Discussion
The aim of the present systematic review with metaregression analysis was to assess the impact of MMA on PA dimensions and AHI in the treatment of OSA, as there is limited evidence regarding their exact correlations.17–24 Indeed, it has been widely reported that MMA increases PA and decreases AHI in the context of OSA, but additional multidisciplinary studies assessing aspects other than PA and AHI are needed to determine which types of maxillary, mandibular and chin movements (eg, advancement, rotation, impaction and descent) are best for enlarging the PA in its specific compromised levels and for finally reducing AHI, as well as patient characterisation in terms of OSA severity, comorbidities and facial profile, among other factors.5 41–43
With regard to MMA surgery according to the analysed articles,17–24 the positive effect of the intervention was clearly evidenced by the surgical SR obtained (87.5%). However, while most of the included studies19 20 22 obtained SR values of 100%, Jones et al
18 recorded the lowest rate (65%). Specifically, a mean final AHI of 12.4 events/hour (95% CI 7.18 to 17.6, p<0.01)17–24 was achieved in all of the literature reviewed. Hence, orthognathic surgery in application to OSA ensures surgical success with a final AHI of <20 events/hour and an AHI reduction of at least 50% according to the criteria defined by Riley et al.34 However, some patients would still require ongoing CPAP treatment after MMA, since OSA may not be cured (AHI <5 events/hour),5 34 and would eventually have more difficulty in adhering to CPAP after surgery.44 None of the included studies reported the number of patients requiring ongoing CPAP after MMA.17–24 However, the surgical success criterion remains subject to controversy.5 44 In this regard, some authors suggest that surgical success in OSA should be assessed on the basis of improvement or resolution of the clinical signs and symptoms of OSA, the normalisation of sleep, AHI reduction (AHI <20) and quality of life.44 On the other hand, surgical CRs (AHI <5 events/hour) were assessed by only two studies (Fairburn et al
17 and Veys et al,23 with CRs of 50% and 40%, respectively (table 2).17 23 Thus, we were not able to draw definitive conclusions on the impact of MMA on CRs.17 23
Scarce data are available on the required MMA advancement to benefit patients with OSA.5 42 In terms of the amount of surgical movement achieved, to date, an MMA of 10 mm has been considered the gold standard orthognathic surgery treatment in patients with OSA.34 Nevertheless, the combination of MMA with counterclockwise rotation has proven to be the movement with the strongest impact on PA.1 7 8 13 17–24 34 41–43 However, there is not enough evidence to establish the magnitude and direction of maxillary or mandibular movement required in order to cure OSA.5 Our results in this meta-analysis showed that for each additional 1 mm of mandibular advance, the final AHI is reduced by 1.45 events/hour on average,17 22 but further in-depth investigations would be helpful to carry out patient-tailored surgeries, depending on their skeletal facial profile, PA shape, OSA characteristics and patients’ comorbidities.45 46
The surgical treatment of OSA through MMA is occasionally performed in combination with additional procedures such as septoplasty, turbinectomy, tonsillectomy, adenoidectomy, UPPP or genial tubercle advancement (GTA).5 35 41 42 As specified by the inclusion criteria, studies where patients underwent turbinectomy and/or septoplasty as adjunctive procedures were included since it is considered that these procedures do not modify PA dimensions.18 19 21 23 Hs, tonsillectomy, adenoidectomy or UPPP as adjunctive procedures were excluded since they may alter PA dimensions.18 19 21 23 Regarding GTA and Gp, these procedures were included, provided that the magnitude of advancement was reported.18 19 21 23 However, in order to discard any independent effect or impact of Gp in MMA in terms of AHI reduction, variation in PAS and PAV gains of the two group analyses assessing MMA alone and MMA with Gp were carried out.17–24
In the past decades, the effectiveness of MMA in modifying PAS and PA has been evaluated using 2D or 3D methods, respectively.14 All of the studies17–24 assessed PAV by means of CBCT or cephalometry, both techniques (2D and 3D) being considered a safe and predictable way to measure PA, though the former lacks the option of evaluating the transverse dimension.17–24 The PA was assessed two dimensionally in three of the included studies,18 19 24 taking the minimum distance between the base of the tongue and the posterior pharyngeal wall, though not all of them indicated the exact landmarks/reference points used.18 19 24 A significant difference of 4.75 mm (95% CI 3.15 to 6.35) between preoperative PAS and postoperative PAS was found. Particularly, mandibular advancement was seen to be statistically significant when considering PAS gain (p<0.001): 1 mm of mandibular advancement implied 0.5 mm gain in PAS.17–19 21 24 However, only Hsieh et al
22 and Veys et al
23 reported 3D airway measurements, and these were evaluated at three different levels with respect to the limits of the PA subregions: nasopharynx, oropharynx and hypopharynx.14 Taking into account that orthognathic surgery impacts three dimensionally and in different subregions of the PA,14 further studies reporting volumetric data with different PA levels of measurement are needed, in addition to those included in our review17 22 23 41 43 Thus, it is important to standardise the PA measurements for homogeneity purposes and thus be able to draw relevant conclusions.14 45
Regarding the correlations between changes in PAS/PAV and AHI reduction in terms of MMA, a statistically significant association between PAS gain and final AHI was found in four of the studies included in the meta-analysis (p=0.023).17 19 21 24 For each 1 mm PAS gain, AHI was reduced by 3.58 events/hour.17 19 21 24 With regard to the 3D studies, PAV gain and AHI reduction were positively correlated (r=0.75, 95% CI 0.65 to 0.85),20 24 reflecting a strong relationship between changes in both dimensions. Thus, the greater the volume gain, the greater the AHI reduction.
OSA severity and its clinical signs and symptoms, as well as special patient features such as comorbidities and facial profile, among others, should be considered when dealing with patients with OSA.5 Regarding OSA severity, to date, MMA is indicated only in moderate to severe cases and not in mild OSA cases (AHI of <5).5 All of the included articles established the type of OSA as moderate to severe in their inclusion criteria17–24 (table 1). However, it should be noted that two studies17 22 reported AHI values at baseline that moved further away from the average (mean 57.9 events/hour, range 35.7±18.017 to 69.2±35.8).22 Thus, further studies are needed in order to evaluate the impact of MMA in patients with mild OSA. Another relevant issue is the importance of a comprehensive assessment of the global OSA symptoms of the patient for diagnostic and disease monitoring purposes.4 EDS and quality of life can be subjectively evaluated through the use of multiple clinical tools and questionnaires, such as the Epworth Sleepiness Scale (ESS) and the OSA Functional Outcomes of Sleep Questionnaire, respectively.3 5 23 Improvement of daytime sleepiness assessed by ESS was reported by one of the included studies.23 A significant decrease in EES from 14 (10–18) to 6 (4–7), preoperatively and postoperatively, was observed (p=0.0014).23
Moreover, anatomical factors such as body mass index (BMI) are relevant factors that compromise OSA.5 47 In our review, only two studies17 18 addressed preoperative and postoperative BMIs. In this context, a 10% of weight loss has been associated with a 26% decrease in final AHI.47 Nonetheless, untreated obesity is also considered a major risk factor for the progression of OSA.5 47 Another crucial factor is the patient’s facial profile, since the maxillomandibular complex sustains the PA soft tissues. Facial analysis of many patients with OSA evidences maxillary or mandibular hypoplasia, which generally can be corrected by orthognathic surgery.48 Accordingly, mandibular advancement devices—apart from being an option for treating mild to moderate OSA—are also useful in deciding which patients may benefit from surgical mandibular advancement in the context of OSA. Unfortunately, no similar maxillary devices for predicting the impact of maxillary advancement on OSA are available.5
The importance of non-anatomical factors in relation to sleep disturbance surgery outcomes has been underscored, including neuromuscular tone, rostral fluid shift, airway collapsibility and loop gain.46 49 Li et al
49 attributed an average of 61% of the recorded variation in postoperative AHI to these parameters (r=0.47, p<0.01).49 Therefore, anatomical and non-anatomical factors are of great value in the diagnosis and treatment of patients with OSA.45–47 Hence, the current literature suggests that a multidisciplinary strategy is strongly advisable, taking into account all the related factors in order to ensure the long-lasting success of surgical treatment.5 45 49
Finally, our study has a number of significant limitations: (1) the main limitation is the fact that none of the included studies were randomised controlled clinical trials25; (2) few articles were included in the meta-analysis; (3) definitive generalisations cannot be made, given that of the eight studies included,17–24 only two were prospective; the remainder were retrospective and therefore subjected to the usual biases and limitations of retrospective studies40; (4) there was a lack of homogeneity among the studies regarding the PA measurements (2D or 3D); (5) some of the studies did not directly provide mean values or SD, such data being calculated directly from the tables reporting individual patient values; (6) regarding the PSG parameters, most of the studies used the AHI17 19 24; however, one publication18 used the respiratory disturbance index; and (7) no firm conclusions on the impact of MMA on surgical CR can be stated since only two studies reported CRs.