Although total hip replacement (THR) and total knee replacement (TKR) are two of the most successful operations in the UK, a peri-prosthetic joint infection (PJI) is a potentially devastating complication associated with high levels of patient morbidity, substantial financial costs and frustration for clinicians.

The use of prophylactic antibacterial agents, ultra-clean air and laminar flow systems has reduced the rates of PJI in THR and TKR from about 10 per cent when the operations were first introduced¹ to 0.5-1 per cent for THR and 1-2 per cent for TKR. 'Ringfencing' of elective arthroplasty beds is also beneficial,² but this protocol is not universally applied.

However, as the UK's population is ageing the number of primary arthroplasty operations and revisions is rising, and the prevalence of PJI is expected to increase.

Presentation

The majority of early wound problems are seen in primary care.

Following a joint replacement, the patient is normally discharged from hospital three to seven days after surgery. The wound should be dry by day seven, and surgical clips and/or sutures can be removed from the hip on day 10, and the knee between days 14 and 21.

C-reactive protein (CRP) is an acute phase protein linked to systemic inflammation. It has a key role in determining patient recovery and potential infection.³

The CRP level rises dramatically after arthroplasty, peaking on postoperative day two. It should return to normal (<10mg/l) by the third or fourth week unless inflammation caused by an infection, or another cause, supervenes. A normal CRP level is a powerful negative predictor of infection, but there is no absolute level of elevation that indicates infection; the CRP level depends on the amount of tissue dissection and patient physiology. The trends in CRP levels are therefore extremely helpful in monitoring the course of infection, the treatment and/or surgery, and the resolution or recurrence of the infection (see table 1, attached).

You should assume that a superficial wound infection indicates a deep infection until proven otherwise. Superficial wound infections usually occur within 30 days of the procedure and involve only the skin and subcutaneous tissues. Either pus or organisms may be isolated from an aseptically obtained culture, or one of the signs of infection - pain, swelling and/or erythema may be present. The classification does not include stitch abscess.⁵

An early superficial wound infection is the most important risk factor for a PJI,⁶ with an odds ratio (OR) of 36. Predisposing risk factors are listed in table 2, attached.

Aetiology

A PJI is a deep infection, in which organisms are found associated with implanted material or discharging via a sinus tract to the skin. There may be a 'woody' character to the oedema around the surgical site. Systemic upset may or may not occur with a PJI. In some cases joint pain and a persistently elevated CRP may be the only features.

Investigation should include a serial CRP, and blood cultures if the patient is febrile. Wound swabs are worthless and should not be taken. They invariably grow colonising organisms, and even if a pathogenic bacterium is cultured it may not be responsible for the PJI.

The key to making an organism-specific diagnosis is microbiological culture. Culture of aspirated joint fluid offers the best preoperative approach and has a sensitivity and specificity of 80 per cent and 94 per cent respectively.¹² Joint aspiration must be done under strict asepsis and cultured immediately to achieve an optimal bacterial yield. It is therefore not an appropriate investigation to be carried out in primary care and patients should be referred directly to the orthopaedic team.

Antibiotics should be withheld until appropriate specimens are obtained as they can frustrate subsequent attempts at culture and therefore greatly complicate management.

Delay in referral¹³ or the injudicious use of antibiotics before the pathogen is isolated is potentially disastrous, and it is important that the GP liaises directly with the surgical team and patients are referred urgently (see table 3, attached).

Gram-positive cocci are the predominant causative organisms (see table 4, attached). They include coagulase-negative staphylococcus, the most common skin commensal and culture contaminant. When associated with biomaterials, these organisms are capable of forming a slimy biofilm. In this state they are largely resistant to antibodies and antibiotics.¹⁴ That is why antibiotic therapy alone is rarely successful in the treatment of a PJI.

Treatment

The classification and treatment of PJIs are based on the assessment of patient-related variables, the length of time since the surgery and the expected treatment goals (see table 5, attached).

In the majority of patients aggressive, repeated debridements and an exchange of the prosthesis are required to eradicate the infection successfully.

However, in patients with an acute postoperative infection (less than three weeks) or a late haematogenous seeding in a well fixed arthroplasty, an early debridement and four weeks of intravenous antibiotics have been shown to eradicate the infection in around 60 per cent of cases. Retention of the prosthesis is rarely successful if debridement occurs more than two to three weeks after the onset of symptoms.¹²

If comorbidity precludes major revisional surgery, long-term suppressive antibiotic therapy may be used and a surgical sinus can be created to allow drainage.

Overall, successful treatment depends on the identification of the organism, prompt surgical debridement and clear lines of communication between the GP and orthopaedic team.