Introduction: Aim of the study was to give an overview about the main macro- and microstructure differences of commercially available resurfacing hip implants. The effect of the manufacturing process and the subsequent heat treatment leads to variable microstructures of implant materials. It is undisputable that a low surface roughness and high sphericity improves the wear behaviour. But the radial clearance, the manufacturing process and heat treatment are discussed controversially.

Methods: Resurfacing hip implants with a 46mm head diameter and corresponding cups were analyzed. Commercially available hip resurfacing implants from 10 different manufacturers were included in this investigation. The heads and cups were measured in a coordinate measuring machine (Mahr Multisensor MS 222). A best fit sphere was created from the point clouds and evaluated using analysing software (Imageware 12.1, UGS Corp.). Head and cup radial clearances were measured and sphericity deviation calculated and graphically plotted. Measurements on surface roughness were carried out three times per implant (Mahr Pertometer M2). The microstructures of the heads and cups were inspected by SEM (LEO 440). Surface images were taken using the scanning electron mode. The back scatter electron mode was used to get element weighted images.

Element analysis was performed by EDX (Oxford D. 7060) to identify carbides and the alloy composition. Element distribution maps were taken to separate the single elements.

Results: The mean radial clearance was found to be 85.53μm. The range was from 49.47μm to 120.93μm. We classified all implants into three groups (low, midrange and high clearance). The low clearance group ranged from < 50μm to 75μm, midrange from 75μm to 100μm and high clearance from 100μm to > 125μm.

All implants showed a sphericity deviation less than 10μm. On average the heads tended to have a higher spherical deviation of 4.1μm (SD: 2.3μm) compared to the cups 2.7μm (SD: 1.4μm). Based on the SEM and EDX inspection the manufacturing process, heat treatment and carbide distribution could be clarified.

Discussion: This study gives an overview about the main macro- and microstructure differences of commercial available resurfacing hip implants.

The characteristically unspheric formations of the heads may be due to the cooling process after manufacturing the implant and there is also a relation between the wall thickness of the implant and the unspheric formations. With decreasing wall thickness the implant cools faster locally. Additionally a cup with a thin wall may deform under loading condition and a very tight clearance could be detrimental.

This study will help to understand clinical observations. It still has to be proven that these biomechanical factors influence the clinical performance of hip resurfacing implants.