Lip Incompetence and its Relationship to Skeletal and Dental Morphology—an Electromyographic Investigation

Cephalometry is widely used by orthodontists in assessing the growth pattern or the effect of orthodontic procedures on the cranio-facial skeleton. While a static position of the related soft tissue may be measured from the same cephalometric radiograph, the orthodontist would have a more comprehensive picture of the presenting malocclusion if quantitative measurements of facial muscle activity were available. rn investigating the relationship between lip activity and the form and positions of the jaws and teeth, research workers and clinicians have sought methods of measuring muscle activity quantitatively. Pressures from the cheeks, lips and tongue and their effect on the dental arches have been recorded by Winders (1956), Luffingham ( 1969) and Proffit et al. ( 1964) using intra-oral pressure gauges and by Gould and Picton ( 1962) using the strain gauge while Marx (1965) chose to study the circumoral musculature using an electromyographic technique. The purpose of this paper is to examine the relationship between the circumoral musculature and skeleto-dental factors and test the hypothesis suggested by Walther (1967) that the causes of lip incompetence are: