Healthcare Utilization and Costs of Inpatient versus Outpatient Interspinous Process Device Placement: A National Claims Database Study

Introduction

The prevalence of lumbar spinal stenosis (LSS) is steadily rising in an aging population, making it one of the most common conditions affecting older adults.¹˒² The standard surgical management of LSS in the past has often been bony decompression with or without lumbar fusion. When stabilization is indicated, fusion procedures are associated with greater blood loss, longer operative times, and higher complication rates compared to decompression alone.³ More recently, interspinous process devices (IPDs) have emerged as a motion-preserving treatment option.⁴ IPDs were developed as an alternative to lumbar fusion and provide indirect decompression by distracting spinous processes and restoring foraminal height.⁵ Furthermore, they allow for dynamic stabilization that serves to preserve natural motion in comparison to the rigid stabilization provided by lumbar fusion.¹˒⁶ Given these characteristics, IPDs have been increasingly incorporated into the surgical management of LSS. 

In response to rising healthcare expenditures in recent years, outpatient surgery has been well documented as a cost-effective alternative for orthopedic procedures due to shorter operating times, lower facility fees, and decreased supply costs.³˒⁷ However, the absence of immediate access to comprehensive hospital resources and postoperative monitoring necessitates careful patient selection based on criteria such as age, BMI, ASA score, comorbidities, surgery complexity, and patient motivation and support network.⁸˒⁹ Prior studies comparing open decompression in inpatient and outpatient settings have shown equivalent clinical outcomes, with some reporting reduced complication rates and costs in the outpatient cohort.¹⁰˒¹¹ However, contemporary analyses leveraging large administrative databases with longitudinal follow-up are lacking.¹¹˒¹² 

As healthcare systems face increasing economic pressures and shift to value-based care, defining the real-world safety and economic profile of outpatient IPD placement is critical for surgeons, payers, and policymakers. This study aimed to compare complications, healthcare utilization, and costs for outpatient and inpatient single-level IPD procedures.

Methods

Database and Study Population
The PearlDiver (PearlDiver Inc., Colorado Springs, Colorado, USA) M170Ortho database contains over 170 million deidentified patient records and was retrospectively queried in this study. An Institutional Review Board (IRB) review and approval were not required for secondary data analysis of the database. Using the Current Procedural Terminology (CPT) code 22867, adult patients who underwent single-level IPD placement between 2017 and 2022 were identified, since this CPT code became effective in 2017.¹³ Patients were excluded if they underwent two-level IPD placement identified by CPT-22868, concurrent lumbar fusion, had diagnoses of trauma, metastasis, or infection within 90 days prior to surgery, or lacked postoperative activity in the database beyond 90 days. With the service location filter, eligible patients were stratified and matched 1:4 in age range, sex, and the Elixhauser Comorbidity Index (ECI) to create the final inpatient and outpatient cohorts.

Outcome Measures and Statistical Analysis
Outcomes included intraoperative and 90-day postoperative medical and surgical complications (arrhythmia, myocardial infarction, stroke, atelectasis, respiratory failure, pleural effusion, pulmonary embolism, urinary retention, urinary tract infection, renal failure, and sepsis), 30-day and 90-day emergency department (ED) visits, 30-day and 90-day inpatient readmissions, day-of-surgery reimbursement, and 90-day global reimbursement. Where appropriate, Welch’s t-tests, Chi-squared tests, and Kruskal-Wallis tests were performed for between-group comparisons. Kaplan-Meier survival analysis with log-rank testing was performed to assess two-year survival probabilities to subsequent lumbar fusion. All analyses utilized the R software integrated within the PearlDiver interface. All hypothesis tests were two-sided, with P < 0.05 denoting statistical significance.

Results

Utilization Trends
The annual volume of single-level IPD procedures with decompression declined but the proportion performed in the ambulatory setting increased over time from 71.6% in 2017 to 87.4% in 2022 (Figure 1). This shift in service location motivated the comparison of outcomes between settings that follows.

Cohort Characteristics
Patients undergoing inpatient and outpatient procedures were matched 1:4 on age, sex, and ECI to limit confounding by patient factors known to influence site-of-service selection, yielding 412 patients (20.6%) in the inpatient cohort and 1587 (79.4%) in the outpatient cohort. Groups were well-balanced with respect to age (mean ± SD; 69.7 ± 9.0 vs. 69.7 ± 8.7 years; p = 0.893), sex (50.5% vs. 50.7% female; p = 0.993), and Elixhauser Comorbidity Index (mean ± SD; 3.0 ± 3.2 vs. 3.0 ± 3.0; p = 0.721), with no individual comorbidity differing significantly between cohorts (Table 1).

Clinical Outcomes and Healthcare Utilization
No intraoperative or 90-day postoperative complication differed significantly between inpatient and outpatient cohorts, including sepsis (0.5% vs 0.3%, p = 0.790), pseudarthrosis (7.3% vs 5.6%, p = 0.245), and dural tear (1.7% vs 0.9%, p = 0.298). Because complication codes alone do not fully reflect the resource burden of postoperative events, ED visits and inpatient readmissions were also assessed; rates were similar at both 30 days (ED visits, 5.8% inpatient vs 6.0% outpatient, p = 0.858; readmissions, 2.2% inpatient vs 2.2% outpatient, p = 1.000) and 90 days (ED visits, 9.7% inpatient vs 9.0% outpatient, p = 0.732; readmissions, 3.4% inpatient vs 3.4% outpatient, p = 1.000). Kaplan-Meier analysis did not demonstrate differences in two-year fusion-free survival following inpatient or outpatient IPD procedures (log-rank p = 0.30).

Costs
Reimbursements were compared to determine whether the observed shift to ambulatory care carried an economic impact. Associated costs were higher in the inpatient setting both on the day of surgery (mean ± SD; inpatient: $2505 ± $4804 vs. outpatient: $1976 ± $3391; p < 0.001) and over a 90-day period postoperatively (mean ± SD; inpatient: $5234 ± $6539 vs. outpatient: $4197 ± $5514; p < 0.001). Taken together, outpatient IPD placement demonstrated lower costs with no significant differences in complication or fusion-free survival rates.

Discussion

Interspinous process devices (IPDs) have emerged as a less invasive alternative to traditional open decompression with fusion for the treatment of lumbar spinal stenosis (LSS), offering symptom relief with dynamic stabilization and less tissue disruption and expedited recovery.¹˒⁴⁻⁶ Given these characteristics, IPD procedures are well-suited to the growing shift of spine surgeries toward outpatient and ambulatory care settings.³˒⁸˒¹²˒¹⁴ 

During this study period, the total volume of IPDs performed decreased annually, while the proportion done in ambulatory settings increased steadily. This drop may partly reflect the withdrawal of early IPD devices like X-STOP in 2015, or it may also represent the continued debate on IPD outcomes compared to those of gold-standard open decompression in treating LSS.⁴⁻⁶ Prior studies have reported mixed long-term outcomes following IPD placement, including higher reoperation rates and device-related failures in some patient populations.¹⁵˒¹⁶ Tamburrelli and colleagues identified errors in indication as a leading contributor to IPD failure, underscoring the importance of adherence to appropriate clinical criteria when considering these devices.¹⁷ 

Importantly, the decline in overall procedure volume observed in this study was not uniform across settings. Inpatient volume fell much more steeply than ambulatory volume over the study period, indicating that the rising ambulatory proportion reflects the contraction of inpatient IPD placement rather than an increase in outpatient procedures. As a result, the smaller number of procedures still performed in the inpatient setting represent an increasingly selected group, and characterizing whether that selection is warranted remains relevant even as overall utilization declines. 

In this claims-based analysis, outpatient IPD placement was associated with lower costs, while rates of short-term complications and healthcare utilization were similar between sites of service. These findings are consistent with Ortega et al., who reported similar reoperation rates for IPDs across operative environments, as well as with broader literature supporting outpatient spine surgery as a cost-effective alternative.⁸˒¹⁰⁻¹² In these cases, cost savings have been attributed to differences in care pathway and length of stay.¹⁸ Notably, the cost differences observed here persisted after matching on age, sex, and comorbidity burden. Comparable patients incurred different costs depending on operative setting, which suggests that structural factors, including reduced facility fees and the elimination of overnight stays, contribute independently of patient complexity.⁷ Spine surgery remains a significant financial burden on the healthcare system, yet extensive postoperative monitoring is not required in certain cases.¹⁹˒²⁰ Therefore, procedural setting is an important consideration in value-based care.²¹ With appropriate patient selection, ambulatory care settings could offer a reasonable alternative for patients receiving IPDs for LSS. 

Certain populations may benefit from IPDs over more invasive procedures. For instance, Gong and colleagues reported that Medicare patients experienced a 300% increase in IPD placement, trending toward outpatient and ambulatory care centers and away from inpatient settings from 2017 to 2020.¹⁴ Given that ambulatory care settings have a predilection for less complex patients, it may be the case that IPDs are being targeted toward these populations.¹⁹ On the other hand, more complex patients, such as elderly individuals with lumbar degeneration, may be better suited for inpatient and more traditional techniques due to more advanced pathology and higher comorbidity burdens.² 

This patient-selection pattern may also contribute to the perceived superiority of outpatient spine surgeries to inpatient spine surgeries. Mooney et al. observed that their inpatient cohort undergoing lumbar decompression had significantly higher rates of comorbidities when compared to the outpatient cohort. These comorbidities put them at greater risk of an extended hospital stay if their length of stay was already past 72 hours.¹¹ Taken together, these insights suggest that apparent differences between operative settings may partially reflect differences between the patients selected for each rather than the settings themselves. The present study attempted to control for this bias using the Elixhauser Comorbidity Index (ECI), a validated predictor of comorbidity burden and perioperative complications in spine surgery.²²˒²³ However, this measure may not fully account for important factors affecting site-of-service decisions, such as disease severity, functional status, surgeon judgement, or other unmeasurable clinical and surgical parameters.¹⁹ As a result, a potentially less complex outpatient population in this study may make these matched cohorts clinically inequivalent. 

Other limitations of this study were inherent to the reliance on claims-based data, which are susceptible to coding errors and inconsistencies across institutions.²⁴ Furthermore, these datasets are often of limited granularity, as claim records do not contain comprehensive preoperative labs and imaging, or intraoperative details.²³ Studies investigating heterogeneous diseases or patient populations, like those seen undergoing surgery for LSS, may therefore have difficulty extracting factors like the severity of disease that may help further stratify outcomes. For similar reasons, costs are estimates of charges billed rather than actual payments received by providers or patients’ out-of-pocket expenses, and they do not account for variation in payer mix, reimbursement structure, or regional pricing. Cost comparisons are similarly vulnerable to confounding, as surgical complexity, disease severity, and device selection influence reimbursement independent of operative setting. Lastly, claims-based datasets are of limited generalizability, as their patient population may not be representative of the broader population.²⁵ 

Nevertheless, this study provides the first national comparison of inpatient and outpatient single-level IPD placement using a procedure-specific billing code, and benefitted from a large sample size and broad scope. Future studies may focus on identifying appropriate patient selection criteria and defining the factors contributing to cost differences between procedure settings.

Conclusion

Outpatient single-level IPD insertion with decompression is associated with lower costs without significant differences in intraoperative or postoperative complications, ED visits, or hospital readmissions when compared to inpatient procedures. These findings support that site-of-service decisions may represent an opportunity to reduce costs, though residual differences between cohorts limit conclusions about the independent effect of operative setting on safety.

IRB Approval

This study used deidentified, publicly available administrative claims data. Institutional Review Board (IRB) review and approval were therefore not required