Before we can understand how joints go wrong in arthritis, we must first review what a normal joint looks like and what it does. What has pumping up a bicycle tyre got to do with understanding how joints work? Read on!
Dog joints can be broadly classified as either:
Synovial – These allow free movement e.g. hip, knee, shoulder or
Synarthroses – Joints with limited or no movement e.g. sacroiliac joint, intervertebral discs
Synovial joints are movable links between bones. They are amazing natural pivot points that give our dogs tremendous flexibility and mobility.
There are 5 major components of a synovial joint that enable it to do what it does:
1. The bones either side of the joint
2. The articular cartilage
3. The joint capsule
4. The synovial membrane
5. The synovial fluid
The bone ends are covered in a thin layer of smooth articular cartilage. The joint is surrounded by the joint capsule. This is a tough, fibrous sac which is firmly attached to the bone ends and creates a joint cavity that contains a sticky fluid known as synovial fluid.
The inner lining of the joint capsule is a delicate layer called the synovial membrane. This acts as a filter and is populated by white blood cells that perform surveillance for any unwanted microorganisms such as bacteria and viruses. Some of the synovial lining cells produce hyaluronic acid, which acts as a sticky lubricant.
A Closer look at Articular Cartilage
This is the smooth surface of the joint and it is only 1-2mm thick. It is a smooth, virtually friction-free surface that allows smooth joint motion. The unique structure of articular cartilage also makes it an excellent shock absorber. It has no blood supply and there are no nerves in it.
The articular cartilage consists of cartilage cells called chondrocytes (Kondro-sites), which produce and maintain the cartilage substance. This substance which is what you see and feel when you examine cartilage is referred to as the extracellular matrix or ground substance. This makes up the bulk of articular cartilage and is responsible for its unique structural properties.
What’s it made of?
Some of the main components of the cartilage are chemicals called glycosaminoglycans or GAGs. These compounds are a type of complex sugar and they are very good at ‘sucking’ in water around themselves to form a gel. Examples of glycosaminoglycans are chondroitin sulphate and keratin sulphate.
GAGs effectively suck water into the cartilage. Now, if that was all that happened, the articular cartilage would just swell up and become a soggy gel – not much good for its main role as a shock absorber and an ultra-smooth, low- friction surface suitable for effortless gliding motions! Something is missing.
Back to the bicycle tyre!
The additional, essential structural factor is collagen. Collagen fibres are like long, tough, rope-like strands running through the glycosaminoglycan gel. They form organised loops. The collagen strands won’t really stretch and so this sets a limit on how much the cartilage gel can swell i.e. how much water it can take in. Because the cartilage gel is trying to pull in more water and swell but the strong collagen framework is resisting, cartilage develops what is known as swelling pressure which makes it stiff.
It is like blowing up a bicycle tyre – the rubber tyre represents the collagen framework and the air represents the water. As the tyre is pumped up with more air it becomes stiffer and more able to take your weight and do its job. Similarly, articular cartilage is beautifully designed to take the stresses and strains of running and jumping without getting damaged.
So, in summary, there are 5 vital components of dog joints. The bone ends provide structure; the joint capsule holds everything together; the synovial fluid is a lubricant and nourishes the articular cartilage; the cartilage is the smooth gliding surface and shock absorber and the synovial membrane acts as surveillance and produces the synovial fluid.
Why is articular cartilage are so important in the progression of arthritis?
The chondrocytes are the living components of articular cartilage. These cells can be viewed as tiny factories producing the glycosaminoglycans and collagen gel in which they are embedded. Not only do they produce the cartilage but, by producing a sophisticated range of chemicals, they are responsible for maintaining it in a healthy state by breaking up and digesting any damaged areas and replacing them with new, healthy cartilage. One of the reasons that articular cartilage gradually falls apart in arthritis is that these control mechanisms go wrong.
|To find out more, see the Free Arthritis in Dogs video series at http://www.arthritisdogs.net
Dr Andrew Coughlan, founder of The Veterinary Expert, is a specialist small animal orthopaedic surgeon. Dr Coughlan’s mission is to help pet owners to make sense of what can be very complex issues and to help them get the best veterinary care for their pets.
Article Source: http://EzineArticles.com/?expert=Andrew_Coughlan