Consequences of patient position in the radiographic measurement of artificial disc replacement angles

Accurate clinical measurement of spinal range of motion (ROM) is essential in the evaluation of artificial disc performance. The effect of patient placement with respect to the X-ray beam source is yet to be reported and may be an influencing factor in radiographic artificial disc angle measurements. This study aims to evaluate how radiographic patient placement influences artificial disc angle measurements. An anatomically accurate synthetic L4-L5 motion segment was instrumented with an artificial disc and two pins. The instrumented motion segment was mounted onto a frame allowing for independent rotation and elevation while holding the artificial disc angle and anatomical position between L4 and L5 fixed. Analyses included descriptive statistics, evaluation of uncertainty, intra- and inter-observer, and a 2-way analysis of variance (ANOVA). The mean angle measurement range at the various positions was 1.26° for the pin, and 2.74° for the artificial disc endplates. The centered patient position had the highest inter- and intra-observer reliability. ANOVA results showed elevation effects to be statistically significant (P = 0.021), and rotational effects to be extremely statistically significant (P < 0.0001) for the pin angles. In terms of the mean artificial disc angle, however, the ANOVA showed a highly statistically significant interaction term (P = 0.002). A significant difference was found in the angle measurements of a fixed artificial disc prosthesis based on a sample of patient radiographic placement positions. Since it is important to assess the success of an artificial disc replacement by evaluating the relatively small ROM present, it is crucial to aim at minimizing the error by placing the patient parallel to the plate with the beam centered not at the mid lumbar spine, but at the level of the arthroplasty, for both flexion and extension views.