Polyaxial bone screw with compound articulation

Abstract

A polyaxial bone screw assembly includes a threaded shank body having an upper capture structure, a receiver and a retainer having spherical inner and outer surfaces. The retainer is compressible, having an opening with two ends that may be pressed toward one another. The retainer spherical surfaces slidingly mate with both the receiver and the capture structure, providing multiple or compound polyaxial articulation.

1 . In a polyaxial bone screw assembly for surgical implantation and including a shank having an upper end and a threaded body for inserting into a bone and a receiver having a channel for receiving a rod within the channel, the improvement comprising:
 a) an articulation structure located between the shank upper end and the receiver and having a multiple polyaxial articulation formed between the shank upper end, the receiver and the articulation structure during positioning of the shank relative to the receiver, the retaining structure engaging and located between both the shank upper end and the receiver whereby the shank polyaxially articulates through the retaining structure with the receiver so as to provide compound polyaxial articulation.   
   
   
       2 . The improvement of  claim 1  wherein the articulation structure has first and second ends in spaced relation, the articulation structure being compressible. 
   
   
       3 . In a polyaxial bone screw assembly for surgical implantation and including a shank having an upper end and a threaded body for inserting into a bone and a receiver having a channel for receiving a rod within the channel, the improvement wherein the shank upper end has a convex articulation surface spaced in relation with respect a concave articulation surface of the receiver by an articulating structure positioned between the shank and the receiver that polyaxially articulates both with respect to the shank and the receiver during positioning so as to provide compound polyaxial articulation between the shank and the receiver. 
   
   
       4 . The improvement of  claim 3  wherein the receiver has first and second arms defining a channel for receiving a rod, the first and second arms having a discontinuous flange form thereon. 
   
   
       5 . The improvement of  claim 3  wherein the receiver has first and second arms defining a channel for receiving a rod and the shank upper end is sized and shaped for frictional engagement with the rod. 
   
   
       6 . A polyaxial bone screw assembly method comprising:
 a) expanding a resilient and open retaining structure about a neck of a bone screw shank, the neck disposed between an upper portion having a curved convex surface and an elongate shank body for fixation to bone;   b) compressing the retaining structure at the bone screw neck and inserting the bone screw upper portion, while the retaining structure is disposed on the neck of the shank and the compressed retaining structure into a receiver cavity; and   c) expanding the retaining structure about the upper portion within the receiver cavity, such that the the retaining structure is located between the shank and the receiver and is in sliding engagement with the convex surface after shape recovery, thereby capturing the upper portion within the receiver and providing for compound polyaxial articulation between the bone screw and the receiver during positioning.