Highly-versatile variable-angle bone plate system
A bone plate system for internal fixation of bone fractures includes a bone plate having a plurality of bone plate holes. The holes are constructed to receive either a non-locking, locking, or variable-angle locking bone screw. The holes have discrete columns of teeth or thread segments arranged around the inner surface of the hole for engaging threads on the heads of locking and variable-angle locking bone screws. Conventional locking bone screws engage the bone plate coaxially with the central axis of the bone plate hole. Variable-angle locking bone screws can engage the bone plate at a selectable angle within a range of selectable angles relative to the central axis of the bone plate hole. The head of the variable-angle locking screw is at least partially spherical, and the thread thereon has a profile that follows the arc-shaped radius of curvature of the spherical portion of the screwhead.
1. A method of forming a thread in an outer surface of a head of a bone screw that is elongate along a central axis, the method comprising the steps of:
orienting a thread cutter in a first orientation with respect to the bone screw, the thread cutter including a cutting bit having an apex, wherein the cutting bit extends along a longitudinal axis that normally intersects the apex of the cutting bit, and when the thread cutter is in the first orientation the longitudinal axis is perpendicular to a central axis of the bone screw;
after the orienting step, rotating the bone screw at least one complete revolution about the central axis relative to the cutting bit while 1) maintaining the thread cutter in the first orientation with respect to the bone screw; 2) maintaining contact between the cutting bit and the outer surface; and 3) translating the bone screw relative to the cutting bit with respect to the central axis a first distance per revolution; and
after the rotating step, further rotating the bone screw another at least one complete revolution about the central axis relative to the cutting bit while 1) maintaining the thread cutter in the first orientation with respect to the bone screw; 2) maintaining contact between the cutting bit and the outer surface; and 3) translating the bone screw relative to the cutting bit with respect to the central axis a second distance per revolution that is different than the first distance per revolution,
wherein both of the rotating steps result in the forming of a cross-sectional profile of the thread, the cross-sectional profile having a plurality of peaks, a plurality of troughs each of which are positioned between adjacent ones of the plurality of peaks, and a plurality of flanks each of which connect one of the plurality of peaks to an adjacent one of the plurality of troughs, each of the plurality of peaks being spaced farther from the central axis than adjacent ones of the plurality of flanks, each of the plurality of flanks being spaced farther from the central axis than the adjacent one of the plurality of troughs.
2. The method of claim 1 , wherein both of the rotating steps result in the plurality of peaks each being positioned on a non-linear curve.
3. The method of claim 2 , wherein the non-linear curve forms a portion of a circle.
4. The method of claim 1 , wherein both of the rotating steps result in the plurality of troughs each being positioned on a non-linear curve.
5. The method of claim 4 , wherein the non-linear curve forms a portion of a circle.
6. The method of claim 1 , wherein both of the rotating steps comprise moving the bone screw while the cutting bit remains stationary.
7. The method of claim 1 , wherein both of the rotating steps comprise moving the cutting bit while the bone screw remains stationary.
8. The method of claim 1 , wherein both of the rotating steps comprise moving the bone screw and the cutting bit.