Men's Osteoporosis Support Group

Bisphosphonates and bone properties

Bone. 2009 Nov 16. [Epub ahead of print]. Bisphosphonate Treatment Modifies Canine Bone Mineral and Matrix Properties and their Heterogeneity. Gourion-Arsiquaud S and others. PMID: 19925895. This research was done in dogs to study the microscopic effects of bisphosphonates on bones using a technique called Fourier Transform Infrared imaging (FTIRI). The specifics of this technique are complex and well beyond my abilities to fully understand. But, in general, it appears to be similar to a microscopic X-ray (but that uses infrared rather than X-rays) that can detect very fine differences in both the hard and soft tissues of bone. The soft tissues would be collagen, the matrix, and the hard tissue would be the mineral content. The dogs were given either a vehicle (placebo) or low or high doses of either alendronate (Fosamax) or risedronate (Actonel) for one year. The alendronate and risedronate increased the mineral content and the collagen maturity mainly in cancellous bone and at the endocortical surface. [The inner bone is cancellous with more of a woven and less dense nature, the outer (cortical) bone is much more dense. The inner surface where the two types of bone meet is the endocortical surface, which would be like the inside of a straw.] These are the areas where bone turnover is higher and which would be expected to be influenced by bisphosphonates which suppress bone turnover. Since the outer (cortical) bone turns over much less frequently, it is not effected as much by bisphosphonates. The authors conclude, "These positive effects for decreased fracture risk are also associated with a loss of bone heterogeneity that could be one factor contributing to increased bone tissue brittleness and micro-crack accumulation."

Editor's comments. It is felt by some that brittleness and micro-crack accumulation can be risk factors for spontaneous fractures after many years of bisphosphonate use. The loss of bone heterogeneity (diversity in kind and nature) that occurs when bone turnover is suppressed may account for this brittleness. That is, normal bone would be a combination of old and new collagen and hydroxyapatite as there is always some removal of old materials while some new materials replace them. When bisphosphonates are given the old materials are not removed, or are removed only minimally, but new material is built up. Eventually this could become a vast amount of old, homogeneous bone and thus subject to micro-crack accumulation and possible spontaneous fracture. See this recent Update for articles covering cases of atypical spontaneous femoral fractures that could be related to this research finding. This might be another reason to take a drug holiday, as is suggested regarding dealing with or preventing osteonecrosis of the jaws. There is more research needed to know exactly how great the risk of spontaneous fracture is in relation to the length of time using a bisphosphonate. For now it doesn't appear to be a major problem, but just something to watch for new research and guidance as more is learned on this topic.

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