A system for forming a spinal prosthesis in situ within an intervertebral space located between first and second adjacent vertebrae includes at least one mold having at least one internal compartment adapted to receive at least one flowable biomaterial. The system also includes a retaining member adapted to secure the mold between the first and second vertebrae, the retaining member including first and second portions adapted to be engaged with first and second surfaces of the first and second vertebrae, respectively. The retaining member also includes an intermediate body operatively coupling the first portion to the second portion, the intermediate body adapted to be positioned in or adjacent to the intervertebral space. A biomaterial delivery apparatus is in fluid communication with the mold at a pressure sufficient for the mold to engage with the retaining member. The spinal prosthesis selectively position the first vertebrae relative to the second vertebrae.