Influence of structure support printing parameters on surface finish of PLA hemispherical cups for emulation of ceramic hip prostheses

Abstract

Hemispherical hip ceramic prostheses require certain degree of porosity on their external surface so as to fix them by means of osteointegration. This can be achieved with porous mesostructures. In contrast, internal surface needs to be smooth in order to assure appropriate sliding of femoral head. Such specific shapes can be obtained by means of 3D printing. However, in certain printing processes, structure supports are required when overhang exceeds a certain angle. In this case, once supports are removed, joints between supports and prostheses will produce irregularities which will increase roughness on the internal surface of the prostheses. In the future, bimaterial prostheses are to be printed in ceramic with plastic structure supports, which are cheaper than ceramic ones. For doing this, double head printing machines will be used. In the present work, as a first step of research, both prostheses and supports were printed in plastic material. Specifically, PLA, which is a biocompatible polymer, was used. Influence of printing variables for supports on surface finish of internal surface of hemispherical cups after removing supports was studied. Prostheses were obtained by means of Fused Deposition Modelling (FDM) technology. Full factorial design of experiments was performed, with three printing variables: Support Pillar Resolution, Horizontal Offset from Part and part, and Dense Infill Percentage. Regression analysis was carried out. Results showed that Support Pillar Resolution and Horizontal Offset from Part are main parameters factors influencing roughness parameters Ra and Rz. In order to obtain low roughness values, high Support Pillar Resolution should be selected. In case low Support Pillar Resolution was necessary, then high Horizontal Offset from Part would be recommended. In the future, research presented in the present work will help selecting proper values for printing parameters in order to obtain smooth internal surfaces of ceramic hemispherical hip prostheses. This will reduce or even avoid subsequent polishing time of the internal surface of the prostheses.