Bone anchor assemblies

1. A bone anchor assembly comprising:

a bone anchor having a proximal head and a distal shaft configured to engage bone,

a receiver member for receiving a spinal fixation element to be coupled to the bone anchor, the receiver member having

a proximal end having a pair of spaced apart receiver member arms defining a recess therebetween, the receiver member arms including an inner thread,

a distal end having a distal end surface defining an opening through which at least a portion of the bone anchor extends, and

a central passage extending between the proximal end and the distal end, the central passage communicating with the opening in the distal end surface, the central passage having a central longitudinal axis extending between the proximal end and the distal end,

a compression member positioned within the central passage of the receiver member, the compression member having a proximal end and a distal end, the proximal end of the compression member having a pair of spaced apart compression member arms defining a U-shaped seat for receiving the rod, each compression member arm having a proximal surface, each compression member arm including an outer wall and an inner wall, the proximal surface of the compression member arm connecting the outer wall and the inner wall, the distal end of the compression member having a distal surface engageable with the proximal head of the bone anchor,

an outer set screw positionable between and engaging the receiver member arms, the outer set screw including a first outer thread for engaging the inner thread, the outer set screw having a distal surface engageable with the proximal surface of the compression member arms, the outer set screw having a set screw central passage from a top surface of the outer set screw to a bottom surface of the outer set screw, the set screw central passage having a second internal thread, and

an inner set screw positionable within the set screw central passage, the inner set screw having a second outer thread for engaging the second inner thread, the inner set screw operable to act on the spinal rod to fix the spinal rod relative to the receiver member;

wherein engagement of the outer set screw with the receiver member arms results in the distal surface of the outer set screw engaging the proximal surface of the compression member arms and the outer set screw thereby delivering a distal force to the compression member to fix the bone anchor relative to the receiver member, the proximal surface of each of the compression member arms being convex in shape and having a constant radius, —to restrict relative movement of the compression member arms both towards and away from each other and the distal surface of the outer set screw having a concave shape and a constant radius equal to the radius of the convex proximal surfaces.

2. The bone anchor assembly of claim 1 , wherein the bone anchor is movable relative to the receiver member.

3. The bone anchor assembly of claim 2 , wherein the bone anchor is movable relative to the receiver member in at least a first direction at a first angle to relative to a central longitudinal axis and in at least a second direction at a second angle relative to the central longitudinal axis, the second angle being greater than the first angle.

4. The bone anchor assembly of claim 1 , wherein the proximal end of the receiver member includes a proximal end surface that defines a first plane and wherein the distal end surface defines a second plane, the first plane and second plane intersecting one another.

5. A bone anchor assembly comprising:

a bone anchor having a proximal head and a distal shaft configured to engage bone,

a receiver member for receiving a spinal fixation element to be coupled to the bone anchor, the receiver member having

a proximal end having a proximal end surface and a pair of spaced apart receiver member arms defining a recess therebetween for receiving a spinal rod, the arms including a first inner thread, the proximal end surface defining a first plane,

a distal end having a distal end surface defining an opening through which at least a portion of the bone anchor extends, the bone anchor being movable relative to the receiver member, the distal end surface defining a second plane, the first plane and the second plane intersecting one another,

a compression member positioned within the receiver member, the compression member having a proximal end and a distal end, the proximal end of the compression member having a pair of spaced apart compression member arms defining a U-shaped seat for receiving the rod, each compression member arm including an outer wall, an inner wall and a proximal surface connecting the outer wall and the inner wall, the distal end of the compression member having a distal surface engageable with the proximal head of the bone anchor,

an outer set screw positionable between and engaging the receiver member arms, the outer set screw including a first outer thread for engaging the first inner thread, the outer set screw having a distal surface engageable with the proximal surface of the compression member arms, the outer set screw having a set screw central passage from a top surface of the outer set screw to a bottom surface of the outer set screw, the set screw central passage having a second internal thread, and

an inner set screw positionable within the set screw central passage, the inner set screw having a second outer thread for engaging the second inner thread, the inner set screw operable to act on the spinal rod to fix the spinal rod relative to the receiver member;

wherein engagement of the outer set screw with the receiver member arms results in the distal surface of the outer set screw engaging the proximal surface of the compression member arms and the outer set screw thereby delivering a distal force to the compression member to fix the bone anchor relative to the receiver member, the proximal surface of each of the compression member arms being convex in shape and having a constant radius, to restrict relative motion of the compression member arms both towards and away from each other, the distal surface of the outer set screw having a concave shape and a constant radius equal to the radius of the convex proximal surfaces.