Effects of Four First Premolar Extraction on the Upper Airway Dimension in a Non-Growing Class I Skeletal Patients: A Systematic Review

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Effects of Four First Premolar Extraction on the Upper Airway Dimension in a Non-Growing Class I Skeletal Patients: A Systematic Review


Jae Yong Choi1* and Kenneth Lee2

1Dentist, Orthodontic and Dentofacial Orthopaedics Speciality Masters Program Universitat Jaume I (Spain), Bachelor of Dental Science (University of Queensland), Australia

2Professor Universitat Jaume I, Castellon, BDS (Syd), MSc Oral Implantology (Goethe), MSc Orthodontics (Castellon), FICD, FPFA, Private practice, Sydney, Australia

*Corresponding author: Jae Yong Choi, Student, Jaume I University, Orthodontic and Dentofacial Orthopaedics Specialty Masters Program, Brisbane, Australia.
Citation: Choi JY, Lee K. (2022) Effects of Four First Premolar Extraction on the Upper Airway Dimension in a Non-Growing Class I Skeletal Patients: A Systematic Review. J Oral Med and Dent Res. 3(1):1-16.

Received: March 22, 2022 | Published: May 10, 2022

Copyright© 2022 by Choi JY, et al. All rights reserved. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Objective: Orthodontic treatment aims not only to treat one's malocclusion and facial aesthetics but also to maintain or improve the patient's airway patency. This review aims to evaluate post-treatment changes in the position of hyoid bone and oropharyngeal airway size and dimensions associated with fixed orthodontic treatment with four first bicuspid extractions in non-growing Class I skeletal bimaxillary protrusion individuals.
Methods: Electronic databases including Embase, Web of Science, PubMed, and Scopus were used to research published articles. Included studies assessed the post-treatment effects of four first bicuspid extractions with maximum anchorage on pharyngeal airway dimensions in non-growing patients. Relevant data were obtained, summarised, and analysed from the included studies.
Results: Six articles were selected in this systematic review after meeting the requirements of the inclusion criteria.
Conclusions: Four bicuspid extractions followed by maximum anchorage involves predominant retraction of the anterior segment of the arches in non-growing skeletal Class I bimaxillary protrusion cases. Extraction of four first premolars led to retraction of the anterior segment and the retroclination of anterior teeth, which narrowed the pharyngeal airway dimensions. After such treatment, the hyoid bone position changes remain inconclusive, which warrants further studies.


Airway; Bicuspid extraction; Bimaxillary protrusion; Hyoid bone; Non-growing; Orthodontics


The airway, also known as the respiratory tract, is a vital anatomical structure responsible for airflow during ventilation [1]. The airway is subdivided into two zones: the upper airway and the lower airway [2].  Anatomically, the upper airway can be divided into three sections: nasopharynx, oropharynx, and laryngopharynx, which serve a vital function of human survival-breathing [3] (Figure 1). Among those, the oropharynx is the narrowest part of the airway. It is also the most predisposed to change during and or after the orthodontic treatment. Respiratory disorders such as obstructive sleep apnoea (OSA) can arise because of upper airway constriction.

Figure 1: A diagram of the upper airway (pharynx). Three sections: nasopharynx, oropharynx, and laryngopharynx. The hyoid bone is also shown. ©2020 Terese Winslow LLC, U.S. Govt. has certain rights.

OSA is one of the sleep-breathing disturbances caused by collapsing of the upper airway during sleep, characterised by cessation of airflow with persistent respiratory effort, oxygen de-saturations, sleep arousals and sleep fragmentation [4]. Although OSA is a multifactorial disorder associated with obesity, age, facial morphology, airway collapsibility, and neuromuscular feedback, the size of the oropharyngeal airway is highly associated with OSA severity [5-12].

Recent research exhibited relationships between narrowed upper airway and obstructive sleep apnoea due to retrognathic mandible, high mandibular plane angle, and dorsally positioned tongue [13,14]. Although extraction orthodontic treatment is not limited to bicuspid extractions, it has been the choice for treating dental crowding cases for a long time. Since the birth of orthodontics, the extraction versus non-extraction orthodontic treatment method has been a long-discussed topic. However, over the last few decades, there has been a paradigm shift from extraction orthodontics, a traditional method of solving dental crowding, to a more functionally driven non-extraction approach. This emphasises the correction of malocclusion (how the study models occlude) and stability and the soft tissue profile aesthetics, the health of temporomandibular joints (TMJ), and the upper airway volumes [14,15].

It is well known that the bicuspid extraction technique is used to solve dental crowding issues. It is also a well-known fact that bicuspid extractions can reduce dental arch lengths due to retraction mechanics resulting in retraction and retroclinication of lower incisors [14-16]. Williams et al. [17] reported that approximately 66.5% of the available premolar extraction spaces were taken up by anterior segment retraction with lower incisor retraction. The rest of the space was taken up by posterior segments drifting mesially [17].  An anchorage design must be established during the diagnosis when orthodontic treatment is planned with extractions. However, not all extraction orthodontic treatment is the same. Extraction space closing mechanics can be divided into different four types [18] (Figure 2).
1. Group A: anchorage technique involving 25% of anchorage loss from the posterior segment and 75% retraction of the anterior segment,
2. Group B: anchorage technique involving an approximately equal amount of posterior and anterior movements, 
3. Group C: anchorage technique involving 75% of protraction of the posterior segment and 25% of retraction in the anterior segment,
4. Absolute: anchorage technique involving 100% retraction of the anterior segment, also known as the maximum anchorage.

Figure 2: Anchorage classification: Group A space closure includes, on average, 25% of posterior anchorage loss and 75% of anterior retraction; Group B space closure includes more equal amounts of anterior and posterior tooth movement; Group C space closure includes, on average, 75% posterior protraction and 25% of anterior retraction. Absolute anchorage involves 100% retraction of the anterior segment [18].

It was shown that when maximum anchorage (absolute) mechanic design was used for the space closure, narrowing of the upper airway dimensions was resulted [19]. First bicuspid extractions with maximum anchorage design are commonly used in skeletal Class I bimaxillary protrusion cases to improve the soft-tissue protrusive profile. [20]. Some studies suggested that extraction orthodontics, which involved retraction of teeth, thus altered upper airway dimension, predisposed patients to sleep breathing disturbances such as obstructive sleep apnoea (OSA) [13-15]. The reduction in dental arch lengths in sagittal dimensions contributed to a decrease in airway volume and oral cavity dimension resulting in posterior displacement of the soft palate and the tongue. 

Fukuda et al. [21] clinically found that orthodontic extraction patients displayed a higher Apnoea-Hypopnea Index (AHI) than untreated controls. On the other hand, Larsen et al. [19] showed no difference between patients who underwent extraction orthodontic treatment and the controls. While changes in hyoid bone position and decrease in airway dimension were reported after orthodontic extractions [20,22,23]. Other studies contradicted the findings where no changes were detected in hyoid bone position and the airway space following extraction orthodontic treatment [24-26].

Such inconsistent conclusions about whether extraction orthodontic impacts the upper airway may be due to natural craniofacial and pharyngeal growth that we see in growing patient samples. During this period, the rapid growth of the airway may partly disguise the effect of extraction on the changes in pharyngeal airway dimension. Varying growth rates in adolescent patients may lead to misinterpretation of the actual impact of dental extraction on the airway [27,28]. On the other hand, multiple studies reported that the upper airway ceased its growth in adult patients [19,24,29].


Above all, the most crucial aspect of the treatment objective should be improving or maintaining a patient's health. Each patient who will walk into our clinics will have their chief concerns that we will try to address and help. As treating orthodontic clinicians, we are trained and often hardwired to look at teeth only and undoubtedly, we straighten and solve malocclusion very well. However, we often forget about the possible negative impact our treatment may have on the patient's health, such as compromising one's airway, hence one's health. As doctors, we should never forget that, above all, we should avoid causing harm to our patients. 

This systematic review attempted to explore the effects of maximum retraction of anterior teeth using maximum anchorage mechanics on pharyngeal airway space and hyoid bone position after four first bicuspid extractions with anterior incisor retractions in Class I skeletal bimaxillary protrusions non-growing individuals.

Materials and Methods

This systematic review was conducted with a study design including comparative studies analysing the association between four bicuspid extractions with maximum anchorage involving anterior incisor retractions and upper airway dimension in skeletal Class I bimaxillary protrusive non-growing patients. Participants included in this review included patients with extractions and non-extraction orthodontic treatment. Eligible selected studies assessed the changes in the hyoid bone position and upper airway dimension as outcome measures.

Search Strategy

This systematic review was conducted using relevant publications from January 2006 to November 2021, from the following electronic databases: PubMed, Web of Science, Scopus and Embase. The keywords used to search these following databases are outlined in Table 1. The reference lists from included articles were manually searched using their full titles.

Table 1: Search strategies for each database.

Study Selection

All articles searched were reviewed by reading titles and abstracts to eliminate and exclude irrelevant articles.
Selection Criteria
The inclusion criteria are as follows: 
(1) Human Studies
(2) Study Type: retrospective clinical studies or randomised controlled trials
(3) Research Sample: late adolescents and adults with skeletal Class I bimaxillary protrusion without limitation to gender or race. 
(4) Intervention: fixed orthodontic treatment involving four first bicuspid extractions with maximum anchorage and retraction of anterior incisors. The method used to achieve the maximum anchorage was not a critical factor in decision making. The intervention must include either two-dimensional or three-dimensional pre-treatment and post-treatment radiographic evaluations. 
(5) Comparator: Non-extraction or no treatment. 
(6) Outcome: soft tissue changes in linear and angular measurements in lateral cephalogram and soft tissue changes measured in 3D using computer tomography (CBCT).
The exclusion criteria are as follows: 
(1) Animal Studies
(2) Study Type: case reports, editorials, opinions, letters
(3) Research Sample: orthodontic treatments other than interventions mentioned in the inclusion criteria, including orthopaedic treatment, mandibular advancement, bimaxillary protraction, orthognathic surgery and patients undergoing growth modification were excluded.
(4) Patients with previous orthodontic treatment history.
(5) Patients with medical histories including pharyngeal pathology, pre-existing OSA, craniofacial deformities, adenoidectomy, tonsillectomy, allergies, nasal obstructions, and chronic mouth breathing.

Data Collection and Analysis

Data Collection

The following data were collected: author, year of publication, study design type, imaging modality, participant information including sample size, gender and age, skeletal and dental malocclusion, teeth extracted, anchorage design (mechanics), outcome and conclusion. Dimensional measurement changes in any region of the pharyngeal airway, which included nasopharynx, oropharynx, and laryngopharynx, were collected. Only measurements from immediate pre-treatment and immediately post-treatment were accepted.

Quality Assessment

This systematic review used the Joanna Briggs Institute’s Critical Appraisal Checklist to assess a study's methodological quality and address any possible bias in its study design, conduct, and analysis. The result of the checklist is described in Table 2, with each question answered as either “yes”, “no”, or “unclear".

Table 2: Assessment of methodological quality.


The article selection process is depicted in Figure 3. The initial total number of citations found was 196 from PubMed, EMBASE, Web of Science and Scopus electronic databases.  A total of 115 duplicates were removed. Further exclusion of 46 articles was completed after screening the titles and abstracts. Thirty-three articles were then selected for full-text assessment. After the full-text appraisal, 26 articles were excluded due to the following reasons:
Article in a different language [30], sample age below 16 years old [24,31-34], Different premolar extractions [35,36], Different anchorage designs [25,37-43], Different malocclusion [44], Non-retrospective studies [16,19,22,45-50].

Figure 3: Flow Diagram for systematic search and article selection process.

Six eligible retrospective study articles were selected in this systematic review [20,22,23,51-53]. Table 2 summarises the quality assessment of the selected eligible articles. Table 3 outlines the characteristics of the included studies. The primary outcomes and conclusions from selected articles are summarised in Table 4. All six articles studied skeletal Class I bimaxillary protrusion cases involving four first bicuspid extraction using maximum anchorage mechanics, which resulted in retraction and retroclination of maxillary and mandibular incisors. Extraction indications were identical in these studies [20,22,23,51-53].

Table 3: Characteristics of selected articles.