A threaded intervertebral implant or cage which includes a cylindrical or tapered body made from a biocompatible metal such as titanium, or a radiolucent plastic material. The cage includes a pair of opposing, threaded arcuate contact portions oriented in the superior/inferior direction for threading, supporting engagement with the endplates of adjacent vertebrae when the cage is implanted. Additionally, the cage includes a threaded lateral side having an arcuate profile, and a medial side having a concave channel extending along the length of the cage. The channel allows two cages to be implanted in a side-by-side manner between adjacent vertebrae with the channel of a first implanted cage receiving the arcuate portions of an adjacent second cage while the second cage is implanted. In this manner, a pair of adjacent cages may be implanted very close to one another. Also, when two cages are implanted in a side-by-side manner, the concave channels of the medial sides of the cages face one another to define a substantially enclosed intercage space between the cages in which bone growth-promoting material may be packed to facilitate fusion between adjacent vertebrae.