What is A 12 year old female patient came to the dental clinic for a dental check up, but is mainly concerned about the outward appearance of the upper front teeth.  On discussion with the parents, they revealed that the patient had a habit of thumb sucking till 9 years of age. Which of the following statements is least explanatory for the constricted maxillary arch in such cases ? and explain in detail?
Although negative pressure is created within the mouth during sucking, there is no reason to believe that this is responsible for the narrowing of the maxillary arch that usually accompanies sucking habits.
Instead, arch form is affected by an alteration in the balance between cheek and tongue pressures. If the thumb is placed between the teeth, the tongue must be lowered, which decreases pressure by the tongue against the lingual of upper posterior teeth. At the same time, cheek pressure against these teeth is increased as the buccinator muscle contracts during sucking. 
Cheek pressures are greatest at the corners of the mouth, and this probably explains why the maxillary arch tends to become V-shaped, with more constriction across the canines than the molars. A child who sucks vigorously is more likely to have a narrow upper arch than one who just places the thumb between the teeth.