what shall i do please to have high magnification inside pulp canals with high magnification more than 25x

Buy a suitable microscope. No DSLR lens will provide the level of magnification you need, particularly at a focal distance that makes sense for dentistry. Macro lenses get a lot of their ability to magnify just from being able to focus at very short distances -- it's like looking closely at something by holding it close to your eye. Beyond the obvious physical and hygienic problems with sticking a lens into a patient's mouth, getting close enough to get the view you want inside a small space will make it impossible to get enough light on the subject.

For dentistry, you need something that'll give you good magnification from far enough away to be hygienic, and also far enough to give you room to work. There are microscope models made specifically for dentistry that can give you the magnification you want from a distance of 200-300 mm. They often have their own light source, and they sometimes include a camera as well. I expect that the high cost of some of those microscopes can be a problem, but it looks like there are also models that aren't so expensive.