Differentials
Septic arthritis
SIGNS / SYMPTOMS
More prevalent in children.
Inflammatory signs specifically around the joint. Pain is worse on movement of the joint in septic arthritis. Osteomyelitis can co-exist with septic arthritis.
INVESTIGATIONS
Analysis of joint aspirate will demonstrate raised white cell count and infecting organisms.
X-ray may show joint effusion.
Juvenile idiopathic arthritis
SIGNS / SYMPTOMS
Common in children.
Chronic illness > six weeks.
Associated with a rash and morning stiffness.
INVESTIGATIONS
Antinuclear antibodies (ANA) are detected in about one third of children.[58]
Analysis of joint aspirate will demonstrate sterile effusion.
Transient synovitis
SIGNS / SYMPTOMS
A self-limiting inflammatory disorder of the hip that commonly affects young children between 2 and 12 years of age. More common in boys.
Presents acutely with mild to moderate hip pain and limp. Often follows a viral illness.
INVESTIGATIONS
Inflammatory markers may be normal, or slightly raised, whereas in acute osteomyelitis they would be more markedly elevated.
X-ray is typically normal; however, may reveal subtle signs early in the disease process, such as capsular distension, joint space widening, diminution of the definition of soft tissue planes around the hip joint, or slight de-mineralisation of the bone of the proximal femur; ultrasound and MRI may show joint effusion.
Reactive arthritis
SIGNS / SYMPTOMS
A patient may give a history of an antecedent genitourinary or dysenteric infection 1 to 4 weeks before the onset of arthritis. Presenting features include systemic symptoms such as fever, peripheral and axial arthritis, enthesitis, dactylitis, conjunctivitis and iritis, and skin lesions including circinate balanitis and keratoderma blennorrhagicum.
The peripheral arthritis in reactive arthritis (ReA) is usually an asymmetrical oligoarticular arthritis affecting the large joints of the lower limb, although monoarticular and polyarticular arthritis can also occur.
INVESTIGATIONS
There is no specific test for diagnosing ReA.
Synovial fluid may be sterile in cases where reactive effusions are present secondary to juxta-articular osteomyelitis, or may grow the infecting organism.
X-ray may show joint effusion.
Slipped capital femoral epiphysis (SCFE)
SIGNS / SYMPTOMS
Also known as slipped upper femoral epiphysis (SUFE), an adolescent patient typically presents with an acute/insidious onset of pain and limp and external rotation on walking.
Associated systemic disease is common with obligatory external rotation on hip flexion.
INVESTIGATIONS
Plain anteroposterior/frog-leg lateral x-rays show the Klein line not intersecting the femoral head in SCFE.
In osteomyelitis, WBC, CRP, and ESR will be elevated and blood cultures may be positive for infective organism.
Legg-Calvé-Perthes' disease
SIGNS / SYMPTOMS
Children with Perthes' disease usually complain of pain in the groin, the thigh or the knee - particularly after physical activity. They limp and have a restricted range of movement (stiffness) of the hip joint.
INVESTIGATIONS
Bilateral hip x-rays show femoral head collapse and fragmentation in one hip. Bone scintigraphy shows a cold spot in the affected hip in Perthes' disease.
Cellulitis
SIGNS / SYMPTOMS
Cellulitis is more superficial. A patient is usually more unwell if they have osteomyelitis.
INVESTIGATIONS
X-ray may show bone lucency or periosteal reaction after 6 to 7 days in osteomyelitis but may be normal in the early stages of osteomyelitis.
Necrotising fasciitis
SIGNS / SYMPTOMS
Skin inflammation, swelling, discolouration; skin necrosis, bruising or gangrene and numbness, bullous lesions, or pain out of proportion to clinical signs and extending beyond the skin redness.[59]
Subcutaneous tissue may feel hard and wooden, extending beyond the area of apparent skin involvement.[59] A wide red tract may be present indicating the route of the infection proximally.[59] Crepitus indicates gas in the tissues.[59] Often associated with high fever, disorientation and lethargy.[59]
INVESTIGATIONS
Subcutaneous gas may be visible on x-ray, ultrasound or MRI in necrotising fasciitis.
X-ray may show bone lucency or periosteal reaction after 6 to 7 days in osteomyelitis, but it may be normal in the early stages of osteomyelitis.
Metastatic bone cancer or primary bone tumour
SIGNS / SYMPTOMS
History of bone cancer, systemic symptoms (e.g., weight loss), a known primary lesion, and/or bone pain.
INVESTIGATIONS
X-rays of the area of suspected infection would not demonstrate typical sequestration and involucrum formation.
Conventional features of osteosarcoma are destruction of normal trabecular bone pattern, a mixture of radiodense and radiolucent areas, periosteal new bone formation, and formation of Codman's triangle (triangular elevation of periosteum). If no abnormalities are visible on plain x-rays, MRI of the entire length of involved bone is indicated.[60][61]
Cultures examined through 1 or more commonly used methods of testing would confirm the absence of a bacterial pathogen. If tumour is suspected, a preoperative image-guided bone biopsy may confirm this. However, a final diagnosis may only be made definitively at surgery.
Old or new trauma
SIGNS / SYMPTOMS
History of trauma and absence of fever, erythema, and/or inflammation.
There can often be a history of incidental trauma prior to presentation with osteomyelitis.
INVESTIGATIONS
X-ray findings suggestive of trauma.
Non-infected non-union
SIGNS / SYMPTOMS
History of fracture and absence of fever, erythema, and/or inflammation.
INVESTIGATIONS
X-ray changes in non-infected traumatic bone are difficult to differentiate from infected bone because of distortion of bone architecture after fractures or multiple surgeries.[62]
If plain x-rays are inconclusive, MRI will provide a more detailed image of bone inflammation and soft-tissue damage than CT or radionuclide scanning. MRI is of limited value around metallic implants, limiting its usefulness in suspected post-traumatic infections. Fluorodeoxyglucose positron emission tomography-CT may have a role.
Aseptic loosening of implants
SIGNS / SYMPTOMS
Absence of fever, erythema, and/or inflammation.
INVESTIGATIONS
Differentiating infection from aseptic loosening is difficult because both are similar at clinical and histopathological examination.
Radiolucent areas in x-rays will indicate loosening of orthopaedic hardware that may result in pain, swelling, and other symptoms.
Cultures will confirm whether an infection is present.
Radionuclide scan at suspected infection site can be useful in ruling out infection, but is complicated by variations in prosthetics.
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