CoCrMo alloys are contraindicated for allergy sufferers. For these patients, uncemented and cemented prostheses made of titanium alloy are indicated. Knee prostheses machined from that alloy, however, may have poor tribological behavior. Therefore, for allergy sufferers, knee replacements in form of cemented high-strength oxide ceramic prostheses which reveal excellent tribological behavior are suitable. In addition, the rate of particle induced aseptic loosening may be reduced. For adhesion of bone cement, the smooth ceramic surface, however, exposes inefficient mechanical retention spots as compared with a textured metal surface. Undercuts generated by corundum blasting, which in the short and intermediate term are highly efficient on a CoCrMo surface, are not appropriate on a ceramic surface due to the brittleness of ceramics. The corresponding textures may initiate cracks which will weaken the strength of a ceramic prosthesis. Due to the lack of textures, mechanical retention is poor or even not existent. Micromotions are facilitated and early aseptic loosening is predictable. Instead silicoating of the ceramic surface will allow specific adhesion and result in better hydrolytic stability of bonding, thereby preventing early aseptic loosening. Silicoating, however, presupposes a clean and chemically active surface which may be achieved by atmospheric plasma or thermal surface treatment. In order to evaluate the effectiveness of silicoating, the bond strengths of atmospheric plasma versus thermal surface treated and silicoated ZPTA (zirconia platelets toughened alumina) surfaces were compared with “as-fired” surfaces by utilizing TiAlV probes (diameter: 6 mm) for traction–adhesive strength test. After preparing samples for traction–adhesive strength test (sequence: ceramic substrate, silicate layer and penetrated silane, protective lacquer (PolyMA), bone cement, TiAlV probe), they were aged up to 150 days at 37 ℃ in Ringer’s solution. The bond strengths observed for all aging intervals were well above 20 MPa and much higher and more hydrolytic stable for silicoated compared with “as-fired” ZPTA samples. Silicoating may be effective for achieving high initial bond strength of bone cement on surfaces of oxide ceramics and is also suitable to stabilize bond strengths in the long term under hydrolytic conditions as present in the human body. Activation by atmospheric plasma or thermal surface treatment seems to be effective prior to silicoating. Due to the proposed silicate layer migration, micromotions and debonding should be widely reduced or even eliminated.
The authors would like to thank the ISF Welding and Joining Institute of the Technical University of Aachen for measuring the surface energies of polished surfaces of tibial parts of CoCrMo and ZPTA prostheses.
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