Without proper footwear and technique, running can create stress injuries in the foot as well as in the joints of the lower body. Just like a good mechanic can diagnose an imbalance in your car's suspension and wheel alignment by analyzing your tires' tread wear, you can read the bottom of your old running shoes to determine what you should look for when buying your next pair.
Look at the "little toe" side of your running shoes. Wear on the outer edges of your shoe soles is indicative of supination. If your foot supinates when you run, your ankles don't roll inward when your foot strikes the ground and the brunt of the impact is borne by the outer edge of your feet. If your feet supinate, buy well-cushioned shoes when you make your next purchase. The extra cushioning should help alleviate some of the force that is traveling up your leg and could lead to stress fractures. Avoid stability shoes.
Look at the front part of the shoe sole. Excessive wear from the ball of the foot to the toes is evidence of overpronation. In this situation, your foot is rolling too far toward your big toe when your foot hits the ground. This wear pattern is common in runners who have flat arches or do not have enough stability in the hip or knee joints, thus resulting in the excessive inward rotation of the foot. A replacement running shoe should have outstanding arch support to limit the degree of inward roll.
Look at the heel and middle area of the shoe sole. Smooth tread under the ball of the foot and across the heel is characteristic of a normal gait, with an even transition being made from heel to toe with each foot strike. As a result, your weight is evenly distributed throughout your stride. Buy the same type of running shoe.