As you know I love ultrasound. One of the simplest scans we do is the identification of knee effusions in ED. The anatomy is simple and the interpretation usually very straightforward. However, when it comes to draining fluid for diagnosis or pain relief there are a lot of opinions about how it should be done. There are basically 3 schools of thought:
Old school – clinically diagnose effusions and use a landmark approach to tapping the fluid. [this is how i was taught back in my residency]
Use ultrasound to look for and confirm an effusion… plan your approach and then use a landmark ( aka blind / no ultrasound) technique to insert your needle. [this is what I had been doing for a long time]
Use ultrasound to diagnose, localise and then guide the arthrocentesis… [this is what I do now.]
So why have I changed my practice? Well for a few reasons, some based on experience, others on evidence.
The big problems with blind arthrocentesis include:
A. Missing the fluid, or having to dig around to find the pocket and causing a lot of pain. Ultrasound makes this easy – you can see the whole needle with an inline approach and know exactly where the tip is at all times.
B: Pain – when inserting a needle we often miss the “numb spot” or insert the needle too far and end up hitting the periosteum or synovium resulting in pain. Using US and following the needle under direct vision means that you can keep the pointy bevel within the fluid and avoid touching the sensitive surfaces and tissues unnecessarily.
C: Not knowing when you have drained all the fluid. If the fluid stops coming it may be because it has been completely drained or more likely – your needle tip has migrated out of the effusion and you are sucking on tissue. Without scanning – you don’t know which it is.
There is actually a modest amount of data to support my anecdotal experience for all of these outcomes being improved by ultrasound guidance. A series of small studies have been summarised in this metanalysis by Yu et al in 2015:
To learn how to do this simple procedure – check out this neat paper from ACEP Now written by Arun Nagdev and friends.
So – how do I do an US-guided knee tap? I prefer the lateral approach to the suprapatellar space. It is usually easier to set up your machine on the contralateral side of the bed and insert the needle into the near side of the knee.
It is useful to have an assistant to help with a few parts of this procedure. Prepare all your kit before putting on sterile gloves – once you start you don’t want to have to move or take your eyes off the target.
This is how I do it
Setup the sterile field with all the kit you need
position the patient and machine
prep the skin with antiseptic – ideally you need 3 minutes to allow the alcohol to evaporate and sterilise the skin. So do this before you setup the rest
draw up local anaesthetic (lignocaine plus adrenaline)
place a sterile probe cover with your assistant
Scan the knee again (using sterile gel) in long then transverse planes. You want a view with the effusion in the centre and your entry point just posterior to the quadriceps tendon on the screen. This is the crucial step. Once you have the view, lock your probe in place.
plan your trajectory. Ideally the entry point will be posterior to the probe. Usually 1-2 cm depending upon the depth of the effusion. You want the needle to stay parallel to the floor and the probe.
Inject a bleb of local at the Insertion point. You can then infiltrate a little deeper watching the screen to see the needle advance.
watch the spot. A common error is to inject local and then miss that spot when the bigger needle is inserted. Having it all lined up and not moving the probe fixes this error.
finally insert the large needle (16-18 Ga) on a 30-50 ml syringe
watch the needle tip all the way through into the effusion.. Avoid touching the joint lining or bone
Aspirate gently. It can be helpful to have your assistant gently press on the medial knee to fill the lateral compartment and aid drainage.
keep going to try and empty the effusion without removing the bevel of the needle from the space
I am a GP working in Broome, NW of Western Australia. I work as a hospital DMO (District Med Officer) doing Emergency, Anaesthestics, some Obstetrics and a lot of miscellaneous primary care. Also on the web as @broomedocs | + Casey Parker | Contact