Plantar pressure relief under the metatarsal heads – Therapeutic insole design using three-dimensional finite element model of the foot

Abstract

Therapeutic footwear with specially-made insoles is often used in people with diabetes and rheumatoid arthritis to relieve ulcer risks and pain due to high pressures from areas beneath bony prominences of the foot, in particular to the metatarsal heads (MTHs).

In a three-dimensional finite element study of the foot and footwear with sensitivity analysis, effects of geometrical variations of a therapeutic insole, in terms of insole thicknesses and metatarsal pad (MP) placements, on local peak plantar pressure under MTHs and stress/strain states within various forefoot tissues, were determined. A validated musculoskeletal finite element model of the human foot was employed. Analyses were performed in a simulated muscle-demanding instant in gait.

For many design combinations, increasing insole thicknesses consistently reduce peak pressures and internal tissue strain under MTHs, but the effects reach a plateau when insole becomes very thick (e.g., a value of 12.7 mm or greater). Altering MP placements, however, showed a proximally- and a distally-placed MP could result in reverse effects on MTH pressure-relief. The unsuccessful outcome due to a distally-placed MP may attribute to the way it interacts with plantar tissue (e.g., plantar fascia) adjacent to the MTH. A uniform pattern of tissue compression under metatarsal shaft is necessary for a most favorable pressure-relief under MTHs.

The designated functions of an insole design can best be achieved when the insole is very thick, and when the MP can achieve a uniform tissue compression pattern adjacent to the MTH.

Introduction

Therapeutic footwear with custom insoles plays a major role in the prevention of plantar ulceration in diabetic foot (Albert and Rinoie, 1994, Arts et al., 2012), relieving painful forefoot syndromes (e.g. metatarsalgia) (Postema et al., 1998), and accommodating structural deformed foot in rheumatoid arthritis (Silvester et al., 2010). These specially-designed insoles are purported to function by alleviating high pressures from areas beneath bony prominences of the foot, in particular to the metatarsal heads (MTHs).

The principle design features of a therapeutic insole may include the thickness, the geometry contour, and other forms of variations such as the metatarsal pad (MP). In addition to the material׳s stiffness (which is strictly limited by commercial availability (Praet and Louwerens, 2003)), these features directly alter interfacial interactions between the foot and the shoe, and potentially offer a most effective approach to the pressure reduction in the foot׳s tissue (Bus et al., 2004, Mueller et al., 1997, Owings et al., 2008).

In the literature, increases in foot׳s contact area, decreases in peak plantar pressure and force-time integrals have been documented for Total Contact Insole (TCI) used in people with diabetes and peripheral neuropathy (Ashry et al., 1997, Bus et al., 2004, Mueller et al., 1997). However, because of concern over TCI׳s effectiveness on local pressure relief, a combination design is occasionally built where a MP component is constructed directly into an insole (Bus et al., 2004). The MP seeks to achieve enhanced pressure-relief to the bony prominences under MTHs. Unfortunately, according to Owings et al. (2008), the design of such devices could be intuitive and mostly based on the past experience of the pedorthists. A number of studies using pressure measurement have shown variations in MP placement may lead to unwanted outcomes (Hastings et al., 2007, Hsi et al., 2005). For example, a MP placed more distal than as little as 1.8 mm from the MTH leads to an increased peak plantar pressure (Hastings et al., 2007). Other studies comparing insoles with and without a MP in healthy subjects showed the pressure-relief effects of a MP seem to be varied and dependent on subject characteristics (Brodtkorb et al., 2008, Holmes and Timmerman, 1990).

When optimal plantar pressure relief is the goal, the efficacy and mechanism of the effects of insole design on foot structures must be quantified. Spiral Computed Tomography (CT) has been used to examine how a specific MP could affect tissue compression in the forefoot (Mueller et al., 2006, Mueller et al., 1997). However, deviation in device location, such as the difficulty in consistently placing a MP during experiments, poses a challenge to obtain reliable data. Importantly, stress/strain distributions could have been modified throughout the foot by different insole conditions. The insole may induce a complex interplay with the foot skeleton, muscles, ligaments, and fascia structures, many of which has not been extensively investigated, and thus are largely unknown (Mueller et al., 2006).

Finite element (FE) modeling of foot and footwear offers a unique computational tool, as it allows multiple insole design variables to be evaluated in a more controlled and efficient manner than traditional experiments. Foot FE models have been employed to investigate the effects of insoles thickness on peak plantar pressure under the 2nd MTH (Lemmon et al., 1997), the influence of midsole plug׳s material on pressure distributions (Erdemir et al., 2005), and efficacy of a modified TCI on MTH pressure reductions in diabetic foot (Actis et al., 2008). However, the primary problem with these models is lack of geometrical details as only 2D sections of the foot were analyzed. A few have attempted 3D foot–insole FE analysis (Chen et al., 2003, Cheung and Zhang, 2005). These have incorporated anatomical-accurate foot structures, but they often do not consider auxiliary muscle action in the analysis, and focus mostly on standing.

In this study, we presented a 3D FE musculoskeletal model of the foot to be applied for therapeutic insole design. More specifically, we aim to establish relationships between insole modification, including thickness and MP placement, and stress distributions under metatarsal heads (MTH). Precise determination of such relationships may shed new light on the underlying mechanism of foot–insole interaction, and will eventually enhance the scientific basis of therapeutic footwear design and testing.