The interior structure of shoes break down over time, not just their exteriors. You can't fix that without tearing the shoes apart and rebuilding them from the inside. The reason this is worth noting is that, as the whole structure deteriorates it can cause your gait to change in compensation, throwing your spine out of alignment and causing nasty physical problems. A simple analogy would be what can happen to a pitcher in basball whose mechanics are thrown off by a small injury.

New shoes may cost some money, but there comes a time when the cost of keeping old, broken down shoes is more expensive than throwing them away and starting new. If you develop odd pains in your hips, back, legs, knees or ankles, suspect your shoes.