There are a few causes of lumps, bumps or prominence on the outside of the foot. Some of these are just normal anatomy; others are due to abnormal processes. Starting from the fifth toe and working back to the heel:
A common prominence on the fifth toe is due to the formation of a hammertoe. A hammertoe is a structural deformity of the fifth toe causing the joint in the toe to become propionate. Pressure from shoes will cause a thickening of the skin. On occasion, this can become very painful making it difficult to wear a closed shoe. Treatment consists of trimming the thick skin or padding the area to reduce the pressure from the shoe. At times surgical strengthening of the toe is necessary (See hammertoe surgery).
Just behind the fifth toe, at the joint where the toe connects to the foot, a prominence can develop. This is called a Tailors bunion. It is caused by the abnormal function of the foot and can be slowly progressive. Some people naturally have an enlargement in this area. Shoe pressure can make the area painful and at times cause a bursa to form. A bursa is a sack of inflamed tissue that occurs over a bony prominence in response to excessive pressure or friction. The bursa is spongy to the touch and can be very painful. The bursa can be treated with cortisone injections to reduce the swelling and inflammation. Padding the area is also useful to reduce the pressure from the shoes. Selecting the correct shoe can also be helpful (See selecting the appropriate shoe). Surgical correction of the tailor's bunion is a common procedure to reduce the prominence and the pain associated with it (See Tailors bunion surgery). A rare occurrence of gout can also occur in this area. This condition usually presents with a sudden onset of pain and swelling in the area.
Further down, along the side of the foot another bony prominence can observed. This area is the base of the long bone (metatarsal bone) behind the fifth toe called the styloid process of the fifth metatarsal. In children this area has the growth plate to the bone. Injury to the area can cause the area to become enlarged. X-rays will show an open growth plate until the age of 12 - 14. If there has been an injury to this area in a child with an open growth plate, caution must be taken because of the possibility of injury to the growth plate. A very powerful tendon also attaches into this area of the bone. In the event of an ankle sprain this area can become injured as a result of the tendon attempting to prevent the foot from twisting. Pain to the touch at this site indicates either tendon or bone injury, and an x-ray should be taken. A fracture can occur in this area, and if it is displaced or out of alignment then surgery may be required to repair the bone. If the fracture is in good alignment or if the area is just sprained, then a below the knee cast should be applied. It may take six weeks for this injury to heal.
Some people have a natural enlargement of this area. When painful, padding the area to reduce shoe pressure is useful and selecting the correct shoe is also helpful. Rare instances of gout can also occur in this area.
Just behind the area of the styloid and a bit over to the top of the foot there is a spongy, soft area of the foot. This area represents the extensor digatorum muscle. This is the only muscle present on the top of the foot. In some people it can be quite large. Fatty deposits can also occur about the muscle making the area appear larger. On occasion, a ganglion may form. In this instance, the area may enlarge and then shirk and then enlarge again. A MRI is the most useful test to determine if a ganglion is present. If the area is not painful, no treatment is required. If the area is painful and a ganglion has been identified, then treatment might consist of draining the fluid from the ganglion with a needle or surgically removing the ganglion.
Small, soft lumps can be observed around the fat pad of the heel when a person stands. These small, soft bumps are usually found in groups. They are called Piezogenic papules and represent herniations of fat from the heel fat pad. They are rarely painful and no treatment is necessary. Removal of the pziogenic papules should be discouraged. There is no adequate treatment for their removal, and surgery to the area will only make the condition worse.
Another lump seen on the side of the heel is called an inclusion cyst. This lump appears as an elevated, soft mass which is often callused on is surface. These are caused by minor puncture wounds or from prolonged pressure or friction to the area by shoe gear. Surgical excision is often recommended. Padding the area to reduce friction may be of some temporary benefit in painful situations.
Other less common lumps and bumps that can appear on the outside of the foot are lipomas, fibromas, and giant cell tumors. These are non-cancerous tumors that tend to slowly enlarge and have a spongy like consistency. They only present a medical problem if they interfere with the function of a tendon, press excessively on a nearby nerve or cause pain secondary to shoe pressure. Malignant tumors of the foot are exceedingly rare.