Infraocclusion of first permanent molars (FPMs) occurs when a tooth fails to reach the occlusal plane, often due to ankylosis, eruption failure, or mechanical obstruction (Kurol, 1981). This can lead to tipping of adjacent teeth, supraeruption of opposing teeth, and occlusal disturbances (Proffit et al., 2018). Recent research also links unilateral infraocclusion to vertical skeletal asymmetry, emphasising the need for early diagnosis and intervention.
Aetiology of Infraoccluded First Permanent Molars
Infraocclusion of FPMs has a multifactorial aetiology, with contributing genetic, mechanical, and environmental factors.
1. Ankylosis – The most common cause (Kurol, 1981). It may result from trauma, infection, or genetic predisposition affecting bone metabolism.
2. Primary Failure of Eruption (PFE) – A genetic defect (PTH1R mutation) affecting the eruption pathway, causing non-ankylosed eruption failure that does not respond to orthodontic forces (Frazier-Bowers et al., 2007).
3. Mechanical Obstruction – Physical barriers to eruption, including overlying dense cortical bone (Baccetti, 2000).
4. Bone Overgrowth & Systemic Conditions – Localised excessive bone formation or conditions like Cleidocranial Dysplasia, Hypothyroidism, and Osteopetrosis can impair eruption (Proffit et al., 2018).
5. Genetic & Familial Predisposition (Bjerklin & Kurol, 1983).
Classification of Infraoccluded First Permanent Molars
1. Severity-Based (Kurol & Magnusson, 1984):
• Mild: Slightly below occlusal plane.
• Moderate: Below contact points, requiring intervention.
• Severe: Subgingival, leading to occlusal and periodontal complications.
2. Aetiology-Based (Baccetti, 2000):
• Ankylosed Infraocclusion: Fusion to bone.
• Non-Ankylosed Infraocclusion: Eruption failure due to mechanical/genetic factors.
3. Timing-Based (Proffit et al., 2018):
• Early: Before root completion.
• Late: After root maturation.
Management Strategies
1. Monitoring (Kurol, 1981): Mild cases require observation but no immediate treatment.
2. Orthodontic Extrusion (Baccetti, 2000): Fixed appliances and TADs can reposition non-ankylosed FPMs.
3. Occlusal Build-Up (Kjær, 2004): Composite/onlays/crowns restore occlusion in moderate cases.
4. Surgical Luxation & Forced Eruption (Lindqvist & Thilander, 1982): Osteotomy can break ankylosis for orthodontic repositioning.
5. Extraction & Space Closure (Proffit et al., 2018): Required for severe, non-restorable cases.
New Evidence: Infraocclusion and Facial Asymmetry
A 2025 AJODO study by Fishman et al. found that unilateral infraocclusion of FPMs is linked to skeletal asymmetry, assessed using the Vertical Asymmetry Index (VAI).
1. VAI >3% indicates one side of the face is growing differently, increasing skeletal imbalance risk.
2. Age-dependent asymmetry suggests infraocclusion worsens with growth if untreated.
3. Early intervention can prevent long-term facial asymmetry and reduce the need for surgical correction.
Understanding the Vertical Asymmetry Index (VAI)
VAI is measured using cephalometric radiographs or 3D imaging by comparing vertical distances of skeletal landmarks (e.g., Menton, Gonion, Condylion) on both sides relative to a horizontal reference plane. The VAI is calculated as the percentage difference between the right and left sides, with a value >3% indicating clinically significant asymmetry.
Conclusion
Infraocclusion of first permanent molars requires early recognition and management to prevent malocclusion and skeletal asymmetry. The Vertical Asymmetry Index (VAI >3%) serves as a diagnostic tool for detecting early skeletal imbalances, reinforcing the need for proactive orthodontic intervention. Early correction minimises long-term complications and optimises facial symmetry and occlusal function.
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