Degenerative lumbar pathologies are among the common causes of disability, and they frequently cause significant pain (back and/or radicular) as well as disruption in function and quality of life (QOL) [1]. When conservative measures fail to relieve symptoms of lumbar degenerative disease, operative treatment may be required. Lumbar interbody fusion (LIF) is a common procedure used by spinal surgeons that is effective in lordotic correction and biomechanical stabilization.1 The anterior (anterior lumbar interbody fusion [ALIF]) and transforaminal (transforaminal lumbar interbody fusion [TLIF]) approaches are two important LIF techniques that have been extensively studied in the spinal literature. Numerous studies have also compared the two procedures, concluding that each has advantages and disadvantages, despite having similar arthrodesis rates [1–3]. In prior comparative studies, however, had a limitation in that they did not control for patient-specific factors. Obesity, a condition in which metabolic derangements and resulting inflammation may precipitate or exacerbate spinal disease, is an important patient characteristic that may play a role in outcomes [4]. While many authors have compared the outcomes of non-obese and obese (defined as body mass index [BMI] ≥30.0 kg/m²) individuals undergoing spine surgery, none, to our knowledge, have attempted to answer the following question: Which procedure is best suited for an obese patient population? As a result, we present a longitudinal study comparing minimally invasive (MIS) TLIF and ALIF for perioperative safety and postoperative success (as measured by patient-reported outcome measures [PROMs] and minimal clinically important difference [MCID] achievement) in obese surgical recipients.

Low back pain (LBP) is a common condition that affects approximately 80% of people at some point in their lives [9]. While conservative measures are frequently tried when LBP is presented, non-operative management has variable outcomes, with up to 62% of patients experiencing recurrence [10]. Patients experiencing refractory symptomatology may require surgical intervention with LIF. The predominant approaches for fusion at L5/S1 are transforaminal (TLIF) and anterior (ALIF) [11,12]. These procedures were pioneered to improve patient outcomes and demonstrate efficacy in achieving lumbar arthrodesis while alleviating signs and symptoms of lumbar spondylolysis [3].

Certain patient factors, such as age, smoking status, and BMI have been shown to increase the risk of developing LBP [13]. One meta-analysis found that obese patients (≥30.0 kg/m²) were significantly more likely to experience LBP than overweight (25.0 kg/m²–29.9 kg/m²) and non-overweight (<25.0 kg/m²) individuals, even after controlling for publication bias and confounding covariates [14]. One explanation for the higher prevalence of LBP is that obese patients are at a higher risk of developing degenerative disc disease and spinal stenosis secondary to adipose-induced inflammation and/or biomechanical instabilities [15,16].

When providing surgical care to this patient population, providers must consider the effects of obesity on perioperative and postoperative outcomes. While previous studies have compared perioperative and postoperative outcomes in non-obese and obese lumbar procedures cohorts, no study to our knowledge has directly compared two LIF procedures specifically for this patient population. The current study compares perioperative characteristics, PROMs, and MCID achievement in obese patients undergoing single-level MIS TLIF versus ALIF at L5/S1. We hope that by conducting this study, we will be able to provide evidence-based findings that will assist spine providers in selecting an appropriate surgical method for this patient group.

The relationship between obesity and perioperative characteristics during spine surgery is not well established in the literature. A meta-analysis by Goyal et al. [17] included 23,415 patients from 32 studies to compare such metrics between obese and non-obese patients. Obese patients had higher EBL and operative time during lumbar spine surgery, according to their findings. However, the authors point out that, while their findings were statistically significant, the effect sizes were quite small and may not have been clinically meaningful. One possible explanation for their findings is that larger dissections are required when operating on obese patients. Nonetheless, such perioperative characteristics in the context of ALIF and MIS TLIF remain unknown because Goyal et al. [17] included a variety of surgical techniques with varying degrees of fusion. In contrast, Jacob et al. [3] compared the perioperative characteristics of MIS TLIF and ALIF. Their analysis of a large patient cohort revealed that patients undergoing MIS TLIF had shorter operative times and EBL than those undergoing ALIF [3]. When we restricted the selection criteria to an obese patient population, we found contrasting results. There was no difference in EBL, operative time, or LOS between obese patients undergoing MIS TLIF or ALIF. As a result, spinal surgeons performing lumbar fusion may be reassured that the perioperative safety of MIS TLIF and ALIF in the operative treatment of obese individuals is comparable. Furthermore, prior research has found comparable fusion rates between the two procedures, a finding supported by our study of an obese patient population [2].

In terms of clinical outcomes, tools like the VAS are commonly used to measure pain intensity. As pain is the most common presenting symptom for those undergoing MIS TLIF or ALIF, postoperative pain measures are a useful metric for determining treatment success. Furthermore, scores can be restricted to specific areas of the patient’s body, such as the back and leg [18]. A study conducted by Kotani et al. [19] on 550 patients found that ALIF resulted in greater VAS leg improvement when compared to TLIF. Furthermore, Jacob et al. [3] found that those who received an ALIF had a significantly greater rate of MCID achievement for the VAS leg when compared to those who received a TLIF. Those who had ALIF showed significantly higher VAS leg scores at the 2-year mark (p=0.017) in our study. This finding, however, may be limited because a 2-year gap in follow-up may have resulted in selection bias. Furthermore, MCID achievement varied over time, including at the 2-year mark. There was no difference in mean scores or MCID between the two cohorts based on VAS back measurements. Our findings are inconsistent with those of Kotani et al. [19] and Jacob et al. [3] but they are consistent with those of Ajiboye et al. [20]. In their meta-analysis, they pooled 811 patients from seven studies and found no significant mean differences in VAS scores between TLIF and ALIF. In terms of VAS score improvements from baseline, ALIF patients did not improve significantly at 2 years for VAS back or 6 weeks/12 weeks for VAS leg. Meanwhile, MIS TLIF patients improved at nearly every time point for both VAS scores (except for 1 year postoperatively for the VAS leg). A plausible explanation would be that there was a smaller sample size for ALIF versus MIS TLIF patients at these time points, requiring a greater magnitude of recovery for the former cohort to reach statistical significance. Furthermore, only one patient had postoperative as well as preoperative VAS scores at 1 year (for VAS back/leg) and 2 years for VAS back. As a result, at those time points, the significance of improvement among ALIF patients could not be calculated. Nonetheless, because we found no difference in MCID achievement at any time point for VAS leg and VAS back, our findings generally indicate that patients in these cohorts experience comparable pain levels. As a result, obese patients who are concerned about pain relief after surgery will experience similar results whether they undergo MIS TLIF or ALIF.

Patients considering spine surgery should consider postoperative functional levels because LBP can impair physical function (PF). Assessing postoperative PF after spinal surgery is an important clinical tool for determining surgical success [21]. The PROMIS and the SF-12 PCS were used in our study to assess PF [22,23]. PF can be assessed within the PROMIS system to assess a patient’s physical well-being [22]. When compared to MIS TLIF patients, obese ALIF recipients had higher PROMIS-PF scores at 6 weeks (p=0.014) and 12 weeks (p=0.030), according to our functional outcomes analysis. This difference, however, did not persist for later time points and did not affect MCID achievement. As a result, we believe it is a unique finding for our cohort and is not generalizable, especially since the 12-week difference dissipated after adjusting for baseline intergroup differences in BMI. Our findings at the majority of postoperative time points are thus broadly consistent with the findings of Jacob et al. [3], who found no difference in PROMIS-PF following MIS TLIF when compared to ALIF. Our SF-12 PCS analysis followed a similar pattern, with no clinical difference in mean scores between the two cohorts. This finding is also consistent with the literature. A study conducted by Divi et al. [24] retrospectively reviewed 391 patients undergoing lumbar fusion and found that surgical technique made no difference in short-term SF-12 PCS mean scores or MCID achievement. While their study compared multiple surgical techniques, MIS TLIF and ALIF were both included in their analysis [24]. At the 1-year time point, Jacob et al. [3] found that the ALIF cohort had higher mean PCS than the MIS TLIF cohort. However, the authors concluded that there was no difference in MCID achievement between the two LIF approaches. Finally, our findings indicate that clinically meaningful physical health recovery among obese patients receiving MIS TLIF or ALIF is comparable. This finding may encourage surgeons because it broadens the scope of available options for improving physical health in obese LIF patients. Evidence-based findings from other PROMs, patient characteristics, and patient preferences should all be taken into account when deciding between approaches.

The ODI is a patient-reported functional disability assessment. The ODI assessment consists of 10 sections, and patients are given a maximum score of 50, with a higher score indicating a worse condition [25]. Ajiboye et al. [20] found no statistically significant pooled mean differences in ODI between MIS TLIF and ALIF. Furthermore, Phan et al. [26] conducted an eight-study meta-analysis and found that TLIF and ALIF resulted in similar postoperative ODI scores. While these studies encompass the general population, others do postulate ODI in obese patients. Djurasovic et al. [27] retrospectively compared 109 obese patients and 161 non-obese patients who received lumbar fusion. There was no difference in ODI scores between the cohorts, according to their findings [27]. Our study examined slightly different parameters but found no differences in ODI at any time point among obese patients undergoing MIS TLIF or ALIF at L5/S1. While ALIF patients did not significantly improve at 6 weeks (and there was no data on PROM improvements beyond 6 months), MIS TLIF patients improved significantly from 6 weeks to 2 years. The relatively smaller sample size of ALIF recipients may have made it more difficult to achieve statistical significance, especially since the mean ODI score was still >10.0 points lower at 6 weeks compared to preoperative for this group. Furthermore, at the 1-year and 2-year time points, there was only 1 ALIF patient who had a postoperative score as well as preoperative data. As a result, an improvement from preoperative to postoperative ODI score for ALIF at these time points could not be calculated. Despite this, there were no statistical differences in MCID achievement between the two cohorts at any time point. Patients who already have debilitating LBP due to obesity should not expect dissimilar ODI scores after either MIS TLIF or ALIF.

A significant limitation of our study is the small sample size, which was reduced by more than half after performing a coarsened exact match analysis to eliminate confounders. More studies with a larger number of subjects provide more statistical power. All operations were performed in an academic setting by one orthopedic spine surgeon, limiting the external validity of the results presented. As longitudinal outcomes were assessed, loss to follow-up at later timepoints resulted in limited available data at 1-year and 2-year intervals which may have contributed to selection bias in our presented results. Furthermore, radiographic measures such as sagittal alignment parameters were not assessed, which is an important direction for future research. Finally, because PROM questionnaires are subjective, response bias may have had a significant impact on our findings.

MIS TLIF and ALIF had comparable operative times, EBL, LOS, and 1-year arthrodesis rates among obese recipients, with no difference in acute postoperative pain or narcotic consumption. Obese patients also improved for both procedures at the majority of time points. When mean PROM scores were compared after adjusting for differences in baseline BMI, only one difference was observed across all time points studied (PROMIS-PF at 6 weeks), with the higher score among ALIF recipients also translating to greater PROMIS-PF MCID achievement at this timepoint. Nonetheless, no other postoperative PROMs or MCID achievement rates differed significantly between procedures at any other timepoint. As a result, we conclude that MIS TLIF and ALIF appear to be equally effective in terms of perioperative safety, 1-year fusion, and QOL patient-perceived outcomes of pain, disability, and physical health.