Periprosthetic infections of the shoulder - Part 1

Introduction 

We humans each contain trillions of microorganisms, outnumbering human cells by 10 to 1. A person weighing around 154 pounds carries about 4 pounds of bacteria. The point being that the presence of bacteria does not indicate an infection. While Cutibacterium is the most common cause of periprosthetic infections of the shoulder, it is first and foremost a normal resident of the pilosebaceous glands of the normal dermis from which it maintains normal skin health.   

Our skin incision transects a large number of the abundant pilosebaceous glands on the chest allowing the bacteria they contain to continuously enter the surgical field during the conduct of the arthroplasty. Cutibacterium are commonly recovered from shoulders having routine shoulder arthroplasty for arthritis (see link, link, link, link), yet very few of these contaminated joint replacements become infected.  

NB:It may come down to the interaction between the organism and the human.  A certain species or strain of bacteria may be "virulent" for one host and inconsequential for another.

When an infection does occur, it is usually from the organisms found on the patient's skin.  The loads of Cutibacterium are higher in young healthy males with greater levels of testosterone.

There is lack of consensus on "what is an infection?"; from the above we recognize that the mere presence of bacteria does not equate to an infection.

A practical definition is "bacteria doing harm".  Whether or not bacteria in a shoulder are causing harm depends on the virulence of the bacteria, the number of active bacteria, and the adequacy of host response. In the usual arthroplasty, the surgeon irrigates the surgical field, reducing the bacterial load, and the host defenses contain the remaining organisms so that harm is not done.

When shoulder periprosthetic infections occur they are costly, resulting in almost $500,000,000 per year and causing a high rate of morbidity and mortality. 

Risk Factors For Periprosthetic Infections

There are many factors that are associated in increased risk of periprosthetic shoulder infections.  These patients may merit consideration of increased prophylaxis and close observation after arthroplasty:

Low volume surgeon

Male sex (link, link, link, link, link)

Young age and young age

Low ASA score

Higher levels of Cutibacterium on the unprepared skin

Higher testosterone levels (link, link)

Recent cortisone infection (link, link, link, link, link)

Recent arthroscopy (link, link, link), cuff repair, or other types of surgery (link, link)

Smoking 

    Cotinine test

Non-tobacco nicotine use

Cannabis use

HIV infection

Recent COVID infection

Anemia

    Hgb<12 g/dL

Low abumin

    <3.5g/dL

Low lymphocytes

    <1500 per microliter

Rheumatoid arthritis

Vitamin D deficiency

Metabolic syndrome

Prior non-shoulder periprosthetic infection

Sarcopenia

BMI <18 or >40 (link, link, link)

Antibiotic allergies

Poor glucose control

    Frutcosamine vs HGB A1C

Perioperative anticoagulation

Transfusion

Arthroplasty for trauma

Reverse total shoulder

Albumin <3.5 mg/dL

Perioperative dexamethasone and GLP-Agonists have not been found to increase the risk of periprosthetic shoulder infections.

What Interventions May Reduce The Risk Of Periprosthetic Infections

Most of the commonly used measures have not been proven to be clinically significantly effective in reducing the rate of periprosthetic infections. These inlcude:

Home washes

Preoperative doxycycline

Skin preps (link, link, link, (link).

Space suits

Laminar flow

Electrocautery for the skin incision 

The two interventions with the strongest evidence of effectiveness in reducing the risk of  periprosthetic infections are:

Preoperative intravenous cephalosporins (link, link).

In wound Vancomycin

Diagnosis         

 

Preoperative evaluation

In considering the diagnosis of periprosthetic infections of the shoulder, it is important to consider separately the two presentations of this condition: the obvious presentation and the stealth presentation.

 

The obvious presentation is typically characterized by the acute onset of shoulder pain, swelling, tenderness and erythema. Blood tests for inflammation (white blood cell count, percent neutrophils, erythrocyte sedimentation rate, and C-reactive protein) are commonly elevated. Aspiration of the joint commonly reveals cloudy fluid with elevated inflammatory markers and cultures that turn positive within a few days for organisms such as Cutibacterium, methicillin sensitive Staphylococcus aureus (MSSA), and methicillin resistant Staphylococcus aureus (MRSA). In sum, diagnosis of an obvious infection is rarely a challenge.

The challenge lies in the diagnosis of a stealth infection. Typically, stealth infections present as otherwise unexplained onset of stiffness and pain after an initially satisfactory recovery (we call this the "honeymoon period") lasting months or years. The physical examination may show only stiffness of the glenohumeral joint. Plain x-rays may reveal loosening of a previously well-fixed humeral component.

A number of preoperative tests have been found to have low sensitivity and specificity for stealth infections including

PET cans

Serum D-Dimer (link, link)

C-reactive protein (link, link, link)

Erythrocyte sedimentation rate and white blood cell count.

In contrast to the situation with obvious infections, attempting a joint fluid aspiration in a case of suspected stealth infection often yields no fluid. When fluid is aspirated the results have low predictive value/reliability (link, link, link, link), and poor concordance with intraoperative cultures (link, link).

Synovial fluid WBC>2800/mm^3 appears to have good sensitivity and specificity, however, synovial fluid Il-6, leukocyte esterase, and alpha-defensin are of uncertain benefit in diagnosing periprosthetic infection (link, link, link, link, link). 

Organisms can be recovered using preoperative tissue biopsy by a needle or by using an arthroscope (link, link, link, link, link, link).

The assessment of the value of preoperative tests has been confounded by the fact that studies frequently co-mingle obvious and stealth infections, potentially inflating the apparent value of these tests in diagnosing stealth infections. 

                                                      

 Intraoperative evaluation        

Obvious infections frequently show cloudy or purulent joint fluid and inflammatory synovitis with a frozen section with more than five white blood cells on multiple high power fields. 

However  in stealth infections, the diagnosis rests on the results of cultures that are not available at the time of revision surgery. Thus, the surgeon must determine the appropriate surgical and medical treatment without knowing whether the shoulder is infected or not.

The recommended approach to obtaining specimens for culture at revision surgery is that five deep tissue and explant specimens be sent for aerobic and anaerobic cultures that are observed for at least two weeks (link, link, link).

Cultures with shorter time to positivity (link, link, link) and higher strength of positivity indicate greater bacterial load.

We'll consider treatment of shoulder periprosthetic infections in Part 2

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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link).
The total shoulder arthroplasty (see this link).
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link).
Shoulder rehabilitation exercises (see this link).