Spaceflight and Fractures: 5-Year-Fracture Incidence Upon Return From Spaceflight

Abstract

Introduction

Exposure to spaceflight and microgravity environments has been implicated in large reductions in astronaut bone mineral density (BMD). While low BMD is a known risk factor for fracture, there have been no reported cases of in-flight fracture, and very little documentation of post-flight fracture. The present study sought to review the incidence of fractures within 5 years of return to spaceflight.

Methods

Using NASA's Lifetime Surveillance of Astronaut Health epidemiology database, a retrospective cohort study was conducted to identify the incidence of fracture in the 5-year post-flight period. All astronauts who participated in spaceflight with 5-year post-flight medical data were included. Demographics were compared between the fracture and nonfracture cohorts.

Results

Of the 242 astronauts who met the inclusion criteria, 7 (2.9%) sustained fractures within 5 years post-flight. Three post-flight fractures occurred in the hip or spine. Six of the 7 fractures occurred within 2 years of return from spaceflight. Spaceflight length, age, and time from spaceflight were not statistically significantly associated with increased risk of fracture.

Conclusion

Fracture upon return to the gravitational environment is a serious risk for astronauts that can significantly jeopardize astronaut health and mission success. Fractures of the hip and spine, specifically, are associated with decreased BMD. While the incidence of these fractures is roughly 1%, the negative implications, including high 1-year mortality rates and functional implications, emphasize the need to optimize bone health and fracture treatment protocols.

Introduction

Space exploration has entered a new era of ambitious missions and increased accessibility. As humanity sets its sights on distant planets and longer-duration spaceflights, the challenges to human health and performance faced by astronauts have come to the forefront of scientific inquiry.

The National Aeronautics and Space Administration (NASA) Artemis program is currently seeking to return to the moon with the possibility of establishing a space station orbiting the moon. This could then serve to facilitate future Martian exploration. Longer missions in space, such as the 3.5-year round-trip mission to Mars, will subject astronauts to extended periods of microgravity with exposure to partial gravity environments. Therefore, it is important to understand the impact that prolonged spaceflight has on the musculoskeletal system.

Concurrently, space travel is becoming more accessible. The emergence of space tourism and commercial spaceflight has begun to democratize access to space, exposing a broader range of individuals to the unique environment beyond Earth's atmosphere. This shift from exclusively government-sponsored missions to a growing array of space travelers further necessitates a comprehensive understanding of the orthopedic impacts of spaceflight on various populations, given that commercial space travel may not have the same stringent health standards as NASA and other space agencies.

While access to spaceflight is increasing, and plans for longer-distance and longer-duration spaceflight are in motion, there remain areas where the impact of spaceflight on orthopedic health has not been fully elucidated. The most well-documented orthopedic manifestation of exposure to spaceflight surrounds bone mineral density (BMD). Exposure to the spaceflight environment is known to rapidly decrease BMD. Findings from astronauts who have undergone 4- to 6-month missions suggest astronauts experience a 1 to 1.5% decline in BMD per month in space for normal weight-bearing skeletal sites, even with countermeasures such as resistive exercise. ¹ While leading theories attribute this BMD loss to microgravity and unloading-induced alteration of homeostasis between osteoblasts and osteoclasts, additional theories have been proposed that implicate altered vestibular function and radiation in accelerated bone loss.^2,3

BMD losses in the spaceflight environment have the potential to pose a significant risk to an astronaut's health and limit an astronaut's ability to perform critical mission-related tasks. On Earth, decreased BMD is a known risk factor for fracture, specifically fragility fracture. ⁴ These fragility fractures not only significantly limit ambulatory and functional capacity but also have significant morbidity and mortality associations. ⁵ Risk and implications of fracture in spaceflight require special consideration, as limited means of treatment are available in the spaceflight environment, and immediate medical evacuation to Earth is not feasible on longer missions to the moon or Mars. Of particular concern are fragility fractures of the hip and spine, which could render an astronaut unable to mobilize, thus jeopardizing chances of mission success and endangering the astronaut’s life. Microgravity also alters fracture healing patterns and impairs hemostasis, complicating bone regeneration after initial injury. In space, bone regeneration produces smaller calluses, a less fully bridged fracture gap, and weaker consolidation. ⁶

While BMD loss during spaceflight has been well-documented, the risk of fracture from spaceflight-induced BMD change is currently derived from theoretical models, as no fractures have been reported in spaceflight. From published models, anatomic locations at high risk of fracture in spaceflight are similar to those on Earth and include the femoral neck, lumbar spine, and distal radius. ⁷ However, the absence of concrete outcomes reported in this area hampers our ability to accurately assess and mitigate the risk of fractures during extended space missions.

While no fractures have occurred in spaceflight, the future of long-duration spaceflight and the increasing number of people engaging in short-duration spaceflight necessitate a better understanding of the effects of microgravity on BMD and eventual fracture occurrence in the post-flight period. This is especially important when considering plans for missions to Mars, which will require long-duration spaceflight followed by extended exposure to a partial gravity environment before return to Earth. The purpose of this study is to review the incidence of fractures within 5 years post-flight to better understand outcomes and risks associated with BMD reduction during spaceflight.

Methods & Materials

A retrospective cohort study was performed to compare and quantify the rate of fracture after return to Earth among astronauts based on mission duration. NASA's Lifetime Surveillance of Astronaut Health (LSAH) epidemiology database was queried to provide data on fracture incidence for astronauts upon return from space. Inclusion criteria were all persons who participated in spaceflight, regardless of age or spaceflight mission time. All injuries occurring greater than 5 years after return to Earth were excluded. NASA epidemiologists reviewed and compiled the astronaut medical records, encompassing 1,210 total person-years.

Due to government-regulated data-sharing confidentiality restrictions surrounding the distribution of identifiable protected health information, astronaut epidemiological information was parsed into dichotomous categories by NASA LSAH that met the criteria to be considered de-identified before the study team was given access. Subjects were categorized by age greater than or less than 45 years at the time of spaceflight, duration of single spaceflight mission greater than or less than 6 months, and time to fracture from spaceflight of less than 1 year, 1 to 2 years, and 2 to 5 years.

Statistical Analysis

With the available data, patient demographics were compared between fractured and nonfractured cohorts. Descriptive statistics were utilized to analyze differences in fracture rates for each cohort. Student's t-test was used to determine differences in continuous variables. Chi-square was used to determine differences in categorical variables. If the assumptive conditions of the chi-square were not met, Fisher's Exact Test was used to determine statistical differences. Alpha was set to a significance level of 0.05 for all analyses. Statistical analyses were performed with IBM SPSS Statistics V.27.0 (IBM Corp, Armonk, NY) and R software 3.6.1 (www.r-project.org).

Results

A total of 242 astronauts met the inclusion criteria for review. Of the 242 astronauts, 7 (2.9%) sustained fractures in the 5 years post-flight, 3 of the post-flight fractures occurred in the hip or spine, and 6 of the 7 fractures occurred within 2 years of return from spaceflight. See Figure 1 for anatomic distributions of all fractures noted (Figure 1). Spaceflight length, age, and time from spaceflight were not statistically significantly associated with increased risk for fracture (Table 1).

Figure 1.

Distribution of astronaut fractures by location.

Table 1.

Astronaut Fractures Post-Spaceflight.

	Fracture	No Fracture	p-Value
Age
<45 yrs	0	84	0.0994
>45 yrs	7	151
Duration of Spaceflight
<6 mos	6	194	1
>6 mos	1	51
Time from Spaceflight to Injury
<1 yr	1	n/a*
1–2 yrs	3	n/a*
>2 yrs	3	n/a*

*Years from spaceflight until injury could not be determined for the uninjured group.

Discussion

This study identified 7 fractures sustained within 5 years post-flight out of a sample of 242 astronauts. Three out of 7 fractures occurred in the hip or spine. There is a significant body of literature establishing the deleterious effects of microgravity environments on bone.^8–10 Such effects can extend for years after exposure to such environments.^10–12 The clinical implications of this, however, remain largely theoretical, as there is little literature assessing actual occurrences of fractures in this population. ¹⁰ The risk of fracture during or post-flight has been postulated to be minimal, as no in-flight fractures have been observed over decades of space travel. ¹ While the rate of post-flight fractures in this study is low, this is one of the first studies to document their occurrence.

Three out of 7 fractures occurring in the hip or spine are particularly relevant given that they are 2 of the most common fragility fractures—injuries typically seen in older patients with poor bone quality. ⁵ Hip fractures are significantly more common in elderly patients with risk factors such as female gender, decreased mobility, and low bone density. ¹³ These demographics differ considerably from those of astronauts who are younger and historically comprised of more males than females. While hip fractures in younger patients are typically thought to be the result of higher energy trauma, even these patients have been found to have lower bone mineral density regardless of trauma mechanism. ¹⁴ Hip fractures are also associated with significant morbidity and mortality, with estimated 1-year mortality rates of 20% to 30%.^13,15 While hip fracture–related mortality rates tend to be higher in elderly patients with significant medical comorbidities, there is still a strong association between hip fractures and morbidity and mortality in younger, healthier populations. The hip fractures noted in this study are not necessarily surprising given the known effects of spaceflight on the proximal femur. Pre- and post-flight imaging data have shown that proximal femur strength and bone mineral density are significantly decreased after 4–6 months in space, without full recovery 1 year later.^9,16

Similar to hip fractures, risk factors for vertebral fractures include older age, female gender, and low bone mineral density.^17,18 Vertebral fractures are associated with their own significant deleterious health effects, including increased morbidity and mortality and an increased risk of subsequent vertebral and nonvertebral fractures. ¹⁹ Spaceflight has been associated with a decrease in vertebral strength that persists 4 years after return to Earth, increasing the risk for vertebral fractures. ⁸ As 90% of hip fractures occur in patients over 50 years old, and 97% of vertebral compression fractures occur in patients over 60 years old, further research with an appropriately matched cohort is needed to see if hip and vertebral fractures occur at higher rates in astronauts post-flight compared to a matched cohort of the general population and, if so, what further steps can be taken to mitigate these risks.^20,21

This study found that spaceflight length as well as time from spaceflight were not significantly associated with increased risk of fracture. This finding is limited by sample size, given the low rate of total fractures in the study. Dual-energy X-ray absorptiometry measurements of astronauts undergoing 4–6 month missions have shown an estimated 1–1.5% decline in BMD per month.^22,23 Studies modeling the risk for longer missions such as to Mars have estimated astronauts may lose 33% of their BMD in the femoral neck and could return to Earth with a T-score below NASA's permissible limit of −2. ²⁰ While bone strength does increase on return to Earth, it does not necessarily return to pre-flight levels, with studies showing persistent decreases 12 months and even 4 years after return to Earth.^8,12,24 Though these findings could predispose astronauts to higher fracture risk, this data has yet to be correlated clinically with an increased rate of certain fractures in astronauts relative to the general population. More research must be done to further delineate the potential effects of spaceflight length and time after spaceflight on fracture risk.

While the fractures in this study occurred after returning to Earth, it is worth considering the implications of fractures occurring while in space. Such an injury could have significant consequences for an astronaut's health and ability to perform their mission. While the low-gravity environment of space provides a protective factor against fractures, there remains a risk—especially for the wrist, due to increased use of upper extremities for movement around spacecraft. ⁷ Animal studies have also shown that microgravity could slow or prevent bone healing. ¹⁰ In-flight training systems to help astronauts address certain fractures have shown promise; however, more research is needed to better understand and prepare for fracture management in space. ²⁵ Rigorous resistive exercise programs have had some success in mitigating bone loss in spaceflight. ¹⁰ The addition of pharmacologic interventions such as bisphosphonates has been shown to further mitigate negative effects on BMD; however, neither of these have been able to eliminate the substantial effects of microgravity on the skeletal system.^10,26

The present study is not without its limitations. The retrospective nature of the study creates bias, which we attempted to minimize. Furthermore, to maintain appropriate astronaut confidentiality, all information related to astronaut fractures was bucketed into broad categories, including astronaut age, time in spaceflight, and time to injury. Information related to the mechanism of injury, specific anatomic location, need for operative intervention, etc., was not provided. The small sample size also limits the ability to extrapolate conclusions from these findings. Awareness and use of this information would help appreciate the true implications of the findings in the present study. Additionally, astronaut demographic information, such as gender and race, was not provided, which may influence results. Lastly, astronaut fracture was only recorded within 5 years of spaceflight. Analysis of data greater than 5 years out would provide additional insight into the long-term implications of spaceflight on fracture risk.

Conclusion

Fractures upon return to a gravitational environment pose a serious risk to astronauts, potentially significantly jeopardizing astronaut health and mission success. Fractures of the hip and spine, specifically, are associated with decreased BMD. While the incidence of these fractures is roughly 1%, the substantial negative implications, including high 1-month and 1-year mortality rates and functional implications, emphasize the need to optimize bone health and fracture treatment protocols.