Here is information from the Yorkie Foundation on Legg-Calve-Perthes. Scroll down about halfway:
Health (Foundation of the Yorkshire Terrier)
Since is is believed to be genetic, this is what the Yorkie Foundation recommends:
Prevention of the disease is only possible through genetic means. Affected animals should not be bred. Breeding stock should have their hips X-rayed to insure that they are not affected with mild LPD, the symptoms of which went unnoticed during the dog's adolescence. Extreme caution should be used when considering breeding animals that have produced LPD, or have LPD affected littermates.
Inheritance: Autosomal recessive