During time that the core of the mandible rotates forward an average of 15 degrees, the mandibular plane angle, representing the orientation of the jaw to an outside observer?

During the time that the core of the mandible rotates forward an average of 15 degrees, the mandibular plane angle, representing the orientation of the jaw to an outside observer, decreases only 2 to 4 degrees on the average. The reason that the internal rotation is not expressed in jaw orientation, of course, is that surface changes (external rotation) tend to compensate. This means that the posterior part of the lower border of the mandible must be an area of resorption, while the anterior aspect of the lower border is unchanged or undergoes slight apposition. Studies of surface changes reveal exactly this as the usual pattern of apposition and resorption. On the average, then, there is about 15 degrees of internal, forward rotation and 11 to 12 degrees of external, backward rotation producing the 3 to 4 degree decrease in mandibular plane angle observed in the average individual during childhood and adolescence. Ref: Contemporary orthodontics Proffit , 4th ed pg 116