Effect of premolar extractions on occlusal contacts and chewing efficiency: A case-control study

Background

This study aimed to evaluate the effect of premolar extractions on masticatory performance and occlusal contact areas in patients who underwent orthodontic treatment with or without premolar extractions.

Methods

Seventy-five patients (aged 18-40 years), who had completed their treatment ≥6 months prior, were divided into groups: maxillary 2-premolar extractions (n = 22), 4-premolar extractions (n = 22), and nonextraction (n = 31). A control group of 31 individuals with no history of orthodontic treatment was also included. Posttreatment quality was assessed using plaster models evaluated according to the American Board of Orthodontics objective grading system (ABO-OGS). Intraoral scans obtained with the iTero Element 5D scanner (Align Technologies, San Jose, Calif) were analyzed in OrthoCAD (version 5.9.1.50; Align Technology, San Jose, Calif) to determine posterior occlusal contact areas (near and tight) at maximum intercuspation. Masticatory performance was evaluated using a gum chewing test. The chewed samples were scanned with a color desktop scanner, and color mixing was analyzed using ViewGum software (version 1.4; dHAL Kifissia, Greece).

Results

Significant differences were found between groups in terms of age, gender, ABO-OGS scores, and occlusal contact areas ( P <0.05), whereas masticatory performance did not differ significantly ( P >0.05). No significant correlations were found between ABO-OGS scores and occlusal contact areas, masticatory performance, or the time since treatment the lasted. Similarly, masticatory performance showed no significant correlations with contact areas and posttreatment time. A positive correlation was found between occlusal contact areas and time since treatment lasted (near contact: r = 0.440; tight contact: r = 0.429).

Conclusions

Although differences in occlusal contact areas were observed, premolar extraction did not appear to negatively affect masticatory performance.

Highlights

  • •

    Two-premolar, four-premolar extraction, nonextraction, and control groups were compared.

  • •

    Occlusal contacts analyzed by scans; masticatory performance by 2-color chewing gum.

  • •

    Premolar extraction affected only near occlusal contacts, not the tight ones.

  • •

    Masticatory performance was similar among all groups.

  • •

    Positive correlation between occlusal contacts and the time since treatment completion was observed.

The choice between extraction and nonextraction treatment strategies in orthodontics has been a subject of ongoing debate. Edward Angle, regarded as the founder of modern orthodontics, and his followers strongly opposed premolar extractions because of their invasive nature, advocating for the preservation of all teeth. However, evolving perspectives in orthodontics have gradually replaced this debate with evidence-based practices. Numerous studies have demonstrated that tooth extraction is an effective treatment option for various malocclusions, especially in patients with crowding, increased overjet, reduced overbite, and correction of incisor inclinations. When considering extraction-based orthodontic treatment, it is recommended to evaluate the effects on dentofacial structures, such as occlusal stability, arch form, and facial esthetics, and select the most appropriate approach tailored to the individual needs of the patient. ,

In extraction protocols, the first or second premolars are most commonly removed because of their favorable position on the arch and eruption timing. These teeth are ideally located to resolve anterior and posterior crowding, and their crown morphology allows the remaining premolars to establish proper contacts with adjacent canines and molars.

Regardless of whether an extraction or nonextraction protocol is employed, orthodontic treatment serves as a form of oral rehabilitation that affects all components of the masticatory system, a complex functional unit comprising the jaws, muscles, teeth, temporomandibular joint, and nervous system. Occlusion, defined as the contact between maxillary and mandibular teeth, plays a crucial role in this system. One of the primary goals of orthodontic treatment is to achieve ideal occlusion and ensure occlusal stability, which helps prevent relapse and eliminates premature contacts during maximum intercuspation and eccentric movements. ,

Indices evaluating orthodontic treatment outcomes are used to assess treatment success and quantify posttreatment changes. The American Board of Orthodontics (ABO) introduced the objective grading system (OGS), which evaluates posttreatment dental casts using a specially designed ruler based on 7 criteria (alignment, marginal ridges, buccolingual inclination, overjet, occlusal contacts, occlusal relationships, and interproximal contacts) and 1 criterion from panoramic radiographs (root angulation). A total score of ≤20 indicates successful treatment.

Although the ABO-OGS provides a standardized method to evaluate static orthodontic treatment outcomes, understanding how these outcomes relate to functional parameters, such as masticatory performance, is also essential. Several factors affecting masticatory performance have been investigated, including posterior tooth loss, prosthetic rehabilitation, occlusal surface area, number of posterior functional units, malocclusions, bite force, age, gender, salivary flow rate, and oral motor function. , Chewing function is closely related to the occlusal contact surfaces of posterior teeth. Once food enters the oral cavity, it is guided by the tongue toward the posterior teeth for fragmentation and softening. Larger food particles are broken down between the premolars and molars and mixed with saliva to form a bolus. Studies have shown that patients with ideal occlusion demonstrate higher masticatory efficiency than those with nonideal occlusion. , Patients with malocclusion tend to have smaller contact areas and reduced efficiency in food breakdown.

Masticatory performance can be assessed subjectively via questionnaires or objectively through standardized tests. Objective methods include sieve analysis, measuring color change or glucose loss in chewing gum, and evaluating the degree of color mixing. Two-colored chewing gum or paraffin wax is commonly used as a test material. The 2-color chewing gum method is a well-established, clinically applicable technique that provides reliable and reproducible quantitative data when analyzed with dedicated software.

Because of the limited number of studies comparing the effects of maxillary vs bimaxillary premolar extractions on occlusal contact areas and masticatory efficiency, a comprehensive investigation was warranted. The present study aims to evaluate the potential effects of premolar extractions on chewing efficiency and occlusal contact areas among patients treated with and without premolar extractions, as well as untreated controls. The findings may provide valuable insights for borderline extraction cases, particularly regarding the impact of treatment decisions on functional outcomes.

The null hypothesis of this study is that premolar extractions do not significantly affect occlusal contact areas or masticatory performance.

Material and methods

The protocol of our study was approved and followed by the noninterventional research ethics committee of Bezmialem Vakif University (decision No. 2022/308; date: November 11, 2022). The records of patients who underwent fixed orthodontic treatment between 2011 and 2024 were retrieved from the archives of the Department of Orthodontics at Bezmialem Vakif University. The patient files were checked, and patients who were considered to potentially meet the inclusion criteria based on their treatment methods were contacted and invited to the clinic for examination. The participants were examined by the researcher (M.A.), and the records of those who met the inclusion criteria were collected ( Fig 1 ; Table I ). Because occlusal contact changes may occur during the settling period after orthodontic treatment, only patients whose treatment had been completed at least 6 months prior were included in the study group. Informed consent forms were obtained from all participants included in the study. The research process was conducted in accordance with the principles of the Helsinki Declaration.

Fig 1

Flowchart of the study.

Table I

Inclusion and exclusion criteria

Inclusion criteria Exclusion criteria
Patients without systemic diseases Patients with systemic or neuromuscular diseases
Participants aged 18-40 y Patients who have undergone treatment with clear aligners
≥6 mo have passed since the completion of orthodontic treatment Patients outside the specified age range
Patients with an ABO-OGS score of <20 Patients with an ABO-OGS score >20
Patients without any dental pain, regardless of the cause Patients with temporomandibular joint disorders
Patients without missing teeth, except for premolars and third molars Patients with posterior restorations involving large chewing surfaces and cusps
Patients without dental crowns or fixed prostheses Patients with bruxism
Patients without implants Patients experiencing any pain in their teeth for any reason
Patients without caries who have caused tooth substance loss or fractures Patients who have had extractions of teeth other than premolars during orthodontic treatment

The study included 4 groups: group 1: patients treated with the extraction of 4 premolars, group 2: patients treated with the extraction of maxillary 2 premolars, group 3: patients treated without extraction, and group 4: participants having no orthodontic history.

Sample-size estimation was performed using preliminary data because the literature-based parameters produced unrealistically small sample-size requirements because of methodological differences and noncomparable outcome measures. Data from 10 patients who met the inclusion criteria were initially analyzed for each group. After evaluating the occlusal contact data of 40 patients, a power analysis was performed using the G∗Power software (version 3.1; Heinrich Heine Universität, Düsseldorf, Germany). Based on the observed group means and the pooled standard deviations (SDs) from this pilot sample, the calculated effect size for a 1-way fixed-effects analysis of variance (ANOVA) was F = 0.40 (The mean values of these variables were group 1: 6.27, group 2: 3.95, group 3: 9.55, and group 4: 8.02, with a within-group SD of 5.19). The analysis indicated that ≥18 patients per group were required to detect statistically significant differences with 80% power and α = 0.05. Considering potential dropouts or errors, a total of 106 patients were included in the study: 22 patients in group 1 (15 females and 7 males), 22 in patients in group 2 (18 females and 4 males), 31 patients in group 3 (19 females and 12 males), and 31 patients in group 4 (18 females and 13 males) formed the groups.

When participants came to the clinic, intraoral and extraoral photographs were taken using a Canon EOS 650D DSLR (Canon Inc, Tokyo, Japan) camera. Alginate impressions (Kromopan, Lascod, Italy) of the maxillary and mandibular jaws were taken using appropriately sized plastic trays, and dental cast models were fabricated. These dental casts were used for ABO-OGS scoring. Seven measurements from the ABO-OGS were performed on the orthodontic models using a specially designed ruler, based on the guidelines. The eighth measurement was conducted using a panoramic radiograph obtained at 64 kV and 6.3 mA with a Planmeca device (Helsinki, Finland). Scores were recorded on the OGS scoring sheet.

Digital scans were obtained using the iTero Element 5D (Align Technology, San Jose, Calif). iTero is a 3-dimensional (3D) scanner that uses parallel-confocal imaging technology and point-and-stitch reconstruction to generate digital impressions. The scans were performed with the patient lying supine in the dental chair, starting from the distal of the mandibular left buccal quadrant and progressing mesially with overlapping scan segments. Two bite registrations in maximum intercuspation were taken for both sides. For bite registration, 2 scans in maximum intercuspation were obtained for 10 patients, and consistent contact areas were observed; based on this, bite registrations for the remaining patients were obtained once, with careful attention to stability.

Measurements were conducted on the digital models by accessing the scans via the My iTero website and importing them into the OrthoCAD (version 5.9.1.50, Align Technology, San Jose, Calif) software. Using OrthoCAD (Align Technology), the intercanine width was measured between the cusps of the mandibular canines for calibration purposes. Occlusal contact areas in maximum intercuspation were identified using the occlusogram view feature, and screenshots were taken. The software calculates the distance between the occlusal surfaces of maxillary and mandibular teeth to determine occlusal contact positions. Contact areas were color-coded: red indicated tight contact, and orange indicated near contact ( red contact areas: <0 mm and orange contact areas: 0-0.2 mm). Contact distance values <0 mm, as reported by the software, do not indicate actual physical penetration between occlusal surfaces. Instead, they arise from algorithmic overlap and tolerance thresholds during surface alignment and should be interpreted as values below the detection limit of the software. The screenshots, saved in JPEG format, were imported into ImageJ software (version 1.52a; National Institutes of Health, Bethesda, Md) for Macintosh. In ImageJ (version 1.52a; National Institutes of Health), calibration was performed by drawing a line between the mandibular canine cusps and inputting the intercanine distance measured with OrthoCAD (Align Technology). Once calibrated, the borders of the red and orange contact areas on the mandibular molars (excluding third molars) and premolars were outlined using the freehand selections tool at 125% magnification, and the areas were calculated in square millimeters. Orange contact areas also included red contacts ( Fig 2 ). According to previous studies, digital models obtained from intraoral scans show significantly smaller deviations in the mandible compared with the maxilla; therefore, to avoid double-counting, only mandibular contact areas were recorded. After measuring the red and orange contact areas on each tooth, the average area per tooth was calculated.

Fig 2

Calculation of contact areas in square millimeters using ImageJ software.

For the assessment of masticatory performance, a 2-colored chewing gum test was used. The test gum, Vivident fruit swing with watermelon and acai grape flavors (Perfetti van Melle, Turkey) (43 × 12 × 3 mm), consists of 2 inseparable layers: 1 green and 1 purple ( Fig 3 ). Participants were asked to sit comfortably in a dental chair to ensure no changes in muscle tone and were instructed to chew for 20 cycles, as described in previous studies. , After chewing, the gum was carefully taken, dried with a paper towel, and placed into a resealable polyethylene bag. It was then flattened with a glass plate to a 1 mm thickness ( Fig 4 ). Flattening the chewed bolus is essential for eliminating high-contrast regions and ensuring accurate digital analysis of the mixed colors.

Fig 3

Unchewed 2-colored gum specimen.

Fig 4

Gum was chewed for 20 strokes and thinned to a thickness of 1 mm.

Both sides of the flattened gum were scanned at 300 DPI resolution using a desktop scanner (HP DeskJet Ink Advantage Ultra 4828 All-in-One; HP Inc, Palo Alto, Calif). Scanning was performed within 24 hours to prevent discoloration caused by saliva. A specialized software program, ViewGum (version 1.4; dHAL Kifissia, Greece), was used to evaluate the mixing degree. The assessment followed a method similar to those described by Schimmel et al. Results were presented as text and in a Gaussian curve graph in z-score format and were directly transferred to a Microsoft Excel (Microsoft, Redmond, Wash) spreadsheet. The H_SD (variance of hue) value was used for analysis.

All measurements were evaluated on a 13.3-inch monitor. Calibration and measurements were repeated twice by a single researcher, and the average values were used.

Statistical analysis

Data were analyzed using SPSS (version 28; SPSS Inc, Chicago, Ill) for Windows. A P value <0.05 was considered statistically significant at a 95% confidence interval. To assess intraobserver reliability, occlusal contact area measurements from 20 randomly selected patients (5 from each group) were reanalyzed 2 weeks after the initial measurement. The intraclass correlation coefficient was used to assess intraobserver agreement. The Shapiro-Wilk test was used to assess normality. The selection of statistical tests was based on normality assessments. Before the comparative analyses, the distribution characteristics of all variables were evaluated using the Shapiro-Wilk test. Age, ABO-OGS scores, occlusal contact area variables, and the duration elapsed after treatment demonstrated non-normal distributions; therefore, nonparametric tests were applied for these outcomes. Chewing-performance (2-colored gum mixing) data were normally distributed and analyzed using ANOVA after confirming homogeneity of variances with the Levene test.

One-way ANOVA, Kruskal-Wallis tests, t tests, Mann-Whitney U tests, and post-hoc Dunn-Bonferroni multiple comparison tests were used for intergroup comparisons. Chi-square tests were used to analyze gender distribution. Spearman correlation was used for correlation analysis, with coefficients interpreted as follows: 0-0.24 (weak), 0.25-0.49 (moderate), 0.50-0.74 (strong), and 0.75-1.00 (very strong).

Results

The comparison of age and gender distributions among the groups is presented in Table II . A statistically significant difference in gender distribution was observed only in group 2 ( P <0.05), whereas significant differences in age distribution were noted in both group 2 and group 4 ( P <0.05; Table II ).

Table II

Comparison of age and gender distributions among the groups

Demographic data Group 1 (n = 22) Group 2 (n = 22) Group 3 (n = 31) Group 4 (n = 31) Total (N = 106)
Min/Max Med Min/Max Med Min/Max Med Min/Max Med P value
Age, y 18/37.7 22.6 18/29.9 20.8 18.1/35.5 22.9 21.2/28.8 24 0.001
Gender
Female 15 68.2% 18 81.8% 19 61.3% 18 58.1% 0.002
Male 7 31.8% 4 18.2% 12 38.7% 13 41.9%

Note. The Kruskal-Wallis test was applied for age, and the chi-square test was used for gender.

Min , minimum value; Max , maximum value; Med , median value.

Post-hoc Dunn-Bonferroni multiple comparison test was used for pairwise comparisons: P 2-4 = 0.001∗∗.

Table III shows the comparison of the time elapsed after treatment and ABO-OGS scores among the groups. A statistically significant difference in the ABO-OGS scores was identified between group 2 and group 4 ( P <0.05; Table III ). No statistically significant differences were observed among the groups in terms of the duration elapsed since the completion of fixed orthodontic treatment ( P >0.05; Table III ).

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Jun 27, 2026 | Posted by in Orthodontics | Comments Off on Effect of premolar extractions on occlusal contacts and chewing efficiency: A case-control study

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