While most cases of pain on the bottom of the heel are caused by plantar fasciitis, some are not. A common alternate cause of heel pain is a condition called bursitis, and it can lead to severe pain directly on the bottom of the heel.
Bursitis is inflammation of a bursal sac, which is a pad of tissue that can be found throughout the body, protecting bony prominences as well as other sensitive tissue. In the bottom of the heel, the bursa is not necessarily a naturally occurring object. In general, unlike many other bursas, this bursa is more reactive, meaning that it forms as a result of abnormal pressure and irritation under the heel. The bursa becomes irritated when the heel bone of a person with a flat foot rotates excessively as it will naturally do during walking, generally exposing the bottom of the heel tissue to a more irritating part of the heel bone that is not usually prominent on the bottom of the heel. Eventually, a bursal sac will form to protect the irritated soft tissue under the heel bone, and in turn this will become irritated itself. Other causes for bursitis include walking for extended periods on rocky or bumpy surfaces, high arched feet with poor shock absorption capabilities, as well as poor heel fat pad in older people with a prominent heel bone resulting. I have even often seen this condition in people who have a more angular shape to the bottom of the heel bone as opposed to a more normal rounded shape.
Bursitis can often be present at the same time as plantar fasciitis.
Treatment of bursitis under the heel is relatively simple. Steroid injections often take care of the inflammation in and around the bursa, usually more effectively than anti-inflammatory medication taken orally. Ice helps, as well as increased shock absorption via gel heel inserts or orthotics with soft heel padding. Surgery is typically not an option, as the attempt to find and remove the bursa can lead to more damage to sensitive tissue such as nerve bundles within the heel, making things worse.