The Journal of Arthroplasty 30 (2015) 468–474

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Reduced UHMWPE Wear Using Magnesia-Stabilized Zirconia Instead of CoCr Femoral Components in a Knee Simulator Marcel E. Roy, PhD a, Leo A. Whiteside, MD a,b, David S. Tilden, BS a,c, Oscar F. Noel, DO a,d a

Missouri Bone and Joint Research Foundation, St. Louis, Missouri Signal Medical Corp., St. Louis, Missouri InSightec, Dallas, Texas d Des Peres Hospital, St. Louis, Missouri b c

a r t i c l e

i n f o

Article history: Received 2 May 2014 Accepted 23 September 2014 Keywords: total knee arthroplasty wear cobalt chromium ceramic roughness

a b s t r a c t Magnesia-stabilized zirconia (Mg-PSZ) is stable and maintains a scratch-resistant surface in hip replacement, but is untested in knees. We assessed whether using Mg-PSZ instead of cobalt-chromium (CoCr) femoral components resulted in less tibial insert wear, and evaluated changes in topography and roughness of the femoral components. Inserts bearing against CoCr or Mg-PSZ were tested using standard (9 Mc) and aggressive (6 Mc) waveforms. Femoral component surface topography and roughness were evaluated before and after testing by optical profilometry. When bearing against Mg-PSZ, UHMWPE wear rate decreased by 73% (standard) and by 59% (aggressive conditions). After 15 Mc, CoCr components featured deep scratches, and roughness increased five-fold, while Mg-PSZ components were unchanged. Mg-PSZ femoral components may be indicated for highdemand patients and those with metal sensitivity. © 2014 Elsevier Inc. All rights reserved.

In total knee arthroplasty (TKA), the distal femur is typically resurfaced with a component made of cast cobalt chromium alloy (CoCr) [1], which bears against a tibial insert made of ultra-high molecular weight polyethylene (UHMWPE). Unlike the wrought CoCr used in modern hip components, which contain less carbon and typically exhibit small carbides as inclusions on the surface, cast CoCr is characterized by larger carbides that protrude from the surface, which become excavated during use by adhesive wear and corrosion. Once released, these hard carbides become third-body wear particles, which can gouge the somewhat softer surrounding base metal, producing scratches with raised edges that in turn gouge the UHMWPE tibial insert. Although these raised edges are eventually polished away during use, more carbides also become excavated during normal use, and the process

Conflict of Interest Statement: This study was funded by the Missouri Bone & Joint Research Foundation. Signal Medical Corp. donated test specimens and manufactured custom fixtures for the knee wear simulator. The first author (MER) has served as an unpaid consultant for Signal Medical Corp., while the second author (LAW) receives support from and is a shareholder of Signal Medical Corp. The second author (LAW) has also received royalty support from Smith & Nephew Orthopaedics, and is a shareholder in Xylon Ceramics LLC. The other authors (DST, OFN) were affiliated with educational institutions (Saint Louis University and Rocky Vista University, respectively) while this work was performed, and have nothing to disclose. Ethical Review Committee Statement: IRB approval was not needed, as no human tissue was used and patient medical records were not accessed as part of this biomechanical study. This work was performed at the Missouri Bone & Joint Research Foundation. The Conflict of Interest statement associated with this article can be found at http://dx. doi.org/10.1016/j.arth.2014.09.027. Reprint requests: Marcel E. Roy, Ph.D., Missouri Bone & Joint Research Foundation, 1000 Des Peres Rd., Suite 150, St. Louis, MO 63131. http://dx.doi.org/10.1016/j.arth.2014.09.027 0883-5403/© 2014 Elsevier Inc. All rights reserved.

repeats [2]. In addition to the increased likelihood of UHMWPE wear, systemic effects of released Co and Cr ions can occur. The delayedtype hypersensitivity-like reaction known as aseptic lymphocytedominated vasculitis associated lesion (ALVAL) has been linked to metal-on-metal hip bearings [3], but perivascular lymphocytic infiltration has also been documented in patients with metal-on-UHMWPE knee bearings [4]. Metal sensitivity has also been documented in patients with metal-on-UHMWPE knee bearings [5–7], with sensitization to metals including cobalt, chromium, nickel, and manganese. However, such cases are not common in the knee. While ceramic materials have been available for use in TKA in Japan and in Europe [6,8,9] ceramics are less frequently used in the USA [10]. Oxidized zirconium (OxZr) [11] has been successfully used in TKA [12], but is not a completely monolithic ceramic, consisting of a thin zirconia outer surface on a zirconium–niobium alloy substrate [13,14]. The softer metallic substrate makes OxZr prone to damage from contact with the posterior tibial tray during implantation in TKA [15] and following dislocation in THA [16,17]. Zirconia ceramics were introduced as a bearing surface for arthroplasty in the 1980s, as a more fracture resistant alternative to alumina [18]. Magnesia-stabilized zirconia (Mg-PSZ) [19] has long been used as a bearing surface in the hip [20], but has not been tested in the knee. Unlike yttria-stabilized zirconia (Y-TZP) [21], which is susceptible to roughening due to phase transformation in the presence of water [22,23], Mg-PSZ has been shown to be stable in artificial aging tests and in vivo[24,25], maintaining a hard, scratch resistant surface. In addition, Mg-PSZ exhibits superior wettability compared to wrought CoCr alloy [25]. Ceramic femoral components may also be suitable for patients with metal sensitivity [6], especially in patients with known hypersensitivity due to a failed metal-on-metal hip.

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Fig. 1. Typical surface topography of (A) CoCr and (B) Mg-PSZ femoral components before testing began. CoCr components were characterized by positive features (mainly carbides) protruding from the surface, while Mg-PSZ components were characterized by negative features. Each image is about 427 μm × 320 μm. Magnification = 10×.

The purpose of this observational study was to compare the wear performance of Mg-PSZ and CoCr femoral components bearing against conventional UHMWPE using standard ISO waveforms, and then using high kinematics/high load waveforms. We hypothesized that Mg-PSZ femoral components would maintain a smoother articular surface and have superior wear characteristics compared to those made of CoCr alloy. Thus, our research questions are: (1) Does the use of Mg-PSZ femoral components result in less UHMWPE wear, compared to tibial inserts bearing against CoCr femoral components? (2) How does the surface topography and roughness of CoCr and Mg-PSZ femoral components change during the test, relative to pre-test values and compared to each other?

Materials and Methods Stock CoCr [1] and custom Mg-PSZ [19] femoral components, and tibial inserts made of non-cross-linked UHMWPE (machined from compression-molded rods of GUR 1020; Symmetric Total Knee, Signal Medical Corp.), were donated for testing. The articular surface of the tibial inserts was stock, while the backside of the inserts was modified with a dovetail allowing them to be locked into a custom tibial tray for testing but then unlocked and removed to measure gravimetric wear. All components were packaged and then sterilized in ethylene oxide gas. After opening the packaging, the tibial inserts were soaked in distilled water until weight gain from fluid absorption had ceased. Surface topography of the femoral components was evaluated before the test began using optical profilometry and red light phase shifting at 10× magnification (632 μm × 475 μm scan area; ZoomSurf 3D; Fogale Nanotech, Nîmes, France). Each condyle was scanned at 0, 15, 30, and 45 degrees of flexion, for 8 scans/specimen. Basic roughness measurements (average roughness Sa, root-mean-square roughness Sq, peak roughness Sp, and valley roughness Sv [26]), surface polarity measurements (skewness Ssk and kurtosis Sku [26],which were also combined as the polarity ratio 3*Ssk/Sku [27]), and 3D functional parameters (core roughness depth Sk, average height of the peaks above the core roughness Spk, and average valley depth below the core roughness Svk [28]) were derived from each scan after subtracting the macroscopic curvature using Mountains Map software (Digital Surf, Besançon,

Table 1 Summary of Pre-Test Measurements of Basic Roughness, Surface Polarity, and Functional Roughness From CoCr and Mg-PSZ Femoral Components. Basic Roughness (nm)

Surface Polarity

Functional (nm)

Material Type

Sa

Sq

Sp

Sv

Ssk

Ratio

Sk

Spk

Svk

CoCr Mg-PSZ P-value

22.8 17.7 0.13

30.7 26.9 0.45

198 183 0.54

136 547 0.03

0.591 −3.62 0.04

0.321 −0.305 0.02

36.4 25.3 0.02

31.8 9.13 0.02

14.6 33.6 0.08

The data shown are mean values (n = 3 per group, 8 scans per specimen).

France). Calibration of the optical profilometer was verified by scanning a 9.1 nm step specimen (VLSI Standards, Inc., Milpitas, CA). The roughness and surface polarity parameters selected for analysis are useful to describe the initial surface, and how the surface changes with wear. The average roughness Sa is based on the arithmetic average of deviations in height on a surface, and is commonly used as a basic measure of surface finish (for example, the average roughness of metallic femoral components must be less than 100 nm [29]). RMS roughness Sq represents the root-mean-square average of the deviations in height of a surface [26]; thus, Sq N Sa, and Sq can be described as a more conservative or cautious measure of surface finish. Maximum peak height Sp and maximum valley depth Sv describe the magnitude of the highest and lowest points on a surface, respectively. However, because they are based on a single point, Sp and Sv are very susceptible to noise (in optical profilometry, “spikes” of reflected light). Surface polarity was measured because Sa and Sq do not differentiate peaks and valleys; thus, two surfaces with the same average roughness Sa but opposite polarities can have markedly different wear properties [30]. For example, a surface characterized by raised edges (positive Ssk) will be abrasive when worn against a softer counterface, while a surface characterized by plateaus and valleys (negative Ssk) will tend to trap lubricant in the voids and may result in lower wear [30–33]. The kurtosis Sku evaluates the distribution of peaks and valleys, where a surface with a perfectly normal distribution of features will have Ssk = 0 and Sku = 3. However, both Ssk (based on the cube of the height of all measured points) and Sku (based on the 4th power) are highly sensitive to extreme points, so the surface polarity ratio 3*Ssk/Sku [27] was used to reduce the influence of noise. Finally, the functional roughness parameters are derived from the material ratio curve [34] and are intended to help assess the operational behavior of stressed surfaces [28]. Sk serves as a measure of “core” roughness, Spk is a relative measure of abrasiveness vs. a softer counterface (such as UHMWPE), and Svk is a measure of the surface's capacity to entrap lubricant. Pre-test optical profilometry scans revealed that the topography of CoCr femoral components was characterized by positive features (mainly carbides) that were about 20–30 μm in diameter and 100–200 nm tall. However, these dimensions may underestimate the actual size of a typical carbide, as only the portion that protruded from Table 2 Comparison of the Standard ISO Waveform Parameters to the High Kinematics/Load Waveforms Used in Final 6 M Cycles.

Parameter Flexion range Peak Fz AP displacement range IE rotation range

ISO 14243-3 (0–9 M Cycles)

High Flexion/High Load (9 M–15 M Cycles)

0°–58° 2.6 kN 0–5.2 mm −5.7° to 1.9°

0°–58° 3.6 kN −1.5 to 10 mm −4.9° to 5.0°

The sign convention for AP displacement was reversed as our simulator moves the femoral side relative to the tibial side, while the sign convention used for IE rotation is for left knees.

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Fig. 2. Plot of gravimetric wear (corrected to load/soaks) of tibial inserts bearing against CoCr (o) and Mg-PSZ (x) femoral components for the first 9 M cycles, using standard input waveforms. UHMWPE wear was reduced by a factor of 3.8 when bearing against Mg-PSZ femoral components. The data points represent mean values (n = 3 per group), while steady state wear rates are indicated by the slope of each linear regression line.

the surface could be observed. In contrast, the surface of Mg-PSZ was characterized by negative features up to 1000 nm deep, with most of them on the order of 200–400 nm deep (Fig. 1). Pre-test roughness measurements revealed that the CoCr and Mg-PSZ femoral components have similar average (Sa), RMS (Sq), and peak (Sp) roughness values, but different valley roughness (Sv) and completely opposite polarities (Ssk and surface polarity ratio; Table 1). Comparison of the functional roughness parameters suggested that the surface of CoCr femoral components, which had a higher Sk and Spk but a lower Svk than that of MgPSZ femoral components, would be more abrasive to UHMWPE. Wear tests were performed on a six-station knee simulator (Advanced Mechanical Technology, Inc., Watertown, MA) at a frequency of 1 Hz, up to a total of 15 M cycles. Both sides of the simulator had three wear stations (which limited n = 3 per group) and two load/ soak stations. At rest, the vertical load Fz was equally distributed over both condyles, but the tibial fixtures of the wear stations also allowed medial-lateral sliding across its pivot point (up to 5 mm in either direction, relative to the center of the knee). This allowed the tibial insert to better follow the femoral component during articulation and promote normal loading. Over the course of the test, each wear couple was periodically rotated to a different station, so that the six wear couples spent the same number of cycles at each wear station on the simulator. Throughout the test, a solution of 25% bovine serum (Donor Adult Bovine Serum, HyClone, Logan, UT) with 20 mM EDTA and 0.3% NaN3 was used as a lubricant, and the bovine serum solution was changed every 500,000 cycles. The serum temperature within each wear station was maintained at about 37 °C by using chilled water to cool the baseplate of the serum reservoirs. For the first 9 M cycles, wear tests were performed using standard ISO displacement-controlled waveforms [35] to simulate normal use from walking. For the final 6 M cycles, the anterior–posterior (AP) displacement and the internal–external (IE) rotation waveforms were modified to match the “natural knee” kinematics [36,37], Table 3 Summary of the Steady-State Gravimetric UHMWPE Wear Rates for Each Protocol Used. Steady-State Wear Rate (mg/Mc) Material Type

A. ISO 14243-3

CoCr Mg-PSZ P-value

0.60 ± 0.055 0.16 ± 0.11 b0.0005

B. High Kinematics/High Load 8.8 ± 0.24 3.6 ± 0.61 b0.0001

The data shown are mean values ± standard deviation, which is appropriate as the steadystate wear rate was derived from linear regression of multiple measurements of wear (10 measurements using the ISO protocol, 7 measurements using the high kinematics/high load protocol) and three specimens per group, for 28 and 19 degrees of freedom, respectively.

and the vertical load (Fz) waveform was scaled to a peak of 3.6 kN (Table 2), to simulate the demands of a younger, more active patient. The wear test was periodically paused (every 500,000 cycles for the first 3 M cycles, and every 1 M cycles thereafter) to measure gravimetric wear of the tibial inserts, corrected by the fluid absorption of the tibial inserts at the load/soak stations [38]. Tibial inserts were otherwise stored in distilled water between test runs to prevent them from drying out. The sizes of the wear scars on the articular surface and backside surface of each tibial insert were periodically evaluated by photogrammetry [39], and the minimum thicknesses of the tibial inserts at each condyle were periodically measured with a micrometer (IP65, Mitutoyo Corp., Japan) that was modified by affixing a hemispherical (to better follow the concave surface of each articular condyle) insert made of UHMWPE to one of its flat jaws. The depth of the articular wear scar in each condyle was measured relative to the plastic deformation exhibited by the load/soak inserts. These indirect measures of wear may help evaluate whether similar measurements on explanted components (on which gravimetric wear cannot be measured) are useful when comparing femoral components made of two different materials. Finally, the roughness and surface topography of the femoral components were evaluated at the end of the test (15 M cycles) using the same procedure as the pre-test roughness measurements, except white light interferometry was used when deep scratches prohibited the use of red light phase shifting. Throughout the study, Excel software (Microsoft, Inc., Redmond, WA) was used to perform heteroscedastic unpaired t-tests and also to plot gravimetric wear, while Prism software (GraphPad Software Inc., La Jolla, CA) was used to perform linear regression of the gravimetric wear data and compare their slopes (steady-state wear rates). A significance level of P b 0.05 was used for all analyses. Results During the wear test using standard waveforms, the tibial inserts initially gained mass due to protein adsorption, but steadily lost mass after 1.5 M cycles (Fig. 2). After 9 M cycles, inserts bearing against CoCr femoral components lost an average of 4.1 g, while those bearing against Mg-PSZ femoral components lost an average of 0.8 g. Plotting the wear data (Fig. 2), the steady-state wear rate for inserts bearing against CoCr femoral components was higher than that of inserts bearing against Mg-PSZ, by a factor of 3.8 (P b 0.0005; Table 3A). After 9 M cycles, the topography of CoCr femoral components was characterized by deep scratches (Fig. 3), while that of Mg-PSZ femoral components did not change (Fig. 4). Over the 6 M cycles of the high kinematics/high load test (Fig. 5, from 9 M to 15 M cycles), the use of Mg-PSZ femoral components reduced steady-state UHMWPE wear rate by a factor of 2.5 compared to inserts bearing against CoCr (P b 0.0001, Table 3B). Among the indirect measurements of wear, the total size (area) of the articular wear scars in each tibial insert increased after each run, but appeared to plateau as each condyle was “worn in” to fit its femoral component. Total worn articular area was usually larger for inserts bearing against CoCr femoral components, relative to inserts bearing against Mg-PSZ (P = 0.096 after 15 M cycles; Table 4). Backside damage of the custom dovetailed inserts was evaluated after 9 M and 15 M cycles, but no differences were observed (Table 4). The depth of the wear scar in each tibial insert's condyle was measured after 9 M and 15 M cycles. Tibial insert wear scar depth was somewhat larger for inserts bearing against CoCr femoral components (P = 0.11 for the medial condyle after 15 M cycles; Table 5). In addition, the medial condyle tended to be worn more deeply (relative to the plastic deformation measured in the load/soak inserts) than the lateral condyle, regardless of femoral component material (Table 5). UHMWPE wear was marginally eccentric at 9 M cycles (P = 0.51 and 0.93 for inserts bearing against CoCr and Mg-PSZ femoral components, respectively), but was more pronounced after 15 M cycles (P = 0.12 and 0.53, respectively).

M.E. Roy et al. / The Journal of Arthroplasty 30 (2015) 468–474

Fig. 3. The anterior end of a typical scratch in a CoCr femoral component after 9 M wear cycles, illustrated as a surface plot (top; 633 μm × 475 μm, vertical axis amplified by 10%) and as a profile perpendicular to the scratch direction (bottom). Magnification = 10×.

After 15 M cycles, the surface topography of CoCr femoral components was still characterized by deep scratches, while that of the MgPSZ femoral components was unchanged. Basic roughness measurements (Sa, Sq) of CoCr femoral components were generally higher than that from Mg-PSZ specimens, while the polarity of CoCr femoral components became negative due to scratches (Table 6). Among CoCr femoral components, the roughness parameters Sa, Sq, and Sv each increased by more than a factor of 5 compared to pre-test measurements, while the measurement Sp increased by a factor of 1.6. Comparing the functional properties, the surface of CoCr femoral components became more abrasive (Spk increased from a pre-test value of 31.8 nm to 102 nm, P = 0.21), and the scratched surface led to an increase in Svk from 14.6 nm to 292 nm (P = 0.14). The surface of Mg-PSZ femoral components exhibited a nominal increase in core roughness Sk (from 25.3 nm to 30.8 nm, P = 0.04) but otherwise did not change.

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components remained stable and did not exhibit scratches, with nearly identical roughness metrics before and after the test. The main limitation of this study was that the sample size of n = 3 per group was pre-determined by the number of available wear stations on the knee simulator. Given the time and other resources required (from the first wear cycle to the last, this test took just under 18 months to complete), we believe it was best to run one longer test as an observational study instead of two identical but shorter tests. However, even with n = 3 we were able to document a significantly lower wear rate when using Mg-PSZ femoral components, in part because gravimetric wear was measured after each stage, resulting in 28 degrees of freedom per group under the ISO protocol, and 19 degrees of freedom per group during the aggressive wear protocol (Table 3). Another limitation was that a complete evaluation of the changes in the topography of the articular surface of the femoral components was not possible; such an analysis may have revealed significant differences in basic roughness measurements, at least within the range of flexion of the input waveforms. In addition, this study did not measure completely stock wear couples, as the backside of the tibial inserts was modified to allow them to be removed for cleaning and gravimetric wear measurements. Although backside damage was observed on the inserts, which may have contributed to wear and mass lost, the amount of backside damage observed in the two groups did not differ at 9 M or 15 M cycles. Finally, knee wear simulators represent an oversimplification of in vivo conditions, in that bovine serum is used instead of synovial fluid as a lubricant, there are no soft tissues or a patella tendon/quadriceps mechanism for constraint of the joint, and no possibility of scratching by bone debris. They are also unrealistic in that humans do not walk for several days at a time, but this limitation is lessened by controlling the temperature of the bovine serum used as lubricant. Compared to CoCr femoral components, the use of Mg-PSZ reduced UHMWPE wear by a factor of 3.8 using standard waveforms, and by a factor of 2.5 under aggressive wear conditions. Direct comparisons to other simulator studies in the literature are difficult due to different test conditions used. However, since other simulator studies also used

Discussion The purpose of this study was to compare the wear of tibial inserts bearing against femoral components made of CoCr alloy or Mg-PSZ ceramic, using two different sets of waveforms in a knee simulator. We also investigated changes in the surface topography and the roughness of the femoral components during the test. The use of Mg-PSZ femoral components resulted in a 73% reduction in the wear rate of conventional UHMWPE compared to CoCr femoral components, using standard displacement-controlled input waveforms. With high-kinematics/high load waveforms, the use of Mg-PSZ femoral components led to a 59% reduction in wear rate. The differences in wear rate were likely influenced by the strongly positive initial surface polarity of the CoCr femoral components, as well as the subsequent scratching of CoCr femoral components during the test, resulting in a five-fold increase in roughness parameters such as average roughness Sa and RMS roughness Sq after 15 M wear cycles. In contrast, the topography of Mg-PSZ femoral

Fig. 4. Typical Mg-PSZ topography after 9 M wear cycles, illustrated as a surface plot (top; 633 μm × 475 μm, vertical axis amplified by 10%) and as a profile (bottom; vertical axis identical to the profile in Fig. 3). Magnification = 10×.

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Fig. 5. Plot of gravimetric wear (corrected to load/soaks) of tibial inserts bearing against CoCr (o) and Mg-PSZ (x) femoral components, using high kinematics/high load input waveforms from 9 M to 15 M cycles. UHMWPE wear was reduced by a factor of 2.5 when bearing against Mg-PSZ femoral components. The data points represent mean values (n = 3 per group), while steady state wear rates are indicated by the slope of each linear regression line.

CoCr femoral components as a de facto control, an indirect comparison is possible if the UHMWPE wear rate using CoCr femoral components in each study is considered to be a baseline. In a pair of knee simulator studies using somewhat different conditions (e.g., 90% bovine serum), OxZr femoral components tested under ISO and “aggressive” conditions reduced UHMWPE wear by factors of 1.7 and 2.2, respectively, relative to the UHMWPE wear rate of inserts bearing against CoCr [40,41]. Tsukamoto et al evaluated the performance of Y-TZP ceramic femoral components against 35 kGy and 70 kGy cross-linked UHMWPE, finding that Y-TZP reduced volumetric wear of 35 kGy UHMWPE by a factor of 4.1 compared to CoCr, while 70 kGy inserts coupled with Y-TZP femoral components still exhibited a net weight gain at 10 M cycles [42,43]. Another knee simulator study revealed that non-crosslinked inserts bearing against Y-TZP or alumina femoral components had less linear wear compared to those bearing against CoCr, by factors of 1.7 and 1.6, respectively [8]. Finally, a knee simulator study with bone cement added as third-body wear particles revealed that the use of zirconiatoughened alumina femoral components reduced UHMWPE wear by a factor of 2.4 compared to CoCr, although they did not report data for normal conditions [44]. While previous clinical studies revealed no clear advantage in using alumina instead of CoCr femoral components [8,45], other clinical studies using Y-TZP [10] and zirconia-toughened alumina [9] have been promising. The low UHMWPE wear rate offered by Mg-PSZ femoral components compares favorably to other materials used in TKA, and suggests Mg-PSZ may be suitable for use with younger and more active patients. The indirect metrics of UHMWPE wear examined in this study were less definitive, with no significant differences observed between the two groups. Examining the articular wear scar on each condyle, both the total area and the depth of the articular wear scar were larger for inserts bearing against CoCr components, which is not surprising as these inserts also exhibited a higher gravimetric wear rate. However, no statistically significant differences in articular wear scar size or depth Table 4 Summary of Articular Wear Scar and Backside Damage Scar Size Measurements on Tibial Inserts Bearing Against CoCr and Mg-PSZ Femoral Components. 2

Articular Wear Scar (mm ) No Wear Cycles (M)

CoCr

Mg-PSZ

P-Value

3 6 9 12 15

412 443 492 776 835

398 419 448 706 746

0.57 0.44 0.24 0.087 0.096

The data shown are mean values (n = 3 per group).

2

Backside Damage (mm ) CoCr

Mg-PSZ

P-Value

were observed. Comparing the wear scar depth on the lateral and medial condyles on each tibial insert, the apparent increase in eccentricity (as measured by MC depth/LC depth) of UHMWPE wear from 9 M to 15 M cycles was not the same for inserts bearing against Mg-PSZ (increasing from 1.06 to 1.10, or 3.3%) as it was for CoCr (increasing from 1.38 to 1.54, or 12%). Hence, the placement of the implants was likely not an issue, and the apparent eccentricity is likely due to the geometry of the femoral component along with normal wear of the tibial insert. Like many femoral components used in TKA, the design used in the current study has a gradual curve from zero degrees of flexion to the anterior flange on the lateral side, while on the medial side the radius of curvature is more acute. As the tibial insert wears, more of the transition zone between the lateral condyle and the anterior flange comes in contact with the tibial insert at zero degrees of flexion, spreading out the load over a greater area on the lateral side compared to the medial side. Finally, the lack of any differences in backside damage among the two groups of tibial inserts suggests that the non-standard backside design was not a confounding variable in this study. Although the basic roughness parameters of the CoCr and Mg-PSZ femoral components were initially very similar, their surfaces were markedly different, with CoCr exhibiting a strongly positive polarity due to protruding carbides while Mg-PSZ was strongly negative. After 15 M wear cycles, the surface of CoCr femoral components was characterized by scratches similar to those observed on retrieved specimens [8,46], which increased roughness while decreasing surface polarity. However, the CoCr components exhibited a net increase in the functional parameter Spk, which indicates that the surface remains abrasive, likely due to raised edges next to scratches and new carbides excavated during normal use. Mg-PSZ femoral components were not scratched and their roughness parameters did not change, aside from a small increase in core roughness Sk. Other investigators comparing ceramic and CoCr femoral components have reported similar results. After 10 M wear cycles, Tsukamoto et al [43] also reported a five-fold increase in average roughness of CoCr femoral components, with no change in

Table 5 Summary of the Wear Scar Measurements (Net Condyle Thickness, Relative to that of the Corresponding Load/Soak Specimens) on the Medial (MC) and Lateral (LC) Condyles From Tibial Inserts Bearing Against CoCr and Mg-PSZ Femoral Components. Wear Scar Depth, MC (mm)

261

247

0.86

498

553

0.26

No. Wear Cycles (M) 9 15

Wear Scar Depth, LC (mm)

CoCr

Mg-PSZ

P-Value

CoCr

Mg-PSZ

P-Value

−0.041 −0.232

−0.032 −0.147

0.57 0.11

−0.030 −0.150

−0.030 −0.134

0.99 0.47

The data shown are mean values (n = 3 per group).

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Table 6 Summary of Post-Test Measurements of Basic Roughness, Surface Polarity, and Functional Roughness From CoCr and Mg-PSZ Femoral Components. Basic Roughness (nm) Material Type CoCr Mg-PSZ P-value

Surface Polarity

Functional (nm)

Sa

Sq

Sp

Sv

Ssk

Ratio

Sk

Spk

Svk

130 18.5 0.14

185 27.9 0.14

324 207 0.19

886 684 0.61

−1.09 −4.11 0.03

−0.263 −0.214 0.53

71.1 30.8 0.22

102 10.8 0.14

292 32.5 0.16

The data shown are mean values (n = 3 per group, 8 scans per specimen).

the roughness of the Y-TZP femoral component used. Recent retrieval studies have reported higher roughness values among CoCr femoral components compared to alumina [8] and OxZr femoral components [47,48]. Finally, the loss of metal from CoCr femoral components may lead to a clinically significant reaction [5–7], although such cases are uncommon. While the clinical benefits of using Mg-PSZ as a bearing surface in TKA have not been evaluated, the lack of Co, Cr, or Ni in MgPSZ ceramic femoral components may indicate its use in patients that are sensitive to those metals. This study describes the first knee wear simulator test using Mg-PSZ femoral components. The significant reduction in UHMWPE wear achieved using Mg-PSZ femoral components under “normal” loads and kinematics was likewise observed in high kinematics/high load conditions. Given its low wear rate, the use of Mg-PSZ femoral components may be indicated for younger/more active/high demand patients, and might also be useful to patients with metal sensitivity. Acknowledgments This study was funded by the Missouri Bone & Joint Research Foundation. The authors gratefully thank Louise M. Jennings, Ph.D., and John Fisher, Ph.D. (IMBE, Leeds, U.K.) for sharing their “natural knee” waveform data with us. We also thank Juan C. Hermida, M.D. (Shiley Center for Orthopaedic Research and Education at Scripps Clinic, La Jolla, CA) for technical assistance with the knee wear simulator used in this study. Finally, the authors thank Diane Morton, M.S., for technical assistance with manuscript and poster preparation. References 1. American Society for Testing and Materials. ASTM F75-12: Standard Specifications for Cobalt-28 Chromium-6 Molybdenum Alloy Castings and Casting Alloy for Surgical Implants. West Conshohocken, PA: American Society for Testing and Materials; 2012. 2. Davidson JA. Characteristics of metal and ceramic total hip bearing surfaces and their effect on long-term ultra high molecular weight polyethylene wear. Clin Orthop Relat Res 1993;294:361. 3. Willert H, Buchhorn G, Fayyazi A, et al. Metal-on-metal bearings and hypersensitivity in patients with artificial hip joints: a clinical and histomorphological study. J Bone Joint Surg Am 2005;87:28. 4. Ng VY, Lombardi Jr AV, Berend KR, et al. Perivascular lymphocytic infiltration is not limited to metal-on-metal bearings. Clin Orthop Relat Res 2011;469:523. 5. Thomsen M, Rozak M, Thomas P. Pain in a chromium-allergic patient with total knee arthroplasty: disappearance of symptoms after revision with a special surface-coated TKA—a case report. Acta Orthop 2011;82:386. 6. Bergschmidt P, Bader R, Mittelmeier W. Metal hypersensitivity in total knee arthroplasty: revision surgery using a ceramic femoral component—a case report. Knee 2012;19:144. 7. Thakur RR, Ast MP, McGraw M, et al. Severe persistent synovitis after cobaltchromium total knee arthroplasty requiring revision. Orthopedics 2013;36:e520. 8. Oonishi H, Ueno M, Kim SC, et al. Ceramic versus cobalt-chrome femoral components; wear of polyethylene insert in total knee prosthesis. J Arthroplasty 2009;24:374. 9. Bergschmidt P, Bader R, Ganzer D, et al. Ceramic femoral components in total knee arthroplasty—two year follow-up results of an international prospective multicentre study. Open Orthop J 2012;6:172. 10. Bal BS, Greenberg DD, Buhrmester L, et al. Primary TKA with a zirconia ceramic femoral component. J Knee Surg 2006;19:89. 11. American Society for Testing and Materials. ASTM F2384-10: Standard Specification for Wrought Zirconium-2.5Niobium Alloy for Surgical Implant Applications. West Conshohocken, PA: American Society for Testing and Materials; 2010. 12. Innocenti M, Civinini R, Carulli C, et al. The 5-year results of an oxidized zirconium femoral component for TKA. Clin Orthop Relat Res 2010;468:1258. 13. Hunter G, Dickinson J, Herb B, et al. Creation of oxidized zirconium orthopaedic implants. J ASTM Int 2005;2(7). http://dx.doi.org/10.1520/JAI12775.

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