In Part 1 of our series about bone strength, you learned about mechanical load. Today we'll look at the other half of the equation: the mechanical behavior of bone. When we talk about bone strength, we refer not just to the structure of bone, but to how it behaves under stress and strain.
By understanding the material and structural properties of bone, we can learn what makes bones resilient and resistant to damage and fracture. This information also sheds light on the detrimental effects of osteoporosis drugs.
Despite what patients are told, bone density is not the only parameter for measuring bone strength.
The Composition Of Bone
Bone is a complex tissue. It is composed of both organic components and inorganic components. The balance of these materials plays a significant role in determining bone's mechanical behavior and strength.
Two important measures of a bone's composition are mineralization and porosity. Taken together, they describe the apparent density of bone.
Mineralization describes how much of a bone's mass is composed of mineral material. Porosity describes the presence of pores within the bone matrix– the amount of bone composed with the spaces between that mineral material. Those spaces contain the organic material of bone, primarily collagen.1
The distribution of mineral material and porosity plays a role on how a bone responds to load. It determines stiffness, elasticity, toughness, and ultimate strength. Here's a breakdown of what those four terms mean:
- Stiffness is a bone's ability to resist deformation, preventing strain
- Elasticity is a bone's ability to absorb stress, which allows for strain without damage
- Toughness is a bone's ability to absorb energy
- Ultimate strength is how much stress and strain a bone can withstand before it breaks
Like most systems in our body, these properties of bone work together to maximize ultimate strength. If a bone has too much elasticity and not enough stiffness, it will suffer damage too easily, leading to fracture. But conversely, if a bone has too much stiffness and not enough elasticity it will become brittle and unable to absorb any shock, also leading to fracture.
One of the primary failures of osteoporosis drugs is that they ignore this fact. Bisphosphonates make bones stiffer, while ignoring the other qualities that increase ultimate strength. Overly stiff bones grow brittle and more prone to fracture.
Bone is composed of organic and inorganic compounds. Mineralization describes the inorganic mineral part of bone, and as this increases a bone's stiffness increases. Porosity describes the porosity of bone– the spaces between mineral bone containing organic material. The proper balance of mineralization and porosity creates both stiffness and elasticity to increase the ultimate strength of bone. Bisphosphonates increase stiffness at the expense of elasticity. Overly stiff bones become brittle and more prone to fracture.
Cortical And Trabecular Bone
There are two different kinds of bone tissue that serve different functions in a bone. Cortical bone, also known as compact bone, has higher mineralization and density. Trabecular bone, sometimes called spongy or cancellous bone, has high porosity and thus is less dense.
Here are some facts about cortical and trabecular bone:2
- Cortical bone, due to its high mineralization, is stronger than trabecular bone
- Cortical bone makes up the outer layer of bones, giving them their structural strength
- About 80% of bone mass is cortical
- Only 20% of trabecular bone is composed of bone, the rest is marrow and fat
- Trabecular bone hosts metabolic activities like the production of red blood cells and bone remodeling processes
- Vertebrae are largely trabecular bone, having only a thin cortical shell
- The hip is largely a site of cortical bone
- The femur's strength comes from its combination of cortical and trabecular bone
Within that stiff outer layer of compact cortical bone tissue, spongy trabecular bone tissue harbors bone marrow and a variety of metabolic activities like the production of red blood cells and bone remodeling processes.
Bone degeneration caused by aging inactivity, or certain health conditions, leads to an increase in porosity of both trabecular and cortical bone. This change in the microarchitecture of bone drastically reduces stiffness, compromising mechanical integrity. This deterioration accounts for about 90% to 75% of the loss of bone strength commonly experienced during aging.1
Cortical bone is the compact bone tissue that makes up the outer layer of bones. It has high mineralization and density. Trabecular bone tissue is on the interior of bones. It is more porous and holds marrow and metabolic processes. Degeneration of bone microarchitecture increases the porosity of both cortical and trabecular bone, reducing mechanical integrity.
Mineralization is the development of mineral content within bone. It happens in two phases, a primary and secondary biomineral phase.
Newly deposited bone begins to mineralize within about five to ten days, creating about 60% of its total mineral content. Secondary mineralization takes place over approximately 30 months, during which time, calcification completes. The extent to which new bone has completed this maturation process impacts the structural flexibility and stiffness of bone.
The degree of mineralization isn't the only factor influencing the mechanical behavior of bone. The quality of the mineral crystals within the bone matrix also plays a role. That measure is called crystallinity.
Increases in crystal size, number, and distribution change the properties of bone. A certain level of crystallinity is essential for bone strength, but just like with mineralization, if crystallinity is too high bone becomes excessively stiff and brittle.
Bisphosphonates have been found to cause the growth of abnormal, oversized crystals which contribute to bone brittleness. That's another way that osteoporosis drugs increase fracture risk.3
Mineralization occurs in two phases, a fast primary phase, and a much longer secondary phase. As this process takes place, the mechanical properties of bone change. Crystallinity describes the size, number, and distribution of the mineral crystals in bone. If crystallinity is too high bone becomes excessively stiff and brittle. Bisphosphonates have been found to cause high crystallinity.
Bone Mineral Density Doesn't Equate To Bone Strength
It is refreshing to see a medical journal publish a review article that so clearly articulates the failure of the Medical Establishment to devise and use accurate means of assessing bone quality and strength.
Read what the study authors have to say about the inadequate and misleading industry standard for assessing bone strength, bone mineral density:
“…all measures of bone mineral density inherently neglect structural properties of bone (architecture, morphology, geometry), which substantially influences mechanical behavior, and greatly contributes to bone strength and fatigue resistance. Although bone density provides valuable modifiable and measurable insights into bone quality; it is only one of several determinants of bone strength, and should therefore form part of a wider investigative framework which includes structural quantities.”
“Increases in bone mass are not the only way for bones to increase their stiffness and strength. Specifically, bone modifies its structure by adjusting its size (thickness and diameter), shape (contour and dimensions), and architecture (alignment and distribution) to increase cross-sectional area (CSA) and cross-sectional moment of inertia (CSMI) as mechanisms to improve load tolerability and fatigue resistance.”1
As the second half of that quotation states, when bone adapts to mechanical load, it can build strength in ways other than just increasing mineral density. These other qualities also determine bone strength. As described in Part 1 of this series, bone can sense strain. When it meets certain parameters, that strain results in adaptation and growth.
This has been the message of the Save Institute from the very beginning; that bone density is not the only way to assess bone health. Big Pharma and the Medical Establishment ignore the other parameters of bone strength to push osteoporosis drugs on unsuspecting patients.
Bone mineral density is not the only measure of bone strength. By only assessing BMD, the Medical Establishment is ignoring the other qualities that comprise bone strength. Fortunately, we know that mechanical load can apply strain to bone that triggers growth. That growth, even when it doesn't impact BMD, can improve the ultimate strength of bone.
The Role Of Muscle In Bone Strength
Muscle plays an active role in bone strength. Furthermore, bone growth and muscle growth are closely linked. Consider this data on the relationship between bone growth and muscle growth:
“Specifically, when immobilized; muscle cross-sectional area, volume, and strength significantly reduces after ~5 to 7 days; whereas bone thickness, volume and strength significantly reduce after ~14 to 21 days. Conversely, when mechanically loaded; muscle cross-sectional area, length, and strength significantly increase after ~20 days; whereas bone diameter, thickness and volume significantly increases after ~40 to 80 days.”1
We can see how the loss of muscle mass is shortly followed by the loss of bone size and strength. Conversely, this data shows that gains in muscle lead to an increase in bone mass.
This relationship is largely explainable by the mechanical relationship between muscle and bone. Our muscles apply the stress and strain to bones that trigger growth. This relationship is described by Wolff's Law of Bone Formation. So as muscles become larger and stronger, they apply more stress and strain. If muscles shrink, they cease to provide enough strain for bones to maintain their mass.
Muscle and bone also communicate with each other via endocrine-paracrine compounds called myokines. Myokines are small proteins produced by muscle cells in response to muscle contraction. This means that muscle and bone interact and communicate in ways beyond their mechanical relationship. Their interconnectedness highlights the importance of considering the strength of muscle and bone together.
Data tracking muscle mass and bone mass have shown that muscle loss leads to bone loss, and that muscle growth leads to bone growth. That's because per Wolff’s Law, muscle is responsible for the strain that bones require to maintain or gain mass. Muscle and bone also communicate via compounds called myokines.
What This Means To You
This deep analysis reveals how complex and multi-faceted bone strength is. That complexity stands in stark contrast to how Big Pharma and the Medical Establishment perceive and communicate about bone health. Their narrow focus on bone mineral density and prescription drugs blatantly ignores the mechanics of bone composition and behavior.
In stark contrast, the Osteoporosis Reversal Program applies an integrative approach to bone health. And it uses the interconnectedness of our bodily systems (such as the link between muscle and bone) to recommend actions that improve bone health.
Every new understanding and every healthy choice is another step on your journey to stronger bones and healthier living.