Radiographic total disc replacement angle measurement accuracy using the Oxford Cobbometer: precision and bias

Abstract

Total disc replacement (TDR) clinical success has been reported to be related to the residual motion of the operated level. Thus, accurate measurement of TDR range of motion (ROM) is of utmost importance. One commonly used tool in measuring ROM is the Oxford Cobbometer. Little is known however on its accuracy (precision and bias) in measuring TDR angles. The aim of this study was to assess the ability of the Cobbometer to accurately measure radiographic TDR angles. An anatomically accurate synthetic L4-L5 motion segment was instrumented with a CHARITE artificial disc. The TDR angle and anatomical position between L4 and L5 was fixed to prohibit motion while the motion segment was radiographically imaged in various degrees of rotation and elevation, representing a sample of possible patient placement positions. An experienced observer made ten readings of the TDR angle using the Cobbometer at each different position. The Cobbometer readings were analyzed to determine measurement accuracy at each position. Furthermore, analysis of variance was used to study rotation and elevation of the motion segment as treatment factors. Cobbometer TDR angle measurements were most accurate (highest precision and lowest bias) at the centered position (95.5%), which placed the TDR directly inline with the x-ray beam source without any rotation. In contrast, the lowest accuracy (75.2%) was observed in the most rotated and off-centered view. A difference as high as 4 degrees between readings at any individual position, and as high as 6 degrees between all the positions was observed. Furthermore, the Cobbometer was unable to detect the expected trend in TDR angle projection with changing position. Although the Cobbometer has been reported to be reliable in different clinical applications, it lacks the needed accuracy to measure TDR angles and ROM. More accurate ROM measurement methods need to be developed to help surgeons and researchers assess radiological success of TDRs.

Fig. 1

The Oxford Cobbometer

Fig. 2

a Graph showing both precision (± 1.0°) and bias (0.6°) for the ten repeated TDR angle readings at the 0 cm,0° imaging position. The Oxford Cobbometer was most accurate (95.5%) at this position. b Similar graph showing precision (± 3.2°) and bias (3.8°) for the −15 cm, + 15° imaging position. Readings at this position were the least accurate (75.2%)

Fig. 3

Digital radiographs illustrating the: a most accurate imaging position (0 cm, 0°) allowing for better precision in repeated TDR angle measurements as compared to: b the less accurate −15 cm, + 15° position showing an oblique view of the TDR making it more difficult to perform repeated readings of the same TDR angle

Fig. 4

Expected and Cobbometer measured TDR angle trend with changing motion segment rotation at the 0 cm elevation