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 articulations have the potential for poor wear performance and more conforming, mobile-bearing UKR designs have been advocated. This study evaluates the wear performance of unconstrained UKR polyethylene bearings retrieved at revision knee arthroplasty.
Forty-two UKR with fixed polyethylene tibial bearings were retrieved. Patient age and time in-situ averaged 73 (45–89) years and 7 (1–19) years, respectively. All knees had intact cruciate ligaments at index surgery. Revision reasons included loosening (45%), progressive arthritis (17%), polyethylene wear (17%), instability (5%), and other (17%). Retrospective radiographic review of radiolucent lines and component alignment was completed using Knee Society guidelines. Polyethylene articular damage size (% of articular surface area), location and damage mode incidence were measured using microscopy and digital image analysis.
Damage area was centrally located and averaged 65%+22%. The largest damage areas consisted of abrasion (19%) and scratching (17%). Revision for loosening or wear was significantly correlated with greater damage area (Spearman Correlation, p=0.049). The incidence of scratching, pitting and abrasion each exceeded 70%, including 29 inserts with peripheral abrasive damage consistent with impingement between the polyethylene and extra-articular cement or bone. Anterior damage location and abrasion were significantly correlated with component position (p< 0.001). Concave surface deformation due to femoral component contact was externally rotated (24 inserts), consistent with tibial external rotation relative to the femoral component, neutrally aligned (11 inserts), internally rotated (4 inserts), and indeterminate (3 inserts).
Despite initial tibiofemoral incongruity and concerns of high contact stress, round-on-flat UKR offers a durable knee arthroplasty. The relatively unconstrained tibiofemoral articulations allowed freedom of placement on the resected bone surfaces and a range of tibio-femoral rotation during activity, as demonstrated by the rotated concave surface deformations. Such deformation may reduce polyethylene contact stresses by increasing the tibio-femoral contact area. However, similar to retrieved mobile bearing UKR which show a 63% incidence of impingement, abrasive damage on these fixed bearing UKR has consequences for polyethylene debris generation and the transmission of shear forces to the bone-implant interface. Rigorous attention to conventional and minimally invasive surgical technique, including cement fixation and component position, is needed to reduce the incidence of abrasive polyethylene damage.
Theses abstracts were prepared by Professor Roger Lemaire. Correspondence should be addressed to EFORT Central Office, Freihofstrasse 22, CH-8700 Küsnacht, Switzerland.
