[Bone Cement Adhesion on Ceramic Surfaces - Deactivation of Surfaces and as a Consequence Inefficient Retention of Knee Prostheses Because of Adsorption of Atmospheric Water].

Aim: CoCrMo alloys are contraindicated for sufferers from allergy. For these patients, uncemented and cemented prostheses made of non-allergenic titanium alloy are indicated. Knee prostheses machined from that alloy, however, may have poor tribological behaviour, especially in contact to UHMWPE inlays. Therefore, high-strength oxide ceramics may be especially suitable for knee replacement in allergy patients with mobile bearing prostheses. For adhesion to bone cement, the ceramic surface only exhibits mechanical retention spots that are less adequate than those with a textured metal surface. Generating undercuts by corundum blasting is highly efficient for mechanical adhesion to a CoCrMo surface, but is not possible on a ceramic surface, due to the brittleness of ceramics. Lack of retention of bone cement promotes micromotions of the prostheses. As a consequence of micromotions, early aseptic loosening is predictable. Silicoating (silicate and silane layering) of the ceramic surface would allow specific adhesion and hence would result in increased and hydrolytically more stable bonding between the bone cement and the prosthesis surface - thereby preventing early aseptic loosening. Silicoating, however, presupposes a chemically active surface that is not blocked by a layer of chemisorbed molecules, e.g. water. Desorption of this chemisorbed layer is mandatory and can be attained by baking out the surface.

Method: In order to evaluate the effectiveness of surface activation via thermal treatment by baking out, with subsequent silicoating of the surface, the bond strengths of thermally treated and silicate layered ZPTA samples were compared with thermally treated surfaces that had not been silicoated. In our study of thermal surface treatment for baking out the surface, we focused on the question of whether there is a minimal "critical" temperature Ts for effective desorption of a chemisorbed water layer. The samples were prepared for the traction-adhesive strength test (sequence: ceramic disc, silicate and silane layering, protective lacquer ("PolyMA" layer), bone cement, TiAlV probes for the traction-adhesive strength test) and their traction-adhesive strengths were then measured.

Results: The bond strength was measured as a function of temperature for ceramic discs that had been baked out and subsequently silicoated. This was graduated, exhibits with a pronounced increase in the bond strength at a baking temperature of Ts ≈ 350 °C. The observed bond strengths before the step are ≤ 20 MPa and after the step ≥ 30 MPa.

Conclusion: Silicoating is effective in achieving high bond strength of bone cement on surfaces of oxide ceramics and can also stabilise the long term behaviour of the bond strength, provided the surface has been thermally treated prior to silicoating. Because of the proposed migration of the silicoating layer, micromotions and debonding should be widely reduced or even eliminated.