Patient-reported outcomes unbiased by length of follow-up after lumbar degenerative spine surgery: Do we need 2 years of follow-up?

BACKGROUND: In modern clinical research, the accepted minimum follow-up for patient-reported outcome measures (PROMs) after lumbar spine surgery is 24 months, particularly after fusion. Recently, this minimum requirement has been called into question.

PURPOSE: We aim to quantify the concordance of 1- and 2-year PROMs to evaluate the importance of long-term follow-up after elective lumbar spine surgery.

STUDY DESIGN: Retrospective analysis of data from a prospective registry.

PATIENT SAMPLE: We identified all patients in our prospective institutional registry who underwent degenerative lumbar spine surgery with complete baseline, 12-month, and 24-month follow-up for ODI and NRS back and leg pain.

OUTCOME MEASURES: Oswestry Disability Index (ODI) and numeric rating scales for back and leg pain at 1 year and at 2 years.

METHODS: We evaluated concordance of 1- and 2-year change scores by means of Pearson's product-moment correlation and performed logistic regression to assess if achieving the minimum clinically important difference (MCID) at 12 months predicted 24-month MCID. Odds ratios (OR) and their 95% confidence intervals (CI), as well as model areas-under-the-curve (AUC) were obtained.

RESULTS: A total of 210 patients were included. We observed excellent correlation among 12- and 24-month ODI (r = 0.88), NRS-LP (r = 0.76) and NRS-BP (r = 0.72, all p < 0.001). Equal results were obtained when stratifying for discectomy, decompression, or fusion. Patients achieving 12-month MCID were likely to achieve 24-month MCID for ODI (OR: 3.3, 95% CI: 2.4 to 4.1), NRS-LP (OR: 2.99, 95% CI: 2.2 to 4.2) and NRS-BP (OR: 3.4, 95% CI: 2.7 to 4.2, all p < 0.001) with excellent AUC values of 0.81, 0.77, and 0.84, respectively. Concordance rates between MCID at both follow-ups were 87.2%, 83.8%, and 84.2%. A post-hoc power analysis demonstrated sufficient statistical power.

CONCLUSIONS: Irrespective of the surgical procedure, 12-month PROMs for functional disability and pain severity accurately reflect those at 24 months. In support of previous literature, our results suggest that 12 months of follow-up may be sufficient for evaluating spinal patient care in clinical practice as well as in research.