Authors Harry Hothi, Anna Di Laura, Johann Henckel and Alister Hart, of The Royal National Orthopaedic Hospital, discuss their two published studies examining retrieved hip implants manufactured with ceramic coatings
Background
In the past three years in the UK alone, more than 510,000 patients received a hip or knee replacement [1]. These devices often consist of components made of cobalt-chromium (CoCr), titanium and stainless-steel alloys, as well as ceramic and polyethene components. The challenges in the use of CoCr metal-on-metal (MOM) hips have been widely documented; these have primarily been linked to wear of the CoCr alloy leading to adverse reactions in patients and ultimately revision of the implant. Whilst the use of MOM hips has virtually ended, CoCr alloy is still widely used in hip and knee implants as part of metal-on-polyethylene articulations. The risk of CoCr wear in these still exists, particularly in the presence of 3rd body debris (albeit at a lower risk than in MOM hips). Additionally, it is believed that around 15% of the population has some level of metal hypersensitivity to nickel, and a small percentage to cobalt, chromium and/or molybdenum, which are the constituent metals of CoCr alloy [2].
Ceramic Coatings
CoCr alloy remains the preferred metal within these bearing surfaces due to its high hardness and wear resistance (relative to titanium and stainless-steel). Manufacturers have for several decades made use ceramic coatings in orthopaedics and we have seen examples of various hips, knees and other devices coated with Titanium Nitride (TiN), which has a distinctive gold colour, Figure 1.
Figure 1: Examples of different TiN coated implants for (a) total knee replacement, (b) partial knee replacement, (c) hip resurfacing, (d) shoulder replacement [3] and (e) a dual-mobility hip [4].
These coatings have been designed to serve as a ‘protective layer’ between patient tissue and the underlying CoCr, theoretically minimising the risk of CoCr wear ions being released (and also reducing the risk of wear of the opposing polyethylene component) or protecting patients with metal hypersensitivity [5]. Ceramic coatings offer other theoretical advantages compared with using CoCr alone: greater hardness, low surface roughness, increased wettability and a lower coefficient of friction, helping to improve the lubrication between the metal and polyethylene components.
Wear simulator testing has shown these coatings to perform highly favourably in terms of enhancing wear resistance and maintaining the integrity of the coating itself, however a clear clinical benefit of coatings is not yet conclusively proven [6]. Retrieval studies have shown however that simulators are not always able to predict what will happen to an implant whilst in the patient.
Our group has published two studies examining retrieved hip implants manufactured with cera