Fifth metatarsal stress fracture in elite male football players: an on-field analysis of plantar loading

Participants

Fourteen elite male soccer players participated in the study. Seven players (professional and/or international players) who had suffered a primary stress fracture of the fifth MT (MT-5 group—age 25±5 years, weight 74±6 kg, height 178±6 cm) and seven matched healthy (no injury) players (control group—26±4 years, 76±4 kg, 179±5 cm) were recruited. All MT-5 injured players underwent surgery at Aspetar Sports Medicine and Orthopaedic Hospital (Doha, Qatar) by the same orthopaedic surgeon (PD) for intramedullary screw fixation±bone graft from the pelvis. Postoperative care comprised 3 weeks in a non-weight-bearing cast and 3 weeks in a partial weight-bearing boot. Physiotherapy was started after cast removal with combined hydrotherapy and reduced gravity treadmill (Alter-G) in the initial phase. After 6 weeks, all players progressed to full-weight-bearing. For all MT-5 injured players, the affected foot was at their stance or non-dominant limb when kicking with 86% (6/7) of the players reporting prodromal symptoms prior to stress fracture. All stress fractures occurred with insidious onset and were not frank or acute traumatic fractures2 (figure 1). Playing positions comprised four midfield players, one wing, one striker and one defender.

Inclusion in the current study occurred only after MT-5 players had returned to play following complete radiographic union of the stress fracture and completion of rehabilitation programme with end-stage field-based return-to-play tests at the rehabilitation department in Aspetar Sports Medicine and Orthopaedic Hospital. MT-5 group players were on average 240±60 days after surgery during on-field biomechanical testing.

Healthy matched control participants were injury-free for 6 months prior to the study with no previous history of injury to MT-5 or anterior cruciate ligaments. Healthy participants were matched for playing position, body mass, height and level of competition.

Testing protocol

Participants were fitted with appropriate-sized firm ground soccer boots (Nike Tiempo Genio leather II; Nike, Beaverton, Oregon, USA) for field-based biomechanical testing (figure 2). Natural Bermuda grass (Cynodon dactylon) surface over-seeded with rye grass (Lolium perenne) with a predominantly sand rootzone at the Aspire zone (Doha, Qatar) was used for testing. Ground staff maintained the surface to have consistent mechanical properties for the duration of the study.13 Surface hardness (69±6 g using FIFA-approved 2.25 kg Clegg hammer), rotational resistance (43±7 Nm using FIFA-approved studded disc apparatus) and temperature (26±6 °C using Kestrel 4400 heat stress tracker, USA) were recorded.

Plantar loading parameters were collected using the Pedar-X in-shoe system (Novel, Munich, Germany). Each insole is 1.9 mm thick and contains 99 capacitive sensors, which were calibrated prior to testing (Trublu Calibration; Novel). The validity and reproducibility of Pedar-X is excellent for running.9 14

Briefly, the Pedar-X insoles relay data sampled at 100 Hz to a data logger (carried in a custom-made back-pack) and then to a laptop via Bluetooth technology. Insoles are placed bilaterally with no foot orthotics in place so that the Pedar-X insoles were flat. A global positioning system (GPS) with accelerometer (Catapult, Australia) was used to verify running speed of the trials.

Standardised ‘warm-up’ protocol consisting of progressing running speeds, lower body resistance exercises (bodyweight, BW) and dynamic stretching was conducted by the same physical performance coach (RA). Following this, participants completed three soccer football-specific movement tests (figure 3). For each test, three familiarisation trials were first performed, followed by three trials of each test while kinetic and spatiotemporal data were collected via the Pedar-X and GPS unit as follows:

1. Set-piece kick: Participants were instructed to hit the furthermost top corner of the goal posts from a spot 10 m adjacent to the corner of the 18 yd box during three trials of curved set-piece kicks at 75% of maximum effort while data were collected from the stance leg.

2. Curved run with ball interplay: Participants performed three curved runs to mimic running into space onto a pass at around 75% of maximum effort. Participants dribbled the football to a cone where they passed to a stationary team-mate (RA) who sent out a subsequent pass for the participant to run onto while following the arc of the centre circle (figure 2). Participants were instructed to accelerate into the curved run, after passing the ball, at 75% of maximum effort.

3. Forward straight-line run: Participants ran 60 m at a speed of 5.5 m s⁻¹ (19.8 km/h). Running speed was controlled using audio cues in which the participant should pass each 10 m distance marker cone as the audio cue (beep) sounds. Speed was checked with the GPS system and any trials outside ±10% were discarded.

For the straight run, plantar loading data from the stance phase of a minimum of six consecutive footfalls were extracted for both the left and right feet and were averaged for subsequent analysis using Novel evaluation software (Groupmask Evaluation; Novel). For the curved run with ball interplay, the maximum force (F_max) of the inside foot (closest to the curve) was averaged over the three trials. For the set-piece kick, the F_max at the non-dominant stance leg was averaged over the three trials. The F_max was normalised to each participant’s bodyweight to facilitate between-participant comparison and was examined for the whole foot as well as anatomical regional areas (‘masks’) for each task (figure 4).15 F_max recorded by Pedar-X is a proxy measure of vertical ground reaction force (vGRF) and has been shown to correlate well with a Kistler force platform.8 The between-limb difference for F_max was calculated subtracting the value of the MT-5 injured limb from the uninjured limb for each MT-5 participant, and arbitrarily for the healthy participants as right leg subtracted from the left leg. Data for the whole foot and also anatomical masks were analysed with a focus on the lateral foot. The masks examined were ‘lateral midfoot’, ‘lateral forefoot’ and ‘lateral toes (2-5)’.

Statistical analysis

Between-limb and between-group differences were examined with an analysis of variance and subsequent post hoc testing with p <0.05 set as indicating statistical significance. Between-group differences were reported using Cohen’s d.16 The differences were reported as small, medium, large and very large when they reached 0.2, 0.5, 0.8 and 1.2, respectively.17

Set-piece kick

Significant differences (stance leg of MT-5 group−stance leg of control (CON) group) are reported for each lateral anatomical region.

Lateral midfoot: Increase in F_max compared with the control group with moderate ES (MT-5 1.0±0.01 BW, CON 0.8±0.02 BW, ES 0.7, p>0.05).

Total foot: Overall, for the total foot, the MT-5 group produced higher F_max than the control group during a set-piece kick (MT-5 3.3±0.6 BW, CON 2.9±0.3 BW, ES 0.8, p>0.05).

Lateral toes 2–5: Substantial increase in F_max (MT-5 0.2±0.06 BW, CON 0.1±0.05 BW, effect size (ES) 1.4, p=0.03) with very large effect size when kicking.

Curved run with ball interplay

Significant differences (MT-5 injured limb−control group inside limb when running towards curve) are reported for each lateral anatomical area.

Lateral forefoot: MT-5 group showed higher F_max with very large effect size at the lateral forefoot of the inside (closest to curve) limb when running a curve to receive a ball (MT-5 injured−CON=0.01 BW, ES 1.5, p=0.004).

Lateral toes 2–5: Higher F_max with large effect size compared with the control group (MT-5 injured−CON=0.005 BW, ES 0.9, p>0.05).

Total foot: Overall for the total foot, the MT-5 group had higher F_max with large effect size than the control group (MT-5 injured−CON=0.02 BW, ES 0.9, p>0.05).

Forward straight-line run at 5.5 m s⁻¹

High index of suspicion with prodromal signs

Prodromal symptoms (such as vague lateral foot pain) might provide an important window of opportunity to intervene and manipulate an individual’s loading variables following intense blocks of training.2 20 Eighty-six per cent (6/7) of participants in the MT-5 group reported prodromal symptoms prior to full stress fracture. It appears that these symptoms are frequently encountered: Ekstrand and Van Dijk2 reported 45% of players who sustain a MT-5 fracture reported prodromal symptoms at the lateral foot and Popovic et al 21 noted all 17 players had prodromal symptoms prior to stress fracture in a surgically managed cohort. Provided medical teams have a ‘player wellness’ monitoring system in place, early intervention may be possible. From the current findings, it is suggested that accelerating into curved runs towards the injured foot or performing set-piece kicks that curl towards a target such as high ball velocity crosses, corner kicks and set-piece penalty kicks may substantially increase lateral loading and should be monitored until symptoms have resolved (table 1). Previous research suggests cross-over cutting may also be viewed with caution due to increases in lateral foot pressure.22

Maximum plantar force alone is likely not a sufficient metric to be used as an indicator for when stress fracture will ensue due to the multifactorial nature of lower limb injuries, and it is suggested that the volume of kick type23 as well as running and direction change demands24 should be individualised to player position when considering return to sport programming. These data may provide further insight into pathogenesis of MT-5 stress fracture when combined with the current data and other factors (eg, sleep quality, training load, vitamin D status, match congestion, anatomical variation including local vascularity, and player age).1–3 25

Toe-flexor strength and management of ground reaction force

Decreased toe-grip strength measured with a digital dynamometer in a large prospective cohort of male football players (n=273) was found to be a prospective risk factor in players who went on to develop MT-5 stress fracture.3 The lateral toes 2–5 anatomical region showed much larger magnitudes of F_max in the MT-5 group for all three movements tested here, peaking with the set-piece kick (figure 3A). This suggests the external vGRF is greater at the lateral toes in injured players, which might be a result of kicking technique (more foot inversion and staying lateral through roll-over progression to toe-off) at the stance leg during kicking. External GRFs must be absorbed and managed by internal forces via the lateral plantar fascia, peroneus brevis/tertius muscle–tendon unit attachments and bending moments at the MT-5 bone itself. Decreased toe-flexor strength may incur higher bending moment forces on the MT-5 bone due to inability to manage the external GRF that generate large torsion, tension and axial loads especially when the foot is inverted prior to contact such as during full effort set-piece kicking.26 27

Football boots

All players wore identical soccer boots (figure 2). While this helped control for the effect different footwear might have on plantar loading, it also means that footwear was not tailored to the individual’s foot anatomy or preference. Given the large magnitude of lateral loading with the football-specific tasks, it is suggested that stud plate outsole width at the midfoot and forefoot must be wide enough to prevent lateral ‘overhang’ of the MT-5 bone in an attempt to offer some form of protection from the playing surface. Queen et al 22 suggested the addition of midsole cushioning, increased number of studs and decreased stud length reduced forefoot pressure during two football-specific tasks (side cut and cross cut) when using turf shoes instead of football boots. Additionally, clinical experience of the authors suggest footwear companies should perhaps work to incorporate forefoot cushioning into the outsole of football boots as many international and professional players remove the football shoe insole (sock liner) completely in attempt to improve ‘feel’ for the ball by wearing very tight shoes.

Further studies are required, with larger sample size, to assess if early intervention and manipulation of loading variables following prodromal symptoms does indeed reduce progression to stress fracture or if these findings extend to other populations (women, adolescents, older players and different playing levels).

An obvious limitation of this study in that ‘in-shoe’ systems measure vertical force and hence we miss the medial–lateral and anterior–posterior components of GRF. Cross-sectional design of this study should also be considered when interpreting the results as we cannot discern whether the higher lateral loads seen in the MT-5 group were the cause of stress fracture or a consequence of the injury.

However, even after completing a graduated rehabilitation programme and having returned to previous level of competitive football, the MT-5 injured players examined here displayed large differences in plantar loading at the lateral foot. These data should implore practitioners to exercise a high index of suspicion should prodromal symptoms occur in a player with history of previous MT-5 stress fracture and manipulate football movements accordingly.