An investigation of the viability of bonnet polishing as a possible method to manufacture hip prostheses with multi-radius femoral heads

Recent research has indicated that the use of multi-radius femoral heads can be advantageous in terms of the articulating zone lubrication and tribological performance for metal-on-metal (MoM) hip implants. The emergence of this multi-radius femoral head is a new challenge in manufacturing research because conventional machining technologies are not normally agile enough to realize high-accuracy machining of the new design of femoral heads. This paper attempts to demonstrate the manufacture of a multi-radius femoral head from a typical single radius femoral head using a bonnet polishing method. The sample used in this investigation is an 18-mm radius cobalt chrome (CoCr) alloy, the most common material for MoM hip prostheses. The paper outlines the process of producing a target of 19-mm radius zone in the articulating region. Post polishing measurement of the radius of the articulating zone showed the radius to be between 18.98 and 19.017 mm. Additionally, the peak-to-valley (PV) value of the spherical error map was reduced from 103 to 1.36 μm and root mean square (RMS) was from 29.65 to 0.318 μm. The machined surface roughness obtained through the process was Sa = 16.1 nm which is well within the recommendation of ISO7206-2:2011 standard for metal hip joint prosthesis value of 50 nm. The experimental result confirms that bonnet polishing is a potentially viable choice to finish the new design multi-radius femoral heads systems.