Foot assessment in people with diabetes: A quantitative diagnostic approach

Background: Diabetic foot ulcers are a serious and costly complication of diabetes. The leading causes of diabetic foot ulceration are mechanical trauma and the breakdown of plantar soft tissues. Biomechanical factors linked to an increase in diabetic foot ulceration are changes in plantar soft tissue mechanical properties and increased plantar pressure. These represent important internal and external risk factors for ulceration that are not commonly assessed within clinical practice due to a lack of clinically applicable measurement techniques.

The measurement of Shore hardness has been identified as a potential method to assess these internal and external biomechanical factors due to its previous use in various soft tissue applications and its simplicity, ease of use, and low cost. However, key questions remain regarding the physical meaning of Shore hardness when used within biological soft tissues to assess the mechanical properties of the plantar soft tissues of the foot. In addition, the clinical relevance of Shore hardness when applied to the diabetic foot needs further exploration. Finally, the association between Shore hardness and plantar pressure in people with diabetes has not been fully investigated. Nevertheless, Shore hardness presents a potential method to assess the external risk factors associated with ulceration.

Aim: The primary aim of this research was to investigate if the measurement of Shore hardness can be used within a clinical setting as a method to assess the mechanical properties of the plantar soft tissues. The secondary aim of this research was to investigate if the measurement of Shore hardness is associated with changes in plantar pressure during walking in people with diabetes and, if so, can Shore hardness in combination with other biomechanical measurements be used to predict these changes.

Methods: Finite element (FE) analysis was conducted to investigate the physical meaning of Shore hardness using an anatomically accurate model of the heel pad. Additionally, the ability of Shore hardness to individually assess the mechanical properties of skin and subcutaneous soft tissue was investigated.

The clinical relevance of Shore hardness was assessed within a cohort of 40 adults with diabetes and diabetic peripheral neuropathy classified as having a high risk of foot ulceration. The average age of participants was 63(±9) years, with an average duration of diabetes of 15(±9) years. To assess the clinical relevance of the measurement of Shore hardness, Spearman’s rank correlation tests were performed between Shore hardness and the previously established parameters found to increase the risk of mechanical trauma to the foot, such as blood biochemistry, loading, and age.

The association between Shore hardness and plantar pressure as the external risk factor for ulceration was also investigated within this cohort of 40 adults using multiple regression analyses. Specifically, the ability of Shore hardness in combination with measurements of 2D sagittal plane range of motion, to predict regional changes in plantar pressure and loading was assessed.

Results: The results of the FE analysis showed that the measurement of Shore hardness offers an assessment of stiffness that is a combination of both the mechanical behaviour of the skin and the underlying subcutaneous tissue. It was concluded that, on its own, the measurement of plantar soft tissue Shore hardness does not provide an assessment of the stress-strain behaviour of the heel pad’s constituent layers but instead offers an assessment of the bulk tissue’s overall capacity to deform. As a result, differentiating between the stiffness of skin and that of the subcutaneous tissue based on the conventional assessment of Shore hardness remains a challenge.

Additionally, through FE analysis, it was found that by altering the size of the Shore hardness indenter within the currently available limits, the measurement of Shore hardness cannot independently assess the mechanical properties of the skin or subcutaneous soft tissue. However, the results of the FE analysis also highlighted that an indenter that is less than 2mm in diameter and 1mm in length might potentially be able to infer differences between the mechanical properties of the skin and subcutaneous soft tissue.

The clinical relevance of Shore hardness was shown by confirming correlations with age, blood biochemistry, and loading, whereby an increase triglyceride levels was associated with increases in tissue hardness. In contrast, an increase in loading causes a decrease in plantar tissue hardness. These results were all found to align with current literature indicating that Shore hardness can indeed be a clinically viable approach for assessing the internal risk factors associated with ulceration.

Finally, Shore hardness, in combination with foot and ankle range of motion, was able to predict changes in peak plantar pressures and pressure time integral within the midfoot region. A reduction in midfoot dorsiflexion and an increase in Shore hardness at the midfoot are predictive variables for an increase in peak plantar pressure and pressure time integral. These results thus highlight the potential usefulness of the assessment of Shore hardness as a method to monitor changes in the external risk factors associated with ulceration.

Conclusion: These findings show that Shore hardness can be a simple, cost-effective and reliable method for assessing both the internal and external biomechanical risk factors associated with diabetic foot ulceration within a clinic setting. This is specifically relevant to low resource settings where access to sophisticated equipment such as ultrasound elastography or plantar pressure platforms can be limited.