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
In the past three years in the UK alone, more than 510,000 patients received a hip or knee replacement . 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 .
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.
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 . 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 . 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 ceramic coatings. The first study reported on three MOM hips received at our centre that had been coated with titanium niobium nitride (TiNbN) in order to minimise wear of the underlying CoCr . All three hips were revised within 33 months after being implanted and two patients were found to have levels of cobalt and chromium in their blood that were significantly above ‘normal’ levels, measuring a maximum of 55 and 46ppb of cobalt and chromium ions respectively. All three were measured as having experienced significantly more wear than would be expected for a well-functioning implant. Macroscopically there will clear regions of coating loss (Figure 2).
The second study involved 43 retrieved MOP hips which had a TiNbN coating applied to the CoCr femoral heads . 15 of these had clear macroscopic evidence of coating loss and this was associated with increased wear of the corresponding polyethylene liners, compared with liners in which the head coating was intact.
These two studies present examples of the clinical impact of coating failure in patients. The underlying cause of coating removal in these cases is unclear however they point towards a need for optimising the coating application method. There are several different methods which can be used, including physical vapour deposition (PVD), chemical vapour deposition (CVD) and plasma spraying . These all have advantages in their use such as the ability to coat complex shapes or their relative costs however all are associated with challenges in delamination or low initial adhesion.
The use of titanium-nitride ceramic coatings on CoCr bearing surfaces can create a hard biocompatible layer, minimising wear against polyethylene components. Indeed, there are several coated implant designs that demonstrate good medium to long-term clinical success (their advantage over not coating is still unclear). These coatings also enable patients with metal allergy, in particular to nickel, to be able to safely use these implants. Retrieval data does show however that coating loss can occur during use in patients, which can adversely impact function and require revision surgery. Further enhancements of this coating technology and its application methods are necessary to fully optimise their clinical use.
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