Septic arthritis is a rheumatological emergency. It is associated with considerable morbidity and mortality (up to 15%), so a high index of suspicion should always be maintained.

Clinical features

A septic joint is exquisitely painful and is often held rigid in the most comfortable position. Systemic features of infection such as fever and constitutional upset are typical but occasionally patients may be afebrile and appear deceptively well. Although the classic presentation of bacterial arthritis is an acute monoarthritis, up to a third of cases are polyarticular.

• Patients with abnormal, damaged or prosthetic joints.
  
• The immunocompromised (including diabetics).
  
• The elderly and the very young.

Diagnosis

The diagnosis hinges on joint aspiration and analysis of the synovial fluid. In non-gonococcal bacterial arthritis, the yield from joint cultures is >95%, falling to 50% yield from blood cultures. In high-risk or clear-cut cases it is prudent to commence antibiotic therapy as soon as joint fluid and blood cultures have been taken; antimicrobial therapy can be adjusted once the microbiological results and sensitivities are available.

The commonest causative agents are divided into:

In children, Streptococcus and Haemophilus influenzae are the commonest pathogens.

Occasionally gonococcal arthritis can be clinically differentiated from non-gonococcal disease and this can prevent considerable diagnostic delay – microbiological yield in gonococcal disease is notoriously poor with up to 75% of synovial fluid cultures being negative. In addition, positive blood cultures are exceedingly rare. Gonococcal septic arthritis tends to occur in younger, healthier patients, the arthritis may adopt a migratory pattern and tenosynovitis and skin lesions are a frequent feature. It is important to remember that the antecedent infection may be asymptomatic, particularly in women.

Treatment

The treatment of choice for septic arthritis is prompt and appropriate antibiotic therapy – empiric choices must cover staphylococcal and streptococcal species and most patients receive 6 weeks of antibiotics. Surgical wash-out of a joint (especially prosthetic ones) may be warranted. Septic arthritis in children may damage the growth plate, causing limb shortening on the affected side.

Osteomyelitis is infection of bone or bone marrow and usually presents with deep-seated bone pain, with or without septic features. The majority of cases in adults are caused by Staphylococcus aureus, but tuberculosis, salmonella and pseudomonas are also encountered.

Diagnosis

Diagnosis relies on appropriate imaging. X rays are unhelpful early on in disease, but ultrasound may demonstrate pus in the subperiosteal region. Bone marrow oedema and periosteal abscess formation can be visualised on MRI, whereas CT is better at demonstrating the development of vascular congestion or thrombosis. Remember that the white cell count and inflammatory response may be normal despite active infection.

Treatment

Treatment is similar to that for the infected joint, with prolonged courses of appropriate antibiotics. Occasionally, surgical debridement is required.

Secondary tumours are far more common than primary bone malignancy. The commonest carcinomas to spread to bone are breast, bronchus, thyroid, prostate and kidney and the commonest sites for deposit are the vertebral column, ribs, proximal femur and humerus.

• Metastatic deposits tend to erode bone (lytic lesions) and may present with vertebral collapse or pathological fracture. Extensive destruction may cause hypercalcaemia, but more frequently the hypercalcaemia of malignancy is due to the production of parathyroid hormone-related protein by tumour cells.
  
• Prostate and occasionally breast secondaries may induce reactive new bone formation, giving rise to osteosclerotic metastases. These patients will have a high serum alkaline phosphatase.
  
• Bone pain from malignancy is unremitting. Night pain is a classic ‘red flag’ symptom for malignancy.
  
• Treatment centres on management of the hypercalcaemia, surgical stabilisation of deposits at risk of fracture, radiotherapy and pain control.

The commonest primary malignant diseases of bone are myeloma and osteosarcoma.

Myeloma

• Multiple myeloma is caused by neoplastic proliferation of plasma cells in the bone marrow leading to bone pain and pathological fractures. If the marrow is replaced by tumour cells a pancytopaenia may result.
  
• Radiologically, myeloma causes multiple ‘punched-out’ lesions; if the disease is widespread a diffuse osteoporosis may occur. Bone scans in myeloma are cold and inconclusive.
  
• Classically myeloma patients have a high erythrocyte sedimentation rate (ESR), elevated calcium and alkaline phosphatase. Diagnosis relies on protein electrophoresis, demonstrating a monoclonal protein in the serum (and Bence-Jones immunoglobulin protein in the urine); bone marrow biopsy reveals a plasma cell infiltrate.
  
• Treatment is chemotherap.

Primary bone tumours

These are rare and can arise from any of the cell types involved in the skeleton. Chondrosarcomas arise from cartilage cells, myosarcomas from muscle, liposarcomas from fat and osteosarcomas from bone cells.

Osteosarcoma

• Osteosarcoma affects young people, with most cases occurring between 10 and 25 years of age.
  
• Patients typically present with pain and swelling but pathological fracture is relatively unusual.
  
• The majority of osteosarcomas arise in the metaphysis of the long bones. Classic sites include the knee, proximal humerus and femur and distal radius.
  
• Treatment involves chemotherapy and surgery.

Ewing’s sarcoma

• Ewing’s sarcoma is the second commonest primary tumour, again affecting children.
  
• Patients present with systemic upset and warm erythematous swelling, commonly at the diaphysis. Chrondrosarcoma may mimic infection or eosinophilic granuloma.
  
• Treatment involves neoadjuvant chemotherapy, radiotherapy and surgery.

Chondrosarcoma

Chondrosarcomas affect adults over 40 years of age. They are relatively insensitive to adjuvant therapy and the treatment is surgical.