Reginald Lee

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.

Tribological Considerations in Primary and Revision Metal-on-Metal Arthroplasty

BACKGROUND Metal-on-metal hip bearings undergo biphasic wear, starting with a short period of high wear (bedding-in) and followed by low steady-state wear. Bedding-in is the process by which the cup wears locally to conform to the geometry of the head. This process reduces the maximum contact stress and allows for appropriate lubrication. A critical area of conformance and wear is required for the bearing to reach a low steady-wear state. Cups were analyzed in this study after primary and revision wear scenarios to determine this critical area for this specific bearing. METHODS Forty and 56-mm cobalt-chromium resurfacing bearings with 150 and 400-microm clearances were wear tested in a hip simulator for 5 million cycles. The cups underwent an additional 5 million cycles of testing against new heads, simulating a revision scenario. The revision heads were manufactured to cause the highest mismatch with the pre-worn cups, resulting in polar or local annular contact. Cup wear area was determined from weight-loss measurements after each phase of testing. RESULTS All bearings experienced a biphasic wear performance with a short period of high wear followed by low steady-state wear. A consistent critical area of conformance was reached by all bearings after primary and revision testing conditions, regardless of bearing size, bearing clearance, or contact mode. CONCLUSIONS An area of conformity (wear) reduces contact pressures, is beneficial for lubrication, and is critical to reach a low steady-state wear rate. This study shows that this critical area is consistent regardless of bearing size, clearance, or contact mode. Bearing designs that allow the proper formation of this conformance area should bed-in and reach a low steady-state wear rate.

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 Metal-on-Metal Bearings at High Inclination Angles

Although metal-on-metal hip bearings generally experience low wear in vivo and in simulator testing, high cup inclination angle has been shown to dramatically increase wear. A recent study has shown that metal-on-metal (MoM) bearings converge to a specific contact area regardless of bearing size, clearance, or even contact mode. This evidence points to a relationship between contact pressure and wear rate such that as the contact pressure is reduced (due to the formation of a conforming surface contact, aka. the wear scar) the wear rate will approach a low-steady state value. This research suggested that the run-away wear that leads to extremely high MoM wear may be due to the inability of the specific bearing to reach a low contact pressure.

Effect of the support systems’ compliance on total hip modular taper seating stability

Assembly of a femoral head onto the stem remains non-standardized. The literature shows altering mechanical conditions during seating affects taper strength and lower assembly load may increase fretting corrosion during cyclic tests. This suggests overall performance may be affected by head assembly method. The purpose of this test was to perform bench-top studies to determine influence of peak force magnitude, load rate, and compliance of the systems support structure on initial stability of the taper. Custom manufactured CoCrMo femoral heads and Ti-6Al-4V taper analog samples were assembled with varying peak force magnitudes (2–10.1 kN), load rates (quasi-static vs impaction), and system compliance (rigid vs compliant). A clinically-relevant system compliance design was based off of force data collected during a cadaver impaction study. Tensile loads were then applied to disassemble the taper and quantify initial taper stability. Results indicated that taper stability (assessed by disassembly forces) increased linearly with assembly force and load rate did not have a significant effect on taper stability. When considering system compliance, a 42%–50% larger input energy, dependent on assembly force, was required in the compliant group to achieve a comparable impaction force to the rigid group. Even when this impaction force was achieved, the correlation between the coefficient, defined as distraction force divided by assembly load, was significantly reduced for the compliant test group. The compliant setup was intended to simulate a surgical scenario where patient and surgical factors may influence the resulting compliance. Based on results, surgical procedure and patient variables may have a significant effect on initial taper stability.

Simultaneous Hip Head-Stem Taper Junction Measurements of Electrochemical Corrosion and Micromotion: A Comparison of Taper Geometry and Stem Material

Disclosures: L. Scholl: 3A; Stryker Orthopaedics. 4; Stryker Orthopaedics. V. Swaminathan: 3A; Stryker Orthopaedics. 4; Stryker Orthopaedics. R. Lee: 3A; Stryker Orthopaedics. 4; Stryker Orthopaedics. L.K. Raja: 3A; Stryker Orthopaedics. 4; Stryker Orthopaedics. A. Faizan: 3A; Stryker Orthopaedics. 4; Stryker Orthopaedics. M. Thakore: 3A; Stryker Orthopaedics. 4; Stryker Orthopaedics. K. TenHuisen: 3A; Stryker Orthopaedics. 4; Stryker Orthopaedics.

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.