Three-dimensional assessment of upper airway changes associated with mandibular positional deviations following fibula free flap reconstruction

According to a recent review of the literature (02/2024), this study is one of the first to evaluate correlations between airway anatomy and mandibular position after reconstructive surgery. Changes in mandibular position after microvascular fibula flap reconstruction were analysed using a method that allows three-dimensional evaluation by analysing three relevant areas: the mandibular condyles, whose position is highly relevant for mandibular movement and masticatory function [13, 17], and the symphysis, whose position is important both for later prosthodontic rehabilitation and for respiratory function, as the genioglossus muscles are attached here [1, 18]. The method proved to be highly reliable, which can be explained by several facts: all measurements were made using as much three-dimensional information as possible, the segmentation process was double-checked by volumetric measurements and the points for bone position analysis were not selected on the bony surfaces but in the centres of the regions of interest [19]. The analysis of the upper airway was performed as proposed by a meta-analysis in the field of orthognathic surgery [10] and supplemented by the measurement of the minCSA, as narrow upper airways are more prone to obstructive events [20].

Deviations and rotations of the condyles showed similar results to the previous study by the working group [15]. In most cases, the condyles’ movements were considered minor, while significant positional changes occurred in some cases. One of these more inaccurate cases might e.g. be explained by its large defect size requiring four fibula segments for reconstruction, since larger defects come with higher complexity [21]. Deviations in the position of the symphysis were greater than those of the condyles, especially in the posterior direction, indicating setback-like movements of the mandible. Matching our expectations, these setback-like movements correlated with a decreasing minCSA of the hypopharynx, which is a main finding of this study. However, no correlation was evident between mandibular setback movements and upper airway volume reduction, though both occurred simultaneously in the majority of cases. In the field of orthognathic surgery, there is huge consensus on the correlation between mandibular setback and a long-term decrease in upper airway volume and posterior airway space (PAS) [10, 22, 23]. While maxillary movement affects the nasal region, the effects of mandibular movement appear more caudally [24], which is in line with our results. Reconstructive surgery is bearing many other possible influencing factors, though, making further research mandatory before making simultaneous conclusive statements.

The limitations of this study are its retrospective design on the one hand, and the complex and multifactorial surgical procedure on the other hand. Due to the retrospective design, jaw position was not controlled during the CT scans and the time intervals between surgery and CT scans varied widely. Other factors that might have had an influence were considered in the basic analyses in this study and did not show systematic effects on the outcomes. Some significant correlations were found that we are unable to interpret logically and therefore considered them as potentially false positive findings. These type I errors may occur due to the high number of statistical tests performed. All of them are mentioned in the results section but are not further discussed to avoid frantic speculation. Furthermore, group sizes varied widely, and the brief analysis of these many influencing variables was primarily designed to rule out serious bias but would be worth a separate study, each. Though not statistically significant (P > 0.05), our results are e.g. suggestive of a tendentially larger airway volume if dental occlusion was preserved at least partially on both sides, than with unilateral or without occlusion. However, the preservation of dentition is highly determined by the defect size and thus difficult to evaluate separately. Previous analyses on possible risk factors for upper airway impairment after head and neck surgery have mostly been focusing on the immediate postoperative period. Cameron et al. [13] attempted to predict the need for tracheostomy after major head and neck surgery. They found that the need for a mandibulectomy and a microsurgical reconstruction were risk factors, as were tumour location and bilateral neck dissection. When performing neck dissection, the lymphatic drainage system is removed, which is associated with significant postoperative swelling [13]. However, Moubayed et al. [18] state that segmental mandibulectomy and microvascular reconstruction is possible without tracheostomy in carefully selected patients. They identified tongue and pharynx resection, defects of the central mandible – and thus genial attachment – and bilateral neck dissection as risk factors. In the present study, neither neck dissection nor tumour location showed relevant correlations with the airway anatomy. This may be due to the fact that postoperative CTs were mainly performed after the critical period of time regarding postoperative swelling, which may explain the lack of correlations and would be in consistence with a study by Du et al. [25], who also considered long-term airway changes after reconstructive surgery. Therefore, the present study explicitly does not allow the assessment of the need for tracheostomy after mandibular reconstruction. However, the measurement method combined with clinical data could be used for such an analysis in the future.

Literature on upper airway changes following reconstructive surgery is scarce and Du et al. [25] are one of the very few to describe airway changes following reconstructive surgery of the mandible. In contrast to the present study, their focus is on the influence of the tumour diagnosis rather than the (neo)mandibular position and they only included tumours of the anterior mandible. Analysing airway volumes and cross-sectional areas, their main finding was that malignant tumour resection and reconstruction resulted in widening of the upper airway, whereas this effect did not occur in benign tumours. They explain this with the deficit of soft tissue after extensive resection around a malignant tumour. In the present study, the diagnosis did not have an effect on the airways, but the size of the groups varied greatly, making it impossible to draw any conclusions. Comparison of the studies is further hampered by the fact that they analysed spiral CT scans taken with the patient in an upright position, whereas the present study evaluated only supine CT scans. The dimensions and volumes of the PAS differ depending on the patient’s position [26]. An effect on the mandible’s position, on the other hand, seems to only be apparent in the occipital-caudal axis but not in the anterior-posterior axis [26]. Mandibular movement as a function of the patient position is another potential bias that needs to be investigated in future analyses before definitive statements can be made. In contrast to orthognathic surgery, where spiral CT scans are often sufficient and where most research on this topic has been conducted to date, patients undergoing reconstructive surgery often require more extensive CT imaging. Research should therefore not be limited to analysis in the upright position, but should also extend to the supine position and to the differences between the two.

Another study addressing the influence of mandibular reconstruction on the PAS was recently published by Winnand et al. [27]. They also state that the mandible was intentionally reconstructed in a retrognathic position, when the symphysis area was involved. While this related to almost all patients of our study, they had a sufficient comparative group of cases where the central mandible had been preserved. This may be explained by the fact that they did not limit their data to fibula flaps, which are commonly used for larger defect sizes, but also included deep circumflex iliac artery flaps used for smaller defects. One of their main results depicts a decrease in the cross-sectional areas of the PAS, when the symphysis region was reconstructed. Combined with the fact that symphysis reconstruction was always accompanied by a posterior repositioning, this is in line with the main result of the present study and emphasizes the need for more awareness of this matter. In both their and our study, however, the posterior movement did not significantly affect the total airway volume. Interestingly, they also found a significant effect of preoperative irradiation, which seemed to counteract the tendency for airway narrowing. In our study, conversely, the only significant effect of irradiation was a reduction in the volume of the nasopharyngeal airways.

Given all this and the many factors that differ from patient to patient individually, more research is needed to make conclusive statements. Clearly, creating ideal conditions for masticatory rehabilitation should remain a primary goal of mandibular reconstruction. However, the impact of the position of the graft on the anatomy of the upper airway should not be overlooked, as respiratory function is also highly relevant to the patient’s quality of life.