Finding the right fit: studying the biomechanics of under-tapping with varying thread depths and pitches

Ehsan Jazini, Carmen Petraglia, Mark Moldavsky, Oliver Tannous, Tristan Weir, Comron Saifi, Omar Elkassabany, Yiwei Cai, Brandon Bucklen, Joseph O'Brien, Steven C. Ludwig

Research output: Contribution to journalArticlepeer-review

6 Scopus citations


Background Context 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. Purpose 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. Study Design This study is a polyurethane foam block biomechanical analysis. Materials and Methods 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. Results 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. Conclusions 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.

Original languageEnglish (US)
Pages (from-to)574-578
Number of pages5
JournalSpine Journal
Issue number4
StatePublished - Apr 1 2017


  • Axial pullout
  • Biomechanics
  • Osteoporotic bone
  • Pedicle screw purchase
  • Screw pullout strength
  • Tap design
  • Thread depth and pitch

ASJC Scopus subject areas

  • Surgery
  • Orthopedics and Sports Medicine
  • Clinical Neurology


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