THE STRENGTH OF PLATE FIXATION IN RELATION TO THE NUMBER AND SPACING OF BONE SCREWS

Our purpose was to study the relationship between the number of plate holes filled and the spacing between the screws and the resultant stre ngth of plated constructs. Broad regular DC plates were anchored with 4.5-mm cortical screws to blocks of polyurethane foam. Six constructs were tested: (a) screws in holes 1, 2, and 3; (b) screws in holes 1 an d 3; (c) screws in holes 1 and 4; (d) screws in holes 1 and 5; (e) scr ews in holes 1 and 6; (f) screws in holes 1, 3, and 5. The strength wa s quantified using a material-testing system. In cantilever and four-p oint bending, the constructs were loaded in both gap-closing and gap-o pening modes. Screws in holes 1, 2, and 3 were tested against other co nstructs. For cantilever bending (gap opening and gap closing), con st ruct (a) was stronger than construct (b), as strong as construct (c), but weaker than the constructs with more widely spaced screws (p < 0.0 001). In terms of four-point bending, for gap opening, the standard fi xation (construct (a) was stronger than construct (b) but weaker than the more widely spaced constructs. For gap closing, construct (a) was stronger than constructs (b) and (c) but weaker than the rest. Regardl ess of the spacing of screws and the plate length, strength in torsion was dependent on the number of screws securing the plate. In a labora tory fracture model of plate-bone constructs tested to failure by scre w pullout, wider spacing of bone screws increases the bending strength of screw-plate fixation and can be more effective than increasing the number of screws. Torsional strength is independent of screw placemen t in plates of a given width and depends on the number of screws used.