MANAGING THE INFECTED KNEE: AS GOOD AS IT GETS

Abstract

An acute infection in the first few weeks postoperatively or an acute haematogenous infection in a previously well functioning and well-fixed prosthesis can be managed with open debridement and postoperative intravenous antibiotics for 4 to 6 weeks. Infrequently, elderly patients with a well-fixed prosthesis, absence of drainage, and acceptable pain can be treated with aspiration and chronic oral antibiotic suppression. Treatment of chronic infection requires implant removal and assessment of functional requirements, soft-tissue envelope status, extent of bone loss, and the integrity of the extensor mechanism. Disruption of the extensor mechanism or a poor soft-tissue envelope usually suggests arthrodesis. Definitive resection arthroplasty or above-the-knee amputation is rarely required.

If the decision is made to proceed with reimplantation, a delayed two-stage approach is preferred and strongly recommended. After resection, antibiotic-impregnated spacers are implanted using an antibiotic that will be effective for the offending organism. The most common antibiotics used include a combination of vancomycin and tobramycin in a ratio of 3 g of vancomycin and 3.6 g of tobramycin powder per 40 g batch of bone cement. Most patients are treated with a 4–6 week course of intravenous antibiotics and also receive erythropoeitin alpha to improve their haemoglobin level between the time of resection arthroplasty and reimplantation.

Reimplantation of another prosthesis is performed as soon as it is convenient after the conclusion of the intravenous antibiotics. If there is concern about persistent infection, aspiration or debridement for retrieval of tissue culture, with delayed implantation until culture results are available, can be performed. Most patients are empirically reimplanted based on the appearance of tissues at revision surgery and histological analysis of fresh-frozen tissue samples. Antibiotic-impregnated bone cement is used for prosthesis fixation with the antibiotic choice based on sensitivity tests from the original offending organism(s). Vancomycin and tobramycin are most commonly used in a ratio of 1 to 2 g per batch of bone cement as higher dosages weaken the mechanical strength of the cement.

Currently, most reimplantation prostheses are posterior stabilised or constrained condylar designs. Bone graft is avoided if possible. Postoperatively, antibiotics are continued until results from intraoperative cultures are available and if negative, all antibiotics are discontinued. Positive cultures with the same organism are treated with a 4-week course of intravenous antibiotics. If positive culture results are deemed to be a laboratory contaminant, additional antibiotics are not recommended. Patients are evaluated with annual clinical examinations, erythrocyte sedimentation rate, C-reactive protein level, and plain radiographs. Currently a success rate of 90% is likely with a two-stage technique.