Total hip replacement (THR) is one of the most successful and most frequently performed elective surgeries and more than 40% of femoral stems worldwide are implanted using cement (PMMA, polymethylmethacrylate). Aseptic loosening is still the main reason for failed THRs. In most cases, the adhesive bond between the stem and the PMMA is lost shortly after surgery, a process known as the debonding. After debonding, micro movements lead to aseptic loosening caused by wear mechanisms in the stem-PMMA interface. In contrast to conventional implant alloys like CoCrMo and TiAlNb, a long-term increase of the shear bond strength of PMMA on tin (Sn) can be achieved using adhesion-promoting phosphoric acid primers.
Sn coating of implant alloys may therefore result in a combination of the well-established mechanical properties of CoCrMo and TiAlNb with the long-term adhesive properties of PMMA on Sn.
In this study the shear bond strength for PMMA adhesion on uncoated and Sn-coated CoCrMo and TiAlNb alloy substrates was determined. To mimic the microstructured surface design of THRs, substrates were modified by either polishing or Al2O3 blasting. Subsequently, thin Sn layers (3.5 µm) were galvanically deposited on substrates with a 0.5 µm gold interlayer. Tin coatings of polished surfaces resulted almost in a twofold increase in cement to hip stem alloy bond strength. This increase, however, was temporary and significantly decreased to the level of the bond strength of uncoated alloys after 150 days in NaCl solution at 37°C.