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

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

Background: We have noted a concerning number of early failures (as defined by revision) for Preservation medial mobile-bearing uni-compartmental knee replacements (UKR’s) implanted in our hospital. This study retrospectively reviewed the postoperative radiographs to see if these were as a result of surgical technical failure. Methods: Between 2003 and 2004, 43 medial mobile-bearing Preservation UKR’s were implanted into 39 patients. The average age of the patients at the time of the index procedure was 61.4 years (range, 46–85), (20 males). The immediate post-operative radiographs were reviewed by 2 independent orthopaedic consultants and a registrar, who were blinded to the patient outcomes, using the radiographic criteria used for the Oxford UKR. We however, particularly tried to identify any medio-lateral offset between femoral and tibial components due to the constrained nature of the prosthesis. A compound error score for all other technical errors was also calculated for each patient. Results: Six (13.9%) of 43 knees were revised (5 for persistent pain, 1 for tibial component subsidence). Technical errors were few and no correlation was found between post-operative radiographic appearances and the subsequent need for revision. The mean compound error score (maximum value 18) was 4.5 (range, 2–9) in the revision cases and in the non revised cases 3.2 (range, 0–8). Conclusions: We believe this study gives credence to the opinion that the DePuy Preservation mobile-bearing implant has design faults and is over-constrained leading to early failures in some cases