Osteomyelitis   

Osteomyelitis is an infection of medullary bone that results in the progressive inflammatory destruction of bone and the apposition of new bone. Pus spreads in vascular channels, increases intraosseus pressure and causes a decrease in blood flow. Ischemic necrosis follows and the devascularized bone is known as sequestra.
     
In children Hematogenous spread of infection is common and presents as a high fever, malaise, local pain and swelling. The plain radiographs show a central lytic defect with surrounding sclerosis, termed a Brodie's abcess.  The clinical picture may be equivocal, and the concern about the possibility of a tumor may arise.  The most common organism in patients over the age of three is coagulase positive staph aureus. Hematogenous osteomyelitis occurs most commonly in the distal femur, proximal tibia, proximal femur and proximal humerus, all areas of rapid growth and trauma.
    In adults hematogenous  osteomyelitis is rare except IV drug users and the elderly. The clinical presentation can be quite deceptive, since  fever, elevated white count, and history of a possible source of infection are often lacking.  In addition, the relatively aggressive radiographic appearance of the lesion may give rise to concern about a primary or metastatic bone tumor. IV drug users often have unusual organisms such as pseudomonas. Older patients may have gram negative bacteria in the spine secondary to organisms that originate as a urinary tract infection and travel through Batson's plexus. Spinal osteomyelitis may present as back pain with negative blood cultures.
     Osteomyelitis may be caused by direct innoculation secondary to trauma or surgery. Osteomyelitis secondary to trauma is often poly- microbial. Complete immobilization maybe necessary to protect the vascular channels necessary to promote healing.
     Osteomyelitis can also be the result of contiguous spread from an abscess or sinus tract. Chronic osteomyelitis may occur secondary to syphilis and can cause bony destruction known as a gumma. Squamous cell carcinoma is a known complication of chronic long term draining fistula sites due to osteomyelitis.
     Suspected osteomyelitis may not be positive on plain x-ray initially. Later, a mixed lytic and sclerotic lesion is seen, which has a wide zone of transition and a variable amount of periosteal reaction.  The lesion may appear quite aggressive by tumor criteria.  Bone scan has poor specificity. CT scan or MRI may show edema, medullary destruction, periosteal reaction, soft tissue mass or damage or articular involvement. On MRI scan, T1 weighted images demonstrate infection as a low signal with ill defined margins. T2 images show infection as a bright signal. MRI shows marrow replaced by edema and inflammatory cells but is not useful if hardware is present.
     Microscopic examination reveals micro-organisms, neutrophils, congested or thrombosed blood vessels, and necrotic bone. Sequestra has no living osteoblasts within lacunae.
     Treatment for osteomyelitis is difficult. l Surgical sampling or needle biopsy is necessary for diagnosis. Infected hardware should be removed if the bone is healed and stable. Acute osteomyelitis is treated with irrigation and debridement as necessary,  followed by four to six weeks of antibiotics. Chronic osteomyelitis is best treated with thorough debridement, antibiotics,  and local flap coverage if necessary.

References
Lew, DP and FA Waldvogel, Osteomyelitis, New England Journal of Medicine, 336(14):999-1007, April 3, 1997.

Deely, DM and ME Schweitzer, MR Imaging of Bone Marrow Disorders,
Radiologic Clinics of North America, 35(1):193-211, January, 1997.

Bullough, Peter, Orthopedic Pathologv (third edition), Times Mirror International Publishers Limited, London, 1997.

Huvos, Andrew. Bone Tumors: Diagnosis. Treatment and Prognosis, W.B.
Saunders, Co., 1991.

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