Scientific Publications by FDA Staff

Spine J 2015 Nov 1;15(11):2425-32

Impact of screw location and endplate preparation on pullout strength for anterior plates and integrated fixation cages.

Nagaraja S, Palepu V, Peck J, Helgeson MD

BACKGROUND CONTEXT: Numerous integrated fixation cages (IFC) have recently been introduced to the market with "zero-profile" designs that incorporate screw fixation through the vertebral endplate. It is unclear whether differences in bone quality and quantity in this insertion location may affect fixation compared to screws used in traditional anterior plate fixation. Moreover, endplate preparation for IFC implantation may affect fixation strength. PURPOSE: To compare pullout strength of screws used in IFCs with screws used for anterior plate (AP) implantations. STUDY DESIGN: A biomechanical cadaveric study. METHODS: T12 and L1 vertebrae from 13 human cadaver spines were prepared for pullout testing. Endplates in T12 vertebrae were scraped according to surgical practice for fusion procedures. Conversely, endplates in L1 vertebrae were kept intact (unscraped). IFC screws were implanted at a 45 degrees angle into the endplate and AP screws were implanted horizontally into the same vertebral body. Pullout testing was performed on all screws and peak pullout force (PPF) and work were compared between groups to determine fixation strength. In addition, micro-CT imaging was used to assess bone quantity and quality parameters such as trabecular bone volume fraction, endplate thickness and anterior cortex thickness around the screw insertions, endplate mineralization, and anterior cortex mineralization. RESULTS: PPF for IFC screws (176 +/- 68 N) with scraped endplates was similar (p=0.26) to AP screws (192 +/- 84 N). However, PPF for IFC screws (231 +/- 75 N) with unscraped endplates was significantly greater (p<0.01) than AP screws (176 +/- 50 N). PPF for IFC screws with scraped endplates was significantly lower (p=0.03) than IFC screws with unscraped endplates. Scraped endplates group (0.17 +/- 0.05 mm) were thinner (p=0.05) than unscraped endplates (0.21 +/- 0.06 mm) by approximately 40 microns on average. CONCLUSIONS: This data indicate that integrated fixation cage and anterior plate screws exhibited similar fixation behavior when the endplate is prepared according to common surgical practices. However, endplate scraping reduces endplate thickness by 20% on average resulting in a decrease in fixation strength when compared to the unscraped and provides bounds for IFC screw fixation strength.