Anterior fixation in the osteoporotic spine: cut-out and pullout characteristics of implants

A new concept for the anchorage of anterior fixation implants in the osteoporotic thoracic and lumbar spine is presented. The Spiral-Blade has been proposed as a suitable device for use in the osteoporotic spine, due to its broad, flat surface, which should provide resistance against cut-out of the implant through the vertebral body under dynamic loading. The cut-out and pullout characteristics of this implant were tested. The SpiralBlade was tested with and without a supplementary insertion guide screw. Two other commercial implants were tested for comparison: the VentroFix and the MACS-TL HMA (hollow monoaxial) screw. All implants were tested in osteoporotic human cadaveric vertebrae, using a modified in vitro testing protocol which simulated a full corpectomy model. Dynamic cyclic loading of 100 N, 200 N and 400 N was applied to the implant for 1000 cycles at each load level, and the subsidence of the vertebral body relative to the implant was measured. Following cyclic testing, the pullout strength of the implant was measured. No significant differences were found in the cut-out performance between the SpiralBlade with guide screw and the VentroFix. The SpiralBlade inserted without a guide screw was prone to cutting-out and a substantial loss of angular alignment of the vertebral body. Cut-out of the HMA screw was significantly greater than with the other implants. Two HMA screws fractured during testing. The VentroFix, with an average pullout force of 1166 N, has a significantly higher resistance to pullout than the SpiralBlade with guide screw (417 N), the SpiralBlade (332 N) and the Aesculap HMA screw (298 N). The SpiralBlade may be an alternative to anterior screw fixation in the osteoporotic spine, offering the same cut-out resistance with one implant rather than two screws.