Orthopedic Teaching Cases 2023, Jan, 20 | 12:00 am

infected TKR

A 72-year-old female with a history of diabetes mellitus underwent uncomplicated total knee replacement. She developed wound drainage 3 months postoperatively which was treated by her original surgeon with irrigation and debridement and liner exchange followed by intravenous antibiotics. Intraoperative cultures revealed

MRSA. Her original surgeon eventually moved out of the country. She presents to you with recurrent drainage and erythema and distal wound dehiscence.

The patient had difficulty finding a new orthopaedic surgeon following the departure of her initial surgeon. Upon presentation in your clinic, the patient has had ongoing drainage and wound dehiscence for the past 2 months. You perform a knee aspiration in the office. What are the diagnostic criteria for deep prosthetic joint infection based upon synovial fluid analysis?

  1. 100,000 WBC, 80% neutrophils

  2. 1,700 WBC, 65% neutrophils

  3. 2,500 WBC, 75% neutrophils

  4. 22,000 WBC, 60% neutrophils

 

Discussion

The correct answer is (B). In the setting of a chronically draining sinus, the diagnosis of prosthetic joint infection (PJI) is clear in this patient. However, in the absence of a draining wound the diagnosis of deep prosthetic joint infection can be difficult to make in many situations. When infection is not clinically obvious, the diagnosis is made based on a collaboration of data to include serum inflammatory markers and synovial fluid analysis as well as histopathologic analysis and imaging. The synovial fluid analysis can be very helpful in this setting. In the setting of the current patient, where infection is established based on the presence of a draining sinus communicating with the prosthesis, synovial fluid analysis is still helpful to confirm the diagnosis and to identify an organism and its antibiotic sensitivities. Synovial leukocyte count is much lower in prosthetic joint infection than it is in native joint infections. The diagnostic criterion for chronic PJI of the knee is a synovial leukocyte count of 1,700/cm2 or 65% or more neutrophils.

The aspirate of the knee grows MRSA. What is the probability of MRSA eradication with irrigation and debridement and liner exchange/component retention?

  1. 100%

  2. 80%

  3. 20% to 40%

  4. 8%

  5. Less than 1%

Discussion

The correct answer is (D). The treatment of PJI has been shown to be less successful with component retention than with two-stage exchange arthroplasty for the treatment of infected total knee replacements. In most reports, I and D with retention of the implant results in approximately 20% to 40% control of infection.

However, in patients with MRSA the success rate with component retention has been shown to be as low as 8%. Two-stage exchange arthroplasty results in control of infection in approximately 90% of patients.

What is the standard of care treatment at this stage?

  1. Repeat I&D and liner exchange

  2. Wound VAC dressing over the region of dehiscence

  3. IV antibiotics followed by long-term suppression

  4. Two-stage exchange arthroplasty

  5. One-stage revision arthroplasty

 

Discussion

The correct answer is (D). This patient has had a draining sinus over a total knee replacement for more than 3 weeks and thus by definition has a chronic infection. Furthermore, she has a virulent organism (MRSA). The standard of care in this setting is removal of the components, placement of an antibiotic spacer followed by intravenous antimicrobial therapy for 6 weeks followed by reimplantation if infection is eradicated at that time. In patients who have had symptoms of infection for less than 3 weeks, do NOT have a draining sinus, have an appropriate nonvirulent microorganism, or those with hematogenous spread and the prosthesis is well-fixed and functioning, these patients may be candidates for irrigation and debridement, exchange of the polyethylene liner and retention of the components.

Which of the following is FALSE as it pertains to antibiotic spacers?

  1. Static spacers in the knee make exposure at the time of reimplantation difficult due to quadriceps shortening

  2. Commercially available cement containing antibiotics should NOT be used for antibiotic laden cement spacers because they contain insufficient antibiotic concentration

  3. The PROSTALAC has been shown to be superior to static spacers

  4. There has been no appreciable difference in infection recurrence and functional outcomes in patient that have had static spacers in comparison to those that have had articulating spacers

Discussion

The correct answer is (C). Articulating spacers such as the PROSTALAC have not been shown to definitively improve function or infection rates in comparison to nonarticulating spacers. Articulating spacers have been found to offer no functional advantage over static spacers. Antibiotic laden cement spacers should contain 2 to 4 g of antibiotic per 40 g bag of cement, which is a higher dose than that available in commercially available cement containing antibiotics which is used for primary and revision joint replacement in absence of active infection.

Following debridement and removal of the components you notice that the extensor mechanism is disrupted at the site of the patellar tendon. What is the best method by which to reconstruct this?

  1. Allograft extensor mechanism

  2. Megaprosthesis—attach the quads tendon to the prosthesis

  3. Suture the patellar tendon end-to-end

  4. Gastrocnemius rotational flap

 

Discussion

The correct answer is (D). The best method of reconstruction of the extensor mechanism in the setting of wound dehiscence and prior infection is to use autologous tissue (not allograft due to the risk of recurrent infection). A gastrocnemius rotational flap will provide soft tissue coverage, can replace the missing/incompetent capsule to prevent ongoing soft tissue problems, and can also be connected to the remnant extensor mechanism and attached to the quads tendon to reconstruct the extensor mechanism. Suturing the patellar tendon end-to-end will not be successful as there is often significant retraction and the tissue tends to be too fragile to obtain a strong repair. A mega prosthesis is an option, however it is

optimal to have a bony fragment of attachment (i.e., the tibial tubercle attached to the patellar tendon) to attach to the prosthesis to get bony ingrowth. In the setting of a midsubstance patellar tendon tear, this will not be the case. Furthermore, this option does not have the added benefit of soft tissue coverage to manage the defect in the capsule. A persistent defect in the joint capsule will result in subsequent drainage and wound problems.

 

Objectives: Did you learn...?

 

 

The diagnostic criteria of an infected TKR? Treatment of infected TKR?