Infectious Arthritis

It is inflammation of synovial membrane with purulent effusion into the joint capsule. It can be caused by various micro-organisms like bacteria, mycobacteria, spirochetes (Lyme arthritis), fungi, and viruses. Among, infections due to Staphylococcus aures are mostly common.

Etiology

• Causative organism is usually Staphylococcus aures.

• In infants, group B Streptococci, gram negative organisms and Staphylococcus aures are common.
• In children under the age of 2-5 years, Haemophilus influnzae is common. Due to the advent of Haemophilus influnzae vaccine, S. aures, Streptococcal pyogens are being the common cause.
• In adolescent, Neisseria gonorrhoae, S aures are commonly implicated organisms.
• Other organisms like spirochetes, Borrelia (Lyme disease), Treponema (Syphilitic arthritis), mycobacteria (Tubercular arthritis), Fungi (Fungal arthritis), Viruses and parasites can cause infective arthritis. Infective arthritis due to these organisms are relatively uncommon.

• After surgical procedures, prosthetic joints and arthsocope (coagulase negative Staph. aureus).
• Human and animal bite (anorebic organisms with facultative aerobic organisms) decubitus ulcers and intra-abdominal abscesses spread to adjacent joint.
• Penetration injury with sharp object through shoe (Pseudomonas auriginosa arthiritis of foot).
• Patients with rheumatoid arthirits, Diabetes Mellitus, glucocorticoid therapy, hemodialysis and malignancies (Drugs used can cause decreased immune strength).
• HIV paitents (Pneumococci, H. influenzae).

Pathogenesis

• Dissemination of the pathogen via bloodstream from distant site is most common. Bacteria may also enter from contagious site of infection, dissemination from acute osteomyelitis focus, from adjacent soft tissues, penetrating trauma, human or animal bite may occur.
• The bacteria then provokes neutrophilic infiltration of synovium. There is an acute synovitis with purulent joint effusion and synovial membrane become edematous, swollen and hyperemic and produces increased amount of cloudy exudates containing leucocytes and bacteria.
• As the infection spreads through joint, articular cartilage is destroyed as degradation of cartilage begins as a result of increased intra articular pressure release of protease and cytokines; from chondrocytes and synovial macrophages and invasion; of cartilage by bacteria and inflammatory cells.
• If the infection is not arrested, the cartilage may be completely destroyed and pus may burst out of the joint to form abscess and sinuses.
• Bacterial factors (like surface associated adherins in S. aureus) permit adherence to cartilage and endotoxins that promote the chondrocyte mediated breakdown of cartilage.
• In severe cases the infection extends to the sub–chondral bone.

With healing there will be:           

• Complete resolution and return to normal.
• Partial loss of cartilage and fibrosis.
• Bone ankylosis.
• Bone destruction and permanent deformity.

Clinical features:

Acute pain and swelling in the single large joint (mostly knee in adults and hip in children), however any joint can be affected. The affected joints in order of most common to less common are:

• Knee > Hip > Shoulder > wrist > elbow > intra phalangeal joint > stenoclavicular > sacroiliac.
• Small joints of hand, feet after the direct inoculation or bite.
• Infection of sternoclavicular, sacroiliac and spine is common in IV drug users.

Symptoms in Infants:

• Emphasis is on septicemia rather than joint pain.
• Irritability, fever, refuse to feed, rapid pulse.
• Pseudo paralysis, unable to move limb with infected joint.
• Cry during movement of infected joint (e.g. during changing diaper).

Symptoms in children:

• Acute pain in single large joint.
• If joint is superficial, it is tender, warm and swollen.
• Fever (101-103oF).
• Pseudo paresis, restricted movements.

Symptoms in adults:

• Intense joint pain with acute onset.
• Joint swelling and redness.
• Unable to move the limb with infected joint
• Low grade fever.

Examination

• Decreased or absent range of motion.
• Inflammatory sings like swelling, warmth, tenderness and erythema.
• Patient lie on position of comfort to minimize pain.

• Hip: abducted, flexed and externally rotated.
• Knee, ankle and elbow: Partially flexed.
• Shoulder: abducted, internally rotated.
• Cellulitis, bursitis and acute osteomyelitis can produce similar clinical picture, so should be ruled out. In those conditions there will be the greater range of motion and less than circumferential swelling.
• Extra articular infections (boils, pneumonia) should be sought.
• In infants, infection is usually suspected but it could be carefully felt and moved to elect signs of warmth, tenderness and resistance to movement.
• Umbilical cord should be examined for possible source of infection.
• If the baby is distressed and is not moving the leg, then hip infection is thought.