Septic Knee

Ovid: 5-Minute Orthopaedic Consult

Editors: Frassica, Frank J.; Sponseller, Paul D.; Wilckens, John H.
Title: 5-Minute Orthopaedic Consult, 2nd Edition
> Table of Contents > Septic Knee

Septic Knee
Michelle Cameron MD
John H. Wilckens MD
  • A septic knee is an infection of the synovial lining of the knee joint.
  • Predisposing factors include arthritis, intravenous drug abuse and alcoholism, steroid use, and any form of immunosuppression (1).
  • Common
  • May occur in infants, children, adults, and the geriatric population
Risk Factors
  • Intravenous drug abuse and alcoholism
  • Trauma, surgery
  • HIV disease
  • Steroids
  • Immunocompromised hosts (e.g., those with diabetes, HIV)
  • Staphylococcus aureus is the most common cause.
  • Other common organisms:
    • Hemolytic Streptococcus
    • Pneumococcus
    • Gonococcus
    • Meningococcus
    • Salmonella
    • Brucella
    • Haemophilus influenzae (infants)
Signs and Symptoms
  • Monarticular erythema
  • Swelling
  • Fluctuant joint capsule
  • Pain on ROM or weightbearing
  • Possible fever and leukocytosis
Physical Exam
  • The key findings are:
    • Joint effusion
    • Painful ROM
    • Erythema
  • The complete blood count may show leukocytosis with a left shift, and the ESR is always almost elevated.
  • C-reactive protein also is helpful.
  • The primary test is analysis of fluid aspirated from the knee joint.
    • Opinions vary on the leukocyte count that is diagnostic of a septic joint.
    • Most authors agree that an aspirate with >100,000 white cells with >90% polymorphonuclear cells is strongly suggestive.
    • The fluid will have low glucose and high protein levels.
    • The aspirate should be sent for Gram stain and culture.
  • The fluid also should be sent for crystal evaluation to rule out gout or pseudogout.
  • Any patient suspected of having septic arthritis should have 2–3 blood cultures drawn before administration of antibiotics.
  • The ESR also is elevated and may be helpful in following the disease course.
  • Chronic, low-grade septic arthritis can be difficult to diagnose.
  • MRI scans may show osteomyelitis and a Baker cyst in addition to a large effusion and hypertrophy of the synovium.
Pathological Findings
  • If the infection is not recognized and treated early, destruction of articular cartilage will occur.
  • Cartilage erosion leads to degenerative changes in the joint.
    • The amount of bone destruction depends on the virulence of the organism and the length of time infection has been present.
  • Long-standing septic arthritis can progress to fibrous or bony ankylosis and septicemia.
Differential Diagnosis
  • Acute osteomyelitis
  • Periarticular cellulitis
  • Prepatellar bursitis
  • Gout
  • Pseudogout
  • Acute rheumatoid arthritis
  • Hemophilia
  • Lyme disease


General Measures
  • In general, management usually is surgical: Open or arthroscopic débridement (2).
    • If the infection is discovered early, 1
      nonoperative treatment can be tried, using intravenous antibiotics and
      serial aspirations.
  • Early diagnosis and prompt treatment are indicated to prevent severe and permanent damage.
  • The joint should be irrigated and débrided urgently; multiple débridements may be necessary.
  • If present, a popliteal cyst may need to be excised because it can reinoculate the knee joint.
  • Immobilization of the leg in a knee immobilizer also is recommended throughout the acute episode.
  • Once the infection has subsided, gentle active and passive ROM exercises can be started.
Special Therapy
Physical Therapy
Gentle active and passive ROM exercises of the knee should be initiated after the acute episode has cleared.
First Line
  • Institute antibiotic therapy as soon as appropriate specimens are obtained for culture.
  • Empiric antibiotic therapy should cover Gram-positive organisms.
  • Antibiotic coverage should be modified appropriately when Gram stain and culture results are available.
  • Irrigation and débridement consist of
    opening the knee arthroscopically or through a standard knee approach
    and washing the joint with multiple liters of normal saline (3).
  • All loculations are found and broken to allow complete drainage.
  • With early treatment, prognosis usually is good.
  • Outcomes are poor if the diagnosis is delayed substantially.
  • Fibrous or bony ankylosis of the knee
  • Osteomyelitis
  • Septicemia
  • Degenerative joint disease
1. Brashear
HR, Jr, Raney, RB Sr. Infections of bones and joints. In: Handbook of
Orthopaedic Surgery, 10th ed. St. Louis: CV Mosby, 1986:110–139.
2. Thiery JA. Arthroscopic drainage in septic arthritides of the knee: A multicenter study. Arthroscopy 1989;5:65–69.
3. Ivey M, Clark R. Arthroscopic debridement of the knee for septic arthritis. Clin Orthop Relat Res 1985;199:201–206.
711.96 Septic arthritis, knee
Patient Teaching
  • Patients are encouraged to work on early ROM.
  • Stiffness will occur and can be permanent without ROM exercises.
Q: What laboratory studies should be requested on an aspirate suspicious for septic knee?
In addition to culture and sensitivity studies, the aspirate should be
sent for Gram stain, cell count, and analysis for crystals. White cell
counts >50,000 with a “left shift” are strongly suspicious for
infection; counts >100,000 essentially are diagnostic for infection.

Q: Does a septic knee always require surgical irrigation and débridement?
Early surgery is recommended for all septic knees. In rare instances,
repeat aspirations may be considered in some pediatric patients and
when surgical evaluation is delayed.

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