An assembly for the in situ formation of a prosthesis in an intervertebral disc space between adjacent vertebrae of a patient. At least one retention structure is located in the intervertebral disc space. A distal end of at least one lumen is located proximate the at least one retention structure. One or more in situ curable biomaterials are delivered to the intervertebral disc space through the first lumen and into engagement with the retention structure. The retention structure serves to retain at least a portion of the biomaterial in the intervertebral disc space by surface tension, adhesion, mechanical capture, friction, viscosity, and/or a variety of other mechanisms. The at least partially cured biomaterial and the at least one retention structure cooperate to comprise the prosthesis.