Comparison of the Effects of Rapid Maxillary Expansion Caused by Treatment with Either a Memory Screw or a Hyrax Screw on the Dentofacial Structures – Transversal Effects

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The aim of the present study was to investigate and compare the effects of conventional Hyrax screw treatment and memory screw treatment on skeletal and dentoalveolar structures. Thirty-two patients with maxillary transversal deficiency were divided into two groups. The memory-screw group included 17 patients (nine females and eight males), while the Hyrax-screw group comprised 15 patients (eight females and seven males). Mean ages of the subjects in the memory-screw and Hyrax-screw groups were 13.00 ± 1.29 and 12.58 ± 1.50 years, respectively. Plaster models and postero-anterior cephalograms were taken from the patients at the beginning of the treatment (T1) and at the end of expansion (T2) and retention periods (T3). The mean expansion period was 7.76 ± 1.04 days in the memory-screw group and 35.46 ± 9.39 days in the Hyrax-screw group. ‘Shapiro–Wilk Normality test’ was used to determine whether the investigated parameters were homogeneous or not. To determine the treatment changes within the group, ‘paired t-test’ and ‘Wilcoxon signed-ranks test’ were applied to the homogeneous and non-homogeneous parameters, respectively. Comparison between the groups was carried out using ‘Student’st-test’ for homogeneous parameters and ‘Mann–Whitney U-test’ for the rest. Rapid maxillary expansion was carried out successfully in both the groups. However, the use of memory screw may be advantageous because it shortens the maxillary expansion period, provides additional expansion in the retention period, and generates light forces relative to the conventional Hyrax screw.

Introduction

Interest in rapid maxillary expansion (RME) has increased during the past four decades. In spite of the recommended use of the RME (Sarver and Johnston, 1989) for different aims, its fundamental objective is to increase the transverse width of the upper dental arch at the apical base. Although the major treatment effects could be observed in the dental area, transversal expansion of the skeletal structure may be considered as an additional contribution.

Usually, appliances with fixed jack screw are used for RME and they produce heavy forces (Isaacson and Ingram, 1964Zimring and Isaacson, 1965). RME occurs when heavy forces aplied to the maxillary anchorage teeth and alveolar structures exceed the limits required for orthodontic movement. Pressure caused by the applied forces compresses the periodontal ligament of the anchorage teeth and subsequently produces orthopaedic movement by opening the midpalatal suture (Haas, 1961) and orthodontics movement by tipping the upper posterior teeth and alveolar structures (da Silva Filho et al., 1991Ciambotti et al., 2001Oliveira et al., 2004Kılıç et al., 2008). The aims of the RME treatment are to achieve minimal dental and maximum skeletal effects (Haas, 1961), and many different RME appliances have been developed for this purpose.

In a recent paper, Wichelhaus et al. (2004) described and evaluated a new maxillary memory palatal split screw that includes nickel–titanium open coil springs in the screw bed in order to lessen massive expansion forces. The screw could be activated six times a day to produce a constant force level of 12–14 N (1224–1428 grams) providing effective and rapid expansion. According to these authors, this new screw could produce rapid, constant, and physiological expansion forces, thus making the expansion procedure more effective, more physiological, and well tolerated by patients. However, Halıcıoğlu et al. (2010)showed that memory palatal split screw produced significant increases in the interpremolar and intermolar distances and reduced the nasal airway resistance. Although studies about memory screw have increased in the past years, till date no study has been carried out to compare the effects of RME caused by memory screw and Hyrax screw on the dentofacial structures.

The aim of the present study was to investigate and compare the transversal changes in subjects treated with a conventional Hyrax screw and memory screw on plaster models and postero-anterior cephalograms.

Subjects and Methods

The material of this study consisted of plaster models and postero-anterior radiographs of 32 patients aged between 11 and 14.5 years. All the subjects had bilateral maxillary crossbites and caused by basal apical narrowness, which underwent RME at the Department of Orthodontics, Faculty of Dentistry, Atatürk University, Erzurum, Turkey.

The subjects were randomly assigned to two groups: memory-screw and Hyrax-screw groups. Maxillary first premolars and first molars were banded and four-armed expansion screws were soldered to the bands in both groups. Expansion was carried out by means of midpalatal jackscrews. Memory-screw group included 17 patients (nine females and eight males), while Hyrax-screw group comprised 15 patients (eight females and seven males). The mean ages of the subjects in the memory-screw and Hyrax-screw groups were 13.00 ± 1.29 and 12.58 ± 1.50 years, respectively. In the conventional Hyrax-screw group, the patients were instructed to activate the jack screw (Product number: 167-1633—Palatal split screw type ‘N’; Forestadent, Pforzheim, Germany; Forestadent USA, St Louis, Missouri, USA) twice (0.225 × 2 = 0.45 mm) a day until the suture was opened and then one turn per day for the remainder of the RME treatment as suggested by Zimring and Isaacson (1965) and Isaacson and Ingram (1964). In the memory-screw group, the patients were instructed to activate the jack screw (Product number: 167M1529—Memory expander type ‘N’; Forestadent; Forestadent USA) six times (0.2 × 6 = 1.2 mm) a day: two in the morning, two after lunch, and two in the evening (Figure 1).

Figure 1

Occlusal photographs with radiographs at the beginning of the treatment (T1) and at the end of expansion (T2) and retention periods (T3) of rapid maxillary expansion caused by memory screw.

 

These screws were activated until the occlusal aspect of the maxillary lingual cusp of the upper first molars contacted the occlusal aspect of the facial cusp of the mandibular first molars, thus producing the desiring expansion. The mean expansion period was 7.76 ± 1.04 days in the memory-screw group and 35.46 ± 9.39 days in the Hyrax-screw group. The average total screw turning activated in the memory-screw group was 46.52 ± 6.42 turns and that in the conventional Hyrax-screw group was 40.46 ± 9.39 turns; however, it was not fixed for both the screws after expansion. Furthermore, the mean retention period was 6.42 ± 0.59 months in the memory-screw group and 6.17 ± 0.32 months in the Hyrax-screw group.

The splitting at the suture palatine media was observed on different days of the expansion treatment. In all the patients, sutural opening without any problem and suture palatine media that was filled with bone after the retention periods were determined using occlusal radiographs (Figure 1).

Plaster models and postero-anterior cephalograms were taken from the patients at the beginning of the treatment (T1) and at the end of expansion (T2) and retention periods (T3).

The evaluation of the models was carried out according to the method described by Oktay and Kılıç (2007). Briefly, a line (1 mm in diameter) was drawn on the upper plaster models using a brush and barium sulfate solution. The line began from the gingival margin of the mesiobuccal cusp of the maxillary right first permanent molar, passed through the tips of the mesiobuccal and mesiopalatal cusps of that tooth, crossed the palatal vault between the first molars, and ended at the vestibular gingival margin of the left first permanent molar. After drawing, the models were placed in a cabinet plastic box that permitted X-rays to pass freely and then a radiographic image of the box was obtained in a standardized manner. The films were taken by a cephalometric unit manufactured by Siemens Corporation (Erlangen, Germany), which was operated at 65 kVp and 15 mA with an exposure time of 15 seconds.

The radiographs were scanned (Epson Expression 1860 Pro, Seiko Epson Corp., Nagonaken, Japan) under ×100 magnification and digitized by one of the authors using Quick Ceph 2000 (Quick Ceph systems, San Diego, California, USA). Eight points were used in the evaluation of dentoalveolar inclination. Three angles were measured on the radiographs, which included right molar crown tipping, left molar crown tipping, and alveolar process inclination (Figure 2).

 

Figure 2

The reference points used for dentoalveolar inclination: (1–4) right and left mesiobuccal and mesiopalatal cusp tips, (5 and 7) right and left upper alveolar tipping points (midpoint of the junction between alveolar process and palatal gingiva of the first molar), and (6 and 8) right and left lower alveolar tipping points (midpoint of the junction between alveolar process and palatal shelf). The references angles used for dentoalveolar inclination: α 1 and α 2, inner angles between the transversal occlusal line connecting the mesio-palatal cusp tips of the right and left molars and the lines passing through the mesio-buccal and mesio-palatal cusp tips of the molars. α3, inner angle between the right and left alveolar lines connecting the upper and lower alveolar tipping points in each side.

 

In addition, maxillary expansion was evaluated at T1, T2, and T3 by measuring the changes in the width on the study models. All the measurements were carried out by one of the authors using high-precision digital callipers with an accuracy of 0.01 mm (Digimatic Calliper CD-6 inCX; Mitutoyo American, Plymouth Michigan, USA).

Postero-anterior radiographs were scanned using the same equipments under ×100 magnification and digitized by one of the authors using Quick Ceph 2000. Fourteen points and two planes were used in the evaluation of transversal changes, and eight distances were measured on the radiographs (Figure 3).

Figure 3

Postero-anterior cephalometric landmarks: Lateroorbitale (Lo): the intersection of the lateral wall of the orbit and the greater wing of the sphenoid. Zygomatic (Z): the most lateral point of Zygomatic arch. Maxillary (Mx): the point located at the depth of the concavity of the lateral maxillary contour. Nasal (Nc): the most lateral point of the nasal cavity. Antegonion (Ag): the point located at the antegonial notch. Upper molar (Um): the most prominent lateral point of the buccal surface of the upper first molar. Lower molar (Lm): the most prominent lateral point of the buccal surface of the lower first molar. ‘Postero-anterior cephalometric planes’: frontal face plane (FFP) and occlusal plane. ‘Postero-anterior cephalometric measurements’: (1) face width (Zgr–Zgl), (2) maxillary width (Mxr–Mxl), (3) nasal width (Ncr–Ncl), (4) mandibular width (Agr–Agl), (5) right maxillomandibular width (Mxr–FFP), (6) left maxillomandibular width (Mxl–FFP), (7) right molar relation (Umr–Lmr), and (8) left molar relation (Uml–Lml). (7) and (8) measurements are called posterior overjet.

 

This research was approved by the local ethics committee (23.06.2006–2006.3,1/20).

Statistical Analysis

Fifteen randomly selected radiographs and plaster models were retraced and remeasured by the same investigator 2 weeks after the initial analysis. The error of the method was examined using the coefficient of reliability calculated for each measurement: coefficient of reliability = 1−Se2/St2, where Se2 is the variance due to random error and St2 is the total variance of the measurements (Houston, 1983).

The data were analysed using SPSS for Windows, version 10.0 (SPSS Inc., Chicago, Illinois, USA). ‘Shapiro–Wilk Normality test’ was used to determine whether the investigated parameters were homogeneous or not. To determine the treatment changes within the group, ‘paired t-test’ and ‘Wilcoxon signed-ranks test’ were applied to the homogeneous and non-homogeneous parameters, respectively. Comparison between the groups was carried out using ‘Student’s t-test’ for homogeneous parameters and by Mann–Whitney U-test for the rest.

 

Results

The value of the coefficient of reliability was above 0.90 for all the measurements.

Changes within the Group

The means and standard deviations of the changes on the dentofacial structures in the memory-screw and Hyrax-screw groups and their within-group comparisons are shown in Tables 1 and 2, respectively.

 

Table 1

Descriptive statistics of the measurements at the beginning of the treatment (T1) and at the end of expansion (T2) and retention periods (T3) in the memory-screw group (N = 17) and their within-group comparisons.

Memory-screw groups T2–T1 T3–T2 T3–T1
T1 T2 T2 T3 T1 T3
Parameters Mean SD Mean SD t Mean SD Mean SD t Mean SD Mean SD t
Upper interpremolar width 29.82 3.33 37.57 3.88 −17.556*** 37.57 3.88 38.04 3.54 −2.461* 29.82 3.33 38.04 3.54 −18.709***
Upper intermolar width 41.22 2.75 49.36 3.35 −3.621*** 49.36 3.35 50.17 3.14 −3.053** 41.22 2.75 50.17 3.14 −3.621***
Lower intercanine width 26.08 1.82 26.23 1.83 −4.021*** 26.23 1.83 26.57 1.86 −3.362** 26.08 1.82 26.57 1.86 −4.790***
Lower interpremolar width 33.91 1.96 34.16 1.84 −3.137** 34.16 1.84 34.53 1.79 −2.950** 33.91 1.96 34.53 1.79 −4.147***
Lower intermolar width 49.70 3.03 49.99 3.03 −3.116** 49.99 3.03 50.41 2.82 −2.975** 49.70 3.03 50.41 2.82 −4.575***
Molar crown tipping, right (α1) 4.46 3.39 13.41 4.12 −12.534*** 13.41 4.12 13.23 4.71 −0.425 4.46 3.39 13.23 4.7 −9.248***
Molar crown tipping, left (α2) 10.74 7.23 18.77 5.89 −7.388*** 18.77 5.89 19.03 6.47 −0.445 10.74 7.23 19.03 6.47 −7.068***
Alveolar process inclination (α3) 66.04 10.81 74.37 9.71 −9.865*** 74.37 9.71 74.66 9.73 −1.025 66.04 10.81 74.66 9.73 −11.514***
Zr–Zl 130.77 5.06 131.44 5.06 −5.685*** 131.44 5.06 132.19 4.80 −3.895*** 130.77 5.06 132.19 4.80 −6.109***
Mxr–Mxl 61.58 3.11 65.11 2.99 −19.011*** 65.11 2.99 65.32 3.06 −3.038** 61.58 3.11 65.32 3.06 −21.206***
Ncr–Ncl 30.71 2.62 33.88 2.69 −24.667*** 33.88 2.69 33.83 2.64 1.089 30.71 2.62 33.83 2.64 −23.786***
Agr–Agl 89.51 4.33 90.22 4.63 −4.762*** 90.22 4.63 91.47 4.63 −5.310*** 89.51 4.33 91.47 4.63 −6.639***
Mxr–FFP 13.65 1.43 12.79 1.56 8.433*** 12.79 1.56 12.80 1.53 −0.126 13.65 1.43 12.80 1.53 10.042***
Mxl–FFP 12.92 2.00 11.70 2.34 −3.625*** 11.70 2.34 11.84 2.21 −0.986 12.92 2.00 11.84 2.21 −3.626***
Umr–Lmr −2.22 1.41 2.44 0.67 −13.239*** 2.44 0.67 2.54 0.660 −3.367** −2.22 1.41 2.54 0.660 −13.918***
Uml–Lml −2.88 1.13 2.34 0.76 −17.423*** 2.34 0.76 2.46 0.76 −2.970** −2.88 1.13 2.46 0.76 −18.121***
  • † Wilcoxon test (z → t).
  • *P < 0.05; **P < 0.01; ***P < 0.001.

Table 2

Descriptive statistics of the measurements at the beginning of the treatment (T1) and at the end of expansion (T2) and retention periods (T3) in the Hyrax-screw group (N = 15) and their within-group comparisons.

Hyrax-screw groups T2–T1 T3–T2 T3–T1
T1 T2 T2 T3 T1 T3
Parameters Mean SD Mean SD t Mean SD Mean SD t Mean SD Mean SD t
Upper interpremolar width 30.09 2.83 38.37 4.38 −14.155*** 38.37 4.38 38.11 3.99 1.686 30.09 2.83 38.11 3.99 −16.036***
Upper intermolar width 41.18 2.89 50.06 3.84 −13.723*** 50.06 3.84 49.60 3.60 2.876* 41.18 2.89 49.60 3.60 −15.570***
Lower intercanine width 25.93 1.91 26.33 1.93 −3.267*** 26.33 1.93 26.56 1.91 −3.551*** 25.93 1.91 26.56 1.91 −3.574***
Lower interpremolar width 33.93 3.54 33.96 2.62 −0.045 33.96 2.62 34.39 2.48 −5.847*** 33.93 3.54 34.39 2.48 −1.115
Lower intermolar width 48.48 3.20 48.91 3.15 −5.609*** 48.91 3.15 49.21 2.97 −1.350 48.48 3.20 49.21 2.97 −2.967**
Molar crown tipping, right (α1) 7.27 6.22 15.95 7.04 −5.279*** 15.95 7.04 15.58 6.14 −0.337 7.27 6.22 15.58 6.14 −7.237***
Molar crown tipping, left (α2) 9.49 6.39 17.86 6.94 −5.295*** 17.86 6.94 17.34 6.63 0.382 9.49 6.39 17.34 6.63 −5.290***
Alveolar process inclination (α3) 62.51 11.10 73.51 9.62 −8.580*** 73.51 9.62 73.24 9.78 0.616 62.51 11.10 73.24 9.78 −7.949***
Zr–Zl 128.95 5.20 129.48 5.016 −4.654*** 129.48 5.016 130.07 4.90 −5.024*** 128.95 5.20 130.07 4.90 −5.163***
Mxr–Mxl 62.36 2.12 66.02 1.80 −19.398*** 66.02 1.80 66.16 1.85 −2.881* 62.36 2.12 66.16 1.85 −20.381***
Ncr–Ncl 31.85 3.41 34.62 3.30 −12.632*** 34.62 3.30 34.58 3.27 0.705 31.85 3.41 34.58 3.27 −12.964***
Agr–Agl 90.28 3.32 91.33 3.18 −6.384*** 91.33 3.18 92.80 3.41 −4.885*** 90.28 3.32 92.80 3.41 −8.065***
Mxr–FFP 13.97 1.63 13.02 1.61 12.954*** 13.02 1.61 13.13 1.66 −1.677 13.97 1.63 13.13 1.66 8.060***
Mxl–FFP 13.00 1.58 12.11 1.70 9.805*** 12.11 1.70 12.22 1.58 −2.416* 13.00 1.58 12.22 1.58 9.097***
Umr–Lmr −2.07 1.42 2.71 0.53 −14.162*** 2.71 0.53 2.63 0.57 2.103 −2.07 1.42 2.63 0.57 −13.655***
Uml–Lml −2.26 1.64 2.05 0.80 −10.346*** 2.05 0.80 2.06 0.77 −0.459 −2.26 1.64 2.06 0.77 −10.767***
  • † Wilcoxon test (z → t).
  • *P < 0.05; **P < 0.01; ***P < 0.001.

Changes in the Memory-Screw Group

During treatment (T1–T2).

Right and left maxillomandibular width (Mxr–FFP) and Mxl–FFP demonstrated a statistically significant decrease (P < 0.001), while all the other postero-anterior parameters showed statistically significant increase (P < 0.001) during the treatment.

Furthermore, the upper interpremolar, upper intermolar, lower intercanine width, right and left crown tipping, and palatal angulation angles (α1, 2, and 3) significantly increased (P < 0.001). In addition, the lower interpremolar and intermolar width also increased (P < 0.01).

Retention Period (T3–T2).

At the retention period, the face width (Zr–Zl) as well as mandibular width (Agr–Agl; P < 0.001), maxillary width, and right and left molar relation (Umr–Lmr and Uml–Lml) increased (P < 0.01), while the nasal width (Ncr–Ncl) and right and left maxillomandibular width (Mxr–FFP and Mxl–FFP) showed no statistically significant changes.

Furthermore, the upper intermolar, lower intercanine, interpremolar, intermolar widths (P < 0.001), and upper interpremolar width also increased (P < 0.01). However, right and left crown tipping and palatal angulation angles (α1, 2, and 3) showed no statistically significant changes.

Total Treatment Period (T3–T1).

At the end of all the observation periods, the right and left maxillomandibular width (Mxr–FFP and Mxl–FFP) showed statistically significant decrease (P < 0.001), while all the other postero-anterior parameters demonstrated statistically significant increase (P < 0.001). Furthermore, all the parameter measurements in the model demonstrated statistically significant increase (P < 0.001).

 

Changes in the Hyrax-Screw Group

During Treatment (T1–T2).

During treatment, while the right and left maxillomandibular width (Mxr–FFP and Mxl–FFP) showed statistically significant decrease (P < 0.001), all the other postero-anterior parameters demonstrated statistically significant increase (P < 0.001). In addition, except lower interpremolar, all the other parameters significantly increased (P < 0.001).

Retention Period (T3–T2).

At the retention period, the face width (Zr–Zl) and mandibular width (Agr–Agl; P < 0.001), as well as maxillary width and right and left molar relation (Umr–Lmr and Uml–Lml) increased (P < 0.05), while the nasal width (Ncr–Ncl), right and left maxillomandibular width (Mxr–FFP and Mxl–FFP) as well as right and left molar relation (Umr–Lmr and Uml–Lml) showed no statistically significant changes.

Furthermore, lower intercanine and intermolar width significantly increased (P < 0.001), while the upper intermolar width decreased (P < 0.05). However, right and left crown tipping, palatal angulation angles (α1, 2, and 3), upper interpremolar, and lower intermolar showed no statistically significant changes.

Total treatment Period (T3–T1).

At the end of all the observation periods, right and left maxillomandibular width (Mxr–FFP and Mxl–FFP) showed statistically significant decrease (P < 0.001), while all the other postero-anterior parameters showed statistically significant increase (P < 0.001).

Furthermore, lower interpremolar showed no statistically significant changes, while lower intermolar width (P < 0.01) and all the other parameters increased (P < 0.001).

 

Comparison between the Groups

The means and standard deviations of the treatment changes observed and their comparisons between the groups are shown in Table 3.

 

Table 3:

Descriptive statistics of the differences in the measurements at the beginning of the treatment (T1) and at the end of expansion (T2) and retention periods (T3) and their between-group (N = 32) and within-group comparisons.

Parameters T2–T1 T3–T2 T3–T1
Memory Hyrax Memory Hyrax Memory Hyrax
Mean SD Mean SD t Mean SD Mean SD t Mean SD Mean SD t
Upper interpremolar width 7.74 1.82 8.29 2.27 −0.755 0.48 0.81 −0.26 0.60 2.921** 8.23 1.81 8.03 1.94 0.299
Upper intermolar width 8.14 1.78 8.87 2.51 −0.961 0.80 0.81 −0.46 0.62 4.920*** 8.94 1.73 8.41 2.09 0.788
Lower intercanine width 0.15 0.15 0.39 0.44 −1.964* 0.34 0.41 0.23 0.45 −0.208 0.49 0.42 0.62 0.66 −1.851
Lower interpremolar width 0.24 0.32 0.02 1.62 −1.209 0.38 0.53 0.44 0.29 −1.416 0.62 0.61 0.46 1.58 −1.001
Lower intermolar width 0.30 0.40 0.43 0.30 −1.852 0.42 0.58 0.30 0.87 −0.925 0.72 0.65 0.73 0.96 −0.076
Molar crown tipping, right (α1) 8.95 2.94 8.68 6.37 0.155 −0.18 1.77 −0.37 4.22 0.165 8.76 3.91 8.31 4.45 0.306
Molar crown tipping, left (α2) 8.03 4.48 8.37 6.12 −0.183 0.26 2.40 −0.52 5.27 0.549 8.29 4.83 7.85 5.75 0.232
Alveolar process inclination (α3) 8.34 3.48 10.99 4.96 0.087 0.29 1.16 −0.27 1.68 0.170 8.62 3.08 10.73 5.23 0.280
Zr–Zl 0.66 0.48 0.53 0.44 0.294 0.76 0.80 0.59 0.46 0.848 1.42 0.96 1.12 0.84 0.197
Mxr–Mxl 3.53 0.77 3.66 0.73 −0.492 0.21 0.28 0.14 0.19 0.771 3.74 0.73 3.80 0.72 −0.252
Ncr–Ncl 3.18 0.53 2.77 0.85 1.659 −0.05 0.20 −0.04 0.22 −0.174 3.12 0.54 2.73 0.82 1.641
Agr–Agl 0.71 0.62 1.05 0.64 0.099 1.25 0.97 1.47 1.16 0.777 1.96 1.21 2.52 1.21 0.151
Mxr–FFP −0.86 0.42 −0.95 0.29 0.734 0.01 0.19 0.11 0.25 −1.299 −0.85 0.35 −0.85 0.41 −0.047
Mxl–FFP −1.22 0.56 −0.89 0.35 0.088 0.14 0.54 0.11 0.17 0.970 −1.08 0.50 −0.78 0.33 0.090
Umr–Lmr 4.66 1.45 4.78 1.30 −0.247 0.10 0.12 −0.08 0.15 3.773*** 4.76 1.40 4.70 1.33 0.121
Uml–Lml 5.21 1.23 4.31 1.61 1.796 0.12 0.17 0.01 0.11 2.117* 5.34 1.21 4.32 1.55 2.072*
  • † Mann–Whitney U-test (z → t).
  • *P < 0.05; **P < 0.01; ***P < 0.001.

 

 

During Treatment (T1–T2).

During treatment, all the postero-anterior parameters showed no statistically significant difference between the groups. In the model measurements, only lower intercanine width was significantly different between the groups (P < 0.05).

 

Retention Period (T3–T2).

At the retention period, right molar relation (Umr–Lmr; P < 0.001) and left molar relation (Uml–Lml) showed statistically significant difference between the groups (P < 0.001 and P < 0.05, respectively). Furthermore, in the model measurements, upper interpremolar and upper intermolar width demonstrated statistically significant difference between the groups (P < 0.01 and P < 0.001, respectively).

Total Treatment Period (T3–T1).

Only the left molar relation (Uml–Lml) showed statistically significant difference between the groups (P < 0.05), while in the model measurements, no significant difference between the groups was observed.

 

Discussion

The control group allowed us to differentiate the treatment effects from normal growth. In the present study, the total observation period was 6–7 months for both the groups. Thus, no control group was used because we believe that growth is negligible in this short period.

Generally, RME screw is turned twice during expansion. Zimring and Isaacson (1965) as well asIsaacson and Ingram (1964) suggested turning the screw twice until the suture opens in adolescent people and recommended that slower rates of expansion would allow for physiologic adjustment at the maxillary articulations and prevent residual force. In this study, the screw was activated twice daily until the suture has opened and then once per day in the conventional RME group. In the memory-screw group, screw was activated six times a day during RME. The difference between the screw activation protocols results from the different levels of forces produced by screws. Isaacson and Ingram (1964) measured the forces produced by conventional RME appliance and found that a quarter turns of the screw produced heavy and intermittent forces ranging from 1.5 to 4 kg, as well as expansion force up to approximately 10 kg. However, Wichelhaus et al. (2004) evaluated that memory screw activated six times a day could generate a continuous force ranging from 1.225 to 1.425 grams. Additionally, Zimring and Isaacson (1965) hypothesized that the total expansion might be physiologically stable in a shorter treatment time with the expansion procedures carried out at lower forces. In our study, adequate maxillary expansion was accomplished with the memory screw in 7.76 ± 1.04 days with relatively lower forces than that produced by conventional expanders. Nevertheless, the hypotheses by Zimring and Isaacson (1965) as well asIsaacson and Ingram (1964) have not been confirmed till date, and the results of this study are very important to confirm them.

At the end of the expansion period (T2–T1), the amount of mean intermolar distance was 8.14 ± 1.78 mm in the memory-screw group and 8.87 ± 2.51 mm in the Hyrax-screw group. Furthermore, at the end of the retention period (T3–T1), the amount of mean intermolar distance was 8.94 ± 1.73 mm in the memory-screw group and 8.41 ± 2.09 mm in the Hyrax-screw group. Similar increase occurred in the upper interpremolar distances of both the groups.

Relapse after maxillary expansion is a common issue in the literature (Hicks, 1978Sarnas et al., 1992), and in our study, Hyrax screw was narrowed in the retention period. Furthermore, both the screws were not fixed after expansion because according to the manufacturer’s instructions, the spindle has been designed to prevent the screw from turning back. Therefore, we recommend fixture of the Hyrax screws after expansion. Conversely, the intermolar width continued to increase at this period in the memory-screw group. The memory screw provided additional expansion in the retention period, which might have probably resulted from the activation of the nickel–titanium springs. In other words, in the retention period, nickel–titanium springs integrated in the screw might have resisted the residual forces that are considered to be the source of relapse. Halıcıoğlu et al. (2010) reported similar results when using memory screw for expansion, where the interpremolar and molar width increased in the retention period. Before completing the active phase of RME, it should be taken into account that some amount of increments will be observed in intermolar and interpremolar widths during the retention period.

Braun et al. (2000) demonstrated that the amount of dentoalveolar tipping depends on factors, such as the type of expansion appliance, age of the patient, resistance of the surrounding tissues, and schedule of screw activation. In addition, Alpern and Yurosko (1987) recommended bilateral buccal corticotomy for this purpose. It has been observed that the rigidity of the expansion appliance is one of the factor responsible for decreasing the buccal tipping of the anchorage teeth (Timms, 1981). Although many different RME appliances have been developed to decrease and/or minimize the dentoalveolar inclination (Haas, 1961Timms, 1981Memikoglu and Işeri, 1999Başçiftçi and Karaman, 2002Kılıç et al., 2008), occurrence of dentoalveolar tipping is unavoidable (Wertz, 1970Bishara and Staley, 1987).

Tipping movement of the posterior dental and/or alveolar process caused by expansion has been evaluated on postero-anterior films (Asanza et al., 1997Byloff and Mossaz, 2004), tomography (Ölmezet al., 2007), and plaster models (da Silva Filho et al., 1991Ciambotti et al., 2001Chung and Goldman, 2003Kılıç et al., 2008) using different methods. Recently, Oktay and Kılıç(2007) have developed a new method to evaluate the buccal inclination of the maxillary posterior dentoalveolar structures. This new method provides precise cross-sectional views of the morphology of the molar area and is simple, reliable, and inexpensive. Moreover, in such techniques, the patients are not exposed to excessive X-rays (Kılıç et al., 2008). Therefore, this method was used in this study.

Hicks (1978) found that the angulation between the molars increased from 1 to 24 degrees during slow maxillary expansion (SME). In addition, Ciambotti et al. (2001) and Erdinç et al. (1999) also showed that the SME appliances cause more dentoalveolar tipping than the RME appliances. However, numerous authors have claimed that the tipping of the alveolar processes and/or supporting teeth by the rigid appliance was lesser than that by the Hyrax appliance (Başçiftçi and Karaman, 2002İşeri and Özsoy, 2004Kılıç et al., 2008).

In the present study, both the screws produced statistically significant (P < 0.001) dentoalveolar tipping. At the end of the expansion period (T2–T1), the right crown tipping (α1), left crown tipping (α2), and palatal tipping (α3) values (degree) were 8.95 ± 2.94, 8.03 ± 4.48, and 8.34 ± 3.48 degrees, respectively, in the memory-screw group and 8.68 ± 6.37, 8.37± 6.12, and 10.99 ± 4.96 degrees, respectively, in the Hyrax-screw group. Furthermore, the anchorage teeth are expected to tip during expansion and remain upright at retention (Hicks, 1978). On the other hand, at the end of the retention period (T3–T1), the right crown tipping (α1), left crown tipping (α2), and palatal tipping (α3) values (degree) were 8.76 ± 3.91, 8.29 ± 4.83, and 8.62 ± 3.08 degrees, respectively, in the memory-screw group and 8.31 ± 4.45, 7.85 ± 5.75, and 10.73 ± 5.23 degrees, respectively, in the Hyrax-screw group. However, the right crown tipping (α1) and left crown tipping (α2) increased along with the retention period in the memory-screw group. This again might have resulted from the nickel–titanium springs in the screw, which exert continuous force and prevent molar teeth to remain upright. On the contrary, as expected, at the retention period, these angles decreased in the Hyrax-screw group. However, no statistically significant differences were found between the groups.

Our findings regarding the banded appliances are consistent with the literature. In addition, our crown tipping values are comparable with those presented in the work by in Kılıç et al. (2008) (9.47 and 9.16 degrees, respectively), who employed the same method to evaluate the tipping caused by the Hyrax appliance. However, Asanza et al. (1997) and Ciambotti et al. (2001) found less molar tipping in patients treated with a Hyrax expander. This difference might be owing to the different measurement techniques employed for the determination of inclination. Additionally, in a recent study, Garrett et al. (2008) showed that dentoalveolar inclination lacks significant correlations with the schedule of screw activation but correlate well with the amount of expansion. In the present study, both groups have most maxillary expansion in between these studies that found less molar tipping.

Oliveira et al. (2004) found that the Hyrax expanders could produce more dentoalveolar effects by increasing the palatal angulation (8.77 degree) than the Haas expanders. Kılıç et al. (2008) also demonstrated that the Hyrax appliance (11.30 degree) tipped the alveolar process more than the rigid acrylic bonded expansion appliance. Our findings are consistent with these studies.

Bishara and Staley (1987) concluded that the RME could influence mandibular dentition, but the accompanying changes were neither pronounced nor predictable. However, Gryson (1977) reported that the mean increase in the mandibular intermolar width was 0.4 mm, and this finding is consistent with our study.

In our study, the sutural opening and the subsequent important skeletal and dental expansion were obtained in all patients of both the groups. When viewed from a frontal plane, a pyramidal opening of the maxilla could be observed (Haas, 1961). Our results showed that after RME, the amount of expansion followed a triangular pattern, with the greatest increase in the maxillary arch width, followed by maxillary width, nasal width, and face width. Previous studies have reported similar changes in the dental and skeletal structures after RME (Haas, 1961Wertz, 1970Memikoglu and Işeri, 1999Başçiftçi et al., 2002). Garrett et al. (2008), who investigated the changes after RME with computerized tomography, showed that the nasal width of approximately 2 mm and maxillary width of approximately 3 mm increased to an average of 5 mm after screw activation, and these findings are compatible with our data.

Thus, our study showed that the transversal skeletal width significantly increased after RME and that the results were stable at the end of the retention period. In other words, the RME carried out using the memory screw resulted in stable expansion, similar to the Hyrax screw.

Conclusions

RME procedure has been successfully carried out in both groups. In the memory-screw group, it may be considered as advantageous for the clinician to complete maxillary expansion in a relatively short time. However, long-term stability of this type of maxillary expansion with lighter forces in a shorter time should be evaluated in a larger patient population.

Oxford University Press, The European Journal of Orthodontics

First published online: 29 August 2011

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KorayHalıcıoğlu, İbrahim Yavuz

http://ejo.oxfordjournals.org/content/36/2/140

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