A bone anchor system anchors a support member to a bone. The bone anchor system comprises a body having a shank adapted for insertion into a hole in the bone. A head is formed on the shank. The head has a support engagement portion adapted to secure the support member to the body. A passage extends from an entry point to an exit point formed in the body. An expandable member has a flexible wall at least partially enclosing an anchoring chamber. The expandable member has an expanded state configured to conform to a cavity in the bone. The anchoring chamber is in fluid communication with the exit point. The expandable member is adapted to expand when a filler material is provided into the anchoring chamber. A portion of the expandable member conforms to the cavity and the body and the support member are substantially anchored to the bone.