MECHANISMS OF POLYETHYLENE DAMAGE ON RETRIEVED UNICONDYLAR KNEE REPLACEMENTS

Abstract

There is renewed interest in unicondylar knee replacements (UKR) to meet the increasing demand for less invasive surgical procedures for knee arthroplasty. UKR survivorship exceeds 85% at 10 years, with unconstrained (round-on-flat) designs showing significantly better survivorship than conforming designs. However, round-on-flat articulation shave the potential for poor wear performance and more conforming, mobile-bearing UKR designs have been advocated. The purpose of this study was to evaluate the wear performance of unconstrained UKR polyethylene bearings retrieved at revision knee arthroplasty.

Forty-two UKR (eight designs) were retrieved from 26 female and 16 male patients. Patient age averaged 73+10 (45–89) years and time in-situ averaged 7+4 (1–19) years. Revision reasons included loosening (45%), progressive osteoarthritis (17%), polyethylene wear (14%), instability (5%), and other (19%). Retrospective radiographic review of radiolucent lines and component alignment was completed. Polyethylene damage (severity score, 0–3) and location were measured using optical microscopy and digital image analysis.

81% of the polyethylene inserts had a concave deformation located on the central or posterior third of the articular surface, consistent with damage due to femoral component articulation. Abrasive damage on 29 (69%) inserts was consistent with impingement between the polyethylene and extra-articular cement or bone. There was delamination in the central region of 7 (17%) inserts and on the extreme posterior rim of 3 (7%) inserts. Severity score averaged 2.0+1.2 for abrasion and 0.5+1.0 for delamination. Radiographic component position was correlated with abrasive polyethylene damage.

Despite initial tibiofemoral incongruity and concerns of high contact stress, round-on-flat UKR offers a durable knee arthroplasty. Delamination was infrequent and did not correlate with time in-situ. Rather, polyethylene cold flow increased the contact area during in-vivo function. Rigorous attention to cement technique and component position may reduce the incidence of abrasive damage on UKR polyethylene inserts.