What is the likelihood of impacted maxillary (upper jaw) third molars causing root resorption of second molars compared to impacted mandibular (lower jaw) third molars?

Root Resorption Risk: Maxillary vs Mandibular Impacted Third Molars

Impacted mandibular third molars are significantly more likely to cause root resorption of adjacent second molars compared to maxillary third molars, though maxillary impactions still carry substantial risk when specific anatomical conditions are present.

Comparative Risk Between Jaws

Mandibular Third Molars (Higher Risk)

• Mandibular impacted third molars demonstrate a higher overall prevalence of causing external root resorption (ERR) of second molars 1, 2.
• The mandibular location is associated with more frequent direct contact between third and second molars, which is the primary risk factor for ERR 2.
• Mesioangular and horizontal inclinations in the mandible are particularly problematic, showing the highest propensity to cause resorption 1, 3.
• In one study, 22.88% of mandibular cases showed ERR on CBCT imaging, though only 5.31% were detected on panoramic radiographs 1.

Maxillary Third Molars (Lower but Significant Risk)

• Maxillary impacted third molars cause ERR less frequently overall, but when present, the condition can be severe 4, 2.
• Half of maxillary second molars adjacent to impacted third molars showed some degree of ERR in CBCT studies (36.9% slight, 6.0% moderate, 7.1% severe) 4.
• Transverse inclination in maxillary third molars presents the highest risk for ERR in the upper jaw 4.
• Contact location matters significantly: apical third (73.7%) and middle third (60.6%) contacts show much higher ERR risk compared to cervical third (28.6%) 4.

Key Risk Factors by Location

Mandibular-Specific Factors

• Mesioangular and horizontal impactions carry the highest risk 1, 2.
• Presence of direct M2-M3 contact is strongly associated with ERR 2.
• Short corpus length combined with non-extraction orthodontic treatment increases risk 5.
• Root development stage of the third molar influences resorption potential 2.

Maxillary-Specific Factors

• Patient age is a significant factor, with older patients at higher risk 4.
• Transverse inclination is the most problematic position 4.
• Contact with apical or middle thirds of the second molar root dramatically increases risk compared to cervical contact 4.
• Maxillary location itself is associated with increased ERR frequency when contact is present 2.

Diagnostic Considerations

Imaging Limitations

• Panoramic radiography significantly underestimates ERR in both jaws, detecting only 5.31% of cases compared to 22.88% detected by CBCT 1.
• Agreement between panoramic radiographs and CBCT for diagnosing ERR is only 4.3% 1.
• Level I radiographic investigations (panoramic radiography) are not adequate for identification and characterization of external root resorption 6.

When to Use Advanced Imaging

• CBCT should be indicated when direct contact between second and third molars is observed on panoramic radiographs, especially with mesioangular or horizontal mandibular impactions 1.
• CBCT is essential for assessing relationships between third molars and critical structures including the mandibular canal and maxillary sinus floor 6.
• For maxillary cases, CBCT is particularly valuable when the third molar contacts the apical or middle third of the second molar root 4.

Clinical Management Implications

Prophylactic Extraction Considerations

• Given the propensity of mesioangular and horizontal mandibular third molars to cause ERR, prophylactic extraction should be strongly considered 1.
• For maxillary third molars with transverse inclination and contact with apical/middle root thirds, early surgical removal is warranted 4.
• In cases with short mandibular corpus length and non-extraction orthodontic treatment, regular radiographic monitoring or preventive extraction is necessary 5.

Monitoring Protocol

• Regular radiographic surveillance is essential when third molars are retained, as ERR can occur even in cases initially assessed as low risk 5.
• Orthopantomography should be the first diagnostic examination for suspected impaction 6, 7.
• CBCT should be reserved for cases where level I investigations cannot provide adequate information 6.

Common Pitfalls to Avoid

• Do not rely solely on panoramic radiography to rule out root resorption—it misses the majority of cases 1.
• Do not assume asymptomatic impacted third molars are safe; ERR progresses silently 5.
• Do not overlook maxillary third molars as benign; while less common than mandibular cases, maxillary ERR can be severe when present 4.
• Avoid delaying assessment in orthodontic patients, particularly those treated with non-extraction approaches 5.