1 Introduction

Venous thromboembolism (VTE), encompassing deep vein thrombosis (DVT) and pulmonary embolism (PE), remainsa critical postoperative complication in orthopaedic surgery, particularly following lumbar spine procedures. While the incidence of VTE in lumbar spine surgery (LSS) remains relatively lower than in total joint arthroplasty, recent studies underscore its clinical relevance, with reported rates of DVT and PE ranging from 0 % to 15.5 % and 0 % to 13.1 %, respectively.^1–4 This marked variability not only reflects the enduring burden of VTE in spinal surgery but also highlights the imperative for risk-stratified prophylactic regimens tailored to this patient population.

The elevated thromboembolic risk in LSS often stems from prolonged postoperative immobilization and surgical techniques that compromise vascular integrity.⁵ Anterior and circumferential approaches, for instance, necessitate manipulation of the inferior vena cava, predisposing patients to endothelial injury and venous stasis, which are known contributors to thrombogenesis.⁵ Such procedural nuances, combined with heterogeneous reported outcomes, emphasize the imperative to optimize VTE prevention protocols in spinal surgery.

Current prophylaxis strategies primarily prioritize the utilization of mechanical strategies, such as compressive stockings, owing to concerns that pharmacologic agents may exacerbate hemorrhagic complications, including spinal hematomas with catastrophic neurologic consequences.^6–8 However, given the morbidity and mortality associated with VTE, discussion emerges regarding the adjunct use of pharmacologic prophylaxis along with mechanical methods, particularly in high-risk cohorts.^9–11 Obesity, a well-established risk amplifier in orthopaedic surgery, complicates the equilibrium between thromboprophylaxis efficacy and bleeding risks. Moreoever, morbid obesity (BMI ≥40 kg/m²) further exacerbates these challenges, contributing to prolonged operative times, extended hospital stays, and heightened risks of surgical site infections and systemic complications.^12–14 While elevated BMI is linked to increased VTE incidence in total knee and hip arthroplasty,^15,16 data in spinal surgery remain equivocal, prompting some clinicians to advocate for intensified anticoagulation in obese patients, such as direct oral anticoagulants (DOACs).¹⁷ However, DOACs come withpharmacokinetic limitations in obese patients, including erratic absorption and metabolism, prompting cautionary guidelines from the International Society on Thrombosis and Haemostasis against their routine use in this population.^18,19

Aspirin (acetylsalicylic acid) has thus gained prominence as a cost-effective, low-risk alternative for these patients. The 2022 International Consensus Meeting endorsed aspirin as first-line VTE prophylaxis for various orthopaedic procedures such as total knee and hip arthroplasty, citing its favorable safety profile and minimal monitoring requirements.²⁰ Despite its adoption in arthroplasty, evidence supporting aspirin's efficacy in obese LSS patients remains sparse, with no BMI-specific guidelines.²¹

The purpose of this study was to evaluate whether VTE event rates differ by BMI category among patients undergoing lumbar spine surgery who are uniformly treated with aspirin. We specifically investigated the incidence of DVT and PE associated with aspirin monotherapy in these patient populations, comparing the incidence of outcomes in the obese cohort to those in the non-obese cohort. Additionally, through a subgroup analysis, we also evaluated whether class III/morbidly obese patients (BMI ≥40) are at an increased risk for DVT or PE compared to class I/II obese patients (BMI 30–39.9) when both cohorts of patients are on aspirin monotherapy. Addressing these questions is relevant given the increasing prevalence of obesity and morbid obesity and the relevance of evaluating aspirin as a simple, cost-effective thromboprophylactic option in this high-risk demographic.

2 Methods

3 Results

4 Discussion

With obesity rates climbing globally, spine surgeons are increasingly likely to treat patients with higher BMIs.²⁴ Our study addresses the gap in the literature regarding the impact of obesity and morbid obesity on postoperative VTE risk in the context of aspirin prophylaxis efficacy. At the 90-day follow-up, our analysis revealed that obese patients had significantly increased risks of both DVT and PE compared to their non-obese counterparts; however, morbid obesity did not further amplify these risks at this early interval. At the 6-month follow-up, the initially elevated DVT risk had subsided between obese and non-obese patients, as well as between obese and morbidly obese cohorts, indicating a transient nature of this complication. In contrast, the significantly increased PE risk observed between obese and non-obese cohorts at 90 days persisted through the 6-month follow-up. Notably, the risk of PE was amplified even further in the morbidly obese cohort compared to the obese cohort at 6 months, although the absolute risk increase was relatively modest. These findings suggest that although aspirin prophylaxis may be associated with lower postoperative DVT risk, a residual risk of PE may persist, particularly among morbidly obese patients.

Our results must be interpreted in light of the known variability and overall significance of VTE in spinal surgery. Historically, the incidence of postoperative DVT and PE in lumbar spine surgery has been reported to range from 0 % up to 15.5 % for DVT and 0 %–13.1 % for PE.^1–4 This wide range reflects differences in patient populations, detection methods, and prophylaxis protocols across studies, but it underscores that VTE is a real albeit variable risk after spine procedures. Notably, even though these rates are generally lower than those seen in total joint arthroplasty, the potential severity of PE makes any occurrence clinically important. This has driven a persistent emphasis on VTE prevention strategies in spine surgery; however, the optimal prophylaxis regimen following spine surgery remains debated. Owing to concerns about epidural hematoma and neurological injury, many surgeons rely on mechanical prophylaxis alone such as compression stockings or pneumatic devices, and are cautious with pharmacologic agents.^25,26

There is no universal consensus on post-spine surgery chemoprophylaxis, and guidelines offer only general recommendations, highlighting the need to balance bleeding and thrombotic risks on a case-by-case basis. In practice, this uncertainty is most pronounced for high-risk subsets such as obese patients, where the clinician must weigh the elevated VTE risk against the heightened bleeding potential.^11,21,27 In recent years, aspirin has emerged as an attractive chemoprophylactic option in orthopaedic surgery given its efficacy and acceptable safety profile. Large joint arthroplasty studies have demonstrated that aspirin can be an effective prophylaxis agent in appropriately selected patients, and its use has increased as part of risk-stratified protocols.²⁸ Nevertheless, specific data on aspirin's efficacy in the postoperative spine population, and particularly in obese individuals, has been limited.

Consistent with our findings, a growing body of literature indicates that obesity is associated with increased thromboembolic complications after spinal surgery. Multiple studies have documented higher rates of VTE in obese vs. non-obese spine surgery patients.^29,30 Marquez-Lara et al., for example, reported nearly a twofold higher incidence of both DVT and PE in patients with BMI ≥25 kg/m² undergoing lumbar spine surgeries,²⁹ and similarly, a meta-analysis by Jiang et al. found that obesity (BMI ≥30) conferred over three times the odds of postoperative VTE compared to normal BMI.³⁰ Obesity promotes a prothrombotic state through mechanisms such as chronic low-grade inflammation, impaired fibrinolysis, and increased coagulation activation,²⁴ which could likely explain these observations. It should be noted, however, that these existing studies did not stratify outcomes by obesity class, such as class I/II vs. class III obesity, or had heterogeneous prophylaxis regimens, making it difficult to discern the specific risks for the morbidly obese subgroup or to generalize findings to an aspirin-only prophylaxis context.

Evidence from prior orthopaedic literature suggests that when effective prophylaxis is consistently applied, obesity alone may not always emerge as an independent risk factor for VTE. For instance, Sadeghi et al. demonstrated in a case-control study of total knee arthroplasty that morbid obesity was not significantly predictive of postoperative VTE when robust mechanical prophylaxis and prompt mobilization strategies were utilized; rather, VTE incidents were primarily linked to inconsistent prophylaxis and delayed ambulation.¹⁵ This underscores that thorough and consistently applied prophylactic measures may offset some of the thrombotic risks associated with obesity. In contrast, our study specifically controlled for prophylaxis by uniformly applying aspirin monotherapy across all BMI categories, thus allowing for a clear examination of obesity's impact on VTE outcomes. Our findings revealed that even with consistent prophylaxis, obese patients experienced increased DVT and PE risks in the early postoperative period. At the 6-month follow-up, however, the heightened DVT risk subsided, whereas the elevated risk of PE persisted, notably amplified further among morbidly obese patients. This distinction between transient DVT risk and persistent, BMI-dependent PE risk may have been obscured in earlier studies that aggregated all obese patients or varied prophylactic strategies, highlighting the critical need for targeted thromboprophylaxis protocols in morbidly obese populations undergoing lumbar spine surgery.

The differential effect of obesity on PE versus DVT incidence is an interesting finding with important clinical implications. Despite comparable rates of deep vein thrombosis, morbidly obese patients demonstrated a marked increase in pulmonary embolism events, suggesting that obesity-specific mechanisms, such as chronic inflammation, hypercoagulability, or altered hemodynamics, may preferentially promote thrombus migration or destabilization within the pulmonary vasculature. A comparable pattern has been reported in other orthopaedic procedures, for instance, in a large analysis of hip and knee arthroplasty, overweight and obese patients had higher odds of PE after surgery but no corresponding increase in DVT risk.²⁸ Likewise, in a recent study of aspirin prophylaxis after total knee arthroplasty, morbidly obese patients exhibited a significantly greater 90-day PE incidence relative to non-obese patients, whereas DVT rates did not differ appreciably by BMI category.³¹ These parallels between our spine surgery data and the arthroplasty literature strengthen the notion that standard aspirin prophylaxis may be less effective for preventing PE in the morbidly obese patients.

Aspirin's antithrombotic effect, mediated through platelet inhibition, may be attenuated in morbidly obese patients due to altered pharmacodynamics. Prior studies have indicated that fixed low-dose regimens (≤100 mg) exhibited diminished efficacy in individuals exceeding ∼70–80 kg.³² While orthopaedic prophylaxis typically employs higher doses, often 325 mg daily, physiological factors in morbid obesity including expanded plasma volume, accelerated platelet turnover, and increased volume of distribution may reduce drug bioavailability, leading to suboptimal dosing. Similar pharmacokinetic challenges are recognized with DOACs, as guidelines caution against their use in high-BMI patients due to unpredictable drug levels.^18,19 These mechanisms may possibly explain the residual PE risk observed in morbidly obese patients on aspirin monotherapy. Consequently, standard weight-independent dosing may inadequately address thrombotic risks in this population, necessitating personalized strategies such as weight-adjusted regimens or adjunctive therapies.

This study has several limitations. The retrospective design using administrative claims data may introduce inherent selection bias and relies on ICD coding accuracy for identifying VTE events. While clinically diagnosed DVT and PE were captured, asymptomatic or undocumented cases may have been missed, and coding inaccuracies could influence results. The analysis also did not directly compare aspirin to other pharmacologic agents like DOACs or LMWH, limiting conclusions about whether alternative anticoagulants might mitigate the observed PE risk in morbidly obese patients. Moreover, aspirin exposure was inferred based on prescription data, which does not confirm patient compliance or ensure that aspirin was taken specifically for VTE prophylaxis in the perioperative period. The database does not capture over-the-counter use or specific clinical instructions that might influence timing or intent. Lasty, our study was limited to clinically diagnosed VTE events, as routine postoperative VTE surveillance, such as Doppler ultrasound or CT scans, was not uniformly available, which may have led to underestimation of asymptomatic events. Despite these limitations, our study is strengthened by a large cohort size and propensity score matching,³³ enhancing internal validity and strengthening the reliability of our findings.

5 Conclusions

When aspirin was utilized as the sole prophylactic agent following LSS, obese patients demonstrated higher observed incidences of DVT and PE compared to non-obese patients at the 90-day follow-up, although morbid obesity did not further amplify these early risks. By the 6-month follow-up, the increased DVT risk subsided across all BMI groups, however, the elevated PE risk persisted in obese patients and was further amplified in the morbidly obese cohort. These findings may warrant a more nuanced approach to thromboembolic prophylaxis in this high-risk subgroup. While aspirin has a favorable safety and cost profile, our findings suggest its relative effectiveness may be attenuated in morbidly obese patients, limiting its role in managing postoperative pulmonary thrombotic risk. Future prospective randomized controlled trials across high-volume centers are warranted to better elucidate the risks and benefits of aspirin monotherapy and to inform the development of optimal prophylactic guidelines and strategies for class I/II/III obese patients undergoing LSS.

CRediT authorship contribution statement

Sri Tummala: Writing – original draft, Data curation, Formal analysis, Investigation, Conceptualization, Methodology, Validation, Writing – review & editing, Prepared Tables 1-3, Project administration. Joseph Chavarria: Writing – review & editing, Validation. Jason Alder: Review & Editing, Validation. Ioannis Avramis: Supervision, Project administration, Review & Editing, Supervision, Validation. James M. Rizkalla: Conceptualization, Methodology, Supervision, Project administration, Writing – review & editing, Validation, All authors have read and approved the final manuscript.

Informed consent statement

This retrospective study is exempt from informed consent. The data reviewed represent a secondary analysis of existing data, do not involve intervention or interaction with human subjects, and are de-identified per the de-identification standard defined in Section §164.514(a) of the HIPAA Privacy Rule. The process by which the data are de-identified is attested to through a formal determination by a qualified expert as defined in Section §164.514(b)(1) of the HIPAA Privacy Rule. This formal determination by a qualified expert was refreshed on December 2020.

Institutional review board statement

This study was exempt from IRB approval since the data were de-identified and publicly available.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.