Abstract
The aim of this study was to investigate the effect of porous polyethylene (PPE) in paranasal augmentation on midfacial soft tissue architecture. This retrospective study recruited patients with midface retrusion and mandibular prognathism. Twenty adult patients who had undergone bilateral PPE augmentation (ready-made type, thickness 4.5 mm, Medpor) to the piriform aperture and simultaneous mandibular setback surgery were included in this study. The soft tissue morphology and thickness of the midface were evaluated using three-dimensional reformatted images from cone beam computed tomography done before and 6 months after surgery. The soft tissue outline of the midface was augmented 1–4 mm. The average increase in soft tissue outline near the peri-alar region was 3.1–3.4 mm, which comprised 68–74% of the PPE thickness ( P < 0.01). The nasolabial angle and columellar inclination were increased significantly (2.2° and 1.4°, respectively; both P < 0.05), whereas the nasal tip angle, nasal tip protrusion, columellar length, and bilateral nostril axis angle did not change. The alar base became wider on average by 2.2 mm ( P < 0.01). The results showed that paranasal augmentation with PPE significantly increased the overlying soft tissue outline without influencing the nasal projection and could enhance paranasal aesthetics with minimal morbidity.
The midfacial regions are important for an attractive appearance. Many patients with skeletal prognathism also have a midfacial deficiency. The concavity around the piriform aperture anteroposterior deficiency of the midface causes a ‘dish shaped’ facial profile, acute nasolabial angle, and deep groove in the subnasal area. Therefore, various techniques have been introduced to improve midfacial deficiency. The Le Fort I osteotomy or malar osteotomy, or a combination of the two, has been utilized to improve severe midface retrusion. For mild or moderate midface retrusion, various augmentation techniques using a variety of graft materials have been used. Nowadays, alloplastic graft materials such as porous polyethylene (PPE) have been suggested as the best available facial bone substitute because of their biocompatibility, ease of handling, and reduced operation time. The use of PPE in orthognathic surgery as an adjunctive augmentation method has been reported.
Even though paranasal augmentation with PPE is widely accepted as a useful method for midface retrusion, evaluation of the outcomes has usually focused on postoperative morbidity, or has presented non-quantitative data. Therefore, the exact effect of sub-periosteal PPE augmentation on the overlying soft tissue has not been confirmed yet. At the same time, the effect on the nasal projection or nasal shape has not been investigated previously. This is because of difficulties in the superimposition of three-dimensional (3D) facial soft and hard tissue structures. The recent introduction of cone beam computed tomography (CBCT) and the development of computerized imaging software has allowed a precise analysis of 3D facial landmarks with reliable reference planes. 3D assessment of the overlying soft tissue changes associated with paranasal augmentation has been attempted previously. However, the authors of that study could not suggest a hard to soft tissue response rate because the PPE was not uniform in thickness or shape. To overcome this limitation, the shape and thickness of PPE need to be standardized. By using the ‘ready-made’ type PPE, the overall soft tissue reaction can be calculated quantitatively.
The purpose of this study was to investigate the overlying midfacial soft tissue response after paranasal augmentation with PPE. To determine the rate of augmentation material thickness to soft tissue change, we compared the preoperative and postoperative paranasal soft tissue contour and nasal shape after performing paranasal augmentation with PPE at the time of mandibular setback surgery using CBCT.
Subjects and methods
Subjects
This retrospective study included patients who had undergone paranasal augmentation with PPE and bilateral sagittal split ramus osteotomy (BSSRO) to correct skeletal class III malocclusion and anteroposterior midfacial deficiency. Preoperative and postoperative CBCT data sets were available for all of the patients. The surgery was performed by the same surgeon (TGK) at the study hospital between August 2010 and December 2012. Patients with facial asymmetry of more than 4 mm in chin deviation, any history of facial trauma or scar, or previous experience of adjunctive cosmetic surgery at the midface were excluded from the study. A total of 20 patients (eight males and 12 females) were recruited into the study; the average age of the patients was 21.5 ± 3.3 years (range 17–29 years). This study was approved by the institutional review board.
Surgical technique
After completing the BSSRO (8.1 ± 2.4 mm of mandibular setback; range 6–10 mm at B point), povidone–iodine preparation was again performed to the maxillary vestibular sulcus. A vestibular incision was made above the root of the central incisor to the canine eminence. There was no need to reflect the anterior nasal spine. After sub-periosteal reflection of the paranasal area near the piriform aperture, the ready-made paranasal PPE (Medpor paranasal left #9519, right #9520; width 28 mm, height 26 mm, thickness 4.5 mm; Porex Surgical, Inc., Newnan, GA, USA) was adapted to the depressed recipient site. The thin margin of the PPE can be trimmed slightly according to the individual anatomical structure. The smooth transition of the graft margin to the recipient bone was carefully confirmed.
The centre of the paranasal PPE was immobilized with fixation using a 7–9-mm miniscrew (diameter 2 mm; KLS Martin Co., Tuttlingen, Germany). No additional alar base cinch suture was applied. The overlying mucosa was closed bilaterally with interrupted sutures. Representative preoperative and postoperative 3D images of a patient and the intraoperative position of the PPE are shown in Fig. 1 .
Image acquisition and processing
The patients were subjected to CBCT (Hitachi CB MercuRay CBCT unit; Hitachi Medical, Tokyo, Japan) before and 6 months after the surgery. CBCT images were taken with 19-cm field of view, 120 kVp, 15 mA, and a contiguous 0.4-mm slice thickness. The CBCT data files (DICOM format) were then reformatted to 3D images using 3D imaging software (OnDemand3D; Cybermed Inc., Seoul, Republic of Korea); a multiplanar reconstruction (MPR) image was finally acquired. The definitions of the reference lines and planes were those used in a previous study.
Measurements of the paranasal soft tissue and nasal structure changes
To analyze the paranasal soft tissue changes, a set of preoperative and postoperative CBCT data images were superimposed on the anterior cranial base according to the maximization of mutual information, as described previously.
To measure the paranasal soft tissue changes, nine horizontal and 18 bilateral vertical planes (nine right, nine left) were established, in a similar manner to previous studies. After setting the head position to the Frankfort horizontal (FH) plane and midsagittal reference plane, various axial planes (parallel to the FH plane) passing through the following points were established: pronasale (Pr), labrale superius (Ls), subnasale (Sn), and middle position (M) between Ls and Sn. Superior to the Sn, additional axial planes (X1–X5) at a vertical interval of 5 mm were established incrementally. Vertical grids were made incrementally at 5-mm intervals from the most lateral aspect of the right (R3) and left (L3) alar wing. Finally, a total of 138 measurement points were analyzed for each patient to evaluate the paranasal soft tissue changes after the augmentation ( Fig. 2 ).
The soft tissue changes at each of the 138 sites were measured before and after surgery using the superimposed images ( Fig. 3 ). Because of the curvature of the nasal structure and absence of bony support, the nasal soft tissue contour could not be evaluated in the same manner as the paranasal areas lateral to the piriform aperture; therefore we evaluated changes in the morphology of the nasal structure separately.
The variation in each measurement point in the subjects was measured preoperatively and postoperatively. The definitions of the landmarks are given in Table 1 . By using the axial, coronal, and midsagittal MPR images and 3D volume rendered images from the CBCT data, various angular and linear measurements were analyzed, as shown in Figs. 4 and 5 .
| Soft tissue landmarks | Definition |
|---|---|
| Reference planes and landmarks | |
| FH plane | The plane including both porion and right orbitale |
| Midsagittal plane (MSP) | The plane perpendicular to the FH plane and including nasion and basion |
| Coronal plane | The plane perpendicular to FH and the MSP and including the point of the most superior and posterior clinoid process |
| Soft tissue nasion (N′) | Deepest depression at the root of the nose on MSP |
| Columella (Cm) | Most anterior soft tissue point on the columella of the nose |
| Pronasale (Pr) | Most anterior projection of the nose |
| Subnasale (Sn) | Point at which the columella merges with the upper lip in MSP |
| Labrale superius (Ls) | Most prominent point of the vermilion border of the upper Cupid’s bow |
| Middle position (M) | Vertical midpoint between Ls and Sn |
| Right nostril (Nr(R)) | Lowest point of the right nostril |
| Left nostril (Nr(L)) | Lowest point of the left nostril |
| Right alar base (Ab(R)) Left alar base (Ab(L)) | Most lateral point at the curved line of the right alar base Most lateral point at the curved line of the left alar base |
| Angular measurements (°) | |
| Nasolabial angle | Angle of Cm–Sn–Ls |
| Nasal tip angle | Angle of Sn–Pr line and the parallel line following the bridge of the nose passing through N′ |
| Columellar inclination | Angle of coronal plane to Cm–Sn |
| Nostril axis angle (Rt) | Angle of Pr–Nr(R)–Nr(L) |
| Nostril axis angle (Lt) | Angle of Pr–Nr(L)–Nr(R) |
| Linear measurements (mm) | |
| Alar base width | Distance between bilateral alar base (Ab(R)–Ab(L) distance) |
| Nasal tip protrusion | Distance from Sn to Pr |
| Columellar length | Distance from Sn to Cm |
| Nasal tip–both alar base distance | Perpendicular distance from bilateral alar base to nasal tip (Pr) |
Statistical analysis
The paired t -test was used to evaluate whether there were any statistical differences between the preoperative and postoperative measurements. Statistical comparisons were done with SPSS v. 10 software (SPSS Inc., Chicago, IL, USA).
Results
Paranasal soft tissue measurements
The paranasal soft tissue outline was significantly augmented 1–4 mm anteriorly after PPE augmentation. The areas with significant changes extended to the infraorbital foramen, medial to the zygomatic eminence, masseteric muscle and lateral aspect of the nose, and the nasolabial fold area. The degree of change was higher in the medial than in the lateral midface. In the nearby alar base area, the extent of anterior movement averaged 3.1–3.4 mm ( P < 0.01) ( Tables 2 and 3 ). Because the maximum thickness of the paranasal PPE was 4.5 mm, the suggested rate of PPE augmentation thickness to soft tissue outline change was 68–74% around the peri-alar region.
