Association Between Cumulative G-force Exposure and Cervical Spine Degenerative Changes

ABSTRACT

Introduction

Fighter pilots work in a unique environment and are subject to high G-force loads under simultaneous head movements. Neck pain is reported to be a common health problem among fighter pilots leading to time lost flying and flight duty limitations. The present study aimed to find out if differences in early military flight career G-force exposure levels could increase the risk for degenerative changes in the cervical spine.

Materials and Methods

The study population consisted of 56 20-year-old Finnish Air Force male fighter pilot cadets, who underwent MRI of the cervical spine at baseline and after 5 years. During follow-up, the G-force exposure was measured using the individual Fatigue Index (FI) recordings. The FI data were collected from each flight with BAE Hawks and is determined by the number of times certain levels of G-forces are exceeded during the flights. The incidence or progression of each degenerative change was compared to individual FI values using Pearson correlation coefficients.

Results

The pilots flew on average 220 (±21) hours with high performance aircraft during follow-up, resulting in an average FI of 1.98 (±0.47). A statistically significant progression was found in intervertebral disc (IVD) degeneration sum score with no correlation to corresponding FI values. A statistically significant increase was also found in the prevalence of IVD herniations with a negative correlation with FI values. Other degenerative cervical spine changes did not progress or did not correlate with corresponding FI values.

Conclusions

The prevalence of IVD degeneration and IVD herniations increases in the early phases of fighter pilots’ career. Only the incidence of IVD herniations correlated with FI values and the correlation was negative. The negative correlation may be attributed to avoidance behavior due to neck pain, which was not measured in our study, or other unmeasured confounding factors. This was the first study to compare individual G-force exposure levels to high-quality MRI data over a follow-up of several years. Finnish fighter pilots are known to report increasing cervical symptoms during the Hawk training phase, but longer follow-up periods are likely needed to determine the association between G-force exposure levels and cervical degenerative changes.

INTRODUCTION

A unique working environment requires fighter pilots to move their heads with full range of motion in three directions to observe the airspace around them.¹ The combination of the head movements and simultaneous G-force load while wearing helmets is considered harmful for the structures of the cervical spine.² Neck pain alone is reported to be a significant health problem among fighter pilots affecting 51% of pilots, of whom 39% reported time lost from flying.^3,4 Another recent meta-analysis on fighter pilots concluded that cervical spine is more prone to pain than lumbar spine.⁵

Wearing a helmet with additional modern equipment, such as joint helmet mount cueing system (JHMCS) or night vision goggles (NVG), increases the mass and moves the center of mass forward altering the biomechanics of the neck under high G-force. The effect of JHMCS on cervical net joint moments was studied by using the Musculoskeletal Model for the Analysis of Spinal Injuries with video motion data of Australian fighter pilots. The heavier JHMCS helmet increased the net joint moment at each cervical segment with greatest increase at C1.⁶ Use of JHMCS was found to increase the intensity of post flight neck pain among U.S. Air Force fighter pilots.⁷ The adaptation of JHMCS and NVG’s did increase one-year prevalence of neck pain and were reported as a cause for flight-related neck pain by 88% of surveyed Royal Netherlands Air Force fighter pilots.⁸

A recent study investigated in-flight muscle activity during combat maneuvering.¹ It was found that high G-force combined with awkward head postures (e.g., rotations in combination with extensions) cause a force exceeding the load-bearing capacity of the cervical muscles. This is believed to result in more loading to the IVDs, ligaments and bony structures of the cervical spine. Although there is only low-grade evidence of using physical training to reduce neck and shoulder pain among fighter pilots, a recent meta-analysis implied that physical exercise could have a protective effect on neck pain among fighter pilots when studies with high heterogeneity were excluded.⁹

Cervical spine disorders may lead to temporary or permanent flight duty limitations.^10,11 Disability accompanied by spinal intervertebral disc (IVD) degeneration is reported as the most common reason for aeromedical limitations among Finnish Air Force (FINAF) fighter pilots.³ Fighter pilots’ occupational health and its preservation are important for several reasons. In addition to individual well-being and health benefits, it is fundamentally important to ensure operational readiness of the fighter pilots. Fighter pilot training is also highly expensive, which limits the number of pilot intakes. Therefore, it is important to monitor and research pilots’ occupational health factors.

Fighter pilots’ neck pain is a problem also recognized by NATO. Evidence-based recommendations to minimize flying-related neck pain and optimizing operational performance were gathered in NATO’s Science and Technology Organization report in 2020.¹² Special flying-related physical training, physical monitoring, easy access physical therapy and optimized recovery were promoted. The report also advised to avoid or minimize to the greatest extent possible postures and movements that are most associated with neck pain and injury. Helmets and helmet-mounted equipment should be designed to be compatible with other equipment and cockpit structures to minimize forces generated on neck. Helmet support systems, such as foam wedges and counterweights have been tested but are not being used by FINAF fighter pilots.

The G-force exposure levels may vary among the pilots even during similar missions and flights due to individual human factors. However, there are no available reports on this. Therefore, studying the individual differences in G-force exposure levels between pilots is an object of significant interest. The aim of this novel study was to determine the association between individual cumulative G-force exposure and degenerative cervical spine changes among FINAF fighter pilots over a 5-year follow-up. We hypothesized that this new research layout could produce valuable information on the effects of G-force exposure on fighter pilot training and occupational health.

METHODS

Subjects

The study population consisted of 56 volunteer FINAF male fighter pilot cadets from a total of seven classes. All male fighter pilots from said classes that volunteered were accepted to the study. At baseline, the pilots were 20 years old (±0.7), with an average height of 179.9 cm (±4.7) and an average weight of 75.7 kg (±6.3). All the FINAF fighter pilot cadets have gone through a multistage selection program before the beginning of the training. None of the applicants were rejected due to MRI findings. All the pilots completed the 5-year follow-up.

After completing Phase 1 and 2 of military flight training, the pilots operated the BAe Hawk jets (up to +8 G) during the follow-up period. Fatigue Index (FI) data is accessible from the consecutive military flight training Phase 3. Before the data collection commenced, the procedures were explained to all the participants, who gave their voluntary written informed consent under the Declaration of Helsinki. The study protocol was approved by the Ethical Committee of the Central Finland Health Care District.

MRI Examination

The MRI examinations were performed at the baseline and after a 5-year follow-up, between 2008 and 2020. The imaging was conducted in supine position and the protocols included conventional T1-weighted and T2-weighted fast spin echo sequences. The protocols were presented in depth in a previous study.¹³ An example of T2-weighted mid-sagittal MRI at baseline and at follow-up of one fighter pilot is presented in Figure 1.

Figure 1.

T2-weighted MRIs in mid-sagittal plane of a study subject. On the left is the baseline image and on the right is the 5-year follow-up scan. The signal intensity loss of cervical intervertebral discs can be seen in most cervical intervertebral discs and especially at C6/C7 (arrow) (Pfirrman grade II to IV). Also, a type 2 Modic-change is seen at C6/C7.

The extent of IVD degeneration was evaluated using the Pfirrmann classification, which employs a five-grade system assessing disc space height, disc homogeneity, nucleus intensity, and the distinction between the nucleus and annulus.¹⁴ An IVD degeneration sum score was derived by summing individual scores at each cervical level. Normal discs (grades 1 and 2) were assigned a score of 0, with each subsequent degree of degeneration incrementing the score by one. Consequently, the cumulative score could theoretically range from 0 to 18 for the six cervical discs.¹⁵ IVD herniations were originally categorized as protrusions, extrusions, and sequesters, but were later combined due to the low frequency of extrusions and sequesters.¹⁶ Evaluation of Schmorl’s nodes was conducted across each vertebral body throughout the cervical spine.¹⁷ Osteophytes were appraised on both sides of vertebral bodies and classified as uncovertebral arthrosis, regardless of unilateral or bilateral occurrence.¹⁸ Scoliosis was defined as a scoliotic angle of at least 10°, but spinal rotation was not measured as the medical diagnosis of scoliosis was not the focus.¹⁹ The kyphotic angle, determined using the Harrison posterior tangent method, was recorded if it exceeded 1° in the cervical spine.²⁰ Initial recording of spinal canal stenosis was done at each cervical level, but it was later simplified to encompass the entire cervical spine.²¹ Modic changes (types 1, 2, and 3) were assessed using both T1-weighted and T2-weighted images, following established protocols.²²

The MRI scans were read by two persons including a musculoskeletal radiologist. Both examiners were blinded to the FI values and flight hours of each pilot, and to each other’s findings. The intrarater and inter-rater reliabilities were assessed by means of kappa and weighted kappa statistics in a previous study.¹³ Intra- and inter-rater reliabilities were mostly substantial or perfect.

G-force Exposure

In addition to flight hours, we analyzed individual cumulative G-force exposure with Fatigue Index (FI). FI is a technical measure representing the amount of G-force stress experienced by the plane and the pilot. Flight hours and FI data collected from every flight was obtained from FINAF flight data recordings. FI was originally developed to track aircraft structural fatigue, but recently has been used also for medical research purposes.^3,23 As presented by Honkanen,³ FI is determined by the number of times the vertical G-force levels of +0.25, +2.5, +3.5, +4.5, +5.5, +7.0, and +8.0 G are exceeded during the flights or, respectively, levels lower than −0.5 and −1.5 G are reached. The FI values from each flight were then given a figure representing the amount of G-force exposure during that particular flight. This is calculated using a mathematical formula, which is presented in depth by Honkanen.³

Statistical Analysis

Data from the MRI examinations and FI measurements were analyzed using SPSS Statistics V.25 software for Windows. Descriptive statistics were presented as means and SDs for continuous variables with normal distributions, medians with IQRs for continuous variables with skewed distributions or percentages and frequencies (n) for categorical variables (exceptions are mentioned in table descriptions). Normality was evaluated using Kolmogorov-Smirnov test with Lilliefors significance correction to provide an objective estimate. Statistical significance of incidence for each degenerative change was defined using McNemar test for class variables and paired-sample t-test for continuous variables with normal distributions. The statistical significance of correlation between the FI and incidence of degenerative changes was defined using Pearson correlation coefficients. P-values < .05 were considered statistically significant.²⁴

RESULTS

G-force Exposure

The pilots had an average of 220 (±21) flight hours with the BAe Hawk jet during the follow-up. Mean FI among all pilots was 1.98 (±0.47) (Figure 2).

Figure 2.

Mean cumulative Fatigue Index of each pilot year class during the follow-up.

Degenerative Changes in the Cervical Spine

A statistically significant progression was found in IVD degeneration sum score and IVD herniations. In uncovertebral arthrosis, Schmorl’s nodes, Modic changes, spinal canal stenosis, kyphosis, and scoliosis the change between baseline and follow-up was not statistically significant. It should be noted that some MRI findings were rare among the sample (Table 1).

Table 1.

Change in the MRI Findings During Follow-up.

Finding	Baseline	Follow-up	P for change	Correlation (Pearson)between incidence and Fatigue Index (P-value)
Disc degeneration sum scorea	1.5 (1-3)	3 (2-5)	<0.001	−0.109 (0.424)
Disc herniationsb	9.7 (11)	19.5 (22)	0.035	−0.386 (0.003)
Uncovertebral arthrosis	26.8 (15)	33.9 (19)	0.454	−0.094 (0.492)
Schmorl’s node	5.4 (3)	3.6 (2)	>0.999	0.095 (0.484)
Modic change	0.0 (0)	3.6 (2)	0.500	0.086 (0.527)
Spinal canal stenosis	1.8 (1)	7.1 (4)	0.250	−0.110 (0.419)
Kyphosis	30.4 (17)	21.4 (12)	0.267	0.359 (0.007)
Scoliosis	3.6 (2)	3.6 (2)	>0.999	0.061 (0.653)

The values are percentages (average number of findings per individual 100×) with frequencies (number of findings per group) unless otherwise noted.

^aFirst value is median and in parenthesis is IQR.

^bFirst value is prevalence of IVD herniations in percentages and in parenthesis is the number of pilots with IVD herniations.

FI and Degenerative Changes

There was no correlation between individual FI and IVD degeneration sum score change. Surprisingly, there was a negative correlation of −0.39 (Pearson, P = .003) between FI and the incidence of disc herniations (Figure 3). A positive correlation of 0.36 was found between FI and the incidence of kyphosis (Pearson, P = .007). There was no correlation between FI and uncovertebral arthrosis, Schmorl’s nodes, Modic changes, spinal canal stenosis, or scoliosis (Table 1).

Figure 3.

Correlation between the Fatigue Index and incidence of IVD herniations.

DISCUSSION

According to the present findings, there was progression of IVD degeneration and an increase in the prevalence of IVD herniations over the follow-up. We did not find progression of uncovertebral arthrosis, Schmorl’s nodes, Modic changes, kyphosis, stenosis, or scoliosis during the follow-up. Surprisingly, there was a negative correlation between individual FI values and IVD herniation incidence. Even though there was no increase in the prevalence of kyphotic angles, it appeared to correlate negatively with FI values.

G-force Exposure

Although an average of 220 flight hours is relatively low compared to the total flight hours built up during a full fighter pilot career, the G-force exposure is high within the first years of the training. The military flight training phase 3 includes high levels of G-force due to emphasis on within visual range air-to-air combat training. The average FI value of each cadet class has decreased over the study period. This might be incidental or even due to intentional reduction of high G-force flight training. FI values cannot be compared internationally between different Air Forces because the FI has been used only in Finland. Furthermore, the amount of G-force exposure, even as flight hours, is not commonly reported since the information is classified.

FI and IVD Degeneration

Even though we found IVD degeneration sum score to increase significantly during the follow-up, we found no correlation between the sum score and FI values. IVD degeneration is an age-related process.²⁵ We aimed to investigate the potential effects of varying individual G-force stress levels on this process. Even though correlation with FI values was not discovered, the finding that young healthy fighter pilots had degenerated discs already prior to occupational exposure to G-force load is in line with previous literature. A systematic review on radiological spinal degeneration revealed a prevalence of 37% of intervertebral disc degeneration among 20-year-old asymptomatic individuals.²⁵ Direct head-to-head comparison is of little value because the studies included in the meta-analysis did not stratify the findings by the degree of degeneration as we did.

Occupational spine loading has also been studied in occupations other than fighter pilots. There is some evidence of a relationship between occupational spine loading and IVD degeneration. A 2019 meta-analysis included 17 studies in which the subjects were exposed to repetitive heavy lifting or driving.²⁶ The study suggests that there is moderate grade evidence of an association between occupational loading and IVD degeneration in terms of signal intensity. The evidence of association with occupational loading and other degenerative spine changes is low.

FI and IVD Herniations

We found that the prevalence of IVD herniations increased from 9.7% to 19.5% during follow-up. A meta-analysis estimated a prevalence of 29% for disc protrusions in 20-year-old asymptomatic individuals.²⁵ The meta-analysis estimated the prevalence of protrusions to increase to 31% by the age of 30 years. The prevalence of IVD herniations was lower in our study, which may be explained by the strong selection bias of healthy and fit FINAF fighter pilot population compared to average civilian volunteer population.

The statistically significant negative correlation between FI and the incidence of IVD herniations seems counterintuitive. However, this could be explained by several factors. There might be avoidance behavior to some extent leading to lower G-force exposure with the pilots with a higher incidence of IVD herniations. In-flight neck pain or previous pain sensations could lead pilots to not produce as much G-forces when flying. Also, work disability leading to individual temporal G-force restrictions may affect the results.

Although the correlation between pain symptoms and IVD herniations causing nerve compression is well established, a substantial portion of IVD herniations are asymptomatic.²⁵ While most past studies focus on the point prevalence of IVD herniations, the correlation between the incidence of IVD herniations and neck pain could be a valuable focus for future research. Furthermore, the cumulative G-force exposure data and FI used in this study does not enable the dissection of the highest G-forces, such as >7 G and >8 G. In future studies, the correlation between MRI findings such as IVD herniations and highest G-force exposure could be another target of interest.

FI and Other Degenerative Changes

A statistically significant positive correlation was found between FI and the incidence of kyphosis. However, it should be noted that there was no statistically significant change in the prevalence of kyphosis during the follow-up. We also found no correlation between FI and the incidence of uncovertebral arthrosis, Schmorl’s nodes, Modic changes, spinal canal stenosis or scoliosis.

The fact that the positioning of the pilots and their necks were not standardized is a potential confounding factor, possibly leading to excess variation when measuring kyphotic angles in the cervical spine using posterior tangent method. However, our findings regarding the prevalence of kyphotic angles are consistent with a study on the cervical spine alignment among South Korean fighter pilots.²⁷ In both studies, the measurements were conducted using standing lateral X-rays of the cervical spine. The prevalence of kyphosis was reported to be 18.3% (angles less than −4°), with a mean C2-C7 angle of 2.72°. This is comparable to our observed prevalence of kyphotic angles (less than 0°), ranging from 21.4% to 30.4% measured in supine posture.

Study Evaluation

The use of MRI as the imaging modality for this research purpose is well reasoned. MRI does not inflict ionizing radiation upon research subjects in contrast to computed tomography, X-ray or positron emission tomography. MRI is a reliable and even superior in many details compared to other imaging modalities. However, bony structures such as vertebrae are much more prominent in computed tomography.²⁸ Furthermore, the assessment of cervical spine sagittal alignment is preferably done in standing posture. This could easily be acquired by using standing lateral X-rays as an additional imaging modality but could also be done by using a modern upright MRI device. The standing posture would even enable the use of flexion and extension images to provide a more accurate interpretation of the functioning of the cervical spine.

FI provides a more comprehensive analysis on G-force exposure levels than traditional classification with only aircraft type and flight hours. However, FI is developed to measure the vertical G-force stress of aircraft components and not the effects on pilots. The FI flight recordings only measure how many times certain G-force thresholds have been exceeded, it does not take account of how long a single exposure to G-force is. In future studies with 5th generation fighter jets, the flight data recordings should include both the exact level of G-force and the time of the exposure.

The pilots flew the same planes and were trained in the same training facility reducing the number of confounding factors. The pilots wore the same equipment such as helmets and did not use any neck support systems. The use of heads-up displays or night vision goggles were not monitored in our study, as the precise data is considered classified. However, the training program leaves little to no room for differences regarding the use of optional head gear.

To authors’ knowledge, the use of FI data in combination with MRI data is new. The use of individual cumulative G-force stress data in medical research purposes is overall rather new. The sample size was moderate, and some MRI findings were rare among the sample. However, this is the first study to explore the association between individual cumulative G-force exposure and cervical spine MRI findings among fighter pilots over a 5 year follow-up.

CONCLUSIONS

According to present findings we could not show that individual in-flight behaviors during the first 5 years of FINAF fighter pilots’ career have an adverse effect on cervical spine. Only the prevalence of IVD degeneration and IVD herniations increased during the follow-up. The incidence of IVD herniations correlated with FI values but the correlation was negative. The negative correlation may be attributed to avoidance behavior due to neck pain, which was not measured in our study, or other unmeasured confounding factors.

This was the first study to compare individual G-force stress exposure data with prospectively collected high-quality cervical MRIs. Future studies should incorporate longer follow-up periods to enable a higher total G-force exposure level and greater variance between pilots. Future studies should also include analysis of neck mobility and neck pain and their effects on pilots’ performance. Preventing training injuries and avoiding chronic disease development and progression is paramount for maintaining military readiness and optimal performance.

FUNDING

No external funding.

CONFLICT OF INTEREST STATEMENT

None declared.

DATA AVAILABILITY

No data are available. All data according to Finnish Air Force fighter pilots are considered as classified, and therefore, data used in this study are not available.

INSTITUTIONAL ANIMAL CARE AND USE COMMITTEE

Not applicable.

INSTITUTIONAL CLEARANCE

Institutional clearance approved.

INDIVIDUAL AUTHOR CONTRIBUTION STATEMENT

T.K. contributed to the manuscript by writing the Introduction section, conducting the data analysis and reading the MRIs. P.O. contributed to statistical analysis and revising the text. T.H. contributed to defining the study design, managing the data, and revising the text. J.N. has taken part in defining the radiological variables, reading the MRIs, and revising the methods section. R.S. contributed by revising the text and acquiring the FI data and flight hours. T.L. has had a key role in administrating the study and revising the text. J.K. has broadly taken part in revising the text and discussing the results. All authors read and approved the final manuscript.