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.

In this study was assessed the precision and accuracy of a novel arthroplasty navigation tool. On-Tool Tracking (OTT) is an innovative on-board wireless device for 3D tracking using miniaturized active infrared LED reference frames. It combines proprietary hardware, software and firmware to acquire and process stereo images to track objects in 3D. OTT seeks to address three basic problems encountered in arthroplasty navigation: inconvenient cameras-markers line-of-sight, large OR footprint and high cost. This study tackles the challenging problem of how to experimentally align, independently measure and present the static 3D position of the OTT relative to its tracked target.

Static accuracy was measured by traversing the OTT over a 3D grid covering the tracking volume [Fig. 1] using an MTS 858 Bionix 5-axis test machine, with a working volume of 100×55.0×76.2 [mm] [Fig. 2]. The absolute position errors were estimated from the MTS actuated/measured versus the OTT recorded X,Y,Z coordinates. First, we registered the OTT coordinate system to that of the MTS, using a point-to-point algorithm which yielded a best-fit OTT-to-MTS 3D transformation. The data set comprised 637 points/locations; with 30 samples collected/averaged at each location. The positional error was the Euclidean (scalar) distance between the reference and measured positions. The RMS, mean, standard deviation, 95% confidence interval, and maximum error were calculated for the whole 3D volume along with three XY planes-of-interest within that volume (at 100, 130, and 160mm OTT-to-reference-frame distances). Initial calibration of the OTT stereo vision rig was made on a totally different and independent physical setup.

Table-1 summarizes the 3D errors for three XY planes-of-interest and the entire volume. The histogram in Fig.3 shows the 3D error distribution. The RMS errors increased with the OTT-to-reference-frame distance. To determine whether the error source was potentially a “scaling” problem, we decoupled the 3D error into individual axis errors [Fig.4]. The summary for all planes is shown on the chart of Fig. 5a. Fig. 5 depicts the directional errors contributed by each axis. Overall results for the OTT show a mean static accuracy of 0.481±0.253 [mm].

The results validated the static accuracy of our overall system, to sub-millimeter averages throughout, but reaching >1mm at the extremes of the measuring volume. Our errors propagated from uncertainty in registration and errors in rigid-body detection rather than just the error of localizing a single retro-reflective marking sphere or LED, as many vendors quote. This study also demonstrates the correlation of the error with the OTT-to-reference-frame (perpendicular) distance and with the proximity of the reference frame to the image edges. The error was expectedly highest in the Z-direction. The errors were mostly uniform within a given XY plane; but increased when the reference frame approached the edges of a captured image. The OTT uses very wide-angle lenses, and so the image distortion/aberration correction algorithms could never be perfect. However, the errors at the distances where the actual surgical cuts would be made (≤ 145 mm) are comparable to today's state-of-the-art systems, even with this highly compact and utilitarian technology.

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.

Current projections point to a large increase in the number of arthroplasty surgeries over the next 20 years. Implant manufacturers typically offer each hospital multiple sets of instruments dedicated to one of their implant system models. Each set includes over 100 mechanical alignment and other instruments (jigs), typically housed in multiple trays. These instruments increase engineering and production costs to the manufacturer, the training burden for surgeons, and increase costs for the hospital for sterilization, training and other logistics. Patient specific instruments addresses some of this burden but involve scaleability, potential liability and speed challenges in that some of the custom jigs design and all its manufacture is delayed and removed from the surgeon, and cannot be changed during surgery.

This talk is about a revolutionary freehand navigated bone cutting technology for joint replacement surgery without implant specific mechanical jigs, without expensive and cumbersome robots, and optionally also even without external navigation tracker equipment. It facilitates navigated freehand bone cutting with real-time 3D graphical feedback. It transforms the traditional orthopaedic power instruments (eg. sagittal saws, drills) into “smart instruments” which can track themselves in 3D around the surgical scene, and optionally prevent the surgeon from deviating from the planned cuts. In bench experiments, this cutting-edge technology promises faster, cheaper, easier and more accurate bone cuts. It assists the surgeon naturally with miniaturized electronics and intelligence on-board the same powered bone cutting instruments they are highly used to.

Joint replacement surgery is highly successful currently, but this technology is intended to make it easier, faster, cheaper and better. A solution that increases benefits to the patient and surgeon alike, while reducing infection risks and costs, may transform patient care in an overburdened field which is expected to grow in the coming years.

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.

As reverse total shoulder arthroplasty (RTSA) systems expand with longer durations in vivo, so does the concern and potential complications of wear, debris and osteolysis. Despite some other profound attempts, no wear testing method has stood out to compare implants across systems and labs. The main reasons may have been the diverse sources of forces and motions used in testing, widely different wear amounts which resulted and the general lack of dedicated shoulder simulators. To add a dedicated shoulder simulator to hip and knee simulators would burden the resources of any testing lab. In this study we propose a shoulder wear test method which addresses the above.

Harnessing the wealth of force-motion data from telemetrized shoulder implants from the Bergman's group in Berlin, we synthesized their results to devise a wholistic multi-axes simulation regime for reverse shoulders. The alignment and motions of the humeral cup and the glenosphere were kept anatomically correct (relative to each other) and yielded a physiologically realistic wear-inducing articulation. However, we opted for a very unusual installation/orientation of the whole implant system to allow a twelve station AMTI (hip) simulator to be adapted for this study. The shoulder constructs were aligned with novel fixtures such that the machine's vertical compressive force mimicked the average forces of the shoulder found from the in vivo telemetry data in magnitude and nominal direction. Aligned thus, a patient with a shoulder installed would neither stand, nor lie down, but be oriented in a composite angle relative the simulator original axes. Each anatomic shoulder motion would be achieved by unique computed combinations of the three simulator motion actuators, none of which would be aligned anatomically for the shoulder on its own.

The maximum ranges of cyclic shoulder motion achieved with the constraints of the simulator were 38°–79° of forward elevation repeated in two separate (15°and 45°) elevation planes. The change of elevation plane inherently involved abduction-adduction motion, and simultaneously also involved variation of internal-external rotation within a 57° range. Each elevation rise (twice per cycle) was also accompanied by a sinusoidally rising and falling compressive load in the range 50N–1700N.

The test method was tested (!) by simulating for 2.5 million of the above (double-elevation) cycles and gravimetrically measuring wear of twelve 36 mm size RTSA systems. We compared six systems having vitamin E-infused highly cross-linked polyethylene bearings (100 kGy radiation) to six controls with a medium cross-linked polyethylene of half the radiation dose. Significant wear resulted for the control bearing material (average 17.9 ± 0.851 mg/MC) which was no less than many hips and knees. Multiply (and statistically significantly, p < 0.001) less average wear (3.42 ± 0.22 mg/MC) resulted for the highly cross linked bearings.

The above demonstrated the effectiveness of the test method. Significant wear resulted under physiologically realistic cyclic motion and forces with strong discrimination between two systems whose bearing materials were known to be different in resilience to wear. Using novel fixtures and unusual orientation to utilize a standard commercially available joint simulator promises efficacy of the test method and utility across different labs.

Introduction

Computer aided surgery aims to improve surgical outcomes with image-based guidance. Navigated Freehand bone Cutting (NFC) takes this further by eliminating the need for cumbersome mechanical jigs. Multiple previous experiments on plastic and porcine bones, performed by surgeons with different level of expertise, suggested that the NFC technique was feasible. This study pushes NFC further by using the technique to perform complete total knee replacement (TKR) surgeries on cadavers (including implant cementing of tibia and femur).

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.

Introduction

Navigated freehand cutting (NFC) technology simplifies bone cutting in laboratory trials by directly navigating implants and power tools [1]. Experiments showed that NFC bone cutting was faster than with conventional jigs. However, most delays occurred at the start of each cut [2]. Therefore, we further reduced starting times and gained more accuracy with a NaviPen and a ‘smart’ NaviPrinter [3]. There were used to physically mark a line on the bone surface indicating where each cut should start. (Fig. 1). Further gains are targeted with our introduction of the On-Tool Marker (OTM); a touch-less laser marking technology as a standalone device or mounted on the cutting instrument (e.g. on the saw). The OTM points the desired cut by projecting a laser image on the bone. That image (usually a line or cross) changes dynamically, so that for any given cut the line projection remains stationary on the bone regardless of the relative location of the device.

Introduction

Computer aided surgery aims to improve surgical outcomes with image-based guidance. Navigated Freehand bone Cutting (NFC) takes this further by eliminating the need for cumbersome mechanical jigs. Multiple previous experiments on plastic and porcine bones, performed by surgeons with different level of expertise, suggested that the NFC technique was feasible. This study pushes NFC further by using the technique to perform complete total knee replacement (TKR) surgeries on cadavers (including implant cementing of tibia and femur).

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.

Some mobile bearing knee replacement designs have shown truly excellent long-term clinical results. The higher laxity of a mobile bearing helps reduce the shear forces and torques transmitted to the prosthesis-bone interface, and this could only help reduce the risk of loosening. Some argue that self-alignment of a mobile bearing rotationally can produce more central patellar tracking. However, the most commonly assumed benefit of mobile bearings is the reduction in contact stress, which is typically expected to reduce fatigue and wear. In a rotating platform TKR for example, wear is also expected to be less because the rolling/sliding motion is separated from the transverse rotational motion onto two separate articulating surfaces, thus less cross-paths and less wear. Such expectations may have dominated the thinking and perhaps even clouded the expectations of TKR wear test engineers. Such wear reduction however has not really been categorically proven clinically.

This paper combines in-vitro wear results from two separate laboratories, one in Nebraska USA and one in Germany. These two (industrially unattached labs) possess between them a very large set of in-vitro wear testing results across the widest variety of fixed and mobile bearing TKR designs. Fortunately, the wear testing methodology using the force-control regime used in the two labs was largely similar, and was highly consistent within each lab. The fixed and the mobile bearings were subjected to the exact same force fields, allowing their Anterior-Posterior translation and internal-external rotation kinematics to vary based on the individual TKR design.

Tens of implant designs have been tested, both fixed and mobile, in total (bycondylar) form and unicompartmental, of various sizes. Some mobile bearings had rotating platforms and some were rotating-translating. Some of the tests specifically compared mobile to fixed bearing tibial components using identical femoral components. Between both labs, and across all tests, no statistically significant difference resulted in wear between fixed and mobile bearings. Yet, such differences did clearly feature with known superior bearing materials (for wear) and other favored design features. Also, generally, the force-control test methodology has proven highly discriminatory in its simulation and measurement of wear as a potential clinical failure mode.

The take home message to test engineers is to expect the wear of both mobile and fixed bearings to depend more on the detailed design and materials of the TKR than on the mobility of the bearing. The results of this study re-confirm the need for wear testing to be performed prior to any clinical use on all implant designs, despite seemingly similar predicates or success of some mobile bearings.

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.