Biomechanical properties of pedicle screw fixation augmented with allograft bone particles in osteoporotic vertebrae: different sizes and amounts.

BACKGROUND CONTEXT

A persistent challenge in spine surgery is improving screw fixation in patients with poor bone quality. Augmenting pedicle screw fixation with allograft bone appears to be a promising approach.

PURPOSE

To evaluate the biomechanical properties of screws augmented or revised with allograft bone particles (ABPs) and the effect of different sizes and amounts of ABP on screw-fixation strength.

STUDY DESIGN

Biomechanical in vitro study.

METHODS

Sixty vertebrae were separated randomly into six groups. Groups A1 and A2: one pedicle of each vertebra was selected randomly to be the original pedicle and implanted with a screw. Then, biomechanical tests were performed. Subsequently, the failed trajectory was revised with 1 mm ABP, and the contralateral pedicle was augmented with the same size and amount of ABP. Groups B1 and B2: two pedicles of each vertebra were augmented with different amounts of 1 mm ABP. Groups C1 and C2: one pedicle of each vertebra was augmented with 1 mm ABP to the maximum. The contralateral pedicle of each vertebra was augmented with the same amount of 2 mm ABP. After augmentation and screw insertion, groups A1, B1, and C1 were subjected to the pullout test, whereas groups A2, B2, and C2 the cyclic fatigue test.

RESULTS

Groups A1 and A2: screw augmentation increased the pullout strength by 47%, cycles to failure by 31%, and failure loads by 21% compared with the screw in the original pedicle (p<.05). Screw revision obtained 79% pullout strength, 97% cycles to failure, and 98% failure loads of the screw in the original pedicle (p<.05). Groups B1 and B2: full (100%) trajectory augmentation increased the pullout strength by 39%, cycles to failure by 18%, and failure loads by 12% compared with half (50%) trajectory augmentation (p<.05). Groups C1 and C2: the values of the pullout strength, cycles to failure, and failure loads of the screw augmented with 1 mm ABP were all greater than those in the 2 mm ABP. However, no significant differences were observed between the two treatments (p>.05).

CONCLUSIONS

Trajectory augmentation with ABP can significantly increase the strength of the augmented screws. Full trajectory augmentation can provide greater strength compared with half trajectory augmentation. In patients with osteoporosis, we recommend using 1 mm ABP in full trajectory augmentation (0.3 g ABP for 5.5 mm×40 mm and 0.5 g ABP for 6.5 mm×45 mm) before trajectory fails.