Abstract
Introduction
Titanium nitride (TiN) coatings are used in total hip arthroplasty to reduce friction of bearing couples or to decrease the allergic potential of orthopaedic alloys. Little is known about performance of currently manufactured implants, since only few retrieval studies were performed, furthermore they included a small number of implants manufactured over 15 years ago.
Aim of study
To examine wear and degradation of retrieved TiN coated femoral heads articulating with ultra-high molecular weight polyethylene (UHMWPE).
Methods
We included eight femoral heads with a made od TiAl6V4 alloy and coated with TiN using Physical Vapour Deposition (PVD). All heads (28 and 32 mm) were retrieved after at least 12 months of use (range 12–56). The reason for revision was aseptic loosening in 6 cases, septic loosening in one case and recurrent dislocations (five episodes) in one uncemented prosthesis. One unused head was included as reference sample. All implants were evaluated with light microscopy, Scanning Electron Microscopy (SEM) with Energy-Dispersive X-ray Spectroscopy (EDS). 30 SEM images from each implant were digitally analysed using ImageJ software to compare damage in loaded and non weight-bearing parts of the heads.
Results
Studies with light microscopy revealed severe damage to the dislocated femoral head, with multiple metallic scratches. SEM studies indicated presence of multiple scratches and pinholes with a diameter of 1–10 µm (Fig1a,b,). Residue from the manufacturing process was present in all implants in form of pure Ti droplets found in round voids. In all implants we found irregular areas (diam. 20–50 µm, Fig 1c,d) where the coating was delaminated from the substrate metal with cracks arising from coating defects (Fig1e-h). Some of these debonded fragments were embedded into the PVD layer in weight-bearing parts of all heads. In one head, which was subjected to dislocations we observed deposits of titanium alloy from the acetabular shell (Fig 2a,b). The deposits were accompanied by large patches of delaminated coating as well as multiple cracks (Fig 2c,d). Small fragments of the acetabular titanium alloy damaged the coating in third body mechanism. Surprisingly in three implants we EDS analysis revealed similar spheres (diam. 1–10 µm) containing Niobium (Nb), although this element is not a part of any of the components used in the implants(Fig 2e,f). Interestingly presence of Nb droplets were associated with a higher number of other defect in these heads both in weight-baring and non weight-bearing parts of the heads suggesting inferior coating quality in these cases (Fig 2 g,h; Fig3, cases H2,H5,H8).
Conclusions
Compared to previous studies we did not observe severe wear or the coating, however we observed some degradation of the film in vivo. Our results indicate that dislocation can lead to severe failure of the coating in vivo. Moreover presence of Nb residues and coexisting defects in some implants suggests inferior coating quality in these implants and indicates the need for strict monitoring of the production process.
This study was funded by a grant from the National Science Centre nr 2012/05/D/NZ5/01840
