Aaron Essner

Wear, oxidation and mechanical properties of a sequentially irradiated and annealed UHMWPE in total joint replacement

Ultra high molecular weight polyethylene (UHMWPE) has been used as a bearing surface in joint replacement prostheses for over 40 years. Recently, highly crosslinked UHMWPE (HXPE) materials were introduced based on the finding that crosslinking reduces wear. These first generation HXPE materials were produced by irradiation followed by heating below the melting temperature (annealing) or above the melting temperature (remelting). Both classes of HXPE material have demonstrated greatly reduced wear. However, remelted HXPE materials have reduced fatigue strength while annealed HXPE materials may oxidize when exposed to oxygen. A second generation HXPE material was produced using a sequential irradiation and annealing process (SXL). SXL materials have crosslinking levels equivalent to those of first generation HXPE materials, have fatigue and mechanical strength characteristics of first generation annealed HXPE material and have an oxidation resistance equivalent to that of virgin (unprocessed) UHMWPE. This combination of properties makes SXL HXPE a preferred material for bearing surfaces of joint prostheses.

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.

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.

A biaxial line-contact wear machine for the evaluation of implant bearing materials for total knee joint replacement

The motion in a human knee joint is complex. During normal walking, the relative motion between the femoral condyle and the tibia involves flexion/extension, anterior/posterior sliding and medial/lateral rotation. The resulting frictional force between the condyle and the tibia is multidirectional. However, most wear test machines that have been used to evaluate implant bearing materials intended for knee applications are based on reciprocating or unidirectional motion. The validity of these linear motion machines have been questioned in recent years particularly with reference to the wear of ultra-high molecular weight polyethylene. This investigation presents an alternate wear test machine with a biaxial motion that mimics the kinematics of the knee joint. Design rationales and experimental data are presented. The results are compared to those produced by a knee simulator.

The effect of neutron radiation on conventional and highly cross-linked ultrahigh-molecular-weight polyethylene wear.

The effects of a sarcoma therapy dose level neutron radiation on oxidation and wear were compared between conventional (N2\Vac, Stryker Orthopedics, Mahwah, NJ) and highly cross-linked (Crossfire, Stryker Orthopedics) ultrahigh-molecular-weight polyethylene acetabular liners. Liners were exposed to 15 Gy, a typical sarcoma treatment dose. Wear testing was conducted on a hip simulator. Transvinylene and oxidation indices were measured to determine if significant radiolytic reactions and oxidation occurred after the neutron beam exposure. The neutron bombardment produced further oxidation in both N2\Vac and Crossfire liners. Surprisingly, neutron radiation caused 62% increase in wear for N2\Vac but 0% change for the Crossfire acetabular liners. This study suggested that when joint implants are exposed to neutron beam radiation therapy, the conventional polyethylene liner is at risk for rapid wear.

Characterization of Depth-Dependent Mechanical Properties in Bio-Titanium Hybrid Osteochondral Tissue Engineered Constructs

With cartilage autografts and allografts in short supply, tissue engineered osteochondral (OC) grafts offer an alternative [1]. These constructs are comprised of a chondrocyte-seeded hydrogel region and a porous, bone-like base. Our laboratory has shown growth of more robust osteochondral constructs on clinically-relevant metal substrates (eg. tantalum) as opposed to devitalized bone, and these constructs have been evaluated in vivo [1,2]. Due to the presence of the base, it is expected that transport of nutrients and chemical factors in OC constructs will differ from transport in chondral-only constructs (Fig. 1, bottom-left). Depth-dependent mechanical properties of chondral-only constructs have been measured, yielding a “U-shaped” strain profile, in which the construct is stiffest on the edges and softest in the center. However, depth-dependent properties have not been measured in tissue engineered OC grafts [3].Copyright

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.

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.