Correlation between Toe Spread and Toe Hold Pressure: Influence of Flip-Flop Strap Design

Background: Every year 9 million diabetics develop foot ulcers (DFU) and 1 million of these will result in an amputation every year resulting in a significant disability burden. Footwear design has been implicated as a major cause of these ulcers resulting in international guidelines recommending specialized protective footwear for diabetics. Unfortunately, in low- and middle-income countries (LMIC), where 80% of diabetics reside, sandals or flip-flops are primary footwear as they are affordable. Studies on flip-flops have focused on plantar pressure and ulcers even though evidence supports that 50% of ulcers are caused by straps on the dorsum of the foot particularly at the toehold. Strap redesign, as evidenced by rolled inner seam (RIS) vs standard straight edge straps, has been shown to reduce pressure on the dorsum of the foot. Objective: This study aims to understand the relationship between toe spread and toehold pressure as toe spread is a modifiable factor in flip-flops by adjusting the thickness of toe posts in the strap. Further, it aims to validate findings from a smaller sample size prior study that RIS design resulted in lesser pressure at the toehold when compared to standard straight edge design. Methods: This cross-over study recruited sixteen patients with 64 measures recorded for toehold pressure during a 5-step walking trial. Toes spread was measured by uploading still images to a convolutional neural network keypoint implementation algorithm which marked the midpoints of the great and second toes and measured the distance between them. Pearson correlation and linear regression models were used to assess the relationship between toe spread and toe hold pressure across and within each flip-flop design. Additionally, a t-test was conducted to compare toehold pressures between the two designs to validate prior study findings. Results: Overall, there was a very weak correlation (r=-0.322) between toe spread and toehold pressure. Regression analysis showed toe spread accounted for a modest variance (R² = 0.104) in toehold pressure overall, with straight edge design showing a slightly stronger association (R² = 0.121) compared to RIS design (R² = 0.022). The t-test validated prior study findings of a statistically significant difference in toehold pressure between RIS and standard straight edge designs, with RIS resulting in lower pressures at the toehold. Conclusion: Toe spread minimally impacts toehold pressure, suggesting further investigation is needed. However, the Rolled Inner Seam (RIS) design effectively reduces toehold pressures and should be adopted in flip-flop manufacturing to potentially mitigate foot ulcer risks in diabetic populations.