Synovial Infection is discussed here by Dr Chris Baldwin BVetMed(hons) MRCVS, who is resident equine veterinary surgeon at the Sussex Equine Hospital, Ashington.
The anatomy of a joint can be broken down into a few basic component parts, two opposing bones are encapsulated in a fibrous joint capsule which is lined with an active synovial membrane. At the end of each bone there is a layer of cartilage which facilitates the gliding motion of the bones over one another. Within the joint a fluid is produced by the synovial membrane which lubricates the articulating joints to reduce friction, provides a shock absorption, supplies oxygen and nutrients and removes metabolic wastes from the cells within the surrounding cartilage.
Joints and the synovial fluids are sometimes referred to as immune privileged sites, this is to say they have a limited capacity to deal with infections. This is because the synovial fluid is a hyper-filtration of the blood produced by the synovial membrane. The synovial membrane produces a viscous, low cell, low protein fluid composed of hyaluronic acid, lubricin, proteinases, collagenases and phagocytic cells (removes the debris that results from normal wear and tear in the joint).
The fluid is an excellent medium of nutrients to allow bacteria to grow rapidly and the fluid is devoid of infection fighting antibodies and lymphocytes. This means the introduction of even a very small number of bacteria can result in a septic joint with an infection that cannot be fought off by the horse’s own immune system.
When bacteria enter a joint space, they reproduce rapidly releasing a multitude of toxins and waste products that results in the synovial membrane releasing a storm of inflammatory mediators to recruit infection fighting cells (neutrophils and lymphocytes). The neutrophils and lymphocytes produce bacteria destroying enzymes in order to tackle the infection, however if the host defences are overwhelmed, which in an immune privileged site they often are, then infection will set in.
Infection results in an extremely painful inflammatory response and synovial fluid production increases, which results in joint distension (which is also painful!) resulting in a severe lameness and an enlarged or swollen joint. Internally the process of the infection and the inflammation aimed at tackling the infection results in cartilage degradation, adhesion formation (tendon sheaths) and pannus/fibrin build up.
When determining if a synovial structure is infected or not your vet may take a sample of the fluid and analyse it for changes that may indicate an infection has established. Synovial fluid is normally a clear viscous fluid however when infected it becomes; turbid, less viscous and cell and protein concentrations increase.
If there is a wound close to a synovial structure then at the same time as taking a sample your vet may also inject sterile water into the joint and observe to see if the water comes out of the wound, if it confirms communication between the wound and the joint, if it doesn’t then the joint fluid sample will need to be analysed.
Once a septic synovial structure has been confirmed it is then important to treat the infection quickly and aggressively. Unfortunately, antibiotics alone have a very poor success rate because antibiotics given either into the muscle, vein or in feed have poor penetration into the synovial fluid.
The “gold standard” treatment is a combination of antibiotics and arthroscopy, this involves placing an arthroscope (camera) into the joint and flushing the joint fluid out with sterile saline, this flushing aims to relieve distension, remove bacteria, inflammatory mediators and restore normal synovial health.
The solution to pollution is dilution! This procedure can be done without the camera by placing multiple needles into the joint and performing what is called a “needle flush” however the benefit of using an arthroscope is that this allows removal of the built-up pannus/fibrin (which is heavily infected thick pus like-tissue).
Arthroscopy also allows visualisation of the joint to assess for any joint damage or contamination (dirt, thorns, hair), allowing an improved prognosis prediction. Arthroscopy has a much greater success rate than needle flushing alone because of all the above outlined benefits.
Although joint infections cause a severe lameness and can result in profound damage to a joint surface, if treated appropriately and promptly horses can make an excellent return to full athletic work. The key to success of fast veterinary intervention.
Dr. C. Baldwin, BVetMed, MRCVS