Craniofacial Bones and Teeth in Spacefarers: Systematic Review and Meta-analysis

Abstract

Introduction: Estimating the risk of dental problems in long-duration space missions to the Moon and Mars is critical for avoiding dental emergencies in an environment that does not support proper treatment. Previous risk estimates were constructed based on the experience in short-duration space missions and isolated environments on Earth. However, previous estimates did not account for potential changes in dental structures due to space travel, even though bone loss is a known problem for long-duration spaceflights. The objective of this study was to systematically analyze the changes in hard tissues of the craniofacial complex during spaceflights.

Methods: Comprehensive search of Medline, Embase, Scopus, the NASA Technical Report Server, and other sources identified 1,585 potentially relevant studies. After screening, 32 articles that presented quantitative data for skull in humans (6/32) and for calvariae, mandible, and lower incisors in rats (20/32) and mice (6/32) were selected.

Results: Skull bone mineral density showed a significant increase in spacefaring humans. In spacefaring rodents, calvariae bone volume to tissue volume (BV/TV) demonstrated a trend toward increasing that did not reach statistical significance, while in mandibles, there was a significant decrease in BV/TV. Dentin thickness and incisor volume of rodent incisors were not significantly different between spaceflight and ground controls.

Discussion: Our study demonstrates significant knowledge gaps regarding many structures of the craniofacial complex such as the maxilla, molar, premolar, and canine teeth, as well as small sample sizes for the studies of mandible and incisors. Understanding the effects of microgravity on craniofacial structures is important for estimating risks during long-duration spaceflight and for formulating proper protocols to prevent dental emergencies.

Knowledge transfer statement: Avoiding dental emergencies in long-duration spaceflights is critical since this environment does not support proper treatment. Prior risk estimates did not account for changes in dental structures due to space travel. We reviewed and synthesized the literature for changes in craniofacial complex associated with spaceflight. The results of our study will help clinicians and scientists to better prepare to mitigate potential oral health issues in space travelers on long-duration missions.

Keywords: bone biology; bone loss; dental morphology; evidence-based medicine; microgravity; oral health.

Figure 1.

Systematic review information flow and Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) diagram. (A) PRISMA diagram. (B) Craniofacial hard tissue structures studied in spacefaring vertebrates in selected articles. (C) Number of selected papers per decade.

Figure 2.

Spaceflight-related changes in bones of the skull. (A) Forest plot of changes in calvaria and mandible bone volume/tissue volume (BV/TV) in rodents (ground control [GC]). (B) Changes in skull tissue mineral density (TMD) in humans, and mice with subgroup analysis for TMD in mouse calvarium and mandible. (C) Changes in calvarium thickness in mice. Changes were calculated as the percentage difference between spaceflight (SF) and comparison control animals (CC), which was ground control for rodents and preflight values for humans. Indicated are the species of spacefarers, missions, days spent in space, number of spaceflight to ground control animals (n_SF/n_GC), or number of astronauts. Circles and lines represent mission’s effect size (ES) (%) and 95% confidence interval (CI); the size of the circle is dependent on n_SF. Overall effect sizes and CI are presented by diamonds. I² and H² were calculated for each parameter analyzed.

Figure 3.

Spaceflight-induced changes to mandibular incisors in spacefaring rodents. (A) Forest plots for changes in incisor volume, (B) dentin thickness, and (C) cemento-enamel junction to apical crest distance (CEJ-AC). Indicated are the species of spacefarers, missions, days spent in space, and number of spaceflight animals to ground control animals (n_SF/n_GC). Circles and lines represent the mission’s effect size (ES) (%) and 95% confidence interval (CI); the size of the circle is dependent on n_SF. Overall effect sizes and CI are presented by diamonds. I² and H² were calculated for each parameter analyzed. GC, ground control; SF, spaceflight.

Figure 4.

Changes in alveolar bone resorption in spacefaring rodents. Forest plot for changes in osteoclast surface per bone surface between spaceflight (SF) and ground control (GC) (left) and GC and vivarium control (VC) (right). Indicated are the species of spacefarers, missions, days spent in space, and number of spaceflight to ground control animals (n_SF/n_GC). Circles and lines represent the mission’s effect size (ES) (%) and 95% confidence interval (CI); the size of the circle is dependent on n_SF. Overall effect sizes and CI are presented by diamonds. I² and H² were calculated for each parameter analyzed.