US Patent 7637928 Variable angle locked bone fixation system

A bone fixation system including a fixation device having an upper surface, a lower surface and at least one opening extending therethrough and defining a first axis substantially perpendicular to the upper surface, the opening including a plurality of isolated protrusions formed on an inner surface thereof and a bone engaging member insertable through the opening at a selected angle of orientation relative to the first axis, the bone engaging member having a threaded head portion, wherein upon tightening of the bone engaging member, the threaded head portion of the bone engaging member mates with one or more protrusions of the opening to lock the bone engaging member to the plate at the selected angle of orientation relative to the first axis, and wherein the protrusions exhibit the same pitch as the threads on the bone engaging member prior to insertion of the bone engaging member in the opening.