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Vitamin E Plays Important Role in Stabilizing UHMWPE Material Properties

Contributor

Luis Alvarez
Director of Engineering & Development

Contemporary advancements in polyethylene materials have been introduced with the aim to improve longevity through reduction of wear, oxidation, and degradation.

Evaluating Knee Poly Under Adverse Conditions for Improved Performance Insights

Under normal expected in-vivo or simulated in-vitro conditions it can be difficult to distinguish differences in performance of modern orthopedic bearings. For this reason, it can be beneficial to simulate adverse articular conditions reported in clinical literature to understand how various bearing materials fare in comparison.

Early clinical failures are still reported when conventional and contemporary sequentially irradiated and annealed total knee bearings are exposed to posterior edge-loading thought to be induced by improper tensioning of the PCL ligament in cruciate retaining total knee arthroplasty. 1, 2, 3

The aged Activit-E™ poly completed all three rounds of testing without failure.

New Test Simulates Tight PCL Conditions to Study Knee Poly Durability

Exactech has developed an aggressive wear test designed to simulate articular conditions when the PCL ligament is overly tight, resulting in posterior edge loading of the tibial insert and an increased risk of early insert failure. Due to the aggressive nature of the test, a relatively low cycle count is adequate to induce failure of certain poly formulations.

The tight PCL test is designed to subject the inserts to 50k cycles at 100% of a prescribed load adapted from a standard wear test waveform, followed by another 50k cycles under the load scaled to125%, and finally a third iteration of 50k cycles at a load scaled to 150%. Samples are inspected for failure every 10k cycles. Machined Conventional UHMWPE (MCPE), sequentially irradiated and annealed highly crosslinked (SXL), and Activit-E™ peroxide highly crosslinked, vitamin E polyethylene inserts were subjected to accelerated aging in a pressurized oxygen chamber followed by the tight PCL wear challenge.

Performance of MCPE, SXL and Activit-E Inserts Under Aggressive Wear Testing

The aged MCPE inserts were unable to withstand the initial 50k cycle challenge while the aged SXL inserts began to fracture during the 50k cycle phase under 125% scaled loading.  The aged Activit-E™ poly completed all three rounds of testing without failure.

Figure 1 shows the failed MCPE and SXL materials with both inserts exhibiting fractures of the posterior condylar edge.

sequentially-crosslinked UHMWPE SXL sample and modified carbon paste electrode MCPE sample after 50k cycle
Figure 1: sequentially-crosslinked UHMWPE SXL poly (left) and modified carbon paste electrode machined conventional UHMWPE sample (right) exhibiting fractures consistent with literature reports.

 

Figure 2 illustrates the relatively unaffected articular surface of the Activit-E bearing after completion of the full test.

Exactech Knee Poly Activit-E peroxide highly crosslinked, vitamin E polyethylene
Figure 2: The aged Exactech poly Activit-E after successful completion of the tight PCL test.

Conclusion

Although considered an aggressive test, the method is able to reproduce similar failure modes illustrated in literature and demonstrates the important role vitamin E plays in stabilizing UHMWPE material properties.

References:

  1. D. W. MacDonald, G. B. Higgs, A. F. Chen, A. L. Malkani, M. A. Mont, and S. M. Kurtz, “Oxidation, Damage Mechanisms, and Reasons for Revision of Sequentially Annealed Highly Crosslinked Polyethylene in Total Knee Arthroplasty,” The Journal of Arthroplasty, vol. 33, pp. 1235–1241, Sep. 2017, doi: https://doi.org/10.1016/j.arth.2017.09.036.
  2. K. A. Sonn and R. M. Meneghini, “Early failure of sequentially annealed polyethylene in total knee arthroplasty,” Arthroplasty Today, vol. 6, no. 1, pp. 18–22, Mar. 2020, doi: 10.1016/j.artd.2019.12.005.
  3. M. R. Swany and R. D. Scott, “Posterior polyethylene wear in posterior cruciate ligament-retaining total knee arthroplasty. A case study,” J Arthroplasty, vol. 8, no. 4, Art. no. 4, Aug. 1993, doi: 10.1016/s0883-5403(06)80045-8.