Scientists Discover What Makes Bones Fracture-Resistant (It’s Not What They Thought!)

A team of researchers at Cornell University has unlocked a long-standing mystery of bone mechanics. Aside from elucidating valuable information for supporting bone health, their discovery has far-reaching implications for a variety of fields interested in building durable materials that resist fracture.

They found that measurements of bone density, and even the way that bone strength is typically defined, fail to consider a critical component of a bone's fracture resistance.

Today we'll dive into their research to learn how a misunderstood part of bone microarchitecture plays an important role in fracture prevention.

The Incredible Structure Of Cancellous Bone

A study published in the journal PNAS has uncovered a previously unknown trait of cancellous bone. Cancellous bone is one of the primary types of bone tissue. It is composed of a lattice-like matrix of minerals and has an almost spongy appearance.

Anyone who has learned about osteoporosis has seen side-by-side images of cancellous bone. They usually depict healthy bone alongside bone in which the struts of the mineral lattice called trabeculae have thinned, resulting in bone tissue that has more holes than it does bone.

This lattice-like structure might seem unstable because it isn't filled in with minerals, but this shape is incredibly strong. In fact, engineers have modeled highly durable materials by copying the architecture of cancellous bone.

However, this study has provided a deeper understanding of the physics behind the structure's strength.

Synopsis

Cancellous bone is a type of bone tissue composed of a lattice-like matrix of minerals. The struts of the lattice are called trabeculae. During bone loss, these trabeculae become thin, weakening bone. This study has generated a new understanding of what makes this lattice strong.

The Importance Of Trabeculae

Trabeculae, the struts of the lattice in cancellous bone, generally run in two directions:
longitudinally, in parallel with the force typically applied to the bone; and transversely, perpendicular to that typical force.

Since humans walk upright on two legs, most of the force applied to our bones is vertical. Longitudinal trabeculae tend to be thought of as the most important trabeculae since they bear the brunt of the load placed on human bones. And indeed, in trauma fractures (during a fall or an accident) the amount of force required to break a bone bears a direct relationship to the size and density of those longitudinal trabeculae.

However, that's not the only type of fracture. Fatigue fractures occur not because of a single trauma, but due to the effect over time of cyclical loading. Cyclical loading describes any repeated force applied to a material.

Up until now, it has been assumed that longitudinal trabeculae are also the most important factor for resisting fatigue fracture. But researchers at Cornell University have proven that assumption wrong.

They found that the thickness of the transverse trabeculae dictated the cancellous bone's ability to withstand cyclic loading and prevent fracture.¹

Synopsis

Longitudinal trabeculae, which align with the force typically applied to a bone, have long been thought the most important part of bone strength. This study found that in the case of fatigue fracture caused by cyclical loading, the transverse trabeculate dictated the cancellous bone's ability to resist fracture.

About The Study

To come to this conclusion, researchers first used a machine to test the durability of different donated bone samples under cyclical loading. This experiment showed that transverse trabeculae were associated with increased resistance to cyclical loading– they lasted longer without fracture.¹

Then, the researchers double-checked this result. They used a 3-D printer to replicate samples of cancellous bone that were each identical save one variable: the thickness of their transverse trabeculae.

They tested those 3-D printed samples using the cyclical loading machine and found the same result. As the thickness of the transverse trabecula increased, the ability of the sample to withstand fatigue fracture from cyclical loading increased.

This discovery has enormous implications for fields like aerospace engineering, where super lightweight and strong materials are required, and engineers need to predict how long the components will be safe to use.

It also has a significant bearing on our understanding of bone health and what makes bones strong and durable.

Synopsis

The scientists put bone samples into a cyclical loading machine to see how many cycles of loading it would take before the bone fractured. They found an association between the thickness of transverse trabeculae and the durability of the bone. Then they 3D-printed samples of bone, identical save the thickness of transverse trabeculae. They ran the same durability test and found the same result.

Density Isn't The Best Predictor Of Bone Health

This groundbreaking study reaffirms the fact that bone mineral density is not useful as a sole predictor of fracture risk. The Osteoporosis Reversal Program was founded on clear scientific evidence of this fact. This new research proves the position correct yet again– and offers a new mechanical explanation for why.

Density measurements can help to predict a bone's resistance to fracture from a single overload of force, say from a fall or a car accident. However, the most common type of osteoporosis-related fracture is vertebral fracture caused not by a single incident but by fatigue from cyclical loading.

Here's what the study's authors had to say:

“Preferential loss of transversely oriented trabeculae during aging, therefore, causes reductions in the fatigue life of cancellous bone that are disproportionately larger than the reductions in stiffness and strength, potentially explaining the long-held but poorly described relationship between cancellous bone microstructure and clinical fractures.”¹

DXA scans that offer readings of bone mineral density (BMD) provide information about bone stiffness and strength. But as this study shows, that's not the whole picture.

Osteoporosis drugs that aim to increase BMD are not improving– or even considering– all of what makes bones resistant to fracture. This incomplete approach has incomplete results. Bisphosphonates (the leading class of osteoporosis drugs) create stiff bones that are not durable, which results in one of their most dangerous and counter-intuitive side effects: atypical femoral fractures.

This new study helps us to understand exactly why the osteoporosis drugs that Big Pharma markets are not up to the task of safely improving bone health and preventing fractures.

Synopsis

This study shows how bone mineral density (BMD) is insufficient for measuring the strength, health, and durability of bone. Osteoporosis drugs that aim to increase BMD are not considering the full picture of what makes bones susceptible to fracture.

What This Means To You

Our bones are marvels of engineering, so complex that we're still unlocking the secrets of how they work. That's why it's important to keep learning and adjusting our approach to preserving the health and durability of our bones.

The Save Institute is committed to keeping you up to date. We're continually crafting the best possible approach to preventing and reversing osteoporosis– an approach you'll find in the Osteoporosis Reversal Program.

The ORP is an evidence-based, all-natural way to build stronger, healthier, more durable bones that will last a lifetime. Start with learning, then use that knowledge to choose a safer path to stronger bones.

References

¹ https://www.pnas.org/doi/full/10.1073/pnas.1905814116