Incompetent lip morphology is commonly related to increased facial height, particularly in the lower third of the face. This elongation leads to difficulty in lip closure at rest due to a vertical disproportion between the maxilla and mandible. The increased vertical dimension causes the lips to stretch and fail to achieve proper contact, resulting in "lip incompetence." This is a critical consideration in orthognathic surgery, as correcting the vertical facial height can restore lip competence and improve both function and aesthetics. Proper assessment of facial proportions is essential for surgical planning to address lip incompetence effectively.

Reference: Contemporary Orthodontics, 6th Edition.