The cement you pick and the order you mix it change the antibiotic dose

Design

in vitro comparative study of 60 polymethylmethacrylate discs across five formulations and two mixing sequences

Population

not applicable — laboratory discs loaded with 4 g vancomycin, tested against methicillin-sensitive Staphylococcus aureus

Primary outcome

cumulative vancomycin elution over 28 days, with antimicrobial activity and porosity

Effect

Palacos MV monomer-first 2,897 ± 64 µg/mL, 120% greater than Simplex P monomer-first; polymer-first Simplex P highest first-hour release at 415 ± 33 µg/mL

High-dose vancomycin in a polymethylmethacrylate spacer is standard in two-stage revision for chronic periprosthetic joint infection, and how it is mixed has been left to habit. Sixty discs across five formulations — two brands, two viscosities, with and without premixed gentamicin — were loaded with 4 g of vancomycin using either a polymer-first or a monomer-first sequence, and elution measured over 28 days alongside activity against methicillin-sensitive Staphylococcus aureus and micro-computed tomography for porosity.

The differences were not marginal. Palacos MV, medium viscosity, mixed monomer-first gave the greatest cumulative elution at 2,897 ± 64 µg/mL — 120% more than Simplex P mixed the same way. Within the Palacos range, monomer-first mixing of the medium-viscosity MV released 57% more vancomycin than the high-viscosity R. Simplex P behaved in the opposite direction: polymer-first mixing gave both the highest first-hour release, 415 ± 33 µg/mL, and 52% greater cumulative elution over 28 days than its monomer-first sequence. Premixed gentamicin helped only the high-viscosity formulations. Vancomycin stayed bactericidal in every combination and porosity did not differ.

Two things follow. First, brand and mixing order are not interchangeable variables to be delegated to whoever is scrubbed; the same 4 g delivers very different totals depending on them. Second, the sequence that is best depends on the brand, so a unit cannot adopt one rule for all cement — it has to know which cement is on its shelf.

The choice also depends on what is wanted. A high early peak and a large cumulative total are different targets, and the two brands optimise different ones.

• Write the brand, viscosity and mixing sequence into your spacer protocol rather than leaving it to the scrub team
• For Palacos MV, mix monomer-first; for Simplex P, polymer-first — the best order differs by brand
• Decide whether you want peak early release or maximum 28-day total, and choose accordingly
• Do not assume premixed gentamicin helps; it only increased vancomycin elution in high-viscosity cement here
• Note that mixing sequence did not harm cement integrity or antibiotic activity, so this costs nothing to change

Why it matters

A variable currently decided by habit turns out to change the antibiotic dose delivered by a factor of two.

Don't overread it

This is an in vitro study — greater elution has not been shown to eradicate more periprosthetic infections.

The statistics, in plain English

These are laboratory elution measurements with Holm-corrected comparisons across 60 discs, and the differences — 120% and 57% — are far larger than the standard errors, so they are unlikely to be noise. What the numbers cannot tell you is whether a doubled cumulative elution eradicates more infections. Elution is a surrogate; the outcome that matters is infection-free survival, and no clinical endpoint was measured here.

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