AAOS Now: Much of your work centers on distraction osteogenesis (DO). How is this process applied and managed clinically?
Dr. Morgan: DO is a procedure for lengthening and sometimes reshaping a bone. It is used both clinically and as a research tool. The basic idea, in the simplest fashion, is that a cut is made across the bone, and the two halves are stabilized with hardware. After a waiting period of approximately 1 week, we begin to gradually lengthen the gap between the two halves of the bone, just 1 mm or 2 mm every 12 hours.
Once the desired gap length is achieved, there is a period of weeks or months, known as the consolidation phase, while the gap heals. Then the hardware can be removed and the patient can resume normal activities. It’s not a procedure to be done cavalierly, but overall it is a successful procedure for correcting a bone deformity arising from a congenital problem or injury. DO is also a treatment option for a bone fracture that fails to heal using the normal standard of care.
AAOS Now: Why did you concentrate your research on DO?
Dr. Morgan: I found it interesting that the stretching or tension applied in DO seems to stimulate a very exuberant healing response. The volume of new bone obtained using DO is just massively large compared with what can be tissue-engineered outside of the body.
We used DO as one scenario for studying the effect of mechanical forces on bone healing. The clinical procedure has been around for 100 years, but why it works is still very unclear. We know the stretching is a major contributor, but we don’t know what proteins are involved in sensing the stretching and creating a response that ultimately leads to bone creation. If we knew why it worked, we could tailor protocols based on the type of bone, type of patient, the hardware, and other factors.
DO is also a fertile area for exploring the mechanic and biologic factors and combinations that really are favorable to forming large volumes of bone, which would have implications for fracture patients as well as those with other bone diseases. Bone is the second most frequently transplanted tissue, after blood. The need to find ways to better promote and to augment formation of bone tissue is enormous.
AAOS Now: What did your experiments show?
Dr. Morgan: The novel contribution of our work—which has very much been a collaborative effort with Thomas A. Einhorn, MD, a clinician, and Louis C. Gerstenfeld, PhD, a basic scientist—was to provide empirical data on what sort of mechanical microenvironment the stretching was actually creating in the gap. A lot of previous studies just said, “the original gap was 2 mm; we’ve lengthened it by 1 mm, so we’ve lengthened it by 50 percent.” And that 50 percent was treated as the sole parameter to correlate to how much bone formed, and to how well the procedure worked overall. But as it turns out, that sole parameter doesn’t have enough predictive value for the healing response.
Computer models have predicted a mix of tissues in the gap, some stiffer than others, so not every part of the gap is being stretched by the same amount. To gather the data to validate those models, we measured the strains or deformations created by the lengthening. We found that the amount of deformation varies greatly throughout the gap. We also found a lot of shearing as well, which was very interesting, because shear stress and shear deformation have been shown to affect the behavior of many types of cells.
I think this helps explain some of the contradictory findings in prior research and clinical practice in DO. If the overall amount of lengthening is the only parameter used to predict how well healing will progress, the complex mechanical microenvironment is ignored. I think the microenvironment is critically important for controlling the success of the DO procedure and even the healing of any bone fracture.
AAOS Now: What did your work with pseudarthrosis yield and why did you focus on it?
Dr. Morgan: We compared the mechanical environment of DO to that of a pseudarthrosis—or false joint—scenario. Pseudoarthrosis is an undesired clinical outcome in which a bone fracture doesn’t heal successfully but instead forms something that looks like an immature joint. In DO, a considerable amount of bone forms; in pseudarthrosis, a lot of cartilage forms. Our experiments showed that the mechanical microenvironment in the gap is totally different when a bending, rather than a lengthening, motion is applied.
Here’s what we think happens: At first, an immature, multipotent tissue populated with skeletal progenitor cells fills the gap. These cells can differentiate and form different types of tissue—bone, cartilage, fibrous tissue—depending on the cues they receive in their local environment. We think that one cue is mechanical, and our results suggest that the deformations that this multipotent tissue experiences as a result of the applied motion (whether bending, tension, or something else) influence the subsequent formation of mature tissue. That would be pretty exciting if it holds up, because it would give us guidelines on the mechanical environments favorable to cartilage formation or bone formation.
AAOS Now: What does your research indicate is happening at the molecular level?
Dr. Morgan: This work is just beginning. We are trying to determine what kind of molecular mechanisms are activated by the mechanical environments that produce bone or cartilage. We are focusing on those mechanisms believed to be involved in sensing mechanical stimuli and then translating it into some kind of biologic response.
We looked at focal adhesion kinase (FAK) and the Rho family of proteins. We found that the expression of RhoA, in particular, maps quite nicely to specific sets of mechanical conditions. In pseudoarthritis, the regions with the most rapid formation of cartilage have a fairly specific mechanical environment in terms of shear deformation. They also have the highest expression of RhoA. RhoA helps regulate cell shape, which is likely to be affected if the tissue in which the cell resides is undergoing shear deformation.
At this point, we don’t know if these results mean that RhoA is essential in relating mechanical conditions to cartilage formation or if something else is relating the two. One big next step is to determine whether activation of a given molecular mechanism actually leads to formation of a certain type of tissue.
AAOS Now: What did your work in imaging uncover and what are the clinical implications?
Dr. Morgan: We have focused primarily on contrast-enhanced computed tomography (CT) scanning for imaging cartilage. Normally cartilage is not visible or at least not easily distinguishable in the CT scan. Most fractures heal by forming cartilage first, which is later replaced by bone tissue. Radiographs or plain CT scans can only be used to follow a fracture late in the game.
It takes about 6 weeks before you can tell whether healing is progressing on schedule on a radiograph. Even then, if you don’t see bone tissue forming across the fracture gap, you don’t know if the healing is simply a bit delayed or if there is a bigger problem and the fracture is going to end up as a non-union. A way to visualize cartilage at the fracture site before or at 6 weeks would be helpful. If it’s there, you can be somewhat optimistic that ultimately bone will form. If it’s not, something is wrong and an intervention is needed.
We worked with Mark W. Grinstaff, PhD, a chemist and biomedical engineer. He developed a small molecule specific to cartilage, CA4+, that is visible in a CT scan. That led to a method for contrast-enhanced CT imaging that enables us to track and assess the healing process much earlier.
AAOS Now: Where do we stand now in our understanding of bone healing, and what lies ahead?
Dr. Morgan: Orthopaedic surgeons who treat fractures have vast clinical experience, but no quantitative, proven way of saying that a certain fixator will create a certain mechanical environment for that particular fracture. They don’t know whether that environment will be beneficial.
I think we’ve made headway on two fronts. One is understanding how the mechanical environment can alter or improve bone healing, through biomechanical, biologic, or biochemical mechanisms. We’ve developed some tools to support that inquiry by providing a quantitative way of examining the mechanical environment of bone that is healing.
Second, we can now test various hypotheses about the most favorable mechanical conditions for forming various types of tissues. We can test this in the laboratory and in retrospective studies by looking at how fractures were treated, estimating what the mechanical environment was, and seeing if it links with our predictions for healing or for non-union.
Dr. Morgan reports the following conflicts: Amgen, Wallace Coulter Foundation.
Terry Stanton is senior science writer for AAOS Now. He can be reached at firstname.lastname@example.org
- Tensile motion, as seen in distraction osteogenesis, encourages the formation of bone while a bending motion encourages cells to develop into cartilage, as seen in pseudoarthritis.
- Because the deformation in tissues in the fracture gap varies greatly, a linear relationship cannot be drawn between the amount of lengthening and healing.
- This research found that expression of Rho proteins aligned with regions of cartilage formation, ultimately resulting in new bone formation.
- A new method of imaging allows for earlier determination of the progress and prospects for success of healing.
- Future research will seek to determine which mechanical conditions are most favorable for formation of various types of tissues.