an apparatus for facilitating fusion of adjacent bony structures includes an implant body dimensioned for positioning between adjacent bone structures to maintain the bone structures in desired spaced relation during interbody fusion . the implant body has an outer wall and an external threaded configuration disposed on the outer wall . at least one concave surface at least partially extends along the implant body . the concave surface advantageously reduces the transverse cross - sectional dimension of the implant member thereby facilitating placement of the implant member in restricted intervertebral areas . in addition , the concave surface enables placement of a pair of implants in nested side - by - side relation . preferably , the threaded configuration has portions removed along an arc section of the outer wall thereby defining a series of generally longitudinally aligned concave surfaces in individual turns thereof . a system and method for facilitating fusion of adjacent vertebrae is also disclosed .