Systems, apparatuses and methods for bone fusion are disclosed. In particular, a fixation screw assembly is provided that comprises a bone engagement portion including a shaft and a head member. The assembly also includes a flexible washer member that can be operably attached and secured around the head member of the bone engagement portion. The washer member is polyaxial relative to the bone engagement portion, such that it can assume a variety of angles. The washer member can help prevent back out of the bone engagement portion when the bone engagement portion is implanted into a bone member. The design of the fixation screw assembly can be used in different fusion procedures, including fusion of the sacroiliac joint.