Saline irrigation does not affect bone formation or fixation strength of hydroxyapatite/tricalcium phosphate-coated implants in a rat model

Intramembranous bone regeneration is critical to implant fixation. In cementless joint replacement (as opposed to cemented joint replacement), saline irrigation is not typically performed during surgery so that the osteogenic stimulus provided by the marrow is preserved. Several groups are now using the rat marrow ablation model to study intramembranous bone regeneration and implant fixation. In this model, the marrow contents are mechanically disrupted, and debris is often cleared by saline irrigation, a step that appears inconsistent with the clinical situation. Furthermore, in contrast to conventional wisdom, it has been reported that saline irrigation enhanced bone-implant contact and peri-implant bone formation in the rat model (Ishizaka et al. Bone 1996;19:589-594), although mechanical fixation of the implant was not investigated. Accordingly, the present study was performed to determine if saline irrigation leads to enhanced mechanical fixation of implants in the rat model. Forty-eight 400 to 450 g male rats were divided equally into two groups. The treatment group, in contrast to the control group, received saline irrigation in the ablated medullary canal prior to placement of hydroxyapatite/tricalcium phosphate-coated implants. Eight animals in each group were killed at 2, 4, or 8 weeks after implantation, at which time the specimens were analyzed by micro computed tomography to measure bone formation around the implant, followed by a mechanical pull-out test to measure the strength of fixation of the implant. As expected, there was increased fixation strength over time, but there were no significant differences in peri-implant bone volume, bone-implant contact, or implant fixation strength between the two groups. Thus, we found no effect of saline irrigation on bone formation or implant fixation strength in this study in which the implant had an osteoconductive coating. (c) 2005 Wiley Periodicals, Inc.