Bottom-loading bone anchor assemblies

Abstract

Bone anchor assemblies are disclosed herein and include bone anchors, receiver members, and closure mechanisms. In one embodiment, a receiver member can be configured to seat a portion of a bone anchor and to receive a spinal fixation element, such as a rod. In an exemplary embodiment, the receiver member is bottom-loading and can include a seat having an opening that can be selectively increased and decreased in size so as to allow a bone anchor to be bottom-loaded. A force can be applied to the receiver member to increase a size of the opening, allowing a portion of the anchor to pass through the opening and into the seat. A closure element can be coupled to the receiver member and can apply a force to decrease a size of the seat and a size of the opening, angularly fixing the receiver member relative to the bone anchor.

What is claimed is: 
     
         1 . A bone anchor assembly, comprising:
 a bone anchor having a proximal head and a distal threaded portion;   a receiver member having a proximal portion with opposed arms configured to receive a spinal fixation rod therebetween, and a distal portion having a polyaxial seat formed therein and configured to polyaxially seat the proximal head of the bone anchor, the opposed arms being configured to move toward one another to cause a size of the polyaxial seat to increase, and the opposed arms being configured to move away from one another to cause the size of the polyaxial seat to decrease.   
     
     
         2 . The bone anchor assembly of  claim 1 , further comprising at least one relief slot formed in the distal portion of the receiver member and configured to allow the opposed arms to move toward and away from one another and to allow the size of the polyaxial seat to increase and decrease. 
     
     
         3 . The bone anchor assembly of  claim 1 , wherein the distal portion includes opposed relief slots formed therein and extending proximally from a distal-most end thereof. 
     
     
         4 . The bone anchor assembly of  claim 2 , wherein the at least one relief slot is substantially T-shaped. 
     
     
         5 . The bone anchor assembly of  claim 1 , further comprising a closure mechanism configured to mate to the opposed arms and to cause the opposed arms to move away from one another such that the distal portion of the receiver member compress around the proximal head of the bone anchor to prevent removal of the proximal head from the polyaxial seat in the receiver member. 
     
     
         6 . The bone anchor assembly of  claim 5 , wherein the closure mechanism comprises a threaded set screw having an outer diameter that increases in a distal-to-proximal direction. 
     
     
         7 . The bone anchor assembly of  claim 1 , wherein an outer diameter of the proximal head of the bone anchor is greater than a size of an opening formed in a distal end of the receiver member and extending into the polyaxial seat. 
     
     
         8 . The bone anchor assembly of  claim 1 , wherein the opposed arms of the receiver member define a U-shaped recess configured to seat a spinal fixation rod, and wherein the U-shaped recess includes opposed flat surfaces thereon for seating spinal fixation rods of differing diameters, the opposed flat surfaces extending at an oblique angle relative to a longitudinal axis of the receiver member. 
     
     
         9 . The bone anchor assembly of  claim 1 , wherein the polyaxial seat is configured to frictionally engage the proximal head of the bone anchor when the head is seated therein and no closure mechanism is applied to the receiver member. 
     
     
         10 . The bone anchor assembly of  claim 1 , wherein the receiver member includes opposed flat surfaces formed therein and configured to limit movement of the distal threaded portion of the bone anchor along a single plane of motion. 
     
     
         11 . A method for implanting a bone anchor, comprising:
 implanting a threaded shank of a bone anchor in bone;   positioning a spinal fixation rod within a u-shaped recess in a receiver member movably coupled to a proximal head formed on the threaded shank of the bone anchor;   applying a closure mechanism to the receiver member to cause a distal portion of the receiver member to compress around the proximal head of the bone anchor and thereby lock the receiver member in a fixed position relative to the bone anchor.   
     
     
         12 . The method of  claim 11 , wherein the closure mechanism comprises an outer set screw that is threaded into a proximal portion of the receiver member. 
     
     
         13 . The method of  claim 12 , further comprising threading an inner set screw into the outer set screw to lock the spinal fixation rod within the receiver member. 
     
     
         14 . The method of  claim 11 , wherein the closure mechanism is tapered such that the closure mechanism causes opposed proximal arms of the receiver member to move away from one another thereby causing the distal portion of the receiver member to compress around the proximal head of the bone anchor. 
     
     
         15 . The method of  claim 11 , wherein the receiver member is limited to movement within a single plane of motion relative to the bone anchor.