All interventions evaluated in this study were effective at reducing both maximal plantar pressure and pressure-time integral under the second metatarsal head in healthy individuals. However, the effect of the Fixtoe Device® interventions was higher than with the traditional second metatarsophalangeal joint stabilizing taping technique described by Yu et al. 35
The greatest ∆ was observed in the Fixtoe Device® with metatarsal pad intervention. The lowest ∆ was observed when placing the stabilizing tape. However, the most important variation in ∆ among the interventions was observed when placing the Fixtoe Device® without a metatarsal pad relative to the tape intervention. This intervention showed a strong size effect. When adding the metatarsal pad to Fixtoe Device® a low variation in ∆ was seen. The size effect of this intervention was moderate. Since we are the first to evaluate the effectiveness of this novel stabilization device, we cannot compare our data to previous studies.
The results we obtained with the Fixtoe Device® are similar to those reported by other authors when placing horseshoe discharges and metatarsal domes under central (second and third) metatarsal heads in a healthy population.8 Reduction of peak pressure and the pressure-time integral values were obtained in both situations. Horseshoe discharges and metatarsal domes also showed positive results when investigating pain relief.1,8,13,15,19,31 The research by Poon and Love showed custom-made orthosis with metatarsal dome reduced plantar pressure under central metatarsal heads by up to 13% in metatarsalgia patients.31
Nordisen et al. also reported a significant peak pressure decrease when placing a metatarsal dome (8.4% reduction) under the first metatarsophalangeal joint in asymptomatic pes planus patients, which was the most effective in comparison with other discharge pads.29 Guldemond et al. found the effectiveness of the metatarsal dome in the reduction of peak pressure was higher when combined with a higher arch slope on customized insoles (18% versus 39% compared to the plain insole condition).10
Even though we did not evaluate it, we understand the placement of Fixtoe Device® might not have a relevant influence on the obtained data. While the effectiveness of other devices, such as discharges or metatarsal domes, depends on the precision of their placement in relation to the metatarsophalangeal joints, as Landorf et al. and Martínez-Santos et al. recently pointed out.22,27 Hastings et al. found that maximal peak pressure reduction (32 ± 16%) was achieved when the metatarsal dome was placed 6.1 to 10.1 mm proximal to the plantar aspect of the metatarsal head.13 This location is highly variable. Therefore, we recommend that the placement of metatarsal domes is assessed individually. 2,10,12,22,26,27
Our results show that the combination of both components of Fixtoe Device® was the intervention that generated the lowest values in the peak of pressure and pressure-time integral. Previous studies have shown a decrease in the forefoot’s load when placing cushioning materials (e.g., different Poron® and Plastazote combinations and foams) under the metatarsal heads. 5,12,17 Not only during normal gait, but also when running, metatarsal cushioning pads have been shown to produce a peak plantar pressure decrease in the forefoot, as Hähni et al. reported using instrumented insoles on their investigation in healthy recreational runners.12 Our work supports that the placement of cushioning materials underneath the metatarsal heads – the cushioning metatarsal pad included in Fixtoe Device®- generates a larger reduction in the studied values in that area.
Nevertheless, we did not investigate the isolated effect of cushioning materials. Domínguez et al. found that the placement of different isolated absorbing energy materials (Pedilastik®, Poron Medical®, or Jogtene®) did not decrease mean pressure in the forefoot area or under the metatarsal heads, which they associated with the need to combine them with a discharge fenestration.5 Given our results, the combination of both effects – cushioning and discharge – generated by Fixtoe Device® is more effective at reducing the load in metatarsal heads.
Furthermore, we analyzed the effect of the interventions on the pressure-time integral. The relevance of the pressure-time integral measures is well known, and the duration of the load at a specific point might be more relevant than the magnitude of the pressure itself. Therefore, in the plantar aspect of the foot, the continuous application of a mild pressure trough time would be more significant than the brief application of higher pressures in pain occurrence.28 The possible relationship between pressure-time integral and deformity progression likely supports the clinical relevance of our findings in propulsive metatarsalgia patients, and further investigations should address this.
Our results show that the placement of a stabilization tape according to the traditional technique on the second metatarsophalangeal joint reduced maximal plantar pressure and pressure-time integral in the second metatarsal head relative to the barefoot condition. This could be an explanation for the clinical improvement seen with this treatment by other authors. 3,18,30,35 Nevertheless, we did not find any other studies quantifying the effects of the stabilization tape in terms of maximal pressure or pressure-time integral. In general terms, we also believe the elasticity and movable anchoring in the novel device offers the patient an easier fitting than the traditional tape.
As a final reflection, we chose to carry out the investigation with healthy participants due to the availability of the sample, and since we believed changes should be first seen in individuals without deformity or pain. Therefore, the results presented here are not transferable to real patients. Furthermore, our study did not analyze certain characteristics of the novel device that might have an influence on its effectiveness, such as the possibility that its size, particularly when used with the metatarsal pad, affects its correct placement, and the possible need for a larger space inside the patients’ shoewear. As such, future studies should verify the efficacy of Fixtoe Device® in propulsive metatarsalgia patients, including subjective measures, such as comfort with the novel device, pain, or inflammation relief as a result of measures in relationship with a decrease in maximal plantar pressure, as other authors did before with metatarsal pads.19
To our knowledge, this is the first investigation proving the effectiveness of the recently developed Fixtoe Device® in terms of plantar pressure modification, which leads the way to its use in clinics.