A Novel View of How Frail Bones Break

The common wisdom regarding the fractures and breaks that are sadly common in very old individuals is that they result from hard knocks against - and heavy loads placed on - bones that are made fragile by osteoporosis. A younger person would shrug off a fall or a load that will cause catastrophic structural failure in the bones of an individual in the advanced stages of osteoporosis. The research here suggests that this view is subtly wrong in several important details, and that the progressive harm caused by osteoporosis is in fact much worse than thought. It is an interesting and plausible viewpoint, though one that needs corroborating physiological data.

Either way, what can be done about osteoporosis? The proximate cause is an imbalance between the activities of cells that deposit bone, osteoblasts, and cells that break down bone, osteoclasts. Both are constantly active, but the various forms of change and damage that accompany aging cause the activity of osteoblasts to decline relative to the activity of osteoclasts, and thus bone becomes ever weaker. Senescent cell accumulation and chronic inflammation are in the list of deeper causes for osteoporosis, as is true of many other age-related conditions, but they are not alone. There numerous possible avenues by which this balance can be adjusted, and research groups are at various stages in these lines of work. If the balance is turned back, then the age-related weakness and damage of bone should start to reverse.

To better understand why many elderly people are prone to break a bone in a fall (known as bone fragility fractures), perhaps doctors and researchers should look at the human skeleton in much the same way civil engineers analyze buildings and bridges. A team of researchers believes the bones of an older person, say above the age of 50, become more susceptible to a break due to repeated stress from everyday activities such as walking, creating microdamage that affects the quality of the bone. That is in contrast to the common-held belief that bone breaks in the elderly are largely due to one massive impact or force on the bone, such as a fall.

"It really starts with a small microcrack that grows over time under repeated loading. You need to be doing something like just walking or moving, and the crack is slowly propagating. At some point, the remaining cross-section of the bone that is still connected is too small and will break suddenly." In that case, such fractures in the elderly would be the cause of a fall rather than the result of a fall. The theory that "cyclic loading" (repeated and fluctuating loads) might be a bigger contributor to bone breaks is similar to the study of structures and engineered materials. This type of stress in structures and materials resulted in a rise of catastrophic accidents near the turn of the 20th Century and has led to the development of "fracture mechanics."

"In engineered materials and structures, cyclic fatigue is the most ubiquitous mode of failure. Cyclic fatigue accounts for more than 80 percent of all failures, leading to catastrophic and sudden accidents such as the failure of railway axles, the collapse of metallic bridges, the failure of ships and the cracking of aircraft airframes and engines." The research is based on examining not just the bone's mineral density (bone mass) but its quality, specifically how well the collagen that provides the ductility of the bone deforms to resist fractures. And as one gets older, the more microdamage that person accumulates over time and the weaker the bones get.

Link: https://unews.utah.edu/reaching-the-breaking-point/

Comments

This seems like it would predict that regularly exercise in the elderly increases the risk of fracture, but my understanding is that research has found the opposite. I can't imagine that this hasn't occurred to the researchers, though, so I'm curious how they square their findings with the existing literature.

Posted by: Brandon Berg at January 26th, 2018 9:32 AM

Remember when Jack LaLane expoded like a bag of chalk dust?
Oh wait...

Posted by: Darren at January 26th, 2018 10:25 PM

@Brandon Berg It probably has more to do with averages in the population. If an elderly individual has a typical (low) level of exercise as they age the cyclical loading would cause more calcium and minerals to be leached and microfractures to occur. If individuals exercise regularly and consistently as they age, with proper nutrition, this effect is probably offset by increased bone density. It's an interesting point and I hope they research it more.

Posted by: William at January 27th, 2018 9:00 PM

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