Osteomyelitis

infection of bone. In older children the infecting organism is usually Staphylococcus aureus whereas in neonates and infants group A beta haemolytic streptococcus is common. Mycobacterium, Gram-negative bacteria, syphilis, fungal and viral agents are less frequent causes of osteomyelitis. Salmonella osteomyelitis may occur particularly in children with sickle cell disease. Local trauma may reduce host resistance and predispose to osteomyelitis.

The spread of infection is usually haematogeneous and the commonest site is the metaphysis where blood flow slows in the sinusoids allowing bacteria to adhere to the vascular membranes. Infection commonly spreads from the primary intramedullary focus via the Haversian canals of the cortex to the subperiosteal space forming a subperiosteal abscess. If this ruptures the infection extends into the overlying soft tissues.

Osteomyelitis may be acute, subacute or chronic. With acute osteomyelitis the presenting complaint is usually local pain, swelling and warmth often with associated fever and malaise. Clinical examination reveals pyrexia, local erythema and tenderness. Investigations typically show leukocytosis and an elevated C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR).

Imaging

Radiographs performed early in the course of the illness may show subtle deep soft tissue swelling or oedematous subcutaneous soft tissues but radiographs are often normal in the first 7-10 days. By 10-14 days focal lucency develops in the metaphysis progressing to lytic destruction with associated focal periosteal new bone formation.

Ultrasound is useful in the acute phase and will define the presence and extent of any subperiosteal abscess collection which will require aspiration or drainage, i.e. ultrasound can be helpful in planning surgery.

Bone scintigraphy will show focal uptake at the affected site (Fig.1) and is particularly of value in order to look for other sites of infection as multifocal osteomyelitis may occur, especially in neonates. False negative bone scans may be seen if performed early in the infection or in young babies less than 6 weeks of age. In these cases 67Ga-citrate, (Fig.2) ¹¹¹In-labelled or ^99mTc-HMPAO-labelled leukocyte scans may be more sensitive.

MR imaging shows marrow oedema and the extent of subperiosteal abscess collection. Intravenous gadolinium DTPA enhanced T1-weighted sequences allow differentiation of enhancing hyperaemic inflammatory tissue from central pockets of nonenhancing pus. In infants the disease is generally milder than in older children, subperiosteal abscess formation is more frequent and rapid and rupture into the soft tissues more common. In neonates vascular channels penetrate the cartilaginous epiphysis and allow infection to spread from the metaphysis into the joint causing adjacent septic arthritis, this being demonstrable either on ultrasound or MRI.

A Brodies abscess is a localized form of osteomyelitis which presents in a subacute stage without preceding acute symptoms. Histology shows an intraosseous abscess cavity lined by granulation tissue. The infecting organism is usually Staphylococcus aureus. The condition typically presents with relatively mild pain which is recurrent over several months or sometimes years. The most common sites are the tibial or femoral metaphysis or diaphysis. The infection may cross the growth plate (Fig.3). Local soft tissue swelling is minimal and there is no soft tissue mass. Radiographs in a typical case show a well defined longitudinally orientated ovoid lucency with surrounding sclerotic margin but little or no periosteal new bone formation.

Complications of osteomyelitis include sequestrum formation with recurrent relapses, skin sinus formation, damage to the growth plate causing tethering, physeal bar, and subsequent growth deformity. See Garrè's sclerosing osteomyelitis.

HC-GL

GL