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Normal Joints Look Like This in Dogs and Cats
Revised: May 03, 2023
Published: May 13, 2003

A pet doesn’t have to be a senior citizen to require joint care supplements, pain medication, or physical therapy. Degenerative arthritis can result from an injury or can be the result of genetics/joint conformation. If your pet is stiff or has poor range of motion in a joint, then you will need some education about joint care options and an understanding of what is happening in your pet's joints, especially if surgery is not feasible for one reason or another. The following represents a beginner's overview of joint structure and function so as to provide a foundation for understanding treatment options for the arthritic pet.

Radiograph showing the two mandibles (lower jaw bones) of a dog. The yellow arrows point to the symphysis joint that joins them
This radiograph shows the two mandibles (lower jaw bones) of a dog. The yellow arrows point to the symphysis joint that joins them. Photo by marvistavet.com

The Structure of a Normal Joint

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. The two halves of the lower jaw are held together by a joint called a symphysis, which, unlike other joints, is meant to reduce movement, not facilitate it.

The joints we are concerned with when we talk about degenerative arthritis are the synovial joints, which are 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 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. Furthermore, the joint capsule secretes assorted nutrients, lubricating fluid, and immunological materials that help keep the joints healthy.

The vertebral bones are joined together by round disc joints that allow the back to have flexibility.
Graphic by MarVistaVet

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 you can better choose supplements and understand what you’re giving. For illustrative purposes, we will use the knee, or stifle joint as it is called in animals, as an example. The illustration at the right shows the structures of the shoulder.

Articular Cartilage Surfaces

The articular cartilage surfaces of the joint are the cartilage caps on the ends of the bone. These are the smooth surfaces that must glide across each other. Cartilage is made up of cartilage cells called chondrocytes and the matrix in which they live.

The cartilage matrix is the material in which the cartilage cells are suspended (think of fruit suspended in a matrix of Jell-O.) The matrix consists of collagen and proteoglycans. We have all heard of collagen, the tough support proteins that act as the steel girders of the body, holding everything from bone to skin in the shape it is meant to hold. Proteoglycan is not a word familiar to the general public but since most joint nutritional supplements relate to it, it is important to know what it is. Proteoglycans are the materials surrounding collagen fibers.  They consist of long-core protein molecules with glycosaminoglycans, or GAGs, growing off their sides like bristles on a hair brush. These hair brush bristles consist of keratin sulfate and chondroitin sulfate, both of which are included in many popular over-the-counter joint supplements. The core proteins carrying their bristles connect to a central GAG called hyaluronan, which is also used in joint support products.  These GAG bristles allow the proteoglycan molecule to soak up water like a little sponge. It is this sponge characteristic that allows the cartilage to be soft like a mattress, yet slippery like a Slip n’ Slide when the two bones of the joint move across each other.   

These are the cells that actually secrete the cartilage matrix. The matrix is the soft (relative to the bone beneath it) material that represents 95% of the cartilage. While the cells represent only a small portion of the cartilage, they must remain healthy so as to produce new matrix when the old matrix is damaged.

 Why Should You Know these Complex Words?

Many of you reading the word glycosaminoglycan may simply let your eye pass over it and not even try to pronounce it in your mind. If you want to know about the nutrition of joint care, though, glycosaminoglycan is an important word to know. The three GAGs that can make up a proteoglycan molecule are:

  • Chondroitin sulfate
  • Keratan sulfate (which is made in the body from a biochemical called glucosamine)
  • Dermatan sulfate
    Proteoglycan depiction: core proteins in green, GAG bristles in red, hyaluronic acid in blue. Graphic by MarVistaVet

When you buy a bottle of some sort of joint supplement, the chances are it will contain chondroitin sulfate and/or glucosamine. Now you know what you are buying and what these supplements are supposed to do in the body.

The last GAG that you should be familiar with is Hyaluronan. Unlike the GAGs listed above, it is not sulfated and it is not part of a proteoglycan. It simply binds and connects the proteoglycans together in the matrix. (In the graphic above, it is shown as a sort of core to which the proteoglycans are attached.) Hyaluronan is also a major component of the joint lubricating fluid, which we review in the next section.

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.

Degenerative Arthritis

When there is an injury or simply poor conformation, the cartilage becomes roughened and can chip, flake off, or even wear down. The joint capsule becomes inflamed and thickened and no longer functions normally. Impurities enter the joint, the lubricating fluid loses its natural properties and ultimately a progressively abnormal and painful joint is created. In providing treatment, our goal is to alleviate the pain and inflammation and provide the biochemical building blocks that allow the joint to heal itself. In the next sections, we will review medications, supplements, and even exercises that may help with the arthritic patient.

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