Joint medication: Part 1 - What is a joint? Main causes of arthritis? How do we feel joint pain?
During this 4 part blog series, I am going to delve into joints, joint injections and other medications which can help with joint health.
This first part of the series will look into what a normal joint looks like, discuss the most common causes of the most frequently diagnosed joint disease - osteoarthritis and how the brain detects pain and discomfort.
The second part will look at some of the most commonly medicated joints, the third will look at joint medications and the fourth will look at other medications and supplements which can be beneficial for joint help.
What is a joint?
Appreciating what makes up a joint is essential in being able to understand how different medications work.
A joint generally consists of two opposing bones - the bone just below the joint is called the subchondral bone, and this portion of bone consists of a very high amount of blood vessels to provide oxygen and nutrients to the joint, whilst removing any waste products. The surface is covered in a material called hyaline cartilage, also known as the articular cartilage. This is composed out of approximately:
10% proteoglycans (mainly polysulfated glycosaminoglycans).
The water content is controlled by the collagen and proteoglycans. This water content is in turn, responsible for the resiliency and compressive stiffness of the cartilage. The chondrocytes function in secreting substances which maintain the matrix (building blocks) of the cartilage, and this is regulated by the blood supply from the subchondral bone.
The synovial membrane is a vascular connective tissue which lines inside of the joint capsule. This means that a lot of blood vessels run through it. The membrane also has special cells called synoviocytes which are responsible for removing unhealthy material from within the joint, whilst producing hyaluronate and anti-inflammatories. The synovial membrane also produces joint fluid, and this consists of a protein-rich blood derivate and hyaluronic acid.
The articular capsule is double-layered, composed out of a tough, fibrous outer-layer which inserts into bone ends at joint margins and an inner layer which supports the synovial membrane.
Causes of arthritic changes
The most common cause of degenerative changes is due to cartilage damage following repeated abnormal mechanical force. Inflammation of the synovium due to excess mechanical force decreases how well the synoviocytes work, resulting in smaller molecules of hyaluron which subsequently decreases the lubrication in the joint, leading to cartilage breakdown.
The second reason is genetics - abnormal properties of the joint surface will result in failure, even despite normal loading.
Another cause lies within the subchondral bone. The subchondral bone acts as a shock absorber, and if it is defective, it cannot adequately protect the articular cartilage.
A less common cause is due to the administration of an antibiotic within the fluoroquinolone antibiotic family called enrofloxacin. This antibiotic is especially good at penetrating bones (and the lungs) making it ideal for pneumonia, bone disease and septic arthritis. They do, unfortunately, also cause joint disease in immature animals (1), resulting in damage on weight bearing surfaces of articular cartilage in foals. It is highly unlikely to have the same effect in pregnant mares.
So if a joint is sore, how does the brain know about it?
The nervous system works to communicate messages between the body, including joints, and the brain and vice-versa. Different stimulation is picked up by different types of receptors - some pick up pressure, others pain, temperature, frequency changes etc. This information is relayed to the brain which provides an appropriate response - this can involve protective measures such as decreased motion/ increased stiffness to restrict how sore the joint can get.
Joint tissues have four different types of receptors which relay information to the brain;
Type 1 - located in the joint capsule, these receptors are relatively sensitive to mechanical force, whilst providing spatial awareness.
Type 2 - found at the junction between the joint capsule and synovial tissue junction, these receptors are activated during motion and also relatively sensitive to mechanical force, whilst providing spatial awareness.
Type 3 - found near the insertions of the joint ligaments, they detect pain and pressure, mainly activated when the joint is at its physiological limit.
Type 4 - found in the synovial membrane - unlike the other receptors, these free nerve endings relay information regarding changes in temperature, abnormal chemicals and pressure from within the joint.
The sensation of pain will, in most cases, be increased by inflammation. In addition to mechanoreceptors stimulated by mechanical influences, they can become hypersensitized by chemical stimuli released during the inflammatory process.
Why is this important to know?
The response to pain and discomfort is different in every horse - they all have different pain thresholds and therefore, they exhibit it in unique ways in different stages of joint disease. Just because a horse has really terrible joint disease, does not necessarily mean that they are always extremely uncomfortable. Subsequently, it is important to remember that horses with minimal changes may still be extremely sore.
Sprecht, T. and Frederick, G., 1991. Quinolone-induced arthropathy in immature equidae. J Am Vet Assoc, 198, p.516.