Crosslinked polyethylene wears well in active TKA patients
Active patients with total knee arthroplasty (TKA) are at risk for polyethylene wear due to repeated exposure to aggressive kinematic conditions. According to the authors of scientific paper 9, crosslinked polyethylene, used in rotating platform (mobile) bearings, had less wear under aggressive testing in a knee wear simulator.
“Mobile bearings are frequently used in younger patients to diminish wear, although the degree of reduced wear is still being investigated,” reported the authors. “The weakness of the rotating platform is the potential for increased wear due to the introduction of an additional articulation. A mobile-bearing design manufactured of crosslinked polyethylene is an attractive combination that simultaneously addresses the disadvantages of the biomaterial and the rotating design.”
Laboratory studies have shown that the use of crosslinked polyethylene can reduce wear in TKA in vitro by 50 percent or more compared to conventional polyethylene. Combining mobile bearings and crosslinked polyethylene could have benefits for younger, more active patients most at risk for early TKA failure when conventional bearings are used.
This study tested six implant sets (tibial trays and femoral components of a cobalt-chromium alloy) in a multidirectional knee wear simulator. Three sets used highly crosslinked polyethylene (XLK) inserts; the others used nominally crosslinked (GVF) inserts of the same design. Wear was measured by the gravimetric method.
To simulate aggressive kinematics, the researchers increased the axial load by 10 percent, the anteroposterior sliding distance by 100 percent and the net axial rotation approximately 40 percent relative to the International Standards Organization (ISO) recommendations.
“Additionally, the concentration of serum in the lubricant was reduced from 90 percent to 25 percent. The combined effect of these conditions was to increase wear 10 times in the GVF group relative to that reported for the same design under benign (ISO-recommended) conditions,”they reported.
All wear-tested inserts exhibited a typical burnished wear scar with minimal pitting on the upper articular surface; the mobile-bearing implants also showed some burnishing (without visible depression) on their “backsides.” None of the inserts delaminated or showed signs of damage; none of the mobile-bearing posts showed signs of damage.
The aggressive wear simulation increased wear rates in both groups, but uncovered a distinct difference in wear performance.
“Wear rates for control mobile-bearing inserts were 7.49 (±1.35) mg/million cycles; crosslinking the bearings reduced wear rates by approximately 50 percent to 3.84 (±0.33) mg/million cycles (p < 0.05),” they wrote.
The authors noted that the design of their study did not address potential concerns about fracture toughness. “Before the inserts can be unequivocally recommended for widespread clinical use, we believe it is prudent to carry out fatigue testing of the mobile-bearing posts and cruciate-substituting cams under clinically relevant conditions. Testing of the inserts under third-body abrasive conditions is warranted as well,” they concluded.
The research team included Darryl D. D’Lima, MD; Clifford W. Colwell Jr., MD; Juan C. Hermida, MD; Kace A. Ezzet, MD; and Nicolai Steklov, BS—all of LaJolla, Cal. The authors received research support from DePuy.