A bone anchor assembly comprising: a body having an upper opening located at an upper end of the body and a lower opening located at a lower end of the body, the lower opening sized and configured to receive a head of a bone anchor, a bore extending between the upper and lower opening, and a rod receiving channel extending through a side wall of the body, a bushing received within the bore of the body, the bushing including an upper opening located at an upper end of the bushing and a lower opening located at a lower end of the bushing, a bore extending between the upper and lower opening, an interior cavity located within the bore for receiving at least a portion of a head of a bone anchor, a slot extending from the lower end to define flexible arms, the slot permitting a portion of the bushing to expand and collapse about a head of a bone anchor, the bushing including a locking mechanism provided by upper and lower teeth (41a, 41b) which can separately engage a tooth on the body (i) in a first axial position in which the lower tooth on the bushing engages the tooth on the body and the bushing is in an unlocked position with said portion of the bushing in its expanded configuration allowing the head of a bone anchor to be inserted into the lower opening of the bushing and to be removed from the bushing through the lower opening, and (ii) in a second axial position in which the upper tooth on the bushing engages the tooth on the body and the bushing is in a locked position with said portion of the bushing collapsed around the head of a bone anchor which has been inserted through the lower opening of the bushing so as to prevent the head of the bone anchor from being removed through the lower opening, and a locking cap assembly received within the bore of the body to retain a spinal rod within the rod receiving channel, wherein the bone anchor assembly is configured such that downward movement of the locking cap assembly within the bore of the body causes a lower surface of the locking cap assembly to engage an upper end of the bushing to move the bushing into the locked position, and in the locked position a lower surface of the bushing contacts an interior surface of the body securing the bushing within the body. a body having an upper opening located at an upper end of the body and a lower opening located at a lower end of the body, the lower opening sized and configured to receive a head of a bone anchor, a bore extending between the upper and lower opening, and a rod receiving channel extending through a side wall of the body, a bushing received within the bore of the body, the bushing including an upper opening located at an upper end of the bushing and a lower opening located at a lower end of the bushing, a bore extending between the upper and lower opening, an interior cavity located within the bore for receiving at least a portion of a head of a bone anchor, a slot extending from the lower end to define flexible arms, the slot permitting a portion of the bushing to expand and collapse about a head of a bone anchor, the bushing including a locking mechanism provided by upper and lower teeth (41a, 41b) which can separately engage a tooth on the body (i) in a first axial position in which the lower tooth on the bushing engages the tooth on the body and the bushing is in an unlocked position with said portion of the bushing in its expanded configuration allowing the head of a bone anchor to be inserted into the lower opening of the bushing and to be removed from the bushing through the lower opening, and (ii) in a second axial position in which the upper tooth on the bushing engages the tooth on the body and the bushing is in a locked position with said portion of the bushing collapsed around the head of a bone anchor which has been inserted through the lower opening of the bushing so as to prevent the head of the bone anchor from being removed through the lower opening, and a locking cap assembly received within the bore of the body to retain a spinal rod within the rod receiving channel, wherein the bone anchor assembly is configured such that downward movement of the locking cap assembly within the bore of the body causes a lower surface of the locking cap assembly to engage an upper end of the bushing to move the bushing into the locked position, and in the locked position a lower surface of the bushing contacts an interior surface of the body securing the bushing within the body.