Factors in measuring lumbar spondylolisthesis with reference data from NHANES-II

Abstract

Background Although spondylolisthesis, disc height loss, and abnormal disc angles are commonly assessed for in clinical practice and research studies, the factors that influence measurements are incompletely understood, and reference data representing a very large and diverse population are not available. Over 7000 lumbar spine x-rays were collected as part of the 2nd National Health and Nutrition Examination Survey (NHANES-II). This nationwide probability sample may facilitate development of robust reference data to objectively classify spondylolisthesis, disc heights, or disc angles as normal vs abnormal. Analysis of lumbar spine x-rays may also help to document whether disc heights and disc angles must be considered when assessing for spondylolisthesis. Methods Anatomic landmarks were obtained using neural networks and coded logic for L1 to S1 in the NHANES-II lumbar spine radiographs. Nine disc and sagittal plane offset (SPO) measurements were calculated from the landmarks. These data were trimmed to exclude abnormal discs and SPO. The factors that affect SPO were explored along with associations between the metrics and with age, sex, race, nation of origin and BMI. The prevalence of abnormalities was tabulated. Metrics were also calculated for previously analyzed lumbar flexion-extension x-rays to better understand the dependence of SPO on disc angles and disc heights. The errors that occur due to variability in radiographic projection were also assessed. Results SPO and disc metrics were obtained for 35,490 levels from 7,358 NHANES-II subjects who were 25 to 74 years old. Descriptive statistics for nine SPO and disc metrics were tabulated before and after trimming the data. Age, BMI, and sex were statistically significant but explained little of the variance in the metrics. SPO was significantly dependent on disc angle and height, though less so in the NHANES radiographs than in lumbar flexion-extension studies. Errors in the metrics due to out-of-plane imaging are generally small but can be large with extensive out-of-plane x-rays. Discussion The NHANES-II collection of lumbar x-rays allows for establishing reference data for SPO and disc metrics. These reference data allow for easily interpreted standardized reporting in units of std dev from average normal. SPO was significantly dependent on disc angle and disc height although the effect is small when there is limited flexion or extension. If SPO is being assessed from flexion or extension, a simple correction can be made. Caution is needed when interpreting measurements when the x-ray beam passes obliquely through the endplates or posterior wall (out-of-plane imaging). Conclusions The NHANES-II data may facilitate standardized assessments of SPO, disc heights and disc angles. Adjustments should be applied to SPO measurements if made from x-rays with the patient in flexed or extended positions.