The incidence of infection after UKA is about half that after TKA (Liddle et al, 2014). The methods of investigation of suspected infection are the same in OUKA as in TKA except that radionuclide uptake studies are not helpful. After OUKA, activity in the bone beneath the implants persists for several years, so the presence of a ‘hot’ area on the scan is not necessarily evidence of infection (or loosening). The C-reactive protein or erythrocyte sedimentation rate are the most useful diagnostic tests but may not be positive in the first 2–3 weeks. We do not have any experience of using the new synovial fluid markers to diagnose suspected prosthetic joint infection after UKA.
Treatment
Acute infection
In the early postoperative period, acute infection is diagnosed and treated in the same way as after TKA. Early open debridement and change of meniscal bearing and intravenous antibiotics can arrest the infection and save the arthroplasty. The use of arthroscopic lavage is not recommended as it is not as reliable as open debridement and exchange of bearing.
Late infection
Failure of treatment of an acute infection, or infection of later onset, is diagnosed from the clinical and radiological signs and bacteriological studies, as in TKA. The earliest radiological signs may be in the retained compartment.
Figure 11.1 (a) The earliest radiographic sign of infection may be the appearance of subchondral erosions in the retained compartment of the knee. (b) The radiolucencies under the tibial plateau are more than 2 mm thick and are not defined by a radiodense line. They are different from the common ‘physiological’ radiolucent lines and suggest infection and/or loosening.
Figure 11.1(a) shows thinning of the articular cartilage and juxta-articular erosions of the lateral joint margin of an infected knee after medial OUKA; evidence of chondrolysis by the infecting organism and chronic synovitis. (Note that acute rheumatoid synovitis can produce a similar appearance.) The eventual appearance of thick (>2 mm), ill-defined progressive radiolucencies beneath the components (Fig. 11.1(b)), quite different from the thin radiolucent lines with radiodense margins that outline most normally functioning OUKA, is diagnostic.
Treatment is by removal of the implant and excision of the inflammatory membrane, followed by one- or two-stage revision to TKA. We prefer the two-stage procedure, with removal of the implant and excision of the articular surfaces of the retained compartment at the first stage. An antibiotic-loaded spacer is left in the joint to maintain the gap and deliver high doses of antibiotic until the infection is eradicated and the second stage can be safely undertaken. Three types of spacers can be used (Fig. 11.2). We favour a bicompartmental spacer as this allows removal of all infected articular cartilage at the first stage. The spacer can be static or articulating depending on surgeon preference. The second-stage TKA may require a stemmed tibial implant (with additional medial augments) if there is substantial tibial bone loss.
