Movement between an implant surface and overlying soft tissue gives rise to fibrous capsule formation with a liquid filled void. Clinically, this situation is more prevalent with electropolished stainless steel (EPSS) implants compared to commercially pure titanium (CpTi) implants. We hypothesise this is mainly due to lack of microtopography on the EPSS.

Four experimental EPSS surfaces with varying microtopographies were selected by a combination of morphological analysis using the scanning electron microscope and quantitative roughness analysis using laser profilometry. Standard treated EPSS (ISO 5832/1) and CpTi (ISO 5832/2) surfaces were also used. The plates had only one screw hole at either end so that the interaction of the tissue with an intact surface could be evaluated. Six plates of each type were implanted on both the left and right tibia, randomly, of 18 white New Zealand rabbits under the muscle for 12 weeks.

After sacrifice samples underwent standard histological processing. Briefly, fixation, dehydration, embedding in methyl methacrylate, sectioning at 250μm slices (with implant), grinding to 50μm and staining with Giemsa. Digital images were taken with a light microscope and the size of thickening of connective tissue on the implant surface and the presence or absence of a liquid filled void was observed.

Results showed no voids present on the CpTi samples. The standard EPSS had 3/6 plates with a void. The experimental EPSS surfaces were in-between these results. There was no relationship between quantitative measurements of average roughness (Ra) and the presence or absence of a void. There was a relationship between lack of fine microroughness of a surface (as seen with the SEM) and the presence of a void. The size of capsular thickening was not related to the Ra of the surface. These results support that void formation is mainly due to lack of microtopography on the plates.