Subchondral Bone and Osteoarthritis – Reviewing the Microenvironment of Synovial Joints

Goal of the Study?

Pain from osteoarthritis is widespread. However, it is also common for osteoarthritis not to cause pain. So what gives? One anatomical microenvironment that appears to play a critical role is the subchondral zone of joints. This is the bone-cartilage interface that resides under (sub) the cartilage (chondral). Herein lies a vibrant area of nerves and blood vessels that crosstalk between the avascular cartilage and the vascular bone. One central research question is: What causes the initiation and progression of osteoarthritis, and why do some osteoarthritic joints continue to cause pain? In this research paper¹ published in Bone Research (2021), the authors set out to review the micro-anatomical environment of this critical region to expose more answers on mechanisms and possible solutions.

Why are they doing this study?

Osteoarthritis is a disorder involving the whole joint, which includes the bone, cartilage, ligament, joint capsule and synovium. This can affect any synovial joint in the body, including the knee and the spine. In the search for treatments, these researchers peel away the anatomical and chemical contributors to look at what can be done. The pathogenesis of osteoarthritis often involves mechanical factors, but other contributors include age, obesity, systemic diseases, and genetics. Learning about the influential factors can help direct future care in managing this disorder. However, the subchondral microenvironment lies under the cartilage of interest in this review.

 

 

What was done in this study?

Reviewing the literature on this bone-cartilage interface was the researcher’s primary target. One crucial factor in developing painful osteoarthritis is the aberrant mechanical loading of the cartilage across the tide mark. (See Fig 1 below)

Other factors that were looked at included:

1. Subchondral bone marrow edema (inflammation below the cartilage) that is seen on T2 weighted MRI that is non-cystic in nature
2. Bone-cartilage interaction (aka the tide zone). Just like that seen in the intertidal zone, this is an area between the bone and cartilage (land and sea) where a great deal of back and forth of nutrients pass. An absence of blood vessels in the cartilage sets up the importance of diffusion to supply the nutrient-dependent cells of the cartilage. A disruption of material exchange can be catastrophic for the metabolism.
3. Turnover rate. Typically, there is a reasonable turnover rate in the subchondral region, but when abnormal forces penetrate this land under the sea, the bone cells that clean up (osteoclasts) start to get overactive and create a deficiency in the bone microstructure.
4. Hypervascularization occurs. In other words, more blood vessels tend to grow under the cartilage bringing in more innervation.

 

What did they find?

The mechanism of pain seems to pivot on the thickness of the cartilage. Because pain is divided into differing types: nociceptive, nociplastic, neuropathic, and idiopathic (pain that clearly cannot be explained), the psychological processing of pain cannot be ignored. Pain is an experience processed in the brain, and if factors outside the joint need to be addressed, more research should be conducted. However, in this review, the researchers found that abnormal loading increases the unwanted turnover of the bone, bringing in new blood vessels and nerves. Aberrant movement can ramp up nociceptive pain in many people, and healthily managing the load can make a difference. Finding the right balance of exercise can play an essential role in controlling osteoarthritis pain.

 

Why do these findings matter?

The subchondral environment is a complicated anatomical zone with contributing factors in pain osteoarthritis, both mechanical and biological. Learning to manage a healthy dose of loading to recovery time may be the primary focus in years to come in tackling this profound problem on Earth: Osteoarthritis.

 

At Dynamic Disc Designs, we create dynamic human anatomy lumbar models showing the dynamic environment of the subchondral and cartilage interface. We have worked to model viscoelastic hyaline cartilage of the facet joints and fibrillation to help in the patient education of spinal osteoarthritis. Recently, we have teamed up with Active Implants to showcase their newest implant for the knee. For non-pharmacological approaches to spinal pain osteoarthritis, Dr. Jerome Fryer (CEO of Dynamic Disc Designs Corp and practicing chiropractor) recommends using Nordic Poles for walking or using chair-care decompression during the act of sitting for spinal osteoarthritis.