Simple model of arch support: Relevance to Charcot Neuroarthropathy

Abstract

Background: Charcot neuropathy is a common complication resulting from poorly controlled diabetes and peripheral neuropathy leading to the collapse, and ultimately the breakdown, of the midfoot. Mechanically, it is likely that a compromised arch support in this, or any other patient group that experiences foot flattening, would be associated with slippage at the distal and proximal interface regions of the plantar surface of the foot and the adjacent support surface. This slippage, although difficult to quantify with standard motion capture systems used in a gait laboratory, could potentially be assessed with systems for monitoring interface shear stresses. However, before investing in such systems, a correlation between arch flattening and interface shear stresses needs to be verified.

Methods: For this purpose, a sagittal plane model of a foot was developed using a multi-body dynamics package (MSC Adams). This model mimicked a subject swaying back and forth, and was constructed to show the dependence of interface stresses on altered arch support.

Findings: The model's predictions matched typical FootSTEPS data: lengthening of the arch of 1-2 mm, sway oscillations of 0.22-0.33 s and frictional force differences (calcaneus relative to forefoot) of 60 N. Of clinical relevance, when the stiffness of the plantar spring (representing aponeurosis and intrinsic muscles) was reduced by 10%, the frictional force difference increased by about 6.5%.

Interpretation: The clinical implications of this study are that, while arch lengthening of less than 2 mm might be difficult to measure reliably in a gait lab, using shear sensors under the forefoot and hindfoot should allow arch support to be assessed in a repeatable manner.

Keywords: Arch support; Charcot neuropathy; Computational model; Diabetes; Foot.

Figure 1.

Initial foot model developed with MSC Adams software. An upright subject is represented on the left, with a close-up of the foot shown on the right.

Fig 2.

Computational model simulating a 85 kg adult male swaying in the sagittal plane for 1.7 seconds.

Fig 3.

Computational model compared to patient trial data (two representative trials). Note that differences between model and trial data are comparable to the differences from one trial to another.

Fig 4.

Predicted metatarsal displacements as a function of arch stiffness. Foot flattening is initially difficult to observe, due to the small displacement of the forefoot relative to the hindfoot. As arch support deteriorates (as quantified by arch stiffness), the relative displacement increases in a slightly non-linear manner.