[Subsidence and its effect on the anterior plate stabilization in the course of cervical spondylodesis. Part I: definition and review of literature].

A definition of subsidence in terms of spinal biomechanics is presented in the paper. Subsidence is defined as sinking of a body with a higher elasticity modulus (e.g. graft, cage, spacer) in a body characterized by a lower elasticity modulus (e.g. vertebral body), resulting in 3D changes of the spinal geometry. Magnitude of subsidence is directly proportional to the load pressure and to the difference between the elasticity modules, but inversely proportional to the area of the graft-bed interface. Both biological and mechanical qualities of the graft-bed interface are important for the subsidence process. Any excessive subsidence decreases the interbody space and produces both local and general kyphotization of the spine. This may cause destabilization of the screw-plate and/or screw-bone interfaces (e.g. pulling-out, altered angulation or breakage of the screws). A method is proposed of radiological estimating the absolute magnitude of subsidence, based on the real known length of the implanted stabilizer (e.g. plate). Clinical examples of an excessive subsidence and its impact on the stabilizing plate system are presented. Subsidence is inherent in the interbody fusion process. Endplate preservation and a dynamic modification of cervical plates may enables us to control subsidence and reduce the number of complications.