Postoperative Infections

Etiology

Postoperative infection of the hip and knee may occur in a myriad of clinical settings, including systemic infection or illness, blunt or penetrating trauma, or following medical intervention. Infection is both a primary complication and a medium through which deleterious effects may impact bone, muscle, neurovascular, and soft tissue as well as implanted materials. Complications may occur if there is delayed diagnosis of an infection, inadequate antimicrobial treatment of a confirmed infection, or compromise of the patient's soft tissues or bones by an offending organism, despite appropriate treatment.

Prevalence and Epidemiology

At least 250,000 joint replacements are performed annually, with infection rates estimated to be 1% to 2% for primary joint replacement and 3% to 4% for revision surgery. Infection is the most common cause of failure in revision hip arthroplasty, occurring in 30% of revised hips. It is estimated that treatment of infection after arthroplasty is at least $50,000/€38,500 per patient. Allograft infections used in the knee may be secondary to contamination with exogenous microorganisms or to microbes present within the tissue at the time of harvest, and are occasionally seen in patients who have undergone cruciate ligament reconstruction. Those with diabetes are particularly susceptible to musculoskeletal infection (see Chapter 65 ), especially after arthroplasty.

Clinical Presentation

The clinical presentation of postoperative hip and knee infections is varied, but most patients will present with pain. In the setting of arthroplasty, persistent rest or night pain or progressive stiffness should raise the concern for deep infection. Patient history and blood laboratory results (elevated C reactive protein and peripheral white blood cell count) suggest periprosthetic infection but may be difficult to diagnose radiographically; radiographs of an infected joint can be normal. Systemic disease, including deep vein thrombosis, must be considered when patients develop sudden swelling of an infected extremity; septic pulmonary emboli can be life-threatening. Deep vein thrombosis can be detected quickly and noninvasively with ultrasonography, and pulmonary emboli using enhanced CT, ventilation/perfusion scanning, or pulmonary angiography. When restricted to the musculoskeletal system, postoperative infection can lead to failure of soft tissues, infection requiring additional intervention (drainage, surgical débridement, revision, or amputation), prosthesis failure, graft removal, and exceptional tumor.

Imaging Techniques

Examination for possible complications of musculoskeletal infection begins with radiography, which can identify osseous (erosions, lysis) and soft tissue abnormalities (gas, foreign bodies). CT is more sensitive to subtle osseous changes and focal abscesses. MRI is helpful in evaluating subtle soft tissue and osseous complications, but it may be degraded by artifacts related to metallic hardware. Ultrasonography may detect complications of fluid collections and is rapid and widely available for use in guided therapy for complications, such as soft tissue biopsy and fluid collection drainage. It has also been shown to be useful in the diagnosis and treatment of stump neuromas, occurring after amputation for infection (corticosteroid, phenol injection). Scintigraphy is invaluable for the diagnosis of primary musculoskeletal infection but can be nonspecific. Preliminary research on fluorodeoxyglucose-labeled positron emission tomography (FDG PET) indicates that FDG accurately detects musculoskeletal infection. PET scans also can detect hypermetabolic processes, such as tumors occurring secondary to chronic postoperative osteomyelitis or draining sinus tracts.

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