Gordon Hunter

Reduced wear with oxidized zirconium femoral heads.

Wear-related complications have been a major cause of revisions and reoperations following total hip arthroplasty. In the past, several materials have shown promise for reducing polyethylene wear, but they have draw-backs. Elimination of polyethylene from prosthetic designs has not gained wide acceptance because it limits design flexibility and introduces other risks: fractures with ceramic-on-ceramic bearings and metal-ion release with metal-on-metal bearings1,2. In an attempt to address the problem on the femoral side, opposite the polyethylene, ceramic modular heads were introduced because their surfaces are more abrasion-resistant and produce less friction than do metal surfaces, thereby reducing abrasive and adhesive wear of the polyethylene3. The use of oxide ceramics for the articulating femoral head can reduce wear by 25% to 50%4-6. However, ceramics such as monolithic zirconia and alumina are susceptible to brittle fracture, with the need for immediate revision1. On the acetabular side, highly crosslinked ultra-high molecular weight polyethylene was used for liners more than thirty years ago7-9. This practice was ended after a brief time because of commercial considerations, but subsequent clinical results have indicated that these liners undergo substantially less wear than do conventional ultra-high molecular weight polyethylene liners. Recently, manufacturers have introduced new highly crosslinked ultra-high molecular weight polyethylene acetabular liners, which also offer the potential of reducing wear10,11. However, laboratory results have indicated that the relative advantage of such polyethylene can be reduced when it articulates against rougher surfaces12. Metallic cobalt-chromium (CoCr) femoral heads do not have the same risk of fracture as ceramic heads, but their surfaces become roughened as a result of abrasive and oxidative wear3. Evidence of this roughening can be seen on retrieved heads, and both clinical and …

Polyethylene wear performance of oxidized zirconium and cobalt-chromium knee components under abrasive conditions.

The surfaces of retrieved cobalt-chromium (CoCr) total knee arthroplasty femoral components show evidence of roughening ( Fig. 1 ) 1-3. In vitro studies have shown that scratches on the hard counterface, particularly those at an angle to the direction of motion, can increase wear of ultra-high molecular weight polyethylene 4-10. An alternative material, oxidized zirconium (OxZr), was developed to provide an improvement over CoCr in resistance to roughening, frictional behavior, and biocompatibility 11-16. Previous knee simulator testing under clean conditions (without intentional addition of abrasives) demonstrated that articulation with OxZr femoral components resulted in rates of wear of the ultra-high molecular weight polyethylene that were more than sixfold lower than those obtained with CoCr femoral components 17. Because femoral components roughen clinically in a way that can increase wear of the ultra-high molecular weight polyethylene insert, simulator testing under abrasive conditions also was needed to better characterize the performance of the femoral component material. Previously, adding abrasives into the test media during simulation did not produce relevant conditions, so a technique was developed to roughen the surface of the femoral components by tumbling them with alumina powder and plastic cones before simulator testing 18. In the present study, we compared the wear performance of CoCr and OxZr in an anatomic knee simulator under these abrasive conditions. Fig. 1: Hard particles scratch cobalt-chromium surfaces under clinical conditions, plowing up adjacent peaks that can increase abrasive wear of polyethylene, as seen on this interferometer image of a retrieved clinical specimen 1,2. The wear performance of three cast CoCr (ASTM F75) and three OxZr (Oxinium) femoral components were compared. Femoral components were tumbled with 25 μm alumina powder and plastic cone media in a centrifugal finishing barrel 18. The inserts …

Microstructural Investigation of the Oxide Scale on Zr-2.5NB and its Interface with the Alloy Substrate

Oxidized Zr-2.5Nb is being developed as an articular bearing surface for the femoral component in total joint arthroplasty. It has so far demonstrated superior wear performance against ultrahigh molecular weight polyethylene (UHMWPE) with respect to traditional articulating materials such as Co-Cr-Mo alloys. In this investigation, we used thermogravimetric analysis, transmission electron microscopy, and in situ x-ray diffraction techniques to study the microstructure and stress state of the oxide scale grown on Zr-2.5Nb. The oxidation temperature not only determines the kinetics of oxidation but the morphology of the various oxidation products. We have identified the oxidation products of both phases of the two phase alloy and correlated them with the original alloy microstructure. These include not only monoclinic zirconia but also small amounts of tetragonal zirconia and a mixed oxide phase combining both zirconium and niobium. The alloy microstructure both influences the final oxidation products and is reflected in the microstructure of the oxide. The oxide scale itself has a predominantly columnar microstructure which extends from the oxide/metal interface to the outer surface of the oxide. In situ x-ray diffraction measurements revealed that the oxide scale is stressed in compression following cooling and exhibits strong crystallographic texture. The oxide/metal interface is continuous, without pores or voids which might be detrimental to oxide adhesion. In addition, we have identified a phase which develops at the interface between the beta-zirconium grains and the oxide. We have also identified amorphous regions within the oxide scale which serve as sinks for silicon and other impurity elements found in the alloy.

Wettability Analysis of Orthopaedic Materials Using Optical Contact Angle Methods

The wettability behavior of orthopaedic materials influences the fluid film layer that affects both the friction and wear of the articulating surfaces in total joint arthroplasty. This study examined the wettability of various orthopaedic bearing materials such as alumina, zirconia, cobalt chrome (CoCr), and oxidized zirconium (OxZr). Diamond-like carbon (DLC) coating on CoCr was also examined. Additionally, the effect of radius of curvature was examined using OxZr femoral heads of various diameters. The contact angle of the liquid droplet on the surface of the material was measured using a optical contact angle method. Both water and bovine serum with 20 g/L protein concentration were used during testing, with a droplet size of 0.25 -L. The droplet was dispensed from an automated syringe and brought into contact with the sample surface. The contact angle was then measured by fitting polynomial curves to the sample surface and drop geometry. Ten individual drops were analyzed on each test component, with at least three test components for each material. There were no differences in contact angles with changing head size or when using serum compared to water. The alumina, OxZr, and zirconia femoral heads all exhibited a similar contact angle, while CoCr and DLC showed significantly greater contact angles. The smaller contact angles for the oxide ceramic surfaces indicate that they tend to be more wettable than the metals, which may help explain their lower friction and superior adhesive wear performance.

Chemically Textured and Oxidized Zirconium Surfaces for Implant Fixation

Introduction. Implant stability is critical in obtaining good long term succe ss of total joint replacements. Loss of fixation can lead to accelerated wear, pa in, loss of function, or even fracture of the implant, each of which could potentially necessitate revision s urgery. Biological and cemented fixation strengths can be increased by using porous or textur ed surfaces which enhance the potential for mechanical interdigitation at the interface. O xidized zirconium, a material recently introduced for orthopaedic bearing applications (OxiniumTM, Smith & Nephew, I nc., Memphis, TN) due to its beneficial wear and abrasion resistance [1-2], cannot be e asily processed using traditional porous coating techniques. Therefore, an alternative fixation surface whi h could be used with or without bone cement was sought.

Heat Generation and Dissipation Behavior of Various Orthopaedic Bearing Materials

It has been shown that with high interfacial temperatures in hip bearings, it is possible to precipitate proteins, greatly reduce the compressive creep properties of ultrahigh molecular weight polyethylene (UHMWPE), and change the phase content of monolithic tetragonal zirconia. These induced features may alter the wear rate of UHMWPE. It was the objective of this study to examine the interfacial temperatures of oxidized zirconium (OxZr) heads as compared with metallic and ceramic heads coupled with polyethylene in a hip simulator. The interface temperatures were measured by placing thermocouples within 0.5 mm of the interface surface of both femoral heads and acetabular liners, and then articulating the surfaces using a 12-station AMTI anatomic hip simulator. The alumina femoral heads had the lowest average interfacial temperature, followed in increasing order by OxZr, CoCr, and zirconia. The ranking corresponds to the thermal conductivity of each material. A statistically significant difference (p<0.05) was found between all four materials for the femoral head temperature. No difference was seen in liner temperature between the alumina and OxZr groups, but statistical differences were found between all other combinations. Additionally, increasing head diameter, peak load, cyclic frequency, and serum concentration all resulted in statistically significant increases in both femoral head and liner temperatures.