Abstract
Purpose of the study
In a recent study, O'Leary et al. [2005] reported their observations on the patterns of Charité disc prosthesis motion under physiologic loads. The purpose of this study was to investigate whether the motion patterns observed in the in vitro model are replicated in clinical practice.
Methods
55 patients with implanted SB Charité 111 artificial lumbar discs were subjected to flexion extension x-rays. Two consultant spinal surgeons and a neuro-radiological consultant were asked to classify the pattern of motion in the clinical subjects based on the patterns observed in the in vitro model. The results were recorded independently then collated. Following this first round of observations an algorithm was devised and the method of measurement was standardised.
Summary of findings: There was modest correlation amongst the three observers in distinguishing motion from nonmotion (Kappa 5.6). There was less agreement on what type of motion was present. On both counts using the algorithm there was no correlation. The clinical study based on patients' flexion-extension radiographs identified the following patterns of prosthesis motion:
angular motion between both the upper and lower endplates and core, with visual evidence of core motion;
angular motion predominantly between the upper endplate and core, with little visual evidence of core motion;
lift-off of upper prosthesis endplate from core or of core from lower endplate;
core entrapment and deformation; and
no motion. There are difficulties associated with the interprtation of these using only flexion extension views.
Conclusions
In devising artificial discs the requirement would be to replicate spinal motion as closely as possible. O'Leary et al have found non-physiological motions in in vitro models. These have been found in this clinical study but interobserver correlations were far from conclusive. Demonstrating non physiological patterns may have important implications for outcome.
