Caterina: Most ECM proteins are hydrophobic and need to be extracted with 8 M urea or guanidine chloride. The extraction details can be found in articles published in the mid 1990s. Good luck.
I suggest you check calcium amount in your bone tissue. If you eliminate calcium you might decellularize bone tissue in order to get bone ECM. Our team already did similar method in cartilage decellularization. This method also offers relatively harmless decellularization instead of detergent based methods. I think this is worth to consider.