Got a lump behind the knee? Learn what a Baker’s cyst indicates

A Baker’s Cyst is a pocket of fluid that appears as a bulge or lump behind the knee.

It causes an ache behind the knee caused by the stretching of the joint capsule, which gets worse when it’s squeezed (eg. crouching) or stretched (eg. forcing knee straight).

It usually forms a small lump behind the knee, which is not visible, but the lump behind the knee can grow larger over time and become more obvious.

Any Baker’s Cyst will show up on MRI and ultrasound scans but the size on imaging depends on how much fluid is present in the knee that day, so it can change from week to week.

X-rays don’t show fluid or soft tissue but can occasionally show a dark shadow in the space behind the knee caused by a Baker’s Cyst.

It’s not critical to confirm a Baker’s cyst via ultrasound or MRI however it can be helpful for long term monitoring.

Size of an average Baker’s Cyst

The average size of a Baker’s cyst is influenced by a number of factors.

It will vary depending on the amount of fluid present in the knee and how long the knee has been swollen.

Think of the lump behind the knee as an over-stretched elastic band.

The harder you pull on the band to stretch it, the more distended it becomes.

And the more often you stretch it, or keep it stretched, the less likely it is to rebound to its normal size and shape.

As the lump behind the knee is just the end result of stretching, the average size of a Baker’s Cyst will increase over time.

Average size of a Baker’s Cyst in the general population

If we look at the average size of a Baker’s Cyst in the general population, they tend to be quite prevalent when they’re smaller as they go unnoticed initially.

As they grow, people are more likely to turn to anti-inflammatory meds or a knee brace for Baker’s Cysts and actively try to reduce the size.

Aside from long term distention of the knee joint capsule, the size will also fluctuate from day to day depending on how much fluid and pressure there was in the knee in the 2-3 days prior.

Activities that make a Baker’s Cyst worse

The Baker’s Cyst will worsen after any aggravating activity that causes more fluid to accumulate.

These types of activities can include compressive forces (such as crouching down or deep squats), twisting forces (such as field sports or dancing) and unstable forces (such as soft sand walking).

After the swelling peaks on day 2-3 after the activity, the fluid begins to be absorbed and the lump behind the knee begins to reduce on size.

How long does it take for a Baker’s Cyst to disappear?

The speed of the reduction in size of the Baker’s Cyst depends on some key factors.

The fluid is absorbed back into the cartilage and the ends of the bone.

Consistent gentle pressure on the fluid will improve the rate of transfer across the cartilage.

Weight-bearing activity, such as walking, will also help it absorb as it squeezes the cartilage on and off, like a sponge drawing up water.

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But the rate of reduction in the size of the lump behind the knee will slow down if new fluid is constantly being added to the knee.

So for the fastest reduction in the size of a Baker’s Cyst, you need to combine meds, a brace, gentle activity and avoiding aggravating forces on the knee.

It can feel like a bit of a tightrope walk at times but consistency is the key to success.

What causes a Baker’s Cyst to form a lump behind the knee?

The cause of the Baker’s cyst is a separate injury in the knee which is causing fluid to accumulate inside the joint.

The lump behind the knee is always a sign of another pathology or injury, as the Baker’s Cyst can never be a primary pathology (ie. it can’t be the first pathology in the process).

These injuries can include osteoarthritis, meniscal tears or cartilage defects.

So it makes sense that the Baker’s cyst will last as long as the underlying injury is still producing fluid.

That’s why the focus of management of a Baker’s Cyst needs to be on the primary cause of the fluid and not just on reducing the size of the lump behind the knee.

(It’s why draining a Baker’s Cyst with a big needle is very rarely performed these days – it’s like deflating a helium balloon while leaving the helium tank still flowing…)

It’s worth noting that anti-inflammatory medication will stop or reduce further fluid accumulation but won’t get rid of the existing fluid already in the knee.

So meds can be helpful but they won’t get rid of the lump behind the knee straight away.

What does a Baker’s Cyst feel like?

A Baker’s Cyst causes localised pain behind the knee, usually felt as a diffuse ache.

It also causes a range of motion restriction with bending and straightening movements.

It can feel like a pulled muscle or muscle knot behind the knee or an uncomfortable obstruction when bending the knee.

In addition to the symptoms associated with the lump behind the knee, you’ll also feel the symptoms associated with the primary pathology (the injury that’s causing the fluid to accumulate).

What does a ruptured Baker’s Cyst feel like?

A Baker’s cyst can rupture or burst, just like a popped balloon.

It occurs when an overinflated Baker’s Cyst is suddenly put under too much pressure, either when the knee bends and squashes it or the muscle tightens and squeezes on it.

The analogy of a bursting balloon couldn’t be more appropriate here to describe the mechanism behind it.

Unfortunately that’s where the comparison stops though – there’s no “pop” sound and it often doesn’t even feel like a sudden decompression.

You also don’t notice an immediate change in the lump behind the knee as the “balloon” is still there, it’s just a deflated sack now.

The ruptured Baker’s Cyst causes a sudden rush of fluid into the calf along with a fairly rapid onset of pain and tightness in the calf, feeling like a torn calf muscle.

A ruptured Baker’s Cyst is often misdiagnosed clinically as a calf tear or DVT (deep vein thrombosis – a serious blood clot in the larger deep veins), based on the painful swollen calf and mystery cause (ie. there’s no clear reason why you tore a calf muscle).

Due to the potentially dire consequences of a DVT, a doctor will often order an ultrasound scan to exclude any serious or vascular pathology.

If you have a thorough sonographer (person who performs the ultrasound scan), they may venture up to scan the lump behind the knee and discover the source of the fluid.

Despite all the drama above, a ruptured cyst is usually not problematic and heals without intervention.

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The excess fluid in the calf is safely reabsorbed into the system, although this can take several weeks.

Depending on the amount of fluid that has accumulated in the calf, you may experience symptoms like pins and needles or numbness in the foot.


How you can treat your own Baker’s Cyst

When it comes to treating your own Baker’s Cyst, you need to focus on more than just the lump behind the knee.

As mentioned above, you must find and treat the cause of the fluid accumulation to successful “fix” a Baker’s Cyst.

Once you’ve taken away the cause and the excess fluid has resolved, the lump behind the knee will usually shrink and disappear without a trace, although you will be more prone to redeveloping the lump if the fluid returns.

So let’s assume you’ve had the cause of the fluid diagnosed and you’ve started treatment, which will typically involve some strength training and avoiding activities that will trigger the problem.

Managing the lump behind the knee involve two approaches:

  1. Reduce the amount of new fluid being added to the knee
  2. Speed up the rate of absorption of the existing fluid in the knee

Step 1 can be achieved through three main approaches.

Exercise plans will build strength and control for the knee to improve the movement quality and reduce the amount of abnormal forces on the knee.

The plan will include avoiding specific types of exercises that involve compressive and/or twisting positions, such as crouching or pivoting.

The third approach for reducing new fluid is to use anti-inflammatory medication to restrict the body’s ability to produce fluid if you happen to overload or aggravate the primary (fluid-producing) pathology.

Once you’ve stopped new fluid from “re-inflating” the lump behind the knee, you can move on to reducing the fluid that’s already there.

For step 2, there are two approaches that can be combined for best success.

Use a compressive neoprene or elastic knee brace with adjustable pressure to speed up resolution of existing swelling.

The knee braces with velcro straps across the front often work best as you can adjust the pressure over time.

Otherwise, as the lump behind the knee begins to resolve, the brace becomes loose and ineffective.

The second and most important approach is gentle exercise, like walking.

Walking produces gentle compression of the knee cartilage without excessive compression of existing fluid.

Think of the cartilage like a kitchen sponge – gently squeezing it on and off will absorb more fluid into it.

Once the cartilage has absorbed the fluid, that fluid will be absorbed back into the system and away from the knee.

But more is not better when it comes to walking – a 20 minute walk is good but a 2 hour walk might cause aggravation and add new fluid to the knee.

The preference is to conduct regular short walks throughout the day on firm flat ground.

Need to know more about how to fix your Baker’s Cyst?

See our self-management plan here.


Myths about Baker’s Cysts

A compressive knee brace is only helpful when the level of fluid bulges past the bony edges of the joint.

Otherwise it only exerts pressure on the bony bumps around the knee, not on the swelling.

Draining a Baker’s cyst is not painful but it’s usually ineffective as the source of fluid in the knee remains and the fluid pressure rebuilds quickly.

Stretching the muscles around the knee is counterproductive.

Although the knee feels tight, stretching will increase the pressure inside the knee and aggravate the cyst.

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