Lizeth Herrera

A Highly Crosslinked UHMWPE for CR and PS Total Knee Arthroplasties

X3 is a highly crosslinked ultrahigh molecular weight polyethylene (UHMWPE) produced by a sequential irradiation and annealing process. The sequential process results in a material with a free radical content that is 1% that of conventional UHMWPE gamma sterilized in nitrogen resulting in an oxidation resistance similar to that of virgin UHMWPE. Yield strength and ultimate tensile strength exceed American Society for Testing and Materials minimum rates for UHMWPE. Simulator testing of contemporary cruciate retaining (CR) and posterior-stabilized knee inserts (Triathlon) manufactured by the sequential process demonstrated 68% and 64% less wear, respectively, compared to conventionally processed inserts. The wear and mechanical integrity of sequentially processed posterior-stabilized inserts was unaffected by accelerated aging, whereas conventional UHMWPE exhibited increased wear, cracking, and delamination.

Wear Performance Evaluation of a Contemporary Dual Mobility Hip Bearing Using Multiple Hip Simulator Testing Conditions

The dual mobility hip bearing concept combines a small bearing with a large diameter bearing through a dual articulation system, potentially increasing the stability of the hip. Bearings with two articulations introduce concerns of whether or not wear might be increased compared to a conventional bearing. We therefore evaluated the wear performance of a dual mobility hip bearing using sequentially cross-linked and annealed polyethylene under the conditions of impingement, abrasion, and when the mobile liner becomes immobilized at either the inner or outer diameter. We found the wear performance of this dual mobility hip is dictated by the conditions experienced by the smaller inner articulation and by the polyethylene material. The highest wearing group wore 75% less than a single articulating conventional gamma/inert polyethylene bearing.

Surface Oxidized Zirconium Total Hip Arthroplasty Head Damage Due to Closed Reduction Effects on Polyethylene Wear

Recent case studies of surface oxidized zirconium THA heads removed after attempted, closed reduction have shown significant surface damage that has been suggested as potentially deleterious to polyethylene wear. We obtained 4 clinically retrieved specimens, produced well-characterized surface damage on additional heads, and tested them on a hip simulator. After 1 million cycles, the amount of polyethylene wear was related to the extent of surface damage, the most damaged clinical specimen showing more than 50 times more wear than a new head. Although all heads after failed attempted closed reduction(s) should be replaced, surface oxidized zirconium heads are of particular concern; those patients with a successful, simple closed reduction should be monitored for excessive wear.

Dual mobility bearings withstand loading from steeper cup‐inclinations without substantial wear

Steep cup abduction angles with adverse joint loading may increase traditional polyethylene bearing wear in total hip arthroplasties. However, there have been few reports evaluating the effect of cup inclination on the wear of dual‐mobility devices. In a hip joint simulation, we compared the short‐term wear of two‐sizes of modular highly cross‐linked dual‐mobility bearings (28 mm femoral head diameter/42 mm polyethylene insert outer diameter/54 mm acetabular shell diameter; 22.2 mm femoral head diameter/36 mm polyethylene insert outer diameter/48 mm acetabular shell diameter) at 50 and 65° of cup inclination with modular 28 mm femoral head on 54 mm cup diameter metal‐on‐highly cross‐linked polyethylene bearings. Increasing inclination from 50–65° had no changes in volumetric wear of 28/42/54 mm (mean, 1.7 vs. 1.2 mm3/million cycles, respectively; p = 0.50) and 22.2/36/48 mm (mean, 1.7 vs. 1.2 mm3/million cycles, respectively; p = 0.48) dual mobility bearings. At 65°, 22.2/36/48 mm dual‐mobility bearings had lower volumetric loss (mean, 2.2 vs. 6.3 mm3; p = 0.03) and wear rates (mean, 1.2 vs. 2.7 mm3/million cycles; p = 0.02) compared to metal‐on‐highly cross‐linked polyethylene bearings. Modern‐generation dual‐mobility designs with highly cross‐linked polyethylenes may potentially withstand edge‐loading from steeper cup‐inclinations without substantial decreases in wear.

Friction in modern total hip arthroplasty bearings: Effect of material, design, and test methodology:

The purpose of this study was to characterize the effect of a group of variables on frictional torque generated by acetabular components as well as to understand the influence of test model. Three separate test models, which had been previously used in the literature, were used to understand the effect of polyethylene material, bearing design, head size, and material combinations. Each test model differed by the way it simulated rotation of the head, the type of frictional torque value it reported (static vs. dynamic), and the type of motion simulated (oscillating motion vs. continuous motion). It was determined that not only test model may impact product ranking of fictional torque generated but also static frictional torque may be significantly larger than a dynamic frictional torque. In addition to test model differences, it was discovered that the frictional torque values for conventional and highly cross-linked polyethylenes were not statistically significantly different in the more physiologically relevant test models. With respect to bearing design, the frictional torque values for mobile bearing designs were similar to the 28-mm diameter inner bearing rather than the large diameter outer liner. Testing with a more physiologically relevant rotation showed that frictional torque increased with bearing diameter for the metal on polyethylene and ceramic on polyethylene bearings but remained constant for ceramic on ceramic bearings. Finally, ceramic on ceramic bearings produced smaller frictional torque values when compared to metal on polyethylene and ceramic on polyethylene groups.

Tribology of Sequentially Irradiated and Annealed UHMWPE with and without Impingement

Alternative bearing materials for total hip arthroplasty include ceramic-on-ceramic (CoC), metal-on-metal (MoM) and highly crosslinked UHMWPE. Each has benefits and limitations. Ceramics offer substantial wear resistance but suffer design limitations due to the brittle nature of the material (no elevated rims, minimum 5mm thickness). MoM devices are strong and allow many design options with minimum thickness requirements, but the wear process appears to produce metal ions with unknown long term effect unknown. Highly crosslinked UHMWPE offers good wear performance but may have design limitations due to strength concerns.