Influence of design and implantation technique on the risk of progressive subsidence of different cervical interbody fusion devices

The aim of this study was to compare the subsidence of differently designed cervical interbody fusion devices under defined conditions. Forty-five bovine vertebral bodies were dissected from soft tissue and cartilage. The bony end plate was then taken off by 0, 1, and 2 mm. Five vertebral bodies of each abrasion depth were prepared for the uptake of a fusion device. Thus, three different fusion devices of comparable size underwent biomechanic testing in a Zwick testing machine with 4000 cycles of axial compression between 50 and 1000 N. Every 1000 cycles,the subsidence into the vertebral body was measured. Abrasion of the end plate resulted in an increased subsidence. The cage with rectangular shape and the cage with cylindric body and lateral wings showed better resistance to axial compression as long as the end plate remained intact. When the end plate was taken off, the subsidence was as high as in the cylindric cage, of which the subsidence did not correlate to the end plate abrasion. During preparation of the implant bed, the cortical bone of the end plate must be treated carefully. In cases of intact end plate, rectangular supporting areas can decrease the risk of subsidence.