Carbonated apatite cement augmentation of pedicle screw fixation in the lumbar spine

Study Design. Biomechanical testing with human cadaveric lumbar vertebral bodies was used to determine the utility of an injectable carbonated apatite cancellous bone cement for improving the structural performance of pedicle screws subjected to axial pull-out or transverse cyclic loading. Objectives. To ascertain whether augmentation with a carbonated apatite cement can enhance pedicle screw fixation in the lumbar spine. Summary of Background Data. The beneficial effects of polymethylmethacrylate augmentation on pedicle screw pull-out strength have been demonstrated. Cancellous bone cement, however, may provide an attractive alternative in this application, as it is remodelable, biocompatible, and nonexothermic. Methods. Forty-three cadaveric lumbar vertebral bodies were instrumented with pedicle screws. In 20 of these specimens, axial pull-out strength was compared between the control screws and those augmented with cancellous bone cement. In the remaining 23 specimens, the screws were loaded in the superior-inferior direction with a peak displacement of +/- 1 mm at a frequency of 3 Hz for 5000 cycles. Three parameters were calculated from the force-versus-time data: 1)the energy dissipated, 2) the peak force at the start of the test, and 3) the peak force at the end of 5000 cycles. Results. The pull-out strength of the augmented pedicles averaged 68% greater than that of the control side. In response to cyclic loading, all measures of biomechanical performance improved 30-63%. Conclusions. The data suggest that augmentation with this carbonated apatite cancellous bone cement can enhance immediate screw fixation.