With all our guide dog pups we started them on bones as soon as they wanted to gnaw. I have heard some say that one shouldn't give bone to dogs as their teeth can break - i can't believe that more likely as they are losing puppy teeth they shed them. I have never heard of any problems feeding dogs a good sized knuckle bone - if they had an allergy to marrow one could just scrap the majority out. As a dog gnaws on a bone a little is bitten off and swallowed and comes out white in #2... usually makes them a little too hard which helps the anal glands. Ideally one doesn't give them a bone for much longer than an hour and it is necessary to check that they are not getting over enthusiastic.

With a knuckle bone they should start on the cartiledge which is softer and once chewed off the knuckle is a rough surface which is what is used to clean the plaque off teeth.

Try a bone again scrapping out the marrow in the end.