Osteomyelitis

OBJECTIVES

The learner will be better able to:
1) Explain the principles reflected in the history and discovery of osteomyelitis
2) Generate a differential diagnosis given a patient case
3) Describe the distinct features of a radiological image of a patient with osteomyelitis
4) Explain factors that influence the incidence of osteomyelitis in a given population
5) Identify classic features of osteomyelitis given a patient case

INTRODUCTION

Osteomyelitis is defined as an inflammation or an infection in the bone marrow and surrounding bone. The disease is classified as either acute or chronic, depending on the length of time the infection or symptoms persist. Symptoms include pain, warmth and/or swelling in the bone. Chronic osteomyelitis may last for years , with slow death of bone tissue from a reduced blood supply. Signs and symptoms may be absent, however, causing difficulty in diagnosing the chronic infection.

Pathogens infect bone in posttraumatic osteomyelitis after a recent fracture. Bacteria, fungus and other microorganisms are typically the causative agents. The more susceptible a bone is to fracturing, the greater the chances of becoming infected and developing disease. Trauma from recent injuries and diabetes are major risk factors for osteomyelitis.The bone can be directly infected from the wound or indirectly via the blood from another site of infection, called hematogenous osteomyelitis. The vertebrae and pelvis are often affected in adults in this blood-borne variety, while children are usually affected in long bones.

EPIDEMIOLOGY

The incidence of osteomyelitis after open fractures is reported to be 2% to 16%, depending significantly on the grade of trauma and the type of treatment administered. Prompt and thorough treatment help reduce the risk of infection, decreasing the probability of developing osteomyelitis. This is particularly important for patients with the following risk factors: diabetes, altered immune states and recent trauma. The tibia is the most frequent site of posttraumatic osteomyelitis , since it is the most vulnerable bone with the least vigorous blood supply in the body.

The classification of osteomyelitis can be broken down into the following categories: exogenous ostemyelitis (47%), secondary to vascular insufficiency (34%) and hematogenous osteomyeltis (19%). The implantation of an orthopedic device (pins, plate, screws, artificial joint) can also seed infection as a nidus for pathogens, and therefore create post-operative osteomyelitis.

The growing skeleton is also at risk. Any bone can be affected but it is usually the weight-bearing bones before the physis has closed. At the physis on the metaphyseal side, end arteries form a capillar loop which may rupture following minor trauma. This region of blood stasis may attract circulating bacteria ("everybody has bacteria circulating, periodically" -HH Jones) . Once escaped through the vascular system, bacteria can set up shop in surrounding tissues.

ETIOLOGY

The presence of bacteria alone in an open fracture is not sufficient to cause osteomyelitis. In most cases, the body's immune system is capable of preventing the colonization of pathogens. The micro-environment determines whether infection occurs. The timing and extent of treatment are critical in determining whether infection develops. The likelihood of developing ostemyelitis increases with impaired immune function, extensive tissue damage, or reduced blood supply to the affected area. Patients with diabetes, poor circulation or low white blood cell count are at greater risk.

Bacterial or fungal infection cause most osteomyelitis. Infection induces a large polymorphonuclear response from bone marrow, particularly staphylococcus aureus, streptococcus and haemophilus influenza. Staphylococcus infection predominates today and before the era of antiobiotics.

CLINICAL DIAGNOSIS AND MANIFESTATIONS

The diagnosis of osteomyelitis is made from clinical, laboratory and imaging studies. When the skeletal system is involved, pain, fever and leukocytosis (an increase in white blood cell count due to infection or inflammation) occur. The affected area is painful. Initial x-rays are typically normal. As early as 4 days, an area of lucency may be seen on x-ray.
Usually, the changes are not recognized until 10 days or two weeks have passed. Subperiosteal new bone formation in the affected area is present, representing periosteal elevation from encroaching pus. If not successfully treated, pus enlarges the bone appearing as increased lucency, which surrounds sclerotic, dead bone . This inner dead bone is called the sequestrum (sequestered from blood supply), and the outer periosteal reaction laminates to form the involucrum. (See diagram at right.)
Draining sinuses develop when the pressure of pus exceeds the containment of the soft tissue. This further deprives the bone of its blood supply. This in turn harbors more bacteria, and the process cannot be reversed until extensive debridement of the area occurs-until the environment changes to one that promotes healing.

DIFFERENTIAL DIAGNOSIS

Ewing sarcoma
Osteosarcoma
Reactive bone marrow edema
Traumatic or stress fractures
Inflammatory arthritis
Gout

SUMMARY

Osteomyelitis is an infection involving the bone. The bones usually affected are the weight-bearing bones, particularly before the physis has closed. Exogenous osteomyelitis occurs from open trauma, sometimes relatively minor in nature. Hematogenous osteomyelitis occurs from bacteria circulating in the bloodstream. Acute and chronic subtypes are classified according to the timing and duration of the infection.

REFERENCES

1. Dirschl DR, Almekinders LC. Osteomyelitis. Drugs. 1993; 45: 29-43.

2. Ehara S. Complications of skeletal trauma. Radiol Clin North Am. 1997; 35: 767-781.

3. Sammak B, Abd El Bagi M, Al Shahed M, et al. Osteomyelitis: a review of currently used imaging techniques. Eur Radiol. 1999; 9: 894-900.

4. Waldvogel F, Medoff G, Swartz M. Osteomyelitis: a review of clinical features, therapeutic considerations and unusual aspects (I). N Engl J Med. 1970; 282: 198-206.

5. Widmer AF. New developments in diagnosis and treatment of infection in orthopedic implants. Clin Infect Dis. 2001; 33: S94-S106.