Interlabial gap and freeway space at rest position: a cephalometric study

Maximum intercuspal position (MIP) has been defined as the complete intercuspation of opposing teeth independent of condylar position.1 Physiologic rest position (RP) has been defined as the assumed mandibular position when the head is in an upright position and the involved muscles are at equilibrium in tonic contraction.1 This position has provided a mandibular reference for analysis and reconstruction of the occlusion.2 At RP, the teeth are slightly apart and the perioral soft tissue and mandibular posture are both unstrained.3

Zachrisson4 demonstrated that the single most important diagnostic record for treatment might be facial photographs of the lips at RP because it will help compare the amount of maxillary incisor display when the patient is functioning. Yogosawa5 suggested that a pre-treatment relaxed lip posture offered a framework for the prediction of post-treatment facial profile change. Burstone6,7 considered that the most functional and reproducible position was a relaxed lip position which may be used as a guide for tooth positioning. Zachrisson4 and Dindaroglu et al.8 also achieved high reproducibility of rest position by instructing the patient to say “Emma”.

The interlabial gap (ILG)6,9,10 is the space between the relaxed upper and lower lips. It represents the shortest linear dimension between the inferior surface of the upper lip and the superior surface of the lower lip. Burstone reported that the average ILG at RP was 3.7 mm in adolescents.6 Bergman et al.9 further suggested that the average ILG at RP was 4.0 and 3.0 mm in females and males at age 6, respectively, and 2.0 mm at age 18 years. Ghorbanyjavadpour and Rakhshanet10 demonstrated that an appropriate ILG was essential for good profile aesthetics.

Freeway space (FWS) or inter-occlusal distance is the difference in the vertical dimension at RP and MIP. Several authors2,11–17 have reported variable average FWS values from 1.0 to 10.0 mm depending on facial type, malocclusion, and measurement methods. Impinging upon normal FWS can result in undue stimulus to the stretch reflex of the masticatory muscles, leading to excessive muscle contractions which may affect the functional occlusion.14 Therefore, orthodontists need to evaluate FWS in each patient and establish a treatment plan for appropriate vertical control.

Previous studies have assessed the soft tissue profile to emphasise the importance of proper ILG and FWS.10,14 However, to the best of current knowledge, there have been few comprehensive reports on ILG and FWS evaluated by lateral cephalograms at RP. Therefore, the present study aimed to assess the amount of interlabial gap (ILG) and freeway space (FWS) according to gender, age, and skeletal pattern, using cephalometric measurements at MIP and RP to determine changes from MIP to RP relative to the amount of ILG and FWS. The null hypothesis would state that there is no difference between groups related to gender, age, skeletal patterns, or the initial amount of ILG and FWS at MIP and RP.

The ILG was greater in males than in females but without significance, while FWS was significantly greater in males than in females (P < 0.01). The ILG (P < 0.05) and FWS (P > 0.05) were greater in adolescents than in adults. The ILG was smaller and FWS was greater in hypodivergent patients than in hyperdivergent and normovergent patients. The ILG was the greatest in Class II subjects without significance while FWS was significantly greater in Class III than Class I and in Class II subjects (P < 0.05) (Table III).

At MIP, the ILG at RP increased as overjet (P < 0.05) and upper lip to the aesthetic line (P < 0.01) increased (Table IV). At RP, the ILG increased as upper incisor exposure and the lips to the aesthetic line increased (P < 0.001), and FWS decreased as overbite decreased (P < 0.001) (Table V).

From MIP to RP, lip length showed the greatest decrease (P < 0.001) in the large ILG group. Additionally, Bjork sum (the sum of the saddle, articular, and gonial angles), the mandibular plane angle, anterior facial height, and ANB angle showed the greatest increase (P < 0.001), while OB showed the greatest decrease (P < 0.001) in the large FWS group (Table VI). The lip competent group showed the greatest frequency distribution in the small ILG and FWS groups. The frequency distribution of the smile line showed no relationship with the amount of ILG and FWS (Table VII).

The vertical dimension (VD) at RP was determined using facial measurements of soft-tissue landmarks,19 but several authors20,21 have concluded that facial measurements using skin markers are inappropriate for determining FWS because the vertical movement of the mandible is not always equal to the movement of the facial soft tissues. Therefore, the present study used lateral cephalograms at MIP and RP to more accurately measure ILG and FWS, and, in addition, the reproducibility of RP in each patient was confirmed by superimposing a profile photograph over a lateral cephalogram (Figure 1).

In the present study, the amount of ILG and FWS was greater in males than in females and the average ILG and FWS were 3.6 and 1.9 mm, respectively. This supported the results of previous studies.6,9,16,17 A longitudinal study9 of cephalometric soft tissue profile traits in patients between the ages of 6 and 18 years suggested that the ILG decreases with age. Peter et al.22 also found that the ILG decreased from childhood to adolescence. However, in the present study, the ILG increased from childhood to adolescence without significance but significantly decreased from adolescence to adulthood. The possible reason why these results varied from previous studies9,22 might be because the current study was cross-sectional in nature and the age ranges were different. Therefore, a longitudinal study with a greater sample size might be necessary to overcome this limitation.

Iwahashi et al.23 suggested that an increased OJ affected lip competence, which could be improved by decreasing the OJ. This finding was similar to the present results in which the ILG increased as OJ increased and the lip incompetent group was proportionally greater in the large ILG group. Leonardo et al.24 indicated that a long AFH was observed in the group with lip incompetence. In the present study, AFH was greatest in the large ILG group which related to lip incompetence. Hassan et al.25 found that the lip incompetent group had a smaller interincisal angle (IIA) along with a greater dentoalveolar height (DAH) and ANB angle, and U1 to FH, which reflected the present results. Lip competence was more prevalent in the small and medium ILG and small FWS than in the other groups. This might be explained by a more advantageous position and structure of the lips in those groups when the patients had an ability to comfortably seal their lips.

Lindegard26 found that FWS was negatively correlated with AFH but showed a positive correlation with PFH. Peterson et al.27 and Michelotti et al.17 concluded that the low MPA group had a greater opening of FWS at RP. Wessberg et al.28 found that FWS showed a negative correlation with vertical dentofacial morphology at RP. In addition, in the present study, FWS was greater in hypodivergent patients than in hyperdivergent and normovergent patients.

Sakar et al.29 found that the differences in the closest speaking space which is similar to FWS were not significant between horizontal skeletal patterns, while in the present study, FWS was significantly greater in Class III patients than in Class I and Class II patients. Furthermore, the ILG was the greatest in Class II patients but without significance and also smaller in hypodivergent patients than in hyperdivergent and normovergent patients. However, there have been few studies directed at the relationship of the ILG to skeletal patterns. Therefore, more studies should be considered.

In the present study, mento-labial sulcus depth (MLSD) had a negative but insignificant correlation with the ILG. This might be due to the frequency of lip redundancy, and the consequent deepening of the mentolabial sulcus in the small ILG group.6 Bishara et al.30,31 suggested that retruded lips and a small lip thickness (LT) caused a reduced ILG. In the present study, an increased upper lip to E-line (UL-EL) and lower lip to E-line (LL-EL) distances were related to a large ILG but an increased LT was not associated with high ILG.

Burstone7 believed that UIE during rest or smiling would provide a valuable reference for orthodontic tooth movement. It was demonstrated that UIE and the ILG should be considered together with lower-face vertical dimension. In the present study, the ILG increased as UIE increased which indicated that UIE had a significant effect on the ILG. Burstone7 suggested that a relaxed lip position was reproducible, but it was difficult for a posed smile to be reproduced. There can be a large variation in smiles, even in the same patient. Therefore, a smile cannot be used as an accurate guide for tooth positioning and treatment planning.4,7 Similarly, in the present study, the smile line was not correlated with the ILG and FWS.

Previous studies32–35 have evaluated the characteristics of the posed smile which were classified based on UIE and upper lip position into three categories: “low smile” (less than 75% of UIE), “average smile” (75–100% of UIE and the exposure of the interproximal gingiva), and “high smile” (exposure of a band of contiguous maxillary gingiva). Peck et al.34 concluded that factors including anterior vertical maxillary excess, greater muscular ability to raise the upper lip upon smiling, excessive overjet and overbite, and excessive ILG at RP were related to a high smile, and ULL, MPA, and PPA were not associated with the gingival smile line. In the present study, the smile line was not related to the amount of ILG and FWS. The distribution of patients was evaluated using different smile lines according to the ILG and FWS, and it was determined that the average smile line was more prevalent than other smile lines in keeping with the findings of previous studies.32,33

It has been hypothesised that the cephalometric correlations between the ILG and FWS, and several variables could have clinical implications. However, the present study has the common limitation of a small sample size. Due to the ethics of exposing patients to unnecessary radiation doses, the number of patients was, of necessity, kept as low as possible. However, it may be of greater benefit to the patient if a better diagnosis is determined. Patient co-operation in achieving RP was another limitation, especially for children. Patients with an airway problem were excluded, but airway problems are able to affect ILG and FWS, so a future study should be considered to evaluate the changes of ILG and FWS in association with disordered breathing. The present study was prospective but static and so it would be beneficial to investigate the various functional perioral soft tissue movements to enhance orthodontic diagnosis and treatment. Therefore, a future study is recommended to assess the dynamic functional state by applying a similar three-dimensional comparison of pre- and post-treatment outcomes to help clarify the various modality treatment effects on the ILG and FWS.

Cephalometric measurements at MIP and RP were evaluated and compared.

The ILG (P > 0.05) and FWS (P < 0.01) was greater in males than in females.

The null hypothesis was rejected but there were some differences identified between the groups. Knowing the cephalometric measurements at RP would be helpful to more accurately evaluate the ILG and FWS and to achieve a more precise diagnosis and a predictable treatment plan.

The authors declare that there is no conflict of interest.