Segmental bone defects (SBDs) secondary to trauma invariably result in a prolonged recovery with an extended period of limited weight bearing on the affected limb. Soldiers sustaining blast injuries and civilians sustaining high energy trauma typify such a clinical scenario. These patients frequently sustain composite injuries with SBDs in concert with extensive soft tissue damage.

For soft tissue injury resolution and skeletal reconstruction, a patient may experience limited weight bearing for upwards of 6 months. Many small animal investigations have evaluated interventions for SBDs. While providing foundational information regarding the treatment of bone defects, these models do not simulate limited weight bearing conditions after injury.

Orthopedic injuries may result from high energy trauma such as car accidents or battlefield explosions, and are a focus in orthopedic research. Current surgical techniques, however, are not always effective for repairing damaged bone. In many cases, to allow for successful healing the injuries will require a secondary bone healing intervention.

An effective treatment could improve the outcome of military casualties, as 60–70% of modern war injuries involve the musculoskeletal system, predominantly caused by explosive devices. Microgravity is the ultimate disuse environment which can be exploited for biological testing.

Because this environment mimics some aspects of the unloading observed in traumatic skeletal reconstruction patients, we can use it to better assess the efficacy of bone healing age. Indeed, this type of approach has been suggested for development of novel drugs for bone loss diseases such as osteoporosis.

Unfortunately, in many cases promising animal data does not translate into the clinic. About fracture healing, this may be due to the animal models bearing weight much sooner than human patients do, as weight bearing improves fracture healing. Their second goal was to demonstrate that the mice having undergone surgery could successfully ambulate on the wire flooring and successfully be housed in the spaceflight hardware, including being able to access food and water.

Taken together, we demonstrated that mice with a SBD surgery can withstand the forces and vibrations associated with launch, can be successfully housed in spaceflight hardware, and exhibit effects of unloading on bone healing.

In conclusion, it was found that neither launch simulation nor caging provided by the NASA Rodent Research Project impacted bone healing. Removal of weight bearing, as accomplished by hindlimb suspension, resulted in a significant reduction in total bone callus volume, whereas singly housing mice did not impact bone healing in our model.

Therefore, since limited weight bearing is obligatory in microgravity, the RR-4 mission should be a higher fidelity model for human bone healing, improving translation into the clinic.