Polyaxial pedicle screw having a threaded and tapered compression locking mechanism

Abstract

A polyaxial orthopedic device for use with rod implant apparatus includes a screw having a curvate head, a two-piece interlocking coupling element which mounts about the curvate head, and a rod receiving cylindrical body member having a tapered socket into which both the screw and the interlocking coupling element are securely nested. The interlocking coupling element includes a socket portion which is slotted and tapered so that when it is radially compressed by being driven downwardly into the tapered socket in the cylindrical body it crush locks to the screw. The securing of the rod in the body member provides the necessary downward force onto the socket portion through a contact force on the top of the cap portion. Prior to the rod being inserted, therefore, the screw head remains polyaxially free with respect to the coupling element and the body. In a preferred embodiment, the cap portion and the socket portion are formed and coupled in such a way that when the cap portion is compressed toward the socket portion, there is an additional inward radial force applied by the cap portion to the socket portion, thereby enhancing the total locking force onto the head of the screw.

1. A polyaxial screw and coupling element assembly for use with orthopedic rod implantation apparatus, comprising:
 a screw having a semi-spherical head;    a cylindrical body including an axial bore defining a bottom chamber portion at a bottom end thereof and a rod receiving channel at a top end thereof, said bottom chamber portion further defining a tapered lower portion and a constant diameter upper chamber portion, and said top end having a threading thereon;    a two-piece interlocking coupling element including a socket portion having a semi-spherical interior volume for receiving therein the head of said screw, upper and lower sections, and vertical slots formed in said upper and lower sections, at least one of said slots rendering said interior volume expandable and contractable, said lower section having a tapered exterior surface for nesting in said tapered lower chamber portion of said cylindrical body such that forceable advancement of socket portion along the tapered lower chamber portion of the axial bore causes the at least one of said slots to narrow such that the semi-sperical interior volume contract, and    a cap portion having an opening in a bottom thereof and an interior chamber extending upwardly therefrom for joining with, and slideably retaining therein, the upper section of said socket portion, said cap portion initially seating with a portion thereof extending into a bottom portion of said rod receiving channel; and    a top locking nut, mateable with said threading, of locking a rod in said channel and for applying therethrough a downward force onto said cap portion, 
 wherein said semi-spherical head portion is rotationally freely mounted within the semi-spherical interior volume of the socket portion prior to said socket portion being forceably advanced into the tapered lower chamber portion of the axial bore, and  
 whereby downward compression of a rod in said channel portion of said body member, by said top locking nut, onto the cap portion causes the forceable advancement of the socket portion into the tapered lower chamber portion of the axial bore, and locks the screw, coupling element and body relative to one another.  
   
     
     
       2. The polyaxial screw as set forth in  claim 1 , wherein said semi-spherical head of said screw further includes a recess formed therein for receiving therein a screwdriving tool such that said screw may be threadably advanced into a vertebral bone. 
     
     
       3. The polyaxial screw as set forth in  claim 1 , wherein said threading on said top end is on the interior surface of said channel. 
     
     
       4. The polyaxial as set forth in  claim 3 , wherein said cap portion further includes a threading and wherein said cap portion needs to be threadably advanced along the threading to be seated in the bore into its initial position. 
     
     
       5. The assembly as set forth in  claim 1 ,
 wherein said socket portion further comprises a substantially constant diameter upper section having an outwardly annular extending lip at an extreme end thereof,    wherein said opening in the bottom of the cap portion comprises an inwardly directed annular lip, and    wherein at least one of said vertical slots in the upper section thereof to be expandable and contractable such that the upper section of the socket portion may be forceably inserted into the opening in the bottom of the cap portion so that is may be retained in the interior chamber therein by mutual interference engagement of the inwardly directed annular lip of the cap portion and the outwardly extending annular lip of the socket portion.    
     
     
       6. The assembly as set forth in  claim 5 , wherein the interior chamber of the cap portion comprises a tapered surface such that advancement thereof into the hole causes an inwardly directed force against the upper section of the socket portion, therein causing the at least one of said vertical slots in the upper section to narrow and causes the upper section to contract and further lock the head of the screw within the interior semi-spherical volume of the socket portion. 
     
     
       7. An orthopedic implant apparatus having a rod and a plurality of screw and coupling element assemblies, comprising:
 at least one screw having a semi-spherical head;    at least one cylindrical body including an axial bore defining a bottom chamber portion at a bottom end thereof and a rod receiving channel at a top end thereof, said bottom chamber portion further defining a tapered lower portion and a constant diameter upper chamber portion, and said top end having a threading thereon;    at least one corresponding two-piece interlocking coupling element including 
 a socket portion having a semi-spherical interior volume for receiving therein the head of said corresponding screw, upper and lower sections, and vertical slots formed in said upper and lower sections, at least one of said slots rendering said interior volume expandable and contractable, said lower section having a tapered exterior surface for nesting in said corresponding tapered lower chamber portion of said cylindrical body such that forceable advancement of socket portion along the tapered lower chamber portion of the axial bore causes the at least one of said slots to narrow such that the semi-spherical interior volume contract, and  
 a cap portion having an opening in a bottom thereof and an interior portion having an opening in a bottom thereof and an interior chamber extending upwardly therefrom for joining with, and slideably retaining therein, the upper section of said socket portion, said cap portion initially seating with a portion thereof extending into a bottom portion of said rod receiving channel; and  
   at least one corresponding top locking nut, mateable with said threading, for locking a rod in said channel and for applying therethrough a downward force onto said cap portion,    wherein said semi-spherical head portion is rotationally freely mounted within the semi-spherical interior volume of the socket portion prior to said socket portion being forceably advanced into the tapered lower chamber portion of the axial bore, and    whereby downward compression of a rod in said channel portion of said body member, by said top locking nut, onto the cap portion causes the forceable advancement of the socket portion into the tapered lower chamber portion of the axial bore, and locks the screw, coupling element and body relative to one another.    
     
     
       8. The apparatus as set forth in  claim 7 , wherein said threading on said top end is on the interior surface of said channel. 
     
     
       9. The apparatus as set forth in  claim 8 , wherein said cap portion further includes a threading and wherein said cap portion needs to be threadably advanced along the threading to be seated in the bore into its initial position. 
     
     
       10. The apparatus as set forth in  claim 7 ,
 wherein said socket portion further comprises a substantially constant diameter upper section having an outwardly annular extending lip at an extreme end thereof,    wherein said opening in the bottom of the cap portion comprises an inwardly directly annular lip, and    wherein at least one of said vertical slots in the upper section of said socket portion renders the upper section thereof to be expandable and contractable such that the upper section of the socket portion may be forceably inserted into the opening in the bottom of the cap portion so that it may be retained in the interior chamber therein by mutual interference engagement of the inwardly directed annular lip of the cap portion and the outwardly extending annular lip of the socket portion.    
     
     
       11. The apparatus as set forth in  claim 10 , wherein the interior chamber of the cap portion comprises a tapered surface such that advancement thereof into the hole causes an inwardly directed force against the upper section of the socket portion, therein causing the at least one of said vertical slots in the upper section to narrow and causes the upper section to contract and further lock the head of the screw within the interior semi-spherical volume of the socket portion. 
     
     
       12. An orthopedic device for securing immobilizing structures to sequences of bone, comprising:
   a screw having a semi - spherical head and a threaded shaft;        a coupling element having an axial hole extending therethrough, a portion of the axial hole defining an interior volume for receiving therein the semi - spherical head of the screw that the threaded shaft may be moved through a variety of angles relative to the axial hole, the coupling element further including at least one slot rendering at least the interior volume deformable, the coupling element further including a tapered exterior surface, the coupling element further comprising a two - part interlocking coupling element including:        a socket portion containing the interior volume, the interior volume being semi - spherical shaped, the socket portion further including upper and lower sections, and at least one vertical slot formed in each of the upper and lower sections, at least one of the    slots rendering the interior volume deformable, the lower section having the tapered exterior surface; and        a cap portion having an opening in a bottom thereof and an interior chamber extending upwardly therefrom for joining with, and slideably retaining therein, the upper section of the socket portion;        a receiving member including a through hole having an interior wall surface, a portion of the interior wall surface of the through hole being shaped to receive the coupling element and the screw when the semi - spherical head of the screw is mounted within the coupling element; and        wherein advancement of the screw through the through hole relative to the receiving member when the exterior surface of the coupling element engages the interior wall surface of the through hole prevents the coupling element from further advancement through the hole, preventing the semi - spherical head of the screw from advancing further through the through hole, and causing locking of the screw relative to both the coupling element and to the receiving member thereby locking the angle of the screw relative to the axial hole.     
     
     
       13. The assembly as set forth in  claim 12  wherein the semi- spherical head of the screw further includes a recess formed therein for receiving therein a screwdriving tool such that the screw may be advanced into a vertebral bone.   
     
     
       14. The assembly as set forth in  claim 12 , wherein the cap portion further includes a threading. 
     
     
       15. An orthopedic implant apparatus comprising:
   a fixation element having a semi - spherical head and a shaft extending therefrom;        a receiving member including an axial bore defined by an interior surface wall, a portion of the axial bore having a tapered portion;        a socket portion having a semi - spherical interior volume for receiving therein the semi - spherical head, and an exterior surface capable of nesting against the interior surface wall of the tapered portion, the socket portion being located in the axial bore of the receiving member, the socket portion further including:      upper and lower socket sections, and at least one vertical slot formed in at least one of the upper or lower sections, the at least one slot rendering the spherical interior volume deformable, where he exterior surface of the socket portion is tapered and located on the lower section; and        a cap portion having an opening in a bottom hereof and an interior chamber extending upwardly therefrom for joining with, and slideably retaining therein, the upper socket portion;        wherein the semi - spherical head is rotationally freely mounted within the semi - spherical interior volume of the socket portion prior to the socket portion being forcibly advanced against the interior surface wall of the tapered portion, and whereby    after forcible advancement of the socket portion causes the fixation element, the socket portion and the receiving member to be locked relative to one another.       
     
     
       16. The assembly as set forth in  claim 15  wherein the semi- spherical head of the fixation element further includes a recess formed therein for receiving therein a screwdriving tool such that the fixation element may be advanced into a vertebral bone.   
     
     
       17. The assembly as set forth in  claim 15  wherein the cap portion further includes a threading.  
     
     
       18. An orthopedic implant apparatus comprising:
   a fixation element having a semi - spherical head, having a lower and upper curvate surface, and a threaded shaft extending from the semi - spherical head;        a receiving member including an axial bore defined by an interior surface wall, the axial bore having a lower portion and an upper threaded portion;        a socket portion comtaining the semi - spherical head of the fixation element therein and having a lower socket portion and a threaded upper socket portion, the socket portion being moveably located in the axial bore;        wherein the lower socket portion has an interior volume defined by an interior surface which receives the lower curvate surface of the semi - spherical head, such that the threaded shaft is inserted through a hole in the interior surface and is capable of being moved through a variety of angles relative to the socket portion, the lower socket portion further including an exterior surface which seats against the interior surface wall of the lower portion, the lower socket portion further including at least one vertical slot rendering the interior volume deformable; and        the threaded upper socket portion comprising a cap portion having an opening in a bottom thereof and an interior chamber extending upwardly therefrom for joining with,    and slideably retaining therein, the lower socket portion, disposed above the upper curvate surface of the semi - spherical head of the fixation element, whereby downward pressure on the cap portion causes compression of the lower curvate surface against the interior surface of the lower socket portion to lock the fixation element relative to the socket portion, causing the upper and lower socket portions, the semi - spherical head of the fixation element, and the receiving member to be locked relative to one another.     
     
     
       19. The assembly as set forth in  claim 18  wherein the semi- spherical head of the fixation element further includes a recess formed therein for receiving therein a screwdriving tool such that the fixation element my be threadably advanced into a vertebral bone.   
     
     
       20. The implant apparatus of  claim 18  further comprising a locking nut engageable with the upper threaded portion of the receiving member, tightening of the locking nut with respect to the receiving member exerting a downward force onto a rod disposed in the receiving member such that the rod exerts a downward force on the cap portion. 
     
     
       21. A bone anchor and coupling member assembly wherein said coupling member is capable of being selectively positioned and locked at a plurality of angles relative to the bone anchor, said assembly comprising:
   a bone anchor having a curvate head;        a coupling member having an axial bore for receiving said curvate head, said bore having an interior surface, and a channel formed herein for receiving an elongate member, at least a portion of said channel being in spatial communication with said bore;        a first intervening member positioned between the curvate head in the bore and an elongate member positioned in said channel; and        a second intervening member positioned in the bore between said curvate head and said interior surface,        wherein a force applied to said first intervening member urges said curvate head to translate axially within said bore and into locking contact with said second intervening member.     
     
     
       22. The assembly of  claim 21  wherein said curvate head is semi- spherical.   
     
     
       23. The assembly of  claim 21  wherein said bore includes at least two portions having different diameters. 
     
     
       24. The assembly of  claim 23  wherein said second intervening member is positioned within one of said portions having different diameters. 
     
     
       25. The assembly as set forth in  claim 21  wherein said second intervening member circumferentially retains the curvate head. 
     
     
       26. The assembly as set forth in  claim 21  wherein the force applied to said first intervening member is provided through the elongate member positioned in the channel. 
     
     
       27. An orthopedic device comprising:
   a screw having a semi - spherical head and a threaded shaft,        a coupling element having an axial hole extending therethrough for receiving therein the semi - spherical head of the screw that the screw may be moved through a variety of angles relative to the axial hole, the coupling element further including at least one slot; and        a receiving member including a through hole having an interior wall surface, a portion of the interior wall surface of the through hole being shaped to receive the coupling element and the screw when the semi - spherical head of the screw is mounted within the coupling element,        wherein engagement of the coupling element and the interior wall surface selectively prevents the screw from moving relative to the axial hole, thereby locking the angle of the screw relative to the axial hole.     
     
     
       28. The orthopedic device as set forth in  claim 27  wherein the semi- spherical head of the screw further includes a recess formed therein for receiving therein a screwdriving tool such that the screw may be advanced into a vertebral bone.   
     
     
       29. The orthopedic device as set forth in  claim 27  wherein the interior wall surface of the receiving member is tapered. 
     
     
       30. The orthopedic device as set forth in  claim 27  wherein the interior wall surface of the receiving member and the exterior surface of the coupling element are tapered. 
     
     
       31. The orthopedic device as set forth in  claim 27  wherein coupling element axial hole has a concave surface conforming to the semi- spherical screw head.