title: Impact of screw location and endplate preparation on pullout strength for anterior plates and integrated fixation cages.
abstract: BACKGROUND
  Numerous integrated fixation cages (IFCs) 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 with screws used in traditional anterior plate (AP) fixation. Moreover, endplate preparation for IFC implantation may affect fixation strength.
  
  
  OBJECTIVE
  This study aimed to compare pullout strength of screws used in IFCs with screws used for AP implantations.
  
  
  METHODS
  A biomechanical cadaveric study.
  
  
  METHODS
  T12 and L1 vertebrae from 13 human cadaver spines were prepared for pullout testing. End plates in T12 vertebrae were scraped according to surgical practice for fusion procedures. Conversely, endplates in L1 vertebrae were kept intact (unscraped). Integrated fixation cage screws were implanted at a 45° 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 and anterior cortex thickness at screw insertion location, endplate mineralization, and anterior cortex mineralization.
  
  
  RESULTS
  Peak pullout force for IFC screws (176±68 N) with scraped endplates was similar (p=.26) to AP screws (192±84 N). However, PPF for IFC screws (231±75 N) with unscraped endplates was significantly greater (p<.01) than AP screws (176±50 N). Peak pullout force for IFC screws with scraped endplates was significantly lower (p=.03) than IFC screws with unscraped endplates. Scraped endplates group (0.17±0.05 mm) were thinner (p=.05) than unscraped endplates (0.21±0.06 mm) by approximately 40 µ on average.
  
  
  CONCLUSIONS
  These data indicate that IFC and AP 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 with the unscraped endplates and provides bounds for IFC screw fixation strength.
authors.affiliation: U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, MD 20993, USA. Electronic address: srinidhi.nagaraja@fda.hhs.gov.
authors.forename: Srinidhi
authors.initials: S
authors.lastname: Nagaraja
authors.affiliation: U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Science and Engineering Laboratories, Division of Applied Mechanics, Silver Spring, MD 20993, USA.
authors.forename: Vivek
authors.initials: V
authors.lastname: Palepu
authors.affiliation: U.S. Food and Drug Administration, Center for Devices and Radiological Health, Office of Device Evaluation, Division of Orthopedic Devices, Silver Spring, MD 20993, USA.
authors.forename: Jonathan H
authors.initials: JH
authors.lastname: Peck
authors.affiliation: Walter Reed National Military Medical Center, Department of Orthopaedic Surgery, Bethesda, MD 20889, USA.
authors.forename: Melvin D
authors.initials: MD
authors.lastname: Helgeson
mesh_terms: D001696:Biomechanical Phenomena; D001860:Bone Plates / Q000009:adverse effects / Q000592:standards*; D001863:Bone Screws / Q000009:adverse effects / Q000592:standards*; D006801:Humans; D013131:Spine / Q000601:surgery
keywords: Anterior plate; End plate thickness; Integrated fixation; Intervertebral cageScrew pullout; Spine