Patent ID: 8430917

Claim:
A bone anchor assembly, comprising: a receiver extending along a central longitudinal axis between a proximal end and an opposite distal end, said receiver including a distal portion defining a receptacle opening at said distal end and a pair of arms extending from said distal portion along said central longitudinal axis on opposite sides of a passage, said pair of arms including inner surfaces facing one another on opposite sides of said passage and outer surfaces, said pair of arms each including a groove extending into a respective inner surface without extending through a respective outer surface, said grooves extending transverse to said central axis and being curved between opposite end surfaces of said arms such that a middle portion of each groove is disposed more distally than opposite ends of said groove, said receiver including a bottom surface extending along said passage between said pair of arms, wherein said passage opens at opposite sides of said receiver between said pair of arms and said receptacle opens into said passage through said bottom surface; a bone anchor including a distal bone engaging portion and a head at a proximal end of said distal bone engaging portion, said head including a platform extending from said head comprising a pair of rails, said rails protruding axially from said platform and defining a recess therebetween, said head being positioned in said receptacle with said bone engaging portion in a first orientation relative to said central longitudinal axis of said receiver; a saddle positioned in said passage of said receiver adjacent to said bottom surface of said receiver, said saddle being configured to translate within said grooves in said receiver, said saddle including a proximal support surface and a convex distal surface opposite said proximal support surface, said distal surface contacting said pair of rails of said platform as said saddle translates within said grooves and said recess, said proximal support surface being concavely curved between said opposite sides of said saddle and linear between said ends of said saddle; a connecting element extending along a central longitudinal axis, said connecting element being located in said passage and extending through said opposite sides of said receiver; and an engaging member engaged to said pair of arms to secure said connecting element against said proximal support surface of said saddle, wherein said saddle engages said receiver and is limited to movement in said receiver in a single plane defined by said central longitudinal axis of said receiver and said central longitudinal axis of said connecting element while said bone engaging portion remains in said first orientation.