Peak Plantar Shear and Pressure and Foot Ulcer Locations: A Call to Revisit Ulceration Pathomechanics

The sensitivity and specificity of plantar pressure in identifying diabetic foot ulcers (DFUs) are relatively low, leading investigators to label peak pressure (PP) as a “poor” predictor (1). Moreover, pressure-reducing therapeutic footwear is only “meagerly” effective in preventing recurrent ulceration (2). These findings indicate that DFUs have a more complicated pathology. The purpose of this study was to investigate the clinical significance of plantar shear as it relates to ulcer locations using a custom-built stress plate (3). Eight volunteers, each with one recently healed forefoot plantar DFU, walked at self-selected speeds across the custom-built plate, quantifying both PP and peak shear (PS). In five subjects (63%), PP and PS occurred at different plantar sites. Of eight ulcers, five did not develop at PP locations (Fig. 1). Two ulcers developed at PP-only locations, whereas three developed at PS-only sites. One ulcer location matched overlapping locations of PP and PS. Surprisingly, two ulcers occurred at neither PP nor PS locations. Not only does pressure have low predictive value, but according to the only longitudinal study of its kind, only 38% of plantar ulcers develop at PP sites (4). Using the same custom-built device, we previously reported higher shear values in individuals with diabetes (3) and that PP and PS sites differ in 60% of diabetic neuropathic subjects. This study reports, for the first time in the literature, ulcers occurring at PS-only locations, not overlapping PP locations. Furthermore, more ulcers developed at PS locations as opposed to PP locations (50% vs. 38%). Plantar shear may subject the tissue to fatigue failure by inducing forces in opposite directions during the same stance phase. Fatigue failure occurs Figure 1—Relative locations of the healed ulcer, PP, and PS. MTH, metatarsal head.