Multiple studies have indicated that removing premolars can aid in managing and in some cases, lowering the vertical dimension by decreasing the mandibular plane angle. This idea is tied to the "wedge effect," which suggests that extracting premolars allows the posterior teeth to shift forward. As a result, the overall vertical skeletal height or mandibular plane angle may be controlled or reduced by minimizing the wedge-like influence of the posterior teeth.

What does the evidence say?

(Old studies)

• Sassouni and Nanda found that premolar extractions led to a reduction in facial height when compared to non-extraction treatment

• Garlington and Logan showed that enucleating second premolars leads to a reduction in the mandibular plane angle

• JA Staggers "On average, orthodontic treatment in both groups produced an increase in the cephalometric vertical dimensions that were examined."

(More recent studies)

• Aranuchalam and Ashima found no reduction in overbite , her conclusion in her study "Extraction of teeth only to increase the overbite or decrease the mandibular plane angle might not be justified"

• Hafiza et al. actually found that the vertical dimension increased in the extraction group! however it was mentioned in the conclusion that this was mainly due to a lack of attention given to orthodontic mechanics and If caution was taken to control the extrusion of molars after premolar extraction, there will most likely have been no significant change in the vertical dimension of orthodontic patients.

• Georgious et al. in a Systematic Review also examined the vertical dimension with premolar extractions compared with non-extraction in a systematic review and his was conclusion was "4 premolar extractions has no specific effect on the skeletal vertical dimension. Thus, an extraction treatment protocol aiming to reduce or control the vertical dimension does not seem to be an evidence-based clinical approach."

References:

1. Cephalometric assessment of dentofacial vertical changes in Class I subjects treated with and without extraction (Arunachalam Sivakumar , Ashima Valiathan)

2. Effect of orthodontic treatment with 4 premolar extractions compared with nonextraction treatment on the vertical dimension of the face: A systematic review (Georgios Kouvelis , Konstantinos Dritsas , Ioannis Doulis , Dimitrios Kloukos , Nikolaos Gkantidis )

3. Vertical Skeletal Changes after Extraction and Nonextraction Orthodontic Treatment (Hafiza Z Shafique , Rumeesha Zaheer , Abdullah Jan , Alaina T Mughal , Rooma Shahid , Fareena Ghaffar , Tooba Zahoor , Sundas Mehmood , Ramsha Nawaz , Safia Umar , Mehak Hassan , Muhammad A Mudasser )

4. Changes in the Vertical Dimension After Orthodontic Treatment in Response to Different Premolar Extraction Patterns(Hamad Burashed)

5. First or second premolar extraction effects on facial vertical dimension (Tae-Kyung Kim , Jong-Tae Kim, James Mah, Won-Sik Yang, Seung-Hak Baek)

6. Molar intrusion in the management of anterior openbite and ‘high angle’ Class II malocclusions (Cousley)

7. Vertical changes following first premolar extractions (J A Staggers)

8. Vertical changes following orthodontic extraction treatment in skeletal open bite subjects (Aynur Aras)

9. Early tooth extraction in the treatment of anterior open bite in hyperdivergent patients (Márcio Antonio de Figueiredo , Danilo Furquim Siqueira, Silvana Bommarito, Eduardo Kazuo Sannomiya, Larry W White)

However Most of the previous Studies investigated the extraction of the 1st premolars

Extracting 2nd Premolars..

If the "wedge effect" theory is accurate, it would make sense to assume that removing second premolars being closer to the posterior wedge might result in a noticeable reduction in vertical dimension and mandibular plane angle after treatment compared to extracting first premolars.

• A study that investigated the difference between first and second premolar extraction was the study by Kim et al. and no significant difference was found between the 1st premolar extraction and the 2nd premolar extraction , his conclusion was " The hypothesis that P2 extraction in hyperdivergent facial types will result in mesial molar movement and decrease FVD by reducing the wedge effect is invalid."

• Hamad Burashed also investigated the extraction of the 1st premolar vs the 2nd premolar on the mandibular plane and vertical facial height , and the result was that "Extracting first premolars versus second premolars during orthodontic treatment does not influence changes observed to the mandibular plane angle or vertical facial height."

• Also Deguchi et al., 2011; Gkantidis et al., 2011 Mentioned that recent research refutes the ‘wedge-effect’ hypothesis

What about extraction of 1st Molars ?

Based on my research on the topic ,studies on the extraction of the 1st molars are limited , i was only able to find one Prospective study and the rest were case reports

• Aynur Aras Examined the difference between 1st premolar , 2nd premolar and 1st molar extraction in young patients who had a skeletal open bite and her conclusion was that the 1st premolar extraction had no significant effect on mandibular rotation

  However the second premolars and the first molars led to a closing rotation of the mandible in skeletal anterior open bite extending to the posterior teeth

• (Márcio et al) in a case report which investigated the extraction of upper and lower 1st permenent molars and 2nd primary molars for a patient with hyperdivergent (dolichofacial) pattern, Class II Division 1 malocclusion, and anterior open bite and his conclusion was "extraction of permanent or primary maxillary posterior teeth at an early age may be a good option for hyperdivergent patients with excessive posterior dentoalveolar height."

Conclusion:

• Most recent evidences refutes the Wedge effect

• The findings of this article are coincident with one of the statements Dr S Chamberland mentioned in one of his blog posts that The extraction of premolars may reduce, even close an anterior dental open bite, i.e by incisor extrusion and retroclination but will have a small or no effect at all on the skeletal vertical relationship.

• "The extraction of the first molars will have an effect on the closing of the open bite only if they are the only teeth to touch. In adults, this will most likely have a small vertical effect if any at all. In the young patient, it is possible to reduce the skeletal vertical dimension if the extraction is performed before the second molars and the premolars are in occlusion."

• So extraction of molars treatment regime is going to need careful case selection for them to work , other than that an extraction decision should be considered to fix the dental open bite or camouflage skeletal open bite but not to reduce an open bite by decreasing the vertical dimension and mandibular rotation.

What is meant by the wedge effect ?

The concept of the “wedge effect” is predicated on the belief that when premolars are extracted, posterior teeth may be able to move mesially, reducing the “wedge effect” and either controlling or reducing the vertical skeletal height or mandibular plane angle

If the concept of mesial movement of molars to reduce the "wedge effect" and decrease the facial vertical dimension (FVD) holds true, its valid to examine the impact of first (P1) and second premolar (P2) extractions on FVD. (Kim et al)

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