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

Abstract

Many different radiographic grading systems for disc degeneration are described in literature. However, only a few of them are tested for interobserver agreement and none for validity. Furthermore, most of them are based on a subjective terminology. The aim of this study, therefore, is to combine these systems to a new one in which all subjective terms are replaced by more objective ones and to test this new system for validity and interobserver agreement. Since lumbar and cervical discs need to be graded differently, this study was divided into the present Part I for the lumbar and a Part II for the cervical spine. The new radiographic grading system covers the three variables "Height Loss", "Osteophyte Formation" and "Diffuse Sclerosis". On lateral and postero-anterior radiographs, each of these three variables first has 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 44 lumbar discs were compared to the respective macroscopic ones, which were defined as "real" degrees of degeneration. The agreement between observers with different levels of experience was determined using the radiographs of 84 lumbar discs. Agreement was quantified using quadratic weighted Kappa coefficients (Kappa) with 95% confidence limits (95% CL). The validation of the new radiographic grading system revealed a substantial agreement between the radiographic and the "real" macroscopic overall degree of degeneration (Kappa=0.714, 95% CL: 0.587-0.841). The radiographic grades, however, tended to be slightly lower than the "real" ones. The interobserver agreement was substantial for all the three variables and for the overall degree of degeneration (Kappa=0.787, 95% CL: 0.702-0.872). However, the inexperienced observer tended to assign slightly lower degrees of degeneration than the experienced one. In conclusion, we believe that the new radiographic grading system is an almost objective, valid and reliable tool to quantify the degree of degeneration of individual lumbar intervertebral discs. However, the user should always remember that the "real" degree of degeneration tends to be underestimated and that slight differences between the ratings of observers with different levels of experience 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. Finally, the sum of the two anterior distances is defined as actual anterior disc height, and the sum of the two posterior distances is defined as 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. [15](Table 2 )

Fig. 2

To assess the variable “Ostophyte Formation”, the two anterior (e1, e2), two posterior (e3, e4), two right lateral (e5, e6) and two left lateral edges (e7, e8) of the adjacent vertebral bodies are screened for osteophytes. Their number is counted and their length is measured along their long axis beginning at the former border of the vertebral body and ending at their tips (white lines in the edges e1, e2, e5, e6, e7 and e8)

Fig. 3

The variable “Diffuse Sclerosis” is assessed on the lateral radiographs only. The lower half of the upper vertebral body and the upper half of the lower vertebral body are each divided into four 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. In this example, the number of affected regions (asterisk) would be three for the upper and three for the lower vertebral body

Fig. 4

Agreement between the radiographic and the macroscopic “real” degree of degeneration of 40 and 44 lumbar intervertebral discs, respectively. 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. 5

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

Fig. 6

Examples of the four degrees of degeneration