What about this explanation:
Originally posted by Piedmon
No, it's not like there's a layer of adamantium over normal bone. Wolverine's skeleton was molecularly bonded to the adamantium. Meaning his bones contain the metal at a cellular level. Effectively, his bones ARE adamantium.The qualities of adamantium have seeped into the ligaments, tendons and muscles of Wolverine's body over time. That's why he doesn't fly apart when a train hits him. It's not that his 'soft' cells are harder, they're simply more resiliant and take punishment better. (If anything, this makes his tendons/ligaments more flexible than a normal human's.)