A bone fixation assembly includes a side plate for mounting on a bone and a barrel for receiving a portion of a lag screw, and configured to be inserted into a hole in the side plate. A mechanical spring located at the upper portion of the barrel biases a first detent portion toward a locking position with a second detent portion located on the side plate such that rotation of the barrel within the hole from a first position to a second position causes a compressing portion on the detent portion to contact a compressing portion on the second detent portion thereby compressing the mechanical spring so as to allow the barrel to be rotated to a third position wherein the mechanical spring is allowed to bias the first detent portion into the locking position such that a retaining portion of the first detent is adjacent to a retaining portion of the second detent and rotation of the barrel with respect to the side plate is restricted.