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-spherical 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, 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. 
   
     
     
       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 screw 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 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. 
   
     
     
       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 orthopaedic 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 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 directed 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 atapered 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 bones, 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 such 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;   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. 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;   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 advancement of the socket portion causes the fixation element, the socket portion and the receiving member to be locked relative to one another.   
     
     
       15. The assembly as set forth in claim 14, 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. 
     
     
       16. A pedicle screw and rod coupling member assembly wherein said rod coupling member and screw are capable of being selectively positioned and locked at a plurality of angles relative to one another, said assembly comprising:
 a bone screw having a curvate head;   a locking collar disposed around said curvate head and having a slot; and   a rod coupling member having a bore therethrough for receiving said curvate head and locking collar, said bore having an interior surface and a pair of upwardly extending members forming a rod-receiving U-shaped channel, said upwardly extending members having threads thereon for receiving a locking device,   wherein said pedicle screw and rod coupling member assembly assume a first position such that said rod coupling member and screw are capable of assuming a variety of angles relative to each other and a second position with a rod member disposed in said rod-receiving U-shaped channel and said locking device tightened on said threads such that said locking collar engages said rod coupling member interior surface to lock the angle of the screw relative to the rod coupling member.   
     
     
       17. The orthopedic device as set forth in claim 16, wherein the curvate 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. 
     
     
       18. The orthopedic device as set forth in claim 16, wherein the rod coupling member interior surface includes a lower portion that is inwardly tapered, and which engages said locking collar to crush lock the locking collar to the curvate head of the screw. 
     
     
       19. The orthopedic device as set forth in claim 16, wherein the rod coupling member interior surface includes a lower portion that is inwardly tapered, and the exterior surface of the locking collar is tapered to engage said inwardly tapered surface. 
     
     
       20. An orthopedic implant and bone anchoring member assembly wherein said bone anchoring member is selectively positionable and lockable at any one of a plurality of angles relative to said implant, said assembly comprising:
 a bone anchoring member having a curvate head and a threaded shaft;   a rod coupling member having a bore extending therethrough for receiving said curvate head in a lower portion thereof, said bore having an interior surface;   a rod positioned in an upper portion of said bore; and   a locking ring positioned in the lower portion of the bore between said curvate head and said interior surface,   wherein axial translation of said rod relative to said bore urges said curvate head into locking engagement with said locking ring to lock the angle of the screw relative to rod coupling element.   
     
     
       21. The assembly of claim 20, wherein said curvate head is semi-spherical. 
     
     
       22. The assembly of claim 20, wherein said bore includes at least two portions having different diameters. 
     
     
       23. The assembly of claim 22, wherein said locking ring is positioned at least partially within one of said portions having different diameters. 
     
     
       24. The assembly as set forth in claim 20, wherein said locking ring circumferentially retains the curvate head. 
     
     
       25. The assembly as set forth in claim 20, further comprising a locking device for engaging said rod to urge said rod to translate axially relative to said bore. 
     
     
       26. The assembly as set forth in claim 25, wherein said locking device comprises a nut engageable with threads on said orthopedic implant. 
     
     
       27. The assembly of claim 20, wherein the bore includes a tapered portion and the locking ring contacts against the interior surface of the tapered portion. 
     
     
       28. An orthopedic device for securing structures to bone, comprising:
 a screw having a curvate 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 curvate head of the screw such that the threaded shaft may be moved through a variety of angles 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;   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 curvate 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, 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.   
     
     
       29. The assembly as set forth in claim 28, wherein the curvate 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. 
     
     
       30. An orthopedic implant apparatus comprising: a fixation element having a curvate 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 curved interior volume for receiving therein the curvate 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 including a slot with the interior volume being deformable when the socket portion moves along the tapered portion;   wherein the curvate head is rotationally freely mounted within the curved 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 advancement of the socket portion causes the fixation element, the socket portion and the receiving member to be locked relative to one another.   
     
     
       31. The assembly as set forth in claim 30, wherein the curvate 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.