A trajectory guide assembly that guides an instrument relative to a patient includes a mounting seat configured to be supported by the patient. The assembly also includes a trajectory member moveably supported by the mounting seat. The trajectory member defines a trajectory passage. Also, the assembly includes a cap member moveably supported by the trajectory member. The cap member defines a cap opening, and the cap member is selectively moveable to a first position relative to the trajectory member to retain the instrument in both the cap opening and the trajectory passage. Additionally, the cap member is selectively moveable to a second position relative to the trajectory member to release the instrument.