a specific pathology [29], were surgeon-dependent [33] or influenced by patients' preference for one type of surgical procedure [16]. Another surgeon-dependent study reported that the surgical approach used the same operative criteria for all surgical interventions [25]. One study stated that the surgical procedure depended on the timing of surgery; therefore, there was no bias in the severity of symptoms between groups [27]. However, the evidence for selective outcome reporting could be more precise. The risk of bias due to imbalances between the timing of the outcomes was primarily low or unclear.

Main results
The results compared stand-alone ALIF with three posterior fusion approaches ALIF vs. TLIF, ALIF vs. PLIF, and ALIF vs. PLF (Table 2).

Surgical time
Seven studies reported surgical time with high heterogeneity (I 2 = 93%). The surgical time (min) was significantly lower  (Fig. 3).

Adverse events
Adverse events were low and not significantly different between ALIF and TLIF. One study categorised adverse events as serious (n = 642, RR 1.62, CI 0.68, 3.84) and minor (n = 642, RR 1.11, CI 0.59, 2.10), with no significant Fig. 2 The methodological quality of studies summarising the risk of bias

Oswestry disability index
ODI scores up to 18 months (n = 58, MD 5.10, CI − 0.51, 10.71) and two years (n = 74, MD 2.40, CI − 6.40 to 11.20) were not significantly different between ALIF and PLIF. The ODI scores categorised as 'not improved' were also

Adverse events
Postoperative deep vein thrombosis, hematoma, need for revision surgery, and ileus was low and not significantly different between the ALIF and PLIF groups. The rates of adjacent segment degeneration within two years, postoperative complications, need for blood transfusions, and dural tears revealed no significant difference between ALIF and PLIF.

Adverse events
Postoperative complications, the need for further surgery, severe and minor adverse events, and readmission rates were not significantly different between ALIF and PLF (Fig. 10).

Discussion
The results support the hypothesis that stand-alone ALIF offers advantages over posterior approaches by minimising surgical time and blood loss [4]. Any fusion surgery is associated with significant intraoperative blood loss. This can be associated with longer operative times and higher rates of complications, which may increase the LOS and reoperation rates. This could cause a further financial burden for hospitals and healthcare systems. Factors such as surgical approach, operative time, and surgical complexity are essential considerations in perioperative planning, and every effort should be made to minimise blood loss and avoid transfusion [33].
The LOS was shorter for ALIF than TLIF but not for other posterior approaches. Spinal fusion is the most expensive operating room procedure in the USA [34,35]. The fusion rates were similar between the anterior and posterior approaches. The radiographic fusion rates reported in the literature for single-level surgeries vary between 70 and 96% [36]. There is yet to be a consensus on the method and timing of radiographic fusion assessment. This impedes the comparison of treatment outcomes. Additionally, the progression of bone formation over time and how this relates to bone grafting options necessitates more extended followup periods. Pseudoarthrosis may have deleterious effects on postoperative outcomes, including segmental instability, persistent or recurrent pain, and hardware failure.
The PROMs for disability and pain scores were equivocal when comparing ALIF with PLIF/TLIF. Back pain PROMs (VAS, ODI and SF-36 PCS) favoured ALIF over PLF. PROMs are affected by surgical and patient factors and fusion rates. Importantly, validated measures in spinal care are necessary for surgeons, patients, and policymakers to compare the effectiveness of different treatments in the same condition [37].
Adverse events did not differ significantly between the ALIF and posterior approaches, and the rates were generally low. A clinical evaluation by Chen et al. (2017) indicated that surgeons adequately recorded major adverse events. However, they often omitted recording minor events [38] enough, so it is possible that the results of this study underestimated minor adverse events. It is imperative to record and track adverse events and complications to implement programs to reduce the morbidity and mortality associated with spinal fusion. Adverse events may have a negative effect on PROMs and a substantial economic burden on health care.
This study had several limitations. Twenty studies used a non-randomised design which increased the risk of introducing confounding variables. For example, imbalances in the severity of pre-existing illnesses or comorbidities associated with poorer outcomes in spine surgery, such as smoking status, obesity, age, and socioeconomic status, could have affected surgical and PROMs. Such confounders could have been minimised by ensuring that baseline variables were restricted. Selection bias may have occurred during the choice of surgical approach, albeit due to surgeons making pragmatic decisions by the diagnosis, clinical and radiological features, or other considerations to maximise patient well-being.
This study could not analyse several potentially confounding variables explored due to a combination of heterogeneity in the techniques used and varied and limited levels of detail in technique descriptions within the articles assessed. Such considerations include diagnostic subtypes (e.g. degenerative versus isthmic spondylolisthesis), graft material (e.g. autograft, allograft, bone substitutes), interbody device type (e.g. PEEK versus Titanium alloy), fixation type (e.g. integrated screws versus plate-screw constructs), and surgical approach (e.g. open versus minimally invasive).
Limited data are available for PROMs, most findings being equivocal. This is partly secondary to the heterogeneity of the public data available to compare. Available PROMs only provide an analysis of short-term outcomes (1 to 2 years). More mid-and long-term data are needed to improve our understanding of the effects of the fusion approach on PROMs and ASD rates. The use of the best clinical evidence will act as the basis for guidelines for lumbar fusion. There remains variability in the standardisation of reporting of PROMs, adverse events, and specific diagnoses in the literature. In addition, there is a lack of uniformity in the surgical approaches used. Finally, continued efforts to develop higher-quality data from well-designed trials about surgical indications and techniques for lumbar fusion will provide valuable information in healthcare decision-making.

Conclusions
Overall, stand-alone-ALIF benefited from a shorter operative time and blood loss than (PLIF/TLIF) approach. PROMs were equivocal when compared to those of PLIF or TLIF. Back pain and disability scores favoured ALIF over PLF. Adverse events were also equivocal between the ALIF and posterior fusion approaches. Future studies must follow the CONSORT and STROBE guidelines [39] to help elucidate the effects of anterior and posterior fusion, particularly for PROMs.
Funding Open Access funding enabled and organized by CAUL and its Member Institutions. This research received no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.

Declarations
Conflict of interest Relevant financial activities outside the submitted work: consultancy, royalties.
Ethical approval This study was approved by the University Human Research Ethics Committee (0000015881) of Bond University.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.