title: The effect of hubbing on the pull-out strength of lateral mass screws in the cervical spine: a biomechanical experiment.
abstract: METHODS
  This was a cadaveric biomechanical experiment.
  
  
  OBJECTIVE
  To compare the pull-out strength of polyaxial screws that are either hubbed or not hubbed when inserted into the lateral mass.
  
  
  BACKGROUND
  It has been shown in a study on pedicle screws in the thoracic spine that "hubbing" the head of the screw against the dorsal laminar cortex results in significantly lower pull-out strength of the screws.
  
  
  METHODS
  Fifteen segments of the human cervical spine (from C3 to C7) were prepared. Polyaxial screws 3.5 mm in diameter were used. On one side screws 12 mm in length were inserted until the screw head touched the lateral mass; they were then turned 2.5 more times until they were fully hubbed (hubbed screws). On the other side screws 14 mm in length were inserted until the screw head just touched the lateral mass (nonhubbed screws). The 2 mm difference in length was to ensure that the screws were buried to the same length. All screws inserted into the lateral masses underwent tensile pull-out by applying a tensile force down the long axis of the screw. The difference in pull-out strength between the 2 groups was evaluated using a nonparametric paired test (the Wilcoxon signed rank test), which compared side to side on each vertebra.
  
  
  RESULTS
  One specimen was excluded because of cement breakage during the biomechanical test. A total of 14 vertebrae were tested. Four vertebrae in the hubbed group showed small fractures or cracks around the screw hole after screw insertion. In a side to side comparison, the hubbed screws had significantly lower pull-out strengths as compared with the nonhubbed screws (P=0.033).
  
  
  CONCLUSIONS
  Hubbing of lateral mass screws lowers the potential pull-out strength of the screws as compared with the pull-out strength of nonhubbed screws. Thus, hubbing of lateral mass screws, on the basis of the parameters applied in this study, is not recommended.
authors: *Department of Orthopaedic Surgery, Dongguk University Ilsan Hospital, Goyang-city, Gyeonggi, Korea †Department of Orthopaedic Surgery, Emory Spine Center ‡Veterans Affairs Medical Center, Atlanta, GA §Department of Orthopaedic Surgery, Konyang University Hospital, Daejeon-city, Korea.
authors: Ki Hyoung
authors: KH
authors: Koo
authors: S Tim
authors: ST
authors: Yoon
authors: Sang Bum
authors: SB
authors: Kim
authors: William C
authors: WC
authors: Hutton
mesh_terms: D001696:Biomechanical Phenomena / Q000187:drug effects; D001843:Bone Cements / Q000494:pharmacology; D001863:Bone Screws; D002574:Cervical Vertebrae / Q000187:drug effects / Q000503:physiopathology* / Q000601:surgery*; D006801:Humans
chemical_list: D001843:Bone Cements