Bone Anchor Assemblies

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 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 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 the compression member arms having a shape configured 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 shape that is complementary to the shape of the proximal surface of the compression member arms.