Mapping Oxidation in Fractured Knee Implant Posts with FT-IR Imaging

Post fractures occur in fewer than 1% of patients, but they have been reported during activities as routine as golfing or biking, and each one requires invasive revision surgery.¹ At SciX 2026, Petra A. Baylin, a research assistant in the Tissue Imaging and Spectroscopy Lab at Temple University, presented work on why polyethylene posts in posterior-stabilized total knee arthroplasty (PS TKA) implants sometimes fracture.¹ This project was led by Nancy Pleshko, who is a Laura H. Carnell Professor Emerita in Temple University's Department of Bioengineering.

As part of our coverage of the SciX 2026 Conference, Pleshko and Baylin sat down with Spectroscopy to talk about their recent study, explaining how Fourier transform infrared (FT-IR) imaging of ultra-high weight molecular polyethylene (UHMWPE) implants permits oxidation evaluation throughout the sample yielding insight into failure.

Post fracture happens in fewer than 1% of posterior stabilized total joint arthroplasty (PS TKA) cases, but revision surgery is invasive. Can you walk us through why even this small failure rate is worth the level of scrutiny you're giving it?

Post fracture is a rare failure mode, but it completely alters how the knee implant construct functions. The post on a posterior-stabilized total knee tibial insert is a critical structure that allows for the knee to achieve femoral rollback, which in turn allows the knee to flex as closely as possible to a natural knee joint. Once the post fractures, patients experience instability, often hear or sense a “pop” when they walk or climb stairs, and eventually can experience serious consequences of polyethylene wear debris, including inflammation and bone loss.

Post fracture requires revision surgery. In most cases, the surgeon will perform a polyethylene liner exchange without disturbing the rest of the implant components to minimize bone loss from revision surgery.

You used FT-IR spectral imaging rather than the more common point-mode FT-IR. What does FT-IR spectral imaging let you see that point measurements would have missed?

FT-IR spectral imaging gives us a more complete picture of oxidation in the implant compared to point mode. Studies of polyethylene implant oxidation using FT-IR microspectroscopy commonly employ one-dimensional (1D) line profiles, with individual spectra acquired at spatial intervals of approximately 100–200 µm. Depending on the length of the profile, this could result in only ~30–60 discrete measurement locations. In contrast, the spectral imaging approach used here acquires spectra at a spatial sampling interval of 50 µm in two dimensions, yielding approximately 10,000–15,000 spectra across each analyzed region and enabling substantially more detailed characterization of the spatial distribution of oxidation.

Why was it important to boil the microtomed slices in n-heptane before analysis, and how might residual lipids have skewed the oxidation index if that step were skipped?

Historically, heptane extraction was not part of the standard protocol for analyzing oxidation in retrieved UHMWPE implants. However, we observed regions with unexpectedly high oxidation indices and carbonyl absorbance that could not be attributed solely to oxidation of the polyethylene. Further investigation showed that lipids from synovial fluid could be absorbed into the polyethylene and were not adequately removed by routine rinsing. These lipids contain ester carbonyl groups that absorb within the same carbonyl region used to assess UHMWPE oxidation. Because oxidation of UHMWPE can also produce ester carbonyls, as well as ketones, carboxylic acids, and other carbonyl-containing product, the lipid contribution cannot simply be distinguished by the presence of an ester peak. Residual lipids can therefore contribute to the measured carbonyl absorbance and artificially elevate the oxidation index. Boiling the microtomed sections in n-heptane extracts these absorbed lipids, allowing the remaining carbonyl absorbance to more accurately reflect oxidation of the polyethylene itself.

You found that oxidation peaks in the midpoint region of the post height. What's your working hypothesis for why that particular region oxidizes more than the surface or base?

This observation mostly makes sense when you consider that oxidation is a cumulative result of oxygen diffusion into the polymer matrix and repetitive loading. The midpoint of the post is where we expect the femoral component to contact the post repeatedly as the patient goes about their life and flexes their knee. This is the expected biomechanics of this type of knee implant; in most cases, the post can withstand the repeated impact load since we know the post fracture occurrence is 1% or lower. However, in additional samples oxidation was also found on the surface of the fracture region. Further analysis should help us understand the significance of oxidation in these different regions.

How does polyethylene oxidation actually translate into a structural fracture risk at the material level. Is it embrittlement, microcracking, or something else?

Polyethylene oxidation occurs as a cumulative outcome of contact stress and oxygen diffusion. Neither of these factors can be eliminated in the knee joint, but materials technology has evolved to make the recent versions of medical-grade polyethylene particularly resistant to oxidation, whether by crosslinking or by adding antioxidants into the polymer matrix. However, the trade-off here is that the resulting polyethylene can be less ductile and therefore, more brittle. Oxidation also is known to cause subsurface hardening and embrittlement, which ultimately leads to delamination. However, it is difficult to say at this time which individual material properties might have exacerbated post fracture occurrence. The team is investigating implant design factors, such as post height, post cross-sectional area, and post-cam contact location and mechanics, to ascertain which factors contribute to post fracture, besides polyethylene oxidation.

If oxidation mapping can flag a fracture-prone region before it fails, what would need to happen for this kind of analysis to inform implant design, sterilization methods, or patient activity guidance going forward?

If oxidation is found to be a critical factor driving post fracture, we would need to address the polyethylene material properties, and that would involve looking at manufacturing, sterilization, and post-manufacturing heat treatments that dictate the material properties. Altering the post design, for example, increasing the cross-sectional area to create wider posts and modifying the implant design to change post-cam contact mechanics, could also mitigate the fracture risk. However, realistically, since this is not a widespread failure mode for the posterior-stabilized knee insert design, it might come down to appropriate implant selection for specific categories of patients. In our study, the patient cohort skews younger, and male. It might be most feasible to use a different kind of implant for this demographic and appropriately guide them towards less demanding activities.

Reference

(1) Baylin, P. A Spectroscopic Imaging Approach for Oxidation Analysis of a Polyethylene Knee Insert Post-Fracture. Presented at the SciX 2026 Conference in Sparks, Nevada. Available at: https://scixconference.org/onlineprogram