Technical Summary: Positional and Dimensional Characteristics of the Mandible in Lebanese Class II Division 1 Skeletal Malocclusion Patients

Problem Statement

Class II Division 1 malocclusion is a prevalent condition, affecting approximately 19.1% of the Lebanese population. While the condition is well-documented globally, individuals presenting with the same malocclusion classification often exhibit heterogeneous cephalometric features, including maxillary protrusion, mandibular retrusion, or a combination of both. A critical challenge in orthodontic diagnosis is distinguishing between positional retrognathism (where the mandible is displaced posteriorly due to cranial base morphology) and true dimensional mandibular deficiency (where the mandible is physically smaller).

Currently, there is a lack of specific data characterizing these positional and dimensional subtypes within the Lebanese population. Without this specific morphological classification, treatment planning may be suboptimal, as positional and dimensional deficiencies require different therapeutic approaches (e.g., functional appliances vs. growth modification or surgery). This study addresses this gap by aiming to classify mandibular retrognathism in Lebanese Class II Division 1 patients to support accurate diagnosis and treatment rationale.

Methodology

This research employed a cross-sectional study design involving 120 subjects (60 males and 60 females) aged 18–30 years, all of Lebanese heritage for three generations. Participants were recruited from the outpatient orthodontic clinics of Beirut Arab University and met strict inclusion criteria:

• Skeletal Class II Division 1 malocclusion (ANB > 4°).
• Mandibular retrognathism (SNB < 78°).
• Normal maxillary position (SNA ≈ 82°).
• Permanent dentition with no missing teeth, craniofacial disorders, or prior orthodontic history.

Data Collection and Analysis:
Standardized lateral cephalometric radiographs were analyzed using the Coben coordinate system to express facial structures in proportional terms rather than absolute values. The analysis included:

• Angular Measurements: Cranial base angle (N-S-Ba), Gonial angle (Ar-Go-Me), SNA, SNB, ANB, and FMA.
• Linear Measurements: Total effective cranial base length (Ba-N), Ramus length (Ar-Go), Mandibular body length (Go-Pog), and Total effective mandibular length (Ar-Pog).
• Proportional Ratios: To normalize mandibular dimensions against cranial base size, ratios were calculated: Ar-Go/BaN, Go-Pog/BaN, and Ar-Pog/BaN.

Statistical analysis was performed using SPSS version 25.0. Data normality was assessed via the Kolmogorov–Smirnov test, and group comparisons (gender and against normal ranges) were conducted using independent t-tests with a significance level of p < 0.05.

Key Contributions

The primary contribution of this study is the establishment of a morphological classification for mandibular retrognathism specifically within the Lebanese Class II Division 1 population. By utilizing proportional ratios relative to the cranial base, the study isolates true dimensional deficiencies from positional discrepancies. The research provides a gender-specific breakdown of cephalometric deviations, challenging or confirming existing assumptions about sexual dimorphism in this specific malocclusion type.

Results

The study yielded several consistent findings across the sample:

1. Universal Dimensional Reduction:

   • 100% of participants exhibited reduced ramal height and reduced mandibular body length relative to the cranial base.
   • 90.8% of the total sample demonstrated a reduction in total effective mandibular length.
   • These reductions were statistically significant when compared to normal ranges for both males and females.

2. Cranial Base Morphology:

   • 100% of the sample showed an increased total effective cranial base length (Ba-N).
   • 65% of males and 50% of females exhibited an increased cranial base angle (N-S-Ba).
   • The increased cranial base length and angle suggest a posterior positioning of the glenoid fossa, contributing to the apparent mandibular retrusion.

3. Gender-Based Dimorphism:

   • Statistically significant differences between genders were confined to cranial base parameters. Males exhibited a significantly greater total cranial base length, while females demonstrated a significantly larger cranial base angle.
   • No significant gender differences were found in mandibular dimensions (ramus length, body length, or total effective length) or the gonial angle. Both sexes showed similar patterns of mandibular reduction.

4. Gonial Angle:

   • The gonial angle did not show a consistent tendency toward increase. In fact, females showed a significantly smaller gonial angle compared to normal ranges, while males showed no significant deviation. This suggests the gonial angle plays a minimal role in the etiology of Class II Division 1 in this population.

5. Correlation:

   • Pearson correlation analysis revealed no significant relationship between the cranial base angle and the three mandibular dimension variables (ArGo/BaN, ArPog/BaN, GoPog/BaN).

Significance and Clinical Implications

The paper concludes that the mandibular deficiency in Lebanese Class II Division 1 patients is characterized by a universal reduction in mandibular dimensions (ramus and body) rather than being solely a positional issue caused by cranial base morphology. While the cranial base is elongated in these patients, the mandible itself is dimensionally smaller.

Clinical Significance:
The authors emphasize the necessity for clinicians to differentiate between:

• Positional Retrognathism: Which may respond favorably to functional appliance therapy.
• True Dimensional Mandibular Deficiency: Which may necessitate growth modification during the mixed dentition phase or orthognathic surgical intervention in skeletally mature patients.

The study supports the use of early functional appliance therapy to enhance mandibular growth, given the identified dimensional reduction. However, it modestly notes that the findings are specific to the Lebanese population and that the lack of gender variation in mandibular size simplifies diagnostic criteria for this specific demographic, though cranial base differences must still be accounted for.