Study subjects and OA treatment

The study population consisted of 25 Japanese male OSAHS patients. Patients were excluded from the study if there was evidence of severe periodontal diseases or they were edentulous. The study protocol was approved by the Institutional Ethics Committee of our institute, and informed consent was obtained from all patients.

A custom-made OA made of self-cured acrylic resin was used for each patient (Figure 1). After taking impressions of the maxillary and mandibular dental arches, the maxillary and mandibular portions were prepared separately and joined in the patient's mouth so that the mandible would be in an anteriorly protruding position. The degree of mandibular protrusion was set at slightly less than the maximum anterior position so as not to exceed the limit the patient could comfortably tolerate. Usually, the position was determined at 70– 80% of the distance between the intercuspal position and complete protrusion position. If the patient complained of any discomfort in the temporomandibular joints or muscles after using the OA for one week, the maxillary and mandibular portions were separated and joined again at less-protruding positions. This adjustment was repeated until the patient could wear the OA continuously without any problems, usually three times on average, for six weeks, before the sleep study was conducted.

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Polysomnography and evaluation of OA treatment

Split-night polysomnography (PSG) was carried out, and Pes was recorded in all patients. The total recording time for split-night study was nine hours, from nine PM to six AM. After the attachment of the sensors and electrodes, a recording was first carried out without the OA to determine the severity of OSAHS. Diagnostic PSG was usually continued for the initial four hours until the detection of at least one rapid eye movement sleep period. The OA was set for the latter half of the night to estimate the improvement of OSAHS.

Standard polysomnographic equipment (Alice 3: Healthdyne Technologies, Marietta, Ga) was used. The predominant sleep stage was scored according to the international standard criteria of Rechtschaffen and Kales. Potential apneas were identified as airflow <20% of baseline, and hypopneas were identified as airflow of <70% of the baseline associated with either an oxygen desaturation of >3% or an arousal for at least 10 seconds. Arousals were identified according to the criteria of the American Sleep Disorders Association.

Pes was recorded simultaneously using a microtip-type pressure transducer (MPC500: Millar, Houston Tex) that was inserted 35 cm from the nostrils. Signals from the transducer were amplified by a signal conditioner and converted by a four channel A/D converter (Power Lab/4s: ADI Instrument Pty. Ltd., Castle Hill, Australia).

Criteria for classification of good responders and poor responders defined by apnea hypopnea index and mean nadir Pes

The patients were classified into good responders and poor responders according to apnea hypopnea index (AHI) and mean nadir Pes. Good responders defined by AHI were defined as showing a >50% reduction in AHI and an AHI <10 events/h when using OA. Those who did not satisfy these criteria were defined as poor responders. Good responders defined by mean nadir Pes showed <20 cm H₂O when using OA, and those who did not satisfy this criterion were considered poor responders.

Cephalometric analysis

Lateral cephalometric radiographs were taken of the intercuspal position, wide-open mouth position, and OA-wearing position, all in an upright position. The wide-open mouth view was subserved to trace the mandibular condyle and all the teeth in the intercuspal position. To evaluate the craniofacial morphology, 41 cephalometric variables were measured, as described in our previous study.4

To assess the change in mandibular position, the cephalometric tracings at the intercuspal position and the OA-wearing position were superimposed to match the bone structures except the mandible. The change in the mandibular position was represented by the displacement of the lower central incisor tip and the rotation of the mandibular plane angle (Figure 2). The Frankfort horizontal plane (FH) was adopted as the anteroposterior reference plane for the evaluation of the displacement of the lower central incisor tip.

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We also used the functional occlusal plane (FOP) in the evaluation, which was the line connecting the middle point of the occlusal surface of the maxillary first molars and the middle point of the line between the maxillary and mandibular first premolar cusps. Total mandibular displacement (TD), anterior displacement on FH (FH[X]), anterior displacement on FOP (FOP[X]), downward displacement in a direction perpendicular to FH (FH[Y]), downward displacement in a direction perpendicular to FOP (FOP[Y]), and rotation of the mandibular plane angle were measured to evaluate mandibular displacement.

Statistical analyses

All descriptive statistics are presented as median and interquartile range (IQR). Descriptive statistics were calculated for each variable, and unpaired and paired subjects were evaluated by the Mann-Whitney U-test and Wilcoxon t-test, respectively, and all P value tests were two-tailed. A P value of less than .05 was considered to indicate statistical significance.

Logistic regression analysis was adopted to verify the possibility of variables, which were detected by bivariate analysis and were estimated as clinically important, to predict the good response to OA treatment. Statistical analyses were performed on a personal computer using the statistical package SPSS (SPSS Inc, Release 12.1).

Importance of Pes measurement

Cephalometrics has been used in a number of studies to predict treatment effects with OA. However, the results vary between studies. Menn et al5 and Ferguson et al6 have reported that evaluations using cephalometric variables cannot predict the treatment success.

On the contrary, there are many studies that concluded that evaluations using cephalometric variables are helpful to predict response to OA treatment. Eveloff et al7 have reported that mandibular-hyoid distance was smaller in the good responders group. They also stated that posttreatment AHI could be predicted by baseline AHI and four cephalometric variables such as SNA, posterior airway space, mandibular-hyoid distance, and posterior facial height. Mehta et al8 have formulated a congeneric model for outcome prediction using baseline AHI, neck circumference, and two cephalometric variables. Liu et al9 have reported that the OA is more effective in younger and less obese patients with a long maxilla, a small oropharynx, a smaller overjet, less erupted maxillary molars, and a large ratio of vertical airway length to the cross-sectional area of the soft palate. Marklund et al10 reported that apnea reduction using OA is associated with a normal mandibular plane angle and a small anterior lower anterior facial height. Skinner et al11 reported that an increased perpendicular distance between the hyoid bone and a mandibular plane is the only cephalometric variable associated with a good clinical outcome.

In this study, we found that there were significant differences in soft palate length, superior airway space, and inferior airway space between good and poor responders defined by mean nadir Pes but none between good and poor responders defined by AHI. These morphological characteristics of a long soft palate and narrow airway space may promote occlusion with increasing respiratory effort. The upper and inferior airways may collapse both anteroposteriorly and laterally. Therefore, in the case of a small airway, OA treatment is insufficient to maintain patency.

Good responders defined both by AHI and mean nadir Pes had less erupted mandibular molars than poor responders. It may be because the amount of downward component of the mandible was large and the forward displacement of the mandible on the reference plane was small in patients with a high molar height. Although multivariate analysis did not show any significant corrected odds ratio, the comparison study of the severity of OSAHS between good and poor responders defined by mean nadir Pes indicated that AI, HI, and mean nadir Pes were significant in the evaluation of the efficacy of OA treatment compared with AHI.

Good and poor responders defined by mean nadir Pes demonstrated significant differences in these variables, whereas only the slow-wave sleep showed a significant difference between these groups defined by AHI. These results suggest that Pes measurement should be performed concomitantly with PSG to improve the diagnostic accuracy for determining the indications for treatment. The Pes swing at the inspiratory peak during sleep is an absolute value and rarely influenced by interscorer differences or the method of detection. An increasing negativity of Pes directly reflects the energy of respiratory effort during sleep and is thus important in the evaluation of sleep-related breathing events.12

Our previous study demonstrated that upper airway surgeries such as uvulopalatoplasty or tonsillectomy change apneas to hypopneas, reducing Pes rather than AHI. This suggests that AHI alone does not always reflect the severity of the disease in patients with OSAHS.13

Importance of forward displacement of mandible

In this study, the adaptation of FH and FOP was beneficial as reference planes and vector resolution, respectively. The FH was chosen as the base plane of the anteroposterior displacement of the mandible because when the cephalometric radiographs are taken at the natural head position, the FH will be the anatomical plane closest to the true horizontal plane.14 The FOP was chosen because it can eliminate any positional variations of the upper and lower incisors and can be used a standard to determine the mandibular position of OA by the naked eye at the chairside without using a cephalometric tracing. One-dimensional vectors such as TD may not be appropriate because the displacement of the mandible showed complicated three-dimensional pathways such as right/left, forward, and downward directions and rotation. The displacement of the mandible on the sagittal plane should be measured for the OA treatment at least two dimensionally.

On the FH(X) and FOP(X), the degrees of anterior displacement of the mandible were shown to be significantly larger in good responders than poor responders defined both by AHI and mean nadir Pes. Furthermore, logistic regression analysis revealed that the degree of anterior displacement of the mandible showed a significant corrected odds ratio of 1.97.

Although the mechanical influences of mandibular advancement on pharyngeal patency are still unexplained, the reason is speculated as follows. Because the tongue is connected to the mandible, the forward displacement of the mandible caused by OA may improve the airway patency of the retroglossal space by reducing folds and compression in the upper airway. In addition, mandibular advancement was confirmed to enlarge not only the retroglossal space but also the retropalatal space. Isono et al15 have explained the mechanism in their hypothesis that the lateral wall of the soft palate anatomically connects to the base of the tongue through the palatoglossal arch, so the mandibular advancement stretches the soft palate through the mechanical connection, stiffening the velopharyngeal segment.

In this study, TD and downward displacement were not significantly different between good responders and poor responders defined both by AHI and mean nadir Pes. A recent study using OAs inducing four and 14 mm of interincisal opening revealed the absence of correlation between the degree of bite opening and a high treatment efficacy identified in terms of AHI, although vector resolution methods were not performed.16 These findings suggest that longitudinal downward vectors or total displacement vectors cannot predict the efficacy of OA treatment compared with anterior displacement. However, the rotation of the mandibular plane angle was shown to be significantly smaller in good responders defined by mean nadir Pes. It may be influenced by the downward rotation of the mandible, which causes the downward displacement and also posterior displacement on the reference plane, so the forward displacement becomes relatively small.