The canine knee joint:
a common site for arthritis.
(original graphic by marvistavet.com)

This radiograph shows the two
mandibles (lower jaw bones) of a dog.
The yellow arrows point to the
"symphysis" joint which joins them

(original graphic by marvistavet.com)

The vertebral bones are joined together
by round disc joints which allow
the back to have flexibility
(original graphic by marvistavet.com)

There are several types of joints within the body. The fibrous disks that separate the back’s vertebrae and allow the back’s flexibility are specialized joints (shown in the illustration above.) The two halves of one’s lower jaw are held together by a joint called a “symphysis” which, unlike other joints, is meant to reduce movement, not facilitate it (also above.)

The joints we are concerned with when we talk about degenerative arthritis are the “synovial joints” also called “diarthrodial joints." They consist of two bones and a fibrous capsule holding the two bones together; however, the joint is far more complicated than just a hinge made of fiber and bone. The two bones have surfaces that are covered with slippery cartilage that must be able to glide across each other with minimal friction no matter what the patient’s activity level is and they must continue to be able to glide easily in this way repeatedly throughout the patient’s life. Further, the joint capsule secretes assorted nutrients, lubricating fluid, and immunological materials keeping the joints healthy. Further, the joint capsule secretes assorted nutrients, lubricating fluid, and immunological materials keeping the joints healthy.

Prevention of the progression of arthritis is all about maintaining the normal structures of the joint. In many cases, this involves providing the biochemical components of these structures as nutritional supplements. Our purpose here is to review what the structures are and what they are made of so that one can better choose supplements and understand what one is giving. For illustrative purposes we will use the shoulder joint as an example. The illustration at the right shows the structures of the shoulder.

 
The shoulder joint involves the scapula (or shoulder blade)
and the head of the humerus (arm bone). The bone
surfaces are lined with cartilage and the entire joint
is encased in a fibrous capsule called a joint capsule.
(original graphic by marvistavet.com)

The articular cartilage surfaces of the joint are the cartilage caps on the ends of the bones. These are the smooth surfaces that must glide across each other with minimal friction. Any rough spots will ultimately abrade over time and create pain and inflammation.

Cartilage is made up of cartilage cells called "chondrocytes" and the special "matrix" in which they live (think of fruit chunks suspended in a "matrix" of jello). The matrix is made of the all-purpose structural protein called "collagen" (probably a familiar word) and "proteoglycan" (probably an unfamiliar word). The matrix is made to retain water like a sponge thus creating a soft “shock absorber” for the joint.

MORE ABOUT WHAT’S IN THE MATRIX

(because you are probably feeding it to your pet if you use a joint supplement)

So we’ve already said the matrix consists of collagen (steel girders for your body’s structure) and proteoglycan (the spongy stuff). When we give “chondroprotectants” or “cartilage building blocks” as supplements we are basically providing the materials that make up proteoglycan so that the body will have plenty of ingredients to make fresh cartilage matrix.

Here is a depiction of a piece of proteoglycan:

Same depiction of the shoulder joint.
The articular cartilages are shown in grey-blue.
(original graphic by marvistavet.com)

THE JOINT CAPSULE OR “SYNOVIAL MEMBRANE”

This is the capsule that encloses the joint, creating the structure of the “hinge.” The capsule has an outer tough, fibrous layer and an inner layer that secretes joint fluid, a fluid that provides both nutrition and lubrication to the enclosed joint. The joint capsule must keep unwanted proteins and biochemicals out of the joint and only let the desired nutrients inside. After all, the joint must be kept smooth, and the lubricants pure if they are to maintain the joint throughout the patient’s entire lifetime.

The joint capsule has two types of cells: Type A Synoviocytes and Type B Synoviocytes. The Type A cells are all about removing impurities and cleaning up debris. Type B cells produce Hyaluronan (mentioned above), an important lubricant in the joint.

Same depiction of the shoulder joint.
The joint capsule is shown in dark green.
(original graphic by marvistavet.com)