The sesamoid bones are tiny little bones found within tendons that are located in areas of the body where the tendon passes over a joint. Their function is to assist the tendons mechanically, by holding the tendon further away from the joint to make movement easier, essentially acting as a fulcrum. They also provide a degree of protection for the tendons, helping it to achieve a more consistent pull. In contrast to cartilage in the joint which helps to cushion shocks and take the weight, sesamoid bones are purely involved with muscle activity. They can be found at various locations around the body, such as the knee, wrist, hand and foot with the later especially prone to injury. The location of the sesamoid bones in the foot is directly behind the joint of the big toe in the ball of the foot, where the two pea sized bones sit.
The location means that the bones do have a weight bearing role when walking and especially when running and jumping. Injuries occur most commonly during exercise and sports when the bones are required to work with greater forces, with any activity which requires burst speed or jumping more likely to trigger an injury. However, even relatively low impact exercises such as golf can trigger the condition, due to the force placed on the bones throughout the swing.
The most common condition which affects the sesamoid bones is sesamoiditis, which is generally considered a fatigue injury. When the feet get tired from overuse, the muscles and ligaments fail to work correctly, placing a high strain on the sesamoid bones and tendons. Sesamoiditis is the inflammation of the bones and the tendons in which they are located, which is typically felt as a persistent dull ache in the ball of the foot behind the big toe.
Due to their position, the sesamoid bones are placed under greater strain by any footwear which raises the heel, and as such it is a condition associated with wearing high heels and stilettos. The sesamoid bones can also become injured more easily with people who have high arches, which place a greater strain on the bones. High arches prevent the feet from working as efficiently, and instead of the arch cushioning properly, the body weight and force is moved forward to the sesamoids and toes.
Runners and joggers who display a degree of pronation, or who over pronate are also more likely to suffer from sesamoiditis, with the strain on the bones being greatly increased.
In some cases, injuries result from physical abnormalities such as when the bones are larger than normal, which again requires them to take excessive force. As with any weight bearing bones, it is possible to suffer from stress fractures, with a repeated force placed on the bones, and sometimes a single trauma. This is most common with people suffering from osteoporosis, when the bones are weaker. The sesamoid bones are actually the hardest form of cartilage, however even this is not infallible and over time the cartilage can become degraded, leading to osteoarthritis.
To lessen the shock on the bones, custom or over the counter orthotic insoles and metatarsal pads can be of great benefit, both in the prevention and treatment of injuries. Correct footwear with adequate underfoot support and cushioning is vital, as is increased arch support, or arch inserts, for anyone with high foot arches. Changing over to shoes with a lower heel, a wide toe box, and footwear which is the correct length will also ease the strain on the bones.