Effect of hydroxyapatite coating and polymethylmethacrylate on stainless steel implant-site infection with Staphylococcus epidermidis in a sheep model.

We aimed to study the influence of hydroxyapatite (HA) coating and polymethylmethacrylate (PMMA) cement on the risk of development of stainless steel implant-site infection with Staphylococcus epidermidis in a sheep model. Uncoated, HA-coated, and PMMA-cemented stainless steel implants were inserted in the left femur of 30 sheep. For each type of implant, sheep were inoculated with S. epidermidis in the intramedullary canal and one non-inoculated group was used as control. After 6 weeks, infection was evaluated using clinical, radiological, bacteriological, and histological criteria. Radiological and clinical results were normal. Cultures were negative in the control sheep. In the inoculated sheep, interposition tissue and bone cultures were positive in 2 of 6 uncoated, 6 of 6 HA, and 6 of 6 PMMA implants with a mean bacteria count of 5.2 +/- 1.17, 3.5 +/- 0.7, and 3.9 +/- 0.9 log10 cfu/g, respectively (NS), for interposition tissue, and 4 +/- 0.01, 2.9 +/- 0.6, and 2.5 +/- 1.3 log10 cfu/g, respectively (NS) for bone. The polymorphonuclear leukocyte (PMN) score (mean number of PMN per 10 different microscopic high-power fields >or=5) in interposition tissue was >or=3 in 6 of 6 HA, significantly different from uncoated (3 of 6) and PMMA (2 of 6) groups (p = 0.04). The HA and PMMA inoculated groups had a higher infection rate than the uncoated inoculated group (p = 0.06). In this experimental sheep model of S. epidermidis infection at the bone-biomaterial interface, HA seems to be at higher risk of infection compared with uncoated or PMMA-cemented stainless steel, when inoculation is intramedullary and contemporary with implantation.