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 whether 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 varying numbers of 17.0-mm-long, 2.0-mm outer diameter, self-tapping titanium screws (W. Lorenz Surgical, Jacksonville, FL) in various patterns using a positional screw technique. These patterns included one, two, three, four, and five screws in a linear pattern; two screws in a vertical pattern; three screws in an L-pattern, inverted-L pattern, backward-L pattern, inverted-backward-L pattern, and right and left diagonal pattern; four screws in a box pattern; 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 and 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 maximum load, were measured, and means and standard deviations were derived and compared for statistical significance with an analysis of variance (ANOVA) and Scheffe multiple comparison test.
RESULTS: Screw number and pattern of placement affected the mechanical characteristics in resistance to vertical load. Screw numbers less than three and 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 more than three screws in geometric patterns offered no greater benefit than the three-screw L patterns.
CONCLUSIONS: In this in vitro study, the three-screw L patterns were the most effective and efficient when using the positional screw technique.