The Mechanical Properties of the Heel Pad in Unilateral Plantar Heel Pain Syndrome

Plantar heel pain syndrome has been attributed to entrapment neuropathy, plantar fasciitis, calcaneal spurs, and stress fractures of the calcaneus. Although deteriorated mechanical properties of the heel pads may play an important role in the pathogenesis of heel pain syndrome, this has received little notice. In this study, a specially designed compression relaxation device with a push-pull scale and a 10-MHz linear array transducer was used to determine thickness of the heel pad under different loading conditions. Twenty consecutive patients aged 29 to 77 years with unilateral plantar heel pain syndrome were enrolled. Thickness of heel pad bilaterally was measured when the heel pad was compressed by serial increments of 0.5 kg to a maximum of 3 kg and then relaxed sequentially. The load-displacement curve during a loading-unloading cycle was plotted, and the compressibility index and energy dissipation ratio of the heel pad were calculated accordingly. Phase I displacement of the heel pad (from 0 to 1 kg load) on the painless side was greater than that on the painful side (P < 0.01), but there was no statistically significant difference between painless and painful sides in thickness of unloaded heel pads, compressibility index, or energy dissipation ratio (P > 0.05). In conclusion, the affected heel pad in plantar heel pain syndrome was stiffer under light pressure than the heel pad on the painless side. The changed nature of chambered adipose tissue in a painful heel pad may be responsible for its increased stiffness under light pressure.