Sterile But Not Clean: Managing Retained PMMA in Autoclaved Surgical Trays

Polymethylmethacrylate (PMMA) cement is widely used in arthroplasty, and small fragments can adhere to instruments during routine handling. Once cured, PMMA bonds tightly to metal and plastic surfaces, and standard cleaning may not remove it. As a result, tiny PMMA fragments may survive cleaning, pass through autoclaving, and be discovered only when the sterile tray is opened in the operating room. This scenario creates a distinctive tension for the surgical team because they must decide whether to trust autoclaving and proceed, or replace the entire tray for safety. While autoclaving should theoretically eliminate microorganisms on PMMA1, evidence for this specific scenario has been limited, so teams have had to use their best judgment. Thomson and colleagues addressed this question with experimental data. Their findings suggest that retained PMMA without blood or organic debris carries a very low risk of viable microbes, supporting the view that the tray can be used without replacement. The authors created 3 PMMA models—smooth coupons, coupons indented with deep crevices, and threaded screws partially embedded in polymerizing cement—that reflect conditions encountered in real surgery. After establishing mature methicillin-susceptible Staphylococcus aureus and Escherichia coli biofilms, they applied 3 clinical autoclaving protocols: IUSS (immediate-use steam sterilization), ONE TRAY (Innovative Sterilization Technologies), and ANSI/AAMI (American National Standards Institute-Association for the Advancement of Medical Instrumentation) ST79. All protocols reduced viable colonies below the detection limit, and adenosine triphosphate (ATP) activity fell below the accepted threshold for microbial activity. Seven days of post-sterilization enrichment culture showed no regrowth. As in prior studies2, scanning electron microscopy (SEM) revealed residual biofilm structures on PMMA surfaces after autoclaving. This study has several strengths that directly inform clinical decision-making. The 3 PMMA conditions faithfully reproduced the diverse contamination patterns observed on surgical instruments. The combination of colony-forming unit (CFU) counts, ATP assays, and 7-day enrichment culture thoroughly evaluated the possibility of microbial reactivation after sterilization. The authors deliberately omitted the precleaning steps—soaking, hand scrubbing, and sonication—in the ANSI/AAMI ST79 protocol to isolate the effect of steam sterilization alone. These features directly address the practical question of how much risk residual PMMA poses when only the autoclaving cycle is applied. Taken together, the findings with use of these methods support the conclusion that autoclaving provides adequate microbial inactivation on PMMA-contaminated instruments, even in high-concern scenarios. At the same time, the study has clear limitations that restrict the scope of its conclusions. The authors evaluated only S. aureus and E. coli, so the results may not apply to anaerobes, spore-forming bacteria, fungi, or multispecies biofilms. Organic materials, such as blood, bone fragments, and fat, were not tested, although these substrates can physically shield microorganisms and, after heat denaturation, may impede steam penetration. The persistence of biofilm structures on SEM means that the presence of viable but nonculturable (VBNC) bacteria cannot be excluded, leaving room for organisms that CFU counts and ATP assays cannot detect3. Prion resistance to standard autoclaving is also well known4, so instruments that contact the central nervous system fall outside the study’s “safe zone,” as the authors noted. The findings therefore do not justify relaxing sterilization practices; instead, they help to clarify the problem. If a small PMMA fragment without blood or tissue remains after autoclaving, it alone does not require replacement of the entire tray. However, these findings do not support reducing cleaning steps, expanding indications for IUSS, or simplifying pretreatment. Cleaning removes substrates such as organic debris, and autoclaving inactivates residual microbes; safety depends on both steps5. This study offers evidence to reduce unnecessary tray replacement without undermining the rigor of cleaning and sterilization workflows. In conclusion, this work provides an evidence-based and experimentally grounded answer to a long-standing clinical question: whether viable microbes can remain on PMMA after autoclaving. However, several issues still require investigation, including the biologic and immunologic relevance of nonviable biofilm structures and the persistence of prions, which autoclaving cannot inactivate. Future studies should test a broader range of organisms and evaluate PMMA contaminated with organic material. Furthermore, molecular techniques, such as polymerase chain reaction (PCR), should be incorporated to address the detection of VBNC bacteria. The 3 autoclaving protocols studied appear to eliminate viable bacteria on PMMA under the tested conditions, but the central importance of cleaning remains unchanged. If confirmed and extended, these findings may help reduce unnecessary instrument replacement and prevent avoidable surgical delays caused by residual PMMA fragments.