[The mobile segment above fusion].

The question is, if a possible concentration of stress in the segment adjacent to the fusion level might result in disc degeneration. Two series of patients have been evaluated: One group of 70 patients underwent a posterolateral fusion without instruments and the other group of 69 patients underwent a posterolateral instrumented fusion. The follow-up was in average 5.7 years resp. 4.6 years. The operations were divided in monosegmental, floating and polysegmental fusions. In the total group, 10% of the non-instrumented and 28% of the instrumented patients had an osteoarthritis of the facet joints, 10% resp. 23% were unstable and in 9% resp. 1% a disc resorption occurred. There have been no statistical differences in the both polysegemental groups, but this was the case for all osteoarthritis and instabilities in the monosegmental fusions. It is supposed that the changes in kinematics together with material related alterations of elasticity led to higher stress concentration in case of instrumentation.