A bone fixation assembly includes a side plate (12) for mounting on a bone and a barrel (16) for receiving a portion of a lag screw (14), and configured to be inserted into a hole in the side plate. A mechanical spring (90) located at the upper portion of the barrel biases a first detent portion (92) toward a locking position with a second detent portion (54) 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.
