Surprisingly, it seems that enthusiasm around breathing, premolar extraction, and CBCT technology has not yet been quenched. We have already seen a plethora of studies that attempted to measure the effects of premolar extraction on airway size using CBCT technology. It all started in the 2010s. Several research papers then claimed that premolar extraction can decrease the size of the upper airway [1,2]. Other papers, however, declared the absence of this correlation [3,4,5,6].
Last week, I came across a new study on this topic. This time the study concluded that premolar extraction may actually increase the size of the upper airway! Let’s take a look…
The paper is written by a team of academic orthodontists from China and one private practitioner from Spain.
For their study the authors retrospectively selected 57 Class II hyperdivergent adult patients, aged 20-35, who had first upper premolars and second lower premolars extracted during treatment. All the patients were Chinese. The authors divided the patients in two groups: those having normal mandibular length and those having short mandibular length (CoGn Chinese norms were used).
The aim of the study was:
“To compare the volume and minimal cross-sectional area (MCA) of the upper airway and hyoid position before and after the orthodontic treatment in hyper divergent patients with or without favourable mandibular length, and to retrospectively explore the possible underlying correlations between the upper airway and dentoskeletal morphology.”
The authors measured the upper airway size using three-dimensional images and the position of the hyoid bone using two-dimensional lateral cephalograms. All the radiographic images were taken “at the end of swallowing”.
The authors found “significant increase in the glossopharynx, hypopharynx, MCA, and anteroposterior linear distance in patients with favourable mandibular length”.
They also found that hyoid bone “moved forward and upward” after orthodontic extraction treatment in subjects with favourable mandibular length.
They attributed those “improvements” to the counterclockwise rotation of the mandible. According to the numbers provided by the authors, the magnitude of this rotation was about 2 degrees on average.
The authors concluded that “with a comprehensive diagnosis and treatment” premolar extraction in hyperdivergent adults can increase the upper airway volume.
What do I think?
I am sorry to say but this study has so many flaws that its findings look questionable.
First of all, it’s important to understand that CBCT is not a reliable method to measure the size of the upper airway.
CBCT images show only a snapshot of the airway, which undergoes constant changes during exhaling and inhaling, as well as changes between upright and supine body positions and between being awake and being asleep. Furthermore, the process of measuring the airway size demonstrates a great inter-operator error. For instance, a study [7] in which 6 independent operators were measuring the size of the upper airway of 10 patients showed that errors are so significant that they may be misinterpreted as treatment effect.
We also have to understand that the size of the upper airway doesn’t necessarily correlate with the presence or absence of breathing problems. Much more important is the activity of the muscles that determine the dilation of the upper airway during inspiration. Thus, subjects with a narrow upper airway and good muscle activity show no breathing problems. [8]
I am also skeptical regarding the magnitude of changes in the mandibular position the authors achieved. In their discussion they correctly pointed out that Class II hyperdivergent patients have a predisposition to breathing problems due to increased clockwise mandibular rotation [9]. Indeed, such patients sometimes have breathing problems so severe that they resort to orthognathic surgery. However I can not believe that just 2 degrees of mandibular counterclockwise rotation can result in any significant improvements of breathing.
Moreover, I have doubts regarding using the hyoid bone as a reference. Especially, “at the end of swallowing”. During swallowing this bone moves upward and forward [10], exactly the direction the authors claimed to move it with premolar extraction!
Finally, the study was retrospective and this introduces a potential selection bias.
Conclusion
I gather that this study was planned as an answer to nonsensical non-extractionists’ claims about the adverse effects of premolar extraction. However we should not oppose nonsense with more nonsense. We have to adhere to rigid science when we confront charlatans.
Today the golden standard to diagnose breathing problems is polysomnography. We have a rather recent brilliant study based on this method with a massive sample size of over 5000 patients unequivocally showing no correlation between four premolars extraction and breathing problems [11].
Despite the fact that I frequently see an “increase” of the upper airway on radiographs in my premolar extraction patients, I am perfectly aware that this is nothing but a coincidence. I will continue to extract premolars in some of my patients for the reasons of stability, facial esthetics, and periodontal health. Breathing is not on this list!
References
1. Wang Q, Jia P, Anderson NK, Wang L, Lin J. Changes of pharyngeal airway size and hyoid bone position following orthodontic treatment of Class I bimaxillary protrusion. Angle Orthod. 2012 Jan;82(1):115-21. doi: 10.2319/011011-13.1. Epub 2011 Jul 27. PMID: 21793712; PMCID: PMC8881045.
2. Zhang J, Chen G, Li W, Xu T, Gao X. Upper Airway Changes after Orthodontic Extraction Treatment in Adults: A Preliminary Study using Cone Beam Computed Tomography. PLoS One. 2015 Nov 20;10(11):e0143233. doi: 10.1371/journal.pone.0143233. PMID: 26588714; PMCID: PMC4654558.
3. Pliska BT, Tam IT, Lowe AA, Madson AM, Almeida FR. Effect of orthodontic treatment on the upper airway volume in adults. Am J Orthod Dentofacial Orthop. 2016 Dec;150(6):937-944. doi: 10.1016/j.ajodo.2016.05.013. PMID: 27894542.
4. Leslie CL, Harris EF. Oropharyngeal airway volume following orthodontic treatment: premolar extraction versus nonextraction: [master thesis]. Memphis, Tenn: University of Tennessee; 2014
5. Valiathan M, El H, Hans MG, Palomo MJ. Effects of extraction versus nonextraction treatment on oropharyngeal airway volume. Angle Orthod 2010;80:1068-74.
6. Stefanovic N, El H, Chenin DL, Glisic B, Palomo JM. Three-dimen- sional pharyngeal airway changes in orthodontic patients treated with and without extractions. Orthod Craniofac Res 2013;16: 87-96.
7. Jason N Zimmerman, Siddharth R Vora, Benjamin T Pliska, Reliability of upper airway assessment using CBCT, European Journal of Orthodontics, Volume 41, Issue 1, February 2019, Pages 101–108, https://doi.org/10.1093/ejo/cjy058
8. Cheng S, Brown EC, Hatt A, Butler JE, Gandevia SC, Bilston LE. Healthy humans with a narrow upper airway maintain patency during quiet breathing by dilating the airway during inspiration. J Physiol. 2014 Nov 1;592(21):4763-74. doi: 10.1113/jphysiol.2014.279240. Epub 2014 Sep 12. PMID: 25217376; PMCID: PMC4253475.
9. Kim SJ, Ahn HW, Hwang KJ, Kim SW. Respiratory and sleep characteristics based on frequency distribution of craniofacial skeletal patterns in Korean adult patients with obstructive sleep apnea. PLoS One. 2020 Jul 20;15(7):e0236284. doi: 10.1371/journal.pone.0236284. PMID: 32687512; PMCID: PMC7371191.
10. Ishida R, Palmer JB, Hiiemae KM. Hyoid motion during swallowing: factors affecting forward and upward displacement. Dysphagia. 2002 Fall;17(4):262-72. doi: 10.1007/s00455-002-0064-5. PMID: 12355141.
11. Larsen AJ, Rindal DB, Hatch JP, Kane S, Asche SE, Carvalho C, Rugh J. Evidence Supports No Relationship between Obstructive Sleep Apnea and Premolar Extraction: An Electronic Health Records Review. J Clin Sleep Med. 2015 Dec 15;11(12):1443-8. doi: 10.5664/jcsm.5284. PMID: 26235151; PMCID: PMC4661337.
Orthodontic Grammar 