Orthopaedic fixation devices, both internal and external, are frequently coupled to bone by the use of fasteners such as screws or pins.  For example, bone plates can be secured to bone with bone screws inserted through plate holes.  Securing the screws to the plate provides a fixed angle relationship between the plate and screw and reduces the incidence of loosening.  One method of securing the screw to the plate involves the use of so-called "expansion-head screws."  U.S. Pat.  No.  4,484,570 discloses an expansion-head screw with a head that has a recess, the walls of which contain a number of slits.  After the expansion-head screw is inserted into bone through a hole in the fixation device, a locking screw is inserted into the recess to expand the walls of the recess to thereby lock the screw to the fixation device. 
In addition to securing the screw to the fixation device, it is also often desirable to insert the screws at an angle relative to the fixation device selected by the surgeon.  The prior art discloses a number of these so-called "polyaxial" systems, most of which utilize a bushing located in a hole in the fixation device to provide for angulation of the screw relative to the fixation device.  For example, U.S. Pat.  No.  5,954,722 discloses a polyaxial locking plate that includes a plate hole having a bushing rotatable within the hole.  As a screw is being inserted into bone through the bushing and plate hole, a threaded head of the screw engages a threaded internal surface of the bushing to expand the bushing against the wall of the plate hole, thereby locking the screw at the desired angular orientation with respect to the plate. 
The present invention relates to an improved bone fixation assembly that allows for a surgeon-selected angle of a fastening screw relative to the fixation device and locking of the fastening screw once the screw is at the desired orientation.