13.1 Osteomyelitis and septic arthritis in children

Although in the United Kingdom paediatric musculoskeletal infection is a diminishing problem, worldwide, bone and joint sepsis remains a common cause of morbidity. Despite improved imaging, delay to diagnosis remains common and treatment can be challenging. Nelson (1991) warns against ‘a cookbook approach of standardized management for a disease with varied manifestations and a variable course’. With appropriate management, recurrence is unusual. Complications, however, are difficult to treat and constitute major morbidity. This chapter outlines the pathology, diagnosis, and treatment of paediatric musculoskeletal infections.

For osteomyelitis, infection usually occurs by the haematogenous route, with the metaphysis being the most common site for colonization; very rarely, osteomyelitis may involve an epiphysis. Trauma increases the likelihood of osteomyelitis subsequent to bacteraemia which may explain the increased incidence in males. Initially there is an acute inflammatory response in the cancellous bone. The formation of pus may lead to perforation of the metaphyseal cortex with subperiosteal accumulation of pus which in turn may breach the periosteum to track externally. A subperiosteal collection may also deprive some cortical bone of its blood supply; the dead cortical bone forms a sequestrum.

Although most bones in the body can be a focus for osteomyelitis, the commonest are the tibia and femur. Similarly, the commonest joints affected by septic arthritis are the knee and hip. The acute inflammatory response involves the influx of neutrophils into the synovial fluid and subsequent rapid destruction of the articular cartilage. Septic arthritis is, therefore, a surgical emergency and joint washout the key treatment.

In joints that have an intra-articular metaphysis (the femoral neck, proximal humerus, proximal radius, and distal lateral tibia), metaphyseal osteomyelitis can transgress directly into the joint. For these joints, particularly, an initial negative aspirate should not provide false reassurance. If the clinical picture deteriorates, secondary infection of the joint may have occurred.

In the neonate, bone and joint infection may present late because of the lack of clinical signs. Furthermore, transphyseal blood vessels allow epiphyseal extension of a metaphyseal osteomyelitis with a subsequent septic arthritis: 60–100% of neonates with septic arthritis have a contiguous osteomyelitis.

The presentation and severity of acute haematogenous osteomyelitis (AHO) can vary considerably, sometimes making it difficult to treat. Prompt diagnosis and appropriate intravenous (IV) antibiotics can lead to complete resolution with no sequelae. However multiple complications may occur, if diagnosis and treatment are delayed, or if the patient’s response is poor. Three salient factors must be considered (see Box 13.1.3).

Bone infection occurs predominantly in lower socioeconomic classes where delayed presentation is common. As a result, the infection is well established within the bone before treatment commences and the subsequent clinical course is more likely to be complicated. Other patients also present late, including the immunocompromised, those with neuromuscular disabilities (whose ‘bone pain’ has been misdiagnosed as a fracture), and the neonate with a pseudoparalysis (Figure 13.1.1). If AHO is diagnosed ‘early’ when the infective organism is causing bone pain as a result of local inflammation, then prompt appropriate IV antibiotics will usually result in complete resolution. If there are significant comorbidities, the clinical course is less predictable and patients with diabetes mellitus, haemoglobinopathies, chronic renal disease, and rheumatoid arthritis will often require IV antibiotics for 2–3 months.

Staphylococcus aureus is the commonest infective organism in AHO but other organisms are relatively more common at specific ages and in certain patient groups. This knowledge influences antibiotic selection. Bactericidal levels of an appropriate antibiotic need to be delivered to the site of infection as swiftly as possible to halt progression (Table 13.1.1).

If the patient is slow to improve, other organisms like Haemophilus influenza and Kingella kingae need to be considered. In some areas, community-acquired methicillin resistant Staphylococcus aureus (CA-MRSA) is a significant cause of AHO

The most common sites of osteomyelitis are the proximal tibial and distal femoral metaphyses. Infection in these areas is relatively straightforward to diagnose and treat. Extension of the infection along the diaphysis is uncommon if treated early, thus whole bone osteomyelitis and its sequelae are rare. Infection in the pelvis, clavicle, and calcaneus is less common, and therefore diagnosis is often delayed. These three bones are largely cancellous and eradication of the infection can take longer.

Bone pain, tenderness, and fever suggest osteomyelitis until proven otherwise. A painful limp and general malaise are also features. However, in immunocompromised and neonatal patients there may be few signs until the condition is well advanced. Pelvic osteomyelitis has a large differential diagnosis that includes psoas abscess, urinary tract infection, hip joint sepsis, appendicitis, and gynaecological conditions.

Blood investigations are: C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), full blood count (FBC), and blood cultures. The CRP is raised in 98% of cases on admission). A normal value does not preclude infection especially in neonates, the immunocompromised or anaemic patients, or in sickle cell disease. The ESR is higher than 20mm/h in 70–90% of cases. The white blood cell count (WCC) may be useful when the differential shows a left shift but it is an unreliable indicator of bone infection, being elevated in only 35–40% of cases. In a young child with bone pain and fever, leukaemia is a possible diagnosis. A blood film and WCC will help determine this diagnosis. Blood cultures are only positive in 30–50% of patients with AHO.

Imaging includes plain radiographs and although these are normal in the initial 7–10 days of AHO, they help with the differential diagnosis (Box 13.1.4). After this early phase there may be periosteal elevation with underlying resorption and/or new bone formation. Other imaging options include the whole body bone scan: a cold scan may indicate early disease or hypoperfusion/osteonecrosis. Magnetic resonance imaging (MRI) is the most sensitive test for osteomyelitis (97–100%) and has a specificity of 73–92%. Furthermore it allows definition of soft tissue extension, intraosseous collection, joint involvement, and aids in planning the surgical approach (Figure 13.1.2).

Bone aspiration can identify the infecting organism accurately in 75–80% but reservations concerning this technique within the overall management are outlined in the following section.

The aims of AHO treatment are to treat promptly and appropriately with IV antibiotics in order to prevent overwhelming sepsis, to cure the local disease as rapidly as possible, and to prevent the secondary sequelae of growth plate damage, pathological fracture, osteonecrosis, deep vein thrombosis, and chronic osteomyelitis.

Antibiotics should be given on the assumption that osteomyelitis is present. It is controversial whether or not the bone should be aspirated first. Some surgeons advocate aspiration before starting parenteral antibiotics. However, aspiration usually involves sedation or a general anaesthetic which delays treatment. Therefore in our institution, we do not routinely aspirate bone before starting antibiotics. Antibiotics are given as soon as the diagnosis is presumed and the child is then investigated with either MRI or bone scan. The bone scan is especially useful if multiple sites of infection are present, or if the site is hard to localize. Beware the ‘cold’ scan which may represent early disease or the coexistence of infection and osteonecrosis. If an abscess is present that requires surgical drainage, specimens are attained at that stage, acknowledging that antibiotic treatment may mean that the culture result is negative.

Conversely, in areas where AHO due to MRSA is common, bone aspiration may be useful. Initial treatment is with vancomycin, but subsequently, antibiotics can be modified if/when organisms other than MRSA are identified.

The patient needs to be monitored closely in a hospital environment and clinical improvement, in terms of vital signs, range of movement, and pain, assessed. Not all children respond well. The role of the orthopaedic surgeon is to establish why and reinvestigate. The patient may require repeat imaging, surgical debridement, or a change in antibiotics. In the situation where the infection is not resolving quickly and the child remains symptomatic, a repeat MRI is used to look for further collections (intraosseous or subperiosteal) or spread of infection. Ultrasound scans can also be useful. Infected deep vein thromboses may be visualized in vessels abutting AHO which may result in septic embolization and require anticoagulation (Figure 13.1.3).

The change in CRP values during treatment for AHO is useful in monitoring control of the infection. In conjunction with the clinical findings, a declining CRP can indicate when a change from IV to oral antibiotics is appropriate. For uncomplicated AHO, the CRP is likely to have normalized by day 9. IV antibiotics may be discontinued once the child has improved clinically and biochemically (often <5–7 days) although oral treatment should be continued for a total of at least 3–4 weeks.

The current controversies relate to duration of treatment and when to switch antibiotics to oral (Box 13.1.5). Prolonging IV therapy requires specialized ‘long lines’ that have their own inherent risks. Unless adequate home support is available the child has to remain an inpatient. Each patient with osteomyelitis is unique because of the factors outlined earlier. Therefore the clinical course must be closely monitored, with antibiotics and surgical interventions used appropriately. In resistant/intractable cases and compromised hosts, IV antibiotics should be continued until all clinical features are resolved (and total antibiotic treatment may need to be extended to 6–12 weeks).

The timing of surgical intervention is crucial to prevent secondary complications (Box 13.1.6). If pus is encountered on aspiration or an abscess localized by MRI (Figure 13.1.4), then surgical debridement is mandatory. Decisions concerning surgical approach are helped by accurate imaging. Occasionally small abscesses in difficult locations can be drained percutaneously with the help of ultrasound or computed tomography (CT) guidance. This is especially useful in pelvic osteomyelitis where, for example, a deep abscess in the iliacus muscle would otherwise require an extensive surgical approach. Rarely does the bone need to be ‘drilled’ as part of the surgical debridement, as the intraosseous collection has already decompressed itself into a subperiosteal collection. Following debridement, large drains are left in the wounds and the skin edges closed loosely with sutures. A ‘second look’,48h later, enables a repeat washout/debridement or definitive closure to be performed. If a large amount of pus is still present, the wound is left open and a suction dressing applied.

Depending on the clinical course, patients are seen regularly after discharge, for a clinical and radiographic review and blood tests (WCC, ESR, CRP). For patients on flucloxacillin, weekly liver function tests and neutrophil counts are advisable, as there can be a high rate of flucloxacillin-associated neutropenia (Box 13.1.7).

A patient with septic arthritis usually presents with signs of sepsis and a hot swollen joint with restricted movement due to pain. Swelling in some joints such as the hip is difficult to detect. For pyogenic septic arthritis the onset and progress is usually rapid and severe. The joint rests in the position of maximum capsular laxity. Such a picture is indicative of a septic arthritis until proven otherwise by appropriate investigations (usually aspiration and microscopy). The picture may be less overt if the patient is on steroids or has been on antibiotics. Limb pseudoparalysis in a neonate, often without overt signs of sepsis, is a typical presentation of septic arthritis.

Aspiration is the key to diagnosis with fluid being sent for microscopy, culture, and cell count. Preoperative investigations should include FBC, CRP, ESR, blood cultures, and plain radiographs. In certain circumstances, ancillary imaging (ultrasound, MRI, and bone scan) can help localization, indicate an effusion, or define an associated osteomyelitis. However, these tests should not delay aspiration. An ultrasound can confirm the presence of an effusion but cannot distinguish between infected and sterile fluid.

Microscopy of the aspirated fluid may show organisms, confirming the diagnosis. If no organisms are seen, a WCC of more than 40.0 × 10⁹/L has greater than 90% sensitivity and specificity in detecting a septic arthritis. However, WCC of greater than this value may be found in acute inflammatory arthritis. Furthermore, if the disease is detected early or has been partially treated with antibiotics, the WCC may be equivocal. If there is any doubt, proceed to immediate washout.

Differentiating between transient synovitis and septic arthritis can be difficult. Four factors are useful predictors to aid clinical judgement: history of fever, inability to weight-bear, ESR greater than 40mm/h, and WCC greater than 12.0 × 10⁹/L. The CRP rises more rapidly than the ESR in early infection and therefore, is often preferred (Box 13.1.8).

A septic joint must be washed out urgently by arthrotomy with high volumes of fluid. For the hip, a Smith–Petersen approach is preferred so that the posterior vessels supplying the femoral head are protected. Tissue biopsy should be performed for histology as well as microbial culture. Parenteral antibiotics, usually for 3–4 weeks, should cover the most likely organisms, and are then adjusted to culture results. The CRP is a useful guide to resolution. There is a trend towards a reduced duration of IV antibiotics.

Antibiotics have good joint penetration after surgical washout. If the condition is diagnosed and treated early, the prognosis is good. Neonatal sepsis, particularly of the hip, has a worse prognosis and may be complicated by limb length discrepancy, angular deformity, avascular necrosis, and joint dislocation.

Patients with sickle disease have an increased risk of osteomyelitis, and are more prone to complications. It can be difficult to distinguish a sickle crisis from AHO. Fever and raised ESR are suggestive of infection. Staphylococcus aureus remains the most common pathogen in sickle cell disease but Salmonella must also be covered.

Osteomyelitis is considered chronic if it has been present for more than 3 months. The definition of a sequestrum, by both CT and MRI scanning, is important with the aim of aggressive surgical debridement of infected material and sequestrectomy.

Primary osteomyelitis of the epiphysis is rare and usually subacute. It presents with mild pain, minimal decrease in function, and little in the way of systemic reaction. The differential diagnosis is large and a gadolinium-enhanced MRI scan is useful in making the diagnosis, localizing and defining the extent of infection, and planning the surgical approach if required (Figure 13.1.6).

Neisseria gonorrhoeae is a common cause of septic arthritis in the sexually active population. The major differential includes Reiter’s disease. It is usual for more than one joint to be involved. The aspirate WCC is not as high as in pyogenic arthritis. Appropriate antibiotics are IV penicillin or third-generation cephalosporins for resistant strains.

This injury is common in children: washout, debridement, and tetanus prophylaxis are required. Staphylococcus is the commonest infecting organism. Pseudomonas which has a predilection for cartilage may also be involved, causing a stubborn infection that requires surgical debridement and IV antibiotics.

Infection of the sacroiliac joint presents with pain, abnormal gait, and pyrexia and may be mistaken for a variety of other conditions. Flexion–abduction–external rotation test and direct sacroiliac compression (with the patient prone) are useful in localizing pathology to the sacroiliac joint. A bone scan is also helpful (Figure 13.1.7). Blood, stool, and urine cultures should be obtained. Anti-staphylococcal IV antibiotics are the treatment of choice, and surgical debridement/drainage is not usually necessary.

Panton–Valentine leukocidin (PVL) is a virulence factor secreted by certain strains of Staphylococcus aureus. PVL is capable of destroying host neutrophils, and has also been associated with increased intravascular coagulopathy. PVL is thought by some authorities to be responsible for increased severity of sepsis. In practice, PVL status has not yet been proven to be a useful guide to treatment.