Understanding Dog Joints

Normal Joints

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 other joint care options and an understanding of what is happening in your pet's joints, especially if surgery is not an option 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.

The Structure of a Normal Joint

There are several types of joints within the body. The fibrous discs that separate the back’s vertebrae and allow the back’s flexibility are specialized joints. 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.

The joints we are concerned with when we talk about degenerative arthritis are synovial joints, also called diarthrodial joints. They consist of two bones and a fibrous capsule holding the two bones together but the joint is far more complicated than just a hinge made of fiber and bone. The two bones surfaces 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.

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 radiograph at the top of this page shows the bones that make up a dog’s knee.

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:

Matrix
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 mean 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 material surrounding the collagen fibers. It consists of a long protein molecule (the proteo part of proteoglycan) with strands of what are called glycosaminoglycans or GAGs, growing off its sides like the bristles on a hair brush. 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. Two important GAGs are keratan sulfate or chondroitin sulfate, both of which are common nutritional supplements available for joint support in most veterinary hospitals, drug stores, and even grocery stores.