Where does osteoarthritis come from?

Figure 1

What is YOUNG’s Modulus? Why is it important? Where does osteoarthritis come from?

Austin Yeargan MD

Regenerative Medicine Clinic, Wilmington, North Carolina

Young’s modulus measures the amount of elasticity of a material. You may see it referred to as just ‘Young’s modulus’ or ‘Young’s modulus of elasticity’.

Elasticity, or stiffness, is a “material property.” That means every material has its own property of elasticity or how much it will yield, given a certain amount of displacement.

To simplify, it means how hard you have to push on something (stress the material) before it starts to ‘give away’ (bend the material). This is known as the ‘elastic modulus’ in science and math and is represented by a ‘stress-strain’ curve that measures how much deformity there is in material with increasing applications of stress (or load).

Understanding what the types of bone are and how the elastic modulus differs between the two sheds light on how arthritis arises and progresses. There are two types of bone for our discussion:

1. Cortical bone, or “compact bone”

2. Cancellous bone, or spongy “trabecular bone”

Cortical bone is the very thick outer layer of bone that encases the spongy trabecular type of bone that is responsible for absorbing the impact under loading conditions. Together these types of bone allow the human organism to live their lives in a bipedal position relative to gravity.

Perhaps the most amazing thing about bone is its ability to change under loading conditions to become more or less stiff depending on the loads imposed by gravity and impact. This adaptation is the mechanism that can protect the joint from breakdown unless it proceeds too far.

Once past the “point of no return”, the cartilage matrix that caps the ends of the bone has become so overloaded that it can no longer withstand the impact and suffers breakdown. This transfers the load from the joint to the subchondral bone, which begins to stiffen in response to the overload.

This ‘stiffening’ of the subchodral (under the cartilage) bone is the rate limiting step for the development of arthritis. If this does not happen, no one gets osteoarthritis. It’s that simple.

The Terracon Sequenced MRI views seen from GE provide an excellent demonstration of this concept.

See figure key for illustration.

Figure 1. In our knee example, as with all other synovial joints, the concave side of the joint receives the impact from the convex side (1). Protein ‘antennae’ embedded in the cell membranes are displaced and through immunologic signaling pathways prompt a resorption front. The resorption front comes from osteoclastic activity that is followed by an osteoblastic response and new bone formation. This additional ‘layering’ of bone causes the bone to become more stiff, always on the concave side initially. In our example, this is the tibial side. This represents a change in Young’s modulus and the material properties of the bone a the concave side becomes more and more stiff. Imaging a baseball glove (concave) receiving a thrown ball (convex). The glove (concave) accepts the load from the thrown ball (convex). All of these changes are adaptive. Arthritis is not a disease.

Once the concave side of the bone (1) has become thicker and more stiff, perfusion through the bony tissues is compromised due to increased pressure within the bone, which prompts additional subchondral bone remodeling and a more cortical bone structure throughout the bone that is underneath the weight-bearing cartilage.

Next, the load is reflected back to the convex side of the joint, where the same process occurs (2). Ultimately there are two opposing, very stiff articulating surfaces that quickly destroy the remaining articular cartilage.

Bone spurs (3) that form on the overloaded side of the joint represent an attempt to increase the surface area of the contact surface to more evenly displace the load. An analogy would be if you were to walk across a hardwood floor with a stiletto type of high-heeled shoe. The force at impact from body weight is concentrated in the heel and is likely to ding the floor. However, a flat-bottomed heel shoe, with its wide surface area of contact distributes the load more evenly, avoiding the ding in the floor imposed by the high-heel shoe. Surface area of contact plays a big role in how the brain and the body’s cells respond to loading conditions.

So why is any of this important?

Understanding how and why arthritis occurs is key to reversing the process and it can be accomplished with the appropriate patient. When patients present with knee pain, regardless of Xray findings, they are approximately four years away from destruction of the affected side of the joint, somewhat less with corticosteroid injections due to the destructive effect they have on cartilage and the loss of tacit proprioceptive sense. Paying strict attention to subchondral bone involvement in the opposite joint compartment (4) and the patellofemoral compartments is key to counselling patients on the likely longevity of their immunobiologics procedure like our Nanoplasty And Mechanical Axis Deviation (NAMAD) protocol. Arthritic disease always proceeds in a stepwise fashion from the concave to the convex side of the joint as noted in the illustration. Having knowledge of this fact allows not only being able to objectively classify disease, but guide treatment appropriately.

S. Austin Yeargan III, MD Orthopedic Surgeon Molecular Immunobiologist

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