Abstract
The effect of screw design and bone mineral density on transpedicular fixation in human vertebral bodies R.H. Wittenberg, M. Shea, K.S. Lee, A.A. White III, and W.C. Hayes Orthop. Biomech. Lab., Beth Israel Hosp., Harvard Med. School, Boston The objectives of this study were to investigate the effect of bone density on pullout force and cyclic vertical loading force. Human lumbar spines were obtaiped from 60 to 94 year old humans. Each vertebral body was analyzed by quantitative computed tomography and the equivalent bone mineral densities were calculated. Cyclic loading or pull out testing was performed. For the cyclic loading the screws were displaced perpendicular to their axis 2 mm cephalad and caudal from the zero load position for 5000 cycles. The mean equivalent density in the Steffee group was 82 + 39 mg/cm3 and in the Kluger 86 + 41 mg/cm3. The pullout force for the Steffee screws was 574 + 274 N and for the Kluger screws 656 + 331 N. The vertical loading force for the Steffee screws inserted into human vertebral bodies was 189 + 46 N , for the Kluger screws 205 + 105 N. A positive correlation existed between the density and the pullout and vertical loading force. The bone mineral density seems to be more important in the bone/metal interface strength in pedicular screws than the screw design. There were no significant differences in the screw designs, but the density correlated positively with the fisation strength.
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