Posterior disk displacement: morphologic assessment and measurement reliability-lumbar spine.
Magnetic resonance imaging is often used to assess for disk displacement after manipulation, but limited information about the true incidence of iatrogenic herniations exists. To design a study that evaluates for a causal relationship, preliminary data must be obtained relating to the size of different types of disk displacement. The reliability of chiropractic radiologists in assessing disks and a comparison of different measuring devices should also be evaluated. To identify average measurements for normal and displaced disks and to assess the reliability of measurements by chiropractic radiologists. Intraobserver and interobserver reliability study assessing disk displacement on magnetic resonance scans. Three evaluators assessed the disks on 122 magnetic resonance scans from two imaging centers. Six categories were graded, and digitizer and ruler measurements were compared. Forty-four scans were reassessed for intraobserver agreement. Intraobserver and interobserver variations were measured with intraclass correlation coefficient and kappa statistical analysis. Measurement device correlation was assessed with Pearson's r. Clear size differences between different types of disk displacement were noted. Interexaminer measurement reliability was 0.78 to 0.84. Agreement concerning the presence of disk displacement was 85% (kappa = 0.68), and the classification of disk displacements was 76% (kappa = 0.60). Intraexaminer measurement reliability was 0.40 to 0.49. Intraexaminer agreement concerning the presence of disk displacement was 76% (kappa = 0.52), and the classification of disk displacements was 62% to 69% (kappa = 0.38 to 0.46). Normal versus bulged disk distinctions demonstrated the most disagreement. The ruler and digitizer correlation coefficient was 0.968. Different disk types demonstrated distinct size averages. Interexaminer agreement was good concerning disk assessment and measurements. Intraexaminer agreement was lower than expected. A millimetric ruler is an acceptable alternative to digital measurement devices.