Intra-articular injection of long acting corticosteroid is a corner stone in rheumatological treatment. The injected intra-articular corticoid is more effective when correctly placed.1 ,2 Injection of radiographic contrast material has shown that fewer than half of the injections are correctly placed in the joint space after blind injection.1

Generally, the clinical application of ultrasonographic examinations can be enhanced by contrast agents.3 The most commonly used technique is creation of microbubble contrast agents. Such agents, applied to the bloodstream, have been used for hepatic, nephrologic, cardiologic, and transcranial examinations.4 Obviously, the risk of air embolism depends on the anatomical site of the injected air contrast. Transient ischaemic attacks are described after echocardiography with air contrast5 and in animal models haemodynamic effects during venous air infusion can be measured.6 Intra-articular injection of air and subsequent lateral and posterior radiographs have shown that this technique can enhance the precision of the procedure.7 The disadvantage of this method is that the result can first be seen after the injection, and that a correction can only be made with a new injection. In the joint space the air is separated from the vascular system and when only small amounts of sterile air are used the risk of venous air embolism is negligible. Air is a very effective contrast medium in ultrasonography. Air sonography has been used for the diagnosis of meniscus lesions in knee joints8 and for rotator cuff lesions in the shoulder9.

We expand the applicability of this method to all joints, not only for diagnosis, but also for the correct placement of the needle before injection of medicine (steroid, osmium acid, viscosupplementation). The sterile air that is contained in the capped vial with lidocain or steroid is used as contrast medium. The needle is guided into the joint space of the distended capsule by ultrasonography.

When the steroid and lidocain are mixed in the syringe a small volume of air will be in the needle itself (∼0.05 ml). The air in the needle is clearly seen when the injection is started and will secure the correct placement of the needle. With this technique, it is not necessary to use two separate syringes and the inclination of the syringe will not cause the air to move from the needle to the bottom of the syringe.

If the knee is injected, injection directly into the recess of the knee is recommended, which will make the small volume of air momentarily visible.

Figure 1 illustrates the ultrasonography of a metatarsophalangeal joint in a patient before and after injected air. The intra-articular air is clearly seen. We have made over 1000 ultrasonography guided intra-articular injections without any complications. This method is easy, inexpensive, without risk and radiation, and should be used routinely in rheumatology. Chemical synovectomy of the knee, especially, should always be guided by ultrasonography, and with this technique smaller joins can also be considered for chemical synovectomy.

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Figure 1

(A) Ultrasonography of a metatarsophalangeal (MTP) joint before injection. The distended capsule is seen (black arrows). (B) Ultrasonography of MTP joint after injection of sterile air (white arrows) and 2 ml steroid. Only the volume of air that is in the needle itself is injected (∼0.05 ml).