Simulating Microgravity in a Large Animal Model

MATERIALS AND METHODSThe microgravity environment encountered during spaceflight has numerous deleterious effects on the human body, with one of the most drastic being decreased bone mass due to mechanical unloading. These alterations in bone mass and skeletal strength are one of the foremost limitations of future space exploration. Due to the cost of long-duration space missions, it is critically important to develop ground-based models of the microgravity environment encountered during spaceflight to investigate possible countermeasures to maintain skeletal integrity. Most ground-based microgravity models utilize rodent hindlimb suspension to simulate how reduced loading affects isolated physiologic systems (1,2). Unfortunately, results derived from these studies are difficult to directly translate to the human condition due to major anatomic and physiologic differences between rodents and humans. Specifically, the differences in rodent and human bone structure become increasingly important when studying orthopaedic issues such as bone maintenance and healing during spaceflight. For example, the basic microstructure of rodent bone, known as “plexiform” bone, lacks the osteons (Haversion systems) that are the main micro-architectural feature of human cortical bone. Furthermore, it is known that the osteogenic and healing potential of rodent bone far exceeds that of adult human tissue. Due to these limitations in current ground-based microgravity models, there is a need to develop a ground-based, large animal model of simulated weightlessness that more closely approximates the human condition. Thus, the aim of this study was: (1) to implement an external fixation-based unloading apparatus in a large animal long bone model and (2) to confirm that the unloading method utilized to simulate weightlessness induces the concomitant physiologic orthopaedic changes associated with microgravity. A previously characterized transarticular hybrid fixator was applied to the right hindlimb of seven skeletally mature sheep for 8 weeks (Microgravity Group, n=7). The device was designed such that unloading of the metatarsus was accomplished by attaching the external fixation device to the proximal phalanges and distal tibia via fixation pins (3). An Earth gravity test group (Sham Group, n=3) was simulated by implanting the associated fixation pins while removing the external fixation connecting rods from the construct, thus ensuring full loading was transmitted through the metatarsal bone. Efficacy of the unloading technique was evaluated by measuring the strains in the external fixation device and a standard orthopaedic locking plate implanted on the metatarsus. The relative unloading of the device was calculated by relating the amount of load transferred through the metatarsus to the total load transferred through the bone and device. Dual-energy x-ray absorptiometry (DEXA) bone mineral density (BMD) measurements were performed on the treated metatarsus, contralateral metatarsus, and tibia of the treated limb at the time of surgery and every two weeks until sacrifice (8 weeks post-operative). Post-sacrifice biomechanical, radiographical, and histological outcome parameters were measured on the treated metatarsus and compared to its contralateral control metatarsus. Mechanical competency was evaluated via four-point bending and diametral compression tests. Trabecular microarchitecture was quantified via micro-computed tomography (µCT), and bone formation parameters and cellular activity were quantified using static and dynamic histomorphometry techniques. Statistical differences between groups were determined via either a paired t-test or one-way ANOVA with a Student-Newman-Keuls