Purpose: The combined effects of metaphyseal filling and implant surface (smooth or rough) on stress applied to the cement was studied using finite element analysis.

Material and methods: The cement-metal interface was modelised in stable then unstable situations at different degrees of metaphyseal filling. For each case, stress applied to the cement sheath and femoral corticals were tested as were rotation displacements of certain nodes chosen at critical sites in the proximal part of the stem and the bone. A first model produced a 3D representation of the femur exposed to physiological weight-bearing. The second mode represented a femoral metaphysis with the cement sheath exposed to rotation forces, critical for femoral prosthesis stability.

Results: With implants with a rough surface (cement adherence), bone stress increased with decreasing implant size and was basically concentrated in the distal part of the bone (diaphysis). On the contrary, for polished surface implants, bone stress increased with the size of the implant and was basically concentrated in the proximal part of the bone (metaphysis). Stress in the cement sheath was lower for rough surface implants. With the smooth implant, they were greatest in the metaphyseal part of the cement sheath, especially with prosthetic configurations we the most filling. When a rotation force was applied to the implant, shear forces in the cement were greater with a rough than a smooth surface implant. Compression forces in the proximal and anterior part of the cement sheath increased with the size of the implant. Proximal discontinuity of the cement sheath did not produce deleterious mechanical effects. The smooth surface implants produced higher compression forces than rough surface implants in the proximal part of the cement sheath. In addition, with smooth surface implants, traction stress was zero in the proximal part of the cement sheath.

Discussion: Use of smooth surface implants with optimal metaphyseal filling increases the rotational stability and allows a better fixation of the femoral stem in total hip arthroplasty, with a more physiological transmission of the stress forces to the proximal part of the bone. It produces greater compression force on the cement but less traction stress.