The effect of surface texture and calcium phosphate coating on bone-implant reaction

Aim of the study

One disconcerting question posed by the authors of this study concerns the superior speed and quality of integration of implants coated with calcium phosphate (CaP). Is this due to the specific material used or to irregularities of the surface ?

Materials and methods

Four types of implants were used, two titanium and two coated with CaP. In the titanium group, one had a smooth surface and the other matt. The same applied to the CaP group, one smooth (coated by RF Magnetron) and the other not (plasma sprayed). 96 implants (24 of each type) were placed in the left and right femoral condyles and the tibial diaphyses of rabbits. The histological study was carried out over a 2 to 12 week period.

Results

Histomorphometric measurements showed different results for the two types of implant. In the tibia, a significantly better percentage of bone-implant contact was obtained with matt CaP implants (84.5 %), compared with smooth CaP (75.4 %) or with matt titanium (76.1 %). They themselves were better than smooth titanium (63.2 %). In the femoral condyles, significantly better scores were obtained with matt CaP (84.7 %) and smooth CaP (83.6 %) compared with matt titanium (76.4 %) or smooth titanium (76.4 %). Histological assessment at 2 weeks showed identical results for the 4 groups. At 12 weeks, the smooth titanium groups differed from the other three, with fewer bone-implant contacts and the presence of a zone of fibrous tissue and inflammatory cells. The other three groups showed more bone-implant contacts and those areas without contact remaining healthy.

Conclusion

A CaP coating, even when smooth (RF Magnetron) has a favourable effect on bone response. This response is affected by the site of implantation.

Commentaries

Several years ago, Maxian compared the behaviour of smooth and matt titanium and CaP implants. He concluded that the crystallinity of the CaP and the irregularity of the implant surface, played a determinant role (Maxian et al., J Biomed Mat Res, 1993). The authors of this study tended to confirm this result, displaying higher scores for implant-bone contact for matt CaP surfaces (in tibia) and the same for smooth surfaces (in femoral condyle). The benefits of a CaP coating are from now on undeniable and the quality of osseointegration is, in the case of plasma sprayed implants, not due to solely to the irregularity of the surface. A further unknown factor remains. Plasma sprayed CaP coatings display stability when they have a mean thickness of 75 µm and a marked crystallinity. They show at their periphery a slight reduction (less than a few mm) which determines the bioactivity and the development of bone epitaxis. But what is it about smooth CaP implants coated with a layer of about 4-5 µm ? Does the deposited layer of CaP remain stable as long as the bioactivity is effective ? A sufficiently long period of observation is necessary in order not to repeat the disappointments experienced with the first hydroxyapatite coated implants (of variable thickness, crystallinity and porosity !) and which received such a poor press.