Three-dimensional analysis of labial morphology: Effect of sex and age

Abstract

Morphological changes in the lips during ageing, including labial volume, area and thickness were assessed in healthy individuals. 40 healthy volunteers (20 men and 20 women) were selected and divided into two groups according to age: 21–34 years (‘youthful’ group) and 45–65 years (‘aged’ group). Three-dimensional (3D) stone labial models were made, digitized, and 3D virtual reproductions obtained. Labial thickness, vermilion area, and volume of the upper and lower lips were measured from the digital reconstructions. The data were compared using three-way ANOVA. All data were significantly larger in men than in women ( p < 0.05), the lower lip was thicker than the upper lip ( p < 0.001). A significant effect of age for labial thickness and area were found ( p < 0.001 and p = 0.014): the youthful group had higher values than the aged group. Upper to lower lip ratios were not significantly different between sexes or ages. Generally, young people have a larger lip area and thickness than aged ones and men had larger lips than women. The upper/lower lip ratios for area and volume were similar in the two genders.

The physical characteristics of an individual’s face have a significant effect on social life, because the face is the centre of attention. As the mouth is strongly connected with facial attractiveness, the lips play an essential role in facial beauty; size, shape, position and colour directly influence mouth morphology .

Lip morphology, teeth and gingival structural design are modified during smiling and speech, and are important aspects to be evaluated in orthodontics, dentofacial surgery and aesthetic dentistry . Changes in lip morphology, such as the position of the lip lines, lip thickness, lip tonicity, lip length, or a reduction in the vermilion border are a problem during ageing .

New concerns of beauty allied with recent therapies for the ageing face are being investigated. Various studies have shown that the characteristics, morphology and aesthetics of the lips are singular for different ages, places, and races. Research has verified that in the elderly, in the sixth or seventh decade of life, the smile becomes elongated and less pleasing . Several investigators have underlined the fact that fuller lips are signs of beauty and fertility . Thus, surgical and medical lip procedures are being used in aesthetic and cosmetic plastic surgery, as patients seek voluptuousness, sensuality, and youthfulness.

To help in diagnosis and treatment planning, different methods of analyzing lip morphology are used including lateral cephalograms, conventional profile and frontal photographs video images , and three-dimensional (3D) systems . Some of these allow 3D quantitative assessment of cutaneous surfaces without submitting the patient to ionizing radiation . Magnetic resonance and stereophotogrammetry appear the most promising techniques, but they cannot record the anatomy of the underlying supporting hard tissues effectively, impeding the assessment of the relationship between the soft and hard tissues. Also, magnetic resonance should be performed in a supine position, which causes a significant alteration in the normal relationships between the facial soft tissues, especially in the aged . Depending on the treatment plan, the quantitative analysis of labial anatomy should include assessment of the relevant supporting dental tissues . Currently, the relationship between occlusion, function and aesthetics is increasing, and an accurate appraisal of the relationship between soft and hard tissues is essential for a correct diagnosis and to provide the best treatment.

In a previous study, the authors developed a noninvasive method for the 3D analysis of labial characteristics as they related to the supporting dental tissues. The method allowed a detailed analysis of labial characteristics in all three dimensions, and a first set of normative data were provided for healthy young adult men and women. In the current study, the same method was used to quantify the morphological changes in the lips during ageing, and data on the labial volume, area and thickness in healthy men and women in their sixth decade of life were compared with those previously collected in young adults .

Materials and methods

A group of 40 healthy volunteers (20 men and 20 women) were selected and divided into two groups according to age: 21–34 years (‘youthful’ group; men, mean age 26 years, SD 4; women, mean age 25 years, SD 4) and 45–65 years (‘aged’ group; men, mean age 53 years, SD 4; women, mean age 54 years, SD 5). All subjects were Caucasians, and had sound, full permanent dentition including the first molars (at least 24 teeth in occlusion, with a complete eruption), with bilateral angle Class I first permanent molar and canine relationships, overjet and overbite ranging from 2 to 4 mm, no anterior cross-bite, no cast restoration or cuspal coverage, no previous or current orthodontic treatment, no previous craniofacial trauma or surgery, and no temporomandibular or craniofacial disorders.

All subjects gave their informed consent to participate in the experiment. All procedures were noninvasive.

Lip reproductions

The method is described in detail elsewhere . In brief, it comprised the reproduction of the dental arches, and of the cutaneous (external) and mucosal (internal) labial surfaces with stone models. The stone models were digitally reproduced, and several 3D measurements obtained .

For each subject, a maxillary and mandibular dental reproduction was obtained using an irreversible hydrocolloid (Tropicalgin, Zhermack SpA, Badia Polesine, Italy) and cast with a type 3 model dental stone (Elite Model, Zhermack SpA, Badia Polesine, Italy); the models were set in maximal intercuspidation. A 0.5 mm thick custom-fitted plastic tray (Plastiche Originali Brega, Effegi di Gelfiori F&C Sas, Sarmato, Italy) was built on the vestibular surface of each couple of dental models for the labial mucosa impression (internal surface) using a moulding procedure. Two orthodontic plaid wires were inserted in the tray through double small drill holes to maintain the relative position between the intra-oral and extra-oral parts of the impression. The mucosal labial surface reproduction was made by introducing the plastic mask in the oral vestibule; subsequently 3 ml of silicon impression material (President Plus, Coltène Whaledent, Altstatten, Switzerland) was put on the tray surface facing the mucosal labial area. The subject was asked to perform labial movements to distribute the impression material. Afterward, to obtain the cutaneous impression (external lip surface), a pre-made metal mesh was used to arrest the silicon impression material placed on the skin and the orthodontic plaid wires connected to the intraoral plastic sheet with the impression material. After the material hardened, the two parts of the impression were removed from the subject’s mouth and skin.

The labial models were cast with a type 3 model dental stone (Elite Model, Zhermack SpA, Badia Polesine, RO, Italy); for each subject, the casts (lips plus dental models) were put in the correct anatomic relationship ( Fig. 1 ).

Fig. 1
Dental (lateral view) and labial (lateral and frontal views) stone models.

A computerized electromechanical digitizer (Microscribe G2X, Immersion, San José, CA, USA) was used to make digital reproductions of the models . Calibration of the instrument was checked before each data collection session using a 3D object of known dimensions. A grid of standardized horizontal and vertical lines was drawn on each model: cutaneous labial surface, mucosal labial surface, and maxillary and mandibular vestibular dental surface. Several points on these lines were digitized using the digitizer, and the files of the 3D coordinates were elaborated using commercial software for 3D modelling (Rhinoceros 3.0, Robert McNeel & Associates, Seattle, WA, USA) ( Fig. 2 ). Additionally, the occlusal plane (passing through the maxillary interincisive and mesial first molar papillae) and the facial axes of the clinical crown of the maxillary anterior teeth were digitized. The 3D digital reproductions were combined in a single file.

Fig. 2
3D digital reconstruction of the labial cutaneous surface (red), labial mucosal surface (white), vestibular dental surfaces (blue), and occlusal plane (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Digital measurements

To avoid any error introduced by the presence of the plastic sheet, the distance between the mucosal virtual surface and the dental surface was automatically reduced by 0.5 mm.

To measure labial thickness (mm) slices were created parallel to the occlusal plane passing by the vermilion and mucosal surfaces using Rhinoceros software. Two different sections were prepared for each lip: superior lip: upper limit of the maxillary vermilion; and upper limit of the cervical borders of the maxillary incisors; inferior lip: lower limit of the mandibular vermilion and inferior fornix. For each lip, the mean value between the two sections was further computed. Lip thickness was measured as the shortest distance between each section of vermilion and mucosal surfaces, neighbouring incisors and canines .

For the vermilion areas (mm 2 ), the labial area was comprised between the upper and lower vermilion–skin interfaces . The cheilion-cheilion line separated the upper and lower vermilion areas.

To measure the labial volumes (mm 3 ), the vermilion border (cutaneous surface) was projected on the mucosal surface using the geometric centre of the occlusal plane as a focus. The volume between the mucosal and the cutaneous projection was calculated. The upper and lower lip volumes were obtained separately (limit: commissural line).

To measure the labial ratios (%) for each subject, the area and volume ratios were calculated between the superior and inferior lips:

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='Ratio(%)=100×upper−lowerupper+lower’>Ratio(%)=100×upperlowerupper+lowerRatio(%)=100×upper−lowerupper+lower
Ratio ( % ) = 100 × upper − lower upper + lower

where zero represents equality, positive numbers show a prevalence of superior lips and negative numbers an inferior dominance.

Statistical analysis

Means and standard deviations were computed for labial volumes and ratio, labial areas and ratio, and labial thicknesses, separately for men and women in the youthful and aged groups. Data analysis was carried out using SPSS software (SPSS 17.0, SPSS Inc., Chicago, IL, USA).

A mixed model ANOVA was applied for data evaluation (between subject factors: age, two levels, youthful and aged; sex, two levels, men and women; interaction: sex × age; within subject factor: lips, two levels, upper and lower lip; interactions: age × lips, sex × lips, lips × age × sex). For labial ratios, the within subject factor was omitted.

To assess age-related modifications better, correlation analyses with age were performed for all analysed variables.

For all analyses, a p value of 0.05 or less was used to assess statistical significance.

Results

On average, the aged group was significantly older than the youthful one, without sex differences or sex × age interactions (2-way ANOVA, age p < 0.001; sex, p < 0.05; interaction, p < 0.05).

Tables 1–3 report the descriptive statistics and the significance values obtained for the labial dimensions analysed; the 3D reconstructions of the lips of two men are reported in Fig. 3 . Labial thickness, area and volume were significantly larger in men than in women, as shown by ANOVAs (factor sex, all p values < 0.05). In all age and sex groups, the lower lip was significantly thicker than the upper lip (within subjects, p < 0.001), whilst no similar differences were found for labial area and volume.

Table 1
Descriptive statistics: mean and standard deviation (SD) of labial dimensions.
Lips Sex Age Thickness (mm) Area (mm 2 ) Volume (mm 3 )
Mean SD Mean SD Mean SD
Upper F Youthful 9.5 0.9 466.9 75.2 1742.8 392.1
Aged 7.9 1.8 432.7 72.5 1845.8 557.9
Total 8.7 1.6 449.8 74.0 1794.3 472.3
M Youthful 11.8 1.1 508.1 120.4 2459.8 719.1
Aged 10.1 0.8 444.3 98.0 2122.0 611.0
Total 10.9 1.3 476.2 111.8 2290.9 672.2
Total Youthful 10.7 1.5 487.5 100.0 2101.3 673.1
Aged 9.0 1.8 438.5 84.1 1983.9 586.8
Total 9.8 1.8 463.0 94.5 2042.6 626.1
Lower F Youthful 15.0 4.0 490.5 69.2 1764.3 569.7
Aged 10.3 2.1 400.4 83.4 1562.7 553.7
Total 12.7 3.9 445.4 87.7 1663.5 556.5
M Youthful 17.7 4.2 573.0 110.5 2949.7 677.8
Aged 11.4 1.7 506.6 84.6 2092.5 747.4
Total 14.6 4.5 539.8 101.6 2521.1 822.0
Total Youthful 16.4 4.2 531.7 99.2 2357.0 860.9
Aged 10.8 1.9 453.5 98.3 1827.6 695.5
Total 13.6 4.3 492.6 105.2 2092.3 817.7
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Feb 5, 2018 | Posted by in Oral and Maxillofacial Surgery | Comments Off on Three-dimensional analysis of labial morphology: Effect of sex and age
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