Effects of spaceflight and skeletal unloading on bone fracture healing

The physiological adaptation of humans to harsh extracorporeal conditions such as those endured during space flight missions is incompletely understood and deserves considerably more attention to better comprehend and prepare space flight personnel for these alterations. Notable and well-documented changes include the loss of musculoskeletal homeostasis and impairment of bone remodeling. These deleterious effects promote loss of bone mass and mineral density and could lead to insufficient reparative processes such as fracture healing. Knowing this, the question arises as to whether extended exposure to space flight would promote altered musculoskeletal physiologies that impinge upon the normal reparative response should a fracture occur during, or some time after, space flight. This review will provide a survey of existing literature concerning fracture healing, as it pertains to space flight and ground-based unloading analogs, as well as introduce experimentally derived mechanistic hypotheses to address the ramifications of space flight on bone fracture healing. Overall, the consensus from this literature concludes that fracture healing is delayed substantially in animals exposed to space flight or unloading. The use of ground-based models of physiological adaptation to microgravity coupled with valuable insights gleaned from actual space flight missions may lead to the development of novel countermeasures that could alleviate deficiencies in musculoskeletal homeostasis and bone remodeling, among other physiological alterations, for use in future longduration space missions. © Springer Science+Business Media, LLC 2011.