Swelling is something you probably routinely look for during your knee assessments…

But what does it actually tell you?

Clearly it tells you that your patients knee is not very happy and hence swollen…

But as with any great assessment as a physio you want to know why.

Because if you know why you can start to plan how you can help fix this issue…

Rather than just treating your patients symptoms i.e. the swelling, without knowing the diagnosis and cause of this issue.

The patella sweep test (also known as the bulge test, effusion wave test, or hydro’s test) is a simple way to assess for the presence of a knee effusion…

And is my choice of test to assess for the presence of a knee effusion.

As simple as this test might be, it can give you super important information about your patient’s injury and help you make the right injury diagnosis.

A positive patella sweep test will tell you that there is fluid present within your patient’s knee joint (intra-articular).

If the knee is unhappy and irritated, the knee makes more fluid in the joint.

This is to keep to the joint lubricated and functioning in the best way it can.

Clearly this can have the opposite effect if there is too much fluid in the joint as a swollen knee can cause pain, a loss of ROM and strength and impede function.

Whilst a positive sweep test will not give you a diagnosis straight away, it will tell you that there is excess fluid within the knee joint and in turn…

That something inside the knee is not happy.

For your patient to have a knee effusion and swelling within the joint, indicated by a positive sweep test, then something within the knee itself must be causing this issue.

Sounds obvious hey, but once you get your head around this, it makes your knee differential diagnosis much easier. 

For swelling to be inside the knee the problem must be within the knee capsule.

And if this is so, there can only be a few potential structures that could be causing this.

Structures inside the knee include the cruciate ligaments (ACL/PCL), the bony surfaces (tibia and femur) or the meniscus.

These structures are what we call intra-capsular i.e. they are within the joint capsule. 

Structures like the patella or quadricep tendons, ligaments like the MCL or LCL are extra-capsular and sit outside the joint capsule… 

And are unable to cause an internal knee effusion.

The only exception to this is when there is an injury to the joint capsule itself which allows fluid from an injury occurring outside of the knee joint (capsule) to pass into the joint itself. 

Capsular injuries are less common and are usually associated with other major knee trauma.

So whilst a positive sweep test will not give you a concrete diagnosis…

It greatly helps you focus on a smaller number of structures for the remainder of your objective assessment to determine what could be injured.

Taking into account the story of events, paying particular attention to their mechanism of injury…

You can then use other special tests like your Lachman’s Test (ACL), Posterior Draw Test (PCL), McMurrays Test (Meniscal) or joint ROM testing to determine whether or not you have a bony surface injury and identify the actual cause of injury.

Always remember…

The presence of a knee effusion is NOT normal. 

If your patient has an effusion then something within the knee joint is not happy, causing the synovial capsule to secrete more fluid.

It is then up to you to identify what specific structure is aggravating the knee, so you can manage your patient in the best possible way.

Hope this helps.

Andy Barker

The New Grad Physio Mentor

PS. If becoming more competent and confident at your knee assessments is something you would like to learn more about…

Then get in touch!

Learn all the knee special tests that you need to make the right patient diagnosis and ensure that you do not miss anything serious.

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Plus how to treat and rehab knee injuries from ACL’s to MCL’s, patella tendon issues to adolescent injuries like Osgood-Schlatters and Sinding Larson syndrome.

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