Peak plantar pressure and shear locations: relevance to diabetic patients.

D iabetic foot ulcers burden the U.S. health care system with an annual cost of approximately $6 billion (1). Based on the mechanical etiology of diabetic foot lesions, investigators tried to establish a relationship between ulcer occurrence and plantar pressures. Mostly, peak pressure has been chosen as an ulcer predictor. However, previous studies have yielded only moderate correlations between peak pressure and the occurrence of diabetic foot lesions (2– 4). Surprisingly, in one study that examined whether plantar ulcer locations matched peak pressure sites (4), only 38% of the ulcers developed under the peak pressure area. Therefore, foot pressure was labeled as a “poor” predictor of diabetic ulcer occurrences and their location (3). Effectiveness of diabetic ulcer prediction and prevention depends on an understanding of plantar soft tissue mechanics and the complete nature of foot-ground interactions. Further investigation of plantar shear in addition to pressure is essential to minimize the neuropathic ulcer prevalence. The purpose of this study was to find whether the peak pressure and shear under the feet of diabetic patients occur at different locations. If confirmed, shear distribution may explain the deviation between peak pressure and ulcer locations and potentially help researchers design more effective interventions. RESEARCH DESIGN AND METHODS — Thirty volunteers were recruited, among whom 10 had diabetic neuropathy. The remaining nondiabetic subjects served as control subjects. Subjects with gross foot deformities (except minor toe clawing), prior foot surgeries, and foot pain in both feet were excluded. The protocol was explained to the volunteers who signed an informed consent form approved by the institutional review board. A custom-built shear and pressure platform (5), 80 sensors (12.5 12.5 mm) arranged in an 8 10 array, was used to collect local barefoot forces. The forefoot region was of primary interest because diabetic ulcers most frequently occur in this area (6 –7). The two-step method was preferred because of its characteristics similar to the midgait method (8). Five trials on a surgery-free foot were conducted for each subject. Resultant shear forces were calculated from anteroposterior and mediolateral components. Peak vertical and shear force magnitudes and their sensorwise locations were extracted throughout the stance phase. Division of forces by sensor area provided peak pressure and shear stress values. Differences in peak pressure and shear locations were quantified by the distance between their corresponding Cartesian coordinates (D). A t test ( 0.05) was performed on D to reveal differences between peak pressure and shear sites due to diabetes. RESULTS — Of the 30 volunteers, 12 were women. The diabetic group had a mean SD age of 64 9 years and weight 97 27 kg, whereas the control group were aged 50 17 years with weight 70 12 kg. D values were found to be 2.3 1.2 and 2.5 0.9 cm for diabetic and control subjects, respectively, with no significant difference (P 0.718). In 20% of the diabetic patients, peak shear occurred at the same site as the peak pressure. Six (60%) had their peak shear site 2.5 cm apart from the peak pressure site. In control subjects, the ratio for occurrence at the same site was zero. D was 2.5 cm in seven control subjects (35%). Figure 1 displays a diabetic subject’s representative peak stress profiles, where peak pressure occurred under the second metatarsal head, whereas peak shear was under the hallux (D 4.6 cm).