Abstract: Systems, devices and methods are provided for implanting medical devices into patients. The systems, devices and methods are particularly useful for orthopedic implants, such as spinal implants that facilitate fusion of bone segments. A fixation assembly for an implant comprises a screw having a head and a shaft, and a washer having at least one outer surface with one or more frictional elements for engaging a surface of the implant. The shaft includes one or more projections that cooperate with the washer to prevent the washer from sliding back down the screw, while allowing for easy application of the washer to ensure optimal placement and desired contact with the screw head. The washer and/or the screw head may also include interlocking cams that create a wedge effect that inhibits the screw from backing out or loosening from a hole in bone or other tissue.