Posterior motion preserving implants evaluated by means of intervertebral disc bulging and annular fiber strains

BackgroundThe aims of motion preserving implants are to ensure sufficient stability to the spine, to release facet joints by also allowing a physiological loading to the intervertebral disc. The aim of this study was to assess disc load contribution by means of annular fiber strains and disc bulging of intact and stiffened segments. This was compared to the segments treated with various motion preserving implants.MethodsA laser scanning device was used to obtain three-dimensional disc bulging and annular fiber strains of six lumbar intervertebral discs (L2–3). Specimens were loaded with 500 N or 7.5 Nm moments in a spine tester. Each specimen was treated with four different implants; DSS™, internal fixator, Coflex™, and TOPS™.FindingsIn axial compression, all implants performed in a similar way. In flexion, the Coflex decreased range of motion by 13%, whereas bulging and fiber strains were similar to intact. The DSS stabilized segments by 54% compared to intact. TOPS showed a slight decrease in fiber strains (5%) with a range of motion similar to intact. The rigid fixator allowed strains up to 2%. In lateral bending, TOPS yielded range of motion values similar to intact, but maximum fiber strains doubled from 6.5% (intact) to 13.8%. Coflex showed range of motion, bulging and strain values similar to intact. The DSS and the rigid fixator reduced these values. The implants produced only minor changes in axial rotation.InterpretationThis study introduces an in vitro method, which was employed to evaluate spinal implants other than standard biomechanical methods. We could demonstrate that dynamic stabilization methods are able to keep fiber strains and disc bulging in a physiological range.