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True or false: the synovial membrane is epithelium?
False - epithelium lies on a basement membrane but the synovial membrane does not. Therefore the synovial membrane is better described as a specialised connective tissue with secretory capacity.
What two primary cell types are present on the synovial membrane?
Macrophages and fibroblasts
What is the main cell type found in a normal joint sample?
Describe the formation of synovial fluid
Plasma dialysate (fluid passed through a membrane) which is modified by secretion from cells in and around the joint and contains a small number of cells e.g. lymphocytes, monocytes and neutrophils.
What are the main functions of synovial fluid?
- Lubrication - boundary lubrication of both of the synovial membrane and articular cartilage
- Nutrient transport - as articular cartilage is avascular most of it is in close apposition to a synovial membrane which produces synovial fluid. This synovial fluid helps to 'fill the gaps' where nutrients cannot be transported directly from tissue to tissue.
List some indications for arthrocentesis
- Joint effusion
- Suspect infection - sudden onset, pain, pyrexia, surgery
- Multiple joints
- Pyrexia of unknown origin
True or false: arthrocentesis can be performed on a conscious animal?
False - it should be done under GA or sedation. After radiography is a good time to perform arthrocentesis.
What type of needle/syringe should you use for arthrocentesis in small animals?
- Blue (23g 0.6mm) or green (21g 0.8mm). Length depends on the size of the joint.
- Dog - 2 or 5ml, Cat - 1 or 2ml
Which joint is the easiest to take a sample from?
The stifle joint
What tube do you put your arthrocentesis sample in if you wish to run cytology?
What should you do if you do not get a lot of fluid in your sample?
Make an air dried smear
What can you put your sample in if you suspect infection?
Blood culture bottles - they provide bacteria with a friendly environment so they can be cultured and possibly identified
How do you tell the difference between iatrogenic and pathological bleeding in a joint sample?
When the sample is still in the syringe barrel, if there is iatrogenic blood there will be distinct separation between blood and synovial fluid. Whereas, if it is a pathological cause the blood will be mixed with the joint fluid and it will have a homogenous appearance.
What is the normal volume of joint fluid samples in cats and dogs?
- Cats = 0-1ml
- Dogs = 0.1-1ml
The amount of fluid you can take for a sample will increase/decrease in diseased joints?
What does normal joint fluid look like?
Clear/egg white in colour and it will form a gel in the sample tube.
What does joint fluid look like in a) DJD b) inflammatory disease c) septic joints?
- a) clear to pale yellow
- b) the turbidity of the fluid increases
- c) very abnormal in colour, blood is often present, often have a bad smell
What is the viscosity of normal joint fluid?
Viscosity will be increased/reduced in inflammatory disease?
Reduced - normal joint fluid should not be watery
How do you prepare a synovial smear?
Put a small drop of synovial fluid on your slide. Take another slide and place it on top of your sample. Do not apply pressure - you just want to sit the two plates on top of each other or you will squash all the cells. Swipe the top plate steadily across the bottom plate to evenly spread the sample. The smear should be air dried very quickly to preserve all the cells. The slide can be stained using Diff Quik, any standard Romanowsky stain or any routine haematology stain.
What should you be assessing when looking at cytology of a synovial smear?
- Always eyeball the smear before putting it under the microscope
- First examine the background - the polysaccharides should be a pink haze
- Assess the amount of red cells - high number indicates iatrogenic or pathological bleeding
- Platelet clumps will occur if there is bleeding at the time of sampling
- Count numbers of large mononuclear cells, lymphocytes, neutrophils and eosinophils.
- Finally assess the nucleated cell count
What will be seen on cytology if there has been acute haemarthrosis/longstanding haemorrhage?
- Acute haemorrhage - platelets
- Longstanding haemorrhage - no platelets, increased numbers of white cells, erythrophagocytosis, haematoidin crystals within macrophages and haemosiderin deposits within macrophages
What do the following changes in biochemistry indicate about a joint sample? a) Increase in total proteins b) Reduction in glucose levels
- a) Inflammatory arthropathies
- b) Bacterial infective arthritis
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