From AKC's Rules Applying to Dog Shows:
11, Section 8-A.
A dog is considered changed in appearance by artificial
means if it has been subjected to any type of procedure
that has the effect of obscuring, disguising or eliminating
any congenital or hereditary abnormality or any
undesirable characteristic, or that does anything to
improve a dog’s natural appearance, temperament, bite
or gait.
Even procedures which are absolutely necessary to the
health and comfort of a dog shall disqualify that dog from
competition if the former had the incidental effect of changing
or even improving the dog’s appearance, bite or gait.
Procedures that would in and of themselves be considered
a change in appearance by artificial means and make
a dog ineligible for shows include, but are not limited to:
1. The correction of entropion, ectropion, trichiasis or
distichiasis
2. Trimming, removal or tattooing of the third eyelid
(nicitating membrane)
3. The insertion of an eye prosthesis
4. Correction of harelip, cleft palate, stenotic nares, or
an elongated soft palate resection
5. Any procedure to change ear set or carriage other
than that permitted by the breed standard
6. Restorative dental procedures, the use of bands or
braces on teeth, or any alteration of the dental arcade
7. The removal of excess skin folds or the removal of
skin patches to alter markings
8. Correction of inguinal, scrotal or perineal hernias
9. Surgery for hip dysplasia, O.C.D., patellar luxation
and femoral head resection
10. Alteration of the location of the testes or the insertion
of an artificial testicle
11. Altering the set or carriage of the tail