An implant for fusing a joint between first and second bone portions. The implant includes a screw member, and a washer polyaxially rotatable relative to the screw member. The screw member includes a head, a lag zone, and a threaded engagement zone. The implant includes a fusion zone for joint compression, extending from the washer to the proximal end of the engagement zone. Fenestrations may be present in the fusion zone. The length of the fusion zone ranges from about 10 mm to about 37 mm. Different surface finishes including roughened and non-roughened may be applied selectively to selected portions of the implant. In an embodiment, the joint is a sacro-iliac joint, and upon implantation the implant extends from the exterior of the ilium, across the joint and into the sacral vertebral body. Instrumentation and methods for preparing the joint and implanting the implant are disclosed.