When A Knee Cyst Won’t Aspirate

If you aspirate enough cysts around the knee, most become fairly predictable.

You enter the collection, apply negative pressure, and yellow synovial-type fluid fills the syringe.

Every so often, though, the needle enters the cyst and almost nothing comes back.

Not because the needle is misplaced.

Because the fluid is too thick.

I have encountered this phenomenon a number of times, and a recent case was a particularly good example.

The patient had a firm prominence along the posteromedial aspect of the left knee.

Ultrasound demonstrated a cystic structure measuring approximately 2 × 3 × 2 cm.

Its exact origin was not completely clear. Based on the location and sonographic appearance, the differential included an atypically positioned Baker cyst, a parameniscal cyst, or another ganglion-like cystic process.

After numbing the skin and subcutaneous tissues, I entered the collection under ultrasound guidance with an 18-gauge needle.

The initial aspirate was yellow, but it did not behave like ordinary synovial fluid. It was extraordinarily viscous and came into the syringe almost in globules.

Eventually, aspiration stopped.

The needle was still clearly within the collection.

The problem was not access.

The material was simply too thick to pass through the needle.

I injected approximately 7 cc of lidocaine into the cyst and allowed it to mix with the remaining contents. The material remained thick and globular, but it became easier to aspirate and I was able to complete the evacuation.

After decompression, I injected 20 mg of triamcinolone into the cyst. I also treated the knee intra-articularly with ropivacaine and 30 mg of triamcinolone.

The case raises a few useful questions:

Why does cyst fluid sometimes become so thick?

What is actually in that material?

And what should we do when it will not pass through the needle?

Why does the fluid become so viscous?

Normal synovial fluid is already a viscoelastic material.

Much of that behavior comes from hyaluronic acid, a large glycosaminoglycan whose long polymer chains contribute to the characteristic stringiness of normal synovial fluid.

But some cystic lesions contain much more concentrated mucinous material.

This is especially well recognized in ganglion cysts, which can contain highly viscous, gelatinous material rich in hyaluronic acid and other mucopolysaccharides.

Meniscal cysts appear to have similarly interesting biology.

A classic light and electron microscopic study found mucinous, acid-mucopolysaccharide-rich material within meniscal cysts and among meniscal collagen bundles. Some cystic spaces were lined by synovial-type cells, leading the investigators to conclude that at least some of the mucopolysaccharide accumulation resulted from active local secretion, rather than simply passive collection of joint fluid. Hernandez, 1976​

That means a parameniscal cyst may be more than a reservoir of synovial fluid that escaped through a meniscal tear.

It may develop its own concentrated, mucin-rich environment.

Chronic retention may further concentrate the contents over time, but I would not call these lesions simply “dehydrated synovial fluid.”

A better model may be:

fluid entry + macromolecular retention + local mucin production + time

That may explain why two apparently similar cysts can produce dramatically different aspirates.

The aspirate may be giving you diagnostic information

A Baker cyst is fundamentally a distended synovial bursa and typically contains synovial-type fluid.

Ganglion and parameniscal cysts can contain much more gelatinous, mucin-rich material.

The consistency of the aspirate certainly does not establish the diagnosis, but it can be another clue when interpreted alongside anatomy and imaging.

If a presumed simple synovial cyst produces material that behaves almost like gel, it is worth reconsidering exactly what type of cyst you are treating.

What do you do when an 18-gauge needle is not enough?

This is where the biology becomes practical.

When aspiration suddenly stops, the first thought is often technical:

Did the needle move?

Am I against the wall of the cyst?

Is the lesion septated?

Ultrasound lets us answer those questions.

If the needle remains clearly within the collection and nothing moves, the problem may simply be rheology.

The contents are too viscous to move efficiently through the lumen.

A larger-bore needle is the obvious solution when one is available.

In this case, 18 gauge was the largest needle immediately available, so I diluted the remaining contents with lidocaine.

The material became substantially easier to aspirate.

This is not a standardized technique, but dilution can make highly viscous cyst contents easier to remove. Saline may be another option.

Treat the cyst, the joint, or both?

After aspiration, I injected 20 mg of triamcinolone into the cyst and 30 mg intra-articularly.

There is precedent for direct cyst corticosteroid injection.

Ultrasound-guided aspiration followed by corticosteroid injection has produced reductions in pain and cyst size in patients with Baker cysts, although the improvements may be similar to those reported after intra-articular steroid injection alone. Di Sante et al., 2010​

Meniscal cysts have also been treated with aspiration followed by local anesthetic and corticosteroid injection. In one small series of 18 patients, 10 reported complete symptom resolution and another two had a sustained satisfactory response at approximately 10 months. Macmahon et al., 2007​

But aspiration and steroid do not necessarily address the mechanism that created the cyst.

If the process is being driven by intra-articular synovitis, treating the joint may matter.

If a parameniscal cyst is being fed through a meniscal tear, aspiration may empty the reservoir without closing the pathway.

That is why I think the more useful question is not simply:

Should the cyst be injected?

It is:

Where is the process actually being driven from?

Sometimes that may be the cyst.

Sometimes the joint.

Sometimes both.

What these cases teach us

When something unexpected happens during a procedure, it is easy to think only about technique.

But sometimes the procedure itself is giving us biological information.

An unusually viscous aspirate may reflect high concentrations of hyaluronic acid and other mucopolysaccharides, local mucin production, chronic retention, or some combination of these processes.

And once the biology changes, the procedure may need to change with it.

Not all cyst fluid is created equal.

If aspiration unexpectedly becomes difficult, confirm needle position, consider whether the contents themselves are the problem, and adjust the technique accordingly.

Sometimes the most useful information from an aspiration is not simply what comes out of the needle.

It is how it comes out.