Spinal arthrodesis is accomplished by implanting an intervertebral spacer made of a biocompatible material that promotes tissue ingrowth and bony fusion. The spacer is implanted into a graft bed formed between adjacent vertebral bodies. The spacer includes a body portion having anterior and posterior end faces. Opposed flanges project outwardly from the body portion at axial locations adjacent the anterior and posterior end faces. The flanges taper radially outwardly to an outer edge that is sharp enough to penetrate vertebral end plates. After a graft bed has been formed, and after the vertebral bodies have been distracted, the spacer is inserted into the graft bed and positioned such that the flanges are adjacent the vertebral end plates. Upon release of the distraction, the vertebral bodies will move toward each other and the flanges will be pressed into the end plates. The effect of the pressing action is to secure the spacer tightly in place within the graft bed such that migration of the spacer is prevented.