Increased Pedicle Screw Pullout Strength with Vertebroplasty Pedicle Augmentation

John Sarzier, MD
Avery Evans, MD
Mark Allen, MD
David W. Cahill, MD (Tampa, FL)

Study Design:

A biomechanical study evaluating pedicle screw pullout strength in osteoporotic cadaveric vertebrae comparing polymethylmethacrylate (PMMA) augmented pedicle and hemivertebrae with the non–augmented contralateral side acting as a control.

Objectives:

To evaluate the effects of injected PMMA vertebroplasty on increasing the pullout strength of pedicle screws in vertebrae weakened by osteoporosis to ascertain if augmentation improves the integrity of the construct.

Summary of Background Data:

Injection of limited amounts of PMMA has been used in revision of failed pedicle screws in the past with the majority of the injection being placed in the pedicle or failed pedicle screw tract. While the majority of the strength of pedicle screws lies in the purchase of the cortex of the pedicle, a significant amount of the length of the screw is purchased in the vertebral body. In osteoporotic spines, pedicle and body strength is decreased. Both can be augmented with PMMA vertebroplasty increasing resistance to further compression fracture and reducing pain possibly by structural enhancement or thermal neurolysis of pain fibers. Methods: Six Formalin fixed lumbar spines were obtained, 3 male and 3 female, and evaluated for mineral bone density at each level. The four spines chosen were at least 2 SD below the mean bone density for age, and were free of evidence of compression fracture as evaluated by CT scan and plain x–rays. 21 vertebrae were found to meet these criteria. Each vertebra underwent bilateral placement of 6mm pedicle screws under fluoroscopy with one pedicle/hemivertebrae serving as the control for the contralateral pedicle/hemivertebrae undergoing hemi–vertebroplasty with an average of 4 cc of PMMA. The pedicle screws were then evaluated for pullout resistance biomechanically.

Results:

For the intact non–augmented vertebrae, the average pullout strength was 454 N (144N – 896N). The augmented group demonstrated an average pullout strength of 885 N (352N – 1504N) with a peak pullout strength occuring at a linear displacement greater than double that of the control (avg of 1.735 cm vs 4.00cm). The average increase in pullout strength was 191% (124% – 333%) in the augmented side. The above results were evaluated by the paired Student's T–test and were significant @p less than 0.005. The vertebral body and not the pedicle screw failed in 9 of 21 augmented hemivertebrae while the remaining 12 demonstrated screw pullout. Increased pullout resistance in augmented vertebrae does not appear to correlate with the baseline bone mineral density, depth of screw placement, or the vertebral level.

Conclusions:

This study suggests that the injection of PMMA into osteoporotic vertebrae via vertebroplasty in conjunction with initial pedicle screw placement can significantly increase pedicle screw pullout resistance. This can subsequently reduce the risk of instrumentation failure or pseudoarthrosis as well as adding structural resistance to compression fractures.