Abstract: Configurations are described for conducting minimally invasive medical interventions utilizing elongate instruments and assemblies thereof to stabilize and/or fixate a sacro-iliac joint. In one embodiment, a tool assembly may be advanced from a posterior approach into the SI junction and 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. After a defect is created, the tool assembly may be retracted and a prosthesis deployed into the defect.