Titanium nitride gives surgeons enhanced biocompatibility

By using titanium nitride coatings on its medical knee replacement system, a prosthesis manufacturer can now offer patients two long life, wear resistant knee joint designs. Dean Palmer reports In order to offer surgeons and patients more options in terms of enhanced biomedical compatibility for its knee joint replacement and rotation system, leading prosthesis manufacturer Finsbury Orthopaedics of Leatherhead, decided to use titanium nitride coatings on its load bearing joint components, supplied by Cambridge-based coatings manufacturer Tecvac. Finsbury's design engineers chose physical vapour deposition (PVD) titanium nitride coatings and this means the firm can now offer patients a choice of two knee joint designs, each with its own special advantages. One option for patients is Finsbury's 'Medical Knee Rotation System' which combines enhanced joint stability with 'natural' kinematics, using a combination of advanced polyethylene plastics and cobalt chrome castings to replace the natural mechanism affected by arthritis, while still achieving positive engagement of the knee components, to transfer locomotion loads evenly and with minimum wear. This design ensures that the muscles above and below the knee have a positive fulcrum to make walking and running an efficient, 'natural' process, so that the knee feels stable and normal to the patient throughout the normal range of motion. The alternative design is the 'dual bearing knee', or DBK, which achieves exceptional life by 'dual surface articulation'. This provides a high degree of contact area within the joint and allows a wide envelope of free translation and comfortable movement, while spreading the natural loads over a relatively wide contact area within the knee joint. In both knee joint designs, the vertical loads are transmitted through a highly polished and precisely profiled cobalt chrome casting, which moves against a polyethylene pad. Both designs have their own special virtues for surgeons and patients, but the issue of long life, wear resistance and tissue compatibility are vital considerations for both. Once a typical prosthesis recipient's mobility is restored, the knee joint is put through a huge amount of stress over a long period of time - typically, more than one million movements per year for 20 years or more. And loads of up to 200kg can be transmitted through the joint. With these factors in mind, Finsbury specified Tecvac's PVD titanium nitride coating for the cobalt chrome casting. The chrome castings are precision made using advanced CAD/CAM software, starting with an investment casting, followed by CNC profiling and then accurate polishing to a 'better than mirror' surface finish. The manufacturing process is critical for long term knee joint performance and Finsbury's engineering team had to obtain very high quality profiles and very smooth surfaces, to avoid internal abrasion and to ensure free, delicate and comfortable knee movements. Tecvac's unique PVD coating process uses electron beam evaporation to ensure very smooth coatings. It also meant that Tecvac could faithfully replicate the very accurate surface forms designed by Finsbury's engineers to give high quality articulation. Tecvac's coating, 2500 HV, is only a few microns thick and offers a smooth, biologically inert surface while ensuring that any patient who is sensitive to alloy constituents in the cobalt chrome casting, such as nickel, experiences maximum tissue compatibility throughout the life of the knee joint. With knee joints today costing anything from £7k to £9k, Tecvac's coating is playing a vital role in ensuring that long term costs are reduced while giving patients and surgeons with comfortable, effective prostheses and joints.