Effect of Packaging on Polyethylene Fractures, Implant Survivorship and Cyst Formation in Mobile Bearing Total Ankle Replacements

Abstract

Category: Ankle Arthritis; Ankle Introduction/Purpose: The Scandinavian Total Ankle Replacement (STAR) is the most widely used mobile bearing total ankle arthroplasty (TAA) in the US. The polyethylene component is ultrahigh molecular weight polyethylene and is designed to articulate with both the tibial and talar component. Initial data on the STAR in the US reported polyethylene fracture rates of greater than 10% as well as a concern for cyst formation due to osteolysis. The manufacturer changed the packaging to foil to prevent potential oxidation of the polyethylene, which could predispose the polyethylene to failure. The objective of the study was to evaluate a cohort of patients with STAR performed before and after converting to foil packaging to determine if there is a difference in polyethylene survivorship and cyst formation. Methods: All primary TARs performed by the senior author from 2010-2017 were reviewed. Patients with a minimum 48 month follow up were included in the study. The two groups were stratified by those performed prior to August 2014 (PRE) and those performed after August 2014 (POST). Preoperative and most recent postoperative alignment was determined on weight bearing radiographs. The outcomes included polyethylene fractures and cyst formation in the two groups. Cysts were defined as those measuring greater than 1cm 2 on plain radiographs. Patients were recommended to have surgery if cyst volume was greater than 2cm 2 . Chi-squared tests were used to detect significant differences in the rate of cyst formation between the two groups. Paired t- tests were used to determine difference in preoperative versus postoperative alignment, follow up length, and polyethylene component thickness and patient age at time of procedure between the two groups. Results: The PRE group had 35 patients with an average follow up of 75 months (range 48-118). The POST group had 15 patients with average follow up 55 months (range 48-60). There was no difference in preoperative versus postoperative radiographic alignment between the groups (p>0.05). Cysts were present in 9 (26%) patients in the PRE group versus 4 patients (26%) in the POST group. Nine patients (26%) needed a second surgery due to cyst formation in the PRE group versus two (16%) in the POST group (p>0.05). Four patients (11%) had a fracture of the polyethylene in the PRE group with an average thickness of 6.7mm (range 6-9) and 77 months (range 68-84) versus zero in the POST group. The average time to polyethylene exchange for cyst(s) or polyethylene fracture was 58.3 months. Conclusion: This study suggests that changes in packaging to limit oxidation of the polyethylene from plastic to foil did not appear to represent a difference in periprosthetic cyst formation. However, there was a trend for additional surgery related to periprosthetic cysts and increased risk for polyethylene fracture in the PRE group. These results could suggest the potential for oxidation changes that alter the wear characteristics and fragility of the polyethylene component. Confounding factors could include postoperative time and component thickness. Further investigation and longer follow-up will be needed to further delineate oxygenation impacts on TAA polyethylene components.