We report on an innovative surface grafting to highly crosslinked (HXLPE) bearing for THA using a biocompatible-phospholipid-polymer poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC). Such hydrophilic surfaces mimic articular cartilage and are hypothesized to improve lubrication and thereby reduce friction and wear.

We performed in vitro testing of wear and friction of ceramic-on-polyethylene THRs with the PMPC treatment, and compared them with untreated controls. Highly cross-linked UHMWPE bearings, gamma-ray-irradiated at different levels with and without vitamin E (HXL Vit. E: 125 kGy, HXL: 75 kGy, respectively) were divided so half were PMPC treated (n=3 for all four groups). All were paired with identical 40 mm diameter zirconia-toughened-alumina ceramic heads. Testing was carried-out on an AMTI hip simulator for 10 million simulated walking cycles with standard lubricant and conditions (ISO-14242-1). Wear was measured gravimetrically at 21 intervals, and so was frictional torque with a previously described and tested methodology.

PMPC treatment produced a statistically significant 71% in wear reduction of HXL poly (1.70±1.36 mg/Mc for PMPC vs. 5.86±0.402 mg/Mc for controls, p=0.013). A similar significant wear reduction was found for PMPC treated HXL with Vit. E liners (0.736±0.750 mg/Mc, vs. 2.14±0.269 mg/Mc, p=0.035). The improvements were associated with 12% and 5% reductions in friction of the HXL and Vit. E HXL respectively (statistically significant p=0.003, and marginal p=0.116, one tailed).

These results were an important step in the quest for lower wearing, thin and strong UHMWPE liners for larger diameter femoral heads with the potential benefit of longevity and less risk of dislocation after surgery.

To improve the longevity of total hip replacements (THR), it is necessary to prevent wear of the ultra-high molecular weight polyethylene (UHMWPE) bearing, as wear debris can cause osteolysis and aseptic loosening. Highly cross-linked UHMWPE reduces wear, sometimes stabilized with vitamin E to preserve its mechanical properties and prevent oxidative degeneration. An extra novel solution has been grafting the surface of UHMWPE with poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC). This treatment uses a hydrophilic (wettable) phospholipid polymer to improve lubrication and reduce friction and wear of the bearing material.

We set out to test the wear and friction of ceramic-on-polyethylene (COP) THRs that had the PMPC surface treatment, or left untreated for control. Four groups of UHMWPE bearings were tested against identical 40mm ceramic heads (zirconia-toughened alumina). The UHMWPE bearings were highly cross-linked with/without vitamin E (HXL Vit. E: 125 kGy radiation dose / HXL: 75 kGy). In each group, half underwent the PMPC treatment (n = 3 for all four groups).

Testing was conducted on an AMTI hip simulator for 10 million walking cycles of ISO-14242-1, at 1 Hz, with diluted bovine serum (30 g/L protein concentration) as lubricant, at 37ºC, and with fluid absorption errors corrected with active soak controls. Using a previously published method, frictional torques and a frictional factor around three orthogonal axes about the femoral head were measured/computed, by data processing of the measurements of a 6-DOF load cell on each station of the hip simulator. Such friction measurements and stops for specimen weighing were carried out at regular intervals throughout the wear test.

The HXL liners without and with the PMPC treatment wore at 5.86±0.402 mg/Mc and 1.70±1.36 mg/Mc, respectively (p=0.013) (Fig. 1). The HXL Vit. E liners without and with the PMPC treatment wore at 2.14±0.269 mg/Mc and 0.736±0.750 mg/Mc, respectively (p=0.035). The wear rates of the untreated HXL and HXL Vit. E liners were significantly different (p=0.0002) but no difference in wear rate was found between the two PMPC treated groups (p=0.179), although, as mentioned above, the PMPC treatment very significantly reduced wear in each case. The ceramic femoral heads showed little wear (weight loss) themselves.

In general, the THRs showed decreasing friction over the 10 Mc, with the PMPC types showing a slight increase in friction towards the end of the test (Fig. 2). PMPC HXL liners showed the lowest friction factor (0.022±0.001) which was significantly lower (p<0.001) than the friction of the untreated liners (0.028±0.002) (Fig. 3). The PMPC HXL Vit. E liners showed lower friction factors than the untreated HXL Vit. E liners (0.034±0.002, 0.036±0.004, respectively), although this difference was not significant (p=0.116). Overall, the liners with the PMPC treatment displayed statistically significantly lower friction factors (p=0.003) than those untreated. The coincidence of some reduction of surface friction with larger wear reduction obviously suggests some but not necessarily full causality.

PMPC successfully reduced both the friction and the wear in these COP THRs during this extended 10 Mc test. This likely would translate to improved implant longevity in patients.

For any figures or tables, please contact authors directly (see Info & Metrics tab above).

Wear testing of THR has chaperoned generations of improved UHMWPE bearings into wide clinical use. However, previous in vitro testing failed to screen many metal-on-metal hips which failed. This talk tours hip wear testing and associated standards, giving an assortment of THR wear test results from the author's laboratory as examples.

Two international hip wear-simulator standards are used: ISO-14242-1 (anatomic configuration) and ISO-14242-3 (orbital-bearing). Both prescribe 5 million (MC) force-motion cycles involving cross-shear synchronized with compression simulating walking gate of ideally aligned THRs. ISO-14242-1 imposes flexion (flex), abduction-adduction (ad-ab) and internal-external (IE) rotations independently and simultaneously. An orbital-bearing simulator more simply rotates either a tilted femoral head or acetabular component, switching from flexion-dominated to ad-ab-dominated phases in each cycle with some IE. In the latter, the acetabular component is typically placed below the femoral head to accentuate abrasive conditions, trapping third-body-wear debris.

Wear is measured (ISO-14242-2) gravimetrically (or volumetrically in some hard-on-hard bearings). Wear-rate ranges from negligible to >80mg/MC beyond what causes osteolysis. This mode-1 adhesive wear can therefore “discriminate” to screen hip designs-materials in average conditions.

Stair-climbing, sitting, squatting and other activities may cause THR edge-loading and even impingement with smaller head-to-neck ratios or coverage angle, naturally worse in metal-on metal hips. Deformation of thin acetabular components during surgical impaction may cause elevated friction or metal-metal contact, shedding more metal-ions and accelerating failure. Surgical misalignments in inclination angle, version and tilt can make this worse, even during modest activities in hard-on-hard bearings. Abrasive particulate debris from bone or bone-cement, hydroxyapatite, neck-impingement, normal wear, or corrosion can compound the above. Such debris can scratch the femoral head surface, or embed in the UHMWPE liner compromising the wear of even metal-on-plastic hips.

Much of the belated standardization activity for higher demand hip testing is in response to the metal-metal failures. ASTM F3047M is a recent non-prescriptive guide for what more rigorous testing can generally be done. Third-body particulate can be intentionally introduced or random scratching of the femoral component surface in extra abrasion testing. Also, the compressive load can be increased, more frequent start-stops to disrupt lubrication, and steepening acetabular shell-liner angles to reduce contact area and cause edge-loading, made harsher when combined with version misalignment. Transient separation can occur between head and liner during the swing phase in a lax THR joint with low coverage angle and misalignments; the separated head impacts the liner rim when reseating. An edge-loading ISO test is currently being discussed where (so-called) “microseparation” to a known distance is directly imposed by a lateral spring force in a hip simulator.

Friction testing of a THR in a pendulum-like setup undergoing flexion or abduction swings is being discussed in the ASTM, and so have multi-dimensional THR friction measurements during a long-term wear test simultaneously measuring and separating friction of three rotational (flex, ad-ab, and IE) axes.

THR wear test methods continue to evolve to address more challenges such as novel duo-mobility THR designs, where UHMWPE bearings cannot be removed for gravimetric wear measurements.

Unicompartmental knee replacements (unis) offer an early option for the treatment of osteoarthritis. However there is no standard method for measuring the wear of unis in the laboratory. Most knee simulators are designed for TKA, for which there is an ISO standard. This study is about a wear method for unis, applied to a novel unicompartmental knee replacement (design by PSW). It has a metal-backed UHMWPE femoral component to articulate against a monoblock metallic tibial component. The advantage is reduced resection of strong bone from the proximal tibia for more durable fixation. The femoral component resurfaces the distal end of the femur to a flexion arc of only 42°, the area of cartilage loss in early OA (Fig. 1). We compared this novel bearing couple to the same design but with the usual arrangement of femoral metal and tibial plastic. Our hypothesis was that the wear of the reversed materials would be comparable to conventional and within the range of TKR bearings.

The test was conducted on a 4-station Instron-Stanmore force-controlled knee simulator. Both specimen groups (n=4 each) were highly crosslinked UHWMPE stabilized with vitamin E. On each of the four stations, one uni system was mounted on the medial side and one on the lateral, as if a standard TKR was being tested. The ISO-14243-1 walking cycle force-control waveforms were applied for 5 million cycles (Mc) at 1Hz, but with the maximum flexion during the swing phase (usually 58°) curtailed to 35° to maintain the contact within the arc of the femoral component. In-vivo this implant would be inlaid into the distal medial femoral condyle and the articulating surface immediately transitions into native cartilage. In our test set-up there was no secondary surface as such. The reduced flexion occurred during the swing phase where compressive load was low and the effect on the wear would be negligible. Wear was measured gravimetrically at many intervals and corrected by the weight gain of extra two active soak controls per group.

After 5 Mc, the average rates of gravimetric weight loss from the UHMWPE femoral and tibial bearings were 4.73±0.266 mg/Mc and 3.07±0.388 mg/Mc, respectively (statistically significantly different, p=0.0007) (Fig. 2). No significant difference was found in wear between medial and lateral placement for specimens of the same type, although the medial side generally wore more. Although the plastic femorals of the reverse design wore more than the plastic tibials, the wear was still low at <5 mg/Mc. The range for typical TKRs using ultra-high molecular weight polyethylene, tested under the same conditions in our laboratory has been 2.85–24.1 mg/Mc.

In summary, we adapted the ISO standard TKA wear test for the evaluation of unis, and in this case, a uni with reversed materials. Based on the wear results, this type of ‘early intervention’ design could therefore be a viable option, offering simplicity with less modular parts as well as load sharing with the native articular cartilage.

Introduction

Computer aided surgery aims to improve surgical outcomes with computer guidance. Navigated Freehand bone Cutting (NFC) takes this further by eliminating the need for cumbersome mechanical jigs, while decreasing cutting time and complexity. To reduce the footprint of the NFC tracking system (currently NDI Polaris) we designed and implemented “On-Tool Tracking” (OTT), a novel miniaturized tracking system that mounts onto the cutting instruments (Fig. 1). This study investigates the accuracy of the 3D-measurements of the OTT system.

Testing wear durability of UHMWPE joint replacement bearings under abrasive conditions (mimicking in vivo conditions when metallic components become scratched from bone or cement debris) is useful in screening new bearing materials or alternative processing methods. Adding third body particle debris in testing brings the complications of minimal (if any) increase in wear with particles lodging into the plastic bearings potentially causing unknown errors for gravimetric wear measurements. Alternatively, testing those bearings against already scratched metallic components may provide a cleaner route without such complications. This requires a method to reproducibly create scratches resembling the damage seen on retrievals. This study introduces such a method, and investigates wear of UHMWPE bearings against metallic femoral hip components that have been intentionally scratched.

In this technique, femoral hip heads were pressed and sunk into a bed of abrasive beads under a known load (712N, one body weight), and this created longitudinal scratches. Latitudinal scratches were generated by rotating the sunken femoral heads ± 90° about their polar axis while under the same load. This process (pressing into the abrasive beads and then turning ± 90°) was repeated 10 times on each femoral component which resulted in thousands of random scratch patterns, but with statistically repeatable overall severity and similar visually to retrievals (Fig. 1). We then evaluated the technique through a hip wear study.

Twelve UHMWPE liners (40 mm I.D.) were tested against CoCrMo femoral heads on a 12-station hip simulator (AMTI). Liners were three materials: a) Three conventional (GUR1020, gamma-sterilized 3.5 Mrad), b) Three highly cross-linked (HXL) (GUR1020, 10 Mrad, annealed, EtO-sterilized, artificially aged), and c) Six HXL w/vitamin-E (GUR1020, 12 Mrad, annealed, EtO-sterilized, aged). The test comprised three phases. Phase-I: standard clean (non-abrasive, non-scratched) test for 5 Mc; Phase-II: Pulverized PMMA was added to serum at 700 mg/L (to introduce abrasive conditions); however, effects were minimal after 2 Mc (7 Mc total). Phase-III: Femoral heads were scratched using our method. Phase-III lasted for 1 Mc, for a testing total of 8 Mc (ISO-14242-1 waveforms). All specimens were lubricated with bovine serum (37°C, 30g/L protein). Plastic liners were cleaned and weighed at standard intervals, and wear was corrected with active loaded soak controls.

The wear results are shown in Fig. 2. The conventional liners showed the highest wear (Phase-I: 55.7 ± 3.00 mg/Mc, Phase-II: 49.2 ± 0.520 mg/Mc, Phase-III: 124 ± 28.9 mg/Mc) while HXL liners displayed much lower wear (Phase-I: 2.58 ± 0.969 mg/Mc; Phase-II: 4.93 ± 1.22 mg/Mc; Phase-III: 9.92 ± 4.64 mg/Mc). Vitamin-E HXL liners also showed very low wear (Phase-I: 5.97 ± 0.50 mg/Mc, Phase-II: 8.89 ± 1.40 mg/Mc, Phase-III: 11.9 ± 2.70 mg/Mc). Addition of the PMMA powder during Phase-II increased liner wear, but the surfaces did not appear damaged like retrievals. Wear rates between Phase-I and Phase-III doubled due to scratching the femoral heads for all material types, a statistically significant increase (p < 0.05).

Our results confirm that the scratching procedure successfully created a severe wear situation for the bearings. Future work will involve abrasive testing on knee components to determine if the method is successful there too.

Damage to metallic femoral heads can occur in vivo. Testing of hip prostheses under abrasive conditions is one among various efforts needed towards more realistic and harsher testing. Abrasion likely increases both wear and friction at the head/liner interface. This study investigates if our novel friction measurement technique can detect damage to femoral heads during extended wear testing of metal-on-plastic (MOP) THRs of various material combinations using both scratched and as-new femoral heads.

Friction was measured based on equilibrium of forces and moments measured by a 6-DOF load cell on each test station of an AMTI hip simulator. The force and moment data from the load cells was utilized to calculate the frictional torque about each of three rotational axes (flexion/extension, abduction/adduction and internal/external rotation). The frictional torques were transformed to account for the offset in load cell position from the hip center and were then vector summed to yield an overall frictional torque about the femoral head. The friction factor was then computed by dividing the overall frictional torque by the applied compressive load and the femoral head radius. The waveforms specified in ISO-14242-1 were used. Diluted bovine serum at 37°C with 30 g/L protein concentration lubricated the specimens.

Twelve UHMWPE liners (40 mm I.D.) were tested against CoCrMo femoral heads. Liners were of three materials: a) Three conventional (GUR1020, gamma-sterilized 3.5 Mrad), b) Three highly cross-linked (HXL) (GUR 1020, 10 Mrad, annealed, EtO-sterilized, artificially aged), and c) Six HXL w/vitamin-E (GUR 1020, 12 Mrad, annealed, EtO-sterilized, aged).

The test consisted of three phases were as follows: –

Phase-I: Standard clean (non-abrasive) test for 5 Mc.

The friction results are shown in Fig. 1. Friction factors of the three THR types tested were similar for the first 5 Mc (0.062 ± 0.0084) and increased only marginally after the PMMA powder was added (0.066 ± 0.0066). The PMMA powder did not appear to damage the heads much visually, and therefore the insignificant increase was not surprising. However, once heads were intentionally scratched at 7 Mc, the friction factor rose on all three THR types: a) 0.11 ± 0.0077, b) 0.082 ± 0.0049, c) 0.087 ± 0.022.

This friction technique successfully detected when femoral head damage had occurred. Higher friction was clearly observed after femoral heads had been scratched.

Background

The constraint of total knee replacement (TKR) implants is not simply defined and many of the factors that influence it are not well understood. Variability in the constraint of different TKR implants designed for the same indication (e.g. cruciate-retaining, or posterior-stabilized) have been previously demonstrated, but these differences among implants have yet to be simply quantified. Furthermore, the relative importance of several variables on the implant constraint remains unknown. The purpose of this study was to quantify the differences in constraint that exist between different implant designs, and to examine the effects of axial load and flexion angle on the constraint of current cruciate-retaining (CR) TKR components.

The addition of vitamin E has been shown to improve wear performance in highly crosslinked (HXL) ultra high molecular weight polyethylene (UHMWPE) total knee replacements (TKR) [1]. We set-out to verify if a new type of vitamin E stabilized HXL UHMWPE would substantially improve wear performance, and we present our new results together with our previous ones to tell a fuller story. This paper therefore reports in vitro wear of tibial bearings of both conventional and HXL UHMWPE (with vitamin E) for a total of 16 specimens covering both ends of the TKR size spectrum, very large and very small.

Different designs, sizes and four material types/processes of UHMWPE were tested. In material type 1, tested previously, the polyethylene was machined from isostatic molded GUR1020 bar stock, crosslinked with 10 Mrad, and then doped with vitamin E. From this material, 4 samples of large posterior stabilized (LPS1) TKRs were tested. Material type 2 was HXL where vitamin E was blended into the polyethylene (GUR1020) at the powder stage and the final irradiation was to 9 Mrad. From this material, 2 large cruciate retaining (LCR2) samples and 2 small cruciate retaining (SCR2) samples were tested. The above sample groups from both material types 1 and 2 were compared in the same simulator testing to corresponding identical design, size and sample numbers of conventional UHMWPE not highly crosslinked and with no vitamin E (material types 3 & 4 respectively).

Each test was run on a significantly upgraded (in house) 4-station Instron-Stanmore force-controlled knee simulator. The machine simulated flexion with anatomically realistic joint reaction forces and torques between tibia and femur, and included a spring-based system to simulate soft-tissue restraining forces and torques. The force-control waveforms of the walking cycle specified in ISO-14243-1 were applied for 5 million cycles (Mc) at 1Hz, with bovine serum lubrication with 20g/l protein concentration at 37°C). The tibial bearing inserts were weighed at various intervals standardized between all tests.

No gross delamination or fracture of the tibial inserts was observed in any tests, but all inserts showed measurable wear. The vitamin E stabilized material exhibited an 85% reduction in wear for the LPS1 designs (p < 0.05, ANOVA) compared to its corresponding conventional poly control material. The LCR2 and SCR2 designs with the new vitamin E material exhibited wear reductions of 61% and 77%, respectively when compared to their corresponding conventional bearings (p < 0.05, ANOVA).

The vitamin E highly crosslinked UHMWPE tibial bearings significantly reduced overall wear when compared to conventional tibial bearings of the same design. Such level of wear reduction should translate to worthy clinical significance in preventing osteolysis. Highly crosslinked UHMWPE stabilized with vitamin E appears to be promising for use as a bearing surface in TKR, from at least two different technologies/processes.

Sub-micron polyethylene wear particles have been identified as a cause of osteolysis frequently found in the bone surrounding total hip replacements (THR). However, the wear of the hard femoral components is much less understood and is often assumed to be negligible; yet, metal particulate and ionic debris are of rising clinical concern. This study investigates not only the wear rates of ultra high molecular weight polyethylene (UHMWPE) acetabular liners, but also the wear rates of metallic femoral heads in several THR designs and sizes, which until now have usually been ignored in this type of wear study.

Conventional UHMWPE liners (three 40mm, three 44mm I.D.), highly cross-linked (HXL) UHMWPE liners (three 40mm, three 44mm I.D.), and HXL UHMWPE liners with vitamin E blended (four 36mm and six 40mm I.D.) were tested against CoCrMo femoral heads, appropriately sized and matched to the particular THR design, on a 12 station hip simulator (AMTI, Boston). The specimens were mounted in a physiologically correct manner on custom made fixtures, lubricated with bovine serum (20g/L protein, 37°C) and subjected to the walking cycle specified in ISO-14242-1 at 1Hz for 5 million cycles (Mc). The femoral heads and acetabular liners were carefully cleaned and gravimetrically weighed at standard intervals, and the wear was corrected with the weight gain of active load soak control heads and liners, and calibration weights.

The conventional UHMWPE liners showed the highest wear (40mm: 55.7±3.00mg/Mc, 44mm: 72.0±2.81mg/Mc) while HXL liners displayed much lower wear (40mm: 2.58±0.97mg/Mc, 44mm: 14.2±3.57mg/Mc) as expected. Vitamin E liners also showed very low wear (36mm: 20.1±2.00mg/Mc, 40mm: 5.97±0.50mg/Mc). Interestingly however, the CoCr femoral heads also showed measurable wear for all liner types and designs (Conv. 40mm: 0.28±0.16mm³/Mc, 44mm: 0.22±0.014mm³/Mc, HXL 40mm: 0.041±0.0060mm³/Mc, 44mm: 0.21±0.0024mm³/Mc, Vit-E 36mm: 0.029±0.0097mm³/Mc, 40mm: 0.064±0.019mm³/Mc). Heads in a previously reported 44mm metal-on-metal test [1] showed burnishing and scratching (0.22±0.022 mm³/Mc, liners: 0.16±0.013 mm³/Mc). The burnishing of the metal femoral heads from all tests (including the MOM test) can be seen in Fig. 1 [Fig. 1 here]. An example showing the circular scratching patterns seen on nearly all femoral heads is shown in Fig. 2, of a 40mm femoral head that was paired with a HXL vitamin E liner [Fig. 2 here].

Our simulator results confirm low wear for HXL UHMWPE acetabular liners both with and without vitamin E. Wear of metal femoral heads, although much less in weight than liner wear, was still clearly detectable and measurable for CoCr heads articulating against all types of UHMWPE liners. Therefore, in wear studies focusing on hard-on-soft material couples such as MOP, the metal head wear should not be ignored.

Unicompartmental knee replacement components have gained favor because they replace only the most damaged areas of articular cartilage and the less invasive operation results in a faster patient recovery than traditional TKR. Additionally, they can provide a solution when a full TKR is not yet needed. However, the wear magnitude of such implants is not well understood, primarily due the variation in design and the difficulty of testing them in knee simulators designed to test full TKRs. Modern innovative partial cartilage replacement knee components which are typically even smaller and more bone conservative than unicompartmental implants, are even less common in testing with added challenges. This study investigates the fatigue characteristics of partial cartilage replacement knee components, and the wear of the UHMWPE bearing of a new, truly less invasive unicompartmental design by Arthrex Inc./Florida.

Fatigue testing was performed on MTS 858 MiniBionix machines. Two 12mm diameter UHMWPE tibial components were cemented into jigs at 0° posterior slope and were axially loaded at 2Hz for 10 million cycles (Mc) with a sinusoidal profile peaking at 60% of 8 average human bodyweights (3800N) and a load ratio R of 0.1. Two femoral components were tested with the same load profile at 10Hz for 10 million loading cycles (Mc). The femoral components were mounted at 15° flexion and only the anterior half of the implant was supported, replicating a worst-case scenario where fixation had failed on the posterior half of the implant. This resulted in a large bending moment when force was applied that would fatigue the femoral implant. Following the fatigue test, two full wear simulation tests were conducted on four 12mm and four 20mm unicompartmental components on a four-station Instron-Stanmore force-control knee simulator. The spring-based system to simulate soft-tissue restraining forces and torques was adapted to operate the machine in a displacement control mode to achieve the motions of the medial compartment based on ISO 14243-3. The specimens were lubricated with bovine serum (20g/L protein, 37°C) and the simulator was operated at 1Hz. Liquid absorption was corrected through passive-soak-control bearing inserts. The tibial specimens were cleaned and weighed at standard intervals with the usual ISO test protocols.

After 10Mc of fatigue testing, both tibial components had deformed by some flattening out but were able to sustain the full load without failure and displayed average stiffness (over the whole 10Mc) of 27,600±1,180 N/mm. Neither partially supported femoral component failed, and the femorals displayed average stiffness (over 10Mc) of 37,500 ±3,280N/mm. After 5Mc of wear testing, the 12mm tibial components displayed a wear rate of 4.56±1.45mg/Mc while the larger 20mm size wore at a lower 2.80±0.39mg/Mc.

The results from the fatigue test suggest that this unicompartmental cartilage replacement design will not fail under simple axial loading, even under the extreme case where the tibial implant is receiving the entire share of the load, and the femoral component is only partially supported. In the clinical application, of course some load-sharing with the native unworn cartilage would occur, reducing the stresses on the implant. The results from the wear test showed very low wear for tibial components of this design, lower than many successful TKRs. The larger size tibial components wore less likely due to reduced contact stress. Based on the results of this test, an implant of this type could be a viable option prior to TKR.

To eliminate UHMWPE debris, hard-on-hard bearing surfaces are regaining favor, and metal-on-metal (MOM) is one such combination. To further improve the performance of MOM THRs, a titanium nitride (TiN) coating is sometimes applied through pressure vapor deposition to femoral heads and acetabular liners. This coating has sufficient hardness and therefore may resist abrasion and reduce overall wear, or at least not prohibitively compromise them. One such coating applied commercially was tested on a hip simulator, with coated and uncoated (control) implants supplied by the same implant manufacturer. This study investigates the wear rates of MOM THRs with and without the TiN coating over a 5 million cycle (Mc) in vitro wear test.

Six MOM THRs with 44mm diameter CoCr femoral heads, acetabular liners, and acetabular shells were simultaneously tested on a hip simulator (AMTI, Boston). Three of the six had heads and liners coated in TiN, and the remaining three were uncoated for control. The specimens were mounted anatomically and were lubricated with bovine serum diluted with deionized water to have 20g/l protein concentration at 37°C. The THR specimens were subjected to the loading and rotations of the walking cycle in ISO-14242-1 at 1Hz for 5Mc, without distraction. The loading and rotations were continually observed to ensure consistency with the desired waveforms. The femoral heads and acetabular liners were carefully cleansed and gravimetrically weighed at standard intervals.

Over 5Mc, the uncoated heads displayed a wear rate of 1.84±0.18mg/Mc while the coated femoral heads wore at 4.37±2.01mg/Mc. Wear results were similar in the case of the uncoated and coated metal acetabular liners (1.35±0.11mg/Mc and 4.16±2.06mg/Mc, respectively). However, most interesting was the observation that all three TiN coated THR specimens displayed a loss of coating on both the head and liner in the articulating region. The area where the coating wore away increased in size as the test progressed. The higher wear observed on the coated specimens was due to the removal of the coating, and perhaps the coating particles causing third body wear (evidenced by numerous scratches on coated components). The loss of coating occurred early in the experiment (after only 0.25Mc) in the case of one specimen which caused severe scratching and high wear to that specimen. After this “breakaway wear” occurred, the wear on that specimen stabilized. The difference in wear between the coated and uncoated femoral heads was not statistically significant. The difference in wear between the two types of metal acetabular liners was not statistically significant until 3Mc, after which it became marginally significant (p< 0.05).

Our simulator results confirm small wear overall for MOM THRs, however, we did find extreme “run-in” wear on one TiN coated specimen. The eventual loss of the TiN coating on all three coated specimens is of concern, as this coating is marketed commercially in some parts of the world. It is possible that the coating process was conducted improperly, which resulted in poor adhesion to the substrate, or perhaps resulted in thin application/deposition in the area where the coating did not last.

Sub-micron polyethylene particles produced by the wear of metal on ultra-high molecular weight polyethylene (UHMWPE) in artificial joints have been identified as a principle culprit in the osteolysis frequently found in the bone surrounding these implants. To eliminate UHMWPE debris, highly crosslinked (HXL) UHMWPE and hardon-hard bearing surfaces have been developed. This study compares the wear rates of 14 designs and/or material combinations (total of 48 specimens) tested on a hip simulator in the biomechanics lab at the University of Nebraska Medical Center.

Twelve ceramic-on-metal (COM) (six 36mm and six 28mm, of high and low clearance (HC, LC)), twelve metalon-metal (MOM) (44mm, 3 TiN coated, 3 uncoated standard, and 6 resurfacing components), eighteen metal-on-UHMWPE (MOP) (36mm: six with CoCr-coated heads and six uncoated standard heads with conventional UHMWPE; 44mm: 3 conventional UHMWPE and 3 HXL), and six ceramic-on-UHMWPE (COP) (three 44mm and three 32mm all with conventional UHMWPE) were tested on a multi-station hip simulator (AMTI, Boston). The specimens were lubricated with bovine serum diluted to 20g/l protein concentration at 37°C and were subjected to the loading and rotations of the walking cycle as specified in ISO-14242-1 at 1Hz (for 5 million cycles (Mc) except where specified otherwise). The liners (and heads where specified) were cleaned and weighed at 0, 0.25, 0.5, and every 0.5Mc afterwards.

For 36mm COM liners the wear rates of HC and LC were the lowest observed (−0.019±0.118mg/Mc and −0.061±0.044mg/Mc, respectively). All three 28mm COM HC and one LC liner exhibited “break-away” wear in that they would lose several milligrams (HC: 5.99mg, 6.37mg, 8.50mg, LC: 10.22mg) after showing nearly no measurable wear (HC: 0.905±0.467mg/Mc, 28mm LC: 0.422±0.982mg/Mc). (Note that COM heads weighs were not quoted here but none of them lost weight). TiN-coated MOM THRs (heads and liners) showed higher wear than the uncoated MOM THRs (8.53±4.07mg/Mc, 3.19±0.281mg/Mc, respectively) as the TiN wore away from all three coated heads and liners. The MOM resurfacing components showed wear rates of 2.77±1.27mg/Mc over 2Mc. The 36mm MOP liners (CoCr-coated and uncoated heads) showed wear rates of 55.6±4.26mg/Mc and 44.5±4.46mg/Mc, respectively, as the coating wore away from the metal heads. Wear rates of the 44mm MOP conventional and HXL liners were 72.0±2.81mg/Mc and 14.2±3.57mg/Mc respectively. For COP, the larger size wore at a higher rate than the smaller size (44mm: 97.4±3.08mg/Mc, 32mm: 51.3±12.2mg/Mc) over 2Mc. The 44mm COP THR displayed the highest observed wear rate.

Our simulator results confirm low wear for hard-on-hard bearing couples (MOM, COM) except where coating failure had occurred. Size-36mm LC COM bearings faired the best of the four COM types tested (showing no measurable wear and no “break-away” wear). MOP THRs showed better wear performance when HXL UHMWPE was used, and also showed a sensitivity to femoral head coating removal. COP THRs showed high wear in the large 44mm size, and less in the smaller size. Simulator wear testing was able to successfully discriminate and characterize wear rates of different material bearing couples and different sizes/designs.

A tibial insert with choices in posterior slope, size, and thickness is proposed to improve ligament balancing in total knee arthroplasty. However, increasing slope, or the angle between the distal and proximal insert surfaces, will redistribute ultra-high molecular weight poly-ethylene (UHMWPE) thickness in the sagittal plane, potentially affecting wear. This study used in-vitro testing to compare UHMWPE wear for a standard cruciate-retaining (CR) tibial insert (STD) and a corresponding 6° sloped insert (SLP). Our hypothe sis was that slope variation would have little effect on wear.

Two of each style inserts were tested on an Instron-Stanmore knee simulator with a force-control regime. The gait cycle and other settings followed ISO 14243-1 & 2, except for the reference position, which was posteriorly shifted 6 mm to simulate the worst-case scenario. The STD insert was tilted 6° more than the SLP to level the articular surfaces. Wear was gravimetrically measured at intervals according to strict protocol.

No statistical difference (p=0.36) was found between wear for the STD (9.5 ±1.8 mg/Mc) and SLP (11.4 ±0.5 mg/Mc) inserts.

The overall wear rate measured was higher than previously published rates using implants similar to the STD inserts. This may relate to the shift in the reference position and the 6° slope, leading to increased shear loads. This is the first time the effect of tibial insert slope on wear has been evaluated in-vitro. When limited to 6°, wear testing suggests that al tering the tibial insert slope will have a minor effect on UHMWPE wear.

With the boom in metal-on-metal hip resurfacing and hard-on-hard total hip replacements (THRs) with extremely low wear, accurate tribological measurements become difficult. Characterizing THR friction can help in this, especially if the progress of such friction can be tracked during wear tests. Friction measurement can also be used as a tool to study the effects of acetabular-liner deformation during insertion, and possible femoral head “clamping”. This study presents estimates of friction during extended wear testing on THRs of the same size but with different material combinations, using a technique (previously introduced) based on equilibrium of forces and moments measured in the simulator.

All tests were based on five million cycles (Mc) and samples of size-44mm (head diameter). Samples included 6 metal-on-UHMWPE (MOP) (3 with conventional UHMWPE and 3 with highly-cross-linked (HXL) UHWMPE liners), 6 metal-on-metal (MOM) (3 TiN-coated and 3 uncoated), 6 MOM resurfacing (3 standard and 3 with small pockets for lubrication transport), and 3 ceramic-on-UHMWPE (COP) THRs (MOM resurfacing and COPs for 2Mc only). All were lubricated with diluted bovine serum with 20g/l protein concentration at 37°C, and subjected to the loading and rotations of the walking cycle in ISO-14242-1 on a twelve-station hip simulator (AMTI, Boston).

The conventional and HXL MOPs had steady friction factors of 0.045±0.009 and 0.046±0.003 over 5Mc, explained by the stability of wear rates of both these MOP types (72.0±2.81mg/Mc and 14.2±3.57mg/Mc, respectively). However, during the “bedding-in” period (first 0.5Mc), the conventional MOP friction factor rose from 0.047±0.004 to 0.057±0.004 while high wear was occurring (147.1±10.08mg/Mc). The TiN-coated and uncoated MOMs displayed initial friction factors of 0.124±0.117 and 0.039±0.003 respectively. The high standard deviation for the coated THRs was due to coating removal on one specimen which caused scratches and scuffs on its articulating surfaces. This specimen had a friction factor of 0.260 at 0.028Mc. By 1Mc, the TiN coating wore away on the other two coated specimens (friction factors at 1Mc: coated 0.081±0.036, uncoated 0.050±0.014). Over the 5Mc test, average friction factors for the coated and uncoated THRs were 0.097±0.020 and 0.049±0.014 respectively. The 44mm standard and “pocketed” MOM resurfacing THRs displayed initial friction factors of 0.038±0.009 and 0.059±0.026 respectively that increased to the same level at 2Mc (0.094±0.020 and 0.094±0.029, respectively). No difference in wear was detected between the two resurfacing head types (wear rates over 2Mc: standard 3.32±0.25mg/ Mc, pocketed 2.22±1.76mg/Mc), but curiously, both types exhibited an equal level of scratching and scuffing on their articular surface. Finally, the three COP THRs exhibited high liner wear over 2Mc (97.44±3.08mg/Mc), which slowed after the “bedding-in” period. The friction factor also decreased from 0.091±0.005 to 0.070±0.008 over the same period as the UHMWPE liner conformed to the ceramic head.

The method utilized here facilitates on-line sampling throughout the progress of a prolonged wear test, and therefore allows predictions on THR performance/wear to be made. When high friction factors were observed, a high wear rate was occurring and measured on the THR specimens, or damage to articulating surfaces was seen.