Overview of Knee Arthrocentesis
Knee arthrocentesis is a simple operation where a doctor removes fluid from your knee joint. This procedure can be used both to find out what’s causing a problem in your knee (diagnostic) and to help relieve any discomfort or swelling by removing extra fluid (therapeutic). When done correctly, it’s generally safe, but this procedure can always carry some risk of harm to blood vessels, nerves, and tendons. It should only be performed by a professional who has a deep understanding of the knee’s structure and is properly trained and qualified for the operation.
Your knee is the largest joint in your body that contains a lubricating fluid (synovial fluid), and it can be accessed from either the inside (medial) or outside (lateral) part and either above or below the midpoint of the kneecap (patella). You can be lying down (supine) or sitting during the operation, but it is easier for the doctor to remove the fluid if you’re lying down. To make sure the procedure is as safe as possible, your knee will be extended with just a slight (20°) bend while being guided by an ultrasound. This method has been found to increase the amount of fluid that can be removed, precision, and lessen pain levels during the procedure.
Anatomy and Physiology of Knee Arthrocentesis
If your doctor needs to get a sample of the fluid in your knee, this is done through a process called knee arthrocentesis. Here’s what you typically need to expect during the procedure:
- You will usually be either lying down (supine position) or sitting.
- The doctor can approach and puncture your knee from the inside (medially) or outside (laterally) of your knee cap (patella), or above (superiorly) or below (inferiorly) your knee cap.
- You should try to straighten out your affected knee as much as possible or relax the muscles in the front of your thigh (quadriceps) at a slight bent (20-degree flexion).
- Your doctor will place their thumb on your knee cap and move it aside as they insert the needle.
- If they’re coming in from above the knee cap, they should aim 1 cm to the side (lateral) and 1 cm above the top of the knee cap.
- Once they’ve found the middle point of your knee cap, they can insert the needle either 1 cm to the inside or outside.
- Your doctor then aims the needle to the space behind and above your knee cap and gently presses both sides of your knee to help get the fluid out.
This procedure helps diagnose different diseases and conditions by examining the fluid in your knee joint. Your doctor will guide you through the entire process, so don’t hesitate to ask them any questions you may have.
Why do People Need Knee Arthrocentesis
This procedure can be performed in a doctor’s office, an emergency room, or a surgical space. It is typically done for one of two main reasons:
1. To relieve pain (therapeutic): This is often done if you have a large and painful swelling in your joint. After the fluid has been drained for relief, the doctor will often inject medications directly into the joint to further reduce pain. These may include steroids, a local numbing medication called lidocaine, or a treatment called platelet-rich plasma (PRP).
2. For diagnosis: If you suddenly develop arthritis and the cause is unclear, this procedure can help doctors understand what’s going on. It’s also used to distinguish between septic arthritis (an infection in the joint) and an inflammatory condition causing bloody joint swelling.
When a Person Should Avoid Knee Arthrocentesis
There are no absolute reasons why someone can’t have this procedure, but there are a few situations where it might not be the best option:
Having an infection on the skin above the area where the procedure will be done. This can potentially allow bacteria to get into the joint which would make the infection worse.
Having a condition that causes the blood to not clot properly or having a bleeding disorder. These conditions can make the procedure more risky.
If the patient has a joint prosthesis, it would be preferred to have an orthopedic surgeon (a doctor who specializes in muscles, bones, and joints) perform the procedure.
If the patient has a recent broken bone, this might make the procedure more difficult or risky.
Having a bone infection near where the procedure will be done can increase the risk of complications.
If the patient is agitated and cannot cooperate during the procedure, it can make it more difficult and increase the risk of complications.
Equipment used for Knee Arthrocentesis
Before any medical procedure, it’s important that all necessary supplies are at hand.
In this case, your doctor will have:
* Skin cleaning liquid (like betadine or chlorhexidine) to disinfect the area before the procedure
* A skin marking pen to mark the specific area for the procedure
* Sterile gloves to ensure cleanliness during the procedure
* Sterile gauze to clean and cover the area afterward
* 1% lidocaine, a common local anesthetic that numbs a specific area of the body, which can be mixed with or without epinephrine, a medication that constricts blood vessels and reduces bleeding
* 10 cc syringe to inject the lidocaine
* An 18 g needle for aspiration, which is used to remove fluid or air
* A smaller 27 g needle to inject the anesthetic
* Specimen tubes for lab analysis. This could include counting cells, identifying bacteria (gram stain), finding out which antibiotics can fight the bacteria (culture and sensitivity), and looking at crystals to diagnose conditions like gout and pseudogout
Furthermore, an ultrasound machine might be used. This device uses sound waves to create images of the inside of your body. It can help to identify the precise body part to be treated. Research has shown that using ultrasound can make a procedure more accurate, enables more fluid to be removed, and can make patients feel more comfortable two weeks after the procedure.[3][4]
Who is needed to perform Knee Arthrocentesis?
A skilled doctor can carry out the procedure on their own. However, if the patient is feeling anxious, or if there are exceptional circumstances, it is recommended to have help with things like ensuring the knee is in the right position, collecting fluid, giving medication, or helping the patient feel relaxed during the procedure.
Preparing for Knee Arthrocentesis
Having fluid drained from your knee might make some people nervous, even though it’s a pretty straightforward procedure. If a patient is particularly anxious, in pain, or finds it difficult to stay still during the procedure, they might need help to stay calm. They could be given medication to help them relax, either in pill form or through an IV. Numbing the area with a local anesthetic is strongly recommended.
Most doctors use a simple, no-additive form of a numbing medicine called lidocaine. But there’s also no problem with using a stronger lidocaine solution, one mixed with a medicine called epinephrine, or another anesthetic called bupivacaine, if there’s a need for them. However, it’s important that the numbing medicine isn’t injected too deep. If it goes into the joint space, it can change the results of tests done on the drained fluid.
How is Knee Arthrocentesis performed
Before performing any medical procedure, your doctor will first ask for your consent. It is important to ensure they have the correct patient and the correct joint. To start, you’ll be asked to lay down in a comfortable position with the knee that’s causing issues being either fully straightened or slightly bent. A rolled towel will be placed under your knee. This position helps the muscles in your thigh to relax, making the procedure easier.
There are two main positions doctors use for this procedure: sitting or lying flat on your back. Various studies have shown that doing this procedure with the patient lying flat with a straight knee tends to have more successful results compared to when the patient is sitting. If the doctor is using a technique that involves drawing fluid from the top outer portion of your knee, it works best when your knee is straight, not bent.
However, the success rate is about the same when you’re lying down with your knee bent if the doctor is using a technique that puts pressure on the top part of your knee.
Here are the steps to the procedure:
- Your doctor will first locate your kneecap and mark it.
- They will then clean the area and cover it.
- After deciding on the best approach, they will numb both the skin and deeper tissues in the area they plan to draw fluid from.
Next, the doctor will insert a needle (attached to a large syringe) about a half-inch away from your kneecap, and aim it towards the area where your thigh bone meets your shin bone. There are different ways this can be done, and it will depend on your specific situation. If you’re sitting upright, they will insert the needle slightly below the kneecap itself. During this, they will gently pull back on the syringe and stop once they start drawing out fluid. They will aim to remove as much fluid as possible, and could potentially use a technique called “milking,” or pressing on the joint, to help get more fluid out. They will then fill tubes with this fluid while keeping everything sterile.
Now, if they’re not planning to inject anything else into the joint, they will remove the needle and syringe together. However, if they need to inject additional medication, they will replace the needle head while keeping things clean. Finally, they will dress the area based on any surrounding injury.
Possible Complications of Knee Arthrocentesis
Complications can sometimes occur due to physical injury during medical procedures, such as damage to surrounding structures, pain, infection, and the return of fluid buildup. If the needle isn’t correctly inserted or if the lining of your joints is too thick, the extraction of fluid may fail. This might require the doctor to move the needle or change the way they’re doing the procedure. If the needle’s direction needs to be altered during the procedure, it’s first pulled back to the surface of the skin. Changing the angle too deep in the soft tissue or knee joint might cause the needle to rip through soft tissue and other structures near it, causing tissue to tear and bleeding.
Bleeding into the joint, known as hemarthrosis, can happen if a large needle harms a blood vessel after multiple attempts. Usually, hemarthrosis will cause joint pain, stiffness, and swelling a few hours after the procedure. However, in most cases, the bleeding resolves itself in a few weeks. A study showed that less than 1% of patients taking a specific blood-thinner, Dabigatran, experienced hemarthrosis. And because this risk is so low, it’s generally not necessary to stop blood thinning medications before the procedure.
An uncommon complication where there’s abnormal connection between an artery and the joint, is called an arterioarticular fistula. If a patient has a clotting disorder, they might need to have it corrected before the procedure, which may involve consultation with a blood specialist (hematologist). In some circumstances, such as when doctors suspect a serious, infectious arthritis, the procedure might still need to be done regardless of whether the patient is taking blood thinning medications.
If a procedure to remove fluid from a joint, known as an arthrocentesis, is done through an area of infected skin to check for an infected joint, antibiotics are usually given. However, if there are signs of an infected joint, the risks associated with not treating it are higher than the risks of removing joint fluid through overlying infected tissue. However, even if an arthrocentesis is done through clean skin, there’s still a small risk of introducing infection into the joint, though this is quite rare, occurring in about 1 out of every 2034 to 3500 procedures.
What Else Should I Know About Knee Arthrocentesis?
Knee arthrocentesis, also known as knee aspiration, is a procedure done to figure out what’s causing knee problems, lessen discomfort, and drain extra fluid from the knee. It’s important for the healthcare provider to know knee anatomy well to avoid any risks. Using the correct process can lower the chance of problems happening.
After knee aspiration, the fluid removed is usually tested for crystals. These tests help determine the cause of your knee pain, whether it be gout (identified by a certain type of crystal), pseudogout (another type of crystal), or an infection (which usually involves white blood cells). Just because there are crystals in the fluid doesn’t mean that you can’t also have an infection in the joint.
If an infection is causing the trouble in your knee, the analysis of the fluid can show different results. A healthy knee joint should have very few to no white blood cells. As the white blood cell count increases, it gets likelier and likelier that there’s an infection. There are certain cutoffs of white blood cell counts used to detect an infection. However, not all infections cause a high white blood cell count.
The procedure also involves staining the fluid to give more details for treatment. This step may not catch every infection, especially in its early stages or if the infection is from certain types of bacteria or fungi. In cases where the fluid test isn’t clear, a biopsy or sampling of the tissue within your knee joint might be recommended to check for conditions related to your body’s immune system and to better identify unusual bacteria or fungi.
