Prosthetic joints: infection challenges

Since the earliest hip replacements, pioneered by Sir John Charnley in the early 1960s, joint replacement (arthroplasty) has become a common procedure. The Health Protection Agency (HPA) points out that infection rates are now much lower than when joint replacement was first introduced. However, there is still a risk associated with each procedure. According to the HPA, this is estimated to be around 1% for elective hip and knee replacements and 4% for emergency hemiarthroplasties.1 At the 8th International Healthcare Infection Society (HIS) Conference and Federation of Infection Societies (FIS) annual conference, approaches to diagnosis, treatment and prevention were discussed, with Professor Hooper, head of orthopaedics, University of Otago, New Zealand providing a surgeon’s perspective. “Because of our ageing society and the raised expectations of this population, joint replacement is undergoing rapid increase. By the year 2030, hip arthroplasty is going to increase by 157%, while knee arthroplasty is going to increase by 673%. As a result, the incidence of revision is also going to increase. Each revision costs approximately four times the cost of a primary implant. The expenditure on health is going to be enormous,” he warned. He pointed out that worldwide infection rates vary from 0.2% to 2.2%, while the New Zealand joint registry shows there is a difference between hip and knee arthoplasty outcomes. “This is common throughout the world – knee arthroplasty has a higher risk of infection than hip replacement,” he commented. The New Zealand joint registry shows that the top reason for revision for total knee replacement is pain, followed by joint infection. Several years after receiving a joint prosthesis, patients may be exposed to other infections, as they age, such as urinary tract infections and pneumonia. Such infections can be associated with the development of prosthetic infections as a secondary problem. New Zealand set up its joint replacement registry in 1999, and the majority of infections are currently seen within the first two years, post-surgery. However, as time passes, the registry is expected to highlight a wave of later secondary infections.

Diagnosis

Prof. Hooper commented that diagnosing acute infection in the early post-operative period is relatively easy; it is often associated with a temperature, wound dehiscence and leakage from the wound. This is also commonly associated with a secondary haematoma – patients who have received anticoagulation for prevention of DVT and PE often go on to develop haematoma, which is a significant issue. “If you have a joint with a clinical infection, it is suggestive that the joint should be aspirated to find an appropriate organism. However, it is not that easy,” said Prof. Hooper. “Only 13% of the diagnoses can be confirmed with an early aspirate. It is often complicated by the patient having antibiotics in the intervening period. White cell count can provide a reasonable sensitivity and positive predictor factor for joint infections but nothing is 100%. If there are increased polymorphs or C-reactive protein, there is an increased chance of infection, but, generally, in the acute situation, it is a clinical diagnosis.” In relation to microbiological testing, most of the common infective organisms are staphylococcus, but studies also show that, in a significant number of cases, no organisms are grown. This presents a dilemma when dealing with chronic infections, Prof. Hooper pointed out. According to the American Academy of Orthopaedic Surgeons, based on the following criteria, a diagnosis of prosthetic joint infection can be made when the following conditions are met:2 1 A sinus tract communicating with the prosthesis; or 2 A pathogen is isolated by culture from two separate tissue or fluid samples obtained from the affected prosthetic joint; or 3 Four of the following six criteria exist: • Elevated serum erythrocyte sedimentation rate (ESR) or serum C-reactive protein (CRP) concentration. • Elevated synovial white blood cell (WBC) count. • Elevated synovial neutrophil percentage (PMN%). • Presence of purulence in the affected joint. • Isolation of a microorganism in one culture of periprosthetic tissue or fluid. • Greater than five neutrophils per high-power field in five high-power fields observed from histologic analysis of periprosthetic tissue at 400 times magnification. Prof. Hooper put forward another view, commenting: “If you see pus, the joint is infected. I do not care what the other parameters tell me. If you have a clinically infected joint, and you can grow one organism in microbiological tests, the orthopaedic surgeon becomes very interested in that organism. “For example, a 57 year old patient came to me two years after a total knee replacement. Immediately after the surgery, she had good function and mobility; however, two years later she was complaining of pain, stiffness and some swelling, but no other systemic symptoms. The implant bond was perfect, so the number one diagnosis had to be infection. “I often perform a bone scan during the early diagnostic regime to see what is happening. After two years you would expect the scan to be relatively neutral. You do not expect to see increased uptake around the implant after two years. When diagnosing infection, the reality is that you will not grow an organism all of the time – often the result will be negative. However, in many cases, you only need to exclude other common causes – such as loosening of the component, for example; the next option becomes infection.” In New Zealand, the use of sonication was investigated as a possible approach to aid diagnosis (this process is designed to dislodge microorganisms from the implant, to assist in microbiological testing). “We were investigating how many patients with aseptic loosening, grew an organism, and were in fact infected. The study found that there was a clinically significant rate of positive culture results in the ‘aseptic loosening’ group of around 15%. Ultrasound sonication was less sensitive than conventional sampling techniques in the study, even in the infection group,” commented Prof. Hooper. The findings suggested that unrecognised infection is potentially present in a significant proportion of aseptic loosening cases undergoing revision surgery but do not support the routine use of ultrasound sonication for its detection.3