Bottom-loading bone anchor assemblies

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.