, Introduction
With the global population aging rapidly, the number of surgeries performed in elderly patients continues to rise. In this demographic, surgical procedures are typically associated with higher risks of morbidity and mortality. As a result, growing attention has been directed toward the development of tools that can more accurately predict postoperative outcomes based on a patient's overall health status rather than age alone.
The concept of the frailty index (FI) was developed in response to this need. FI is a quantitative measure of a patient’s physiological reserve, calculated based on the accumulation of deficits across up to 70 variables encompassing medical history, physical examination findings, and functional ability [1,2].
Numerous studies across various surgical fields—including general surgery, gynecology, thoracic surgery, and orthopedics—have demonstrated that higher FI scores are significantly associated with increased rates of complications and mortality [3–6]. These findings underscore the utility of FI in preoperative risk stratification and screening. By identifying high-risk individuals before surgery, clinicians can improve survival outcomes through optimized perioperative management and interdisciplinary approaches [7].
In the field of spine surgery, similar associations between frailty and surgical outcomes have been reported. Studies involving adult spinal deformity correction, anterior cervical discectomy and fusion, and lumbar fusion have revealed that higher FI scores correlate with poorer outcomes, including increased complication rates, mortality, and prolonged hospitalization [8–10].
Unilateral biportal endoscopic (UBE) decompression has recently gained traction as a minimally invasive alternative for the treatment of lumbar spinal stenosis (LSS). Compared with microscopic decompression, UBE decompression offers more rapid pain relief and shorter hospital stays [11]. Given these characteristics, UBE decompression may be particularly beneficial for frail elderly patients who are at higher risk from conventional open surgery. However, no study has directly investigated the relationship between FI and outcomes following UBE decompression for LSS. The aim of this study was to examine the correlation between the FI and clinical outcomes following UBE decompression, and to evaluate whether this minimally invasive procedure is a suitable surgical option for frail patients with LSS.
Material and Method
Study design and patient selection
This retrospective study included 85 patients aged 60 years and older who underwent sinlge level UBE unilateral laminectomy with bilateral decompression (UBE-ULBD) at single-center between January 2015 and December 2020. Inclusion criteria were patients diagnosed with LSS who underwent UBE-ULBD and had complete preoperative and postoperative clinical data. Exclusion criteria included patients with multilevel surgery, prior lumbar spine surgery, spinal infections, tumors, or incomplete medical records.
Frailty and age group classification
Patients were categorized based on their frailty status and age. The FI was assessed using the modified FI-11 (mFI-11) which was calculated based on preoperative clinical assessments.
mFI-11 incorporates comorbidities and functional impairments, including diabetes mellitus, hypertension requiring medication, congestive heart failure, chronic obstructive pulmonary disease or pneumonia, history of myocardial infarction, history of percutaneous coronary intervention or cardiac surgery or angina, peripheral vascular disease or rest pain, impaired sensorium, cerebrovascular accident with or without neurological deficit, and functional dependency (partially or totally dependent) [12]. Each deficit is assigned a score of 1 if present and 0 if absent. The total score is calculated by summing the present deficits and dividing by 11, resulting in a continuous frailty score ranging from 0 to 1.
Patients were also stratified into 3 age groups: 60s, 70s, and 80s. This classification was used to examine the differential impact of frailty and aging on postoperative outcomes.
Surgical procedure
All patients underwent UBE-ULBD performed by experienced spine surgeons using a standard minimally invasive technique. The procedure involved unilateral access through 2 small incision, followed by laminotomy and contralateral decompression. Intraoperative parameters such as estimated blood loss, operative time, and any complications were recorded.
Complication and outcome measures
Clinical outcomes were assessed using the visual analog scale (VAS) for pain, the Oswestry disability index (ODI), and Macnab’s criteria for patient-reported satisfaction. The length of hospital stay (LOS) was also recorded. These outcome data were collected at postoperative a year. Complications evaluated included dura tear and epidural hematoma.
Statistical analysis
Descriptive statistics were used to summarize baseline characteristics. The Kruskal-Wallis test was used to compare differences in non-normally distributed variables among groups. One-way analysis of variance (ANOVA) was applied to analyze normally distributed continuous variables. Post hoc analyses were performed using the Bonferroni correction when significant differences were observed. Categorical variables were analyzed using the chi-square test or Fisher’s exact test as appropriate. Correlation analyses were performed to evaluate associations between mFI-11, age, and clinical outcomes. A significance level of P<0.05 was considered statistically significant. Statistical analyses were conducted using IBM SPSS ver. 22.0 (IBM Corp.).
Comparison by age and frailty index
To evaluate the relationships between age, mFI-11, and postoperative outcomes, Pearson’s correlation analysis was performed for continuous variables, and categorical variables were analyzed using the chi-square test or Fisher’s exact test as appropriate.
Age group analysis
Patients were stratified into 3 age groups (60s, 70s, and 80s and older), and intergroup comparisons were performed using the Kruskal-Wallis test for non-normally distributed variables and one-way ANOVA for normally distributed variables.
Subgroup analysis based on complications
Patients were further divided into 2 groups based on the presence or absence of postoperative complications. An independent t-test was used to compare continuous variables, and the Mann-Whitney U test was applied for non-normally distributed data. Chi-square or Fisher’s exact test was used for categorical variables.
Results
Patient demographics
A total of 85 patients aged 60 years and older were included in this study. The mean age of the patients was 72.2±7.9 years. The proportion of patients in their 60s, 70s, and 80s was 33 (38.8%), 32 (37.6%), and 20 (23.5%), respectively. The preoperative and postoperative clinical characteristics, including VAS, ODI, and Macnab’s criteria, are summarized in Table 1. Patients were stratified into 3 groups based on their mFI-11 scores: 0.00 (n=20, 23.5%), 0.09 (n=38, 44.7%), and ≥0.18 (n=27, 31.8%), as presented in Table 2.
Comparison by age and mfi-11
Age showed a significant positive correlation with hospital stay duration (r=0.265, P=0.015), indicating that older patients tended to have a longer hospital stay. Age was also significantly associated with postoperative VAS scores (r=0.312, P=0.004), suggesting that older patients experienced less pain improvement. Macnab’s criteria demonstrated a significant correlation with age (r=–0.298, P=0.007), indicating that older patients reported lower satisfaction rates after surgery. No significant correlation was found between mFI-11 and hospital stay duration (r=0.052, P=0.637), postoperative VAS scores (r=0.121, P=0.263), or Macnab’s criteria (r=–0.144, P=0.184). Among the complications, there were 2 cases of dura tear and 3 cases of epidural hematoma, but the number of patients with complications was not statistically significant (Table 3).
Age group analysis
Postoperative VAS scores differed significantly among age groups (P=0.012). Post hoc analysis using Bonferroni correction revealed that patients in their 80s had significantly higher postoperative pain scores compared to those in their 60s (P=0.008). Macnab’s criteria also varied significantly by age group (P=0.021), with post hoc analysis showing that patients in their 80s had significantly lower satisfaction scores than those in their 60s (P=0.017). LOS increased with age (P=0.033). Post hoc analysis confirmed a significant difference between the 60s and 80s groups (P=0.029).
Subgroup analysis based on complications
There was no statistically significant difference in age between the complication and non-complication groups (P=0.136). mFI-11 did not significantly differ between these 2 groups (P=0.262). These findings suggest that age is a significant factor influencing postoperative pain, functional recovery, and hospital stay duration following UBE-ULBD surgery, whereas mFI-11 does not appear to be a strong predictor of postoperative prognosis.
Discussion
In this study, we found that mFI-11 was not significantly correlated with postoperative outcomes following UBE-ULBD, including hospital stay duration (r=0.052, P=0.637), postoperative VAS scores (r=0.121, P=0.263), and Macnab’s criteria (r=–0.144, P=0.184). These findings contrast with previous studies on spine surgery, which have reported a strong association between frailty and postoperative morbidity. For example, Shin et al. [9] investigated the relationship between frailty and outcomes in posterior cervical fusion and found that complication rates significantly increased with higher mFI-11 scores. Patients with mFI-11 ≥0.36 had a 41.26-fold increased risk of major complications (odds ratio [OR], 41.26; P<0.001). Similarly, in the study by Leven et al. [10] on adult spinal deformity surgery, while age ≥60 was predictive of certain complications, mFI-11 ≥0.18 was more strongly associated with complications (OR, 1.6; P=0.01) and reoperation (OR, 2.3; P=0.017). In particular, patients with an mFI-11 score of 0.27 experienced complication rates up to 60%, mortality rates of 10%, and reoperation rates of 15%. Furthermore, in multilevel posterior spinal fusion for spinal deformity, mFI-11 ≥0.09 was significantly associated with increased rates of complications, reoperations, thromboembolic events, and mortality (P<0.05).
The lack of a significant association in our study may be attributed to the minimally invasive nature of UBE-ULBD, which is characterized by shorter operative time, reduced intraoperative blood loss, and less surgical trauma. These factors may contribute to favorable postoperative outcomes, even in frail patients [13,14].
Another noteworthy finding is the significant correlation between age and postoperative outcomes, including hospital stay duration (r=0.265, P=0.015), postoperative VAS scores (r=0.312, P=0.004), and Macnab’s criteria (r=–0.298, P=0.007). Previous studies have explored the effect of age on outcomes in endoscopic spine surgery. Son et al. [15] compared the efficacy of transforaminal endoscopic lumbar discectomy in patients aged ≥65 years versus younger patients and found no significant differences in postoperative pain relief, radiological outcomes, operative time, blood loss, or LOS. Additionally, there was no significant difference in intraoperative complications between the 2 groups (4.46% in the younger group vs. 6.38% in the elderly group; P=0.578, Pearson chi-square test). Chernysh et al. [16] evaluated pain and functional outcomes following full-endoscopic spine surgery in patients with degenerative lumbar disease aged ≥70 years compared to those <70 years. Both groups showed significant improvement in VAS back, VAS leg, and ODI scores at 3 months postoperatively (P<0.001), with no significant difference in the degree of improvement between groups (P>0.05).
The main distinction between previous studies and the present study lies in both the age stratification method and the proportion of older adults included. While earlier studies typically divided patients into 2 broad groups using a single age cutoff (e.g., 65 or 70 years), our study included a larger proportion of elderly patients aged ≥60 years and further subdivided this cohort into decade-based age groups. This approach allowed for a more detailed analysis of age-related trends within the elderly population. As a result, we identified a significant correlation between age and surgical outcomes—particularly in patients in their 60s and those aged ≥80 years—with respect to postoperative VAS scores, hospital stay duration, and Macnab’s criteria. These findings suggest that postoperative outcomes in endoscopic spine surgery may vary substantially across age subgroups within the elderly population. In the context of a globally aging society, our study provides meaningful insights by emphasizing the importance of age-specific analysis, rather than treating the elderly as a homogeneous group.
One possible explanation for the poorer surgical outcomes observed in the very elderly population is the technical challenge of performing endoscopic spine surgery in the presence of advanced degenerative changes. Compared to open surgery, endoscopic procedures offer a narrower field of view and more limited access for instruments [17]. In elderly patients, degenerative changes such as increased scoliotic curvature, vertebral rotation, spondylolisthesis, and facet hypertrophy can further restrict the visual and working corridor, potentially resulting in suboptimal decompression and poorer outcomes [18,19].
Furthermore, due to the widespread nature of age-related degenerative changes in the spine, even when the primary symptomatic lesion is successfully addressed, residual pain or suboptimal patient satisfaction may arise from pathology in adjacent or remote segments.
There were 2 cases of dura tear, with one patient each in their 60s and 70s. According to mFI-11 stratification, one case occurred in the 0.09 group and the other in the ≥0.18 group. One patient was managed with bed rest, while the other underwent dural repair surgery. Three cases of epidural hematoma were identified, with one case each in patients in their 60s, 70s, and ≥80s. Based on mFI-11 grouping, one occurred in the 0.09 group and 2 in the ≥0.18 group. All cases were treated conservatively, including steroid administration. Due to the small number of complications, statistical significance could not be established.
This study has several limitations. First, its retrospective design and the lack of a matched control group may introduce selection bias and limit causal inference regarding the relationship between mFI-11 and surgical outcomes. Second, the limited availability of detailed preoperative data on physiological comorbidities may have affected the accuracy of mFI-11 scoring. Third, the follow-up period of 12 months may be insufficient to capture long-term outcomes, especially reoperation rates.
Future studies should employ prospective, randomized controlled trial designs with standardized frailty assessments and extended follow-up periods. Establishing a comprehensive preoperative data registry would also facilitate more accurate risk stratification and the development of predictive models for outcomes in minimally invasive spinal procedures.
Conclusion
This study found no significant association between mFI-11 and postoperative outcomes following UBE-ULBD. However, age showed a significant correlation with pain improvement, hospital stay duration, and patient satisfaction. These results suggest that chronological age may be a more relevant predictor of surgical outcomes than frailty in elderly patients. Age-stratified assessment should be considered when evaluating candidates for endoscopic spine surgery.
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