The purpose of this study was to evaluate the potential impact of orthognathic surgery on hearing function. Several studies showed the relation of orthognathic surgery and alteration of hearing ability. Most of these studies considered the maxillary osteotomy more critical than the mandible. Changes in the anatomy of the soft palate and nasopharynx muscles as well as a change in direction and tension of para-tubular muscles, especially in advancement movements, are more emphasized when exploring maxillary surgeries [10]. A surgical procedure such as improper osteotomies of the pterygoid area, which is near the attachment of the hearing tube and operative muscle of the Eustachian tube can also interfere with normal auditory function. Factors such as trauma and the scar of the muscles around the hearing tube can reduce the hearing capacity [10].
Mandibular movements may probably affect the hearing capacity because of the proximity of the condyle and TMJ to the ear structures and common neurovascular systems of these areas [3, 11]. Furthermore, maxillary repositioning can change the hearing capacity through generating tension and edema in the masticatory muscle and, consequently, excess pressure on the adjacent structures [8, 9].
The impact of some general factors such as edema in the areas surrounding the hearing tube, which are of osteotomy surgery side effects or some procedures such as nasal intubations, on the function of the Eustachian tube is confirmed [10]. Some of the anesthetic agents in prolonged surgeries can cause the dysfunction of the Eustachian tube cilia and resultant hearing dysfuction [12].
The noises of the saw application and other rotary instruments during the osteotomy can negatively impact the hearing capacity [13]. The use of maxillo-mandibular fixation (MMF) and the immovability of the patients after the surgery can result in a reduction of the natural function of the nasopharynx area (like swallowing, speech, yawning) [4, 14].
One of the strengths of this study, in comparison with numerous similar articles, was the long follow-up period, so that the patients have nearly fully overcome the compulsory limitations of the surgery such as reduced physical activity and other functional limitations and have been living their routine lifestyle after 6 months. Also, both ears have been separately evaluated in this study, which was novel in comparison with other studies.
According to the PTA test, all of the patients, whether in 6 weeks or 6 months follow-up, presented a normal hearing threshold (>20 dB) although some fluctuations were observed. In the general evaluation (without considering the type of vertical or horizontal movement or a specific class of deformity), patients did not present tangible changes in their hearing threshold. In other words, orthognathic surgery did not have a long-term impact on the hearing threshold (Table 3). However, statistical evaluation was performed according to the type of vertical and horizontal movements; the patients presented better results in the right ear in the vertical movement group. The vertical movement in this study only included the maxillary impaction (patients did not undergo inferior maxillary reposition in this study); it seems that upward repositioning of the maxilla can act as a positive factor. Similar to the results of Wong et al.’s [9] study, in which they stated that maxillary elongation has a negative impact on hearing capacity, they mentioned maxillary advancement and elongation could cause hearing symptoms [9]. They also reported that among 74 ears that did not have any hearing loss or ear effusion, three ears consistently tested negative during the 6-month period of the study. They also reported less than 22.2% of ears had aural symptoms at 6 months post-operative period. The ears that experiencing fullness and otalgia returned to pre-operative status, while perceived hearing problem and tinnitus have significantly reduced. In their study, some of the patients reported persistent aural symptoms [9]. Algudkar et al. [10] reported a 22-year-old female with persistent bilateral middle ear effusion for more than 2 years after orthognathic surgery. The patient’s hearing loss was treated with grommet insertion while her rhinitis was treated with nasal steroids.
According to tympanometry, the A diagram was subdivided into Ad, An, and As for further evaluations. In our study, similar to Yaghmaei et al. [4] and Bayram et al. [1], all of the cases presented type A tympanometry before the surgery, but in 6 weeks of follow-up after the surgery, 13% of our cases presented type C tympanometry, while in this time period, Yaghmaei et al. [4] and Bayram et al. [1] presented 15% of type C tympanometry, similar to the results of the current study. In our study, after 6 months of follow-up, only 9.7% of the patients (in the left ear) still presented type C tympanometry, which is not statistically significant. In the study above, follow-ups were not longer than 1 to 2 months.
According to ETFT, this study showed similar results in pre-surgery tests and 6 months of follow-up tests based on Eustachian tube function. In other words, orthognathic surgery did not affect the function of the Eustachian tube. However, the function of the left Eustachian tube was reduced in the 6-week follow-up (29%). It could be the result of some factors such as edema, swallowing, and speech limitations since in 6 months of follow-up, the results of ETFT were similar to the pre-surgery measures. A 37% reduction in Eustachian tube function was observed among Yaghmaei et al.’s [4] patients, in the 6-week follow-up; one probable reason for this difference in the results of their study with ours may be the application of MMF procedure that was performed for Yaghmaei et al.’s [4] patients. It could be assumed that our patients resumed the natural functions such as swallowing, mastication, and yawning because MMF was not implemented for them. Therefore, the impact of MMF on the hearing threshold can be confirmed.
Generally, the results of this study showed that regardless of the fact that the type of jaw movements can have negative impacts on the hearing threshold of the patients, the nature of orthognathic surgery can cause slight changes in the hearing capacity, during the first weeks after the surgery, but these negative changes will either totally fade or remain in slight values that can be totally disregarded and do not need any medical interventions.
Previous follow-up of 132 orthognathic surgery patients over 10 years showed that 86.4% had no hearing symptoms, and even 7.6% stated that they could hear better than when did not have their surgery done [15]. Minor alteration in hearing sensation is predictable, and patients require information before surgery and reassurance after the surgery. This is consistent with previous studies [1, 4, 6, 9, 11].
Some of the present study limitations is more sample size of the patients should be investigated. Subjective reports of the symptoms could also obtain a more clear sight of the patients’ experience of hearing problems after orthognathic surgery.