The ankle region comprises three joints:

Involvement of any of these three joints will cause ‘ankle’ pain. The tibiotalar joint is a true hinge joint whose movement is almost entirely limited to plantar flexion (downwards) and dorsiflexion (upwards). The fibula articulates on the lateral side of the tibia but does not bear weight. The subtalar joint allows the foot to be inverted or everted. The midtarsal joints, including the talonavicular joint, allow forefoot supination and pronation.

The articular capsule is lax on the anterior aspect but tightly bound on both sides by strong medial and lateral ligaments. All the tendons crossing the ankle joint lie superficial to the articular capsule and are partly enclosed in synovial sheaths (Fig. 7.1B). Disorders of the peroneal tendons (inferoposterior to the lateral malleolus), the tibialis posterior tendon (inferoposterior to the medial malleolus), the tendons that run anterior to the ankle joint (such as tibialis anterior, extensor hallucis longus and extensor digitorum longus) or the Achilles tendon also cause ‘ankle’ pain. Which structure is responsible for the patient’s pain can often be determined by testing specific movements and the patient’s ability to resist movement in specific directions. The tibialis posterior muscle helps to maintain the ankle in a position of inversion, and the peroneal muscles help to maintain eversion.

The major neurovascular bundles of the foot and ankle are the dorsalis pedis artery and anterior tibial (deep peroneal) nerve, which run longitudinally anterior to the ankle and over the dorsum of foot, and the posterior tibial artery and nerve, which course behind the medial malleolus.

An anterior approach to the ankle is usually used. The joint line should be carefully palpated with the help of passive movement of the joint (if the patient actively moves the ankle, then tendon movement obscures the joint line). The point at which the needle enters the skin is approximately 1 cm proximal to the tip of the medial malleolus. The needle is directed posteriorly.

Joint aspiration is usually performed as a clean (not sterile) procedure. Thus, palpation of the joint line and marking the site with an indentation of the skin needs to be performed prior to cleaning the skin. The solution used to clean the skin (e.g. iodine) should be allowed to dry. Gloves are worn to protect the person performing the aspiration against blood-borne viral infection. Unless the person performing the aspiration is very experienced and can confidently place the needle in the joint space at the first attempt, local anaesthetic should be used to anaesthetize the skin and periarticular structures. The needle route should avoid the major tendons anterior to the ankle and the dorsalis pedis artery. If joint fluid is not obtained on the first attempt, then the needle should be slightly repositioned.

The synovial fluid obtained should be examined promptly; including macroscopic appearance, cell count, Gram stain, culture and microscopic examination for crystals. In Chapter 1 we described the characteristics of synovial fluid in normal and abnormal joints. Here, we will consider only those aspects of synovial fluid analysis that apply to crystal identification. Examination for crystals requires only a tiny amount of synovial fluid (even an apparently ‘dry’ tap may yield enough) but the examination should be performed immediately. Polarized microscopy allows the identification of urate crystals (strongly negatively birefringent needle-shaped crystals) and CPPD crystals (weakly positively birefringent rod-shaped or rhomboidal crystals) as shown in Fig. 7.2. The technique is not difficult but the experience of the observer and the adequacy of the microscope (e.g. rotating stage) can make a major difference to accuracy. CPPD crystals are often missed, and thus a repeat examination of a fresh specimen may be appropriate if the diagnosis appears likely, but no crystals are seen initially.

Other investigations can provide useful supplementary information but cannot replace synovial fluid examination. X-rays can detect or help to exclude pre-existing joint or bone disorders, chondrocalcinosis (CPPD deposition in cartilage) and bone trauma. It is sensible to assess laboratory parameters of inflammation, such as white cell count and ESR, for comparison with future results but they assist little in the differentiation of septic from gouty arthritis. The serum uric acid level is of little help in the setting of possible acute gout. A normal level does not exclude gout and an elevated level does not confirm gout. However, the uric acid level is very important in the long-term management of gout.

1. chronic hyperuricaemia

2. urate crystal formation

3. interaction between the inflammatory system and the urate crystals (Fig. 7.3).