New research details link between plantar pressure and internal stress in the foot

SAN FRANCISCO — Researchers at the Cleveland Clinic reported a new testing approach that underscores the link between plantar pressure and internal stress in the foot, which could have benefits for managing patients with chronic foot problems, including diabetic ulcers.

In his presentation at the 54th Annual Meeting of the Orthopaedic Research Society, Peter R. Cavanagh, PhD, said many experts believe that ulcers caused by repetitive trauma "do not begin on the skin surface but rather deep in the tissue, and then progress to the skin surface." Unfortunately, "There has been no way to clinically and accurately assess the internal stress on the foot."

In their study, the researchers made a subject-specific three-dimensional finite element model of the right peak forefoot of healthy male subject (ie, 21 years old, 1.68 meters, 61 kg). The model contained separate sections representing the skin, plantar fat pad, muscle, metatarsals and phalanges. Material property parameters for these tissues were determined using an inverse finite element analysis technique that matched model deformations to those observed in an in-vivo MRI load-deformation experiment of the same subject, according to the investigators' paper.

The investigators simultaneously recorded ground reaction forces and plantar pressure distribution as the test subject took a first step, and the instant of peak metatarsal head regional pressure was extracted for use in model simulations. The researchers applied the ground reaction forces extracted at this instant to the model as he compressed his forefoot against a rigid plane.

The protocol included minimization of differences in predicted and measured peak plantar pressures at 10 regions (5 metatarsal heads and 5 toes) "by changing the plantar/dorsiflexion of metatarsals and toes while keeping the load constant," he said.

The measured peak pressure on the area under the second metatarsal head was 570 kPa. The peak von Mises stress on the plantar skin was well-aligned with the location of peak plantar pressure under the same location.

However, peak von Mises stress in the fat pad was much larger (18.2 MPa), more focal and more laterally located (ie, sub-third metatarsal) than the plantar skin stresses. Likewise, hydrostatic pressure was much more prominent on the fat pad than on the plantar skin.

Ultimately, "This model shows the complexity of the relationship between plantar loading and internal stresses in the foot."

For more information:
Cavanagh PR, Erdemir A, Petre M. A finite element approach to examine the relationship between plantar pressure and internal stress in the foot. Presented at the 54th Annual Meeting of the Orthopaedic Research Society. March 2-5, 2008. San Francisco.