Septic Arthritis

This is infection producing inflammation in a native or prosthetic joint. It can be acute or chronic.

American studies have estimated an occurrence of 20,000 cases of suppurative arthritis per year (7.8 per 100,000 person-years).¹ The figures for Europe are similar. One study in Denmark over the period 2003-2004 found an annual incidence of streptococcal infection of 2.6 cases per 100,000 inhabitants.² The incidence of disseminated gonococcal arthritis is 2.8 cases per 100,000 person-years. The disease is becoming increasingly common among the elderly with multiple morbidities, and in the immunosuppressed. The incidence of prosthetic joint infection among all recipients ranges from 2-10%.¹

Symptoms

The classic picture is a single swollen joint with pain on active or passive movement. The knee is involved in about 50% of the cases, but wrists, ankles, and hips are also commonly affected.³ Septic arthritis may present as polyarticular arthritis in about 10% to 19% of patients.⁴

It is commoner in patients with prior joint damage as in gout, rheumatoid arthritis and systemic connective tissue disorders.⁵ Fevers and rigors are present in the majority of cases, but their absence does not exclude the diagnosis. Bacteraemia is a common finding, and when present may cause prostration, vomiting or hypotension.

Patients with septic arthritis of the sternoclavicular, acromioclavicular, sternocostal or manubrosternal joints may present with chest wall pain.⁶ Infection of the sacroiliac joint may present as buttock, hip or anterior thigh pain.

Apart from pre-existing joint disease, associated conditions include immunosuppressive disease, recent steroid injection, sexually transmitted disease and intravenous drug use.

Signs

The joint is usually swollen, warm, tender and exquisitely painful on movement. An effusion may be obvious. The knee is the commonest joint involved (50% of cases), followed by the hip (20%), shoulder (8%), ankle (7%) and wrists (7%).

Signs may be less marked or poorly localised in the elderly, immunocompromised, and drug abusers and in infections of the spine, hip and shoulder joints.

Infection of a prosthetic joint may show few signs until a drainage sinus develops. Occasionally an abscess around the joint, or loosening of the implant is indicated by pain.^7,8

• Intercurrent chronic joint disease - septic arthritis in rheumatoid arthritis is associated with an increased mortality of 25-30% due to delay in diagnosis and treatment, as it may mimic an acute flare-up of the disease.⁹
• Previous accidental or iatrogenic trauma (e.g. prosthetic surgery).
• The presence of extra-articular symptoms, e.g. pyrexia, systemically unwell.
• A history of vascular invasion due to catheterisations or intravenous drug abuse.
• A history of sexually transmitted diseases or exposure to ticks (Lyme disease).
• Any condition likely to compromise the host's defence system - e.g. diabetes, liver disease, lymphoma, immunosuppressive treatment, hypogammaglobulinaemia.

The triad of fever (45-60% of cases), pain (75%) and impaired range of motion is typical. Fevers are usually low grade and rigors are only present in 20% of cases.

History and examination may not only yield clues as to the diagnosis of septic arthritis but also to the type of infection that is present. 85%-90% of nongonococcal suppurative arthritis affects one joint.¹⁰ Staphylococcus aureus is the commonest cause of polyarticular arthritis. Other causes include various viral infections, Lyme disease, gonococcal disease, reactive arthritis, and various noninfective conditions.

• Infection of the sternoclavicular and sacroiliac joints is commonly caused by Group B Streptococcus spp. infection.
• Gonococcal disease usually presents with fever, arthralgia, multiple skin lesions (dermatitis-arthritis syndrome), and tenosynovitis of the hand joints, knees, wrists, ankles, and elbows, but it may also present as a monoarticular arthritis in which these other features are absent.
• Lyme disease (caused by infection with Borrelia burgdorferi, may cause swelling disproportionate to the level of pain. It should be suspected in patients with a history of tick bite or who have travelled to endemic areas and present with transient polyarthralgia, typical erythema chronicum migrans, and systemic symptoms. Joint inflammation may present many months after initial infection, and occurs in 60% of untreated patients, mainly affecting the large joints, commonly the knee.
• Hip joint infection may cause pain which does not localise directly and swelling may not be obvious. Specific examination techniques may be helpful such as the Patrick/Fabere manoeuvre (pain in the hip on sequential flexion, abduction, external rotation, extension of the joint with the contralateral knee flexed) may be required.¹¹
• Sacro-iliac (SI) joint infection often presents as buttock, anterior thigh or hip pain. Direct pressure may elicit tenderness. Other diagnostic test is the Gaenslen manoeuvre (pain in the SI joint on hyperextension of the hip and leg while the patient is lying down).¹¹
• Reactive arthritis (including Reiter's syndrome, psoriatic arthritis, ankylosing spondylitis, and arthritis associated with inflammatory bowel disease) often presents with inflammation in a few large joints, distributed asymmetrically. It may develop several weeks after the original infection has resolved, and there may be few concurrent symptoms. Other symptoms of the disease process such as gastrointestinal or genitourinary symptoms, skin lesions, or uveitis may yield diagnostic clues.
• Viral arthritis (eg. rubella, parvovirus, hepatitis C, HIV) often presents with symmetrical involvement of smaller joints such as the hands, accompanied by a rash.
• Tuberculous arthritis may be associated with a joint which feels 'boggy' on palpation due to granulomatous involvement. The symptoms may be quite indolent and the diagnosis may be delayed for many years.
• Prosthetic joint infection may exhibit a prolonged low-grade course with gradually increasing pain. There is often no significant swelling or fever, although high-grade fever, focal swelling and redness occasionally occur. Cellulitis and sinus development are common presenting features.
• Infective endocarditis may be complicated by septic arthritis or bone infection in 15% of cases. About 15% of patients with infective endocarditis have septic arthritis or bone infection. These patients may also present with sterile synovitis or arthralgias mimicking septic arthritis.¹²

Laboratory tests

• Synovial Fluid Examination. If synovial fluid can be aspirated, it should be sent for leukocyte count, Gram staining, polarising microscopy to exclude crystal arthropathy, and culture. Marked leucocytosis may be seen in mycobacterium infection. Culture is the only reliable method of evaluating a potentially infected joint, as the white cell count is not itself diagnostic.¹³ Since time is of the essence, treatment should not be delayed for the results of culture but should be based on Gram stain and polarising microscopy results. An exception to this is where there is a signficant risk of infection elsewhere that could lead to bacteraemia, (e.g. pyelonephritis or pneumonia).
  Fluid should always be sent for culture, irrespective of the screening result, as Gram staining is only 60% sensitive, and a joint damaged by crystal arthropathy may well have co-existent infection. Culture is 80% sensitive for tuberculous infection. Additional staining or cultures may be required as clinically indicated from the list of differential diagnoses. Fluid can be injected directly into aerobic/anaerobic "blood" culture bottles to maximise chance of growing organisms.
• Synovial Tissue Culture. This is predominantly indicated where fungal or mycobacterial infection is suspected. Culture is 94% sensitive for tubercular infection.
• Blood Cultures. At least two blood cultures should be taken to exclude bacteraemia.
• Cultures for gonococcal infection. Rectal, cervical, urethral and pharyngeal swabs should be taken if this condition is suspected.
• Polymerase Chain Reaction (PCR). This is a method of detecting bacterial DNA and has been used with some success in the diagnosis of reactive arthritis due to Yersinia spp., B. burgdorferi, Chlamydia spp., Neisseria gonorrhoea, and Ureaplasma spp.¹⁴ The disadvantage of this test is that it cannot be used to distinguish between live and dead organisms and it is susceptible to contamination. More recently, a PCR test based on the detection of bacterial RNA has been developed; it can deliver a result within three hours.¹⁵
• Tests for Lyme Disease. Silver staining of synovial fluid is positive in approximately 5% of cases. Diagnosis in the early stages is based on the clinical picture (particularly the rash) in a patient with tick exposure a history of travel to an endemic area, and laboratory testing may only be helpful in the later stages.¹⁶ Even then, obtaining accurate laboratory confirmation may prove challenging.
• Inflammatory Markers. and Immunology An ESR or C-reactive protein may be useful in following response to therapy, as well as detecting an acute process in chronically inflamed joints. Serological tests for diagnosis of various rheumatological disorders and vasculitides should be arranged as clinically appropriate.

Imaging

• Plain Radiographs. These are of limited value, and may be normal in the first few days of the disease, but they may show underlying osteomyelitis or periarticular osteomyelitis of the joint itself. Fat pad displacement, swelling of capsule and soft tissue around the affected joint and in some cases joint space widening due to localised oedema and effusion may be seen. In later stages, diffuse joint space narrowing due to cartilage destruction may be evident. Plain radiographs may also be used to identify inadequately treated septic arthritis with generalised joint destruction, joint fusion, subchondral bone loss followed by reactive sclerosis, or calcification of periarticular tissue. The linear deposition of calcium pyrophosphate can sometimes be detected on plain radiograph and is a pointer to the diagnosis of pseudogout.
• Ultrasonography. This is not as sensitive as MRI or CT scan, but is an inexpensive non-invasive method that can be helpful in showing intra- and extra-articular abnormalities not obvious on plain Xray, and it can detect early effusions, and guide joint aspiration and drainage procedures.
• CT and MRI scanning. These are the most sensitive methods for diagnosing periarticular abscesses, joint effusions and osteomyelitis. They are usually reserved for cases of diagnostic difficulty and in specific clinical situations - e.g. sacroiliac or sternoclavicular joint infection, to exclude extension into the mediastinum or pelvis. In experienced hands, specific diagnostic features of Lyme disease can be detected on MRI scan.¹⁷
• Radionuclide scans. Technetium Tc 99m, gallium Ga 67, and indium In 111 leukocyte scans are used to localise areas of inflammation, and although they cannot distinguish infections from sterile processes, they may be helpful in identifying septic arthritis in relatively sequestered areas such as the hip and sacroiliac joints. A refinement of this technique, using radiolabelled antibiotic, is proving to be more specific.¹⁸

Investigation of the prosthetic joint

A high index of suspicion should be maintained for any patient complaining of recurrent pain in a prosthetic joint. Fever and leukocytosis may be absent in elderly patients.¹⁹ Plain radiographs may reveal transcortical sinus tracts and new subperiosteal bone growth. Contrast arthrography can demonstrate loosening of the prosthesis, synovial outpouchings and abscesses.²⁰ CT and MRI scans may also be helpful, as may scintigraphy.^20,21 However, no single imaging technique is 100% sensitive, and if the condition is suspected, joint aspiration or operative biopsy should be performed.^19,21 Aspiration of a prosthetic joint is normally only attempted by an orthopaedic specialist.

Antibiotic therapy^1,3

Treatment should be started empirically before the results of cultures are known, as evidence suggests that a better functional result is obtained the sooner an antibiotic is commenced. Gram-staining, the clinical picture and the background of the patient should act as a guide. 40-50% of Gram stains fail to reveal any micro-organisms, in which case the individual's age and sexual activity become major determinants as to the likelihood of a gonococcal infection. In the absences of infection elsewhere, the antibiotic should at least cover Staphylococcus aureus and Streptococci spp. A microbiologist should also be consulted as the choice of therapy should be based on resistance patterns in the local hospitals and community.

MRSA is becoming an increasing problem, as is penicillin resistance in group B Streptococcus spp. Penicillin plus gentamicin, or later generation cephalosporins are often used. Treatment is generally administered intravenously for 3-4 weeks, except in the case of gonococcal infection, where a switch to oral antibiotics is often made after two weeks. There is no benefit in injecting antibiotics intra-articularly. Antibiotics have good penetration across the synovial membrane, and intra-articular injection has the potential for causing a chemical synovitis.²²

Other medical therapy¹

If the condition fails to respond to 5 days' treatment with an appropriate antibiotic (as evidenced by persistent fever, positive cultures or synovial purulence), the therapeutic approach should be reassessed. Synovial fluid should be re-examined for crystals, Lyme disease serology should be arranged. Consideration should be given to synovial biopsy to exclude fungal or mycobacterial infection, and to the possibility of reactive arthritis requiring the use of non-steroidal anti-inflammatories.

Joint drainage

Repeated percutaneous aspiration may be required if the infection does not respond to antibiotic treatment, sometimes two or three times a day.^1,3 The situation depends to some extent on the joint affected. Joints difficult to access (e.g. hip, shoulder, and sacroiliac joints) may require ultrasound-guided needle aspiration or open arthrotomy.^23,24 Surgical drainage may be required in any infected joint which does not respond to medical treatment, although the decision as to whether to use repeated needle aspirations or arthroscopic lavage is chiefly anecdotal and not supported by a large comparative evidence base.²⁵ One study of 40 patients with septic arthritis of the knee suggested that for this condition at least, needle aspiration should only be performed in the very early stages, arthroscopic debridement being the treatment of choice in all other cases.²⁶ A mini-arthrotomy in which a window is cut into the joint capsule has been attempted successfully in five patients with septic arthritis of the hip.²⁷ Patients with underlying disease such as rheumatoid arthritis, diabetes or the immunosuppressed, benefit from earlier surgical intervention.³ Gonococcal joints rarely require drainage.

Splinting

The limb should be splinted in the position of function (knees in extension, elbows at 90 degrees, wrists in neutral to slight extension, hips in balanced suspension in neutral rotation). Once the infection is under control, immediate joint mobilisation will promote healing of the articular cartilage and prevent contractures.²⁸

Treatment of infected prosthetic joints

The commonest prosthetic joints to get infected are elbow, shoulder and ankle joints (6-9%) followed by hips and knees (0.5-2%).³ Early infection (less than 12 weeks after implantation) is usually caused by skin pathogens such as coagulase-negative Staphylococcus spp. It can often be cured medically providing there is no evidence of periarticular soft tissue involvement or joint instability.²⁹ Late-onset infections (more than 1 year after implantation) are usually caused by haematogenous spread of common organisms such as Escherichia.coli, Proteus mirabilis, Pseudomonas aeruginosa, S. epidermis and S. aureus.
Prosthetic material combined with cement forms an ideal avascular culture medium for bacteria away from the immunologic defenses of the body.³ Removal of the prosthesis, insertion of an antibiotic-loaded spaced, and then replacement with a new prosthesis achieves a high success rate.³⁰ A compromise is to exchange the infected joint with a new one in a one-stage process with concurrent use of antibiotic cement and antibiotic therapy. A 90% success rate has been achieved with this method.³

Fifty per cent of adults with septic arthritis have significant sequelae of chronic pain or reduced range of motions.
Risk factors for a poor outcome include:

• Infection of the shoulder or hip
• Age greater than 60 years
• Underlying rheumatoid arthritis
• Delay of 7 days or more in instituting therapy
• Positive findings on synovial fluid cultures after 7 days of appropriate therapy

Thirty per cent of cases of reactive arthritis may become chronic.