Biomechanical and histomorphometric evaluation of skin integration on titanium and PEEK implants with different surface treatments

Percutaneous implants are frequently affected by bacterial growth at the skin-implant interface. Integration between implant and surrounding skin is important to prevent bacteria from spreading to the underlying tissue. The standard method to evaluate skin-implant integration is by histomorphometry on samples which have been placed in tissue grown in vivo or ex vivo. In this study, a biomechanical method was developed and evaluated. The integration of implants into porcine skin was studied in an ex vivo model, where pig skin samples were cultivated in a nutrient solution. Cylindrical shaped implants, consisting of polyether ether ketone (PEEK) and titanium (Ti) with different surface treatments, were implanted in the skin tissue and the skin was grown in nutrient solution for 2 weeks. The implants were then extracted from the implantation site and the mechanical force during extraction was measured as a quantitative assessment of skin-implant integration. Implants from each group were also processed for histomorphometry and the degree of epidermal downgrowth (ED) and tissue to implant contact (TIC) was measured. A higher mean pullout force was observed for the PEEK implants compared to the Ti implants. Applying nanosized hydroxyapatite (HA) on Ti and PEEK increased the pullout force compared to uncoated controls, 24% for machined and 70% for blasted Ti, and 51% for machined PEEK. Treatment of Ti and PEEK with nanosized zirconium phosphate (ZrP) did not increase the pullout force. The histomorphometry analysis showed correlation between ED and pullout force, where the pullout force was inversely proportional to ED. For TIC, no significant differences were observed between the groups of same material (i.e. Ti, Ti+HA, Ti+ZrP, and PEEK, PEEK + HA, PEEK + ZrP), but it was significantly higher for PEEK compared to Ti. Scanning electron microscopy analysis was done on samples before and after the pullout tests, showing that the ZrP coating was unaffected by the 2 week ex vivo implantation and pullout procedure, no dissolution or detachment of the coating was observed. For the HA coating, a loss of coating was seen on approximately 5% of the total surface area of the implant.