1. Wisdom Teeth
It is named wisdom teeth because they normally came out in the age of 17-21 and usually in the very back of your teeth. Two on the top and two on the bottom. They are also the part of adult 32 teeth.

2.The molars that missing 

Wisdom teeth are molars, the strongest teeth that you use to grind food, but not everyone has all wisdom teeth. In adults mount they are the most commonly missing 

3.  The reasons why are the taken out 

Most likely to be the impaction because when they come out, but there is not enough space for the teeth to come out. It will push the gums or other teeth that may cause the pain. Also when it push other teeth and relocate all the teeth make them not in a proper shape. 

4.  The issues
The wisdom tooth that has the sign of disease should be taken off such as an Infected tooth, cavities, cause to damage the teeth nearby, not enough space to clean or blush.

5. A number of problems

The underlying tooth which under the gum can destroy the tooth nearby by the impact and the root has been eaten. The bacteria and plaque can build up around the tooth that under the gums or only some part out.