Review Question - QID 220097

This patient presents with a chronic left hip prosthetic joint infection and thus would benefit most from an explant with the placement of an antibiotic spacer (answer choice 3). With metal-on-metal hips, as shown in the AP radiograph in Figure A, the synovial WBC cutoff value is >4350/µL and PMNs >85%.

Prosthetic joint infections (PJI) occur at an incidence of 0.3-1.3% in total hip arthroplasty (THA) patients. Patients most commonly present with persistent hip pain and stiffness. They may experience acute onset of pain, swelling, and redness to the area or chronic infections; symptoms may be more subtle with deteriorating function and increasing pain over time. A draining sinus may also present in chronic infections, which is a sign of a definite infection. Typically, patients have a history of recent bacteremia, multiple local surgeries, or soft tissue penetration near the joint. History is important with these patients, and acute symptoms are those that occur within 3-6 weeks of the index procedure (CDC definition <90 days of joint surgery). Chronic symptoms are those that arise more than 3-6 weeks from surgery (CDC definition >90 days from surgery). If there is high suspicion for a PJI, joint aspiration is typically performed, and a synovial fluid analysis is run. WBC >3000/µL and PMN >80% in a THA with implants other than metal-on-metal (MoM) is indicative of a PJI. For THA with MoM implants in place, the cutoff value is >4350/µL and PMNs >85%. With these implants, a manual cell count is necessary to obtain because the fibrinous debris from MoM reactions will falsely elevate automated cell counts. Common operative treatment for PJI includes debridement, antibiotics, and implant retention (DAIR) for acute infections and two-stage revision arthroplasty (gold standard) for chronic infections. In two-stage revisions, explantation of implants with placement of an antibiotic spacer is performed with IV antibiotics for 4-6 weeks (until the infection is cleared), and then revision arthroplasty.

Parvizi et al. conducted a multicenter study at three different academic centers to develop an evidence-based and updated version of the Musculoskeletal Infection Society (MSIS) criteria for PJI. This study helped update the major and minor criteria from the original 2011 recommendations. They found that those patients with a total MSIS score of 6 or greater were considered infected, and scores 2-5 require the inclusion of intraoperative findings for confirmation or denial of infection. They conclude that the new criteria demonstrate a higher sensitivity (97.7%) compared to the MSIS (79.3%) and International Consensus Meeting definition (86.9%), with a similar specificity of 99.5%.

Bryan et al. reviewed the contemporary results of DAIR treatment for acute PJI in primary THA and identified factors that predict failure or success. 90 hips were included in the study with 6-year mean follow-up data. 70% underwent modular head and liner exchange, and 30% underwent irrigation and debridement alone. Those with McPherson host grade A had a failure rate of 8%, host grade B had a rate of 16%, and host grade C had a rate of 44%. They concluded that their success rates were higher than previously reported, and systemic host grade A was predictive of treatment success.

Karim et al. published the 2019 proceedings of the International Consensus on Orthopedic Infections. They explain that the workup of PJI in the setting of adverse local tissue reaction requires a thorough history and joint aspiration. They note that with MoM implants, the automated cell counts routinely performed during synovial fluid analysis can cause a falsely elevated WBC. They note this false positive could be from cellular debris causing the error in automated cell counts. Therefore, they recommend a manual cell count and differential in this setting to properly evaluate a PJI.

Figure A is an AP radiograph of a left hip demonstrating a well-fixed total hip arthroplasty with metal-on-metal implants in place.

Incorrect answer choices:
Answer choice 1: This patient had a THA 10 years ago and has had symptoms for 3 months. Therefore, this is a chronic PJI, and a two-stage revision is warranted.
Answer choice 2: Revision arthroplasty to a ceramic-on-polyethylene THA is a reasonable option in the setting of symptomatic metallosis without a PJI.
Answer choice 4: An MRI of the hip can be performed to further evaluate for metallosis or a pseudotumor when a PJI is not present.
Answer choice 5: Physical therapy and serial hip radiographs would be a reasonable nonoperative option with asymptomatic metallosis and without a PJI.