BACKGROUND: Military aircrew frequently report cervico-thoracic pain and injury. The relationship between risk factors and future pain episodes is, however, uncertain. The aim of this study was to identify risk factors for cervico-thoracic pain and to determine the 1-yr cumulative incidence of such pain.

METHODS: A total of 47 Swedish aircrew (fighter and helicopter pilots and rear crew) without pain in the cervico-thoracic region were surveyed about work-related and personal factors and pain prevalence using the Musculoskeletal Screening Protocol questionnaire. They also performed tests of movement control, active cervical range of motion, and isometric neck muscle strength and endurance. Aircrew were followed for a year with questionnaires. Logistic regressions were used to identify potential risk factors for future cervico-thoracic pain.

RESULTS: Previous cervico-thoracic pain (OR: 22.39, CI: 1.79–280.63), lower cervical flexion range of motion (OR: 0.78, CI: 0.64–0.96), and lower neck flexor muscular endurance (OR: 0.91, CI: 0.83–0.99) were identified as risk factors for reporting cervico-thoracic pain. At follow-up, 23.4% (CI: 13.6–37.2) had reported cervico-thoracic pain during the 12-mo follow-up period.

DISCUSSION: The Musculoskeletal Screening Protocol can identify risk factors for cervico-thoracic pain. The link between cervico-thoracic pain and previous pain, as well as lower performance of neck range of motion and muscular endurance, highlights the need for primary and secondary preventive action. The findings from this study can facilitate the development of such pain prevention programs for aircrew.

Tegern M, Aasa U, Larsson H. A prospective cohort study on risk factors for cervico-thoracic pain in military aircrew. Aerosp Med Hum Perform. 2023; 94(7):500–507.

BACKGROUND: The purpose of this study was to evaluate the bond strength of fiber posts with different mixing methods and insertion into root canal techniques in long-term hypobaric pressure changes.

METHODS: We chose and decoronated 42 teeth with straight and single root canals. After post-space preparation, the posts were cemented with hand-mixed and auto-mixed resin cements that were inserted into the canals with an endodontic file (lentilo), dual-barrel syringe, and root canal tip (14 for each group). After cementation, each group was divided into two subgroups (N = 7): the control (ambient pressure) group and the hypobaric pressure group. The samples were exposed to hypobaric pressure 90 times. They were cut into 2-mm-thick segments and the push-out bond strength test was performed using a Universal Testing Machine. One-way ANOVA tests, Bonferroni tests, and Student-t-tests were used for statistical analysis.

RESULTS: The environmental pressure changes and insertion techniques affected the bond strength values. The auto-mixed with root-canal tip group showed the highest push-out bond strength values in both hypobaric (dual-barrel syringe group: 10.01 MPa; root-canal tip group: 11.61 MPa) and control (dual-barrel syringe group: 12.29 MPa; root-canal tip group: 14.58 MPa) group. In all root segments, the bond strength values of hypobaric groups were lower than atmospheric pressure groups. The most frequent failure type was adhesive between dentin and cement in all groups.

DISCUSSION: Dentists should use auto-mixed self-adhesive resin with a root canal tip for post cementations in patients likely to be exposed to hypobaric pressure changes.

Ozkan Ata S, Ata N, Yazgan E, Akkor T. Ugurlutan R. Push-out bond strength of fiber posts in long-term hypobaric pressure changes. Aerosp Med Hum Perform. 2023; 94(7):508–513.

INTRODUCTION: The 1% rule has long been a standard threshold for aerospace medical risk acceptance, but medical literature has noted multiple shortcomings with this threshold. Previous studies have suggested a risk matrix approach in aeromedical decision-making. General use of risk matrices for risk assessment is already codified in the U.S. Air Force (USAF). Based on this, the USAF School of Aerospace Medicine (USAFSAM) Aeromedical Consultation Service (ACS) generated and evaluated the ACS Medical Risk Assessment and Airworthiness Matrix (AMRAAM).

METHODS: The ACS adapted existing USAF standards to build the AMRAAM, gathered expert feedback, and sampled 100 previously adjudicated cases to compare legacy case dispositions to AMRAAM dispositions using polychoric correlation.

RESULTS: The AMRAAM disposition showed strong agreement with legacy dispositions (ρ* = 0.9424). One case was discarded as it did not meet inclusion criteria. Of the 99 remaining cases, 88 had perfect agreement between legacy and AMRAAM dispositions. With the AMRAAM, eight cases were less restrictive and three were more restrictive (two due to an erroneous omission in the legacy disposition).

DISCUSSION: The AMRAAM produces disposition recommendations that are highly consistent with the legacy approach informed by the 1% rule, with discordant AMRAAM dispositions tending to be more permissive. The USAFSAM AMRAAM allows a more dimensional risk evaluation than the 1% rule, communicates aeromedical risk consistent with nonmedical USAF organizations, and harmonizes aeromedical risk with the level of risk the USAF has defined for all flying systems. The ACS will use the AMRAAM as standard practice in future aeromedical risk assessments.

Mayes RS, Keirns CJ, Hicks AG, Menner LD, Lee MS, Wagner JH, Baltzer RL. USAFSAM Aeromedical Consultation Service Medical Risk Assessment and Airworthiness Matrix. Aerosp Med Hum Perform. 2023; 94(7):514–522.

INTRODUCTION: Astronauts exercise to reduce microgravity-induced bone loss, but the resultant skeletal loading may not be sufficient to reduce fracture risk on an extended Mars mission. Adding additional exercise increases the risk of a negative caloric balance. Neuromuscular electrical stimulation (NMES) induces involuntary muscle contractions, which load the skeleton. The metabolic cost of NMES is not fully understood. On Earth, walking is a common source of skeletal loading. If the metabolic cost of NMES were equal to or less than walking, it could offer a low metabolic cost option for increasing skeletal loading.

METHODS:We measured the oxygen consumed and carbon dioxide produced from 10 subjects during 5-min bouts of walking at 2 mph, 3 mph, and 2 mph on a 6° incline, and of NMES to the legs at duty cycles of 1 s on and 5 s, 4 s, or 3 s off. Metabolic cost was calculated using the Brockway equation and the percent increase above resting for each NMES bout was compared to walking.

RESULTS: Metabolic cost increased 64.9 ± 52.8% from rest in the most intense NMES duty cycle (1 s/3 s) and 120.4 ± 26.5%, 189.3 ± 59.5%, 281.7 ± 66.8%, for the 2 mph, 3 mph, and incline walking, respectively. The metabolic cost did not differ significantly between the three NMES duty cycles.

DISCUSSION: The increase in metabolic cost of the fastest NMES bout was less than that of the slowest walk, indicating that numerous NMES bouts offer a way to increase skeletal loading at a modest metabolic cost. This might allow for more daily skeletal loading cycles, potentially further reducing bone loss.

Abitante TJ, Alemi MM, Newman DJ, Duda KR. Metabolic cost of a proposed NMES spaceflight countermeasure compared to walking in active adults. Aerosp Med Hum Perform. 2023; 94(7):523–531.

INTRODUCTION: Inhalation of hydrazine or hydrazine-derivative (for example, monomethylhydrazine) vapors during spaceflight operations remains a risk to crew and ground support personnel. Here we sought to provide an evidence-based approach to inform acute clinical treatment guidelines for inhalational exposures during a noncatastrophic contingency spaceflight recovery scenario.

METHODS: A review of published literature was conducted concerning hydrazine/hydrazine-derivative exposure and clinical sequelae. Priority was given to studies that described inhalation though studies of alternative routes of exposure were additionally reviewed. Where possible, human clinical presentations were prioritized over animal studies.

RESULTS: Rare human case reports of inhalational exposure and multiple animal studies provide evidence of varied clinical sequelae, including mucosal irritation, respiratory concerns, neurotoxicity, hepatotoxicity, hemotoxicity (including Heinz body development and methemoglobinemia), and longitudinal risks. In an acute timeframe (minutes to hours), clinical sequelae are likely to be limited to mucosal and respiratory risk; neurological, hepatotoxic, and hemotoxic sequelae are unlikely without recurrent, longitudinal, or noninhalational exposure.

CONCLUSIONS: Acute clinical management should focus on likely clinical concerns as supported by existing data; recovery medical personnel should be prepared to manage mucosal irritation and respiratory concerns, including the potential need for advanced airway management. There is little evidence supporting the need for acute interventions for neurotoxicity and there is no evidence that acute hemotoxic sequelae would drive the need for on-scene management of methemoglobinemia, Heinz body development, or hemolytic anemia. Training that overemphasizes neurotoxic or hemotoxic sequelae or specific treatments for such conditions potentially raises the risk for inappropriate treatment or operational fixation.

Hanshaw BC, Ryder VE, Johansen BD, Pattarini JM, Nguyen HN, Nowadly CD, Blue RS. Spaceflight recovery considerations for acute inhalational exposure to hydrazines. Aerosp Med Hum Perform. 2023; 94(7):532–543.

BACKGROUND: The previous short-term (1-h and 24-h) Spacecraft Maximal Allowable Concentrations (SMACs) for benzene were established at 10 and 3 ppm by NASA in 1996, based on a study of mice in which no hematological effects were noted following two 6-h exposures to benzene. When the benzene SMACs were updated in 2008, there was no revision to the short-term SMAC limits. Rather, that effort developed a long-term SMAC (1000-d) for Exploration mission scenarios.

Acute benzene exposures can cause numerous neurological effects, and long-term exposure to low levels is well-known to cause acute myeloid leukemia. Since publication of the original benzene SMACs, the National Academy of Sciences developed interim Acute Exposure Guideline Limits (AEGLs) for unintentional releases of benzene into the air. Based on the data used to establish the AEGLs, we have increased our short-term, off-nominal limits for benzene in crewed spacecraft to 40 ppm and 6.7 ppm for 1-h and 24-h, respectively.

Ryder VE, Williams ES. Revisions to acute/off-nominal limits for benzene in spacecraft air. Aerosp Med Hum Perform. 2023; 94(7):544–545.

BACKGROUND: Exertional heatstroke affects athletes and soldiers and can lead to temporary heat intolerance. The heat tolerance test (HTT) was developed to assist in the return-to-duty decisions of military personnel. Although there are several possible causes for heat intolerance, a soldier who fails the test is prevented from returning to serve in a front-line combat unit, regardless of the underlying reason.

CASE REPORT: A 19-yr-old special infantry male soldier with unremarkable medical history collapsed during an afternoon hand-to-hand combat training session in the gym. The medic on site promptly initiated inefficient tap water cooling and measured a rectal temperature of 38.7 °C; he returned to duty the same evening. A few weeks later, after intensive physical training, he experienced exhaustion during a stretcher-carrying foot march. He was referred by the unit’s physician, who suspected a condition of heat intolerance, to an HTT. The soldier underwent two HTTs, which were found to be positive. Consequently, he was discharged from serving in his infantry unit. No congenital or functional underlying causes could explain the diagnosis of heat intolerance.

DISCUSSION: We discuss the possibility that the only explaining cause for the two positive HTTs was the soldier’s anthropometric measures, particularly the low body surface area to body weight ratio. We raise the question of whether this soldier could have been returned safely to duty.

Rabotin A, Epstein Y, Ketko I. Heat intolerance, body size, and return to duty. Aerosp Med Hum Perform. 2023; 94(7):546–549.

INTRODUCTION: Medical conditions occurring in spaceflight pose risks to the crew and the mission and these risks will be exacerbated during exploration-class missions. Probabilistic risk assessment is a method used at NASA to quantify this risk for low-Earth orbit operations. Informing Mission Planning via Analysis of Complex Tradespaces (IMPACT) is a next-generation tool suite that will perform these assessments for exploration-class missions. It will require a robust list of medical conditions of significant likelihood and/or consequence to exploration-class missions to accurately inform the tool suite.

METHODS: The IMPACT 1.0 Medical Condition List (ICL 1.0) contains 120 conditions selected in the context of a 210-d cis-lunar, Mars analog design reference mission. The conditions were selected via a systematic process that preserved institutional knowledge from nine prior condition lists. Conditions were prioritized for inclusion in the ICL 1.0 based on history of occurrence in spaceflight, concurrence among the nine source lists, and concurrence among subject matter experts.

DISCUSSION: The ICL 1.0 has notable advantages over its predecessor lists in that it is more specific to exploration-class missions, contains a greater number, breadth, and depth of conditions, and was derived via consensus across multiple medical specialties.

Kreykes AJ, Suresh R, Levin D, Hilmers DC. Selecting medical conditions relevant to exploration spaceflight to create the IMPACT 1.0 Medical Condition List. Aerosp Med Hum Perform. 2023; 94(7):550–557.