Total shoulder arthroplasty - what materials should we be using?

 The Biomaterials of Total Shoulder Arthroplasty Their Features, Function, and Effect on Outcomes

These authors review the materials used in total shoulder arthroplasty (TSA). They point out that the 2 main metal alloys used in TSA implants are Ti-6Al-4V (titaniumaluminum-vanadium) and CoCrMo (cobalt-chromium-molybdenum). Ti alloys are softer than CoCr alloys, making them less wear-resistant and more susceptible to damage, but they have improved osseointegration and osteoconduction properties. While some surgeons are interested in "osseointegration" of the humeral component, bone ingrowth is not necessary for secure durable fixation; furthermore, revision of an ingrowth stem carries with it a much higher complication risk than the revision of an impaction grafted smooth stem (see this link).

While metal allergy may be a concern in some patients having problems after TSA, the diagnosis of this metal allergy is difficult. This concern is not sufficiently compelling to merit the routine use of "hypoallergenic" prostheses. Prosthesis without nickel or chromium seem to be mechanically inferior, so avoiding the risk of "metal allergy" may give rise to other problems. For example, while ceramic and pyrolytic carbon humeral heads may have theoretical advantages, they also pose an increased risk of fracture - a complication unknown with cobalt chrome heads. 

On the glenoid side, cross linked polyethylene glenoids have an excellent record of survivorship as shown by the data from the Australian Orthopaedic Association. 

Comment: With the current state the art, total shoulder arthroplasty using standard implants has an established track record of excellent clinical outcomes. Efforts to improve on this track record may result in more costly implants and in unexpected outcomes (see this link).

Our time tested approach to total shoulder arthroplasty is shown in this link.

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Biofilms on titanium - the race to the surface

Strategies to Prevent Biofilm Infections on Biomaterials: Effect of Novel Naturally-Derived Biofilm Inhibitors on a Competitive Colonization Model of Titanium by Staphylococcus aureus and Human Cells

This is a very interesting article that points out that bacteria love to form biofilms on the material we love to use in arthroplasty: titanium and that there is competitive colonization of the surface, either by  host cells or by bacteria = 'the race for the surface'.

Since this article appeared in a journal that shoulder surgeons may not read regularly, we summarize a bit of the article here.

"Antimicrobial resistance is one of the major healthcare challenges that is currently faced by mankind. By switching into the biofilm state, bacteria can withstand antibiotic chemotherapy, and this is increasingly regarded as the most important nonspecific mechanism of antimicrobial resistance. Biofilms are defined as a community of cells encased within a self-produced matrix that adhere to biological or non-biological surfaces. Because implanted medical devices can be ideal substrates for bacteria to attach, biofilm-mediated infections are one of the leading causes of prosthesis implantation failures.

When implanting a biomaterial, the desired outcome is the correct integration of such material with the host tissue. However, this ideal outcome is often impacted by the presence of bacterial cells at the moment of implantation. According to the concept of “race for the surface”, if host cells are able to colonize the surface of the device first, the chances of bacterial cells to adhere to such surface are lower, therefore lowering the risk of implant infection. A frequent route of infection for implants occurs during surgery, as microorganisms can be introduced on the implant surface, providing them with an advantage to colonize the unprotected surface and create a biofilm.

Taking this in consideration, a reasonable approach would be to design an antimicrobial material or coating, which promotes tissue integration. In that direction, it would be advantageous to precondition the material with host cells. Staphylococcus aureus is found asymptomatically on the skin and its presence there enhances the risk of infection in the surgical site, which is why it is egarded as a frequent causative agent of implant-related infections, especially in orthopaedics."

The authors assessed the potential applicability of three anti-biofilm compounds (based on natural compounds) as part of implanted medical devices by testing them on in a competition model based on the co-culture of SaOS-2 mammalian cells and Staphylococcus aureus (collection and clinical strains) on a titanium surface. 

Comment: The search for non-antibiotic approaches to the prevention and treatment of prosthetic infections is of great importance in that it is apparent that antibiotics cannot do the job alone.

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The limited effectiveness of Vancomycin-containing spacers in treating periprosthetic infections

Vancomycin-Loaded Polymethylmethacrylate Spacers Fail to Eradicate Periprosthetic Joint Infection in a Clinically Representative Mouse Model

These authors developed a murine model of the first-stage surgery of a 2-stage revision for periprosthetic joint infection involving a 3-dimensionally printed Ti-6Al-4V tibial implant

 infected with Staphylococcus aureus and treated 2 weeks later with irrigation and debridement of the leg with revision of the implant to an articulating vancomycin-loaded PMMA spacer.

Postoperatively, mice underwent radiography and serum inflammatory-marker measurements. Following euthanasia of the mice at 6 weeks, bone and soft tissues were homogenized to quantify bacteria within periprosthetic tissues. Implants and articulating spacers were either sonicated to quantify adherent bacteria or examined under scanning electron microscopy (SEM) to characterize the biofilm.

They found that vancomycin-loaded PMMA spacers eluted vancomycin for 6 days with retained antimicrobial activity.

Control mice had elevated levels of inflammatory markers, radiographic evidence of septic loosening of the implant, and osseous destruction. Mice treated with the vancomycin-loaded PMMA spacer had significantly lower levels of inflammatory markers (p < 0.01), preserved tibial bone, and no intra-articular purulence. 

Retrieved vancomycin-loaded spacers exhibited significantly lower bacterial counts compared with implants (p < 0.001). SEM identified S. aureus encased within biofilm on control implants,

while vancomycin-loaded spacers contained no bacteria.

However, bacterial counts in periprosthetic tissue did not significantly differ between the groups.

The results suggest that the antimicrobial effects of PMMA spacers are tightly confined to the articular space and must be utilized in conjunction with thorough tissue debridement and systemic antibiotics to manage bacteria in the surrounding tissue.

Comment: This article once again demonstrates the remarkable propensity of titanium to enable biofilm formation. It also shows the effectiveness of Vancomycin spacers to resist biofilm formation and to help manage bacteria in the joint. However, it points to the relative ineffectiveness of spacers in managing infection in the tissues surrounding the joint. While they advocate "thorough tissue debridement," it is not always easy to discern what tissue needs to be debrided during surgical management of a periprosthetic infection. Hopefully a combination of debridement and systemic antibiotics will prove effective in managing bacteria in the tissues surrounding an infected prosthesis.

Interested readers may want to read these related posts:

Quantification of Peri-Implant Bacterial Load and in Vivo Biofilm Formation in an Innovative, Clinically Representative Mouse Model of Periprosthetic Joint Infection

Delayed Propionibacterium acnes surgical site infections occur only in the presence of an implant.

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