Osteomyelitis - Musculoskeletal Infections

Musculoskeletal Infections

OSTEOMYELITIS

Osteomyelitis is an infection of the bone. The bone becomes infected by one of three modes:

·      Extension of soft tissue infection (eg, infected pressure or vascular ulcer, incisional infection)

·      Direct bone contamination from bone surgery, open fracture, or traumatic injury (eg, gunshot wound)

·      Hematogenous (bloodborne) spread from other sites of infection (eg, infected tonsils, boils, infected teeth, upper respiratory infections). Osteomyelitis resulting from hema-togenous spread typically occurs in a bone area of trauma or lowered resistance, possibly from subclinical (nonapparent) trauma.

Patients who are at high risk for osteomyelitis include those who are poorly nourished, elderly, or obese. Also at risk are pa-tients with impaired immune systems, those with chronic illness (eg, diabetes, rheumatoid arthritis), and those receiving long-term corticosteroid therapy.

Postoperative surgical wound infections occur within 30 days after surgery. They are classified as incisional (superficial, located above the deep fascia layer) or deep (involving tissue beneath the deep fascia). If an implant has been used, deep postoperative in-fections may occur within a year. Deep sepsis after arthroplasty may be classified as follows:

Stage 1, acute fulminating: occurring during the first 3 months after orthopedic surgery; frequently associated with hema-toma, drainage, or superficial infection

Stage 2, delayed onset: occurring between 4 and 24 months after surgery

Stage 3, late onset: occurring 2 or more years after surgery, usually as a result of hematogenous spread

Bone infections are more difficult to eradicate than soft tissue infections because the infected bone becomes walled off. Natural body immune responses are blocked, and there is less penetration by antibiotics. Osteomyelitis may become chronic and may affect the patient’s quality of life.

Pathophysiology

Staphylococcus aureus causes 70% to 80% of bone infections.Other pathogenic organisms frequently found in osteomyelitis include Proteus and Pseudomonas species and Escherichia coli. The incidence of penicillin-resistant, nosocomial, gram-negative, and anaerobic infections is increasing.

The initial response to infection is inflammation, increased vascularity, and edema. After 2 or 3 days, thrombosis of the blood vessels occurs in the area, resulting in ischemia with bone necro-sis. The infection extends into the medullary cavity and under the periosteum and may spread into adjacent soft tissues and joints. Unless the infective process is treated promptly, a bone abscess forms. The resulting abscess cavity contains dead bone tissue (the sequestrum), which does not easily liquefy and drain. Therefore,the cavity cannot collapse and heal, as occurs in soft tissue ab-scesses. New bone growth (the involucrum) forms and surrounds the sequestrum. Although healing appears to take place, a chron-ically infected sequestrum remains and produces recurring ab-scesses throughout the patient’s life. This is referred to as chronic osteomyelitis.

Clinical Manifestations

When the infection is bloodborne, the onset is usually sudden, occurring often with the clinical manifestations of septicemia (eg, chills, high fever, rapid pulse, general malaise). The systemic symptoms at first may overshadow the local signs. As the infec-tion extends through the cortex of the bone, it involves the peri-osteum and the soft tissues. The infected area becomes painful, swollen, and extremely tender. The patient may describe a con-stant, pulsating pain that intensifies with movement as a result of the pressure of the collecting pus. When osteomyelitis occurs from spread of adjacent infection or from direct contamination, there are no symptoms of septicemia. The area is swollen, warm, painful, and tender to touch.

The patient with chronic osteomyelitis presents with a con-tinuously draining sinus or experiences recurrent periods of pain, inflammation, swelling, and drainage. The low-grade infection thrives in scar tissue, because it has a reduced blood supply.

Assessment and Diagnostic Findings

In acute osteomyelitis, early x-ray findings demonstrate soft tis-sue swelling. In about 2 weeks, areas of irregular decalcification, bone necrosis, periosteal elevation, and new bone formation are evident. Radioisotope bone scans, particularly the isotope-labeled white blood cell (WBC) scan, and magnetic resonance imaging (MRI) help with early definitive diagnosis. Blood stud-ies reveal elevated leukocyte levels and an elevated sedimentation rate. Wound and blood culture studies are performed to identify appropriate antibiotic therapy.

With chronic osteomyelitis, large, irregular cavities, raised periosteum, sequestra, or dense bone formations are seen on x-ray. Bone scans may be performed to identify areas of infection. The sedimentation rate and the WBC count are usually normal. Anemia, associated with chronic infection, may be evident. The abscess is cultured to determine the infective organism and ap-propriate antibiotic therapy.

Prevention

Prevention of osteomyelitis is the goal. Elective orthopedic surgery should be postponed if the patient has a current infection (eg, urinary tract infection, sore throat) or a recent history of in-fection. During orthopedic surgery, careful attention is paid to the surgical environment and to techniques to decrease direct bone contamination. Prophylactic antibiotics, administered to achieve adequate tissue levels at the time of surgery and for 24 hours after surgery, are helpful. Urinary catheters and drains are re-moved as soon as possible to decrease the incidence of hematoge-nous spread of infection.

Treatment of focal infections diminishes hematogenous spread. Aseptic postoperative wound care reduces the incidence of superficial infections and osteomyelitis. Prompt management of soft tissue infections reduces extension of infection to the bone. When patients who have had joint replacement surgery undergo dental procedures or other invasive procedures (eg, cystoscopy), prophylactic antibiotics are frequently recommended.

Medical Management

The initial goal of therapy is to control and halt the infective process. Antibiotic therapy depends on the results of blood and wound cultures. Frequently, the infection is caused by more than one pathogen. General supportive measures (eg, hydration, diet high in vitamins and protein, correction of anemia) should be in-stituted. The area affected with osteomyelitis is immobilized to decrease discomfort and to prevent pathologic fracture of the weakened bone. Warm wet soaks for 20 minutes several times a day may be prescribed to increase circulation.

PHARMACOLOGIC THERAPY

As soon as the culture specimens are obtained, IV antibiotic ther-apy begins, based on the assumption that infection results from a staphylococcal organism that is sensitive to a semisynthetic penicillin or cephalosporin. The aim is to control the infection before the blood supply to the area diminishes as a result of thrombosis. Around-the-clock dosing is necessary to achieve a sustained high therapeutic blood level of the antibiotic. An anti-biotic to which the causative organism is sensitive is prescribed after results of the culture and sensitivity studies are known. IV anti-biotic therapy continues for 3 to 6 weeks. After the infection ap-pears to be controlled, the antibiotic may be administered orally for up to 3 months. To enhance absorption of the orally admin-istered medication, antibiotics should not be administered with food.

SURGICAL MANAGEMENT

If the patient does not respond to antibiotic therapy, the infected bone is surgically exposed, the purulent and necrotic material is removed, and the area is irrigated with sterile saline solution. Antibiotic-impregnated beads may be placed in the wound for di-rect application of antibiotics for 2 to 4 weeks. IV antibiotic ther-apy is continued.

In chronic osteomyelitis, antibiotics are adjunctive therapy to surgical débridement. A sequestrectomy (removal of enough in-volucrum to enable the surgeon to remove the sequestrum) is per-formed. In many cases, sufficient bone is removed to convert a deep cavity into a shallow saucer (saucerization). All dead, infected bone and cartilage must be removed before permanent healing can occur. A closed suction irrigation system may be used to remove debris. Wound irrigation using sterile physiologic saline solution may be performed for 7 to 8 days.

The wound is either closed tightly to obliterate the dead space or packed and closed later by granulation or possibly by grafting. The débrided cavity may be packed with cancellous bone graft to stimulate healing. With a large defect, the cavity may be filled with a vascularized bone transfer or muscle flap (in which a muscle is moved from an adjacent area with blood supply intact). These microsurgery techniques enhance the blood supply. The improved blood supply facilitates bone healing and eradication of the infection. These surgical procedures may be staged over time to ensure healing. Because surgical débridement weakens the bone, internal fixation or external supportive devices may be needed to stabilize or support the bone to prevent pathologic fracture.