The only thing that has really lasted for Kai is the Galileo chew toy by nylabone. Everything else lasts about 5 mintues until it’s pulverized into little peices! =)
Here’s a link to a picture of it:
http://s3.amazonaws.com/tc-photos/11442/product/giant/504834.jpg
You can buy them at any chain pet store or even the small ones carry them as well. May be a bit pricey, but well worth the money.