This study aimed to evaluate and compare the skeletal effects of monocortical and bicortical mini-implant anchorage on maxillary skeletal expansion (MSE) using cone-beam computed tomography in young adults.
The sample comprised 48 patients (aged 19.4 ± 3.3 years; 19 male, 29 female) treated with maxillary skeletal expander and was divided into 3 groups according to insertion pattern of mini-implants used. G1, 4-all-bicortical penetration (n = 17); G2, 2-rear-bicortical penetration (n = 17); G3, non-4-bicortical penetration (n = 14). Cone-beam computed tomography scans were taken before treatment and 3 months after activation.
The transverse width of maxilla, nasal bone, lateral pterygoid plate, zygomatic bone, and temporal bone increased similarly in G1 and G2. Contrarily, G3 produced less skeletal expansion, having no effects on the temporal bone. Significant increases in width were seen in all 3 groups regarding transverse dentolinear measurements. A triangular expansion pattern was also observed, but G1 and G2 showed more parallel expansion than G3. In addition, G1 and G2 showed less inclination of anchorage teeth compared with G3. The loss of vertical alveolar bone, although only in a small amount, was observed in all groups.
MSE with non-4-bicortical penetration produced fewer orthopedic effects and more unwanted dentoalveolar side effects, whereas MSE with 2-rear-bicortical and 4-all-bicortical penetration showed similar skeletal effects, which means that 2-rear-bicortical penetrating mini-implants were critical to skeletal expansion.
Maxillary skeletal expansion (MSE) showed notable expansion for young adults.
MSE resulted in a negligible clinical loss of vertical alveolar bone.
MSE with 2-rear-bicortical and 4-all-bicortical penetration showed greater expansion.
MSE with non-4-bicortical penetration showed a larger inclination of anchorage teeth.
Transverse maxillary deficiency is a relatively common orthodontic problem, which has been reported to affect 7.9% and 9.9% of individuals aged 12-18 and 18-50 years, respectively. The condition is often accompanied by crowding, mandible deviation, and unilateral or bilateral posterior crossbite, which cannot be self-corrected. Rapid maxillary expansion (RME) is a common and reliable treatment method to correct transverse maxillary deficiencies for prepubertal and adolescent patients, which has a significantly favorable effect on the sagittal occlusal relationships of Class II and III and improves nasal respiration by increasing nasal cavity volume and reducing nasal resistances. However, conventional RME transmits the expansion forces through the teeth, producing some unwanted results such as root resorption, alveolar bone bending, dental tipping, alveolar bone loss, gingival recession, and clockwise rotation of the mandible. , , RME in nongrowing patients has been shown to produce limited skeletal expansion effects because of interdigitation of the midpalatal suture and adjacent articulations. ,
Consequently, surgically assisted RME (SARME) is designed to be used for older patients by releasing the closed sutures resisting expansion force, which increases skeletal expansion efficiency and reduces the side effects mentioned earlier. However, SARME has higher biological and financial costs. Most patients are reluctant to undergo surgical procedures. In addition, SARME inevitably results in tipping of the anchorage teeth, periodontal damage, and higher recurrence. ,
Mini-implant–assisted RME (MARME) has been designed to correct transverse maxillary deficiency in adults based on previous findings that the proportion of ossified tissue in the entire skeletal suture was low in all subjects. , Some studies , have shown that MARME can achieve effective expansion of the maxillary base bone. Lin et al concluded that MARME produced more orthopedic effects and fewer dentoalveolar side effects compared with conventional RME in late adolescents. Choi et al reported that MARME could be a clinically acceptable and stable treatment modality.
There are variable designs for MARME, such as C-expander used by Lin et al, MARME by Lee et al, and maxillary skeletal expansion (MSE) invented by Moon et al. In addition, different investigators have different choices of depth for the placement of the mini-implants. Lee et al used 7-mm mini-implants to penetrate palatal cortical bone only. Moon et al recommended using bicortical mini-implant anchorage to achieve greater orthopedic effects and more parallel expansion in the coronal plane. Other studies did not indicate whether the mini-implant penetrated the monocortical or the bicortical bone. However, bicortical penetrating miniscrew will traumatize the mucosa of the nasal floor, inevitably based on anatomical and physiological principles. We also found that bicortical engagement of the mini-implants resulted in nasal discomfort in many patients. Furthermore, some scholars observed that the mini-implant with monocortical engagement could also achieve the expected skeletal efficiency. ,
Therefore, we wanted to explore whether it is necessary to guarantee bicortical engagement to achieve a similar expansion. No clinical research on comparing the effects of bicortical and monocortical anchorage during MARME has been found. Our objectives in this study were to evaluate and compare the skeletal effects of monocortical and bicortical mini-implant anchorage on maxillary expansion in young adults using cone-beam computed tomography (CBCT), to provide more evidence for the clinician to choose an appropriate expansion strategy with lower biological and financial costs and larger wanted effects.
Material and methods
This retrospective study approved by the Ethical Commission of Stomatology Hospital of Shandong University included 82 consecutive young adults (34 male, 48 female) with transverse maxillary deficiencies, who received MARME from 2017 to 2018 at the Department of Orthodontics, Stomatology Hospital of Shandong University. An informed consent form was signed by each patient. The inclusion criteria for this study were as follows: (1) >15 years old; (2) maxillomandibular skeletal transverse discrepancy of 3 mm or more ( Fig 1 ); (3) no history of expansion treatment or orthognathic surgery; and (4) no severe dentofacial anomalies such as a cleft lip or palate.
Thirty-four patients were excluded because of various reasons. Seven patients stopped treatment because of swelling of palatal mucosa or intolerance to MSE, 14 patients did not take CBCT scans 3 months after activation under consistent shooting condition, and 13 patients had CBCT scans showing only 1 or 3 mini-implants penetrating bicortical bone, or 2 unilateral mini-implants penetrating bicortical bone. Eventually, there were 48 patients (aged 19.4 ± 3.3 years; 20 males and 28 females) who fulfilled the inclusion criteria and were enrolled in the study ( Fig 2 ).
The 48 patients were divided into 3 groups according to the insertion patterns of the mini-implants used, as shown by CBCT scans. The groups were as follows: (1) G1 (4-all-bicortical penetration; n = 17; aged 19.5 ± 3.1 years), 4 mini-implants were applied by penetrating the palatal and nasal cortical bone ( Fig 3 , A ); (2) G2 (2-rear-bicortical penetration; n = 17; aged 19.2 ± 3.5 years), 2 posterior mini-implants were applied by penetrating the bilateral cortical bone, 2 anterior mini-implants were applied by penetrating the palatal cortical bone only ( Fig 3 , B ); (3) G3 (non-4-bicortical penetration; n = 14; aged 19.6 ± 3.5 years) received 4 mini-implants penetrating the palatal cortical bone only ( Fig 3 , C ).
Each patient was treated by MSE type II (BioMaterials Korea, Seoul, South Korea), developed by Dr Won Moon et al at the University of California, Los Angeles, with 2 stainless steel arms soldered to the dental alloy casting crowns on the maxillary first molars. The jackscrew was generally oriented on the palatal region at maxillary first molars. The alloy casting crowns of the expander were bonded to the maxillary first molars, and 4 mini-implants (diameter, 1.5 mm; length, 11 mm; Mplant Series, BioMaterials Korea) were inserted along guided slots under local infiltration anesthesia. The heads of the mini-implants were then attached to the jackscrew with flow resin (3M Unitek Transbond; St Paul, MN) to minimize irritation of the tongue and increase the postinsertion stability of the mini-implants ( Fig 4 ). The jackscrew was activated one-sixth of a turn (0.13 mm) each day until the maxillary skeletal width was no longer less than that of the mandible. The postactivation retention duration was 3 months, allowing bone formation in the separated maxillary suture.
CBCT scans (5G; NewTom, Verona, Italy) were obtained before treatment and 3 months after activation. The CBCT device was set at 7.33 mA and 110 kV, and images were acquired for 4.8 seconds, with an 18 × 16-cm field of view and a standard voxel size of 0.3 mm. The obtained data were analyzed by Dolphin (Dolphin Imaging, Chatsworth, Calif). First, the CBCT images were oriented along the palatal suture, tangent to the nasal floor and parallel to the palatal plane ( Fig 5 ). The measured coronal images were then produced by the coronal line that was positioned at the center of the palatal root canal in the most apical region of the maxillary first molars on the right and left sides (choosing the midpoint if the left and right root canal were not at 1 coronal line; Fig 6 ), then the measurements were taken. The nasomaxillary dentoskeletal and periodontal measurements are shown in Figures 7 and 8 . The width of the lateral pterygoid, zygomatic bone, and temporal bone are shown in Figure 9 .
For the assessment of method reliability, measurements of all variables on 8 randomly selected patients from each group were repeated after 2 weeks by the same investigator. Intraclass correlation coefficients were used to determine measurement consistency. The intraclass correlation coefficients ranged from 0.989 to 1.000, which showed repeated agreement regarding all measurements.
The normality of the data distribution was confirmed using the Shapiro-Wilk test. The homogeneity of group variance was assessed by the Levene test. A paired t test was performed for comparison before treatment and 3 months after activation in each group, and a 1-way analysis of variance and Scheffé post-hoc analysis were performed for comparison among 3 groups. Statistical analysis was performed using SPSS (version 22.0; IBM, Armonk, NY). P values <0.05 were considered statistically significant. The statistical power of considered parameters for the sample size and α of 0.05 was 100%, and the width at the central fossae of the first molars was 77%, which was acceptable. Moreover, the jackscrew opening was 28%, whose mean value was extremely close.
There were no significant differences in the amount of activation of the MSE jackscrew or in the sex, age, and the interval of taking CBCT among the 3 groups, as shown in Table I .
|Variables||Group I||Group II||Group III||P|
|Mean (SD)||19.5 (3.1)||19.2 (3.5)||19.6 (3.5)||NS|
|Mean (SD)||4.0 (0.5)||4.2 (0.5)||4.1 (0.5)||NS|
G1, G2, and G3 showed significant increases in the width of the maxilla, nasal cavity (N-N), zygomatic bone (Z-Z), pterygoid plate (Lpt-Lpt), and temporal bone (T-T; P <0.05; Table II ), with the exception that G3 did not significantly affect the temporal bone after MSE ( P > 0.05; Table II ). In addition, the amount of maxillary expansion indicated a pyramidal pattern of expansion, with the least increase at NF5 and the greatest increase at HP5. G1 showed slightly greater increases in all the skeletal measurements compared with G2, although they were not statistically significant ( P >0.05; Table III ). G3 showed fewer statistically significant increases than G1 and G2 ( P <0.05; Table III ).
|Variables||Group 1||Group 2||Group 3|
|N-N (mm)||31.5 ± 2.3||34.8 ± 2.7||0.000||31.8 ± 2.7||34.7 ± 2.6||0.000||32.6 ± 3.3||35.4 ± 3.4||0.000|
|NF5 (mm)||75.8 ± 7.4||79.2 ± 7.9||0.000||77.0 ± 6.5||80.5 ± 6.4||0.000||77.8 ± 7.8||79.6 ± 7.9||0.000|
|NF (mm)||68.5 ± 5.4||72.7 ± 6.2||0.000||68.4 ± 3.7||72.3 ± 3.9||0.000||70.1 ± 6.2||72.4 ± 6.4||0.000|
|HP (mm)||65.9 ± 4.9||70.5 ± 5.5||0.000||66.2 ± 3.9||70.5 ± 4.2||0.000||65.7 ± 5.6||68.8 ± 5.6||0.000|
|HP5 (mm)||62.1 ± 4.0||67.4 ± 4.7||0.000||62.8 ± 4.3||67.8 ± 4.8||0.000||62.5 ± 4.9||66.2 ± 5.0||0.000|
|IMW-C (mm)||46.2 ± 3.3||53.0 ± 3.8||0.000||48.9 ± 4.4||55.8 ± 4.9||0.000||47.5 ± 4.8||54.7 ± 4.2||0.000|
|IMW-A (mm)||34.3 ± 3.2||39.8 ± 3.8||0.002||35.5 ± 2.5||40.8 ± 3.0||0.000||36.2 ± 4.8||39.8 ± 4.7||0.000|
|Al-L (°)||110.0 ± 7.4||110.4 ± 7.3||0.001||111.0 ± 7.4||112.1 ± 7.5||0.000||108.8 ± 6.4||110.9 ± 6.5||0.000|
|Al-R (°)||110.9 ± 7.9||111.5 ± 7.9||0.000||112.6 ± 8.2||113.6 ± 8.0||0.000||106.8 ± 8.5||108.8 ± 8.5||0.000|
|Tor-L (°)||100.7 ± 7.4||101.3 ± 7.4||0.003||104.6 ± 9.0||105.9 ± 9.5||0.000||99.6 ± 6.6||104.2 ± 7.4||0.001|
|Tor-R (°)||103.5 ± 5.3||104.3 ± 5.1||0.000||105.0 ± 9.0||106.4 ± 9.2||0.000||100.3 ± 5.3||105.3 ± 6.2||0.000|
|Alh-L (mm)||13.4 ± 2.8||12.9 ± 2.8||0.000||12.6 ± 2.8||11.8 ± 2.9||0.000||14.2 ± 2.9||13.8 ± 3.0||0.000|
|Alh-R (mm)||56.1 ± 5.1||13.1 ± 2.6||0.000||12.4 ± 2.4||11.7 ± 2.4||0.000||14.3 ± 2.8||13.5 ± 2.7||0.000|
|Lpt-Lpt (mm)||56.4 ± 5.1||58.2 ± 4.7||0.000||58.4 ± 5.4||59.7 ± 5.3||0.000||55.8 ± 4.1||56.2 ± 4.0||0.000|
|Z-Z (mm)||102.3 ± 4.7||104.4 ± 4.8||0.000||101.7 ± 5.5||103.8 ± 5.7||0.000||104.0 ± 6.8||105.1 ± 6.8||0.000|
|T-T (mm)||119.6 ± 4.3||120.2 ± 4.3||0.000||120.0 ± 5.7||120.5 ± 5.8||0.000||121.9 ± 6.4||122.0 ± 6.3||0.125|