1 Introduction

Rates of lumbar spine surgery in the United States have increased considerably in the past few decades, particularly in the Medicare population.^1,2 Moreover, minimally invasive techniques have become increasingly utilized for the treatment of lumbar spinal stenosis (LSS) and are often selected over traditional approaches as they appear to reduce morbidity, increase operating room efficiency, and potentially decrease costs.³ Minimally invasive approaches have also been associated with shorter hospital stays and time to mobilization, as well as a reduced rate of opioid use, which can greatly impact patient satisfaction and outcome.⁴

A factor that has been found greatly associated with outcome in the treatment of LSS is patient age. Elderly patients often have a greater burden of increasing comorbidities which can ultimately decrease the rate of improvement and/or overall outcome, further confounding this relationship.

Given the current trends, it is important to consider whether differences exist in microdecompression outcomes in elderly patients compared to the general adult population. This study investigated differences between patients <65 and patients ≥65 years of age following microdecompression for symptomatic LSS. Patient characteristics, surgical outcomes, and patient-reported outcomes (PROMs) over a two-year follow-up period are reported. The secondary purpose of this review was to compare costs associated with these procedures between those <65 and those ≥65 years of age, within Medicare and a private payer (Humana).

2 Methods

A retrospective review of patients enrolled in the senior author's surgical outcomes database was performed. Briefly, this database enrolls patients prospectively before surgery and captures a variety of validated PROMs. Patient enrollment began in January 2014 following Institutional Review Board approval of the protocol. The resulting database of consecutive lumbar microdecompression surgeries performed by a single, fellowship-trained orthopaedic spine surgeon is maintained regularly and PROMs are collected with patient visits at regular intervals.

Inclusion criteria for this study were patients ≥18 years of age with symptomatic LSS who underwent elective microdecompression of the lumbar spine. Patient selection for surgery was based on preoperative evaluation by the senior author and included persistence of neurologic deficits after at least 90 days of conservative non-operative treatment (e.g., activity modification, non-steroidal anti-inflammatory drugs, physical therapy, and/or exercises), absence of instability on radiographs, and preoperative magnetic resonance imaging to confirm the presence of pathoanatomy amenable to microdecompression (e.g., central, lateral, and/or foraminal stenosis). Exclusion criteria included deceased patients and non-index microdecompression cases.

All procedures were performed by a single surgeon using previously described techniques.^5–9 Microdecompression procedures included unilateral hemilaminectomy (Fig. 1), unilateral laminotomy with bilateral decompression (Fig. 2), and far lateral decompression of the neuroforamen (Fig. 3). The METRx™ Tubular Retractor System (Medtronic Sofamor Danek, Memphis, TN) was utilized in all cases.

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Fig. 1 Illustration of unilateral hemilaminectomy using a tubular retractor.

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Fig. 2 Illustration of unilateral laminotomy and bilateral decompression using a tubular retractor.

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Fig. 3 Illustration of a far lateral decompression using a tubular retractor.

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PROMs for this study included visual analog scale (VAS) for back pain and leg pain, Oswestry Disability Index (ODI), and EuroQol-Five Dimensions Index (EQ-5D).^10,11 Data were collected during office visits; in some cases, postoperative data were collected via a standardized telephone script. As ODI is a composite score based on responses to separate questions, the following correction factor was applied in cases where one or more ODI questions were left unanswered.¹²CorrectedODI=ODI×(1010−numberofmissingresponses)

EQ-5D indices were calculated using a validated valuation model for United States patient populations.¹¹ No corrections were applied to the scores recorded for VAS.

Patient characteristics, comorbidities, and operative details were retrieved from electronic medical records. The operative time reported is the operative time per level; i.e. two-level decompression operative times were reduced by a factor of two. Spondylolisthesis and scoliosis measurements were obtained from preoperative standing plain film radiographs of the lumbar spine and measured by a single reviewer. All cases of spondylolisthesis in this study were degenerative and non-lytic.

3 Results

The senior author's database of 324 consecutive microdecompression cases performed between January 2014 and June 2019 was queried. 159 consecutive cases would have had two-year follow up at time of analysis. 11 patients were deceased. 4 additional cases were non-index microdecompressions. The analyzed sample size was comprised of 144 living primary microdecompression patients with two-year outcomes available.

The <65 age group was comprised of 67 patients (46.5%). The ≥65 age group was comprised of 77 patients (53.5%). Unilateral hemilaminectomy was performed in 35 patients (24.3%), unilateral laminotomy and bilateral decompression was performed in 77 patients (53.5%), and far lateral or isolated foraminal decompression was performed in 32 patients (22.2%).

3.2

3.2 Patient-reported outcomes measures

Despite patients <65 years of age having greater preoperative ODI (56.93 ± 14 vs. 46.37 ± 19.53, p = 0.005) and one-year back pain VAS (3.66 ± 3.18 vs. 2.43 ± 2.61, p = 0.017), there was no difference in any two-year PROMs between the two age groups. The linear mixed effects models confirmed significant improvement in PROMs compared to baseline at one- (p < 0.001) and two-years (p < 0.001), without decline between one- and two-years (p > 0.05). Age group had a significant effect on back pain improvement (p = 0.016), but not on leg pain (p = 0.377), ODI (p = 0.063), or EQ-5D (p = 0.138) improvement (Table 2, Fig. 4).

Table 2 Comparison of patient-reported outcome measures preoperatively, at 1-year follow-up, and at 2-year follow-up for patients included in this study. A linear mixed effects model was fit to the data in order to accomodate for missing outcomes; the model included a random effects term to account for repeated measures on individual patients. *Denotes significance, p < 0.05, of patient-reported outcome measure differences between age groups at each visit time (marginal), or of longitudinal differences compared to previous visits (preoperative, 1-year), or between age groups (age).

	Back Pain		Leg Pain		ODI		EQ-5D
	Age <65	Age ≥65	Age <65	Age ≥65	Age <65	Age ≥65	Age <65	Age ≥65
Preoperative
Mean ± SD	6.64 ± 3.10	6.46 ± 3.06	7.69 ± 2.05	6.99 ± 2.71	56.93 ± 14.00	46.37 ± 19.53	0.31 ± 0.32	0.40 ± 0.31
n	55	70	58	70	44	57	53	62
p (marginal)	0.770		0.202		0.005*		0.102
1-year
Mean ± SD	3.66 ± 3.18	2.43 ± 2.61	2.43 ± 3.15	1.99 ± 2.86	25.91 ± 18.92	19.90 ± 17.4	0.71 ± 0.32	0.79 ± 0.24
n	62	70	60	70	57	61	61	71
p (marginal)	0.017*		0.379		0.065		0.141
p (preoperative)	<0.001*		<0.001*		<0.001*		<0.001*
2-year
Mean ± SD	3.68 ± 3.09	2.79 ± 2.81	2.91 ± 3.15	2.59 ± 2.93	27.02 ± 19.79	24.38 ± 16.95	0.69 ± 0.31	0.71 ± 0.28
n	57	70	57	69	56	68	51	67
p (marginal)	0.095		0.582		0.365		0.558
p (preoperative)	<0.001*		<0.001*		<0.001*		<0.001*
p (1-year)	0.618		0.306		0.126		0.211
p (age)	0.016*		0.377		0.063		0.138

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Fig. 4 Back pain and leg pain visual analog scale, Oswestry Disability Index, and EuroQol-Five Dimensions Valuation vs. Time, categorized by age group. *p < 0.05 **p < 0.01 ***p < 0.001.

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4 Discussion

This study demonstrates comparable outcomes between elderly and general adult patients undergoing lumbar microdecompression. The PROMs that were collected in this study were selected on the basis of capturing subjective pain reporting, an objective measure of disability, and a valuation of health state.^10,11,13,14 Patients demonstrated significant improvements in PROMs at one- and two-years, without decline between one- and two-years. The current literature defining minimum clinically important differences (1.2, 1.6, and 12.8 for back pain, leg pain, and ODI, respectively) suggests that the improvement observed in this study is not only statistically significant, but also clinically significant.¹⁵ Age group had a moderate effect on back pain improvement over time; the ≥65 age group showed greater improvement. One possible explanation is that the etiology of back pain in the two age groups may be unevenly distributed, perhaps patients <65 years of age more likely to have arthritic back pain and patients ≥65 years of age more likely to have neurogenic back pain that is more amenable to decompression. Medicare charges and reimbursements are comparable between the two age groups, which supports the hypothesis that microdecompression is equally cost-effective in both elderly and general adult populations. Humana reimbursements appeared greater in patients <65 years of age. Given that Humana is a secondary payer, this discrepancy may be explained by patients ≥65 years of age having costs partially covered by Medicare.

Other studies have shown that surgery is superior to non-operative treatment of lumbar spinal stenosis, yielding better improvement in ODI, bodily pain, and physical function scores on the Medical Outcomes Study 36-item Short-Form General Health Survey.^2,16 Surgical treatment results in 1.64 total mean discounted quality-adjusted life-years (QALYs), while non-operative treatment is statistically significantly inferior, resulting in 1.44 total mean discounted QALYs.² Nonetheless, the validity of these findings has not been well documented previously in regards to age. In intervertebral disc herniation, the estimated the cost per QALY gained for surgical treatment relative to non-operative care is $69,403 in the general population and $34,355 in the Medicare population, demonstrating the value-added of these procedures.²

This study has certain limitations: it is a retrospective case series with limited sample size and setting. Nonetheless, evaluating 144 patients with 2-year outcomes of new surgical techniques assessed in a variety of populations provides suggestive findings for larger multicenter studies. Another limitation to this study is the lack of age-matched non-operative controls. Yet there is still debate whether non-operative controls serve better as controls than disease-specific, older controls as performed here, given that non-operative patients may undergo surgery in the future. Other studies have demonstrated high rates of non-adherence to randomized treatment group, which would not only pose logistic challenges, but also ethical challenges.^2,16 Furthermore, a third limitation is the lack of analysis of age specific cut-off points to determine an age where surgery no longer provides value. This is an ongoing research endeavor at our institution as data collection continues, outcomes of patients ≥80 years will be later described.^3,17–19

5 Conclusion

Microdecompression is an effective treatment option for symptomatic LSS in both elderly and general adult populations. Patients ≥65 years of age may experience greater relief in back pain following microdecompression. Nonetheless, pain, disability, and health state are comparable at two-years post-operatively, despite differences between age groups. The charge associated with lumbar decompression is comparable in patients <65 and patients ≥65 years of age, however there may exist differences in reimbursement amounts.

Funding sources

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

Declarations of interest

None.

Institutional review board approval

IRB00042783.