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Aspiration and injection in the knee

Our series on joint injections continues with a look at how to aspirate and inject into the knee


Effusions of the knee joint are commonly seen in general practice, and both aspiration and steroid injection may be confidently undertaken.



Presentation and diagnosis

An effusion is often detected and both knees should be inspected with the patient first standing and then lying down.

Palpate the patella for the following signs:

• With an effusion, the hollows alongside the kneecap disappear, and a suprapatellar bulge may appear that is painful on palpation. The ‘patella tap' may be less painful with smaller effusions, but the fluid can be stroked from one side of the patella to the other.

• Synovial thickening, which may be nodular, indicates synovitis.

• Bony prominences (osteophytes) may occur in osteoarthritis.

• Note the temperature, by placing the backs of the fingers on the patella. In infection and crystal synovitis, there will be warmth, tenderness and redness of the overlying skin.

• Patellar ‘grating' and crepitus both occur in osteoarthritis.

Examine the full active and passive movements of the knee joint and note any quadriceps wasting.


Aspiration and injection therapy

There are three indications for aspiration of an effusion of the knee joint:

1 diagnostic, in septic arthritis, haemarthrosis, traumatic effusion, rheumatoid arthritis, steoarthritis, gout and pseudogout. Send aspirate to the laboratory for analysis.

2 therapeutic, when a tense effusion causes pain and discomfort.

3 (a) steroid injection for an acute flare-up – for example of rheumatoid arthritis, osteoarthritis, psoriasis, Reiter's syndrome, synovitis and soft tissue lesions that occur in trauma.

(b) viscosupplementation with hyaluronic acid preparation in osteoarthritis.


In recent years there has been an increasing interest in viscosupplementation in the treatment of osteoarthritis of the knees.1 It is known that hyaluronic acid in synovial fluid is responsible for absorbing mechanical shocks, producing elastoviscous protection for soft tissues, shielding pain receptors and protecting cartilage against inflammatory mediators and degradative enzymes. Viscosupplementation is a means of injecting into the knee joint hyaluronic acid preparations of high molecular weight with optimal elastoviscous properties. These have the effect of restoring osteoarthritis synovial fluid to healthy levels and reducing pain and improving mobility.


Adverse effects of viscosupplementation

After injection, increased pain and swelling may occur in approximately 2% of patients and can last for a few days. Always send joint aspirate for microscopy and analysis to aid diagnosis and exclude infection. Joint aspirate in normal patients should appear clear and pale yellow in colour. Any turbidity in appearance of the joint aspirate should raise suspicion of infection, in which case steroids must not be injected until infection is excluded.


Analysis of the synovial fluid will usually confirm the diagnosis – see the table below. Inject with steroids no more than once every three months. This is most effective for acute flare-ups of arthropathy, especially those that affect a single joint as in psoriasis or rheumatoid arthritis exacerbations. Unlike steroids, a viscosupplementation course of three injections in three weeks may be repeated twice a year.

Analysis of synovial fluid

Technique of aspiration and injection

The patient lies on the couch with the knee slightly flexed – a pillow behind the knee is helpful. This allows relaxation of the quadriceps and patellar tendon. Carefully palpate the bony margin of the patella, which may be moved freely before the needle is inserted. Injection can be from either the lateral or the medial side of the patella and below the superior border of the patella.



• Prepare a 20ml (or 50ml) volume syringe and a sterile specimen container for diagnostic microscopy and culture. Use a 3.8cm needle.

• Insert the needle horizontally and in a slightly downward (or posterior) direction into the joint, in the gap between the back of the patella and the femoral condyles. When the needle is behind the patella, it is in the joint space. Just before reaching that stage, it should be possible to slide the patella over the femur freely from side to side, ensuring relaxation of the quadriceps.

• If a steroid injection is to follow the aspiration, leave a small amount of synovial fluid in the knee joint. This will allow the steroid to diffuse around the joint cavity more easily.

• It is kinder, but not strictly necessary, to infiltrate 1ml lidocaine 1% plain into the skin at the aspiration site.


• Use 1ml steroid (20mg triamcinolone acetonide, 40mg methylprednisolone or 20mg hydrocortisone acetate) in a 2ml volume syringe. Use a 3.8cm needle.

• Follow the same needle insertion procedure as for aspiration, above.

• Inject steroid into the knee no more than once every three months.

After aspiration or injection, the knee joint should be rested for 24 hours, supported by a firm Tubigrip or elastic crêpe bandage. The short-term benefit of intra-articular steroids in the treatment of osteoarthritis of the knee joint is well established and few side-effects have been reported. Longer-term beneficial effects have not been confirmed.

The response to hyaluronan appears to be more durable.2,3,4,5



1.       Balaz EA and Denliger JL. Viscosupplementation: a new concept in the treatment of osteoarthritis. J Rheumatol 1993;20:3–9

2.       Bellamy N et al. Intra-articular steroid for treatment of osteoarthritis of the knee. Cochrane Database Syst Rev 2006;CD005328

3.        Bagga H et al. Long-term effects of intra-articular hyaluronan on synovial fluid in osteoarthritis of the knee. J Rheumatol 2006;33:946-50

4.       Petrella R and Petrella M. A prospective, randomized, doubleblind, placebo controlled study to evaluate the efficacy of intra-articular hyaluronic acid for osteoarthritis of the knee. J Rheumatol 2006;33:951-6

5.        Gossec L and Dougados M. Do intra-articular therapies work and who will benefit most? Best Pract Res Clin Rheumatol 2006;20:131-44

This extract is taken from Joint and Soft Tissue Injection 5th Edition, by Dr Trevor Silver which is now available to purchase from Radcliffe Publishing with an exclusive 20% discount available until 30th September – simply quote PULSETS12.


-          Confidently diagnose conditions of the joints and soft tissue.

-          Understand the benefits and pitfalls of steroid injections.

-          New content – sports physiotherapy, elbow joint, iliotibial band syndrome and updates on greater trochanter pain syndrome.

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