Delamination and Oxidation in Compression-Molded Polyethylene

At our institution, surgeons were observing cases of failed total knee arthroplasties (TKAs) with surface delamination of the tibial insert fabricated by direct compression molding. The increase in unexpected failure led us to investigate the prevalence of delamination and its causes through the use of retrieval analysis and reviews of clinical, demographic, and radiographic data.

Between 2000 and 2019, a total of 519 Exactech Optetrak posterior-stabilized direct-compression-molded polyethylene inserts had been retrieved. To determine prevalence, we utilized institutional usage data, manufacturer sales to our institution, and hospital records to determine the delamination rate. Eighty-six retrieved specimens (16 with delamination) were assessed for oxidation with use of infrared spectroscopy.

Sixty-four (12%) of the 519 inserts had delamination. The delamination rate was 0.36% across the 20-year period. Osteolysis was the reason for revision in 25% of delaminated cases, compared with 4% of non-delaminated cases. The mean oxidation index of the delaminated inserts was 2.67 ± 1.4 (range, 1.2 to 6.6). Delamination was not associated with surgical factors (cement viscosity and tibial insert thickness) or processes associated with manufacturing and implantation of the inserts into the patients (implantation year, shelf life, and packaging and sterilization dates).

The lack of causative factors for the increase in delamination was perplexing. In 2021, following the completion of our study, the manufacturer determined that since 2004, polyethylene inserts were packaged in "non-conforming" vacuum bags that were missing a secondary barrier layer intended to markedly lessen oxygen permeation. The use of non-conforming bags apparently increased the risk of premature oxidation, delamination, and associated osteolysis.

Prognostic Level IV . See Instructions for Authors for a complete description of levels of evidence.