Configurations are described for conducting minimally invasive medical interventions utilizing elongate instruments and assemblies thereof to stabilize and/or fixate a sacroiliac joint. A system for stabilizing an SI joint, comprises a defect-creating tool assembly configured to be advanced from a posterior approach into an SI junction defined between sacrum and ilium structures of a patient, the tool assembly being configured to create a defect defined at least in part by portions of both the sacrum and the ilium, the defect having a three dimensional shape defined in part by at least one noncircular cross sectional shape in a plane substantially perpendicular to the longitudinal axis of the tool assembly and a prosthesis configured to fit into the defect created by the tool assembly.
