To Be (Infected) or Not To Be (Infected): That is one question when shoulder replacement fails!

Shoulder replacement is a very successful procedure. Once recovery after surgery is complete, most individuals who have undergone shoulder replacement feel no or very little pain; typically, good use of the shoulder is restored as well. However, a small number of patients continue to experience shoulder pain without a clear explanation after an otherwise successful replacement. Other patients will suffer return of pain months to years after the replacement because the implants have become loose off the bone. In these two circumstances (painful replacements and loose implants) it is very important to rule out infection, since recommendations for treatment may change substantially. But…, there is a problem: infections complicating shoulder replacement may be extremely difficult to diagnose.

Why is it that shoulder infections after replacement are so hard to pinpoint?

Infection is a well-known, undesired complication after replacement of any joint (hips, knees and shoulders included). Infection happens when bacteria or other infectious organisms settle and multiply in the joint, creating inflammation, pain, redness, swelling, drainage and other symptoms. How do these infectious organisms get there? Three main ways:  (1) they may have been present in the joint at the time of replacement (typically as a consequence of prior surgeries), or (2) they gained access during surgery (most commonly), or (3) they settled late though blood vessels that nourish the joint. Bacteria can also potentially get in when the replaced joint is pierced with a needle in order to inject a medication or draw liquid, but that is a rare occurrence.

Two common bacteria responsible for periprosthetic shoulder joint infections: Staphylococcus spp and Cutibacterium acnes

The shoulder joint is particularly prone to infections by a bacterium called Cutibacterium acnes (C. acnes, formerly known as Propionibacterium acnes). Our skin contains normal quantities of various bacteria. It turns out the Cutibacterium acnes is highly present in the chest and back areas, in close proximity to the shoulder region. As such, it is relatively easy for this bacterium to get into the shoulder joint while performing replacement surgery. Unfortunately, infections by C. acnes are difficult to confirm for the following reasons:

  • Severe inflammation is uncommon
    • Uncommon symptoms
      • Redness
      • Drainage
      • Fever & chills
    • Inflammatory markers may be normal
      • ESR
      • cRP
    • Joint fluid cells are relatively sparse
      • Relatively low cell count
      • Neutrophils do not predominate
  • Slow bacterial multiplication complicates cultures
    • Positive cultures take longer to grow
    • May never grow in culture

Other bacteria (such as Staphylococcus) multiply very rapidly, creating redness, drainage of purulent material, and severe pain. On the contrary, C. acnes multiplies slowly; as a consequence, shoulder infections with C. acnes present with pain, they may lead to implant loosening, but rarely show drainage or severe inflammation. The degree of inflammation in the body may be assessed with certain blood tests (erythrocyte sedimentation rate [ERS] and c-reactive protein [cRP]); these tests are commonly abnormal in Staph infections, but they are normal in most infections with C. acnes. Finally, because C. acnes multiplies so slowly, it is hard to isolate in cultures, which consist of placing a sample of fluid or tissue obtained from the joint in a sterile plate with nutrients so that microorganisms will multiply and can be identified.

Definite, probable and possible infections

Because infections after shoulder replacement are so difficult to confirm, groups of experts have combined their research and knowledge to create a number of criteria for infection. These criteria may guide surgeons and patients to determine how likely it is that the replacement is infected. I was lucky enough to join one group of shoulder surgeons that agreed on the criteria summarized below. Depending on how many of these criteria are abnormal, we believe the joint is either infected for sure (definite), very likely to be infected (probable), or could be infected but it is actually hard to tell (possible).

Major criteria that define definite periprosthetic shoulder infection (meeting one of these criteria is diagnostic of infection)
• Presence of a sinus tract from the skin surface to the prosthesis
• Gross intra-articular pus
• Two positive tissue cultures with phenotypically identical virulent organisms
Minor criteria to be considered for the scoring system below
Unexpected wound drainage – 4 points
• Single positive tissue culture with virulent organism – 3 points
• Single positive tissue culture with low-virulence organism – 1 point
• Second positive tissue culture (identical low-virulence organism) – 3 points
• Humeral loosening – 3 points
• Positive frozen section (5 PMNs in high-power fields) – 3 points
• Positive preoperative aspirate culture (low or high virulence) – 3 points
• Elevated synovial neutrophil percentage (>80%)* – 2 points
• Elevated synovial WBC count (>3,000 cells/microL) – 2 points

Elevated ESR (> 30 mm/h) – 2 points
• Elevated CRP level (> 10 mg/L)* – 2 points
• Elevated synovial alfa-defensin level – 2 points
• Cloudy fluid – 2 points
Probable infection
• Organism identified in at least one culture and 6 or more points
Possible infection
• Single positive culture with virulent organism
• 6 or more points with no organism identified
• 2 positive cultures with low-virulence organism
Unlikely infection
• Less than 6 points and negative cultures or only a single culture with low-virulence organism

Definition of infection according to the
International Consensus Meeting on Orthopedic Infections.
PMN, polymorphonuclear leukocyte; WBC, white blood cell; ESR, erythrocyte sedimentation rate; CRP, C-reactive protein. *Beyond 6 weeks from most recent surgery.

So, how do surgeons evaluate patients with a painful or loose replacement to determine if there is infection? I am not going to lie to you…, this topic is complicated to understand. As you read this text, please refer to the video and the charts below often. Otherwise, it is difficult to navigate this complex maze of options.

Help me navigate this maze!

If you shoulder replacement is loose or you are experiencing pain after replacement for no apparent reason, your surgeon may recommend drawing fluid from your shoulder joint, a procedure called aspiration. In our practice, this procedure is done under local anesthesia, and an ultrasound device is used to obtain images of your replacement during the aspiration to ensure that liquid is obtained from the right spot. You may be familiar with ultrasound: it is commonly used during pregnancy to assess the baby.

Aspiration of the shoulder joint under ultrasound guidance allows drawing joint fluid
that may be analyzed and cultured

Typically, the liquid obtained by aspiration is analyzed in the laboratory to determine the number of cells floating in the fluid and perform other measurements. In addition, a few drops of fluid are also cultured: fluid is placed in a sterile plate with nutrients to facilitate the growth of any infectious organisms that may have been extracted from the joint with the fluid. These plates are analyzed on a daily basis up to 14 days.

As mentioned before, severe infections such as those by Staph may show growth very quickly, whereas infections by slow-growing organisms like C. acnes may take all 14 days to grow. When there is growth, the culture is read as positive. Another benefit of cultures is that they allow testing in the laboratory which antibiotics are most effective against the particular strain of infectious organism recovered from the joint; this process is called assessment of sensitivities. As such, there may be two different situations: (a) the culture is positive and/or there are criteria for definite infection, or (b) the culture is negative and the criteria for infection are not that strong.

Video-presentation of our diagnostic flowchart

Aspiration culture positive or definite infection

When the replaced shoulder joint is clearly chronically infected, the main decision to make is whether the infection can get cleared with one procedure (one-stage reimplantation) or whether more than one procedure will be needed (two-stage reimplantation). In one-stage reimplantation, the joint is thoroughly cleaned, and the infected implants are removed and exchanged for a brand-new set of implants in a single operation. In two-stage reimplantation, two separate surgical procedures are performed; in the first procedure, the infected implants are removed, the joint is thoroughly cleaned, and a mold made of bone cement with a high dose of antibiotics (polymethylmethacrylate spacer) is placed in the joint. The second operation to implant a new prosthesis is performed typically 8-10 weeks later, once the infection is considered to be cleared. In both one-stage and two-stage procedures, intravenous antibiotics are administered for several weeks.

Flowchart for shoulders with a positive fluid culture or definite infection

Theoretically, two-stage reimplantation provides a higher chance to clear the infection, since there is a period of time when the shoulder has a temporary spacer that will be removed: as such, the real prosthesis is less likely to be exposed to any residual microorganisms. However, studies published to date have not definitely shown differences in infection cure rates between one-stage and two stage procedures.

# planned proceduresOneTwo
IV antibiotics6 weeks6 weeks
Oral antibiotics for lifeMore likelyLess likely
Possible need for reoperation??
Total recovery time5-6 months7-9 months
Chances to clear the infection??
Theoretical and practical differences between one-stage and two-stage reimplantation

Aspiration culture negative

Due to the fact that cultures of fluid obtained by aspiration have a low diagnostic yield, a negative culture does not exclude infection. In these circumstances, if the surgeon and patient are very worried about infection, a two stage-reimplantation may still be the safest option. However, a one-stage revision makes a lot of sense too, since if the cultures of tissue obtained at the time of that revision procedure are negative, no more treatment will be required. On the contrary, if a one-stage procedure is selected and the cultures obtained at the time of surgery are positive, the patient will need 6 weeks of intravenous antibiotics, and there may be higher chances that oral antibiotics are recommended for life, that the infection is not cured, and that more surgery is needed in the future.

Flowchart for shoulders with a negative fluid culture or definite infection

When the concern for infection is substantial but undergoing two more surgeries for a two-stage reimplantation sounds like too much, another alternative is to perform a smaller arthroscopic procedure. Arthroscopy involves introducing a surgical camera through a small stab wound so that biopsies of tissue may be obtained through a second stab incision. Patient recovery after arthroscopy is very fast, and if cultures of tissue biopsies are negative the chances of infection are extremely low. Then, proceeding with a one-stage revision will feel safer in terms of infection.

Tissue samples obtained arthroscopically are sent for culture to aid in the diagnosis of periprosthetic joint infection

Time to think things through…

Deciding the best treatment pathway when there are concerns for infection after shoulder replacement is difficult. It has substantial implications in terms of number of surgeries and chance of success. If you are in this unfortunate situation, revisit the charts offered in this post several times to make sure you and your surgeon are selecting the pathway that will be best for you!

Review of all possible options for shoulders with positive and negative culture of fluid obtained through aspiration

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