Stability determinants of bone-borne force-transmitting components in three RME hybrid expanders—an in vitro study

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The aim was to test which component [wire arm, connecting abutment attachment, and orthodontic mini-implant (OMI)] of the force-transmitting system (FTS) in the anterior palate of three commonly used hybrid expanders (HEs; WILMES-HE, LUDWIG-HE, and WINSAUER-HE) deforms under increasing load.

Materials and methods:

Crude single and double wire arms were tested individually. Non-opening of the maxillae halves was simulated in artificial bone blocks with single wire and double wire FTS specimens. OMIs were inserted 8mm and underwent 6mm of continuous static lateral loading. Deformation angles were measured (X-ray, n = 6) at 0, 3 and 6mm feed. OMIs and abutments were scan electron microscope (SEM) evaluated.


After 1.0mm of loading, the single wire arm of all FTS deformed between 63.4 (16.5) N and 76.2 (18.4) N, and the double wire arm of reinforced FTS (wires positioned ‘side by side’) deformed after 1.0mm between 110.0 (18.4) N and 134.8 (22.3) N. The crude single wire resisted 89 (5.1) N until plastic deformation, whereas the crude double wire positioned ‘on top of each other’ resisted 438 (21.3) N. At 6mm loading, the reinforced WINSAUER-HE FTS withstood a maximum load of 320.9 (31.1) N and the reinforced LUDWIG-HE FTS 19% less, both under great deformation of double wires and OMIs. The screw-fixated WILMES-HE FTS abutment attachment (overlapping OMI head 34%) detached around 250N. The bonded WINSAUER-HE and LUDWIG-HE abutment attachments did not detach. Nor did the modified bonded plus the modified screw-fixated WILMES-HE abutment attachment when overlapping 100%.


Early OMI and single wire arm deformation in HEs are crucial for unsuccessful RME in more mature maxillae. Double wire arms should be obligatory. OMIs with inner diameter greater 1.36mm are recommended. One hundred per cent overlapping abutment attachments do not detach.

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