Immediate effects of rapid maxillary expansion on the naso-maxillary facial soft tissue using 3D stereophotogrammetry
Introduction
Rapid maxillary expansion (RME) has been advocated to increase the transverse width of a narrow maxilla. The expansion facilitates posterior crossbite correction, relief of crowding, increase in airway dimensions and has been used in conjunction with facemask therapy to facilitate maxillary advancement.1, 2, 3 The dental and skeletal effects of RME are well documented in the literature. The main skeletal effects produce separation of the mid-palatal suture, more anteriorly then posteriorly, with vertical expansion extending to varying levels. The expansion is often pyramidal in shape with the greatest expansion around the region of the nasal aperture.4, 5, 6, 7, 8 Studies based on two-dimensional lateral cephalograms have shown that the maxilla displaces downwards and forwards to a varying degree following RME treatment.9 One of the most noticeable dental effects during RME activation is a diastema between the upper central incisors but tipping of the maxillary posterior teeth and alveolar processes laterally have also been reported.9, 10, 11, 12
Since the maxillary bones contribute significantly to the nasal cavity's anatomical structure, the effects of RME are not just limited to the maxilla but extend to the surrounding nasal structures.13, 14, 15, 16 Conventional tomography has been used to evaluate volume changes in the nasal cavity after RME.15, 17 These authors reported that the internal area and volume increased significantly throughout the nasal cavity. These changes, together with the maxillary advancement, may produce clinically significant changes in the morphology of the naso-maxillary soft tissue.
The potential effects of RME may not be limited to skeletal and dental changes but may be expected to affect the overlying soft tissue; in particular around the nasal soft tissue. Most of the studies carried out to quantify the effect of RME on soft tissue have utilised two-dimensional techniques, including lateral cephalograms18 and frontal photographic views or by physical direct measurements.19, 20 Measuring three-dimensional changes from a frontal profile photograph will have inherent errors whilst directly measuring changes in nasal width using calipers is clinically difficult with landmark identification and soft tissue distortion being a problem. A more recent study has used cone beam CT to evaluate the changes in the naso-maxillary complex associated with two types of maxillary expanders.16 However, non-invasive methods of capturing the 3D soft tissue based on stereophotogrammetry are available which have been validated and used for analysis of facial morphology.21, 22
The aim of the study was to investigate the immediate three-dimensional effects of rapid maxillary expansion on the naso-labial soft tissue using 3D stereophotogrammetry. The underlying hard tissue effects of RME of this group of patients have previously been reported.23 The null hypothesis was that there is no difference in linear and angular measurements of the naso-labial soft tissue pre- and post-RME.
Section snippets
Materials and methods
Following ethical approval from the West of Scotland Research Ethics Committee (REC reference number: 09/S0709/40), patients were recruited from the Orthodontic Department of the Victoria Hospital, Kirkcaldy, Scotland, U.K. Patients were recruited on their need for upper arch expansion as part of their full comprehensive orthodontic plan; either for unilateral or bilateral posterior crossbite correction and/or relief of crowding. All patients went on to receive full comprehensive orthodontic
Statistical analysis
The reproducibility of the landmark placement error was assessed. All the images were landmarked on two separate occasions, one week apart, by the same operator. The data was used to determine intra-examiner systemic and random error according to Houston.24 Systemic error was assessed using a Student's t-test and random error was examined using correlation coefficient.
The pre-RME (T0) and post-RME (T1) linear and angular measurements were compared. Significant differences (P < 0.05) were tested
Results
No systemic errors were observed (p > 0.1). All coefficients of reliability were above 90%. Landmark identification error was 0.3 mm ± 0.2 mm for all the landmarks.
In total fourteen patients (7 males, 7 females; mean age 12.6 ± 1.8 years) were recruited. The mean separation of the anterior nasal spine for this group of patients was 3.8 ± 1.2 mm.23
All transverse linear nasal soft tissue measurements increased following RME. The largest median increase in width was seen in the nasal base width
Discussion
Subjects that had a deficiency in maxillary arch width with unilateral or bilateral posterior crossbite were recruited and treated with RME. The study was controlled through the application of inclusion and exclusion criteria. The power calculation had determined that a minimum of 14 subjects were required.
The 3D soft tissue landmarks and measurements used in the present study were decided by the reproducibility of landmark identification and the landmarks and measurements recorded in previous
Conclusions
This study has quantified the subtle changes expected to the naso-labial soft tissue complex following RME. There is an increase in nasal base width, retraction of the nasal tip and flattening of the nasal tip. These changes are small and variable between patients hence it would make prediction of naso-labial soft tissue changes following RME difficult.
Acknowledgements
The authors would like to acknowledge the staff at the Radiology Department at the Glasgow Dental Hospital and School for their support during this study, as well as Mrs Carol McIvor for help in organising the patients.
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