This refers to a type of "aparato de contención" or orthodontic retainer:

"An Orthodontic retainer is an oral appliance which is used to hold the teeth stable and in alignment. Retainers are usually used after orthodontic treatment is completed and a patient is given two retainers, one for the upper, and a second for the lower teeth."
http://braces.com/47601.html
See this page for a clear guide to the different types of retainer used in orthodontics.

The classic Hawley type brace or retainer (see page just cited) is usually called a "placa de retención". "Barra de contención" appears to refer to a type called fixed retainer, attached to the teeth rather than removable, and specifically to the type called a continuous bar retainer. There are other types of bar retainer, referred to for example as "lingual bar retainer", so it might be best just to put "bar retainer", since the context does make it clear precisely what type is involved here.

"¿Cómo puedo pegar yo misma la barra de contención post-ortodoncia que está detrás de los dientes?"
http://espanol.answers.yahoo.com/question/index?qid=20100914...

Here's a "barra de contención lingual" or lingual bar retainer, with illustration:
http://www.clinicaperiodoncia.cl/?p=377

"continuous bar retainer
n.
A metal bar, usually resting on lingual surfaces of teeth, to aid in their stabilization and to act as an indirect retainer after orthodontic treatment."
http://medical-dictionary.thefreedictionary.com/continuous b...

http://books.google.es/books?id=ZGvegIdicUoC&pg=PA765&lpg=PA...

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Note added at 35 mins (2012-03-02 11:38:39 GMT)
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The "sector ántero-inferior de elemento 33 y 43" refers to the front of the bottom front teeth, in everyday language. So this seems to be a bar retainer on the front rather than the back of the teeth.

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Note added at 45 mins (2012-03-02 11:48:34 GMT)
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"Elementos 33 y 43" means the lower canines in the FDI tooth numbering system:
http://users.forthnet.gr/ath/abyss/dep1151_1.htm

So this seems to refer to a fixed bar retainer on the lower teeth from canine to canine, spanning six teeth in all:

"Other Fixed Retainers may span from canine to canine securing six teeth in total."
http://braces.com/47601.html