"Permanencia" is not an error here. It refers to the continued presence of deciduous or primary teeth (popularly known as "milk teeth" or "baby teeth") beyond the stage when they should normally have been replaced by permanent (adult) teeth.
http://en.wikipedia.org/wiki/Tooth_eruption

The word normally used for this is "retention".

"New teeth from old: treatment options for retained primary teeth
S. Robinson1 and M. F. W-Y. Chan2
Retention of primary teeth beyond their expected exfoliation date is encountered relatively frequently"
http://www.mydentalspecialist.co.uk/PDF/newteethfromold.pdf

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Note added at 3 hrs (2013-05-09 07:37:23 GMT)
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"Over-retention" is also used.

Rapid correction of a simple one-tooth anterior cross bite due to an over-retained primary incisor: clinical report
Susan A. McEvoy, DMD, MS
Abstract
Where a permanent anterior tooth cross bite results from over-retention of a primary incisor, a rapid correction method may prove successful."
http://www.aapd.org/assets/1/25/McEvoy-05-04.pdf

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Note added at 3 hrs (2013-05-09 07:39:15 GMT)
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"Retention of primary teeth.
The primary tooth that is most commonly involved is the maxillary second molar"
Pediatric Dentistry: A Clinical Approach, p. 206
http://books.google.es/books?id=pqhzzcjCFH4C&pg=PA206&lpg=PA...

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Note added at 4 hrs (2013-05-09 07:49:43 GMT)
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"Persistence" is also used sometimes, but much less commonly:

"Other dental anomalies include oligodontia, significantly delayed eruption, persistence of primary teeth, and malocclusion."
http://books.google.es/books?id=mSUnOoCvQWAC&pg=PA1107&lpg=P...

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Note added at 7 hrs (2013-05-09 10:52:42 GMT)
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To illustrate the fact that "permanencia" is used in Spanish in the sense of retention:

"El hijo de la hermana de 23 años (3F)* presenta tres agenesias. Estas son: 2.3, 4.3, 4.5, y presenta la permanencia de dientes primarios"
http://www.actaodontologica.com/ediciones/2008/3/oligodoncia...

"PERMANENCIA O PERSISTENCIA o RETENCION PROLONGADA
Presencia en boca de una pieza dentaria de la fórmula temporal en un paciente, que por su edad, ya debería haber sido cambiada por la pieza de la fórmula permanente. Generalmente va asociada a agenesia del sucesor permanente o agenesia de las piezas dentarias vecinas o cuando el sucesor no logra asumir su posición normal de erupción y queda retenida. También puede observarse en casos de anquilosis de la pieza temporal. La permanencia de dientes temporales es frecuente en casos de disostosis cleidocraneal y displasia ectodérmica"
http://www.biblioteca-medica.com.ar/2012/05/anomalias-dentar...

The one thing it never seems to be called in English is "permanence"!