THE CONSEQUENCES OF MUSCULOSKELETAL DECONDITIONING AND INJURY: MARTIAN MEDICINE EN-ROUTE AND ON SITE
Department of Medical Imaging Science, Curtin University of Technology
Musculoskeletal deconditioning is a well-recognised, if not well-understood, consequence of space flight. Musculoskeletal integrity is a requirement for optimal crew performance, and in turn for successful mission completion. The risk of musculoskeletal compromise is significantly higher in space than in analogue terrestrial environments, and it is likely that adaptation to reduced (Martian) gravity will have deleterious effects that are as yet ill defined. The Mars Gravity Biosatellite project will be of great assistance in defining some of the physiological aspects of the changes. The nature of such changes is likely to be multifactorial, including alterations in bone and muscle architecture, vascularity, efficacy of the immune response, the likelihood of infection, and radiation-mediated damage, which may potentially lead to neoplasia (including benign and malignant tumourogenesis). These changes equate to an increased risk of injury, and an associated decrement of personal and crew performance. Whilst the physiological changes are well discussed in the literature, there remains a paucity of work discussing the impact of such events on a mission to Mars. On such a mission, these physiological challenges fall into two distinct phases; those associated with microgravity during the journey to and from the planet, and those associated with the reduced gravity of Mars itself. These two mission phases will be marked by differing crew activities, levels of risk, support facilities and medical infrastructure. The impact of injury will similarly have differing sequelae, depending on the mission phase during which the injury is sustained.
A crewed mission to Mars is one of the most difficult tasks humans have ever contemplated. Impediments to the successful completion of such a mission include philosophical objections (the "why?" question), high cost, possible equipment failure, psychological and psychosocial aberrations, and unknown, potentially fatal biological effects of such a space flight. Whilst societies can choose to go to Mars, and equipment can be made ever more reliable, the medico-social aspects of a mission may represent the most intractable challenge to its successful completion.
A return trip to Mars has been estimated to take a minimum period of approximately three years (Hoffman and Kaplan 2000). The 15 year Earth-Mars orbital cycle dictates that transit times between the planets will vary. However, Hoffman and Kaplan identify that within the next two decades the worst-case scenario will indicate transit times of less than 180 days. In designing the so-called Mars Reference Mission, these authors suggest that a total mission time of 879 days is achievable, with an outward bound time of 150 days, a surface stay of 619 days and a return journey of 110 days. In keeping with the assumptions of the reference mission, this paper presumes that a Mars mission will utilise spacecraft derived from current vehicles, and will therefore be without any form of centrifugal artificial gravity device.
To date, the longest spaceflight by an individual is the 438 day flight on the Mir spacestation by cosmonaut Veleri Polyakov, between January 1994 and March 1995. Mir has been host to ten missions lasting in excess of 179 days, and it is likely that the International Space Station (ISS) will challenge these records in years to come. These and other missions have yielded substantial information regarding the deleterious effects of weightlessness on the human body. Such effects typically fall into two categories: those intrinsic to the hostile influence of the space environment, and those arising from human activity in space. Among the intrinsic risks are radiation exposure, fluid shift and fluid loss from the body, compromised immunity, muscular atrophy, loss of bone mineral, cardiovascular deconditioning and a weakening of the heart. When these effects are combined with activities such as station construction, and long term habitation in the close confines of a space craft, the risk of medically significant events including bone fracture, serious blood loss, heart arrhythmias and infection is commensurately higher. Compounding these issues is the high stress environment of space travel, and the negative impacts on health and well-being which are endemic to such environments (Soares and Grossi 1999)..
Spaceflight: The Medical Perspective
Long-duration interplanetary spaceflight presents mission designers with a unique set of environmental characteristics. From the medical perspective, these may be summarised as below:
TABLE 1 - Medically relevant effects of interplanatery spaceflight. All of these effects will have implications on the viability of a manned mission to Mars, and may threaten not only the affected crewmember, but also the entire crew. (Adapted from Houtchens 1993).
As an integrated system, the human body cannot be reduced to a series of discrete, independent sub-systems. Influences on one element will have consequential effects on numerous others. In the context of the deleterious effects to musculoskeletal tissue, two influences deserve particular mention. The unloading of the body associated with weightlessness leads inevitably to altered bone and skeletal muscle metabolism. These changes are characterised by loss of calcium and phosphorous from the hydroxyapatite (bone) crystal, leading to a thinning of the weight-bearing components and a reduced ability to withstand stress. Skeletal muscle is subject to an analogue of this process, during which the contractile muscle fibres undergo atrophy, and are progressively replaced with fatty infiltrate. This reduces their contractile efficiency and vascularity, and therefore reduces their capacity to function correctly and repair subsequent damage. Whilst these effects are limited to the musculoskeletal system, radiation affects all organ systems, and therefore compounds the deleterious effects specific to a particular body system. More importantly, it is unlikely that the effects of radiation will ever be completely overcome.
The ISS orbits approximately 400 km above the Earth, and is well contained within the radiation shielding offered by the terrestrial magnetosphere. Interplanetary travel of necessity requires humans to be exposed to the environment outside this protection, a feat only accomplished to date by the Apollo missions. The interplanetary medium is filled with charged particles and radiation from the sun, as well as galactic cosmic radiation. This intense and ubiquitous radiation creates an extremely hostile environment in which exposures can readily exceed lethal doses.
Exposure to this environment for the length of time required to reach Mars is beyond our present experience. It is not yet certain that radiation or its effects can be adequately controlled. Robbins and Yang (1994), quoting Petrov (Petrov, Kovalev et al. 1981) have suggested that radiation may represent "..the primary hazard associated with orbital and interplanetary space flight" . Despite advances being made in spacecraft shielding design, the development of chemical radioprotectants and other radiation countermeasures, it is likely that an interplanetary mission will deliver a significant radiation dose to its crew (Robbins 1996).
Radiation exerts its biological effects in a number of ways, although all are related to the deposition of energy into living tissues. This energy has the capacity to disrupt DNA and thereby create cancers and other genetic mutations, to generate damaging chemical species such as free radicals, and to interrupt biochemical processes. Of particular importance are the effects on tissues that have high sensitivity to radiation; the eyes, thyroid, reproductive organs and blood-forming organs. However, any tissue that is undergoing growth or repair is more susceptible to the effects of radiation (a fact put to good use in the treatment of cancer using radiotherapy). Repair following injury is one such example, and radiation effects in musculoskeletal trauma include disorganised bony callus formation and sub-optimal end state repair. During a mission to Mars, these effects will compound those of weightlessness and skeletal unloading, further delaying functional return for the injured crewmember.
Musculoskeletal deconditioning and trauma en-route to Mars
A spaceflight of four to six months, as suggested by the Mars Reference Mission, will result in significant loss of bone and muscle mass. It appears that loss of bone mass is a continuous process in conditions of microgravity. During periods in low-Earth orbit (LEO), previous studies have reported bone loss in the heel bone of up to 19.8% (184 day flight), loss of lean muscle mass in the leg of 6.6%, and increase of leg fat of 15.9% (Nicogossian, Huntoon et al. 1994). As would be expected, the losses occur predominantly in the weight-bearing skeleton, and in the anti-gravity muscles of the legs and back, yet all musculoskeletal tissue appears to be affected. These losses occur even when the individuals followed exercise countermeasure regimes designed to reduce these deleterious effects. These regimes are typically 2-4 hours per day (Martin 1995), and therefore represent a significant proportion of the crewmembers' waking hours. Although the subject of continuous development, it is unlikely that the regimes typical of a mission to Mars will be any less rigorous.
Musculoskeletal injury is one of a wide range of possible medical emergencies that might occur on a mission to Mars. However it deserves particular mention for a number of reasons. Musculoskeletal deconditioning pre-disposes an individual to sustaining injury. Injuries are often sudden, immediately debilitating, require a relatively long recovery period during which a crewmember may be completely or partially incapable of performing their normal duties. Recovery may be complicated by wound infection, caused by the high particle-count, recycled spacecraft atmosphere. Fluid loss (particularly blood loss) reduces the crewmember's capacity to withstand haemorrhage, and therefore the risk of hypovolaemic shock is increased.
Given that the weight-bearing bones are those most susceptible to the deleterious effects of microgravity, these bones may also be most at risk of sustaining serious injury. This implies that the bones of the lower limb, pelvis and spine are at increased risk when compared with bones of the upper limb, thoracic cage, face and skull. Traumatic fractures of the leg or pelvis are often associated with vascular injuries, due to the proximity of large vessels to sharp-edged bone fragments (Figure 1).
FIGURE 1 - Two examples of trauma to weight-bearing bones. Multiple fractures of the femur (thigh bone, left), produce sharp-edged fragments (arrows) which are close to the major arteries supplying the leg (arrowheads). A similar situation exists in the pelvis, where fractures of the pelvic ring (arrows, right) can lead to severance of the major vessels supplying the lower limbs (arrowhead). These injuries are typical of deceleration impacts typical of motor vehicle accidents, and are also encountered as a result of crush injuries.
Traumatic fractures of the lumbar and thoracic spines can lead to crushing of the vertebrae, or shearing fractures which, depending on location, may cause spinal cord injury or severance.Figure 2 illustrates a terrestrially-sustained example of each type.
FIGURE 2- Typical spinal fractures sustained by weight-bearing vertebrae. Note the crush injury sustained by the twelfth thoracic vertebra (arrow, left), typical of longitudinal loading of the spine exceeding the crush strength of the spine. The mis-alignment of the spinal column following a shearing injury sustained in the mid-chest region is typical of a blow to the upper or lower body (arrow, right). Under these circumstances spinal cord damage, including impingement or complete severance, may occur as a result of the spinal canal no longer being axisymmetric either side of the injury.
Whilst these injuries have not yet been sustained in the extraterrestrial environment, it is recognised that crush injury or other blunt trauma are probably the most likely cause of future musculoskeletal injuries in space (McCuaig and Houtchens 1992). . The fractures shown above are typical of those sustained following blunt trauma, as may occur under such conditions. Musculoskeletal trauma, depending on which body part is injured, can lead to rapid blood loss, blood clots in the lungs or brain following leakage of bone marrow fat (fat embolus), and, if not properly treated, can cause permanent disability or death. Terrestrially, skeletal trauma requires approximately six weeks, and often significantly longer, for complete functional return. The deconditioning process which occurs in space probably implies an increased recovery period, although currently recovery time lines are unknown (Hart and Campbell 2002). Because the transit journey is of fixed duration, an injury sustained shortly after launch, with relatively little loss of bone mass and with a long time to heal before landfall, may have a significantly different clinical course than an injury sustained later in the flight, with a greater pre-existing level of compromise and with arrival at Mars likely to occur before healing has completed. The interval between the onset of microgravity and the time of injury is likely to significant in determining the subsequent prognosis, and is the subject of ongoing research.
The six-month trip to Mars will be characterised by continuous microgravity, arduous countermeasures to musculoskeletal deconditioning, increasing isolation, long data transmission times to Earth (of up to 40 minutes), and potentially significant interpersonal conflicts. In a 2001 study, Sandal describes a Mir station simulation of 135 days with three astronauts (Sandal 2001). This study corroborates previous findings that crew tension is likely to be a significant impediment to efficient completion of a mission to Mars. In the medical context this has significant implications.
Musculoskeletal trauma, such as the fractured leg or pelvis illustrated above, requires prompt medical intervention. Terrestrially, trauma care is provided by a highly trained, interdependent team of health care professionals. Typically led by an emergency department specialist, a trauma team consisting of medical doctors, nurses, radiographers and other health professionals will simultaneously coordinate a number of investigations and activities over a period of up to an hour. This requires an integrated approach, readily available diagnostic and interventional equipment, and an eye for the clock. The trauma response team on a mission to Mars may consist of a dysfunctional crew, operating with minimal medical equipment and with expert advice up to 40 minutes away. They will be operating in a physical, technological and social environment vastly different from their terrestrial counterparts; compounded by the likelihood that even if their efforts are successful, the effects on the future of the mission will be profound. This clearly presents a very challenging scenario in which to provide effective medical care, and demands that contingency planning and trauma management protocols must be firmly established and clinically validated prior to any Mars mission.
Martian medicine on site
The Mars Reference Mission (Hoffman and Kaplan 2000) suggests that several launches be used to deliver equipment to the surface of Mars prior to the arrival of the first crew. This equipment, including habitation modules, power generators and a fuelled Earth crew return vehicle, will be checked out before a crew launch from Earth. Assuming, therefore, that the crew arrives safely without incident in orbit around Mars, they will be faced with the prospect of enhanced facilities, including provision for the delivery of a higher standard of medical care. Following exposure to microgravity for six months, they are now also faced with the return of gravity, albeit the reduced gravity of Mars. Martian gravity is approximately 3/8 of Earth's gravity, and therefore making landfall will present a lesser challenge than that faced by astronauts spending extended periods in LEO. This gravitational influence will redistribute body fluids, including blood, to the legs. This increases the chance of post-flight orthostatic intolerance, as occurs in many cases where astronauts return to Earth. These conditions lead to an enhanced risk of fracture, either through reloading of the skeleton, or as a result of falls.
The risk of trauma is also increased during the crew's sojourn on the Martian surface due to the activities that will be undertaken. The nature of these activities has yet to be determined, but will include completion of the base camp and re-acclimatisation to gravity, followed by scientific investigations on and off site. Physical activities such as walking, construction and sample collection carry risk. This phase of the mission is therefore characterised by increased occupational hazards when compared to the transit phase. It is to be hoped, however, that the more familiar gravitation environment, increased space, and availability of medical infrastructure will enable a higher level of care to be delivered under these conditions. With more time available for recovery and reduced radiation intensities, the Martian surface may represent a less hazardous environment than that encountered on the outward and return journeys. However, physiologic responses to Martian gravity are not yet well understood. It is not clear if gravity at the Martian surface will prevent or reverse the musculoskeletal effects of microgravity. The responses of healing processes are likewise unknown. To assist in defining at least some of these answers, a project termed the Translife Mars Gravity Biosatellite will be launched in 2005. Using a centrifuge to create the equivalent of Martian gravity, a generation of mice will be born to pregnant animals in this unique environment. Subsequent analysis of bone and skeletal muscle growth and integrity will allow prospective estimations to be made regarding the likely response to injury sustained on the Martian surface.
The first crewed mission to Mars will represent a triumph of human endeavour. The medical challenges faced by mission designers, mission controllers and the crew themselves are unprecedented in the history of medicine. With sufficient preparation, use of Mars analogue experiences and ongoing technological advancement, it is likely that these obstacles will be successfully overcome.
Hart, R. and M. R. Campbell (2002). “Digital radiography in space.” Aviation, Space and Environmental Medicine 73(6): 601-6.