Cement-Augmentation Of Pedicle Screw Fixation Using Novel Cannulated Cement Insertion Device
Chad Waits, MD,
Douglas Burton, MD, Terence Mciff, PhD, Marc Asher, MD, R. Glattes
University of Kansas Medical Center, Kansas City, KS, USA

INTRODUCTION: Pedicle screw stability is difficult to obtain in osteoporotic bone. Recent studies have looked at various cannulated screws through which polymethylmethacrylate is injected and have shown superior fixation to the standard method of injecting cement into the screw hole prior to screw insertion. The advantage is control of cement placement, limiting extravasation outside the vertebral body. However, extraction of cemented screws via a cannulated system is exceedingly difficult but may be necessary in revision surgery. We developed a cannulated cement delivery device and technique for cementing that allows for placement of a standard pedicle screw into a cement-augmented threaded tract.

PURPOSE: The purpose of this study was to evaluate the efficacy of this cement-augmentation device and technique and to biomechanically compare pedicle screw stability for uncemented screws versus screws augmented with cement via this technique.

METHODS: Thirteen fresh osteopenic cadaveric lumbar vertebrae were instrumented with an uncemented pedicle screw in one pedicle and the novel cemented screw in the opposite pedicle. The novel cannulated pedicle screws were fabricated with fenestrations in the distal one-third of the screws and the heads were adapted to fit a syringe. Vertebral body augmentation was performed by placement of the novel screw and injection of 2.5 ml of PMMA per screw. Prior to final cement hardening, the novel screw was removed and replaced with a standard screw. Cephalocaudal toggling of each screw was performed on an MTS for 10,000 cycles and total displacement was measured. Torque to remove 1) uncemented screws, 2) cemented screws by our technique, and 3) cemented fenestrated screws left in place until cement hardening was also measured.

RESULTS: All specimens tested were osteopenic as evaluated by dual-energy X-ray absorptiometry (DEXA). At the conclusion of 10,000 cycles, the average displacement was 7.3 mm (2.5-16.0) for uncemented screws and 2.7 mm (1.9-3.5) for cemented screws, p<0.001. This represents a 63% decrease in the average motion of cemented versus uncemented screws. Torque required for screw removal was similar for uncemented screws (70-672N-mm, mean=358) versus PMMA-augmented screws via this technique (196-717N-mm, mean=343). However, fenestrated screws cemented into place required an average of 4100N-mm (2838-5287) which may be excessive torque in clinical applications.

CONCLUSIONS: This novel cement-delivery screw and injection technique provides a significant increase in pedicle screw stability and the ability to easily remove the screws if needed. Cement augmentation of pedicle screws, in a manner that allows for placement of a standard pedicle screw that can easily be removed at the time of revision surgery, may be a valuable asset in achieving more stable fixation of spinal implants in osteoporotic spines.





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