Ultrasonic investigation of the circumoral musculature.

Real-time ultrasound with a high frequency probe is used to image the circumoral musculature with the lips relaxed and contracted. The ultrasonic images show that muscle tissue makes up only a part of the total thickness of the lip, and varies among individuals in shape, transonicity (clearness) and thickness. The muscle layer in the upper lip is sandwiched between layers of facial and lingual connective tissue, while the muscle image in the lower lip lies close to the lingual surface. In a relaxed state the muscle layer in the lower lip is significantly thicker than that in the upper lip, and a mean difference also occurs in contraction, but to a considerably lesser degree. Thus the upper lip muscle appears to increase in thickness during contraction relatively more than does that of the lower lip. Furthermore, the contraction in the upper lip seems to occur fairly uniformly along the lip from left to right. The muscular changes in the lower lip are more complex, even though embryologically complexity is ascribed properly to the upper lip. In the lower lip contraction seems to occur in the lateral parts and in the mentalis muscle, but in the midline the orbicularis oris muscle remains quite inactive. More complex arrangements are essential merely to ensure support of the lower lip against gravity.