Shear and pressure under the first ray in neuropathic diabetic patients: Implications for support of the longitudinal arch.

OBJECTIVE: To assess dynamic arch support in diabetic patients at risk for Charcot neuroarthopathy whose arch index has not yet shown overt signs of foot collapse.

METHODS: Two indirect measures of toe flexor activation (ratios: peak hallux pressure to peak metatarsal pressure - Ph/Pm; peak posterior hallux shear to peak posterior metatarsal shear - Sh/Sm) were obtained with a custom built system for measuring shear and pressure on the plantar surface of the foot during gait. In addition, the tendency of the longitudinal arch to flatten was measured by quantifying the difference in shear between the 1st metatarsal head and the heel (Sflatten ) during the first half of the stance phase. Four stance phases from the same foot for 29 participants (16 control and 13 neuropathic diabetic) were assessed.

RESULTS: The peak load ratio under the hallux (Ph/Pm) was significantly higher in the control group (2.10±1.08 versus 1.13±0.74, p=0.033). Similarly, Sh/Sm was significantly higher in the control group (1.87±0.88 versus 0.88±0.45, p=0.004). The difference in anterior shear under the first metatarsal head and posterior shear under the lateral heel (Sflatten ) was significantly higher in the diabetic group (p<0.01). Together these findings demonstrate reduced plantar flexor activity in the musculature responsible for maintaining the longitudinal arch.

CONCLUSIONS: With no significant difference in arch index between the two groups, but significant differences in Ph/Pm, Sh/Sm and Sflatten the collective results suggest there are changes in muscle activity that precede arch collapse.