3.1. Epidemiology and Clinical Relevance
The prevalence of infection of prosthetic hip joints is 0.5-1.3%. Prosthetic knee joints have a prevalence of infection of 1.3-2.9%. Elbows have an infection rate of approx 9%, whereas that of shoulders, wrist, and ankles is approx 1-2%. From 3550% of the patients with an infected joint present within 3 mo of surgery with increasing pain both at rest and with activity (6). Another 30-35% of the patients present from 3-24 mo postoperatively with joint pain. Infection also may spread hematogenously from a distant focus to the joint as well. The average age of patients with an infected prosthetic hip is 67 yr. Elderly patients frequently have rheumatoid or degenerative joint disease (7). Pressure ulcers and urinary tract infections are particularly likely to occur in the elderly and serve as sources of transient bacteremia, which can seed a prosthetic joint.
Elderly patients often require prosthetic joints because of degenerative or rheumatoid arthritis, or fractures. Patients often present complaining of pain (>90%) or altered mobility. Signs of systemic toxicity or fever are often absent. Only 40-45% of elderly patients with prosthetic joint infection may have fever (7). Drainage or erythema may be present at the site of the incision, but infections can be quite indolent (8).
Radiographic signs of loosening of the prosthesis are seen in two thirds of late infections but in less than 50% of early infections. Arthrography (after hip aspiration) may be helpful for determining loosening of the cemented components by showing penetration of the dye between cement and bone. Radiographic and bone scans may fail to distinguish between a low-grade infection and aseptic loosening of the prosthesis. A bone scan following hip surgery may take as long as 6 mo to become normal in the area of the lesser trochanter. The area around the acetabulum and the tip of the prosthesis may show increased activity for up to 2 yr. Infection on bone scan generally shows a more diffuse pattern of abnormality along the prosthetic shaft, and it is usually positive on the two early phases of the scan. False-positive scans may occur in up to 40% and false-negative findings in 5-10% of cases. A gallium scan would suggest osteomyelitis by a focal uptake. However, any inflammatory bone process may cause increased uptake. White blood cell scans are more specific for making a definitive diagnosis of infection but may be less sensitive (9).
The erythrocyte sedimentation rate (ESR) may be elevated in greater than 85% of the patients with infection. If 30-35 mm/h is used as the threshold for infection, then the sensitivity rate will range from 61-88%. With ESR below 30 mm/h, those infections are unlikely and the specificity rate will range from 96-100%. However, ESRs may not accurately diagnosis those patients with an infection in the early postoperative period (8). The C-reactive protein (CRP) is an acute phase protein that can be used to follow the course of acute infections. It rises and falls faster than the ESR. If 10 mg of CRP are used as the threshold for a total hip infection, the specificity and sensitivity for CRP are both about 90%. The CRP should return to normal in about 2-3 wk after hip replacement. The fluid obtained for culture usually has visible organisms on Gram stain in about 30% of cases, and pathogens are recovered by culture in about 85-98% of cases, depending on whether or not the patient had previously received antibiotics. The criteria for infection on histology or frozen section is that there must be 5-10 white blood cells per high-powered field. The sensitivity is about 85% using this criteria, and the specificity is 99% when 10 white blood cells per high powered field are noted. Staphylococci are isolated from about 50% percent of patients with infected prosthetic joints with S. aureus as the most common pathogen in most series, particularly if hema-togenous infections are included. In a series of elderly patients only, coagulase-nega-tive staphylococci and S. aureus were isolated in nearly equal frequency; enterococci were isolated as the third most common isolate (7). Gram-negative bacteria, including Pseudomonas, can occasionally be present. Other fastidious bacteria or mycobacteria can be present and cause indolent infections (8).
To eradicate infection, usually removable of the prosthesis is required (7,8). This may be difficult, as elderly patients are often poor surgical candidates for repeated prosthetic joint procedures. Debridement and retaining of the components can have a high success rate (>50%) in well selected patients, i.e., those in whom the interval between onset of symptoms and surgical intervention is short (2-5 d) (10). Following debridement, antibiotics should be continued for at least 6 wk. If a one-stage reimplantation procedure is performed, it is essential that all foreign material be removed. Many surgeons choose to implant the femoral component with antibiotic-laden cement (11). Prolonged antibiotic therapy should be considered if one-stage procedures are chosen. More often, a two-stage procedure, in which a new prosthesis is implanted after 4-6 wk of antibiotics are given, is chosen (8). Success rates vary from 70-100% for hips, and 85-90% for knees. If a virulent organism such as a Gram-negative bacilli has been isolated, some clinicians advise the delay of up to 1 yr before implanting a new hip prosthesis. As stated earlier, the original prosthesis can be salvaged by debridement and intensive antibiotic therapy in select cases (8,10,12). This approach is successful only when the infection has been diagnosed early (<2 wk), the prosthesis remains securely fixed, and the bone-cement interface is not disrupted by infection.
When removal of an infected prosthesis is not possible, chronic suppressive oral antibiotic therapy has occasionally been successful (8,12). The use of oral agents such as ciprofloxacin combined with rifampin has recently been shown to be beneficial in some cases of prosthetic joint infection (13). The functional outcomes for elderly patients are poor with the majority unable to walk without assistance (7), and 40% may show persistent or recurrent infection (14).
The use of antibiotic prophylaxis for high-risk procedures for patients who have prosthetic joint infections is somewhat controversial. For patients undergoing dental procedures, perhaps prophylaxis might be beneficial for patients who are immuno-compromised or immunosuppressed (15). Other patients who could possibly benefit are patients who have diabetes mellitus, those with a history of a previous prosthetic joint infection, hemophiliac patients, and perhaps patients during the first 2 yr following joint placement (16). Recommendations regarding dental prophylaxis for those patients with prosthetic joints can be found on the Internet at http://www.ninthdistrict.org/ pro-01.html. Preoperative antibiotics are generally given prior to the placement of a prosthetic joint. Usually a first-generation cephalosporin is chosen. The value of "ultra-clean" rooms in decreasing infections is somewhat controversial.
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