Bone Anchor Assemblies and Methods With Improved Locking

Abstract

Various exemplary methods and devices are provided for fixing bone anchors to bone. In general, the methods and devices can allow for a bone anchor to be fixed to a bone at a desired angle to a receiver member. In an exemplary embodiment, a bone anchor assembly is provided that includes a bone anchor configured to engage bone, a receiver member that polyaxially seats a spherical head of the bone anchor, and a compression member seated within the receiver member, proximally of the head of the bone anchor. The compression member can have a distal end with gripping features thereon configured to create at least one line contact and/or one band contact with the head of the bone anchor, thus providing a firm grip on the head of the bone anchor and reducing a risk of slippage of the bone anchor with respect to the receiver member.

What is claimed is: 
     
         1 . A bone anchor assembly, comprising:
 a bone anchor having a proximal head portion and a distal shank portion;   a receiver member having a polyaxial seat formed therein for polyaxially seating the head portion of the bone anchor; and   a compression member configured to be disposed within the receiver member and having a distal end configured such that, when the compression member is locked relative to the receiver member, the distal end grips the head portion of the bone anchor with greater friction as compared with a distal end formed as a negative of the head portion of the bone anchor.   
     
     
         2 . The bone anchor assembly of  claim 1 , wherein the distal end of the compression member includes a sidewall having an inner cylindrical surface, an outer surface, and a planar distal-facing surface extending between the inner and outer surfaces. 
     
     
         3 . The bone anchor assembly of  claim 1 , wherein the distal end of the compression member includes a sidewall having an inner cylindrical surface, an outer surface, and a conical distal-facing surface extending between the inner and outer surfaces. 
     
     
         4 . The bone anchor assembly of  claim 1 , wherein the distal end of the compression member includes a sidewall having an inner spherical surface, the inner spherical surface including a series of grooves that define a plurality of ring-shaped teeth extending inwards from the inner spherical surface. 
     
     
         5 . The bone anchor assembly of  claim 4 , wherein each of the plurality of teeth includes a sharp crest configured to contact the head portion of the bone anchor along a ring-shaped line. 
     
     
         6 . The bone anchor assembly of  claim 4 , wherein each of the plurality of teeth includes a crest defined by a spherical surface configured to contact the head portion of the bone anchor along a ring-shaped band. 
     
     
         7 . The bone anchor assembly of  claim 4 , wherein each of the plurality of teeth includes a crest defined by a conical surface configured to deform into contact with the head portion of the bone anchor along a ring-shaped band. 
     
     
         8 . The bone anchor assembly of  claim 1 , wherein the compression member includes a sidewall having
 a first hemicylindrical portion comprising an inner spherical surface,   a second hemicylindrical portion comprising an inner cylindrical surface,   an outer surface, and   a conical distal-facing surface extending between the inner and outer surfaces.   
     
     
         9 . The bone anchor assembly of  claim 8 , wherein the compression member provides uneven contact with the head portion of the bone anchor, thereby compressing the head portion of the bone anchor between the first hemicylindrical portion and the second hemicylindrical portion. 
     
     
         10 . The bone anchor assembly of  claim 1 , wherein the compression member includes a first inner spherical surface having a first radius, and a second inner spherical surface extending distally from the first inner spherical surface, the second inner spherical sidewall having a second radius that is greater than the first radius. 
     
     
         11 . The bone anchor assembly of  claim 10 , wherein at least one of the first radius and the second radius is less than a radius of the head portion of the bone anchor such that the compression member forms an interference fit with the head portion of the bone anchor. 
     
     
         12 . A bone anchor assembly, comprising:
 a bone anchor having a proximal head portion and a distal shank portion;   a receiver member having a polyaxial seat formed in a distal end thereof and configured to polyaxially seat the head portion of the bone anchor; and   a compression member configured to be disposed within the receiver member and having a polyaxial seat formed in a distal end thereof and configured to polyaxially seat the head portion of the bone anchor, the compression member having a surface feature formed in the polyaxial seat that is configured to make at least one of line contact and band contact with the head portion of the bone anchor to substantially prevent movement of the head portion of the bone anchor relative to the receiver member when the compression member is locked relative to the receiver member.   
     
     
         13 . The bone anchor assembly of  claim 12 , wherein the surface feature comprises at least one ring-shaped tooth having a crest configured to make ring-shaped line contact with the head portion of the bone anchor. 
     
     
         14 . The bone anchor assembly of  claim 12 , wherein the compression member includes a sidewall having a first hemicylindrical portion comprising an inner spherical surface, and a second hemicylindrical portion comprising an inner cylindrical surface, an outer surface, and a conical distal-facing surface extending between the inner and outer surfaces. 
     
     
         15 . The bone anchor assembly of  claim 12 , wherein the polyaxial seat of the receiver member includes at least one ring-shaped tooth having a crest configured to make ring-shaped line contact with the head portion of the bone anchor. 
     
     
         16 . A surgical method, comprising:
 advancing a bone anchor having a proximal head portion and a distal shank portion into a bone, the head portion of the bone anchor being seated in a polyaxial seat formed in a receiver member;   advancing a compression member disposed within the receiver member such that the compression member grips the head portion of the bone anchor with greater friction as compared with a compression member formed as a negative of the head portion of the bone anchor.   
     
     
         17 . The method of  claim 16 , further comprising, after advancing the bone anchor and before advancing the compression member, positioning a spinal fixation rod within the receiver member and proximal of the compression member. 
     
     
         18 . The method of  claim 16 , wherein advancing the compression member comprises applying a closure mechanism to the receiver member to apply a distally directed force to the spinal fixation element and the compression member to thereby lock the spinal fixation element and the bone anchor in a fixed position relative to the receiver member.