The metatarsals are the long bones of the feet. There are five of them, numbered 1-5. The first metatarsal is found at the base of the big toe, running into the middle part of the foot. The other metatarsals correspond with the bases of the other toes, all the way to the 5th toe. The part of the foot where the metatarsals sit can be a source of pain for many different reasons. Since the end of the metatarsals is located at the ball of the foot (base of the toes), they are subject to a great amount of pressure when the foot is propelling forward along the ground.
Pain where the metatarsals are located can be due to many different conditions, but most are related to how the foot is structured and interacts with the ground. Depending on the relative lengths of the metatarsals, excessive pressure can occur to the heads of one or more of the metatarsals. This can lead to long term inflammation of the joint tissue that surrounds the joint the metatarsal head creates with the toe bone, which is felt as pain in the ball of the foot. After a long time, the pressure can also lead to a stress fracture in the metatarsal bone. Both of these conditions are directly influenced by one's foot structure: high arches and flat feet allow for far more pressure to the ball of the foot than normal arched foot structures.
The joints the metatarsals create with the toes can also become arthritic because of damage, although this is fare more common with the big toe joint, in which the 1st metatarsal sits. Tears can form in the tissue under the metatarsal heads if the toe exerts too much backward pressure on it, leading to instability and toe deformity with severe pain in the ball of the foot. There are also numerous body-wide conditions, like rheumatoid arthritis and psoriasis, that can be the source of joint inflammation at the metatarsals, and subsequently cause broad pain in the ball of the foot.
The spaces in between the metatarsals can also be a source of pain. In this area are pads called bursal sacs that can become inflamed, and cause pain. Also, there are nerves in this part of the foot that can thicken and form a condition called a neuroma. This condition causes pain in the ball of the foot and can be mistaken for pain in the bone because it feels like there is hard spot under the foot.
Treatment of metatarsal pain is tailored around reducing the inflammation causing the pain, as well as reducing the pressure or abnormal foot structure that has led to the inflammation in the first place. Common treatment techniques include anti-inflammatory medication, steroid injections, icing, and orthotic inserts to control the foot structure. Surgery is also performed to reconstruct the metatarsal's position if non-surgical measures do not work.