Defining "Paradoxical Instability" and Other Indications for Aseptic Single-Component Polyethylene Revision: A Cohort...

Single-component polyethylene revision (SCPR) is a less invasive approach for addressing polyethylene wear in total knee arthroplasty by replacing only the tibial polyethylene component. While traditionally used for aseptic ligamentous mid-flexion instability and limited arc of motion, this study introduces paradoxical instability as a third indication, characterized by ligamentous laxity with knee effusion causing mid-flexion instability and secondary flexion contracture.

A retrospective study analyzed 58 consecutive SCPR patients treated by a single fellowship-trained surgeon between 2012 and 2024, with a minimum 2-year follow-up. The cohort included 20 men (34%) and 38 women (66%), with a mean age of 70.2 years and average follow-up of 2.83 years. Patients were categorized into 3 groups: mid-flexion instability (n = 43), limited arc of motion (n = 9), and paradoxical instability (n = 6). Outcomes were assessed using clinical examinations and Knee Society Score 2011 and UCLA activity scores.

Clinical success rates defined as good to excellent clinical outcomes were 88.3% (38 of 43) for mid-flexion instability, 88.8% (8 of 9) for limited arc of motion, and 100% (6 of 6) for paradoxical instability patients. No readmissions or reoperations occurred within 90 days postsurgery. The mean hospital stay was 0.87 days.

SCPR demonstrated effectiveness in treating all 3 indications, with particularly promising results for paradoxical instability cases. This study establishes paradoxical instability as a distinct clinical entity characterized by knee imbalance, mid-flexion instability, recurrent effusions, and limited motion arc. This limited motion arc is paradoxically treated with an increase of polyethylene thickness. The findings support SCPR as a viable treatment option for carefully selected patients with these conditions.