A child with lip incompetence has hard time to close the lips.  The muscles in the chin have to contract hard (so called muscle strain, Fig.1 (before; May 24, 2012).

Fig.5-1: after (Decemeber 22, 2014).  There is no lip strain.  The chin also enlarges.

Lip Incompetence (Before), Lingual Retainer (After)

Xin Wei, DDS, PhD, MS 1st edition 12/26/2014, last revision 12/26/2014