THE RANGE OF TREATMENTS
The goal of treatment for an infected total knee arthroplasty is to eradicate the infection, preventing it from spreading locally or systemically, and to leave the patient with a wellfunctioning, relatively pain-free joint. Methods of achieving this goal include antibiotic suppression therapy and various surgical procedures.
Antibiotic suppression therapy has a very low success rate: in a study by Johnson and Bannister, for example, 25 deep infections were treated with antibiotic suppression therapy and only two resolved.2 Nevertheless, this approach is an alternative for patients in whom surgery or anesthesia and associated risks are not feasible or for those who meet certain criteria, including infection with a microorganism of low virulence, infection with a microorganism susceptible to oral antibiotics, ability to tolerate antibiotics without serious complications, and no loosening of the prosthesis.12
Surgical intervention is the most common treatment for infected total knee arthroplasty. The options include two-stage resection arthroplasty, one-stage resection and reimplantation, irrigation and debridement alone, and arthroscopic irrigation and debridement, all discussed later in this article. Fusion of the knee joint and above-knee amputation can also be done, although these are clearly considered salvage procedures and are treatments of last resort, usually performed only after other surgical treatments have failed to eradicate the infection or to treat the patient who continues to suffer with either persistent infection or a poorly functioning, painful knee.
The primary decision to be made with a newly diagnosed infected knee arthroplasty is whether the infection can be treated with retention of components or if the components will need to be resected in order to eradicate the infection. When components are resected, reimplantation on a delayed basis (4 weeks to 3 months) occurs after eradication of infection via repeated debridements and prolonged antibiotic therapy.
TWO-STAGE RESECTION ARTHROPLASTY
The most common and reliable treatment approach for an infected arthroplasty is a two-stage procedure that allows infection to be eradicated before the new, revised joint components are inserted.
Stage I After diagnosis of an infected arthroplasty is made, the patient is brought to the operating room for irrigation, debridement, and resection of components. An incision and arthrotomy are performed; and the polyethylene, femoral, tibial, and patellar components are removed, as is any bone cement that may remain in place. When components are removed, care is taken to preserve as much bone stock as possible to allow a foundation for the reimplantation of new components at a later date. The soft tissues are thoroughly debrided, typically removing the synovium and any additional necrotic tissue or bone. The joint is then copiously irrigated using antibiotic solution.
The second part of stage I of the procedure is to place an antibiotic spacer in the joint to maintain the space for reimplantation of the components in the future and possibly to allow the patient to ambulate until component reimplantation can be completed. The spacers are made with bone cement impregnated with antibiotics. This cement allows the antibiotics to elute into the joint and surrounding tissues over time, usually several weeks, to help eradicate infection. The goal of the spacer is to provide patient comfort and mobility, prevent the loss of joint space due to scarring and tightening of tissues, enhance bone quality, and allow for treatment of the infection locally with time-released antibiotics.13
Spacers can be simple block-shaped (Figure 3) or quasifunctioning articulating (Figure 4) devices. Recent studies suggest that using an articulating antibiotic spacer allows for better functioning for patients between the stage I and stage II procedures.4,14,15 Articulating spacers may be custom made from coating the patient's own sterilized implants with cement at the time of resection or may be constructed from prefabricated cement molds. After the antibiotic spacer is implanted, the incision is closed; and the patient may be allowed to have at least partial weight bearing and range of motion of the joint. The patient is also treated with IV antibiotic therapy, with the choice of antibiotic dependant on microorganism culture and sensitivity results. Courses of antibiotics range from 14 days to 12 weeks.12,16-18
Stage II The second stage of the arthroplasty occurs after the infection is eradicated. The length of time between the first-stage and second-stage procedures ranges from 4 to 58 weeks and depends on the patient's medical condition, the physical condition of the joint itself, and results of repeat aspiration/culture and tests for inflammatory markers.14,16-20 Stage II consists of the removal of the antibiotic spacers and components, repeat debridement, and reimplantation of a revision total knee arthroplasty (Figure 5). This procedure is done using the same incision and approach that was used previously. Once the spacers are removed and bony and soft tissues are thoroughly debrided, revision total knee implants are cemented in place using antibiotic bone cement. Following revision arthroplasty, there are generally no restrictions in weight bearing or range of motion.
The two-stage revision procedure is generally considered to be the gold standard treatment for infected total knee arthroplasty. Cuckler reports no recurrence of infection with an average follow-up of 5.4 years for 44 infected total knee arthroplasties treated with two-stage revision using articulating spacers.18 Additional studies have described success rates that depend on the type of infecting organism. In a study by Hirakawa and colleagues, a success rate of 66.7% was found when high-virulence organisms (Staphylococcus aureus, Enterococcus species, methicillin-resistant S aureus) were involved.16 The success rate was 80% when infection was with low-virulence organisms (Staphylococcus epidermidis, streptococci, Proteus species) and 71.4% with polymicrobial organisms.16 Hart and Jones reported that infections were successfully eradicated in 42 of 48 patients (88%) and that of the six patients with persistent infection, four were successfully treated with further two-stage procedures.19
In addition to eradication of infection, another goal of revision total knee arthroplasty is to provide the patient with a functional knee that allows for ambulation and freedom to perform everyday activities. Outcomes following two-stage treatment, while not as good as those following primary arthroplasty, are certainly acceptable in many studies. Hospital for Special Surgery knee scores were presented in two studies and showed good to excellent results in 90% and 75.6% of patients respectively.14,16 Hofmann and colleagues showed an increase in average range of motion from 6 degrees to 91 degrees before revision to 4 degrees to 104 degrees after revision, an increase of 16 degrees in arc of motion.14 A decrease in the average flexion of 9 degrees, from 92 degrees to 83 degrees, was documented in another study.16