Original Full Length ArticleFundamental differences in axial and appendicular bone density in stress fractured and uninjured Royal Marine recruits — A matched case–control study☆,☆☆
Introduction
Stress fracture is a common overuse injury within military training, where a high level of physical activity involving impact is a key component of the physical training [1], [2]. The prevalence of stress fracture in Royal Marine (RM) recruits (males aged 16–32 y) has typically been of the order of 4–7% [3], [4]. These injuries result in significant morbidity, often requiring lengthy rehabilitation at a personal cost to an injured recruit, as well as considerable expense to the military medical services [5].
RM training is widely acknowledged to be one of the most arduous initial military training programmes in the world. The 32-week programme – which only recruits males – encompasses physical training, classroom-based learning and personal administration skills along with field exercises, weapons training and specific Commando skills such as the assault course and rope climbing. There is a progression of professional and physical assessments (or criterion tests) during the course, culminating in the Commando Tests in week-31.
In contrast with other military programmes – which tend to be of a shorter duration than the 32 weeks of RM training – the most common site of fracture during RM training is metatarsal rather than tibia [6], [7], [8]. Furthermore, fractures in RM training tend to be reported later during the military training programme than in much of the literature [1], [6], [9].
Published cohort studies have indicated gender, age, body weight, height, muscle size/strength, physical fitness, exercise history and smoking to be potential factors influencing stress fracture risk in male military recruits [6], [10], [11], [12], [13], [14], [15]. However, the small number of case–control studies exploring bone mineral density (BMD) and other bone parameters in military populations have been limited by the extent of the control for these risk factors and/or the small number of confirmed stress fractures per se [6], [8], [9], [10], [16], [17]. Furthermore, only one study investigating stress fractures in military personnel has employed Dual X-ray Absorptiometry (DXA), peripheral Quantitative Computer Tomography (pQCT) and Broadband Ultrasound Attenuation (BUA) [15]. Central DXA was only undertaken on seven males with stress fracture, limiting the power of these findings with respect to stress fracture risk.
Bone turnover markers represent a method of assessing the metabolic changes in bone associated with bone remodelling [18]. Previous studies have reported conflicting findings with regards to longitudinal changes in bone turnover markers during military training, as well as with respect to differences between stress fracture cases and controls [19], [20], [21], [22]. Therefore the relevance of bone turnover markers in predicting stress fracture risk remains unclear.
The principal aim of this case–control study therefore was to investigate differences in axial and appendicular BMD using three contrasting but complimentary methods of measurement for bone mass and bone structure (i.e. DXA, pQCT and BUA), and a marker of bone resorption (C-telopeptide of collagen cross-links, CTx), between stress fractured and non-injured RM recruits, that was adequately powered to detect any such differences and with identical matching for age, body weight, height and aerobic fitness.
Section snippets
Study participants
The present study was part of Surgeon General's Bone Health Project, which aimed to identify risk factors for illness and injury in recruits during RM training. Other measures including body girths, skinfolds and assessment of past and present dietary intake were also undertaken, but are not reported here. Recruits from a total of twenty troops, who commenced RM training at the Commando Training Centre (CTC), between Sept 2009 and July 2010, were invited to participate in the study
Stress fracture
From the 1090 recruits who volunteered for the study, a total of 78 recruits (7%) suffered one or more stress fractures. From the study sample, 62 matched pairs of stress fractured recruits and controls were assessed with DXA. Ten stress fractured recruits either opted out of training before it was possible to complete scan visits, or the injury was reported to the CTC Medical Centre in week-31 of training. (By week-31, a recruit would have completed all the criteria Commando Tests, such that
Main findings
This study in male Royal Marine recruits identified lower mean BMD at axial sites and structural differences in the tibiae of the stress fracture group compared with matched controls (P < 0.001). There were no differences in serum CTx concentration or BUA between stress fracture recruits and matched controls. The incidence of stress fracture (7%) was similar to that previously reported for this military population [3]. But this incidence was higher than that reported for male recruits in other
Authors' roles
Authors' roles: Study design: JLF, SLN, AA, TD. Study conduct: TD, JLF, AA. Data collection: TD, AS, RC, PT. Scan interpretation: PT, CC. Serum analysis and interpretation: MORH, TRA. Data analysis: TD. Data interpretation: TD, JLF, SLN. Drafting manuscript: TD, JLF. Revising manuscript content: TD, JLF, SLN, AA. Approving final version: TD, SLN, CC, MORH, TRA, AA, JLF. TD takes responsibility for the integrity of the data analysis.
Disclosures
All authors have no conflicts of interest.
Acknowledgments
The authors would like to thank colleagues at the Commando Training Centre Royal Marines, Lympstone, Devon, UK, for their cooperation and support with this study, and the imaging technicians at the Osteoporosis Centre, Southampton General for accommodating the recruit scans within a busy clinical schedule.
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This work was funded by the UK Ministry of Defence.
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TD was a recipient of a Young Investigator Award at the UK Osteoporosis Conference, Manchester, UK, 4–7th June 2012 for this work. Aspects of this work were presented in the Plenary Poster Session at the ASBMR Conference, Minneapolis, USA, 12–15th Oct 2012.