Does A Change In Foot Position Have An Effect On The Thickness And Stiffness Of The Plantar Fascia Of The Foot?

The plantar fascia (PF) of the foot is complex in both its structure and function. Ultrasound and Shear wave elastosonography has allowed more detailed non-weight bearing examination of this structure. Previous research has focused on the non-weight bearing assessment of plantar fascia thickness (PFT) and stiffness (PFS) but has not linked these measurements to rearfoot function. The assessment of rearfoot function has engaged many different approaches from functional indices to pain scales.

This study aims to assess if a change in the weight-bearing rearfoot foot position has an effect on the thickness (PFT) and stiffness (PFS) of the plantar fascia of the foot. A unique linear actuator driven 3D printed platform was developed, that was able to reliably move the rearfoot through a range of frontal and sagittal plane angles, whilst weight-bearing. An ultrasound probe capable of shear wave elastography was incorporated into the platform for closed chain assessment (weight-bearing) of the PF and a standardised protocol produced for measurement of the PFT and PFS, whilst weight-bearing. The PFT and PFS was collected for 13 (26 feet) participants (11 male; age 20 - 67 years, Mean 35.62, SD 15.04; BMI 22 – 42 kg/m2, Mean 30.31, SD 6.22) from a convenience sample of volunteers from Staffordshire University, subject to exclusion criteria. The data was subject to parametric statistical and, collective and cluster analysis.

The collective analysis shows there was no significant effect on PFT or PFS for changes in the rearfoot frontal or sagittal planes for the group. Individual analysis does show strong polynomial correlation for PFT and PFS for changes in frontal and sagittal plane rearfoot positions for some clusters. The rearfoot sagittal plane cluster demonstrated a negative Poisson ratio in 65.8% of the group, where the PFT and PFS both increased. The frontal plane cluster demonstrated a normal Poisson ratio in 45.8% of the group, where the PFT decreased as the PFS increased.
In conclusion, this study has shown that the closed chain rearfoot can be manipulated through a range of biaxial angles using a novel, accurate and reliable device. The PF can be assessed in the closed chain and the PFT and PFS calculated using a unique validated protocol. The PF does have a specific response to changes in the rearfoot position for individuals which, in some, can show a negative Poisson distribution.

There are several implications for future research and clinical practice.