[Initial instability of different cages for fusion of the cervical spine]

Purpose of the present investigation is to determine the biomechanical behaviour of different cages for monosegmental fusion of the cervical spine. Three commercially available cages (BAK, NOVUS, WING) representing the different principles of intercorporal implants and a combination of intercorporal bone graft together with anterior plating were tested for their resistance and sintering patterns under axial compression conditions. Therefore, FSU (functional spine-units) of 5-months old calfs were used. After preparation, the anterior fusion was performed by an orthopaedic surgeon. Specimen were mounted in a testing machine Zwick 1425 and axial load from 100 N up to 2000 N was applied. The compressed distance was measured and put into relation to the applied load. After that, the device was unloaded and the test was repeated another 2 times to determine the plastic deformation of implant and specimen. There was no significant difference to all of the constructs in the first compression. After repeated compression, the WING-cage, the NOVUS-cage and the plate-construct showed a constant compression pattern as expression of resting stable on the vertebral endplates, whereas the cylindrical BAK-cage had a decrease in compression distance, but increase in sintering into the vertebral body. Intercorporal implants that require the destruction of the vertebral endplates as described in the Cloward dowel-technique may have a higher risk of sintering into the vertebral body and therefore of developing progressive kyphosis. By attaching lateral supporting areas this risk can be reduced and the advantage of sponges contact for fusion is preserved.