The present invention generally relates to bone implants. More specifically, the present invention relates to bone implants used for the fixation or fusion of the sacroiliac joint and/or the spine. For example, a system for fusing or stabilizing a plurality of bones is provided. The system includes an implant structure having stem portion and a head portion, the stem portion having a rectilinear cross sectional area. A tulip or saddle structure can be attached to the head portion, and a rod can be secured within the tulip or saddle structure.