Evaluation of the stiffness characteristics of rapid palatal expander screws
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Abstract
Background
The aim of this study is to evaluate the mechanical properties of the screws used for rapid expansion of the upper jaw.
Methods
Ten types of expansion screw were assessed, seven with four arms: Lancer Philosophy 1, Dentaurum Hyrax Click Medium, Forestadent Anatomic Expander type “S”, Forestadent Anatomic Expander type “S” for narrow palates, Forestadent Memory, Leone A 262010 with telescopic guide, and Leone A 063010 with orthogonal arms; and three with two arms: Dentaurum Variety S.P., Target Baby REP Veltri, and Leone A 362113. A test expander with the mean dimensions taken from measurements on a sample of 100 expanders was constructed for each screw. The test expanders were connected to the supports of an Instron 4467 (Instron Corp., USA) mechanical testing machine equipped with a 500 N load cell, and the compression force exerted after each activation was measured. The mean forces expressed by the two and fourarm expanders were then compared.
Results
After five activations, the forces expressed by the twoarm devices were double than those expressed by the fourarm devices on average (224 ± 59.9 N vs. 103 ± 32.9 N), and such values remained high after subsequent activations.
Conclusions
The expanders tested demonstrated stiffness characteristics compatible with opening of the palatine sutures in preadolescent patients. The stiffness of such devices can be further increased during the construction phase.
Keywords
Rapid Maxillary Expansion Stiffness Characteristic Rapid Palatal Expander Median Suture Expansion ScrewBackground
Normalizing the dimensions of the upper jaw is of primary importance in orthodontics. In fact, an upper jaw of incorrect dimensions may affect both the transversal and sagittal planes [1]. A rapid palatal expander (RPE) is the most popular device of choice in this regard, characterized by safety, predictability, and efficiency [2, 3, 4, 5, 6, 7, 8].
RPEs have a predominantly orthopedic action, although they do bring about a certain degree of dental expansion, in turn provoking labial inclination of the teeth. This effect becomes more pronounced as age advances, since an increase in the interdigitation at the palatine suture increases its resistance to opening and reduces the orthopedic effects in favor of dental effects [9, 10, 11, 12].
Indeed, the change in skeletal transversal dimensions decreases from 50 % to roughly a third of the quantity of RPE screw activation after the pubertal growth peak in initial permanent dentition [13].
Rapid maxillary expansion treatment is able to induce more pronounced transverse craniofacial changes at the skeletal level before the peak in skeletal growth, and skeletal outcomes of greater magnitude and stability can be obtained when the expander is used before the pubertal growth spurt. When RME therapy is performed after the pubertal peak, on the other hand, transverse changes shift to the dentoalveolar level [14, 15].
Various models of screws and operating protocols have been suggested, both for achieving standard expansion and for activating the premaxillary sutures via alternate phases of expansion and contraction [16, 17, 18, 19]. In order to open the median palatine suture to a sufficient degree and contemporaneously avoid a significant dentoalveolar response, RPEs must exert intense levels of force within a short timeframe. Hence, they must possess sufficient stiffness characteristics to enable them to exert such forces without deformation, so as to minimize the inclination of the teeth [20, 21]. Furthermore, the stiffness characteristics of an expander must be increased when a patient presents with a particularly deep palate [22].
In this regard, the use of miniscrews to stabilize RPEs seems to be helpful, especially in late adolescence, and is currently the focus of ongoing research [23, 24]. However, to date, available data is scarce. For example, Muchitsch et al. [25] analyzed only the mechanical characteristics of the arms of RPEs, while Camporesi et al. [26] analyzed the compressive forces developed at each activation of three types of expander screw.
With a view to reducing patient discomfort and facilitating oral hygiene procedures, manufacturers are developing and marketing increasingly less bulky, more streamlined RPEs [27, 28], and we set out to evaluate the stiffness characteristics of several such devices.
Methods
Screw characteristics
Twoarm screws  Max. expansion  Arm ∅ (mm)  Screw body size (mm)  Amount of expansion per activation (mm)  Lot no. 
Dentaurum Variety S.P. twoarms  12  1.48  9.6 × 5 × 3  0.8  435299 
Veltri Target baby REP  13  1.45  11 × 6 × 4.5  0.8  700032 
Leone A 362113 twoarms  13  1.48  10 × 6 × 4.5  0.8  12032901 
Fourarm screws  Max. expansion  Arm ∅ (mm)  Screw body size (mm)  Amount of expansion per activation (mm)  Lot n° 
Lancer Philosophy 1  10  1.55  8 × 8 × 3.5  0.8  RPE OOO440 
Dentaurum Hyrax Click Medium  10  1.48  10 × 11 × 4  0.8  435361 
Forestadent Anatomic Exp. Type “S”  10  1.48  12 × 7 × 3.5  0.8  48297815 
Forestadent Type “S” for narrow palates  10  1.48  12 × 7 × 3.5  0.8  7399006 
Forestadent Memory  10  1.48  15 × 10 × 4  0.8  14957593 
Leone A 262010 with telescopic guides  10  1.48  14 × 11 × 4  0.8  12122001 
Leone A 063010 with orthogonal arms  10  1.48  10 × 6 × 4.5  0.8  13011601 
RPE construction
Each screw was used in the construction of an RPE modeled on average values derived from measurements made on 100 expanders constructed to fit 100 Caucasian patients (54 females and 46 males) aged between 8 and 13. All patients of the sample needed a RPE treatment. The patients already treated by orthodontics were excluded. This age range was chosen because the RPE allows favorable orthopedic changes and it is widely utilized by patients of this age.
Means of measures
1. Measurement  Kind of measure  Mean value 

Length of posterior arms (including screw body)  41.3 mm  
Length of anterior arms (including screw body)  32.5 mm  
Angle between screw body and posterior arms  146.2°  
Angle between screw body and anterior arms  144°  
Distance between anterior and posterior arms  19.8 mm 
The first molars were removed from these models and replaced with analogous metal teeth, joined together by means of a threaded pin to ensure that they remained parallel and that the RPEs constructed around them would be correctly aligned with the mechanical testing machine; an Instron 4467 (Instron Corp., USA) with 500N load cell was to be used for the stiffness testing.
Orthodontic bands (LEONE MOD. E8305 no. 14) were then fitted to the first molars of each model, and the test screw brazewelded on the bands. The RPE constructed for the fourarm screw featured palatal supports, and no bands on the premolars. The twoarm RPEs were welded to the first molars and featured no palatal supports. The constructed RPEs were bonded to the metal teeth using composite cement (ULTRA BANDLOCK, Reliance, USA) and lightcured using an LED curing light (Elipar Freelight 2, 3M Espe, wavelength 430 ÷ 480 nm, intensity 1200 mW/mm^{2}) for 30 s from the occlusal surface, the most effective method of bonding bands with this type of bonding agent [29].
RPE activation procedure
Statistical analysis
The statistical analysis aimed to assess how type (two arms, 2b, or four arms, 4b) and activation (1, 2,…, 26) influence the measured strength. It was performed on 10 unique models that were measured for up to 26 activations (the effective length of measurement varied by model) and the analysis was performed using the growth curve analysis [30] approach. In particular, the strength behavior upon activation was approximated using a threedegree polynomial as a function of activation (baseline model). Polynomial’s coefficients vary by type. To confirm the main conclusions t, a repeated measures ANOVA on the activation range where both 2A and 2B models were measured (1, 2,…, 11) consistently.
The polynomial functional expression of the baseline model is:

A modelspecific intercept (treated as random effect), indicating material specific strength response tendency

A constant intercept that applies when the model is 4b type, indicating the baseline variation of strength due for the sample being of 4b type

The terms β, γ, and δ, respectively, represent the first, second, and thirddegree polynomial coefficients and are supplementary polynomial terms that applies when the model is 4b. The cubic curve tries to approximate the nonlinear behavior of strength against activation, while the 4b terms represent interaction terms that allow the polynomial shape to change between 4a and 4b models which represent an additive error term. The repeated measures ANOVA assumed the type variable as the “between” factor and the activation (levels ranging from 1 to 11) as the “within” factor. A post hoc analysis has been therefore performed to compare difference in strength means between 4b and 2b types by activation level.
The statistical significance was assessed using a 5 % threshold. R software [31] was used throughout the data processing, and the lme4 R software package [32] was used to estimate both the growth curve and the repeated measures ANOVA. Post hoc analysis was performed using lsmeans R package [33].
Results
According to our results, the stiffest fourarm screw is the Leone A262010, which features telescopic guides, at a maximum force of 227 N, followed by the Dentaurum Hyrax Click, which expressed a maximum force of 200.2 N. The other RPEs present curves characterized by a quasilinear trend up to the ninth or tenth activation, followed by a steady reduction down to a plateau at which the force remains almost constant as the activations progress. Among these, that which developed the highest maximum force was the Lancer Philosophy 1 (179.9 N), followed by the Leone A063010 with orthogonal arms (157.5 N), the Forestadent Type S for narrow palates (148.6 N), and, finally, the Forestadent Anatomic Expander (124 N).
Results of stiffness tests
Type  Activations  Number  Mu  SD 

2B  1  3  37  14.6 
2B  2  3  96  24.7 
2B  3  3  147  46.6 
2B  4  3  183  61.3 
2B  5  3  224  59.9 
2B  6  3  256  49.7 
2B  7  3  275  34.0 
2B  8  3  282  11.0 
2B  9  3  286  8.2 
2B  10  3  284  20.7 
2B  11  3  278  17.1 
2B  12  3  271  19.7 
2B  13  3  280  12.0 
2B  14  3  275  14.1 
2B  15  3  265  4.2 
4B  1  7  24  6.6 
4B  2  7  44  11.6 
4B  3  7  66  18.0 
4B  4  7  86  26.2 
4B  5  7  103  32.9 
4B  6  7  118  36.8 
4B  7  7  131  39.2 
4B  8  7  141  40.1 
4B  9  7  151  41.6 
4B  10  7  158  40.9 
4B  11  7  164  42.2 
4B  12  7  165  45.4 
4B  13  7  158  42.8 
4B  14  7  160  41.3 
4B  15  7  162  41.5 
4B  16  7  163  40.6 
4B  17  7  164  41.2 
4B  18  7  164  40.8 
4B  19  7  164  40.4 
4B  20  7  162  39.8 
4B  21  7  162  38.6 
4B  22  7  174  30.7 
4B  23  7  172  29.2 
4B  24  7  149  6.4 
4B  25  7  149  6.8 
The twoarm RPEs assessed in this study expressed more than double the force of their fourarm counterparts, even at five activations (224 ± 59.9 N vs. 103 ± 32.9 N), maintaining far higher values as the activations progressed. Indeed, after 10 and 15 activations, the force remained over 250 N, reflecting the conserved high level of stiffness. However, any more than 15 activations were prevented by structural deformations causing a block in the activation mechanism. In contrast, the fourarm RPEs continue to express a fairly constant force even after 20 activations, albeit at a much lower level.
Growth curve analysis coefficients
Term  Estimate  Std. error  Statistic  Significance 

α  −47  21  −2.2  * 
β  85  6.9  12  * 
γ  −7.1  0.96  −7.4  * 
δ  0.19  0.04  4.8  * 
4b _{ i }  40  25  1.6  
β _{4b }  −56  7.3  −7.7  * 
γ _{4b}  5.5  0.99  5.6  * 
δ _{4b}  −0.16  0.04  −4.1  * 

The linear term β is positive, indicating an initial positive growth. The cubic term γ is negative indicating that the increase in strength levels off as far as activation progresses. The cubic term δ is negative finally. All the three terms are statistically significant indicating a nonlinear behaviors of activation vs. strength.

Activation and type 4b interaction (β _{ab}) is negative and significant. This indicates that the initial increase is less steep for type 4b models.

All the higher polynomial terms interaction are significant (γ _{ab}, δ _{ab}), indicating that the curvature for the growth dynamic of type 4b is different from that of the 2b models.
Modelspecific intercept
(Intercept)  

Dentaurum.Hyrax.Click.Medium  37 
Dentaurum.Variety.SP  1.1 
Forestadent.Anatomic.Expander.type.S  −39 
Forestadent.Anatomic.Expander.type.S.for.narrow.palates  −31 
Forestadent.Memory  0.27 
Lancer.Philosophy.1  5.4 
Leone.A.0630.10.with.orthogonal.arms  −16 
Leone.A.2620.10.with.telescopic.guide  44 
Leone.A.362113  13 
Target.Baby.REP.Veltri  −14 
During the course of the experiments, no breakage or deformation of any of the bands associated with any of the RPEs tested occurred.
Discussion
On the vertical plane, the line of force expressed by the screw is at the same distance from the center of resistance in the two types of RPEs. However, the twoarm devices are susceptible to greater deformation as they have two fewer arms and lack the palatal support that opposes vertical deformation (Figs. 12 and 13). This deformation involves both the screw body and the retention arms and is responsible for the deterioration in the force expressed by the twoarm RPEs after the peak.
Few studies on this topic can be found in the literature. In a recent study, Muchitsch et al. [25] compared the stiffness of the retention arms of 16 commercially available RPE screws and found a difference in stiffness of 37.32 %. Camporesi et al., [26] on the other hand, measured the force expressed by three different fourarm RPEs, like us using the Instron machine, but without any bands or welding; they revealed maximum forces ranging between 215 and 156 N. In contrast, Zimring and Isaacson [21], using intraoral dynamometers fixed to the RPEs mounted in the mouth of a sample of 5 patients, found forces ranging from 73 and 154 N. Sander et al. [34] also found smaller forces, between 70 and 120 N, needed to activate a special precalibrated screw mounted in ten 9 to 13yearold patients via a hyperrapid activation protocol (one or two activations, five times a day).
As regards comparison of two and fourarm types of RPEs, Lamparsky et al. [27] conducted a study to evaluate the difference in their clinical effects, using radiographs to quantify the separation of the median suture and plaster models to measure the intercanine distance, intermolar distance, and arch perimeter before expansion, after the active expansion phase and after removal of the RPE. Their results suggest that there is a little difference in the clinical effects on the median suture and teeth brought about by two and fourarm RPEs.
In light of these studies, our in vitro results show that the forces expressed by RPEs welded to anchorage bands appear sufficient to separate the median palatine suture in preadolescent and adolescent patients, although the force expressed by certain models may be insufficient for this purpose in older patients. In particular, models Forestadent type S for narrow palates and Forestadent Anatomic Expander generate low maximum forces (respectively, 148.6 and 124 N) that may be insufficient for the clinical demands reported elsewhere (Sander et al. [34]: 70–120 N, Zimring and Isaacson [21]: 73–154 N). That being said, it is important to note that the stiffness in vivo will be strongly influenced by clinical factors such as the stiffness of the median palatine suture and the circummaxillary sutures, which is far lower than that of the Instron machine.
Recent literature is ever more frequently proposing the use of RPEs in adult patients, or using protocols of alternating expansion and contraction, which severely test the mechanical resistance of such devices [16, 17, 18, 19]. Hence, manufacturers should take into account the mechanical stiffness of the RPEs being manufactured, as well as comfort, hygiene, and versatility issues. RPEs need to be manufactured in such a way as to maximize skeletal effects and minimize unwanted dentoalveolar effects. That being said, the twoarm models we tested showed a loss of force due to deformation way beyond the levels of force clinically required to separate the median palatine suture and therefore appear to be fit for purpose in terms of stiffness, in agreement with the findings of the clinical trial conducted by Bratu et al. [28].
Conclusions
The twoarm RPEs seem to be stiffer than their fourarm counterparts, and although from a mechanical perspective both are effective means of bringing about rapid expansion of the palate, certain models of the fourarm RPE may not express sufficient force to separate the median suture after puberty. The stiffness of the fourarm RPEs can be increased by using bands or bonding to fix them to the anterior teeth, but in addition to patient comfort, manufacturers should focus on the stiffness of both the retention arms and the screw body, whose propensity to generate a bending moment can be reduced by reducing its size. To this end, more research is needed into the resistance characteristics of RPEs, in particular to assess their suitability for older patients and alternating activation/contraction protocols and to determine how best to enhance skeletal and reduce unwanted dentoalveolar effects.
Notes
Acknowledgements
All authors made substantive contributions to the article and assumes full responsibility for its content.
Authors’ contributions
All authors made substantive contributions to the article. LL and MF have conceived the design of the study. SE and MV have performed the tests and acquired the data. SGA has performed the statistical analysis. LM has drafted the manuscript and revised it critically. SG has given the final approval of the version to be published. All authors read and approved the final manuscript.
Competing interests
The authors declare that they have no competing interests.
Consent for publication
Written informed consent was obtained from the patient’s parent for the publication of this report.
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