Comparison of static and dynamic biomechanical measures in military recruits with and without a history of third metatarsal stress fracture

Abstract

Background

For Royal Marine recruits in training, the third metatarsal is the most common site for stress fracture. Previous evidence regarding biomechanical factors contributing to metatarsal stress fracture development is conflicting, possibly due to the lack of differentiation between the metatarsals. The present retrospective study compares static anatomical characteristics and dynamic biomechanical variables for Royal Marine recruits with and without a history of third metatarsal stress fracture.

Methods

Ten Royal Marine recruits with a history of third metatarsal stress fracture were compared with control subjects with no previous stress fracture occurrence. Selected static anatomical variables were measured to describe the ankle and subtalar joints. Peak ankle dorsi-flexion and rearfoot eversion were measured during running. In addition, peak vertical and horizontal ground reaction force variables were compared for the two study groups.

Findings

No significant differences in static anatomical variables were identified between study groups. During running, peak rearfoot eversion was found to occur significantly earlier for the stress fracture group than for their matched controls, suggesting an increase in time spent loading the forefoot. The peak applied resultant horizontal force during the braking phase was directed significantly more laterally for the stress fracture group. In addition, the peak magnitude of resultant horizontal force applied during the propulsion phase was significantly lower for the stress fracture subjects.

Interpretation

The findings of this study highlight the importance of including dynamic biomechanical data when exploring variables associated with the development of third metatarsal stress fracture and indicate that successful interventions to reduce the incidence of this injury are likely to focus on forefoot function during braking and propulsion.

Introduction

It has been well documented that stress fractures are a major problem in military personnel, with the most common injury sites being the tibia and the metatarsals (Meurmann, 1981, Evans, 1982, Riddell, 1989, Pester and Smith, 1992). Ross and Allsopp (2002) have identified that for Royal Marine Recruits in training, stress fractures are the single most important cause of lost training days. These authors describe how, although the number of cases of stress fracture appears low at around 4% of recruits, between 6 and 12 weeks is typically required for adequate recovery and return to full training. Total time to complete training is therefore increased, together with an increased financial cost through treatment and rehabilitation. Of the stress fracture sites reported in this study of Royal Marines, the third metatarsal was found to be the most common site, accounting for 38% of reported stress fracture injuries (Ross and Allsopp, 2002).

Despite various biomechanical variables previously being associated with the development of lower extremity stress fractures, evidence regarding the cause of stress fractures at specific sites is inconclusive (Beck, 1998). This makes it difficult to identify recruits likely to be predisposed to injury development and thus to confidently apply appropriate interventions to reduce the likelihood of injury. A limitation of previous studies of lower limb characteristics is that they do not typically differentiate between the metatarsals. The specific mechanism for stress fracture of individual metatarsals will likely differ, suggesting that the categorisation of all metatarsal stress fractures together when investigating anatomical characteristics associated with injury may have masked relationships. The present study therefore focuses on biomechanical variables in individuals with and without a history of third metatarsal stress fracture.

Specific lower limb anatomical characteristics previously associated with metatarsal stress fracture have included forefoot varus, limited range of ankle dorsi-flexion and low or high arch height (Hughes, 1985, Simkin et al., 1989). Hughes (1985) identified that army recruits with forefoot varus were 8.3 times more likely to develop a metatarsal stress fracture than those with a neutral or valgus forefoot alignment. Hughes (1985) also identified limited passive ankle dorsi-flexion as placing individuals at a greater risk of metatarsal stress fracture. The mechanisms by which forefoot varus or ankle dorsi-flexion may contribute to the development of third metatarsal stress fracture were not explored. Regarding arch height, whilst an increased overall incidence of lower limb stress fracture has been associated with a high arched foot (Giladi et al., 1985, Brosh and Arcan, 1994), Simkin et al. (1989) found that metatarsal stress fractures were more common in military recruits with a low arched foot than in those with a normal or high arch. Thus, despite evidence relating static anatomical features to metatarsal stress fracture development, the evidence is sometimes conflicting and the mechanism(s) by which stress fractures develop at a specific site is not understood.

Whilst there is some evidence of association between static anatomical measures and metatarsal stress fracture occurrence, in order to understand the mechanism by which these may contribute to injury it is necessary to additionally investigate the resulting movement during running. For example, a common compensation for forefoot varus is rearfoot eversion during stance, suggested to result in altered distribution of load by forefoot structures (Weinfeld et al., 1997). It is therefore desirable to measure the amount of compensatory rearfoot eversion during locomotion, in addition to static forefoot varus, to allow the suggestion of cause–effect relationships. In the same way, knowledge of ankle dorsi-flexion during running gait will reveal whether restricted static range of dorsi-flexion is transferred to locomotion, possibly resulting in an early heel lift and greater loading of forefoot structures compared to an individual with sufficient ankle dorsi-flexion (Hughes, 1985).

In addition to movement patterns, there is some evidence of peak impact force being associated with stress fracture risk (Grimston et al., 1991). However, other studies have found no difference in peak impact force for stress fracture and non-stress fracture populations (Grimston et al., 1994, Bennell et al., 2004). Measurements of pressure beneath the foot during heel–toe running indicate that the metatarsal heads experience peak load during the midstance phase of ground contact (de Cock et al., 2005), highlighting the relevance of loading later than the initial impact phase when investigating metatarsal stress fracture mechanisms. When considering the specific aspects of ground reaction force likely to be associated with stress fracture to the third metatarsal, a study by Arangio et al. (1998) highlighted the importance of horizontal forces. By mathematically modelling the shear and normal stresses through the metatarsals, these authors found that the third metatarsal is less able to withstand horizontal than vertical loads. Thus, the measurement of both vertical and horizontal forces during the stance phase of running may reveal specific loading characteristics associated with stress fracture development at this site.

The aim of the present study was to compare selected static anatomical characteristics and dynamic biomechanical variables for military recruits with and without a history of third metatarsal stress fracture. Based on the literature evidence presented, it was hypothesised that recruits with previous third metatarsal stress fracture would have lower arch height, greater static forefoot varus, lower static range of ankle dorsi-flexion, lower dynamic ankle dorsi-flexion during running, greater rearfoot eversion during running, and greater stance phase ground reaction forces than recruits with no history of this injury.