Preoperative Evaluation of Oswestry Disability Index in Lumbar Spinal Stenosis: New Evidence of Time Independence of Variation Up to 1 Year

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Figure 1
Time evolution of Oswestry Disability Index (ODI) results for the individual patients between the 2 measurement times.
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Figure 2
Time evolution between the 2 Oswestry Disability Index measurement points with the time interval between the 2 points on the horizontal axis.
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Figure 3
Predicted change in the population mean due to interval length between 2 Oswestry Disability Index (ODI) measurements. The shaded area corresponds to the 95% CI. The dotted lines denote the clinically significant difference of 10 ODI points.

Table 1

Inclusion and exclusion criteria.

Variable	Inclusion Criteria	Exclusion Criteria
Diagnosis	Lumbar spinal stenosis with or without spondylolisthesis	Any concomitant spinal disorders (scoliosis, vertebral fracture, isthmic spondylolysis, tumor, and metastases)
ODI scores	Measured at 2 timepoints prior to the operation	Incomplete or missing scores, or the interval was <1 week10

Table 2

Results of the mixed linear model fit.

Variable	Estimate^{^a}	SE^{^a}	P ^{^a}
Fixed effects
Intercept^{^b}	44.0	6.5	<0.0001
Interval (day)	-0.0005	0.011	0.97
Sex (male)	-8.1	2.9	0.007
Age	0.18	0.45	0.32
Concomitant diseases (yes or NA)	0.35	0.34	0.96
BMI	0.68	0.34	0.05
Random effects
Patient	σ = 12.8
Residual	σ = 8.3
Fit quality
R² _mar /R² _con ^{^c}	0.09/0.73

Abbreviations: BMI, body mass index; NA, not applicable; ODI, Oswestry Disability Index.
↵a For the fixed effects.
↵b The intercept reflects the expected ODI score for a 70-year-old woman with no concomitant diseases and a BMI of 29.
↵c Marginal R ² value/conditional R ² value.