Lower lip changes after overjet reduction with and without mandibular incisor retraction

Four (2 maxillary + 2 mandibular) premolar (4P) and 2 maxillary premolar (U4) extractions are commonly employed for Class II malocclusion with excessive overjet. Although upper lip changes have been widely investigated, lower lip changes remain unclear. The soft-tissue responses after maxillary and mandibular incisor movements in each protocol were investigated.

A total of 90 pretreatment and posttreatment lateral cephalograms were digitized and allocated to 3 groups (n = 30 each) according to the initial overjet and treatment: normal overjet with 4P (NJ-4P), excessive overjet with 4P (EJ-4P), and excessive overjet with U4 (EJ-U4). The cephalometric parameters were compared, and the lip-to-incisor change ratios (upper lip-maxillary incisor, lower lip-mandibular incisor, and lower lip-maxillary incisor) were determined.

Soft-tissue parameters revealing significant changes among the groups included upper lip and lower lip positions, mentolabial area, and vermilion lip thickness. Significant differences were observed between NJ-4P/EJ-U4 for lower lip-mandibular incisor ratio and between NJ-4P/EJ-U4 and NJ-4P/EJ-4P for lower lip-maxillary incisor ratio but not for upper lip-maxillary incisor ratio.

Reduction in upper lip protrusion was smaller, whereas that in lower lip protrusion was greater after maxillary incisor and mandibular incisor retraction, respectively, in patients with excessive overjet and skeletal Class II malocclusion undergoing the 4P extraction than in those with normal overjet. Moreover, a reduction in lower lip protrusion can be expected with minimal retraction of the mandibular incisors in patients undergoing U4 extraction. The potential impact of the maxillary incisors on the lower lip, especially in excessive overjet malocclusion, should be noted during clinical examination.