1. Introduction
Periprosthetic Joint Infections (PJI) are a challenging complication in joint replacement surgery that often results in worse outcome for the patients, especially when the causing pathogen is a multi-resistant germ [
1] and/or the patient is at high risk [
2,
3]. PMMA bone cements loaded with a single (SALBC) or two antibiotics (DALBC) are used for PJI prevention or treatment, e.g. for a spacer in the interim period of a two-stage exchange procedure [
4,
5,
6,
7]. Especially DALBC support PJI reduction compared to SALBC [
8]. Commercially available antibiotic loaded bone cements (ALBCs) mainly contain the antibiotics Gentamicin or Tobramycin (amynoglycosides), Vancomycin (glycopetide), Clindamycin (lincosamide) [
9]. The number of PJIs caused by resistant germs, among them Vancomycin-resistant
Staphylococcus aureus (VRSA), Vancomycin-resistant
Enterococci (VRE), Methicillin-resistant
Staphylococcus aureus (MRSA) and Methicillin-resistant
Staphylococcus epidermidis (MRSE) is rising [
10,
11] and the manually addition of vancomycin or commercially available ALBCs lack to cover these germs. The named bacterial species developed resistance mainly against Vancomycin and Methicillin therefore antibiotics used for treatment must be adapted to the resistance pattern [
12]. An antibiotic found to be effective against Vancomycin-intermediate resistant
S. aureus (VISA), VRSA and VRE is Daptomycin; it also shows antimicrobial activity against anaerobic bacteria but none against gram-negative bacteria [
10,
13]. Daptomycin has a unique mode of action and disrupts the cell membrane integrity of bacteria [
14].
There is no Daptomycin ALBC commercially available and the manually admixing of Daptomycin is highly expensive (Cubicin, ~ 780 € / 2 g). In clinical practice, surgeons increase the Vancomycin concentration up to 4 g/40g polymer powder to treat Vancomycin-resistant germs (VRE). But a massively increased antibiotic concentration in PMMA spacers negatively influences their mechanical stability [
15] and the risk of local as well as systemic kidney toxicity is highly increased. According to “Pocket Guide to Diagnosis & Treatment of PJI” from PRO-IMPLANT Foundation (PIF) 2 g of Daptomycin can be added to a fixation cement and 3 g of Daptomycin to a spacer cement [
16]. The addition of more than 2 g Daptomycin results in non-compliant ISO mechanical properties.
With this investigation we wanted to figure out which concentration of Daptomycin can be added to PMMA cement to efficiently inhibit bacterial growth and at the same time deliver mechanical stability according to ISO standards.
3. Discussion
As PJI especially when caused by resistant germs still poses a threat for patients and the health care system, any kind of support to prevent a PJI or treat it in a two-stage revision protocol are needed [
17]. Daptomycin containing PMMA cement is recommended in PJI when combatting vancomycin resistance germs e.g., VRE.
In clinical practice, spacers for two-stage revision procedures are created by using industrially manufactured cement already containing Vancomycin or by manually admixing Vancomycin to PMMA cement [
6,
18,
19]. The use of the commercially available cement COPAL
® G+V is already established in PJI prevention especially against the most frequent PJI germs
S. aureus and
S. epidermidis [
8,
18] – but does not cover VRSA, VISA or VRSE. In general, it's recommended to use a fixation or spacer cement containing two complementary antibiotics to best cover the spectrum of PJI pathogens [
12]. The spectrum of Vancomycin is predominantly limited to gram-positive bacteria and therefore a broad-spectrum antibiotic is needed to also cover gram-negative bacteria, e.g. Gentamicin [
13]. Additionally, the combination of two antibiotics offers a synergistic elution effect that leads to an overall increased antibiotic elution from PMMA bone cement resulting in a stronger antimicrobial effect [
2]. Antibiotic-loaded bone cement with Gentamicin and Daptomycin can make a difference in prevention and treatment of PJI with Vancomycin-resistant pathogens [
20]. Daptomycin offers a good safety profile and has a unique mode of action maybe also effective against bacteria in biofilms [
20] coming with a low resistance profile [
21]. According to Gray and Wenzel, 2020 [
14] knowledge on the exact mode of action is still missing, but they observed that the resistance development of Daptomycin is slower compared to other antibiotics with single protein targets. Rouse et al. [
22] figured out with a rat model that a PMMA cement with Daptomycin may be an option for the local treatment of resistant bacteria causing osteomyelitis. The manual admixture of Daptomycin to PMMA bone cement is suggested for PJI cases caused by VRSA, VRE, MRSA and MRSE with a Vancomycin MIC of greater than 2 µg/mL [
10]. A first case study 2013 already showed the ability of PMMA spacers with Daptomycin to eradicate an infection in a two-stage revision hip surgery [
23]. In our study we found an antimicrobial effectiveness for GD1.5 and GD1.0 whereas a concentration of 0.5 g Daptomycin was not sufficient. This is in line with findings from Eick et al. [
24] who concluded from their inhibition zone testing, that 1.5 g of Daptomycin shows an antimicrobial effect in contrast to 0.5 g. This may also be caused by the combination of two antibiotics Gentamicin and Daptomycin and the resulting synergistic elution effect. The combination of 0.5 g Gentamicin and 1.5 g Daptomycin showed the best antibiotic elution profile, as well as a synergistic elution effect supporting infection prevention [
15]. The overall elution profile of Daptomycin is comparable to the findings from Meeker et al. [
25] showing a peak elution for the first 24 hours. Our observed synergistic elution effect is in contrast to the studies quoted by Antonello et al. [
20] concluding a rather antagonistic interaction of Daptomycin and Gentamicin from their study review. But also studies in the larvae biofilm model
Galleria melonella showed a synergistic effect combining Gentamicin with Daptomycin for Vancomycin resistant
E. faecium [
26]. We observed an effect on the bacterial growth comparable to Webb et al. [
27] indicating an inhibitory effect of Daptomycin on the growth of resistant strains of gram-positive bacteria. Overall, the synergistic elution effect as well as the high Daptomycin release suggested a positive effect in combining 0.5 g Gentamycin and 1.5 g Daptomycin in PMMA bone cement. We also investigated on the Vancomycin elution as COPAL
® G+V was set as reference: despite we found the highest Vancomycin elution for day 1, we assume this concentration is not sufficient to cover Vancomycin-resistant germs. Especially as the elution already decreased by ~85 % on day 2.
To ensure patient safety Daptomycin must be sterilized with either ethylene oxide or gamma radiation. Our findings suggested that Daptomycin-loaded bone cement should be sterilized using gamma radiation to maintain its antimicrobial effectiveness because a sterilization by ethylene oxide reduced the efficacy of Daptomycin significantly.
To best treat MRSE, MRSA or enterococci PIF pocket guide [
16] recommends to increase the Vancomycin concentration of commercially available COPAL
® G+V by another 2 g. But an increase of added antibiotic powder exceeding a total concentration of 10 % results in a spacer cement that does not longer fulfill the mechanical ISO requirements for bone cement [
15,
19,
28]. Despite Lunz et al. [
29] point out, that an antibiotic concentration exceeding 10 % of powder volume significantly reduces the mechanical strength of PMMA spacers, they recommend to manually admix 4 g of Vancomycin to PALACOS
® R+G instead of using the commercially available COPAL
® G+V. These spacers do not comply with the ISO requirements as the bending strength for PALACOS
® R+G + 4 g Vancomycin is below the minimum threshold of 50 MPa and comes with the potential risk of bone cement or spacer fracture [
15,
19,
21,
29] [
15]. Therefore, we assessed the mechanical stability of a Daptomycin-containing bone cement. ISO bending modulus of GD samples was increased compared to the reference samples as admixing antibiotics increases the hydrophilic characteristics of PMMA cements which results in an increased elasticity of the cement [
15]. The mechanical properties of GD1.5, GD1.0 and GD0.5 were all above the corresponding minimum thresholds as the concentration of the added Daptomycin was below 10 % of total PMMA cement powder [
15,
22,
30]. Considering the mechanical properties, a concentration of 1.0 g Daptomycin would be ideal, but does not offer a sufficient antibiotic release needed for infection prevention. As the bone cement GD1.5 showed antimicrobial effectiveness and promising mechanical properties handling characteristics were only assessed for this bone cement sample. ISO setting time, assessed according to ISO 5833:2002 [
31], determines the time point when the bone cement is completely set and cannot be handled any longer. ISO doughing time describes the time till PMMA cement reaches the dough state. ISO setting time as well as ISO doughing time were comparable to PALACOS
® R+G and COPAL
® R+V indicating a slightly faster setting time for GD1.5 which means that the application time window shortens.
Our investigations indicate that the performance of a PMMA bone cement containing 0.5 g Gentamicin and 1.5 g Daptomycin is the choice considering its antibiotic effectiveness, antibiotic release, and mechanical stability (
Figure 7). According to the recommendations from PRO-IMPLANT Foundation 3.0 g Daptomycin can be added to a PMMA spacer cement [
16], but Kühn [
15] suggested not to add more than 2 g Daptomycin, which is in line with our findings. PMMA spacers with manually added antibiotics also must fulfill the legal requirements for medical devices and comply with the ISO standards [
15]. This is rather from a legal perspective important as the surgeon becomes – by admixing antibiotics – the legal manufacturer of the product. We assumed that the concentration of 3 g Daptomycin was recommended because the antimicrobial effect was perceived as not sufficient, so this indicates that the antibiotic release is too weak. It’s also described in literature that the “Daptomycin dose in ALBC for spacer should be 3.3-times the original dose to double the release” [
21]. As the mode of action of Daptomycin is dependent on calcium ions, solely adding more Daptomycin does not necessarily improve the antimicrobial effectiveness [
14]. The mode of action is dependent on a sufficient concentration of calcium-ions [
14] in the surrounding tissue: to increase the inhibitory effect of Daptomycin eluted from PMMA bone cement potentially calcium ions could be added [
30]. We want to investigate on that in a further study. As manually admixed ALBC is mainly used for spacers we recommend investigating a longer time period of more than 14 days to better simulate the spacer interim period of a two-stage revision protocol [
19]. Despite this we also recommend further investigations with Daptomycin-containing PMMA cement in biofilm models.
We want to highlight, that a surgeon ordering Daptomycin from the pharmacy will receive a product other from the industrially used Daptomycin. The clinical available “Cubicin” [
32] contains beside Daptomycin also natrium hydroxide, its potential influence on the mechanical properties and antibiotic elution from PMMA bone cement is not yet clarified. Following the recommendations from PRO-IMPLANT foundation [
16] the addition of Cubicin is costly: for a fixation cement ~ 780 € (2 g) and even more for a spacer cement ~1,500 € (3 g); not yet including the price for a Gentamicin-loaded PMMA bone cement as basis for admixing. From a financial perspective, a commercially available PMMA bone cement with Gentamicin and Daptomycin could be of interest.