Precision measurement of disc height, vertebral height and sagittal plane displacement from lateral radiographic views of the lumbar spine

Objective. To compile a database of disc height, vertebral height and sagittal plane displacement from lateral radiographic views of the lumbar spine, valid for male and female subjects in the age range 16–57 years. The protocols used to measure these parameters compensate for distortion in central projection, off-centre position, axial rotation and lateral tilt of the spine as well as for variation in radiographic magnification and stature.

Study design. The study comprised designing and testing of measurement protocols, together with subsequent data collection from archive radiographs.

Background. Attempts to quantify primary mechanical damage to lumbar vertebrae and discs have been limited due to imprecision when measuring disc height, vertebral height and sagittal plane displacement. Age-related, normative values for these parameters were not previously available. Consequently, important issues like the effectiveness of past and present guidelines for safe manual handling with respect to prevention of overload injuries could not be resolved and judgement on pathological alterations in the morphology of the individual lumbar spine could only be performed in a qualitative, subjective manner.

Methods. Based on the analysis of vertebral contours in the lateral radiographic image of the lumbar spine, new protocols for measuring disc height, vertebral height and sagittal plane displacement were developed. The measured data are virtually independent of distortion, axial rotation and lateral tilt. Furthermore, description of height and displacement using dimensionless parameters guarantees independence of radiographic magnification and stature. Subjective influence in the measurement procedure was minimized by automatic computation of contour-landmarks and derived parameters. Measurement errors were assessed from sets of radiographs of spine specimens and serial flexion-extension radiographs; interobserver and intraobserver errors were assessed from repeated measurements of lateral views. For compilation of a database, measurements were performed of a set of 892 lateral views of the lumbar spine of male and female subjects between 16 and 57 years of age. Data from pathologically deformed vertebrae or discs, or from motion segments exhibiting spondylolisthesis or retrolisthesis were excluded by normal radiological inspection; data from spines showing normal, age-related degenerative changes were included.

Results. The new protocols allow height and displacement of lumbar vertebrae as well as height of lumbar discs to be measured for all motion segments on a lateral view, rather than just those vertebrae or discs close to the central beam, to facilitate recognition of localized abnormalities. Since the results are independent of exposure geometry, retrospective investigations are feasible. The relative measurement error in vertebral height amounts to 2.2%; for a vertebra of 30 mm height this corresponds to an error of approximately 0.7 mm. The error in sagittal plane displacement amounts to 0.015 (measured in units of mean vertebral depth); for a vertebra of 35 mm depth this corresponds to an error of 0.5 mm. The relative error in disc height amounts to 4.15%; for a disc of 10 mm height this corresponds to an error of approximately 0.5 mm. For both genders, the database contains age-dependent, normative values of disc height, vertebral height and sagittal plane displacement. In addition, the database describes intersegmental shape correlation, i.e. the relation between height of neighbouring discs and between height as well as displacement of neighbouring vertebrae. On average, height of lumbar vertebrae is larger in females than in males; height of lumbar discs is larger in males than in females and shows a minute dependence on age in males; in both genders, sagittal plane displacement increases, but only by a small amount, with age.

Conclusions. The new measurement protocols for disc height, vertebral height and sagittal plane displacement, together with the database of normative age-related values, permit quantitative assessment of the prevalence of pathological morphological changes in the human lumbar spine. The new method and the database will serve to explore the effect of potentially detrimental influences such as high spinal loading and to provide quantitative documentation of existing injury to vertebrae and discs in individual cases.