Evaluating Residual Metal Particulate Debris on New Total Hip Arthroplasty Components

“With the advent of 3D printing and additive manufacturing, there is growing concern amongst orthopedic companies, the FDA and other regulatory bodies that the processes could leave residual amounts of particulate unsintered within the material,” says Nadim Hallab, PhD, the Crown Family Professor of Orthopedic Surgery in the Department of Orthopedic Surgery at Rush.

Shifting the focus to small particles

To date, implant-specific standards for acceptable levels of debris have not been developed, and evaluations of pre-operative particle contamination on implants have utilized the US Pharmacopeia (USP) 788 guidelines, developed in the 1970s for injectable fluids and parenteral devices. Because electron microscopes capable of counting particles <10-μm were not widely available when USP788 was developed, the guidelines do not include small particles. Small particles <10-μm and <1-μm — the focus of Dr. Hallab’s 25-year research at Rush — are of concern in orthopedics, in part because they tend to be more pro-inflammatory than larger debris.

For this study, Dr. Hallab and fellow Rush collaborators Robin Pourzal, PhD, and Lauryn Samelko, PhD, aimed to analyze representative predicate total hip arthroplasty (THA) components to determine the presence of smaller residual particles. The team employed scanning electron microscopy (SEM), which can detect smaller particles more effectively than light obscuration or microscopy.

The study builds on earlier work by Dr. Hallab’s team on immune responses to orthopedic implant debris over the lifetime of implants. Previous research by Hallab, et al, has demonstrated that such debris can trigger hypersensitivity in some patients or a more exuberant immune response that can contribute to premature failure of the implant.

The study’s aim and methodology

The researchers aimed to analyze predicate implant components to help establish clinically relevant acceptance criteria for residual implant debris, including particles <10-μm. To do so, they used well-performing, unused and packaged 20-year-old Titanium-alloy (Ti6AI4V) femoral THA stems, comparing hydroxyapatite (HA)-coated and non-coated stems.

The team processed the stems in a particulate-free environment and used SEM following ASTM 1877-16 guidelines to analyze particulate debris. SEM imaging involved capturing 5–10 fields at low, medium and high magnifications, totaling up to 30 fields with >400 particles counted per specimen.

Results

Under SEM, debris from both coated and noncoated implants showed distinct differences. Coated implants produced more and larger debris, and chemical analysis found that >99% of the particles originated from the HA coating, which typically biodegrades without causing toxicity, Dr. Hallab says. For noncoated implants, the particles arose from the THA stem surface or bulk alloy.

Both types of stems met the USP788 criteria for acceptable residual implant debris (>25-μm (<600 particles) and 10–25-μm (<6,000 particles). The team also found that both implant types had 100 times more smaller particles (<10-μm) than those >10-μm. The study did not confirm the team’s hypothesis that including particles <10-μm in a comprehensive USP788 analysis would exceed current acceptance criteria based on established limits for larger particles.

“The good news is that residual particles on these implants are not really a concern because there have been good methods of cleaning implants, prepping the surfaces and packaging them,” Dr. Hallab says. “It’s been a high-fidelity process over the past 50 years.”

Next steps

The results, previously published in the Journal of Biomedical Materials Research, provide baseline contamination levels for packaged implants that can be used for updating guidelines in the future.

Dr. Hallab and his team plan to further study residual debris, particularly on additively manufactured 3D-printed implants.

“Unlike using cast implant materials where you can count particles on that starting material, every new batch of implants from an orthopedic company’s 3D printing shop should be tested,” he says. “For example, a hiccup in the laser sintering process could leave a pouch of powder with access to the surface, which could be problematic for a patient with metal hypersensitivity.”

As these implants become more widespread, he cautions orthopedic surgeons to remain vigilant and monitor for potential red flags for debris, such as powder on an implant surface.