Biomechanics of Pedicle Screw Augmentation with Polymethymethacrylate

David B. Cohen MD,
Dennis Cullinane, PhD,
Masayuki Iizuka, MD, PhD,
Nathan Walk BS,
John P. Kostuik, MD
Baltimore, Maryland, USA

PURPOSE:
Spinal surgery in osteoporotic patients is fraught with failure at the implant bone interface. In long bones, polymethylmethacrylate (PMMA) has been used to augment stripped bone screws. The purpose of this study is to determine the effect of PMMA augmentation of the bone–screw interface of pedicle screws in osteoporotic vertebral bodies.

MATERIALS AND METHODS:
One hundred–two vertebrae from sixteen human cadavers (T10–L5) were used in this study. Pedicle dimensions were manually measured and vertebral bone density (BMD) measured with DEXA scanning. Pedicle screws (6mmX40mm) were implanted in each pedicle and axial pullout performed using an MTS servohydraulic testing machine with a load rate of 10 mm/min. Liquid PMMA (1, 2 or 3 cc) was introduced down the pedicle and the pedicle screws replaced. Following cement hardening, pedicle screws were re–tested in axial pullout.

RESULTS:
The BMD of the specimens used in this study was 0.350 ± 0.109 gm/cm2. The pull–out forces after PMMA injection were significantly increased in all specimens. Non–augmented screw pullout measured 448 ± 220 N. Augmentation with 1, 2 and 3 ccs of PMMA increased strength by 75%, 500% and 700% respectively. Each group significantly increased the pullout strength compared to control (p< 0.01). Multiple regression analysis revealed that pedicle dimensions or vertebral level did not significantly effect pullout. BMD did significantly effect pullout in the control and 1 cc injection groups with a decrease of 120 N for every decrease in BMD of 0.100 gm/cm2. BMD did not effect pullout when either 2 or 3 ccs. of PMMA was used. Both 2 and 3 ccs of PMMA increased pullout strength to level greater than axial pullout strength of 7x40 mm pedicle screws in normal density vertebrae performed in prior studies (~1600 N).

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DISCUSSION:
Pedicle screw augmentation with PMMA significantly increases pullout strength from osteoporotic vertebrae. By using either 2 or 3 ccs to augment a screw surgeons can increase the strength to greater than that of larger diameter screws in normal density bone. This technique may allow surgeons to prevent the bone–implant failures that frequently occur when surgical spinal stabilization is required in an osteoporotic patient.

** The FDA has not cleared a drug and/or medical device for the use described in this presentation. (i.e., the drug or medical device is being discussed in an “off–label" use).