Introduction: Many claim that an inflammatory reaction to wear debris particles is the main cause for prosthetic loosening. We have rat model in which bone resorption can be induced both by fluid pressure and particles. We compared the bone resorptive potency of particles and fluid pressure.

Materials and Methods: The rats received a titanium plate at the proximal tibia. A central plug was inserted. After 5 weeks of osseointegration, the central plug was changed to a piston or a hollow plug with 20mg titanium particles. Commercially pure titanium particles with 90 % of particles lesser than 3,6 microns were used. The pressure piston was subjected to a transcutanous force of 5N. Each episode of pressure comprised 20 pressure cycles at 0.17 Hz, applied twice a day. 60 rats were randomized to 6 groups for particle treatment. One group had particle implantation both at day 1 and 7. Additionally 15 rats were randomized into 3 groups with for pressure treatment. All rats were killed after 5 or 14 days. Bone resorption underneath the piston was evaluated blindedly in hematoxlyin/eosine sections and scored as 0 – 5. Differences between groups were analyzed by Kruskal Wallis and Mann-Whitney U-tests.

Results: Many specimens with titanium particles showed no visible resorption at al, and a few showed dramatic osteolysis. After 14 days, the osteolytic effect was significant. Partice refill made no difference. Titanium particles over 10 μm had minor effect. Fluid pressure always caused bone resorption, and significantly more so than particles both after 5 and 14 days.

Discussion: There was lesser variability in fluid pressure induced osteolysis, which might be due to a different signalling pathway. Titanium particles have an osteolytic effect in this model, but in spite of the massive amount of particles, the effect was less predictable than with pressure.

Soaking bone grafts in a bisphosphonate solution before implantation can prevent their resorption and increase the local bone density in rats and humans. However, recent studies suggest that pre-treatment of allografts with bisphosphonate can prevent bone ingrowth into impaction grafts. We tested the hypothesis that excessive amounts of bisphosphonate would also cause a negative response in less dense grafts. We used a model where non-impacted metaphyseal bone grafts were randomised into three groups with either no bisphosphonate, alendronate followed by rinsing, and alendronate without subsequent rinsing, and inserted into bone chambers in rats. The specimens were evaluated histologically at one week, and by histomorphometry and radiology at four weeks. At four weeks, both bisphosphonate groups showed an increase in the total bone content, increased newly formed bone, and higher radiodensity than the controls. In spite of being implanted in a chamber with a limited opportunity to diffuse, even an excessive amount of bisphosphonate improved the outcome. We suggest that the negative results seen by others could be due to the combination of densely compacted bone and a bisphosphonate.

We suggest that bisphosphonates are likely to have a negative influence where resorption is a prerequisite to create space for new bone ingrowth.