Apparatus for preventing screw backout in a bone plate fixation system

A method for securing a fixation plate having a number of bores therethrough to a bone with a corresponding number of bone screws wherein each of the screws have a headed portion and an elongated shaft extending therefrom defining a first diameter non-threaded portion adjacent the head portion and a second larger diameter bone engaging threaded portion remote from the head portion, the method comprising the steps of:

• (1) forming a number of annular collars constructed of a shape memory alloy to have a final inner diameter less than the second diameter while the crystal structure of said alloy is in the austenite phase;
• (2) cooling said collars sufficiently so that the crystal structure of said alloy shifts to the martensite phase;
• (3) increasing the inner diameter of each of said annular collars while maintaining said collars in the martensite phase to permit passage therethrough of an elongated bone screw shaft;
• (4) mounting the fixation plate to the bone by advancing each of the number of bone screws through a corresponding fixation plate bore, through one of said annular collars and into the bone;
• and
• (5) heating said number of collars sufficiently to cause said inner diameters of said collars to shrink to their final diameters thereby trapping the number of collars between the head. portions and the threaded portions of the corresponding number of bone screws.