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Year : 2009  |  Volume : 20  |  Issue : 4  |  Page : 437-441
In vitro evaluation of force-expansion characteristics in a newly designed orthodontic expansion screw compared to conventional screws

1 Department of Orthodontics, Shiraz Dental School, Shiraz, Iran
2 Department of Mechanical Engineering, Shiraz University, Shiraz, Iran
3 Department of Mechanical Engineering, Tarbiat Modares University (TMU), Tehran, Iran
4 Department of Prosthodontics, Mashhad Dental School, Mashhad, Iran

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Date of Submission24-Sep-2008
Date of Decision29-Jan-2009
Date of Acceptance19-Mar-2009
Date of Web Publication29-Jan-2010


Objective : Expansion screws like Hyrax, Haas and other types, produce heavy interrupted forces which are unfavorable for dental movement and could be harmful to the tooth and periodontium. The other disadvantage of these screws is the need for patient cooperation for their regular activation. The purpose of this study was to design a screw and compare its force- expansion curve with other types.
Materials and Methods : A new screw was designed and fabricated in the same dimension, with conventional types, with the ability of 8 mm expansion (Free wire length: 12 mm, initial compression: 4.5 mm, spring wire diameter: 0.4 mm, spring diameter: 3 mm, number of the coils: n0 ine, material: s0 tainless steel). In this in vitro study, the new screw was placed in an acrylic orthodontic appliance, and after mounting on a stone cast, the force-expansion curve was evaluated by a compression test machine and compared to other screws.
Results : Force-expansion curve of designed screw had a flatter inclination compared to other screws. Generally it produced a light continuous force (two to 3.5 pounds) for every 4 mm of expansion.
Conclusion : In comparison with heavy and interrupted forces of other screws, the newly designed screw created light and continuous forces.

Keywords: Force-expansion, orthodontic expansion, palatal expansion screws

How to cite this article:
Oshagh M, Momeni Danaei S H, Hematian M R, Oshagh M R, Zade A H, Saboori A A. In vitro evaluation of force-expansion characteristics in a newly designed orthodontic expansion screw compared to conventional screws. Indian J Dent Res 2009;20:437-41

How to cite this URL:
Oshagh M, Momeni Danaei S H, Hematian M R, Oshagh M R, Zade A H, Saboori A A. In vitro evaluation of force-expansion characteristics in a newly designed orthodontic expansion screw compared to conventional screws. Indian J Dent Res [serial online] 2009 [cited 2023 Jan 28];20:437-41. Available from:
Palatal expansion is a viable treatment in orthodontics. About 25 to 30% of all orthodontic patients and 95% of class II mal-occlusions can be treated by palatal expansion, correction of first molar angulation and distal drifting of molar teeth. [1] Posterior cross bite is caused by inter-arch discrepancy due to a narrow maxillary arch [2],[3] and palatal expansion is necessary to maintain a harmonious inter-arch and skeletal relationship. [4] Also, regarding the increased tendency toward the use of non-extraction treatments, expansion may be a solution for the space deficiency. [5],[6]

In posterior cross bites fixed or removable appliances are used depending on the age of the patient and the inducing factor (skeletal or dental cross bite). The theory for rapid maxillary expansion was that with rapid force application to the posterior teeth, there would not be enough time for dental movement and skeletal changes would be maximized but it was proved that the overall result of rapid versus slow expansion is similar and with slower expansion a more physiologic response is obtained. [2] Skeletal anchorage has been reported since the early 1980s [7] and dental expansion would be avoided by fixing the screws directly to the hard palate for maximizing skeletal changes. [8] Bone supported maxillary expansion with an implant borne hyrax screw caused skeletal expansion with less tipping of dental segments. [9],[10] Fixed appliances like Hyrax or Haas, apply forces about 3 to 10 pounds, which can reach up to 20 pounds in daily activation. [11] Different studies showed that heavy forces created by these appliances can cause alveolar bone loss and decrease the buccal bone width in posterior segment. [12],[13],[14] However, removable appliances create lighter forces ranging from two to four pounds and are harmless to periodontium. [15] The disadvantage of removable appliances includes dependency on patients cooperation; however, the slow rate of expansion is a noticeable advantage that increases the stability of the outcomes by reducing tissue resistance in naso-maxillary complex depending on the age of the patient. [16],[17] The most common removable appliance for this purpose, is an acrylic based removable appliance with one or two expansion screws. [4],[18],[15] Heavy interrupted forces created after activation of these screws can displace the appliance and frequent activations can also harm the tooth and periodontium. [2]

Some studies have questioned the application of 10 pound or higher forces and believed that slower activations or new designed screws with less expansion would be more helpful. [19] Storey showed that slow palatal expansion not only results in less relapse, but also integrity of the mid-palatal suture would be preserved. [20] Since the first introduction of screws to removable appliances by Shwartz, different generations of screws have been designed. Despite the variety of screws, orthodontists are indeed using just a limited number of them and it is proposed to be more accurate in the selection of screw type. [21] A kind of screw which activates a small spring during activation was first designed by Hausser. This type of screw provides only 0.7 mm of expansion following a complete activation phase and using spring in this screw can provide a continuous force. Very small screws associated with springs are also designed for single tooth movements. The disadvantages of these screws include unavailability in routine practice, lesser expansion rate compared to other screws, and the necessity of patient cooperation in screw activation. In addition, there is no evidence-based study confirming their force-expansion curve. [21]

As previously mentioned, instead of light and continuous forces, conventional screw activation creates heavy forces which dissipate immediately therefore the purpose of the present study was to introduce a spring associated screw for removable appliances which provides light continuous forces. The function of the designed screw and other screws were also compared by force- expansion curves.

   Materials and Methods Top

With contributions from Mechanical Engineering faculty (Shiraz University) and Physics department (Sciences faculty of Tarbiat Modarres University- Tehran), and inspired by screws associated with springs, [21] a screw to create light continuous forces was designed, evaluated and fabricated. After evaluating the wire diameter, number of the coils, and other characteristics of the desired spring, the wire was twisted in a spring constructed machine. Cover of the cylinder was fixed by soldering [Figure 1]. This designed screw, has a metallic cylinder with 3.3 mm outer diameter and a piston, both of which are made of stainless steel, and a stainless steel spring is positioned inside the cylinder (Free wire length: 12 mm, initial compression: 4.5 mm, spring wire diameter: 0.4 mm, spring diameter: 3 mm, number of the coils: 9 material: Stainless steel) [Figure 2]. Some grooves are positioned on the free portion of the piston, and the other part can move within the cylinder. A bead is also placed on the piston, which can rotate and move on the grooves. In this designed screw there is just one axiom within the acrylic base, but regarding to the large surface area of piston, the device possesses proper stability.

To compare force-expansion curve of this screw with other conventional expansion screws, stone casts including different screws were tested in a compression machine. To fabricate hyrax and Haas appliances, after band selection (standard edge wise - Dentaurum band) on dentiforms, pick-up impressions were taken with alginate (Dust free Alginate-Impression Material- Golchai, Reg.Iralgin No: 1/18435) and poured with orthodontic plaster (Dentaurum/Synthetic.stone.plaster/ White- Rapidure). Hyrax, Haas (Hyrax and Haas- Biedermann Dentaurum - stainless steel-602-807-10), two and three axiom removable screws (600- 303-30.Dentaurum) and newly designed screw were fabricated on stone models [Figure 3]. It must be noted that two axiom removable screws are smaller in size than three axiom screws and used for too narrow maxillary width.

After the fabrication process, stone models were cut into two separate parts and comparable to the study of Chaconas et al., each model with associated appliance, placed on the compression test machine (Tec Quipment (TQ) England-Q247013 - SM100). [22] Forces would transmit hydraulically to the jaw of this machine and its amount would be showed in digits. Different forces and their associated movements were calculated and compared [Figure 4].

Screw activation method

In screw fabrication, the spring within the screw gets compressed. Keeping the screw compressed, a ligature wire is soldered to both screw ends. After screw insertion within the acrylic base, cutting the soldered ligature wire would free the compressed spring and the piston is pushed out of cylinder and the expansion began. This caused approximately 4 mm distance between two halves of appliance.

This device has the potential of secondary 4 mm activation after the initial 4mm expansion. To reactivate the device, a wrench is placed over the bead on the free end of piston and rotated for 4 mm . Then the free end of the piston moves out 4 mm from acrylic base and gets into the cylinder which causes compression of the inner spring. This compression of the spring causes re-activation of the design to force the piston out of the cylinder [Figure 2].

   Results Top

Force - expansion curves showed that 0.5 mm opening of the designed screw creates 600 gram force. There was a harmonic increase in created forces related to increase in opening of the screw up to 1000 grams [Figure 5].

Evaluation of the curve showed that in hyrax screw 11 kg force exerted for 0.5 mm opening, and this reached to 30 kg force in 1.5 mm of opening. Haas screw produced the highest incremental forces through the opening (20 kg for 0.5 mm of opening and 5 kg for 1.5 mm). Two axiom and three axiom screws in removable appliances, had similar force-activation curves and created forces for determined opening (both 15 kg in 0.5 mm opening, and 41.55 kg in 1.5 mm of it). It must be noted that all of these appliances broke in this experiments above 1.5 mm expansion [Figure 5] and [Table 1] and [Table 2].

   Discussion Top

It must be considered that the introduced screw in this study is inspired by spring associated screws which are not available anymore. On the other hand, this study has just compared the in vitro comparison of this screw with other conventional screws and its findings should be clinically validated.

The results of the present study showed that, the designed screw produced light and continuous forces about one kilogram in the initial 4 mm expansion.

The comparison between this screw and other screws showed that the designed screw produced considerably less force for comparable amount of activation and had a flatter inclination in force-activation curve. The removable appliances with two or three axioms delivered a heavy force initially which reduced rapidly to zero. Similarity of these two types of curves might be related to their similar structure and components [Figure 1].

Hyrax appliance delivered the least force compared to Hass, two and three axioms removable appliances. The Hyrax appliance, consists of wire parts attached to a screw, the wire shows flexure and decreases the exerted force. However, acrylic does not possess enough flexibility and fractures due to the forces. To increase the strength of the complex or, in other words, rigidity of the device, increased thickness of the acrylic base would be helpful. It is in agreement with Chaconas' study which showed that force created by Hyrax was less than Haas and removable appliances. [22]

Amount of exerted force from Haas appliance was measured higher than all other screw types in this study. The reason might be the device structure which consists of a heavy wire with an acrylic base. Combination of these two parts decreases the flexibility and consequently increases the forces. These findings are consistent with the findings of Chaconas et al. which showed forces created by Haas are greater than Hyrax. [22],[23] However, in a study carried out by Garib et al. which compared the dentoskeletal effect of Haas and Hyrax screws, clinical results of Haas were more reliable. He reported that Hyrax caused alveolar bone loss and decreased the buccal plate thickness. It might be related to the differences in the force measurement by in vitro and in vivo studies. [13]

In this study, Hyrax device created a 11 kg force during 0.5 mm of expansion. This finding is consistent with Zimring and Issacson, who believed that a force of about 16.6 to 34.8 pounds (about 8 to 17 kg) is transmitted to the jaws in palatal expansion using this appliance. [24] The force has been reported as 2-3 kg by Vig et al. [25] In a study by Shetty et al., [26] the Hyrax force was evaluated about 500 gram which is within the orthopedic limits. This difference could be attributed to the variation of evaluating methods. In their study, forces were evaluated by a straining frame though we used compression test machine instead. [26]

The comparison of force-expansion curves for different screws in the present study and Chacona's research, showed the consistency of the findings. The higher amount of forces exerted by two and three axioms removable devices, Hyrax and Haas screws in the present study was the only difference. This might be attributed to the difference in acrylic base thickness, rigidity and strength of the wires, thermal procedures for the fabrication of the devices, adjustment for measurement devices and the fabrication of the appliances. [22]

Since Minn-expander was not available, we could not evaluate its force-expansion for this device. However, in Chacona's study, exerted forced were measured about 1000 gram for 0.5 mm of expansion, and it is twice the forces created by the newly designed screw, but they have a similar curve inclination [Figure 5]. [22] It must be noted that in this study we just evaluated the screws in vitro, so clinical studies must be performed to determine the validity of our findings.

   Conclusion Top

The designed screw created light, continuous forces which are favorable for orthodontic movements. The other advantage of this appliance is elimination of the need for activation by the patient. It must be noted that the results can only be approved by in vivo studies.

   Acknowledgment Top

The authors would like to thank Farzan Institute for Research and Technology for technical assistance.

   References Top

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3.Mills JR. Principles and practice of orthodontics. 2 nd ed. London: Churchill Livingstone; 1987. P. 236-46.  Back to cited text no. 3      
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5.Bassarelli T, Dalstra M, Melsen B. Changes in clinical crown height as a result of transverse expansion of the maxilla in adults. Eur J Orthod 2005;27:121-8.  Back to cited text no. 5      
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25.Vig KW, Shanker SV, Magness MJ. Skeletal and dental considerations in the transverse dimension. In: Bishara SE. Textbook of orthodontics. 1 st ed. Philadelphia: W.B. Saunders Co.; 2001. p. 431-9.  Back to cited text no. 25      
26.Shetty V, Caridad JM, Caputo AA, Chaconas SJ. Biomechanical rationale for surgical-orthodontic expansion of the adult maxilla. J Oral Maxillofac Surg 1994;52:742-9.  Back to cited text no. 26      

Correspondence Address:
Morteza Oshagh
Department of Orthodontics, Shiraz Dental School, Shiraz
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0970-9290.59447

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]

  [Table 1], [Table 2]

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