An in vitro comparison of the effect of number and pattern of positional screws on load resistance

Objective: The purpose of this in vitro investigation was to determine whet her the number or pattern of placement of positional screws affected their ability to resist vertical loads resembling mastication. Materials and Methods: Standardized bone substitutes were secured with vary ing numbers of 17.0-mm-long, 2.0-mm outer diameter, self-tapping titanium s crews (W. Lorenz Surgical, Jacksonville, FL) in various patterns using a po sitional screw technique. These patterns included one, two, three, four, an d five screws in a linear pattern; two screws in a vertical pattern; three screws in an L-pattern, inverted-L pattern, backward-pattern, inverted-back ward-L pattern, and right and left diagonal pattern; four screws in a box p attern; and five screws in a domino pattern. Five models of each group were fabricated along with a control group. The models were secured in a jig an d subjected to vertical loads by an Instron 8511.20 Mechanical Testing Unit (Canton, MA) until failure. Common engineering standards, including yield load, yield displacement, stiffness, maximum load, and displacement at maxi mum load, were measured, and means and standard deviations were derived and compared for statistical significance with an analysis of var-lance (ANOVA ) and Scheffe multiple comparison test. Results: Screw number and pattern of placement affected the mechanical char acteristics in resistance to vertical load. Screw numbers less than three a nd all linear patterns were the least effective. The three-screw L patterns , as a category, provided greater resistance to vertical loads than either the three-screw linear or three-screw diagonal patterns, The addition of mo re than three screws in geometric patterns offered no greater benefit than thr three-screw L patterns. Conclusions: In this in vitro study, die three-screw L patterns were the mo st effective and efficient when using the positional screw technique.