Contents:
1 - Indications for joint arthrocentesis
2 – Equipment you will need
3 - Sedation options for the procedure
4 – ‘How to’ guides on performing arthrocentesis of the various joints with video links
5 - How to interpret the findings in-house, including a video on how to prepare a slide and photos of typical cytological findings
- Stifle
https://www.youtube.com/watch?v=zeY2ahrp4Dw
- Elbow
https://www.youtube.com/watch?v=MCsENBBBHJQ
- Shoulder
https://www.youtube.com/watch?v=QNBUdGzupEc
- Tarsus
https://www.youtube.com/watch?v=uSa7OH7edVk
- Carpus
https://www.youtube.com/watch?v=Ny6r8s9Xnpk
- Hip
Arthrocentesis is where a needle and syringe are utilised to obtain a sample of synovial joint fluid from within a joint. It is a simple procedure that you can perform within your practices!
1 - Indications for arthrocentesis:
Arthrocentesis is a useful and minimally invasive procedure. Evaluation of the cytology and gross appearance of the fluid on it’s own can be extremely beneficial in the diagnosis of many common problems in the joint. Joint fluid can also be sent to the laboratory for culture and cell counts. It is indicated for the following situations;
- Pain with joint manipulation
- Pyrexia of unknown origin, especially if there is any joint pain (always consider immune-mediated polyarthropathy)
- Generalised stiffness
- Shifting limb lameness
- Joint effusion with no diagnosis (e.g. no cruciate ligament rupture, but a stifle effusion – beware though, not all cruciate ligament ruptures have palpable instability, so cruciate ligament rupture may need to ruled out in other ways – a joint tap is still useful in this situation however. If you can rule out suppurative inflammation in the joint, then a stable partial cruciate ligament tear is very likely). We will discuss suppurative versus non-suppurative inflammation below.
2 - The equipment you will need:
Sedation – see the following section (3)
Needles
(a) In terms of needle length, don’t overthink the exact length required – estimate this from the patient – will the needle reach the joint? Use the shortest needle possible.
(b) Use the smallest Gauge needle possible – between around 20-23 gauge depending on the size of the patient. Generally speaking, around the gauge you would use for jugular stick on the same size patient should be adequate.
Syringe
A 2.5ml or 3ml syringe for a smaller patient and a 5ml syringe for a larger patient.
Microscope slides
Slide holders for sending to lab
EDTA (cytology) and sterile pot/blood culture medium (if you require culture)
Sterile gloves
A 22 Gauage spinal needle of appropriate length to reach the joint if tapping the hip or shoulder
3 – Sedation
Sedation or light anaesthesia is required for this procedure. A combination of medetomidine or dexmedetomidine and butorphanol is an option.
Dexmedetomidine – 1ug/kg for moderate sedation (range 0.1 – 10ug/kg)
Medetomidine – 10ug/kg for moderate sedation (range 10-80ug/kg.
Butorphanol – 0.1-0.3mg/kg IM or 0.05mg/kg IV.
Note: we often start with much lower doses than generally reported. Often we will start with a dose of around 5ug/kg IM medetomidine combined with 0.2mg/kg butorphanol IM, then titrate upwards as required. Alternatively, a full general anaesthesia is often a good option, especially if you are also planning to obtain radiographs.
4 – How to perform the Procedure:
a) Detecting effusion in various joints
To palpate for effusion, have the patient in a standing position as this makes it more likely you will detect the effusion, as weight bearing pushes effusion to the joint periphery. You cannot palpate an effusion in the shoulder, hip or spine articular facet joints.
Stifle – The pouch just caudal to the patella ligament will feel puffy (normally concave). The patella ligament itself palpates as less distinct.
Tarsus – In this joint, capsule distention can be palpated on the craniomedial, craniolateral, caudolateral and caudomedial aspects of the joint, adjacent to collateral ligaments.
Carpus – Effusion of this joint is most obvious cranial and medial. This is best palpated with the carpus flexed.
Elbow – Effusion is best palpated on the lateral aspect of the joint between the lateral epicondyle and olecranon.
b) Performing arthrocentesis in various joints
These are demonstrated with the following videos.
- Stifle
https://www.youtube.com/watch?v=zeY2ahrp4Dw
- Elbow
https://www.youtube.com/watch?v=MCsENBBBHJQ
- Shoulder
https://www.youtube.com/watch?v=QNBUdGzupEc
- Tarsus
https://www.youtube.com/watch?v=uSa7OH7edVk
- Carpus
https://www.youtube.com/watch?v=Ny6r8s9Xnpk
- Hip
https://www.youtube.com/watch?v=pOy94PAgUWc
Generally speaking, the needle is inserted in the direction shown in the video, using the palpable landmarks shown.
Once inserted into the joint, look for joint fluid in the hub of the needle. Apply negative pressure gently. Vigorous suction may yield synovium or make blood contamination more likely. If you do not obtain joint fluid, rotate the needle by 90 degrees. Before you remove the syringe, release negative suction. If you have fluid in your syringe, you can leave the needle and disconnect the syringe. This reduces the risk of blood contamination, more likely to be present in the needle.
Place a drop of joint fluid onto a slide and spread it gently with another slide (see video). If you need a culture, submit this in a culture medium. Blood culture medium is the best, but not always available. If there is a small amount of joint fluid, it can be mixed with saline and submitted for culture in culture medium. If no culture medium is available, submit the sample in a sterile pot.
Complications of this procedure are very low. Infection is possible, but is rare, especially using aseptic technique. No antibiotics are required prophylactically. Iatrogenic cartilage damage is common, but is unlikely to be of clinical significance. Insert the needle carefully and hold the syringe stable to reduce the risk of damage.
Trouble shooting – common issues:
1 – Needle is not in the joint – repeat examination
2 – Plug of tissue in the needle – repeat with a shorter bevelled needle or spinal needle. One advantage of a spinal needle is they have a short bevel, so it is more likely to obtain a plug of soft tissue.
5 – Interpreting the findings.
Fluid analysis:
Note that normal joints typically have smaller amounts of fluid, in the realms of 0.05-0.3mls.
Gross appearance.
Blood within the fluid is abnormal, but may occur at sampling (iatrogenic hemorrhage). You can often see a streak of blood when the fluid becomes contaminated. If the fluid is more amber in colour, this can sometimes indicate the blood has been there for some time (abnormal).
Usually if only one joint is affected, an infectious process can be suspected. Infectious diseases that cause multiple joints to be affected (e.g. Leishmania, Borrelia) are very rare.
Cytology:
Probably the most important diagnostic test for this fluid, is cytological analysis of the fluid ie. take a look at your stained slide. For preparation of this slide, follow this link (see video)
Normal Joint Fluid
Inflammatory - nonsuppurative
Inflammatory – suppurative
Trauma, DJD, hemarthrosis
Immune-mediated, infectious/septic
Volume
Small 0.5mls
Slightly increased
Increased
Appearance
Colourless, pale yellow
Colourless, light yellow or orange
White, light yellow or orange
Viscosity
Viscous
Slight loss viscosity
Loss of viscosity
Cell Count – WBC/ul
<3000
3000-5000
5000-370,000
Mononuclear:Neutrophil Ratio
90:10
90:10
10:90
Synovial Fluid Findings: It is important thing to note that you can determine in-house (with a basic microscope and staining) whether your joint fluid is inflammatory or non-inflammatory. This can instantly guide further diagnostics (e.g. culture) and treatment.
1 - Normal joint fluid.
Volume – small 0.5mls.
Appearance – colourless, pale yellow
Viscosity – viscous
Cell count (WBC/ul) <3000
Approximate Mononuclear:Neutrophil ratio. 90:10
Cytological appearance of normal joint fluid under 50x magnification.
Image kindly supplied by Dr Brett Stone, QML Vetnostics
Cytological appearance of normal joint fluid under 50x magnification.
Image kindly supplied by Dr Brett Stone, QML Vetnostics
2 - Inflammatory – nonsuppurative
– DJD (cruciate ligament rupture, elbow dysplasia, hip dysplasia chronic joint trauma), trauma, hemarthrosis
Volume – Slightly increased
Appearance – colourless, light yellow or orange
Viscosity – viscous or slight loss of viscosity
Cell count (WBC/ul) 3000-5000
Approximate Mononuclear:Neutrophil ratio. 90:10
Cytological appearance of non-suppurative mononuclear inflammation under 20x magnification.
Image kindly supplied by Dr Brett Stone, QML Vetnostics
Cytological appearance of non-suppurative mononuclear inflammation under 20x objective
Image kindly supplied by Dr Brett Stone, QML Vetnostics
3 - Inflammatory – suppurative
-Immune-mediated, infectious/septic
Volume – increased
Appearance – white, light yellow or orange
Viscosity – loss of viscosity, more fluid
Cell count (WBC/ul) 5000 – 370,000.
Approximate mononuclear:neutrophil ratio 10:90
20x objective – shows neutrophilic inflammation
50x objective – neutrophilic inflammation
Degenerate neutrophil 100x objective
Intracellular bacteria 100x objective
The key cytological feature of note, is the ratio of mononuclear:neutrophil ratio, however you can very roughly estimate cell counts based on a thin preparation of synovial fluid (see video for slide preparation).
For high power fields (hpf - The 100x lens), 1 WBC per hpf is around 1000 cells/ul
12 WBC per hpf would be 12,000 cells/ul.
So if you are counting 5+ neutrophils at high power,on average (average about 10 fields) you likely have a suppurative process – easy!
Typically, with a nonsuppurative inflammatory process (e.g. osteoarthritis), you will see a few cells with mononuclear cells predominating. With suppurative inflammation, you will see a predominance of neutrophils.
Suppurative Inflammation
If you have (3) - suppurative inflammation (neutrophils), you would be concerned about either a septic (infection) arthritis or an immune-mediated arthropathy (IMPA). The presence of bacteria is uncommon, even in a septic joint (only in 25% will you see bacteria), so it is difficult to distinguish a septic joint from a joint affect by an IMPA.
The key distinguishing feature, between a septic joint and a joint affected by IMPA may be the presence of a polyarthropathy – multiple joints (indicating IMPA versus septic). If only one joint is affected, empirical antibiotic therapy should be started and response-to-therapy monitored. Cephalexin, Amoxicillin-Clavulonic Acid and Clindamycin are good first-line options for common pathogens. Unfortunately, culture results are false-negative in around 30-50% of cases and diagnosis is often made based on response to treatment.
The next steps if you have a suppurative inflammation (3 above), is to send some of the fluid for culture, start antibiotics if you suspect a septic process i.e. one joint, and you may also take some radiographs to evaluate for an erosive immune-mediated polyarthropathy (more on this below).
A note on the Immune-mediated polyarthropathies IMPA:
Immune-mediated polyarthropathies are broadly divided into non-erosive and erosive forms. Non-erosive forms include idiopathic immune-mediated polyarthritis, polyarthritis-myositis syndrome, systemic lupus erythematous, drug-induced IMPA and breed associated IMPA. Immune-mediated polyarthropathies can be secondary to underlying disease e.g. infection remote to the joint, GI disease or neoplasia.
Erosive IMPA, is typically a rheumatoid arthritis. It is a rare, chronic inflammatory disorder that primarily affects synovial joints. It is characterised by a symmetrical polyarthritis with articular cartilage loss, erosion of juxta-articular bone and in most patients IgM rheumatoid factor.
Diagnosis is based on;
- clinical signs of joint pain and swelling, shifting lameness, pyrexia of unknown origin, stiffness that persists after rising.
- Joint tap results! A minimum of THREE joints should be tapped when you are worried about IMPA
- Radiographs may help to diagnose the erosive forms in the later stages – look for signs of erosion, periosteal reaction, subluxation or deformity. Radiographic changes, (apart from effusion) are uncommon in the non-erosive most common forms of IMPA.
- Bloodwork – often neutrophilia with left shift, anemia, possibly thrombocytopenia, elevated CK if combined with myositis, elevated globulins, elevated liver enzymes
- Serology – rheumatoid factors (not specific for canine rheumatoid arthritis), anti-nuclear antibodies in dogs that have at least one other sign of SLE (another form of non-erosive immune-mediated polyarthritis).
Treatment of immune-mediated polyarthropathies:
Around 65% require immunosuppression. Occasionally treatment of underlying disease will resolve the IMPA. Immunosuppresion will lead to a complete cure in around 50% of dogs, continuous medications are required in around 18% of cases, relapses occur in around 31% of cases and are successfully treated around 50% of the time, 15% of patients are euthanased due to their disease.
Immunosuppression generally starts with 1mg/kg prednisolone BID, but other immunosuppressive drugs can be added to the protocol especially if prednisolone is poorly tolerated. Full treatment protocols are outside the scope of this module, however if the dog is prednisolone-responsive, this is also indicative of immune-mediated disease.
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