Polyethylene delamination in the PCA total knee. Material analysis in two failed cases

Analysis of ultrahigh molecular weight polyethylene failure in artificial knee joints: thermal effect on long-term performance

The mechanism resulting in damage to and failure of ultrahigh molecular weight polyethylene (UHMWPE) tibial inserts was investigated on clinically retrieved components. The severity of the subsurface damage increased with the length of time that the component had been implanted. A theoretical analysis was developed to account for the generation of subsurface damage based on a heat transfer model. Friction generates surface heat during articulation of total knee systems. Due to the cooling effect of body fluid on the surface, the rise in temperature on the UHMWPE surface is lower than that below the surface. The peak temperature was estimated to occur on a plane positioned about 1 to 2 mm below the surface. This result was similar to the bulk temperature variation observed during in vivo and in vitro studies by other investigators. Although the difference in temperature on and below the surface is only a few degrees, the thermal effect becomes apparent after a long time and may be explained by the viscoelastic behavior of polymers: the temperature-time equivalence. It is therefore suggested that this thermal effect is another contributory factor to material damage, in addition to high stress and oxidative degradation (in appropriate cases). Therefore, any technological efforts aimed at improving the performance of artificial joint prostheses should minimize the thermal effects at the subsurface of the articular components.

Intercondylar distal femoral fracture. An unreported complication of posterior-stabilized total knee arthroplasty

In an attempt to study the incidence of intraoperative femoral intercondylar fractures, two large series of posterior-stabilized total knee arthroplasty (TKA) systems were reviewed. Eight hundred ninety-eight nonconsecutive primary posterior-stabilized TKAs were compared with a second nonconsecutive series of 532 posterior-stabilized TKAs. Unique to the secondary TKA system is an intercondylar sizing guide to aid in verification of the intercondylar resection size. In the initial series, 40 distal femoral intercondylar fractures were noted (rate = 1:22; nondisplaced, 35; displaced, 5). In the secondary series, one displaced distal femoral intercondylar fracture occurred (rate = 1;532). The difference in the rate of fracture between the two populations was statistically significant (P<.001). Intraoperative distal femoral intercondylar fracture represents a potential complication of TKA and can be avoided with careful resection technique and size verification. No change in the postoperative rehabilitation program is required, however, for those patients identified with nondisplaced and intraoperative-stabilized displaced distal femoral intercondylar fractures.

Cold extrusion deformation of UHMWPE in total knee replacement prostheses

The use of metal-backed tibial plates in total knee replacement prostheses can result in the flow of ultra-high molecular weight polyethylene (UHMWPE) from the tibial insert into a cavity on the metal tray surface. A study of the relationship between the thickness of UHMWPE inserts and the amount of cold extrusion is reported here. An attempt was made to correlate the occurrence of cold extrusion with computer-aided analysis. UHMWPE samples of varying thickness, from 3 mm to 10 mm, were placed over cobalt-chrome (Co-Cr) discs. The Co-Cr discs had a 5 mm diameter hole placed centrally to simulate a tibial tray cavity. A cyclic load was applied at 20 Hz through a Co-Cr spherical indentor for a million cycles. The application of cyclic loading on UHMWPE samples resulted in cold-extrusion values comparable to those reported for retrieval analysis studies. Results after fatigue loading show that the samples do not suffer any gross surface damage. A shiny depression was visible at the load application site and the surface roughness value was decreased. The amount of cold extrusion increased with decreasing UHMWPE sample thickness. From the results, a minimum UHMWPE thickness of 12 mm is required if cold extrusion of UHMWPE is to be eliminated.

A quantitative technique for reporting surface degradation patterns of UHMWPE components of retrieved total knee replacements

A quantitative method of reporting surface degradation of the ultra-high molecular weight polyethylene (UHMWPE) tibial component from retrieved total knee replacements (TKR) was developed. Specific features include a qualitative assessment expressing the patterns in which the damage was detected as well as a quantitative summary of the observed degradation mechanisms. In addition, a method of measuring lower limb alignment changes with time is described and related to the observed damage patterns. Two case studies are presented. One case illustrated that changes in alignment resulted from factors other than wear. The damage observed on the tibial plateau appeared to occur subsequent to the changes in alignment. The second case illustrated that the wear of the UHMWPE tibial insert lead to the changes in the overall lower limb alignment. The methods described provide additional information regarding TKR failure mechanisms compared to reporting methods currently available. In particular, the collection of temporal alignment data at clinical follow-up visits enhanced the assessment of the retrieved TKR.