A provisional tibial prosthesis system (30A, 30B) for a prosthetic knee joint for implantation in a natural knee, the provisional tibial prosthesis system capable of alternatively mimicking the geometry of a first final tibial prosthesis and a second final tibial prosthesis, the natural knee comprising a proximal tibia and a distal femur and having a medial/lateral axis (AM L), an anterior/posterior axis (AA P), and a proximal/distal axis (AP D), the medial/lateral axis corresponding to a medial/lateral direction, the anterior/posterior axis corresponding to an anterior/posterior direction, and the proximal/distal axis corresponding to a proximal/distal direction, the provisional tibial prosthesis system comprising: a tibial base plate (38A, 38B) having a bone contacting surface (152A, 152B) and an opposing base plate superior surface (150A, 150B); a tibial bearing component (32A, 32B) having a tibial bearing component height, said tibial bearing component attachable to said tibial base plate, said tibial bearing component height representing the first final tibial prosthesis; and a shim (36A, 36B) having a shim height the provisional tibial prosthesis system being configured so that, said shim is slidable relative to both said tibial base plate (38A, 38B) and said tibial bearing component (32A, 32B), to be slidably receivable between said tibial base plate and said tibial bearing component in the anterior/posterior direction when said tibial base plate and said tibial bearing component are separated by a distance along the proximal/distal axis equal to said shim height and when the tibial prosthesis system is used in implantation in the natural knee, said shim height cooperating with said tibial bearing component height to represent the second final tibial prosthesis; characterized in that said system further comprises: a base component (34A, 34B) configured to be attached to said tibial bearing component by a lock (64A, 68A, 70A, 72A, 96A, 104A) wherein when said tibial bearing component is attached to said tibial base plate, said lock locks said tibial bearing component to said tibial base plate in the medial/lateral direction and the anterior/posterior direction, whereby significant relative movement between said tibial bearing component and said tibial base plate in the medial/lateral direction and the anterior/posterior direction is prevented, said base component positionable between said tibial bearing component and said tibial base plate and said shim (36A, 36B) slidably receivable between said tibial bearing component and said base component in the anterior/posterior direction when said base component and said tibial bearing component are separated by said distance along the proximal/distal axis equal to said shim height. a tibial base plate (38A, 38B) having a bone contacting surface (152A, 152B) and an opposing base plate superior surface (150A, 150B); a tibial bearing component (32A, 32B) having a tibial bearing component height, said tibial bearing component attachable to said tibial base plate, said tibial bearing component height representing the first final tibial prosthesis; and a shim (36A, 36B) having a shim height the provisional tibial prosthesis system being configured so that, said shim is slidable relative to both said tibial base plate (38A, 38B) and said tibial bearing component (32A, 32B), to be slidably receivable between said tibial base plate and said tibial bearing component in the anterior/posterior direction when said tibial base plate and said tibial bearing component are separated by a distance along the proximal/distal axis equal to said shim height and when the tibial prosthesis system is used in implantation in the natural knee, said shim height cooperating with said tibial bearing component height to represent the second final tibial prosthesis; a base component (34A, 34B) configured to be attached to said tibial bearing component by a lock (64A, 68A, 70A, 72A, 96A, 104A) wherein when said tibial bearing component is attached to said tibial base plate, said lock locks said tibial bearing component to said tibial base plate in the medial/lateral direction and the anterior/posterior direction, whereby significant relative movement between said tibial bearing component and said tibial base plate in the medial/lateral direction and the anterior/posterior direction is prevented, said base component positionable between said tibial bearing component and said tibial base plate and said shim (36A, 36B) slidably receivable between said tibial bearing component and said base component in the anterior/posterior direction when said base component and said tibial bearing component are separated by said distance along the proximal/distal axis equal to said shim height.