Rapid maxillary expansion with Haas-type and Hyrax-type expanders

Upper orthodontic appliance

First described over 100 years ago, rapid maxillary expansion (RME) is a method used to correct a transverse maxillary deficiency.  The aim of this study was to evaluate and compare the immediate effects of RME in the transverse plane with Haas-type and Hyrax-type expanders by using cone-beam computed tomography.

The authors randomised 33 subjects (mean age, 10.7 years; range, 7.2-14.5 years) with transverse maxillary deficiency to treatment with Haas (18 patients) or Hyrax expanders (15 patients).  All patients received an initial activation of 4 quarter turns followed by 2 quarter turns per day until the expansion reached 8 mm.  Cone-beam computed tomography scans were taken before expansion and at the end of the RME phase.  Maxillary transversal measurements were compared by using the mixed analysis of variance (ANOVA) model and the Tukey-Kramer method.

They found that:-

  • RME increased all maxillary transverse dimensions (P>0.0001), with less expansion at skeletal than dental levels.
  • The Hyrax group had greater statistically significant orthopaedic effects and less tipping tendency of the maxillary first molars than the Haas group.

They concluded that:-

Both appliances were efficient in correcting a transverse maxillary deficiency. The pure skeletal expansion was greater than actual dental expansion. The hyrax-type expander produced greater orthopaedic effects than did the Haas-type expander, but this effect was less than 0.5 mm per side and might not be clinically significant.

Weissheimer A, de Menezes LM, Mezomo M, Dias DM, de Lima EM, Rizzatto SM. Immediate effects of rapid maxillary expansion with Haas-type and hyrax-type expanders: A randomized clinical trial. Am J Orthod Dentofacial Orthop. 2011 Sep;140(3):366-76. PubMed PMID: 21889081.


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Derek Richards

Derek Richards is a specialist in dental public health, Director of the Centre for Evidence-Based Dentistry and Specialist Advisor to the Scottish Dental Clinical Effectiveness Programme (SDCEP) Development Team. A former editor of the Evidence-Based Dentistry Journal and chief blogger for the Dental Elf website until December 2023. Derek has been involved with a wide range of evidence-based initiatives both nationally and internationally since 1994. Derek retired from the NHS in 2019 remaining as a part-time senior lecturer at Dundee Dental School until the end of 2023.

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