Changes in Pharyngeal Airway After Mandibular Setback Surgery: A Retrospective Cephalometric Study

Effect on Skeletal Measurements

SNA angle did not show any changes including the follow up. After surgery the SNB angle was found to have decreased on average by 4.6 degree while ANB angle increased on average by 4.6 degree since a mandibular setback surgery was performed. No significant difference was seen from postsurgical to follow-up. These results showed that these skeletal measurements after orthognathic surgery were retained and remained stable in the long term. These changes are similar to previous studies on the stability of orthognathic surgery to correct Class III skeletal deformities.^{8, 10}

Mandibular plane angle increased significantly from T1 to T2 and T1 to T3, indicating that mandible was rotated clockwise during surgery. However, it did not increase significantly from T2 to T3, indicating that there was negligible change in the mandibular plane from immediate postsurgery to follow up. Similarly, the gonial angle increased from T1 to T2 to T3. However, only significant change was from T1 to T3, indicating increase in the inclination of posterior border of ramus to the mandibular plane following surgery. In spite of these changes, stable occlusion was maintained in all cases because of postoperative orthodontic alignment. Other studies have also found similar changes in the mandibular plane angle postoperatively. ¹⁸

Vertically, the mean inferior movement at pogonion was 1.6 mm at surgery and it relapsed back upward during follow-up, but it did not reach the presurgical level. Pg horizontal decreased on average by 8.1 mm indicating the amount of setback of the mandible. There was a relapse of 1.1 mm from postsurgery to follow-up. Similar findings were noted by few other studies.^{12, 19} Eggensperger et al found a skeletal relapse of 1.3 mm measured at pogonion, amounting to 23% of surgical setback after 1 year. ¹⁴ However, Kitagawara et al found no significant horizontal relapse after surgery. ¹⁸

Effect on Pharyngeal Airway Measurements

Most cephalometric studies of airway diameters in OSAS have used conventional skeletal reference points to define lines along which airway diameters were measured. Reference lines dictated by the facial skeletal morphology are very prone to be altered at osteotomy and usually do not represent minimum cross-sections. To rule out these discrepancies, the definitions of airway diameters used in the present study were narrowest diameters of the airway at different landmarks. ²⁰

Since a two-dimensional lateral cephalogram has been used to evaluate three-dimensional pharyngeal airway and hyoid bone, its validity can be questioned and CT might be considered a better option instead. However, Riley and Powell have reported a high reliability of CT scans and cephalograms in determining the posterior airway space with statistically significant correlation between posterior airway space and volume of the pharyngeal airway. ¹² Therefore, lateral cephalograms are still used widely in the assessment of sleep apnea and craniofacial form. Its advantages include wide accessibility, simplicity, low expenditure, minimal radiation exposure, and ease of comparison with extensive normative data and other studies.^9-11

Pharyngeal airway space along the nasal line showed no significant changes from T1 to T2 to T3. Minimum airway distance between the soft palate and pharyngeal wall, and the distance between the uvula tip and posterior pharyngeal wall showed a decrease from T1 to T2 but no change from T2 to T3. This indicated that the airway decrease after surgery although not significant remained stable in the follow-up. Whereas airway measurements at the intersection of tongue to the inferior border of mandible, at the base of the tongue, and at the vallecula showed a decrease in the PAS from T1 to T2 but an increase from T2 to T3. Although the measurements at T3 were larger than T2 they did not reach the T1 values.

Saitoh, Eggensperger et al, and Marsan et al found significant decrease in pharyngeal airway size at all levels after mandibular setback surgery.^{8, 14, 21} Eggensperger et al stated the continuation of decrease in the sizes of the upper and middle pharyngeal airways with time. ¹⁴ However, the lower pharyngeal airway diameter remained almost unchanged.

Chen et al, Kitahara et al,, and Liukkonen et al found a significant reduction in the pharyngeal airway at the oropharyngeal and hypopharyngeal levels.^{10, 11, 22}

Other study which showed contradictory results was that of Kitagawara et al in which the nasopharyngeal depth significantly increased 3 days after surgery and posterior airway space was stable during the whole period. ¹⁸ However, Tselnik and Pogrel noted that mandibular setback surgery resulted in an initial increase in both the area and anteroposterior dimension of the PAS which was followed by a significant long-term decrease in both parameters. ²³

Hochbanet et al have reported larger than average preoperative airway space in patients with prognathic mandible and following mandibular setback surgery airway size decreases to values within a normal range. ²⁴ The findings of this study do not support this postulate, preoperative airway size was found to be on average 11.5 mm at the intersection of the posterior border of the tongue and the inferior border of the mandible to the closest point on posterior pharyngeal wall, thereby corresponding to normal values by Mc Namara (10-12mm). ²⁵ This finding is in accordance with that reported by Liukkonen et al. ²²

Kim et al and Degerliyurt et al reported that the pharyngeal dimensions after surgery did not demonstrate sex dimorphism.^{26, 27} However, few found gender difference in the airway after surgery.^{26, 28} Due to these contradictory results in literature the number of males and females taken in this study were equal to eliminate gender bias.

Effect on Soft Palate Measurements

Soft palate length increased from T1 to T2 to T3. There was significant increase from T1 to T2 and from T1 to T3. Similar changes were noted by Achilleos et al and Saitoh.^{8, 12}

Soft palate thickness reduced from T1 to T2 and increased from T2 to T3 but these changes were not statistically significant. This was in accordance with the findings of Achilleos et al ¹²

Angulation of soft palate to nasal line increased significantly from T1 to T2. Although it decreased from T2 to T3, but this change was not statistically significant. Similar changes were noted by Achilleos et al, Saitoh, Marsan et al, and Kim et al^{8, 12, 21, 26} Significant alteration in the soft palate inclination reflected changes in the oropharyngeal airway space that is as the angle increased from T1 to T2, the minimal airway between the soft palate and the posterior pharyngeal wall decreased. From T2 to T3 as the inclination of the soft palate to the nasal line decreased that is as the soft palate moved away from the posterior pharyngeal wall, this airway measurement increased.

Both the tongue and soft palate positions influence the pharyngeal airway size. Mandibular setback surgery shifts the tongue posteriorly, hence causing the soft palate to extend posteriorly leading to change in its length, angulation, and altering pharyngeal airway measurements. Although some studies attribute mandibular setback group to be at a greater risk of developing obstructive sleep apnea, at the same time the compensatory mechanisms in the soft palate explains their probability of being at a low risk.

Effect on Hyoid Bone Measurements

Following mandibular setback surgery, the hyoid bone moved inferiorly with respect to the mandibular plane as well as the true horizontal. This is consistent with findings of other studies.^{29, 30} This movement is thus a physiologic adaptation to prevent hyoid-related encroachment on the pharyngeal airway following mandibular setback surgery. However, during long-term follow-up, the hyoid bone tended to move up slightly, although not significant. While the hyoid bone tends to return to its preoperative position, it never regained its original location with respect to both the mandibular plane and the true horizontal. Mean displacement therefore from T1 to T3 was also in an inferior direction and changes in hyoid in the vertical plane from presurgical to follow-up were not statistically significant.

However, Achilleos et al, Kitahara et al, and Kitagawara et al found that the hyoid bone moved inferiorly at short-term follow-up, a transient finding as the long-term follow-up showed a significant elevation of the hyoid bone almost to its presurgical level.^{11, 12, 18} There are conflicting views on the degree and duration of the postoperative changes in the hyoid bone position and PAS decrease. Eggensperger et al also found inferior movement of the hyoid bone; however, after 11 years they found that the hyoid bone moved further inferiorly. ¹⁴ Whereas Kim et al noted no inferior displacement of the hyoid in both males and females,^{26 a few} found significant downward displacement of the hyoid bone one year postoperatively in males. ²⁸ Although a similar tendency was seen in females this trend was insignificant.

In this study, the hyoid bone moved posteriorly toward the pharyngeal wall from T1 to T2 to T3, but only changes from T1 to T2 were statistically significant. This is in accordance with the study by Kim et al ²⁶ Significant narrowing of minimal airway distance at the base of the tongue immediately following surgery is due to this posterior and inferior movement of tongue represented by the hyoid bone changes.

Kitahara et al and Eggensperger et al also found significant posterior movement of the hyoid immediately after orthognathic surgery.^{11, 14} But during the follow-up stage they found that the hyoid bone moved forward. It however did not fully regain its original preoperative position, whereas Achilleos et al noted insignificant changes in the anteroposterior relationship of the hyoid bone. ¹²

In contrast, Tselnik and Pogrel observed an early downward and forward movement of the hyoid bone as a part of physiological adaptation to preserve airway patency. ²³

Effect on Head Posture Measurements

SN/OPT showed increased angular measurements between T1 and T2 and decreased from T2 to T3, but none of these changes were statistically significant. However, SN/CVT increased from T1 to T2 to T3, only changes from T1 to T2 and T1 to T3 were statistically significant. It indicated extension of the head which was maintained even at long term after setback. Thus, cervical hyperflexion had taken place after correction of mandibular prognathism.

Achilleos et al and Gu et al also reported that the head posture was raised as a result of mandibular setback surgery.^{12, 30}

The immediate response to setback surgery involves the musculature of the neck and frequently leads to a different head posture. An altered head posture was found to influence the position of the hyoid bone that is as the craniocervical angle increased significantly at follow-up the hyoid bone moved upward toward its original position. This finding could indicate long-term adaptation to maintain airway patency.

Thus, skeletal relapse and the size of the airway can be clinically seen due to postoperative changes in the pharyngeal complex. Suprahyoid musculature relaxation can be attributed to the postoperative positional alterations of the hyoid bone. This decreased suprahyoidal musculature tension may change the muscular balance within the head and neck, thus resulting in an increased forwardly directed force exerted by the neck muscles and hence leading to pulling the mandible forward again. ³⁰ However, the long-term biomechanical adaptation of the tongue, supra and infra hyoid, and all neck muscles balance the stomatognathic system resulting in increase in the airway during the follow-up. The net result was a decrease in the airway from presurgical to follow-up; however, these values were neither clinically nor statistically significant.