“Appliance First” or “Bone First” for miniscrew assisted rapid palatal expansion?

Rapid palatal or maxillary expansion (RME) is considered the optimal procedure to achieve a skeletal widening of the maxilla.^[1,2] Since the anchor teeth distribute the forces to the bony structures through the anchor teeth, distribution of the forces to as many teeth as possible and completion of root growth are considered essential. However, side effects, for example, tipping teeth, root resorption, decrease in buccal bone thickness or dehiscence, and loss of marginal bone height resulting in gingival recessions may occur.^[3-6]

More recently, mini-implants to share the load of the expansion with the anchorage teeth to reduce or eliminate the unwanted dental side effects were used. Wilmes et al. introduced the Hybrid Hyrax expander in 2007 using two mini-implants in the anterior palate and two (deciduous) molars [Figure 1].^[7-10] Similar hybrid expanders were published in the following years by Garib et al.^[11] in 2008, Lee et al.^[12] in 2010, and Moon et al.^[13] in 2015 called MSE [Figure 2, Mini-Screw Assisted Expansion]. Mini-implant-supported expanders with additional buccal extensions [Figure 3] can also be used very successfully to treat growing Class III patients^[14-21] allowing skeletal maxillary protraction without the common dental side effects.^{[13,16,17,22]} In addition, alternating expansion and constriction of the maxilla Alt-RAMEC^[21,23] over 9 weeks can enhance the response of the maxilla to the protraction forces and offer a good response in children with more sutural maturation.^[24-26]

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Figure 1:

Principle of the Hybrid Hyrax: two mini-implants in the anterior palate to support RME: “TADs First” or “Bone First.”

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Figure 2:

Principle of the MSE appliance: Four mini-implants in the posterior palate to support RME: “Appliance First” (Dr. Miguel Hirschhaut).

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Figure 3:

Buccal segmental hooks for maxillary protraction with a facemask.

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For the stability and predictability of the mini-implants (TADs), we need to place them in the areas with the best cortical bone quality. Several CBCT studies have shown that this is in the anterior palate along an area designated as the T-Zone [Figure 4].^[27-29] In the conventional Hybrid Hyrax, the mini-implants are placed paramedian in the anterior palate in the T-Zone. An impression or scan is taken and sent to a laboratory to manufacture the appliance, which is then fitted in a subsequent appointment. More recently, these appliances can be manufactured using selective laser melting procedures.^[30] The available printing metal powders provide a high rigidity of the appliance, which seems very important, especially for expansion appliances. Furthermore, this process can be made more efficient using CAD/CAM manufactured insertion guides, which facilitate a safe and precise insertion of mini-implants in the anterior palate in the areas of the best bone [Figure 5]. Furthermore, these insertion guides allow for the insertion of mini-implants and installing the appliance in a single appointment.^[31,32]

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Figure 4:

Picture of a maxilla of a cadaver: Recommended insertion site (T-Zone) posterior from the rugae. The bone is usually very thin in the posterior and lateral areas.

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Figure 5:

Guided insertion of palatal TADs: (a) Virtual positioning after superimposition of an upper scan and a cephalogram. (b) Design of a CAD-CAM insertion guide.

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In both of the above approaches, with impression/scan and insertion guides, mini-implants are inserted first, and the appliance is installed afterward. This method can be called “TADs First.” A disadvantage of this “TADs First” procedure is the need for two appointments: One to insert the TADs and the second to cement the appliances. Using the insertion guide eliminates this issue, however. Second, the appliances may not fit if there are inaccuracies during the manufacturing process. This problem may also occur when using an insertion guide, especially if there is a change due to tooth eruption or movement from the original scan to the insertion appointment.

As an alternative to the TADs First concept, it was proposed to insert the expansion appliance first and 2–4 mini-implants afterward. For example, with the MSE.^[33] This protocol can be called “Appliance First.” A disadvantage of the conventional appliance first method is choosing the placement site for the mini-implant freely based on bone quality. The prefabricated nature of the appliance restricts the placement site of the mini-implants. It may be possible to shift the mechanisms forward or backward to allow the channels to line up with an area of good bone. Unfortunately, it is unlikely that all 2–to 4 mini-implants will be consistently in the best bone quality areas. This problem is especially pertinent in the posterior molar area where the bone height is only 1–2 mm in paramedian sites.^[27,28] In addition, the connection between the expander and the mini-implants in those appliances is not rigid and angularly stable. As a consequence, mini-implants have some play with the expander and will tip as soon as the expander is activated.^[34]

PROS AND CONS OF “APPLIANCE FIRST” METHOD VERSUS “TADS FIRST” METHOD

Advantages of the “Appliance First” method (MSE)

• Mini-implant insertion and installation of the appliance in just one appointment

• No impression/scan is needed after mini-implant insertion

• No risk of misfitting the appliance and the mini-implants.

Advantages of the “TADs First” method (hybrid hyrax)

• The appliance can be removed and re-installed easily, for example, for modifications or repairs

• Rigid and stable connection between the mini-implants and the expansion appliance

• The customized design allows total freedom in selecting the mini-implant sites to utilize the best available bone. As a consequence, the “TADs First” approach can also be called “Bone First”

• Mini-implants can be used to anchor more than one appliance, for example in the first step for RME and a second step for molar distalization^[35]

• Due to free customization, multipurpose appliances such as the Hybrid Hyrax Distalizer^[36] for RME and a subsequent molar distalization can be designed [Figure 6].

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Figure 6:

Miniscrew assisted rapid palatal expansion and subsequent distalization in one appliance: Hybrid Hyrax Distalizer.

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