A fusion cage 10 having an external thread 12 can be surgically inserted into a threaded bore extending laterally between the adjacent bony structures such as two vertebrae 94, 95 with the thread 12 penetrating into cancellous bone of each of the vertebrae 94, 95. The fusion cage 10 is easily screwed into place by hand without damage to the bony structures 94, 95. Cage 10 is then packed with a bone-growth-inducing substance such as cancellous bone. When a pair of such cages 10 are implanted between adjacent vertebrae 94, 95, patients have been able to sit without pain by the second or third day, much earlier than has been possible in prior spinal fusions except those involving steel plates and screws. Eventually, the ingrowth of bone through perforations 13 in the valley 14 of the thread 12 of the fusion cage 10 forms a permanent interconnection between the two bony structures 94, 95.