What is septic arthritis?
Septic arthritis is a joint infection caused by bacteria. Bacteria usually reach the joint via the bloodstream (a haematogenous route) from other parts of the body, such as a tooth or a skin wound. One theory of how septic arthritis develops suggests it occurs as a direct consequence of acute bone infection (osteomyelitis) in the part of the bone enclosed by the joint capsule. As a result of the infection, bacteria, white blood cells and joint fluid accumulate, together forming pus. If pus remains in the joint for more than 4 days, permanent damage to the joint cartilage, or even to the bone’s growth plate, occurs, with catastrophic consequences for the joint. Children who develop septic arthritis appear unwell and exhausted, with a raised body temperature. They complain of pain in the affected joint, which, in the case of the knee, becomes swollen, warm and reddish. Unlike the knee, changes on the skin over the hip are rarely noticeable. In septic arthritis, inflammatory blood markers – white blood cells, CRP and ESR – are always elevated. For all these reasons, septic arthritis is an emergency in paediatric orthopaedics that requires rapid diagnosis and appropriate treatment. Treatment usually combines surgery – removal of the pus with joint irrigation – and prolonged antibiotic therapy.
What is septic arthritis of the hip?
Septic arthritis of the hip is the most common purulent joint infection in children. It is a very dangerous, urgent condition that must be recognised as early as possible, because bacteria that reach the hip via the bloodstream create pus within the joint, which over time damages the joint cartilage of the femoral head or the acetabulum. Joint damage can be so extensive that, soon after bone growth is complete, children need a hip replacement – a total hip prosthesis. Symptoms in children usually begin with a raised temperature over several days; they have difficulty moving and complain of thigh pain. Because the hip lies deeper within the body, the skin over the hip usually looks normal. Parents describe general malaise and irritability in the child. In the youngest children, the leg may be held in a "pseudoparalysis" position, and changing nappies causes pain and intense crying. A far more common condition affecting children’s hips is transient synovitis, which initially presents with symptoms similar to septic arthritis (limping and pain). However, in transient synovitis children are in good general condition without a raised temperature, although they sometimes have mildly elevated CRP values.
How is septic arthritis of the hip diagnosed?
Septic arthritis of the hip can be suspected based on a discussion in the clinic and a clinical examination. If a child is limping or unable to walk, has a raised body temperature and appears unwell, a hip ultrasound should be performed as soon as possible to look for a joint effusion. Basic laboratory tests should always be carried out, checking white blood cell count, CRP and ESR, which will be elevated. A hip X-ray is also recommended to assess the condition of the bones. Other conditions that can cause similar hip symptoms at onset include juvenile idiopathic arthritis, osteomyelitis of the pelvis or femur, and psoas muscle abscess.
How is septic arthritis of the hip treated?
It is a longstanding surgical principle that any pus must be evacuated from the body. The same applies to septic arthritis of the hip. Current surgical methods include arthroscopic drainage and joint irrigation (using special instruments through small skin incisions), as well as classic opening of the joint capsule – arthrotomy with joint irrigation. During surgery, a sample of the pus is taken and sent for microbiological analysis to identify the causative organism. Antibiotic therapy begins immediately after surgery, initially given directly into a vein (intravenously). During this time, the child needs to stay in hospital. Once inflammatory markers fall, treatment switches to oral antibiotic tablets, and the child can then go home. Total antibiotic treatment can last up to 6 weeks.
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