Peri-prosthetic infections are a serious complication after prosthetic joint replacement, and even if they are eventually managed effectively, the duration of treatment is usually long and the patient experiences a long period of loss of joint function and difficulty in continuing to participate in daily tasks, making the overall outcome poor.
The main reason for this poor outcome is that it is often difficult to accurately diagnose periprosthetic infections early and thus manage them correctly. An understanding of the underlying pathologic changes in periprosthetic infections can help guide their proper diagnosis and management.
Current diagnostic criteria for various infections, including periprosthetic infections, are still largely based on the extrapolations proposed by Koch in 1884 and its various modifications. These criteria include first isolation of pathogenic bacteria from tissues and body fluids, followed by identification of their species and drug sensitivity testing to select the appropriate antibiotic for treatment.
Such a simple measure has been shown to be effective in most cases of infection. However, since most bacteria do not grow naturally as simple colonies on common laboratory bacterial media, including agar, but form as bacterial films.
Research has proposed and thoroughly investigated the biofilm theory of microbial growth and has developed a solid scientific foundation. The biofilm theory has been widely accepted in the fields of marine pollution, water treatment, and the food industry. The American scholar ArnoldWV et al. reviewed the knowledge of biofilm in periprosthetic infections in an AAOS tutorial.
The biofilm theory suggests that bacteria survive and grow in two different ways.
One, these unicellular bacteria are able to survive and grow on a complex biofilm matrix similar in structure and function to the extracellular matrix, which is an important hallmark of high-grade multicellular organisms. Bacterial biofilms are produced by the bacteria themselves, which provide both protection and a tissue architecture for survival, and this tissue architecture facilitates the metabolic activities of bacteria as well as signaling between different bacteria.
Second, bacteria can also exist in the planktonic form, a common form of survival for traditional unicellular organisms. Bacterial monomers living in the planktonic form do not have an organizational structure between them and do not generate chemical media gradients and corresponding microecological environments.
The form of bacterial presence is extremely important for the treatment of infections caused by the corresponding bacteria.
The planktonic state facilitates the spread of infection, but at the same time is vulnerable to attack by the body’s immune system and antimicrobials. In contrast, bacteria in biofilm form are less likely to spread, but are also protected from attack by the immune system and are not sensitive to antimicrobial therapy.
Figure 1 illustrates the typical characteristics of biofilms based on the studies of Boles and Horswill, Otto, Resch, etc., using staphylococci as an example. The blue boxes indicate the main steps of biofilm formation, the yellow boxes indicate the chemical environment in which the bacteria are exposed, and the red boxes indicate the different phenotypes of the bacteria. Planktonic cells inhibit biofilm aggregation by producing pathogenic factors, reducing their adhesion, and increasing bacterial dispersion through the accessory gene regulator (agr) system. eps: extracellular polymer.
It is worth noting that fungi, such as Candida infections, may also be present in biofilm form.
Biological characteristics of infection To develop an infection, the bacteria should first be inoculated at a site with suitable conditions. Normally, Staphylococcus is a commensal bacterium on the surface of the body that may enter the body as a pathogenic bacterium through a surgical incision when undergoing surgery. At this point the bacteria are usually considered to be in a planktonic form.
Upon entry into the body, these bacteria must adhere to the tissue surrounding the prosthesis or to the surface of the prosthesis. The molecular biology of staphylococcal adhesion is as follows: these bacteria secrete adhesion factors belonging to the MSCRAMM (microbialsurface components recognizing adhesive matrixmolecules) family, which facilitate the attachment of these bacteria to various cell-packed matrix proteins.
Staphylococcus aureus (S. aureus) has genes encoding more than 20 adhesion factors. In addition, adhesion factors that bind to fibronectin in the extracellular matrix may help to mediate the entry of these S. aureus into the interior of human cells, where these bacteria may also replicate within the host cell.
When the bacteria adhere successfully, they enter the replication phase, during which they are likely to be most vulnerable. However, for an immunocompetent host, the occurrence of infection depends on the ability of the host immune system to clear the invading bacteria from the body.
An antimicrobial alone cannot clear an infection, but it can clearly assist the organism in fighting colonized bacteria. Likewise, any mechanism that helps the bacteria to escape the attack of the immune system, or to counteract the attack of antimicrobial drugs or the body’s immune system, facilitates the pathogenic bacteria to initiate infection.
Once adhered to the surface of the prosthesis, S. aureus begins the growth and colonization phase and releases septic factors that are virulent to the host. During this phase, the bacteria initiate or shut down specific gene expression in an extremely precise and coordinated step in response to environmental factors.
In addition, the researchers found that a contact system called quorum sensing is closely linked between different bacteria to communicate information that may contribute to the overall growth of the colony and coordinate biofilm formation.
Bacteria are eventually enveloped in a bacterial biofilm composed of polysaccharides, glycoproteins, and extracellular DNA (eDNA). Bacteria present within the biofilm grow in an essentially different manner than bacteria in the planktonic state, and the two can be considered different phenotypes of the same bacteria.
Bacteria within biofilms can tolerate antibiotics at concentrations 100 times higher than those required to kill planktonic bacteria, and are also more resistant to attack by the body’s immune system. However, white blood cells still have the ability to invade the biofilm.
Biofilms also facilitate the exchange of nutrients, and bacteria can be dislodged from biofilms to re-enter the planktonic state or reach other parts of the body in a steady stream of biofilm fragments, even causing acute systemic infections.
Bacteria may exist in a relatively quiescent state within the biofilm in a less virulent manner, but the biofilm still stimulates a series of inflammatory responses that continue to destroy surrounding tissues and eventually lead to clinical symptoms, including pain, and in cases of chronic infection of longer duration, to prosthetic loosening.
Biofilm formation has been detected on the surfaces of medical implant devices such as abdominal catheters, vascular catheters, contact lenses, orthopedic devices, and artificial joint prostheses. In addition, biofilm formation has been found in many non-endophyte related chronic infectious lesions such as: prostatitis, cystic fibrosis, endocarditis, otitis media, and osteomyelitis.
Diagnosis of periprosthetic infections under the biofilm theory
The diagnosis of chronic periprosthetic infections is often difficult and is usually determined by indirect indicators including erythrocyte sedimentation rate (ESR), C-reactive protein (CRP), joint fluid cell count, and joint fluid leukocyte ratio. More recent studies have gone further and measured leukocyte esterase levels in the joint fluid to assist in the diagnosis.
All of these diagnostic tools are essentially tests of the body’s immune system’s clearance response to periprosthetic infection rather than a direct means of identifying the causative agent.
The Musculoskeletal Infection Society and the American Academy of Orthopaedic Surgeons (AAOS) have both published protocols and pathways for the diagnosis of periprosthetic infections.
The inability to isolate the causative organism from the joint puncture fluid in cases of suspected chronic periprosthetic infection is highly unlikely. The difficulties in diagnosis are not difficult to understand when biofilm factors are taken into account.
Bacteria in a planktonic state can easily be successfully isolated and cultured by conventional laboratory testing techniques, but bacteria present in biofilms are difficult to detect successfully by the same means. In contrast, the vast majority of bacteria in chronic infection cases are present in biofilms.
Some of the newer methods for diagnosing periprosthetic infections employ molecular biology techniques including polymerase chain reaction (PCR). Although PCR can demonstrate the presence of bacteria by detecting bacterial-specific ribosomal RNA, it requires amplification with bacterial-specific primers to identify the causative bacterial species.
A combination of PCR and mass spectrometry, known as Ibis technology, has been used to identify pathogenic bacteria and has shown promising applications. A recent study used the Ibis technique to successfully detect the presence of pathogenic bacteria in culture-negative cases of periprosthetic infection and in some cases of revision of what was thought to be aseptic loosening of the prosthesis. Of these, the Ibis technique detected pathogenic bacteria in 15 out of 57 revision cases that were originally thought to be aseptic loosening of the prosthesis.
Such results provide some support for previous speculation by investigators that many aseptic prosthetic loosening actually has a low-grade chronic infection. This novel molecular biology technique may be useful in the diagnosis of periprosthetic infections and in confirming that the infection has been cleared before preparing for reinsertion of the prosthesis.
Management of periprosthetic infections under the biofilm theory
Periprosthetic infections are usually first classified as acute or chronic before a management plan is determined: infections occurring within 4 weeks of the initial surgery are considered acute, whereas infections occurring after 4 weeks postoperatively are considered chronic.
Early postoperative infections are usually associated with pain, poor wound healing, localized redness and swelling, and prolonged wound oozing. This also includes the sudden onset of acute infection in an otherwise well-functioning prosthetic joint, usually after 1 year postoperatively. This acute infection is considered to be a hematogenous secondary infection from an infected lesion elsewhere in the body and usually presents as a painful swollen joint.
Chronic periprosthetic infections are usually less pronounced and may simply present as chronic pain.
Acute infections are usually treated surgically, including debridement and irrigation to preserve the prosthesis, debridement and irrigation to replace the prosthesis (phase I revision), debridement and irrigation to remove the original prosthesis and place an antibiotic cement spacer, and reinsertion of the prosthesis after the infection is controlled (phase II revision).
Chronic infections can be revised in either phase I or phase II. A simple debridement and irrigation with retention of the prosthesis has a very high failure rate in the management of chronic periprosthetic infections.
The wide variation in outcomes in the literature for acute periprosthetic infections treated with debridement and retention of the prosthesis can be explained, in part, by the presence of biofilm. If the biofilm is not completely removed from the infected area, then any surgical approach will ultimately fail.
A simple debridement and irrigation with retention of the prosthesis can be successful if surgery is performed in time before biofilm forms on the surface of the prosthesis, or if the biofilm is adequately removed during surgery. In cases of chronic infection, more thorough intraoperative debridement may be required to remove any biofilm that may have formed in the bone tissue surrounding the prosthesis.
Although second-stage revision is considered more reliable, if the biofilm is not completely removed from the lesion, eventual failure is inevitable. The essence of both second-stage revision and first-stage revision is to remove all biofilm attached to the prosthetic component, including its surface, so that the biofilm can be removed from the tissue surrounding the lesion.
Future Research Directions
Future research goals include elucidating the processes underlying bacterial infection and biofilm formation.
Clearly, the first step in preventing infection formation lies in preventing bacterial adhesion.
From a prosthetic perspective, measures such as developing prosthetic surface structures that do not attract bacterial residency and treating the prosthetic surface with an antimicrobial coating may reduce bacterial biofilm formation. Although some studies have found that prostheses with vancomycin covalently bound to the surface can effectively inhibit the growth of S. aureus while still promoting bone healing, this may also predispose bacteria to develop resistance to the appropriate antibiotic.
Another approach is to treat the surface of the prosthesis with a biosurfactant that adheres to the prosthesis. Some bacteria, such as Lactobacillus, are able to synthesize complexes with dual affinity and may be used to treat silicon-based surface coatings. Acacia alcohol, which is classified as a citrus-derived alcohol, was recently found to inhibit the formation of S. aureus biofilms on titanium alloy surfaces. Some other materials may not be suitable for prosthetic surface coating treatment, but they can be used to remove or disrupt biofilms.
Blocking the population induction between bacteria is another possible treatment. More in-depth studies on the molecular mechanisms of the population-sensing phenomenon have made it more feasible to disrupt the information traffic between bacteria. Some investigators have found that the use of ribonucleic acid III inhibitory peptides is effective in preventing graft-associated infections caused by a variety of Staphylococcus spp. bacteria, including methicillin-resistant strains. The ultimate goal of related research is to find similar inhibitors that can actively cross multiple, rather than specific, bacterial walls.
Additional research is focused on developing vaccines against common pathogenic bacteria such as Staphylococcus aureus. Animal studies have initially demonstrated vaccines against specific biofilm antigens to treat S. aureus chronic osteomyelitis.
The combination of vaccine and vancomycin significantly reduced the infection rate in experimental animals and was even effective against infections caused by methicillin-resistant Staphylococcus aureus. The model also highlights the importance of distinguishing between different phenotypes of bacteria: vancomycin is used to clear the planktonic form of bacteria, whereas the vaccine is used to clear the biofilm form of bacteria.
If antigens of specific bacteria can be isolated and the corresponding antibodies further produced, an effective way to diagnose and treat biofilm infections may be found.
Conclusion
Many of the challenges currently faced in the diagnosis and treatment of chronic periprosthetic infections can be explained by the biofilm theory. Further basic research could help provide insight into the biology of biofilms and thus help improve the diagnostic and therapeutic success of these chronic infections.
Finding a therapeutic regimen that targets not only a specific bacterial or fungal strain, but multiple pathogens remains a great challenge.