Biomechanical Approach to Stability of Intersomatic Implants in Cervical Spine

Abstract

A frequent complication with intersomatic implants (e.g. cages) is still subsidence. The knowledge of the mechanism of these subsidences is not yet well established. The preparation of the endplate is still controversially discussed. The strength of the natural endplate is provided by the combination of a compact cover layer as a shell over underlying spongious bone as an elastic foundation. In the sense of a trade-off, cuts into the endplate are considered necessary to assure biological bridging but shall have minimal extensions to prevent loss of strength. A personal conceptual model of the head and its cervical vertebral column reveals static loads on an exemplary C5 vertebra of 131 N normal and 32 N shear load. Increasing these loads by a load factor of 1.4 to reflect activities of daily living, lifts these loads to 183 N and 45 N respectively. The order of magnitude of these loads is considered rather small and cannot confirm lack of strength as the only (or main) reason for subsidence. Other factors must also be looked at. In the case of a cage, it must be positioned in such way that the shear loads will still be transferred by the facet joints, since shear may be detrimental for an implant-/bone-interface in the early post-operative phase, especially when shear leads to micromotion. The implant must be prevented from tilting with resulting local stress concentration. Due to the new load distribution and the surgery, bone will be subject to remodeling and should be protected in this critical transition phase. Finally also the consequences of the disturbance of the once healthy biological balance between spongious bone, endplates, nucleus pulposus, endplates, spongious bone must be kept in mind.