Arthrocentesis means aspiration of fluid from a joint space which is usually done by placing a needle into a joint space and withdrawing fluid in to a syringe or collecting device. Besides medications can be administered into it by inserting needle i.e. injection to reduce pain, inflammation and infection.

All moveable synovial joints are covered by fibrous capsule lined by synovial membrane and ligaments to maintain structural integrity. The joint space contains synovial fluid and the articular surfaces of bone are covered by hyaline cartilage except those few bones which ossifies in membrane and lined by fibro-cartilage. Under normal conditions most joints contain only few ml of clear, viscous synovial fluid but the volume and character of it change in certain infections, inflammatory and traumatic conditions.

Aspiration
Joint aspiration and injections are done both for diagnostic and therapeutic purposes viz
1.    Diagnostic:
a.    aspiration in suspected septic arthritis, traumatic haemarthrosis, crystal induced synovitis
b.    diagnostic arthrography by injecting contrast media
2.    Therapeutic:
    Injection of steroids, intra-articular for persistent localized synovitis or peri-articular for soft tissue lesions. Sometimes aspiration and injection of antibiotic in septic arthritis

Requirements:
1.    Sterile needle and syringe: size according to site, procedure, type, anatomy of the joint
2.    Antiseptic agent for skin e.g. povidone iodine, isopropyl alcohol
3.    Sterile towel
4.    Sterile gloves – 1 pair
5.    Sterile pack – 1
6.    A simple/crape bandage
7.    Medications: 1%-2% xylocaine (without adrenaline), steroid/antibiotic/contrast media
8.    An assistant

Aspiration of joint can be safely performed in an outpatient department, however in case of large weight bearing joint rest for 24 hours after the procedure is preferable for fear of any bleeding in synovial cavity due to puncture injury. In case of pyogenic arthritis as much as possible the fluid in the joint is withdrawn and then adequate dose of proper antibiotic dilutedddd to half the volume of the aspirated fluid is injected. The procedure may again be repeated after 48 hours.
Steroids
Usually a cocktail of 2 different agents i.e. a local anaesthetic and intra-articular preparation of corticosteroid injected which provide a safe, affective and rapid means to treat local symptoms of pain and inflammation without various side effects of medications when taken orally. Pain and inflammation begins to take off within a day or two and effects last upto several months and sometime cures a frozen shoulder. However the same joint should not be injected more than once in 3 months.

For intra-articular purpose Hydrocortisone Acetate is preferred to long acting preparation e.g. Methylprednisolone Acetate and Triamcinolone. In case of peri-articular soft tissue injections also Hydrocortisone is preferred to long acting preparations for the risk of atrophy of subcutaneous tissue and skin.
Steroid Injection
One should remember the followings regarding corticosteroid injections:
a.    Suitable for clear, local lesions
b.    Exclude infection before giving
c.    Use non-touch simple cleansing technique
d.    Premix anaesthetic (2% Lignocaine, 1-2 ml) and corticosteroid
e.    Long acting expensive steroids have little advantage over Hydrocortisone 25-50 mg (1-2 ml)
f.    Use smallest syringe and finest needle (however for aspiration of thick purulent or chronic effusions 19/21 gauge needle and for finger plus toe joints aspiration 23 gauge needle is used)
g.    Avoid intradensual injection a
s it causes fat necrosis and skin atrophy
h.    Post injection pain due to crystal reaction or tissue damage is rare
Technique
After skin preparation general anaesthesia is given in children but local anaesthesia in adults. After infiltration of skin with local anaesthetic, the synovium is well infiltrated. The surgeon locates the site of maximal pain and tenderness. This can be over lateral epicondyle in tennis elbow, or at the medial epicondyle as in golfer’s elbow (safeguard the ulnar nerve which lies behind the medial epicondyle by palpation), at the lateral tip of acromion for subacromial bursitis or painful arc syndrome.

Aspiration sites
Shoulder Joints  
Anterior Approach – Patient sits with arms relaxed against side of chest. The needle inserted with slight medial angle 1cm below the coracoid process into space between head of the humerous and glenoid capsule
Lateral Approach – Just below acromion between it and greater tuberosity.

Elbow Joints
The elbow flexed at right angle. Position of the head of the radius is checked. The needle is inserted from posterolateral aspect above the head of the radius (fig. B). It can be also inserted in the midline posteriorly immediately above the olecranon.

Hip Joint
Anterior Approach – A lumbar puncture needle is preferred. The needle is inserted perpendicular to skin i.e. directly backwards 1 inch below and lateral to mid inguinal point i.e. lateral to the femoral artery (safeguard femoral artery by palpation Fig. D)

Lateral Approach – Introduce needle just above the tip of the greater trochanter and thrust it upwards and medially in the line of neck of the femur.

Knee Joint
The needle can be inserted into the joint at either side of lower border of patella. For practical purpose the joint is slightly flexed and muscles relaxed and then through the lateral side deep to patella approach is made. The posterior margin of the patella is palpated laterally and the patella pushed gently to the other side so that femoral surface below it is felt.
The needle is now inserted horizontally or slightly downwards in the gap between the femur and the patella. In supracondylar bursitis the needle may be introduced
2.5 cm above upper border of the patella.

Ankle Joint
The joint line is found out by moving foot on tibia. The needle may be entered from anterolateral aspect just medial to lateral malleous and directed backwards and slightly upwards to reach interval between tibia and talus.

Conclusion
Anatomical knowledge of the joint is very important for proper joint aspiration and injection. Over enthusiastic joint aspiration should not be done. The patient should be told that there may be increased pain for a day or two specially after peri-articular and soft tissue injections.
The possible cause of other effusions e.g. Gonococcal, Syphilitic, Tuberculous, Rheumatoid, Osteoarthritic conditions etc. must be kept in mind, which require specific treatment.
In cases of spontaneous haemarthrosis or haemarthrosis following trival injury, Haemophilia should be suspected. Frequent intra-articular steroid in weight bearing joints especially in Osteoarthritis is not recommended. Besides in suspected septic arthritis i.e. if joint fluid is purulent steroid should not be injected until negative bacterial culture have been obtained.

All effusion specimens must be sent for culture and sensitivity and a heparinized sample for cytology and crystal examination using polarized light.
A dried smear should be examined by gram staining for staphylococcus aureus, pneomonococcus, streptococcus and pseudomonas. After aspiration a simple or
crape bandage may be applied to the joint.  (Reprint)