Permanent Retainer. Teeth from shifting?

It would be difficult to provide an answer to your solution because what you are asking in a single paragraph has been taught to us through a 600 page book. But trying to put it in simple words, all I can say is that the retainers allow new bone formation around the new position of the teeth so that they do not relapse. The current position which has been achieved by orthodontic treatment includes proper proximal contacts between the teeth and they help to keep each other in position because splinting makes them behave like a single unit. Some of the tooth movements are self retaining and probably that is the reason retainers have not been advised for your upper jaw. But this is a tentative suggestion going only by the verbal description that you have mentioned and without a look at the clinical condition of the teeth and without knowing what treatment you have undergone till now. Regards.