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OEM Online First, published on September 1, 2014 as 10.1136/oemed-2014-102268
Cervical and lumbar pain and radiological degeneration among ﬁghter pilots: a systematic review and meta-analysis Rahman Shiri,1 Heikki Frilander,1,2 Markku Sainio,1,3 Kirsi Karvala,1,2 Roope Sovelius,4 Tapio Vehmas,1 Eira Viikari-Juntura1 ▸ Additional material is published online only. To view please visit the journal online (http://dx.doi.org/10.1136/ oemed-2014-102268). 1
Disability Prevention Centre, Finnish Institute of Occupational Health, Helsinki, Finland 2 Occupational Medicine Team, Finnish Institute of Occupational Health, Helsinki, Finland 3 Mental Health and Cognition Team, Finnish Institute of Occupational Health, Helsinki, Finland 4 Centre for Military Medicine, Tampere, Finland Correspondence to Dr Rahman Shiri, Disability Prevention Centre, Finnish Institute of Occupational Health, Topeliuksenkatu 41 a A, FI-00250 Helsinki, Finland; [email protected]
ﬁ Received 11 April 2014 Revised 1 August 2014 Accepted 13 August 2014
ABSTRACT To assess the associations of acceleration force indicators (aircraft type and ﬂight hours) with cervical and lumbar pain and radiological degeneration among ﬁghter pilots. The PubMed, Embase, Scopus and Web of Science databases were searched until October 2013. Twentyseven studies were included in the review and 20 in the meta-analysis. There were no differences in the prevalence of neck pain ( pooled OR=1.07, 95% CI 0.87 to 1.33), cervical disc degeneration (OR=1.26, CI 0.81 to 1.96), low back pain (OR=0.80, CI 0.47 to 1.38) or lumbar disc degeneration (OR=0.87, CI 0.67 to 1.13) between ﬁghter pilots and helicopter or transport/cargo pilots. Moreover, the prevalence of cervical (OR=1.14, CI 0.61 to 2.16) or lumbar (OR=1.05, CI 0.49 to 2.26) disc degeneration did not differ between ﬁghter pilots and non-ﬂying personnel. Most studies did not control their estimates for age and other potential confounders. Among high-performance aircraft pilots, exposure to the highest G-forces was associated with a higher prevalence of neck pain compared with exposure to lower G-forces ( pooled OR=3.12, CI 2.08 to 4.67). The studies on the association between ﬂight hours and neck pain reported inconsistent ﬁndings. Moreover, looking back over the shoulder (check six) was the most common posture associated with neck pain. Fighter pilots exposed to high G-forces may be at a greater risk for neck pain than those exposed to low G-forces. This ﬁnding should be conﬁrmed with better control for confounding. Awkward neck posture may be an important factor in neck pain among ﬁghter pilots.
To cite: Shiri R, Frilander H, Sainio M, et al. Occup Environ Med Published Online First: [ please include Day Month Year] doi:10.1136/oemed-2014102268
Cervical and lumbar spine disorders are common conditions in ﬁghter pilots. One out of two ﬁghter pilots reports neck pain and one out of three reports low back pain.1 In addition, nearly 15–50% of ﬁghter pilots show cervical2 and 10–50% lumbar disc degeneration radiologically.3 4 Fighter pilots are exposed to high accelerations during ﬂight. Air combat engagements with currently used ﬁghter aircraft consist of multiple, frequently repetitive excursions to high Gz levels (Gz is a gravitational force that is applied to the vertical axis of the body). Peak levels usually range from +7 to +9 Gz, and approximately 20% of the ﬂying time is spent with Gz loading above +2 Gz.5 Fighter pilots have reported a higher prevalence of neck and low back pain as well as multisite pain than transport or cargo pilots.1 High G-forces have been suggested as an underlying factor.1
A ﬁghter cockpit is a compromise between limited space, the placement of controls and ejection seat requirements. The position of the backrest of an ejection seat more upright than recommended may stress the low back structures.6 Moreover, the cervical spine of ﬁghter pilots is especially susceptible to injuries. High G-forces, frequent turning and tilting of the head and neck as well as the weights of the helmet and oxygen equipment increase the stresses on the cervical vertebrae, especially during neck rotations and extensions.7 To date, a single meta-analysis on the association between exposure to acceleration forces and radiologically conﬁrmed cervical disc degeneration has been conducted and published in a report.2 That meta-analysis comprised seven studies and showed a signiﬁcantly higher prevalence of cervical disc degeneration in ﬁghter pilots compared with other pilots or non-ﬂying personnel. We are not aware of any other meta-analyses on the role of occupational physical factors in cervical or lumbar pain or degeneration among ﬁghter pilots. Our aim was to carry out a systematic review to assess the possible associations of acceleration forces with cervical and lumbar pain and degeneration among ﬁghter pilots and to estimate the magnitude of these associations with meta-analyses.
METHODS Search strategy We conducted comprehensive literature searches in the PubMed, Embase, Scopus and Web of Science databases until October 2013 using the following predeﬁned keywords: (Gravitation OR gravity OR gravit* OR G-force OR pilots OR helicopter OR military OR jet OR ﬁghter OR ﬂight OR ﬂying) AND (spine OR spinal OR neck OR back OR disc OR cervical OR lumbar OR degeneration). We used both MeSH terms and text words in PubMed, and we used Emtree terms and text words in Embase. We checked the reference lists of included articles for additional studies.
Selection of the studies We included cross-sectional, case–control and cohort studies that assessed the associations of exposure to G-force with neck or low back pain or cervical or lumbar disc degeneration. We included studies published in English only. To be eligible for a meta-analysis, the studies had to report quantitative data on the associations of occupational factors with cervical or lumbar disorders. RSh assessed the
Shiri R,employer) et al. Occup Environ MedProduced 2014;0:1–6. doi:10.1136/oemed-2014-102268 Copyright Article author (or their 2014. by BMJ Publishing Group Ltd under licence.
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Review titles, abstracts and full texts of the studies and evaluated the studies for inclusion. Not only case reports, reviews, guidelines, editorials, and letters are excluded, but also studies on lifetime neck or low back pain (one study) and studies with a sample size less than 10 participants (four studies). The results did not change after including those ﬁve excluded studies.
Quality assessment The quality of each included study was assessed by two authors using the modiﬁed Effective Public Health Practice Project tool for observational studies.8 RSh assessed all studies and the second assessments were carried out by other coauthors. Discrepancies were solved by discussion in the group. The following ﬁve main domains were assessed: selection bias, performance bias, detection bias, confounding and attrition bias (see Web table 1).
G-forces We classiﬁed aircrafts as ﬁghters, transport/cargo and helicopters. Fighters included aircraft types F-16, F-15, F-5, F/A-18, A-7 and A-4 as having the capacity to induce exposure to high G-forces on pilots (high manoeuvrability, higher G-developing rate and capability to sustain G-forces above 4 Gz). Other jets named in the studies were classiﬁed as having low G capacity, and they were combined with helicopters and transport/cargo aircrafts.
Meta-analysis We estimated crude or age-speciﬁc ORs with 95% CIs for the studies that reported the prevalence of cervical or lumbar pain or degeneration in ﬁghter pilots vs. helicopter or transport/ cargo pilots, or in ﬁghter pilots vs. non-ﬂying personnel. We calculated Woolf CIs for the estimated ORs.9 We used a ﬁxed model to estimate the overall OR for combining subgroups of a single study, e.g., combining the estimates for different age groups. We used a random model to combine the estimates of a single study on different outcomes, e.g., different signs of disc degeneration. For a study10 that reported the mean ﬂight hours in participants with or without cervical or lumbar disorder, we calculated the standardised mean difference by dividing the difference between the two means by their pooled SD. We then converted the standardised mean difference into OR.11 We used a random-effects meta-analysis to combine the estimates of different studies and assessed the presence of heterogeneity across studies by the I2 statistics.12 The I2 statistic shows the total variation across studies, which is not due to chance. I2 statistic less than 25% indicates small inconsistency and more than 50% indicates large inconsistency between studies.12 The inﬂuence of each individual study on the summary OR and heterogeneity was examined by repeating the meta-analysis leaving one study out at a time. Publication bias was assessed by a funnel plot and Egger’s regression test.13 14 The assumption of publication bias is that studies with signiﬁcant results are more likely to be published than studies with non-signiﬁcant results. In the presence of publication bias, studies with signiﬁcant results are more likely to be included in a meta-analysis. In this review, most of the included studies reported non-signiﬁcant results and the pooled estimates were close to unity. Thus, we assessed publication bias for meta-analyses with positive ﬁndings only. We used Stata, V.13 (Stata Corp, College Station, Texas, USA) for meta-analysis. 2
RESULTS Our searches initially identiﬁed 9879 abstracts (see Web ﬁgure 1). RSh screened the full text of all relevant reports. Fifty-seven relevant studies were identiﬁed. Of those, we excluded 30 studies conducted on pilots other than ﬁghter pilots, and we ﬁnally included 27 studies in the review (see Web tables 2–4). Of these 27 studies, 20 were included in the meta-analyses of neck pain, low back pain or radiologically deﬁned cervical or lumbar disc degeneration. Our quality assessment showed that most of the included studies did not control for potential confounders and 9 of the 27 studies had strong selection bias (see Web tables 2–4).
Cervical pain and degeneration Type of aircraft We used aircraft type to compare ﬁghter pilots with helicopter or transport/cargo pilots and to study the effects of different G-levels. A pooled estimate of four studies on the association between aircraft type and neck pain did not differ from unity (pooled OR=1.07, 95% CI 0.87 to 1.33, I2=0%; ﬁgure 1). A meta-analysis of three studies15–17 comparing neck pain between two high G-performance aircraft F-16 and F-15 pilots showed no difference ( pooled OR=1.19, 95% CI 0.85 to 1.67, I2=0%). However, three studies16 18 19 compared F-16/F-15 with F-5,16 F/A-18 with A-7/A-418 and F-15 with F-1.19 Neck pain was more common in F/A-18, F-15 and F-16 pilots than in pilots ﬂying the less G-capable aircrafts (including F-5 aircraft). A meta-analysis of these three studies showed a threefold (pooled OR=3.12, 95% CI 2.08 to 4.67, I2=0%) increased risk of neck pain for the highest G exposure (ﬁgure 2 and see Web table 2). A meta-analysis of seven studies on cervical disc degeneration conﬁrmed by imaging (ﬁgure 1) showed no difference between ﬁghter pilots and helicopter or transport/cargo pilots ( pooled OR=1.26, 95% CI 0.81 to 1.96, I2=70.8%). Of these studies, the largest one reported a signiﬁcant association between aircraft type and cervical disc degeneration that disappeared after adjustment for age.20 Moreover, there was no difference in cervical disc degeneration conﬁrmed by imagery between ﬁghter pilots and nonﬂying personnel in the meta-analysis of four studies ( pooled OR=1.14, 95% CI 0.61 to 2.16, I2=24.3%; ﬁgure 3).
Cumulative ﬂight hours
The pooled estimate of four studies10 15 21 22 that reported quantitative data on the association between ﬂight hours and neck pain was 1.54 (95% CI 0.99 to 2.39; ﬁgure 4). In all, seven studies10 15 21–25 explored the association between ﬂight hours and neck pain. Four studies21–24 (two included in the meta-analysis) found no association, but three10 15 25 did (two included in the meta-analysis). Of the latter group, one study25 (not included in the meta-analysis) did not control for any confounder. In the remaining two studies10 15 (both included in the meta-analysis), the association between the number of ﬂight hours and neck pain remained signiﬁcant after controlling for some confounders. The pooled estimate of the two studies that controlled for some confounders was 1.95 (95% CI 0.77 to 4.91). A single study26 found an association between the total ﬂight hours and cervical disc degeneration in high-performance aircraft pilots (OR=2.80, 95% CI 1.21 to 6.51 for ≥2000 h vs.