Estimates of the fixed effects in the mixed-effects regression modeling of pain and disability change scores over 13 y.
| Parameter | Coefficient β | 95% CI | P Value |
| VAS-B | |||
| Intercept | 60.3 | (58.3, 62.3) | <0.001b |
| Baseline score (centered on 75) | 0.9 | (0.9, 1.0) | <0.001b |
| Time, y | −0.6 | (−1.0, −0.2) | 0.002b |
| VAS-L | |||
| Intercept | 53.1 | (49.0, 57.3) | <0.001b |
| Baseline score (centered on 60) | 0.9 | (0.9, 1.0) | <0.001b |
| Time, y | −0.2 | (−0.5, 0.1) | 0.27 |
| Age groupa coded 1 (35–45 y) | −3.1 | (−8.3, 2.1) | 0.24 |
| Age groupa coded 2 (>45 y) | −7.2 | (−12.3, −2.0) | 0.007b |
| ODI | |||
| Intercept | 35.0 | (33.5, 36.5) | <0.001b |
| Baseline score (centered on 46) | 0.9 | (0.8, 1.0) | <0.001b |
| Time, y | 0.1 | (−0.1, 0.4) | 0.22 |
| RMDQ | |||
| Intercept | 13.7 | (13.2, 14.1) | <0.001b |
| Baseline score (centered on 16) | 1.0 | (0.9, 1.1) | <0.001b |
| Time, y | −0.03 | (−0.11, 0.05) | 0.45 |
Abbreviations: ODI, Oswestry Disability Index; RMDQ, Roland-Morris Disability Questionnaire; VAS-B, visual analog scale for back pain; VAS-L, visual analog scale for leg pain.
↵a Reference age group coded 0: <35 y.
↵b Statistically significant P < 0.0125. A negative coefficient signifies a decrease in improvement. The intercept coefficient represents the mean change score at year 1 for patients with median baseline for VAS-B, ODI, and RMDQ and for the <35 y age group for VAS-L.