Validity and interobserver agreement of a new radiographic grading system for intervertebral disc degeneration: Part II. Cervical spine

Abstract

A new radiographic grading system for a more objective assessment of lumbar intervertebral disc degeneration has been described and tested in Part I of this study. The aim of the present Part II of the study was to adapt this system to the cervical spine, and to test it for validity and interobserver agreement. Some modifications of the grading system described in Part I were necessary to make it applicable to the cervical spine. Its basic structure, however, stayed untouched. The three variables "Height Loss", "Osteophyte Formation" and "Diffuse Sclerosis" first have to be graded individually. Then, the "Overall Degree of Degeneration" is assigned on a four-point scale from 0 (no degeneration) to 3 (severe degeneration). For validation, the radiographic degrees of degeneration of 28 cervical discs were compared to the respective macroscopic ones, which were defined as "real" degrees of degeneration. The interobserver agreement was determined between one experienced and one unexperienced observer using the radiographs of 57 cervical discs. Quadratic weighted Kappa coefficients (kappa) with 95% confidence limits (95% CL) were used for statistical evaluation. The validation of the new version of the radiographic grading system showed a moderate agreement with the "real", macroscopic overall degree of degeneration (kappa=0.599, 95% CL 0.421-0.786). In 64% of all discs the "real" overall degree of degeneration was underestimated but never overestimated. This underestimation, however, was much less pronounced and the Kappa coefficients were significantly higher for the three variables: Height Loss, Osteophyte Formation, and Diffuse Sclerosis separately. The agreement between the radiographic ratings of the experienced and the unexperienced observer was substantial for the overall degree of degeneration (kappa=0.688, 95% CL 0.580-0.796), almost perfect for the variable, Height Loss, moderate for Osteophyte Formation and fair for Diffuse Sclerosis. In conclusion, we believe that the new version of the radiographic grading system is a sufficiently valid and reliable tool to quantify the degree of degeneration of individual cervical intervertebral discs. In comparison to the version for the lumbar spine described in Part I, however, a slightly higher tendency to underestimate the "real" overall degree of degeneration and somewhat higher interobserver differences have to be expected.

Fig. 1

To assess the degree of “Height Loss”, first, the actual disc height has to be determined. For this purpose the anterior and posterior edges of the adjacent vertebral bodies (small white circles) are defined as those points having the largest distance to the centre of the vertebral body (black points). Then, the distance of each of these four edges to the midplane of the disc (dashed line) is measured. The sum of the two anterior distances finally is defined as the actual anterior disc height, the sum of the two posterior distances as the actual posterior disc height. This procedure is meant to support the estimation of actual disc height but does not have to be carried out using drawings or digitisation. In a second step, this actual height is compared to the respective height before degeneration, which is estimated based on the normal values reported by Frobin et al. 1997 (Table 2)

Fig. 2

To evaluate the variable “Diffuse Sclerosis” in the assessment of cervical disc degeneration, the lower half of the upper vertebral body and the upper half of the lower vertebral body are each divided into two regions. Then, the number of regions is counted, which are covered by sclerosis. Note that a partially covered region is counted as if it was completely covered

Fig. 3

Agreement between the radiographic and the macroscopic, “real” degree of degeneration of 28 cervical intervertebral discs. Each field contains the number of discs rated with 0, 1, 2, or 3 points radiographically (rating of one experienced observer) and with 0, 0.5, 1, 1.5, 2, 2.5, or 3 points macroscopically (mean value of the ratings of two experienced observers)

Fig. 4

Agreement between the radiographic ratings of one experienced and one unexperienced observer. Each field contains the number of cervical intervertebral discs rated with the respective scores

Fig. 5

Examples of the four degrees of degeneration