Introduction
The objective of this study was to identify the soft-tissue profile changes and the potential pretreatment cephalometric parameters that clinicians could use to predict the lip response after premolar extraction treatment in adult patients.
Methods
Pretreatment and posttreatment lateral cephalograms of 75 white patients treated with premolar extractions were analyzed. The following initial cephalometric measurements were recorded: upper and lower lip to E-plane, vermilion thickness, lip length, maxillary and mandibular incisor inclination, and mentolabial and nasolabial angle. Pretreatment and posttreatment radiographs were superimposed using the Björk structural method to record lip retraction and incisor/lip retraction ratio. Pearson correlation and Kruskal-Wallis tests were used to compare lip retraction and incisor/lip retraction ratio with the cephalometric variables. The sample was divided according to different extraction patterns.
Results
The mean upper and lower lip retraction values were 1.4 mm and 1.7 mm, respectively. Vermilion thickness showed a negative and statistically significant correlation ( P <0.05) with lip retraction and incisor/lip retraction ratio. In addition, the mean incisor/lip retraction ratio was 61% and 98% for the upper and lower thin lip, respectively, whereas the mean incisor/lip retraction ratio was 17% and 44% for the upper and lower thick lip, respectively. The comparison among extraction patterns did not highlight any noticeable difference.
Conclusions
The choice of a specific extraction pattern did not impact lip response. The vermilion thickness was the key factor influencing lip retraction: an increase in this parameter was related to a decrease in lip retraction and vice versa.
Highlights
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The choice of a specific extraction pattern did not impact lip response.
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The vermilion thickness was the key factor influencing lip retraction.
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Vermilion thickness showed a negative correlation with lip response.
There have been considerable fluctuations over time in the rate of patients with extraction in orthodontic practice. Low percentages were reported in the early 1900s, followed by a gradual increase to a peak around the 1950s. The subsequent years have displayed a gradual decline to the present day. According to some authors, the introduction of nonextraction philosophies on the basis of indiscriminate arch expansion, such as the Damon technique and clear aligner therapy, may be responsible for this trend. This treatment approach may be perceived as easier for poorly trained clinicians, less invasive, and therefore more acceptable to the patient. Furthermore, this decrease in the frequency of extractions seems to be due to unwarranted clinician concerns about esthetics, stability, and temporomandibular disorder. However, the literature has now largely ruled out any potential negative impact of extractions on these factors.
Regarding the effect on soft tissues, the conclusion that extractions may result in a dished-in or more retruded profile has been reported as unacceptable. In fact, there is no support in the literature for the hypothesis that soft tissue acts as a passive drape that follows dental changes to a predictable degree. Studies have shown that the choice of extraction or nonextraction treatment does not cause variations in profile changes in the medium and long term. , Zierhut et al reported a gradual flattening of the facial profile in growing patients who underwent both extraction and nonextraction treatment, suggesting that maturational changes rather than extraction were responsible for this phenomenon.
However, predicting the behavior of soft tissue after extraction treatment is challenging because of the influence of various factors, including patient age, ethnicity, and skeletal, dental, and soft-tissue characteristics. , In addition, when attempting to predict the outcome of this treatment, key factors such as anchorage, crowding, soft-tissue thickness, and lip strain must be considered.
Because the behavior of soft tissues is still considered unpredictable, this study aimed to identify the profile changes and the pretreatment cephalometric parameters that clinicians could use to predict the lip response after premolar extraction treatment in adult patients. The null hypothesis was that there would be no correlation between lip response and cephalometric measurements.
Material and methods
The design of the present retrospective study was reviewed and approved by the Ethics Committee of the Postgraduate School of Orthodontics, Ferrara University (approval no. 10/2022).
The sample included patients with premolar extraction from 2 experienced orthodontists (E.A. and P.W.).
The inclusion criteria were as follows: white, aged ≥16 years at the start of treatment, no congenitally missing teeth (excluding third molars), presence of high-quality pretreatment (T1) and posttreatment (T2) lateral cephalometric radiographs with good soft-tissue definition. Radiographs needed to be taken with teeth in occlusion. The millimeter ruler was also required.
Patients who had undergone orthognathic/facial surgery (including facial and lip filler injections) and those with syndromes or craniofacial anomalies were excluded from the study.
Lateral cephalograms were digitally traced and analyzed by 1 examiner (L.B.) using Dolphin Imaging software (Dolphin Imaging and Management Solutions, Chatsworth, Calif). The initial cephalometric measurements analyzed were ( Table I ): (1) upper and lower lip to E-plane, (2) upper and lower vermilion thickness, (3) maxillary incisor inclination to the palatal plane (U1/PP), (4) inclination of mandibular incisors to the mandibular plane, (5) upper and lower lip length, (6) mentolabial angle, and (7) nasolabial angle.
| Measurement | Definition | Normal † |
|---|---|---|
| Upper lip to E-plane | Distance of the most anterior point of the upper lip to the E-plane | −6 ± 2 mm |
| Lower lip to E-plane | Distance of the most anterior point of the lower lip to the E-plane | −2 ± 2 mm |
| Upper vermilion thickness | Distance from the most labial surface of the maxillary incisor to the vermilion border of the lip | 13 ± 1 mm |
| Lower vermilion thickness | Distance from the most labial surface of the mandibular incisor to the vermilion border of the lip | 13 ± 1 mm |
| Maxillary incisor inclination to palatal plane | Angle formed by the long axis of the maxillary incisor and the palatal plane | 110° ± 5° |
| Inclination of mandibular incisors to mandibular plane | Angle formed by the long axis of the mandibular incisor and the mandibular plane | 95° ± 7° |
| Upper lip length | Distance between subnasale and the most inferior point of the upper lip vermilion | 23 ± 3 mm |
| Lower lip length | Distance between soft-tissue menton and the most superior point of the lower lip vermilion | 48 ± 4 mm |
| Mentolabial angle | Angle formed by lower lip anterior, soft-tissue point B, and soft-tissue pogonion | 122° ± 11° |
| Nasolabial angle | Angle formed from the subnasale, columella, and upper lip anterior | 102° ± 8° |
Subgroups based on diagnosis were considered for each cephalometric variable measured. For lip position, the retrusive, protrusive, and normal categories were considered. Thick, thin, and normal subgroups were used for lip thickness, whereas short, long, and normal subgroups were used for lip length. For the inclination of the maxillary and mandibular incisors, the categories assessed were proclined, retroclined, and normal, whereas, for both the nasolabial and the mentolabial angles, the categories defined were increased, decreased, and normal.
The T1 and T2 lateral cephalometric radiographs were digitally traced and superimposed by the same investigator using Adobe Illustrator software (version 26.0; Adobe, San Jose, Calif) according to Björk structural method. , Tracings were superimposed on the reference structures in the anterior cranial base; after that, the following measurements were recorded for both maxillary and mandibular counterparts: (1) incisor retraction (in millimeters): horizontal distance between incision points (the incisal edge of the incisor crown) at T1 and T2, and (2) lip retraction (in millimeters): horizontal distance between midpoints of the lip at T1 and T2. If these points were not located on the same plane, one of them was projected perpendicularly.
The millimeter ruler in the radiographs was used as a calibration reference to determine these horizontal distances in millimeter values. Furthermore, the ratio of incisor retraction to lip retraction (incisor/lip retraction ratio), expressed as a percentage, was also derived from these measurements.
Statistical analysis
For both the upper and lower lip, the mean and standard deviation of lip retraction and incisor/lip retraction ratio were calculated.
Pearson correlation test was used to compare the numerical variables of the incisor/lip retraction ratio and initial cephalometric measurements. However, Kruskal-Wallis tests were used to compare the incisor/lip retraction ratio and the subgroups of each of the cephalometric variables. In addition, these tests were used to compare the variables of lip retraction and vermilion thickness. Upper lip retraction and incisor/lip retraction ratio were compared with respective maxillary cephalometric variables; likewise, for the mandibular ones.
The sample was categorized into groups on the basis of different extraction patterns, and the Kruskal-Wallis test was used to compare the incisor/lip retraction ratio and lip retraction. Furthermore, in the case of test significance, pairwise comparisons with Bonferroni corrections were performed to identify pairs with different distributions. The use of Kruskal-Wallis nonparametric tests allowed accurate comparisons despite unbalanced groups.
The method error was determined using Dahlberg’s test. Radiographs from 10 patients were randomly selected, traced, superimposed, and measured at 14-day intervals by the same examiner. Because other similar studies in the literature employed different methods, a post hoc power analysis was performed to determine the reliability of the results obtained with the available sample size.
The level of significance was set at P <0.05 for all statistical analyses. Statistical analyses were performed using SPSS software (version 28; IBM, Armonk, NY).
Results
The study sample consisted of 75 subjects (53 females and 22 males) with a mean age of 22.5 ± 2.1 years at the start of treatment and an average treatment time of 24 ± 1.5 months. A total of 48 patients underwent 4-premolar extractions, whereas 27 patients underwent treatment with only maxillary premolar extractions ( Table II ).
| Group | n | Lip retraction, mean ± SD | Incisor/lip retraction ratio, % | ||
|---|---|---|---|---|---|
| Upper lip | Lower lip | Upper lip | Lower lip | ||
| Total sample | 75 | 1.4 ± 1.1 | 1.7 ± 1.2 | 39 | 71 |
| 2-Premolar extraction sample | 27 | 1.4 ± 1.0 | 1.2 ± 1.2 | 50 | – |
| 15,25 † | 18 | 0.8 ± 0.7 | 0.6 ± 0.7 | 53 | – |
| 14,24 † | 9 | 1.8 ± 1.0 | 1.6 ± 1.3 | 50 | – |
| 4-Premolar extraction sample | 48 | 1.4 ± 1.2 | 2.3 ± 3.3 | 32 | 71 |
| 15,25,35,45 † | 23 | 0.9 ± 0.9 | 1.6 ± 1.0 | 27 | 65 |
| 14,24,34,44 † | 25 | 1.8 ± 1.3 | 2.1 ± 1.4 | 37 | 77 |
† Tooth numbering according to the Fédération Dentaire Internationale system.
The method error, calculated from repeated superimposition and measurements, was 0.1 mm.
Table II shows the mean values of lip retraction and incisor/lip retraction ratio in the total sample and according to the extraction pattern.
Table III shows that the distributions were statistically different among different extraction patterns only for the variables of upper and lower lip retraction ( P <0.05), whereas they were not for the variables of maxillary and mandibular incisor/lip retraction ratio. However, only the pair 15,25-14,24,34,44 for lower lip showed statistically different distributions in pairwise comparisons ( Table IV ). No pair showed such distributions for the upper lip ( Table IV ).
| Variables | Retraction | Incisor/lip retraction ratio | ||
|---|---|---|---|---|
| Upper lip | Lower lip | Maxillary | Mandibular | |
| Test statistic | 11.037 | 11.163 | 5.062 | 1.154 |
| Degrees of freedom | 3 | 3 | 3 | 1 |
| Asymptotic significance (2-sided) | 0.012 ∗ | 0.011 ∗ | 0.167 | 0.283 |
| Sample 1-sample 2 | Test statistic | SE | Standard test statistic | P value | Adjusted P value † |
|---|---|---|---|---|---|
| 15,25-15,25,35,45 ‡ | |||||
| Upper lip | 2.838 | 8.481 | 0.335 | 0.738 | >0.999 |
| Lower lip | −20.389 | 8.522 | −2.392 | 0.017 | 0.100 |
| 15,25-14,24,34,44 ‡ | |||||
| Upper lip | 17.936 | 8.385 | 2.139 | 0.032 | 0.195 |
| Lower lip | −28.129 | 8.426 | −3.338 | <0.001 | 0.005 ∗ |
| 15,25-14,24 ‡ | |||||
| Upper lip | 19.611 | 8.806 | 2.227 | 0.026 | 0.156 |
| Lower lip | 19.833 | 8.849 | 2.241 | 0.025 | 0.150 |
| 15,25,35,45-14,24,34,44 ‡ | |||||
| Upper lip | 15.097 | 6.232 | 2.422 | 0.015 | 0.092 |
| Lower lip | 7.740 | 6.263 | 1.236 | 0.216 | >0.999 |
| 15,25,35,45-14,24 ‡ | |||||
| Upper lip | 16.773 | 6.788 | 2.471 | 0.013 | 0.081 |
| Lower lip | 0.556 | 6.821 | 0.081 | 0.935 | >0.999 |
| 14,24,34,44-14,24 ‡ | |||||
| Upper lip | −1.676 | 6.668 | −0.251 | 0.802 | >0.999 |
| Lower lip | 8.296 | 6.700 | 1.238 | 0.216 | >0.999 |
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