title: A comparison of screw insertion torque and pullout strength.
abstract: OBJECTIVE
  Pullout strength of screws is a parameter used to evaluate plate screw fixation strength. However, screw fixation strength may be more closely related to its ability to generate sufficient insertion because stable nonlocked plate-screw fracture fixation requires sufficient compression between plate and bone such that no motion occurs between the plate and bone under physiological loads. Compression is generated by tightening of screws. In osteoporotic cancellous bone, sufficient screw insertion torque may not be generated before screw stripping. The effect of screw thread pitch on generation of maximum insertion torque (MIT) and pullout strength (POS) was investigated in an osteoporotic cancellous bone model and the relationship between MIT and POS was analyzed.
  
  
  METHODS
  Stainless steel screws with constant major (5.0 mm) and minor (2.7 mm) diameters but with varying thread pitches (1, 1.2, 1.5, 1.6, and 1.75 mm) were tested for MIT and POS in a validated osteoporotic surrogate for cancellous bone (density of 160 kg/m(3) [10 lbs/ft(3)]). MIT was measured with a torque-measuring hex driver for screws inserted through a one-third tubular plate. POS was measured after insertion of screws to a depth of 20 mm based on the Standard Specification and Test Methods for Metallic Medical Bone Screws (ASTM F 543-07). Five screws were tested for each failure mode and screw design. The relationship between MIT and compressive force between the plate and bone surrogate was evaluated using pressure-sensitive film.
  
  
  RESULTS
  There was a significant difference in mean MIT based on screw pitch (P < 0.0001), whereas POS did not show statistically significant differences among the different screw pitches (P = 0.052). Small screw pitches (1.0 mm and 1.2 mm) had lower MIT and were distinguished from large pitches (1.5 mm, 1.6 mm, and the 1.75 mm) with higher MIT. For POS, only the 1-mm and 1.6-mm pitch screws were found to be different from each other. Linear regression analysis of MIT revealed a moderate correlation to the screw pitch (R(2) = 0.67, P < 0.0001), whereas the analysis of POS suggested no correlation to the screw pitch (R(2) = 0.28, P = 0.006). Pearson correlation analysis indicated no correlation between MIT and POS (P = 0.069, r = -0.37). A linear relationship of increased compression between the plate and bone surrogate was found for increasing screw torque (R(2) = 0.97).
  
  
  CONCLUSIONS
  These results indicate that the ability of different screw designs to generate high screw insertion torque in a model of osteoporotic cancellous bone is unrelated to their pullout strength. Therefore, extrapolation of results for POS to identify optimal screw design for osteoporotic bone may not be valid. Screw designs that optimize MIT should be sought for fixation in osteoporotic bone.
authors.affiliation: Washington University School of Medicine, St. Louis, MO, USA. ricciw@wudosis.wustl.edu
authors.forename: William M
authors.initials: WM
authors.lastname: Ricci
authors.forename: Paul
authors.initials: P
authors.lastname: Tornetta
authors.forename: Timothy
authors.initials: T
authors.lastname: Petteys
authors.forename: Darin
authors.initials: D
authors.lastname: Gerlach
authors.forename: Jacob
authors.initials: J
authors.lastname: Cartner
authors.forename: Zakiyyah
authors.initials: Z
authors.lastname: Walker
authors.forename: Thomas A
authors.initials: TA
authors.lastname: Russell
mesh_terms: D001863:Bone Screws; D019544:Equipment Failure Analysis; D005592:Fracture Fixation / Q000295:instrumentation*; D006801:Humans; D008953:Models, Anatomic; D019415:Torque