Polyaxial locking screw plate assembly
A bone fixation assembly includes a fixation device, a cup shaped bushing, and a fastening screw. The fixation device has a through passage with an annular recess. The a fastening screw has a threaded shaft for insertion through the through passage and for threadable insertion into bone. The screw has a head with substantially frustospherical shaped side surfaces and mates with the cup shaped bushing. The annular recess in the through passage of the fixation device is configured and dimensioned to downwardly, directly, and annularly receive an upper end of the bushing under compression with a snap fit to prevent the screw from backing out of the bone fixation assembly.
1. A bone fixation assembly comprising:
(a) a fixation device having a through passage;
(b) an annular recess in said through passage;
(c) a fastening screw having a threaded shaft for insertion through said through passage and threadable insertion into bone, and a head having substantially frustospherical shaped side surfaces;
(d) a bushing having:
(i) an annular cup shaped side wall having upper and lower ends and defining an upwardly open interior bowl for receiving said screw head, said upper end of said side wall having a first circumference when said bushing is not engaged with said annular recess of said through passage, and a second, smaller circumference when said bushing is engaged with said annular recess of said through passage;
(ii) annularly spaced slots in said side wall which depend downwardly from said upper end for allowing inward compression of said side wall at said upper end;
(iii) a centrally located screw shank passage in said lower end;
(iv) said bowl configured and dimensioned for polyaxial rotation of said screw head therein and for preventing back out of said screw head when said side wall is compressed inwardly; and
(e) said fixation device through passage configured and dimensioned to downwardly receive said bushing therein to inwardly compress said upper end of said side wall of said bushing about said screw head,
wherein said annular recess in said through passage is configured and dimensioned to annularly and directly receive said upper end of said annular cup shaped side wall of said bushing under compression with a snap fit when said upper end of said annular cup shaped side wall of said bushing is at said second circumference whereby said screw head is retained in said bowl and prevented from backing out.
2. The bone fixation assembly of claim 1 wherein said screw head is expandable for thereby locking said screw relative to said assembly.
3. The bone fixation assembly of claim 2 , further comprising an expansion means, wherein said screw head is split into segments at its upper or lower end, and the expansion means is received in said screw head between segments for expanding said segments.
4. The bone fixation assembly of claim 3 wherein said expansion means includes radially extending cam ramps.
5. The bone fixation assembly of claim 3 wherein said fastening screw and said expansion means are axially cannulated for receiving a guide wire.