Just like heart attack prevention depends on a lot more than lowering cholesterol, effective fracture prevention is not simply increasing calcium density. A piece of chalk packed full of calcium breaks easily when dropped. Collagen supports the bone matrix flexibility and gives bone the ability to absorb stress and bend with trauma. Two simple ways to improve bone collagen are glucosamine/chondroitin and vitamin K. The former is frequently given as a supplement for osteoarthritis. The best source of vitamin K is green, leafy vegetables, but it also can be taken as a supplement in the form of 45 mg a day of K2, sometimes combined with vitamin D3. Those who are taking warfarin should not take vitamin K.
See John Neustadt, To Prevent Fractures, Consider Bone Density, Not Just Mineral Density, in Holistic Primary Care, Spring, 2011, p. 1-2.