2012 PRORP Investigator Vignette

Title: A Murine Model of Orthopaedic Implant Infection

Investigator: Edward Greenfield, PhD; Case Western Reserve University

The goal of this grant is to develop a mouse model of implant infection so that we can study the novel therapies and treatment strategies to prevent infection and allow orthopedic implants to integrate with the bone.

The worst complication for orthopedic implants is when they get infected. That's the worst thing that can happen to an orthopedic patient. And what we'd like to do is develop new strategies for treating those infections and hopefully even preventing them. The way we're starting out is to develop a mouse model of it, so we can test therapies in mice before we test them in larger animals and then ultimately someday in patients.

So the methodology is we had a mouse model of osteo-integration where we put very tiny metal implants in the femurs of mice, the long bones, and the bone grows into them. And now we've added bacteria to that to mimic an infection. And what happens is the bone doesn't integrate because the bacteria causes massive inflammation and basically the implants often fall out.

So one of the things we took advantage of is bioluminescence, so the bacteria have been engineered to express a gene that under the right circumstances the protein product from that gene produces light and we can detect that. So the intensity of these spots tell you how much bacteria there is, and what we can see is in all cases it's localized over the femur which is where the implant is with the bacteria.

And so what that shows is we're not getting systemic infection; the bacteria are localized, the mice are healthy, but they do have this big infection, but localized. And that's what we're going to try to treat.

And then we take the femurs out and we can do one of two things. We can either test the integration biomechanically and that is we pull on the implant and see much force it takes to the pull implant out. And with bacteria it takes very little force because there's not much very much bone growing around the implant, which you can also see histologically so this is an uninfected implant where there was no bacteria. The bone is grown around that implant and it's holding it in tight.

In contrast where there's low dose or high doses of the bacteria you can see there's very little contact between the implant and the bone which is shown in the pink.

We can measure the amount of new bone both right up against the implant and the bone in the area and again that's inhibited by the presence of high levels of bacteria.

So this is in Staph aureus which is the most common type of bacteria that causes implant infection both in civilian hospitals as well as in military settings. So the other thing we're going to do is use acinetobacter which is a very common type of bacteria in military settings and is starting to be seen in civilian hospitals as well.

So other people have done similar kinds of things; what's different here is that in our model because of the kind of implant we're using we get osteo-integration in the absence of bacteria, whereas other people have used stainless steel pins and things like that which the bone doesn't osteo-integrate in. And that's an important part of what happens in the real world is that you want the bone to integrate with the implant and so we want to have a model that we test that in and that hasn't been done before in mice at least.

What's really been the huge benefit is these bioluminescent bacteria, which initially we weren't planning on using. And the advantage of this strain is the bioluminescent gene is stably incorporated so it stays in the bacteria. And so you can do long-term studies whereas the older versions that the gene would get kicked out of the bacteria once you put them in the mice, and so you could only do short-term studies. And once these became available it really made the study much more doable. Because what we can do-because we don't have to kill the mice to do this; so each one of these dots are the same mice-just over time.

So the model works pretty well. And now we're in a position to start testing these therapies. We're going to use some peptides that have recently been discovered that prevent infections in other parts of the body and that haven't been used before in orthopedic infections. And that's what we're going to try.