Sample Guide Index

Sampling Guide 3

SYNOVIAL FLUID - SMALL ANIMALS AND HORSES.

Examination of synovial fluid is a powerful diagnostic tool in the investigation of arthropathies in dogs, cats and horses. The quality of the information obtained depends upon sampling technique and the preparation and submission of samples.

The level of restraint must be sufficient to allow controlled manipulation of the affected joint. This will range from physical restraint to general anaesthesia depending on the individual animal and the joint to be sampled. Minimal restraint will usually require sedation and local anaesthesia. Routine aseptic technique must be used. Manipulation of the joint, particularly where there is joint distension, may allow identification of the joint space.

In approaching a joint, the flexor surface should be avoided. A 2ml or 5ml syringe is used with a 20g to 22g, 5/8" to 1.5" hypodermic needle for most joints. For small joints (e.g. cat carpus) a 25g disposable spinal needle may be used. The needle is advanced gently through the joint capsule, the syringe attached and negative pressure applied. If bone is encountered then the needle should be withdrawn slightly and redirected. The pressure is released before removal from the joint. The following information is taken from Parry, 1989 and Houlton, 1994. The latter reference includes excellent diagrams illustrating the approaches for athrocentesis of specific joints.

Hip joint
Lateral approach - 1.5" 22g needle. In lateral recumbancy with the affected limb uppermost, slightly abduct and rotate the femur outwards. Insert the needle just cranial to the greater trochanter and advance medio-ventrally towards the joint. Avoid the circumflex femoral artery (crossing the joint capsule) and the sciatic nerve (caudal to the joint). After penetration of the joint capsule release the traction to lock the needle in place.

Stifle joint
1" 22g needle. In lateral recumbancy with the affected limb uppermost, flex the stifle and apply digital pressure to the medial side of the straight patellar ligament. Insert the needle on the opposite side of the straight patellar ligament midway between the patella and the tibial tuberosity and direct it obliquely and distally toward the intercondylar space of the tibia.

Hock joint
1" 22g needle. In lateral recumbancy with the affected limb uppermost, flex the hock joint and palpate the space between the distal fibula and tibia. Insert the needle in the cranio-lateral aspect and advance distally following the calcaneus into the joint space.

Shoulder joint
1.5-2" 22g needle (large and giant breeds). In lateral recumbancy slightly flex the shoulder and apply distal traction to the leg whilst rotating it externally. Insert the needle just cranial and distal to the acromion process of the humerus (just caudal and proximal to the greater tubercle of the humerus). Advance it obliquely ventro-caudally to pass through the biceps brachii and medial to the deltoideus. On entering the joint the traction should be released to lock the needle in place.

Antebrachiocarpal joint
5/8" 25g needle. In lateral or sternal recumbancy, fully flex the carpus and insert the needle from the cranial aspect, between the tendons of the extensor carpi radialis and the common digital extensor tendon. This joint communicates with the middle and carpometacarpal joints.

Manual restraint or sedation may be required depending upon the individual horse and routine aseptic technique should be used. A 5ml syringe and a 1", 18g or 20g needle is suitable for most joints. Local anaesthesia will assist positioning of the needle. The needle is advanced gently through the joint capsule, the syringe attached and negative pressure applied. If bone is encountered then the needle should be withdrawn slightly and redirected. The pressure is released before removal from the joint. Alternatively the needle may be inserted and the synovial fluid allowed to drip from the hub into suitable collecting bottles. The following information is taken from Rose and Frauenfelder, 1982. The article includes a series of photographs illustrating the approaches to individual joints.

Stifle joint
The stifle joint comprises three separate joint spaces:-

Femoropatellar -This is the largest and most often affected articulation. Insert the needle between either the medial and middle or lateral and middle patellar ligaments. Direct upwards and caudally towards the distal edge of the patella.

Medial femorotibial -Insert the needle horizontally between the medial patellar and the medial femorotibial (collateral) ligaments.

Lateral femorotibial -Insert the needle either just caudal to the lateral patellar ligament between the lateral femorotibial ligament and the common tendon of the long digital extensor and peroneus tertius.

Hock joint
The tibiotarsal joint is the joint space usually sampled. Insert the needle on the cranio-medial aspect between the extensor tendons and the medial collateral ligament. Care must be taken to avoid the saphenous vein.

Fetlock joint
The palmer or planter pouches of the joint are usually distended in disease. With the affected leg weight bearing, insert the needle from the lateral aspect into the space formed by the cannon bone, proximal sesamoid and the suspensory ligament.

Coffin joint
Insert the needle 1cm above the coronary band and 1.5cm medial or lateral to the midline. Direct the needle downwards, towards the midline and slightly backwards. Entry to the joint space may be aided by elevating the limb and flexing the joint. Care must be taken to avoid the coronary band plexus and the common digital extensor tendon.

Elbow joint
First palpate the lateral elbow and identify the tense lateral ligament and its points of insertion. Insert the needle cranial to the ligament and direct horizontally into the joint space under the margin of the lateral humeral condyle. The joint is situated between the middle and lower thirds of the lateral ligament.

Carpal joints
Of the three carpal joints, only the radiocarpal and intercarpal joints are commonly affected. Flex the leg to palpate the joints and insert the needle in the most prominent portion, usually the cranio-medial aspect just medial to the extensor carpi radialis tendon.

An assessment of synovial fluid volume and physical appearance should be made during arthrocentesis. The volumes collected from normal canine joints range from 0.01 to 1.0 ml. In horses at least 1ml of fluid can be collected from the major limb joints. Normal synovial fluid is transparent and pale yellow. Discoloration, turbidity and any tendency to clot should be noted. (Normal synovial fluid may become gelatinous on standing but will clear when shaken).

Processing depends upon the volume of fluid obtained. Samples should be prioritised as follows:-

Bacterial cultures
Where the differential diagnosis includes an infected (septic) inflammatory arthropathy, samples should be taken for bacterial culture. This laboratory recommends the inoculation of synovial fluid into a 20ml bottle of blood culture medium (supplied free of charge). First remove the foil cover and change the needle used for arthrocentesis for a sterile needle. Transfer between 1 and 2ml of synovial fluid to the blood culture bottle and submit to the laboratory. Do not remove the cap from the blood culture bottle.

Samples in anticoagulant
Any remaining sample should be placed into anticoagulant. EDTA is recommended for cell counts, protein concentrations, viscosity assessment and sediment smears. Mucin clot tests cannot be performed on EDTA samples and heparin is preferred for this purpose. It is recommended to dispense 1.5- 2ml of synovial fluid into EDTA and any remaining fluid into heparin, if no heparin is available then a plain tube should be submitted.

All samples should be dispatched promptly to minimise the time taken to processing. Synovial fluid samples are much less prone to ageing artefacts than e.g. cerebrospinal fluid or urine samples.

Anon. Some Useful Clinico-Pathological Sampling Techniques. Beaufort Cottage Laboratories. Newmarket. p. 5-6.

Houlton, J. (1994) Ancillary Aids to the Diagnosis of Joint Disease. In: Manual of Small Animal Arthrology. Eds. Houlton,J.E.F. and Collinson, R.W. BSAVA. Cheltenham. p. 22-38.

Maheffey E.A. (1992) Synovial Fluid. In: Cytology and Hematology of the Horse. Eds.. Cowell, R.L. and Tyler, R.D. American Veterinary Publications Inc. California. p. 153-162.

Parry, B.W. (1989) Synovial Fluid. In: Diagnostic Cytology of the Dog and Cat. Eds. Cowell, R.L. and Tyler, R.D. American Veterinary Publications Inc. California. p. 121-136.

ROSE,R.J. AND FRAUENFELDER,H.C. (1982). Arthrocentesis in the horse. Equine Veterinary Journal. 14 (2). pp 173-177