Introduction

Analysis of synovial fluid from joints is a very useful and minimally invasive method of classifying joint pathology that is easily performed in practice. Evaluation of cell count and cytology can determine whether pathology is present in a joint and allows differentiation between degenerative conditions such as osteoarthritis and more inflammatory arthropathies such as immune-mediated arthritis and sepsis (See Table 1). Culture of joint fluid may be used to assist in diagnosing septic conditions.

Table 1. Synovial fluid findings for various arthropathies²

Arthrocentesis - Synovial Fluid Sampling

Arthrocentesis is most easily done with patients heavily sedated or under general anaesthesia. The joint to be sampled is clipped and aseptically prepared - sterile gloves should be worn if repeated palpation of the site is required. In all joints neurovascular structures should be avoided.

For sampling, needles of appropriate length should be chosen relating to the size of the patient being sampled and the anatomy of the required joint. The smallest gauge and shortest needle possible should be used. 20–23 Gauge needles are most appropriate for dogs. For cats and small dogs, 2.5 ml syringes are usually used whilst 5 ml syringes are used in larger dogs. Pre-cleaned microscope slides should be available to smear the sample directly after taking it and EDTA is useful to store any remaining fluid for fluid analysis. Delays in smear preparation may result in vacuolation and nuclear degeneration of large mononuclear cells and should be avoided.¹ Sterile pots and blood culture bottles are required for samples requiring microbial analysis.

Once the needle has been introduced through the skin and into the joint a small flash of synovial fluid is usually seen in the hub of the needle. At this stage gentle suction is applied using the syringe. In some patients, including those with inflammatory arthropathies and degenerative joint disease, relatively large volumes of fluid may be removed. In normal joints and in some diseases a needle hub is all that can be obtained. If suction is applied and no more fluid comes out release the suction before redirecting the needle. Don't be greedy! A needle hub is all that is required to make a cytology smear. Over-zealous suction may lead to aspirates of the synovial membrane or blood-contamination. To remove the needle from the joint following aspiration, release the suction and pull the needle straight back through the skin. There will occasionally be some blood at the puncture site but this usually doesn't persist.

Sites for Sampling

There are a number of different techniques described for joint aspiration. The most common methods used in our clinic are described below. The most easily sampled joints are the stifle and carpus. The hip joint is the most challenging.

Carpus - The antebrachiocarpal joint is the joint aspirated in carpal sampling. Flex the carpus to 90° and palpate the carpal joint space - usually identified as a small depression just distal to the radius. The needle is inserted medial to the common digital extensor tendon and cephalic vein, visible in the centre of the space on the dorsal aspect.

Elbow - The elbow is maintained in a neutral position. The lateral epicondyle of the humerus and the olecranon are palpated and the needle is introduced parallel to the long axis of the ulna, halfway between these two sites. The needle is angled distomedially and should slide between the lateral humeral condylar crest and the anconeal process.

Shoulder - With the shoulder in a neutral position palpate the acromion and insert the needle just distal to this, perpendicular to the skin surface. If the needle touches bone, gently move the needle proximally or distally until the joint capsule is penetrated.

Hock - The talocrural joint space is first identified by flexing and extending the hock joint. The joint is held in a neutral position and the needle is inserted on the dorsolateral aspect, medial to the lateral malleolus of the fibula. The needle is angled in a plantaromedial direction. An alternative approach is to insert the needle into the plantarolateral joint space by directing it parallel to the calcaneus, just medial to the lateral malleolus.

Stifle - The stifle is slightly flexed and the needle inserted midway between the tibial tuberosity and patella to the lateral side of the straight patellar ligament. The needle is angled at approximately 45° to the skin. Once the skin is penetrated the needle is flattened towards the skin and advanced proximally parallel to the straight patellar ligament towards the lateral parapatellar joint pouch.

Hip - The limb is slightly abducted and distal traction is applied to open the joint space. The needle is inserted just dorsal to the greater trochanter, perpendicular to the long axis of the limb. If the needle hits the dorsal acetabular rim it is angled distally until the joint is reached. The caudal aspect of the joint should be avoided to prevent sciatic nerve trauma.

Synovial Fluid Analysis

Normal synovial fluid is clear or colourless to slightly yellow in colour and is viscous so forms a string between your fingers if you stretch it out. Total cell counts of normal joint fluid are <2000x10⁹/litre and cells are mainly (90%) lymphocytes and large mononuclear cells - synoviocytes and macrophages. Neutrophil counts are <5–10% in normal synovial fluid.

Inflammation will reduce the hyaluronic acid content of the fluid leading to reduced viscosity. Blood within the fluid (haemarthrosis) is abnormal although on occasion blood contamination may occur during sampling. A streak of blood is usually visible when contamination occurs. Fractures, rupture of intra-articular ligaments, surgery and some haemostatic disorders may lead to haemarthrosis. Xanthochromia usually indicates previous and resolving joint haemorrhage.

In cases of degenerative joint disease there will be mild inflammatory change evident, characterised by an altered volume (usually increased) of synovial fluid with reduced viscosity and an increased white cell count with mononuclear cells dominating. Inflammatory arthropathy is characterized by significantly increased white cell counts and neutrophils become the more predominant cell type. In immune-mediated conditions more than one joint is usually affected and multiple joints are sampled to confirm the diagnosis.

Culture of joint fluid should be performed if infection with a bacterial agent is suspected. For aerobic culture place the fluid into blood culture medium and for anaerobic culture use a charcoal transport swab.³ Arthropathies caused by protozoal infection (Leishmania) or spirochaetes (Borrelia) usually affect multiple joints and can be difficult to differentiate from immune-mediated polyarthropathy without serological testing.

Complications

The most common complication of arthrocentesis is the inability to obtain a sample. Usually, with experience, the ability to perform a successful aspirate improves but in certain disease states such as severe osteoarthritis, a negative tap will be obtained.²

Blood contamination is fairly common and depending on the experience of the pathologist analysing the sample is unlikely to be a problem. A haematology sample submitted simultaneously will allow the differential white cell count to be compared between the two.

Sepsis is a risk of the procedure although it is extremely unlikely. Providing aseptic technique is used, the procedure is classified in the National Research Council guidelines as clean and no antibiotic cover is required.

Iatrogenic cartilage damage during needle placement is common and can be seen on arthroscopic evaluation of damaged joints. Careful needle placement and gentle sampling will minimise this risk although the damage is likely to be of no clinical significance.

References

1.  Fisher D. Musculoskeletal system. In: Raskin RE, Meyer DJ, ed. Atlas of Canine and Feline Cytology. Philadelphia, PA: WB Saunders; 2001;313–324.

2.  Clements D. Arthrocentesis and synovial fluid analysis in dogs and cats. In: Practice. 2006;28:256–262

3.  Montgomery RD, Long IR, Milton JL, DiPinto MN, Hunt J. Comparison of aerobic culturette, synovial membrane biopsy and blood culture medium in detection of canine bacterial arthritis. 1989;18:300–303.