title: Finding the right fit: studying the biomechanics of under-tapping with varying thread depths and pitches.
abstract: Compromise of pedicle screw purchase is a concern in maintaining rigid spinal fixation, especially with osteoporosis. Little consistency exists among various tapping techniques. Pedicle screws are often prepared with taps of a smaller diameter, which can further exacerbate inconsistency.
  
  
  
  The objective of this study was to determine whether a mismatch between tap thread depth (D) and thread pitch (P) and screw D and P affects fixation when under-tapping in osteoporotic bone.
  
  
  
  This study is a polyurethane foam block biomechanical analysis.
  
  
  
  A foam block osteoporotic bone model was used to compare pullout strength of pedicle screws with a 5.3 nominal diameter tap of varying D's and P's. Blocks were sorted into seven groups: (1) probe only; (2) 0.5-mm D, 1.5-mm P tap; (3) 0.5-mm D, 2.0-mm P tap; (4) 0.75-mm D, 2.0-mm P tap; (5) 0.75-mm D, 2.5-mm P tap; (6) 0.75-mm D, 3.0-mm P tap; and (7) 1.0-mm D, 2.5-mm P tap. A pedicle screw, 6.5 mm in diameter and 40 mm in length, was inserted to a depth of 40 mm. Axial pullout testing was performed at a rate of 5 mm/min on 10 blocks from each group.
  
  
  
  No significant difference was noted between groups under axial pullout testing. The mode of failure in the probe-only group was block fracture, occurring in 50% of cases. Among the other six groups, only one screw failed because of block fracture. The other 59 failed because of screw pullout.
  
  
  
  In an osteoporotic bone model, changing the D or P of the tap has no statistically significant effect on axial pullout. Osteoporotic bone might render tap features marginal. Our findings indicate that changing the characteristics of the tap D and P does not help with pullout strength in an osteoporotic model. The high rate of fracture in the probe-only group might imply the potential benefit of tapping to prevent catastrophic failure of bone.
authors.affiliation: Department of Orthopaedics, University of Maryland, 110 South Paca St, Suite 300, Baltimore, MD 21201, USA.
authors.forename: Ehsan
authors.initials: E
authors.lastname: Jazini
authors.affiliation: Department of Orthopaedics, University of Maryland, 110 South Paca St, Suite 300, Baltimore, MD 21201, USA.
authors.forename: Carmen
authors.initials: C
authors.lastname: Petraglia
authors.affiliation: Musculoskeletal Education and Research Center, A Division of Globus Medical, Inc., 2560 General Armistead Ave, Audubon, PA 19403, USA.
authors.forename: Mark
authors.initials: M
authors.lastname: Moldavsky
authors.affiliation: Department of Orthopaedics, University of Maryland, 110 South Paca St, Suite 300, Baltimore, MD 21201, USA.
authors.forename: Oliver
authors.initials: O
authors.lastname: Tannous
authors.affiliation: Department of Orthopaedics, University of Maryland, 110 South Paca St, Suite 300, Baltimore, MD 21201, USA.
authors.forename: Tristan
authors.initials: T
authors.lastname: Weir
authors.affiliation: New York-Presbyterian/Columbia University Medical Center, 622 West 168 St, New York, NY 10032, USA.
authors.forename: Comron
authors.initials: C
authors.lastname: Saifi
authors.affiliation: Musculoskeletal Education and Research Center, A Division of Globus Medical, Inc., 2560 General Armistead Ave, Audubon, PA 19403, USA.
authors.forename: Omar
authors.initials: O
authors.lastname: Elkassabany
authors.affiliation: Musculoskeletal Education and Research Center, A Division of Globus Medical, Inc., 2560 General Armistead Ave, Audubon, PA 19403, USA.
authors.forename: Yiwei
authors.initials: Y
authors.lastname: Cai
authors.affiliation: Musculoskeletal Education and Research Center, A Division of Globus Medical, Inc., 2560 General Armistead Ave, Audubon, PA 19403, USA.
authors.forename: Brandon
authors.initials: B
authors.lastname: Bucklen
authors.affiliation: Department of Orthopaedic Surgery, George Washington University, 2150 Pennsylvania Ave NW, Washington, DC 20037, USA.
authors.forename: Joseph
authors.initials: J
authors.lastname: O'Brien
authors.affiliation: Department of Orthopaedics, University of Maryland, 110 South Paca St, Suite 300, Baltimore, MD 21201, USA. Electronic address: sludwig@umoa.umm.edu.
authors.forename: Steven C
authors.initials: SC
authors.lastname: Ludwig
mesh_terms: D001696:Biomechanical Phenomena; D008422:Materials Testing; D065289:Pedicle Screws / Q000009:adverse effects* / Q000592:standards; D011140:Polyurethanes / Q000737:chemistry
chemical_list: D011140:Polyurethanes; C028279:polyurethane foam
keywords: Axial pullout; Biomechanics; Osteoporotic bone; Pedicle screw purchase; Screw pullout strength; Tap design; Thread depth and pitch