Polyaxial bone screw with split retainer ring

Abstract

A polyaxial bone screw includes a head member and a shank member. The shank has a capture end and an opposite threaded end for threaded insertion into a vertebra. The head has a U-shaped cradle for receiving a spinal fixation rod and a central bore for receiving the capture end of the shank. An expandible retainer ring with a radial split is snapped onto the capture end of the shank to retain it within the head. The retainer ring has a spherical outer surface which forms a ball joint with a spherical socket cavity within the head to enable the head to be angled relative to the shank. A threaded closure plug is tightened within the cradle to clamp the rod into engagement with a knurled dome on the capture end of the shank to secure the rod relative to the vertebra.

what is claimed and desired to be secured by Letters Patent is as follows: 
     
         1 . A polyaxial head bone screw assembly for surgical implantation and comprising:
 (a) a shank having a threaded body and a capture end;   (b) a head having a channel outwardly open and adapted to receive a rod within said channel; said head having a shank mating side with a shank receiving bore formed therethrough and a seat; said head having a cavity therein open into said bore;   (c) said capture end of said shank being received within said shank receiving bore of said head;   (d) a retainer ring with a split therein to enable resilient expansion and contraction of a diameter of said retainer ring; said retainer ring being received in said head cavity and securely receiving said capture end of said shank while within said cavity so as to retain said capture end within said head; said shank and said retaining ring forming a joined shank and retainer ring structure wherein the retainer ring rotates in combination with said shank in said head seat enabling selective angular positioning of said shank with respect to said head, when in an assembly configuration; and   (e) a closure member operably mountable within said head in such a manner as to close said channel; a rod being received in the channel to directly engage said shank and retainer ring structure in such a manner so as to secure said head from movement relative to such a rod and to secure said head from angular movement relative to said shank and retainer ring structure in a locking configuration.   
     
     
         2 . An assembly as set forth in  claim 1 , wherein:
 (a) said retainer ring has an outer surface, of which at least a portion is substantially spherical; and   (b) said head cavity has an inner surface, of which at least a portion is substantially spherical to enable mating engagement with said outer surface of said retainer.   
     
     
         3 . An assembly as set forth in  claim 1 , wherein:
 (a) said capture end of said shank has a frusto-conical surface diminishing in diameter toward said shank; and   (b) said retainer ring has a frusto-conical retainer bore formed therethrough to enable mating engagement with said capture end within said ring bore.   
     
     
         4 . An assembly as set forth in  claim 1 , wherein:
 (a) said retainer ring has a substantially radial split to enable resilient expansion and contraction of a diameter of said retainer ring.   
     
     
         5 . An assembly as set forth in  claim 4 , wherein:
 (a) said cavity has a first region having a partial hemispherical surface sized and shaped to mate with a partial hemispherical surface on said retainer ring; and   (b) said cavity has a second region directly accessible from said first region and having a larger radius than said first region wherein said ring is expandable as it joins with said shank.   
     
     
         6 . An assembly as set forth in  claim 1 , including:
 (a) a knurled dome formed on said capture end of said shank with a non-slip formation.   
     
     
         7 . An assembly as set forth in  claim 1 , wherein:
 (a) said capture end of said shank has a tool formation adapted to enable engagement by a tool for rotation.   
     
     
         8 . An assembly as set forth in  claim 1 , wherein:
 (a) said head has an internal guide and advancement structure formed therein; and   (b) said closure member has a radially outward surface with an external guide and advancement structure formed thereon which is sized and shaped to rotatably mate with said internal guide and advancement structure of said head.   
     
     
         9 . An assembly as set forth in  claim 1 , wherein:
 (a) said head has an internal mating structure formed therein; and   (b) said closure member has a radially outward surface with an external mating structure formed thereon which is configured to rotatably mate with said internal mating structure of said head.   
     
     
         10 . In a polyaxial head bone screw assembly for surgical implantation and including a shank having a threaded body and a capture end and a head having an outward opening channel adapted to receive a rod within said channel, said head having a shank receiving bore formed therethrough and a seat, the improvement comprising:
 (a) said capture end of said shank being received within said shank receiving bore of said head;   (b) a retainer ring that is loaded separately from said shank into said head and being joined to said capture end of said shank within said head to retain said capture end within said head; said shank and retaining ring forming a single structure and being rotatable in the head seat so as to enable angular positioning of said shank with respect to said head; said shank and retainer ring being joined such that said retainer ring rotates with said shank during the angular positioning of said shank relative to said head; and   (c) a closure member operably engageable with said head and adapted to engage a rod within said channel; the rod being in direct engagement with said shank and retainer ring structure in the channel to produce friction between said ring and said head seat so as to secure said head from movement relative to such a rod and to secure said head from angular movement relative to said shank and said retainer ring structure, when in a locking configuration.   
     
     
         11 . An assembly as set forth in  claim 10 , wherein:
 (a) said retainer ring has an outer surface, of which at least a portion is substantially spherical; and   (b) said head has a cavity with an inner surface, of which at least a portion is substantially spherical and sized and shaped to enable mating engagement with said outer surface of said retainer.   
     
     
         12 . An assembly as set forth in  claim 10 , wherein:
 (a) said capture end of said shank has a frusto-conical surface diminishing in diameter toward said shank; and   (b) said retainer ring has a frusto-conical retainer bore formed therethrough to enable mating engagement with said capture end within said retainer bore upon expansion of said ring with said ring returning to an original diameter after placement on said shank.   
     
     
         13 . An assembly as set forth in  claim 10 , wherein:
 (a) said retainer ring has a radial split to enable resilient expansion and contraction of a diameter of said retainer ring.   
     
     
         14 . An assembly as set forth in  claim 10 , wherein:
 (a) said cavity has an expansion chamber above said head spherical surface that allows expansion of said ring therein as said shank is inserted into said ring.   
     
     
         15 . An assembly as set forth in  claim 10 , wherein:
 (a) said capture end of said shank has a dome formation to enable non-slip engagement with a rod within said channel of said head.   
     
     
         16 . An assembly as set forth in  claim 10 , wherein:
 (a) said capture end of said shank has a knurled dome having a radius and being adapted to enable non-slip engagement with a rod within said channel of said head.   
     
     
         17 . An assembly as set forth in  claim 10 , wherein:
 (a) said capture end of said shank has a tool formation so as to be adapted enable non-slip engagement by a tool.   
     
     
         18 . An assembly as set forth in  claim 10 , wherein:
 (a) said head has an internal guide and advancement structure formed therein; and   (b) said closure member has a radially outward surface with an external guide and advancement structure formed thereon which is sized and shaped to rotatably mate with said internal guide and advancement structure of said head.   
     
     
         19 . A polyaxial bone screw assembly, comprising:
 a) a head including a rod-receiving channel, an inner cavity communicating with the rod-receiving channel and having a seat therein, and lower opening joining the seat with an outer surface of the head;   b) a resilient retainer ring received in the cavity and including an inner bore and an outer seating surface; and   c) a shank including a threaded body and a capture end extending upwardly therefrom, the threaded body being adapted for implantation into a bone, and the shank being sized and shaped to be received through the head lower opening and into the cavity;   d) the retainer ring inner bore resiliently receiving and matingly engaging the shank capture structure when the shank is received through the lower opening and into the inner cavity, the retainer ring outer seating surface slidingly engaging the head seat, such that the shank polyaxially rotates relative to the head while in an adjustment configuration; and   e) a closure member operably received in the head in such a manner as to capture a rod located within the rod-receiving channel.