The toes are vital to walking and stability (see Anatomy of the Foot and Ankle). Toe deformities are commonly caused by a muscle imbalance in the foot as a result of flatfeet, traumatic injury, or disease of the toe joints.
There are at least six sets of muscles that control each toe. Two tendons (the extensor digitorum longus and the extensor digitorum brevis) join on the dorsal aspect (top) of the toes and insert into the middle and distal phalanges (bones) of each toe. On the bottom (plantar aspect) of the toes, there are two muscles that, instead of joining like the extensors, remain separate.
Each muscle stabilizes one of the bones in the toe. The flexor digitorum longus muscle attaches to the bone at the end of the toe (distal phalanx) and the flexor digitorum brevis attaches to the middle bone (middle phalanx). The lumbrical and interossei muscles stabilize the inner toe bone (proximal phalanx). If there is an imbalance in the foot, the smaller muscles can be overpowered by the larger flexor and extensor muscles.
If a foot is flat (pronated), the flexor muscles can overpower the others because a flatfoot is longer than a foot with a normal arch. When the foot flattens and lengthens, greater than normal tension is exerted. The toes are not strong enough to resist this tension and they may be overpowered, resulting in one of two deformities, depending on which muscle contracts first.
If the flexor digitorum brevis contracts first and overpowers the rest of the muscles in the toe, the middle phalanx (middle bone) is pulled downward, causing the joint between the proximal and middle phalanges to buckle upward. This is known as a claw toe.
If the flexor digitorum longus contracts first and overpowers the smaller muscles, it can pull on the proximal phalanx, causing the outer two joints of the toe to bend downward. This results in a hammertoe.
Incidence of toe deformities increases with age and ranges from about 2–20%. Claw toes and hammertoes are approximately 5 times more common in women.