Risk Factors for Failing to Reach a Minimal Clinically Important Difference Following Minimally Invasive Lumbar Decompression

DISCUSSION

Traditionally, patient improvement has been gauged based on both radiographic measures and clinical evaluations. Over time, PROMs have become an additional asset to the evaluation of postoperative improvement. However, while PROMs may provide statistical insight into a significant change in symptoms, MCID may provide a more patient-centered assessment of a clinically important change. Understanding risk factors associated with failure to achieve an MCID may allow surgeons greater insight as to which individuals are most likely to experience meaningful benefits from surgery. While previous studies have established risk factor assessment following both cervical and transforaminal lumbar fusions,5,6 LD constitutes both a different procedure as well as a different subset of indications. This study investigated the demographic and perioperative characteristics that may contribute to a higher risk for failure to achieve an MCID for five different commonly used PROMs. We present the most prevalent risk factors for failure to achieve an MCID as they relate to patient demographics, operative characteristics, and underlying spinal pathology.

Among the 5 PROMs assessed for failure to reach an MCID, all outcome measures demonstrated that a majority of patients (>80%) were able to achieve a clinically important difference. For those patients who were unable to achieve an MCID, the demographic variables which had the most significant impact were largely factors which contribute to comorbidity burden. More specifically, ageless CCI was the most common significant risk factor across all PROMs, except PROMIS PF, and age similarly increased the risk for failure to achieve an MCID for all outcome measures except VAS back. Although few, if any, previous studies have examined MCID risk factors for MIS LD, a number of investigators have suggested that comorbidity burden increases risk for unfavorable outcomes in a number of outcome measures. Paulsen et al20 reported that poorer satisfaction following decompression surgery for spinal stenosis was associated with patient comorbidities, smoking, and duration of symptoms. A similar finding was demonstrated in a study of laminectomy patients where a lower CCI was also associated with an overall favorable functional outcome.21 However, other studies have suggested that CCI does not predict worse outcomes for lower back pain and disability and instead suggest that BMI may be a stronger factor.22 Even with contrasting results, our analysis demonstrated that the comorbidity burden placed patients at increased risk for failed achievement of an MCID for not just one but multiple PROMs. This may indicate that special considerations or patient counseling should be made for patients who carry a larger comorbidity burden.

Although age is a part of the CCI score, we opted to analyze the ageless CCI to determine the effect of advanced age on risk for MCID failure in isolation. Not surprisingly, age, as a risk factor for failure to achieve an MCID, coincided with PROMs that also identified ageless CCI as a risk factor. Although age is a well-known risk factor for poorer outcomes, only a select few studies have established this for LD outcomes. Krutko et al23 reported that younger patients had a “more successful” treatment for lumbar disc herniations and was substantiated by a study that reported that deterioration of outcomes, defined as an 8-point increase in ODI, was associated with age, smoking, ASA score, and previous operations at the same or other lumbar levels.24 This result is of interest because of the age cutoff we set at 50 years. Trends in lumbar disc herniation indicate that the highest incidence of lumbar disc herniation occurs in the lower lumbar spine in patients aged 25–55 years, but other studies demonstrate that it has a smaller impact among elderly patients.25,26 Regardless, it can be inferred that the elderly may need to curb expectations and clinicians may need to adjust their preclinical counseling for MIS LD patients.

Interestingly, although not specifically included in the CCI, BMI was also identified as a risk factor only for PROMIS PF. Previous studies among decompression patients were able to also identify BMI as a risk factor for poorer operative outcomes and failure to reach an MCID following MIS LD.27

One of the most interesting aspects of this study was identifying insurance status as a potential risk factor influencing MCID failure for VAS back. This relationship was also estimated by LASSO and confirmed by postestimation regression analysis. More specifically, being listed as having workers’ compensation was associated with an increased risk for MCID failure, which may be attributed to more severe nonage-related spinal pathology of the intervertebral disc space in this population. Previous MCID studies reported that worker’s compensation was a risk factor for outcomes in transforaminal lumbar fusion patients6 and was similarly identified in a systematic review, which established the association between compensation status and poorer postoperative outcomes.28,29 Other studies also implicate insurance collected as a prognostic factor, as achievement of MCID and sustained improvement of lower back pain following decompression surgery was reportedly associated with lack of compensation claims and absence of narcotic usage.30 Furthermore, Koerner et al31 also demonstrated that having no litigation pending and use of nonworkers’ compensation payment sources were associated with better treatment effects at 2 years. With a depth of evidence, these results, in addition to our results, may indicate that workers’ compensation patients endure a more severe spinal pathology and therefore a lower chance of achieving an MCID. Unfortunately, workers’ compensation status is beyond the control of surgeons and may be relatively nonmodifiable, but our study demonstrated that a number of modifiable operative characteristics were associated with failure to achieve an MCID and may be of greater interest.

Categorization of an LD procedure may vary across different providers; however in the current study, procedures were limited to laminectomy, discectomy, or a combination of both performed in conjunction with foraminotomy and facetectomy. Patients who underwent a laminectomy procedure without discectomy were noted to have an increased risk for failing to achieve an MCID for VAS leg and ODI only. These results also coincide with both central and foraminal stenosis being significant risk factors for failed VAS leg MCID achievement. Previous studies comparing revision rates between decompression and fusion patients noted that at 1 year, rates were higher among patients who underwent an index-level decompression surgery, which included laminectomy alone, as compared to spinal fusion procedures.32 However, authors of the same study noted that revision rates by the 4-year timepoint were identical. It may be inferred by these results that laminectomy with foraminotomy and facetectomy for the treatment of degenerative changes may require additional recovery time to resolve leg pain and associated disability. It is also interesting to note that performing a laminectomy and discectomy demonstrated a protective effect for failing to achieve an MCID for leg pain and physical function; however, only results for VAS leg reached significance. This coincides with the observation that a spinal pathology of HNP also was protective against failed MCID.

Operative characteristics such as blood loss, number of levels decompressed, and underlying spinal pathology are significant risk factors for failure to reach an MCID following MIS LD. Although operative time and blood loss are believed to be associated with one another, there was no direct correlation between operative variables. However, these operative risk factors have both demonstrated negative impacts on postoperative outcomes and been associated with increased incidence of complications. A large spine surgical registry study evaluated over 4000 patients who were treated for lumbar spinal stenosis and determined that in addition to increased VAS back pain at baseline, the vertebral levels involved were associated with negative outcomes.16 Alternatively, there is evidence that the invasiveness of a surgical procedure, number of levels decompressed, fused, or instrumented and approach type may predict risk for poorer outcomes.33 Interestingly, among our results, EBL and number of levels decompressed were suggested as risk factors for only VAS leg, ODI, and PROMIS PF, a finding supported by LASSO postestimations for VAS leg and PROMIS PF only. These results suggest that for a common procedure such as MIS LD, operative characteristics that may prolong the procedure and increase blood loss can result in lower chance of reaching an overall MCID. This finding places further emphasis on the surgeon’s ability to discern the necessity of an additional operative level and may aid in providing guidance for future multilevel MIS LD procedures.

MIS LD is believed to be an effective treatment for symptomatic lower back pain. However, the current study suggests that the spinal pathology clinical symptoms may predict risk of failure to achieve an MCID for a number of commonly used PROMs. Of particular interest is the reduced risk that was associated with having a spinal diagnosis of HNP for all PROMs except for VAS back. This was further supported by our results from LASSO, which suggested HNP was a protective factor for VAS leg and SF-12 PCS. Among the most common spinal diagnoses in our cohort, HNP is believed to be highly amenable to treatment, with numerous studies reporting favorable surgical outcomes. Although our study does support these findings, the severity of herniation must be addressed as a number of studies have suggested its influence on postoperative outcomes. Specifically, transligamentous, caudal migrations, or large annular defects can lead to increased recurrence of herniations and poorer outcomes.34–37 Given that our cohort demonstrated a reduced relative risk associated with HNP, it may be presumed that these patients underwent treatment of relatively uncomplicated symptomatic disc herniations. Collectively, these results suggest that more complex spinal pathologies and procedures may increase the risk for a failed MCID.

Use of MCID to objectively assess postoperative outcomes may provide benefits to the clinician, but it also has its share of limitations. Achievement of MCID may provide context to the magnitude of improvement in terms of patient satisfaction, as prior studies have anchored their established thresholds on satisfaction questionnaires.38–40 Although achievement and nonachievement may represent a “satisfied” and “not-satisfied” patient, there are inherent limitations to this type of analysis, and may not be a suitable replacement for other objective measures such as physical function tests (time to ambulation, dynamometer, range of motion, and gait assessment). Even though the current study provides potential baseline characteristics that are associated with a failure to achieve a meaningful difference, from the perspective of the patient, it may not be a suitable replacement of continuous measures that other clinicians may rely on to determine the acceptable level of postoperative improvement.