Direct injuries to the bottom of the heel can produce pain that is independent of the fascia. The bottom of the heel contains a small network of nerves, as well as fat padding. A small pillow of tissue can also be found in this area, and is known as the subcalcaneal bursa. This pad of tissue, or bursal sac, is part of a system of little shock absorbers that protect prominent bone tissue throughout the body. People have bursal sacs throughout the body, but under the heel one is not born with it. They develop over time after the bottom of the heel receives chronic stress and pressure. I tend to see these form and become inflamed more readily in people that have a heel bone with more of a pointed shape to the bottom of the heel. Gradual pressures causes inflammation to form in these bursal sacs, and direct injury to the heel tissue, such as when one steps on a rock or other hard object, can also result in inflammation to the bursa (bursitis), or simply may cause a bruise to form. The plantar fascia can become injured with this tissue, or the bursitis and bruise can form independently.
The result is the sensation of sharp pain or strong aching when direct pressure is applied to the heel, particularly when it strikes the ground during walking. Pain is rarely present when one is at rest, and generally the pain stays in the direct undersurface of the heel without extending to the arch or ankle.