Intervertebral disc damage and spaceflight

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Astronauts have expressed an increased incidence of back pain during spaceflight and herniated intervertebral discs (IVD) have been diagnosed upon return of Skylab and Shuttle spaceflight participants.

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These conditions and symptoms may be from previous back injury, but the evidence of IVD injuries raises the concern[ by whom? ] that astronauts are at an increased risk of intervertebral disc damage during loading scenarios experienced during exploration missions (re-entry to a gravitational field and activities on planetary surfaces). To date, flight data related to potential back injuries have focused on spine elongation and the well-established effects of mechanical unloading on intervertebral discs.

Causes and current studies

Sixty-eight percent of early astronauts, through 1991, who have flown in space have reported generalized back pain. [1] The pain is considered most painful during early flight and lessens as the flight progresses.

Possible causes of back pain in flight may be associated with:

Regardless of the cause, astronauts may be at an increased risk of intervertebral disc injury or damage when the swollen discs are subjected to excessive forces or torques while working on planetary surfaces. Exploration missions on planetary surfaces may also introduce habitability issues that could induce excessive torsional stress, an established risk factor for herniation of anulus fibrosus. [9]

Currently, there is minimal in- and post-flight data that would characterize the changes in intervertebral discs in crewmembers to assess how these changes would predispose the discs to injury under re-loading. Herniated nucleus pulposus is known to occur in aviators exposed to high G-force environments [10] and has occurred in astronauts after a mission.

The relative risk rate of intervertebral disc damage has only recently been researched, [11] but there is currently no evidence that links the origin of intervertebral disc damage with changes to the disc as a result of spaceflight.

Based upon the intervertebral disc tissue analysis of unweighted animals, biochemical changes to the nucleus pulposus during spaceflight will affect the ability of the osmotic pressure and elasticity of the nucleus pulposus to resist compressive loading. [12] [13] [14] Biochemical changes in the intervertebral discs of crewmembers after flight have not been identified, but there is in vitro research with bovine cartilage explants to use magnetic resonance technology to correlate changes in intervertebral disc proteoglycan content with the T1 rho relaxation of protons. [15] This biomarker will enable non-invasive monitoring of proteoglycan content as a method of assessing the biochemical impact of weightlessness.

Computer based simulation information

Studies applying Finite Element Modeling (FEM) to IVDs under the lower osmotic pressure of the space environment shows that the appearance of a crack in the IVD experiencing lower osmotic pressure will increase the IVD risk for injury. [16] FEM was also used to demonstrate that static loading alone will not promote fluid extrusion from IVDs swollen during bed rest or weightlessness. Fluid expulsion will increase with the increased frequency of loading. [17]

Future work in this simulation capability needs to be pursued.

Risk in context of exploration operational scenarios

Defining the cause of back pain and IVD injury due to spaceflight remains an open issue. Assumptions and presumptions considered include:

Spaceflight evidence

During the 84-day Skylab 4 mission, the spine elongation of one astronaut was measured and recorded to the 1/16th inch (Thornton, 1987). This study described an asymptomatic increase in height during flight that plateaued at 29 days. The total height increase was 1.5 in (3.8 cm), measured at the end of the mission. This elongation is presumed to be due to the expansion of intervertebral discs during weightlessness (axial unloading). The astronaut also reported back pain on landing day that was associated with a herniated intervertebral disc.

Astronaut chart review

A retrospective chart review to evaluate the incidence of intervertebral disc damage after several astronauts developed cervical or lumbar herniated nucleus pulposus in the immediate postflight period. [11] This study specifically compared the incidence of intervertebral disc damage in astronauts to an age-matched control population of persons who have never flown in space. This review also ought to clarify whether there is an increased risk for intervertebral disc damage due to:

It is unclear if changes due to spaceflight increase the risk of intervertebral disc damage since there is evidence that many of the injured astronauts have previously had multiple exposures to excessive G-forces as high-performance jet pilots (6-20 G's) or to vibrational forces as helicopter pilots.

The pathophysiology of intervertebral disc injury after spaceflight has not been clearly identified. The documented expansion of disc volume after spaceflight, together with the intervertebral disc injuries after reloading in Earth's gravity, suggests that the adaptive changes of the intervertebral disc in weightlessness disrupts the balance between osmotic pressure of the nucleus pulposus and the resistive collagen structure of the anuli fibrosi, thereby reducing the ability of the intervertebral disc structure to withstand re-exposure to G forces. Repeated, previous exposures to excessive G forces in high-performance jets may also have weakened the intervertebral disc structures, particularly in the cervical vertebrae, increasing the susceptibility of these discs to damage. The relative risk of spaceflight-induced intervertebral disc injury needs to be delineated by comparing the absolute risks of the astronaut population with that of a terrestrial control cohort with similar flight history.

Ground based evidence

Intervertebral disc volume changes were quantified by magnetic resonance imaging in response to varying scenarios of axial unloading. [18] The cross-sectional areas and the transverse proton relaxation constants (T2) of IVDs were indices used to monitor adaptive changes of the intervertebral discs to overnight bed rest (over 5 weeks and 17 weeks) and after 8 days of spaceflight. The averaged expansion of IVDs with bed rest appeared to reach an equilibrium anywhere between 9 hours and 4 days of unloading with the expansion ranging between 10-40% of baseline, pre-bed rest values (mean = 22%). There were mild increases in T2 relaxation times relative to increases in disc area. Restoration of IVD volumes after unloading was not evaluated systematically but Table 1 provides a relative comparison of the elapsed time in 1 G at which time the measured IVD volumes were no different from baseline measurements; the relative periods of recovery appear to lengthen as the period of IVD adaptation to unloading increases.

Table 1. Relative comparison of the elapsed time in 1G
Period of UnloadingRelative Time Before Recovery
8 days spaceflight< 24 hours
5 weeks bedrestdays
17 weeks bedrest> 6 weeks

See also

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References

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  3. ↑ Ho, CW; Chen, LC; Hsu, HH; Chiang, SL; Li, MH; Jiang, SH; Tsai, KC (15 September 2005). "Isokinetic muscle strength of the trunk and bilateral knees in young subjects with lumbar disc herniation". Spine. 30 (18): E528-33. doi:10.1097/01.brs.0000179307.34310.7d. PMID   16166880. S2CID   30373006.
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  9. ↑ Farfan, HF; Cossette, JW; Robertson, GH; Wells, RV; Kraus, H (1970). "The effects of torsion on the lumbar intervertebral joints: the role of torsion in the production of disc degeneration". J Bone Joint Surg Am. 52 (3): 468–497. doi:10.2106/00004623-197052030-00006. PMID   5425641. Archived from the original on 2014-04-15. Retrieved 2012-10-12.
  10. ↑ Mason, KT; Harper, JP; Shannon, SG (1996). "Herniated nucleus pulposus:rates and outcomes among U.S. Army aviators". Aviat Space Environ Med. 67 (4): 338–340. PMID   8900985.
  11. 1 2 Johnston, SL; Campbell, ML; Scheuring, R (2009). Increased incidence of herniated nucleus pulposus among astronauts (Report).
  12. ↑ Pedrini-Mille, A; Maynard, JA; Durnova, GN; Kaplansky, AS; Pedrini, VA; Chung, CB; Fedler-Troester, J (1992). "Effects of microgravity on the composition of the intervertebral disk". J Appl Physiol. 73 (2 supp): 26S–32S. doi:10.1152/jappl.1992.73.2.S26. PMID   1526953.
  13. ↑ Morey-Holton, ER; Globus, RK (2002). "Hindlimb unloading rodent model: technical aspects". J Appl Physiol. 92 (4): 1367–1377. doi:10.1152/japplphysiol.00969.2001. PMID   11895999. S2CID   15041219.
  14. ↑ Hutton, WC; Yooon, ST; Elmer, WA; Li, J; Murakami, H; Minamide, AS; Akamaru, T (2002). "Effect of tail suspension (or simulated weightlessness) on the lumbar intervertebral disc: a study of proteoglycans and collagen". Spine. 27 (12): 1286–1290. doi:10.1097/00007632-200206150-00008. PMID   12065975. S2CID   24629490.
  15. ↑ Wheaton, AJ; Dodge, GR; Elliott, DM; Nicoll, SB; Reddy, R (2005). "Quantification of cartilage biomechanical and biochemical properties via T1rho magnetic resonance imaging". Magnetic Resonance in Medicine. 54 (5): 1087–1093. doi: 10.1002/mrm.20678 . PMID   16200568.
  16. ↑ Wognum, S; Huyghe, JM; Baaijens, FP (2006). "Influence of osmotic pressure changes on the opening of existing cracks in 2 intervertebral disc models". Spine. 31 (16): 1783–1788. doi:10.1097/01.brs.0000227267.42924.bb. PMID   16845351. S2CID   36871706.
  17. ↑ Cheung, JT; Zhang, M; Chow, DH (2003). "Biomechanical responses of the intervertebral joints to static and vibrational loading: a finite element study". Clin Biochem. 18 (9): 790–799. doi:10.1016/s0268-0033(03)00142-6. PMID   14527805.
  18. ↑ LeBlanc, AD; Evans, HJ; Schneider, VS; Wendt, RE; Hedrick, TD (1994). "Changes in intervertebral disc cross-sectional area with bed rest and spaceflight". Spine. 19 (7): 812–817. doi:10.1097/00007632-199404000-00015. PMID   8202800. S2CID   19464358.

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