A femoral head arthroplasty system comprising: a femoral prosthesis comprising: a mounting plate having a first side and an opposed second side , wherein the first side of the mounting plate is configured to abut a cut proximal end of a femur, and an implant body extending from the first side of the mounting plate , wherein the implant body extends from the mounting plate by a distance no greater than 90 mm, wherein, in cross section in a plane that is perpendicular to a first axis , the implant body has a major cross sectional dimension along a second axis and a minor cross sectional dimension along a third axis that is different than the second axis , wherein the minor cross sectional dimension is less than the major cross sectional dimension, and wherein the implant body has a centerline that is perpendicular to the mounting plate : and a femoral head replacement having a generally spherical surface, wherein one of the femoral prosthesis and the femoral head comprises an adapter , and wherein the other of the femoral prosthesis and the femoral head defines a recess that is complimentarily shaped to receive the adapter , characterised in that the implant body has an outer circumferential surface comprising two hemi-cylindrical surfaces being spaced along the second axis , and two parallel planar faces that are spaced along the third axis and extend between and to the two hemi-cylindrical surfaces along a respective entirety of each of the lengths of the two hemi-cylindrical surfaces, wherein the implant body has a distal end defining a distal surface through which the centreline extends. a femoral prosthesis comprising: a mounting plate having a first side and an opposed second side , wherein the first side of the mounting plate is configured to abut a cut proximal end of a femur, and an implant body extending from the first side of the mounting plate , wherein the implant body extends from the mounting plate by a distance no greater than 90 mm, wherein, in cross section in a plane that is perpendicular to a first axis , the implant body has a major cross sectional dimension along a second axis and a minor cross sectional dimension along a third axis that is different than the second axis , wherein the minor cross sectional dimension is less than the major cross sectional dimension, and wherein the implant body has a centerline that is perpendicular to the mounting plate : and a mounting plate having a first side and an opposed second side , wherein the first side of the mounting plate is configured to abut a cut proximal end of a femur, and an implant body extending from the first side of the mounting plate , wherein the implant body extends from the mounting plate by a distance no greater than 90 mm, wherein, in cross section in a plane that is perpendicular to a first axis , the implant body has a major cross sectional dimension along a second axis and a minor cross sectional dimension along a third axis that is different than the second axis , wherein the minor cross sectional dimension is less than the major cross sectional dimension, and wherein the implant body has a centerline that is perpendicular to the mounting plate : and a femoral head replacement having a generally spherical surface, wherein one of the femoral prosthesis and the femoral head comprises an adapter , and wherein the other of the femoral prosthesis and the femoral head defines a recess that is complimentarily shaped to receive the adapter , characterised in that the implant body has an outer circumferential surface comprising two hemi-cylindrical surfaces being spaced along the second axis , and two parallel planar faces that are spaced along the third axis and extend between and to the two hemi-cylindrical surfaces along a respective entirety of each of the lengths of the two hemi-cylindrical surfaces, wherein the implant body has a distal end defining a distal surface through which the centreline extends. <EOS>