Quantification of third body damage to the tibial counterface in mobile bearing knees

Third body damage and wear in arthroplasty bearing materials: A review of laboratory methods

Third body wear of arthroplasty bearing materials can occur when hard particles such as bone, bone cement or metal particles become trapped between the articulating surfaces. This can accelerate overall implant wear, potentially leading to early failure. With the development of novel bearing materials and coatings, there is a need to develop and standardise test methods which reflect third body damage seen on retrieved implants. Many different protocols and approaches have been developed to replicate third body wear in the laboratory but there is currently no consensus as to the optimal method for simulating this wear mode, hence the need to better understand existing methods. The aim of this study was to review published methods for experimental simulation of third body wear of arthroplasty bearing materials, to discuss the advantages and limitations of different approaches, the variables to be considered when designing a method and to highlight gaps in the current literature. The methods were divided into those which introduced abrasive particles into the articulating surfaces of the joint and those whereby third body damage is created directly to the articulating surfaces. However, it was found that there are a number of parameters, for example the influence of particle size on wear, which are not yet fully understood. The study concluded that the chosen method or combination of methods used should primarily be informed by the research question to be answered and risk analysis of the device.

Explant analysis of a Maestro™ wrist prosthesis and calculation of its lubrication regime

Explant analysis can provide key insights to understanding failures of artificial joints and thus how they might be improved for the ultimate benefit of patients. There are no previous reports of explant analysis of an artificial wrist joint. In this study, an explanted metal-on-polymer Maestro wrist was analysed both in macro and nanoscales to estimate its biotribological performance. The articulation was formed between a cobalt chromium carpal head and an ultrahigh molecular weight polyethylene bearing. The surface roughness values of its articulating surfaces and the backside of the articulation were measured. On average, the articulating surface roughness values were calculated as 0.06 ± 0.02 μm and 1.29 ± 0.63 μm for the cobalt chromium carpal head and ultrahigh molecular weight polyethylene bearing, respectively. Both surfaces had negative skewness, indicating a preponderance of valleys. On the articulating surface of the carpal head, light scratches were observed, and no impingement was observed throughout the component. The polymeric surface had a polished appearance. It had unidirectional scratches at the centre of the articulation, pits of different sizes on its articulating surface, and matt white subsurface regions. The backside of the UHMWPE bearing and the convex surface of the radial body that it was sitting on, were found to have average surface roughness values of 4.23 ± 0.69 μm and 5.57 ± 1.05 μm, respectively. The difference in the means was not significant (p > 0.05). Taking the articulating surface roughness values, the lubrication regime that the explanted Maestro wrist operated under in vivo was estimated for varying physiological conditions, i.e. varying loads, entraining velocities and lubricant viscosities. In every case considered, the explant was found to operate under boundary lubrication.

Third Body Wear of UHMWPE-on-PEEK-OPTIMA™

PEEK-OPTIMA™ is being considered as an alternative to cobalt chrome (CoCr) in the femoral component of total knee replacements. To date, investigations of ultra-high molecular weight polyethylene (UHMWPE)-on-PEEK have shown an equivalent wear rate to conventional implant materials under standard conditions. In this study, the third body wear performance of UHMWPE-on-PEEK was directly compared to UHMWPE-on-CoCr in a series of pin-on-plate studies using two approaches for third body damage. Damage simulation with particles of bone cement showed a significant (p < 0.001), four-fold increase in the mean surface roughness of PEEK plates compared to CoCr. However, wear simulation against the damaged plates showed no significant difference in the wear of UHMWPE pins against the different materials (p = 0.59), and a polishing effect by the pin against the PEEK plates was observed. Scratching PEEK and CoCr counterfaces with a diamond stylus to create scratches representative of severe third body damage (4 µm lip height) resulted in a significantly higher (p = 0.01) wear of UHMWPE against CoCr compared to PEEK and again, against PEEK plates, polishing by the UHMWPE pin led to a reduction in scratch lip height. This study shows that in terms of its wear performance under third body wear/damage conditions, UHMWPE-on-PEEK differs from conventional knee replacement materials.

The influence of a calcium sulphate bone void filler on the third-body damage and polyethylene wear of total knee arthroplasty

OBJECTIVES

Bone void fillers are increasingly being used for dead space management in arthroplasty revision surgery. The aim of this study was to investigate the influence of calcium sulphate bone void filler (CS-BVF) on the damage and wear of total knee arthroplasty using experimental wear simulation.

METHODS

A total of 18 fixed-bearing U2 total knee arthroplasty system implants (United Orthopedic Corp., Hsinchu, Taiwan) were used. Implants challenged with CS-BVF were compared with new implants (negative controls) and those intentionally scratched with a diamond stylus (positive controls) representative of severe surface damage (n = 6 for each experimental group). Three million cycles (MC) of experimental simulation were carried out to simulate a walking gait cycle. Wear of the ultra-high-molecular-weight polyethylene (UHMWPE) tibial inserts was measured gravimetrically, and damage to articulating surfaces was assessed using profilometry.

RESULTS

There was no significant difference (p  >  0.05) between the wear rate of implants challenged with CS-BVF (3.3 mm3/MC (95% confidence interval (CI) 1.8 to 4.8)) and the wear rate of those not challenged (2.8 mm3/MC (95% CI 1.3 to 4.3)). However, scratching the cobalt-chrome (CoCr) significantly (p < 0.001) increased the wear rate (20.6 mm3/MC (95% CI 15.5 to 25.7)). The mean surface roughness of implants challenged with CS-BVF was equivalent to negative controls both after damage simulation (p = 0.98) and at the conclusion of the study (p = 0.28).

CONCLUSION

When used close to articulating surfaces, a low-hardness, high-purity CS-BVF had no influence on wear. When trapped between the articulating surfaces of a total knee arthroplasty, CS-BVF did not scratch the surface of CoCr femoral components, nor did it increase the wear of UHMWPE tibial inserts compared with undamaged negative controls.Cite this article: R. M. Cowie, S. S. Aiken, J. J. Cooper, L. M. Jennings. The influence of a calcium sulphate bone void filler on the third-body damage and polyethylene wear of total knee arthroplasty. Bone Joint Res 2019;8:65-72. DOI: 10.1302/2046-3758.82.BJR-2018-0146.R1.

Influence of third-body particles originating from bone void fillers on the wear of ultra-high-molecular-weight polyethylene

Calcium sulfate bone void fillers are increasingly being used for dead space management in infected arthroplasty revision surgery. The presence of these materials as loose beads close to the bearing surfaces of joint replacements gives the potential for them to enter the joint becoming trapped between the articulating surfaces; the resulting damage to cobalt chrome counterfaces and the subsequent wear of ultra-high-molecular-weight polyethylene is unknown. In this study, third-body damage to cobalt chrome counterfaces was simulated using particles of the calcium sulfate bone void fillers Stimulan^® (Biocomposites Ltd., Keele, UK) and Osteoset^® (Wright Medical Technology, TN, USA) using a bespoke rig. Scratches on the cobalt chrome plates were quantified in terms of their density and mean lip height, and the damage caused by the bone void fillers was compared to that caused by particles of SmartSet GMV PMMA bone cement (DePuy Synthes, IN, USA). The surface damage from Stimulan^® was below the resolution of the analysis technique used; SmartSet GMV caused 0.19 scratches/mm with a mean lip height of 0.03 µm; Osteoset^® led to a significantly higher number (1.62 scratches/mm) of scratches with a higher mean lip height (0.04 µm). Wear tests of ultra-high-molecular-weight polyethylene were carried out in a six-station multi-axial pin on plate reciprocating rig against the damaged plates and compared to negative (highly polished) and positive control plates damaged with a diamond stylus (2 µm lip height). The wear of ultra-high-molecular-weight polyethylene was shown to be similar against the negative control plates and those damaged with third-body particles; there was a significantly higher (p < 0.001) rate of ultra-high-molecular-weight polyethylene wear against the positive control plates. This study showed that bone void fillers of similar composition can cause varying damage to cobalt chrome counterfaces. However, the lip heights of the scratches were not of sufficient magnitude to increase the wear of ultra-high-molecular-weight polyethylene above that of the negative controls.

Experimental testing of total knee replacements with UHMW-PE inserts: impact of severe wear test conditions

Aseptic implant loosening due to inflammatory reactions to wear debris is the main reason for the revision of total knee replacements (TKR). Hence, the decrease in polyethylene wear particle generation from the articulating surfaces is aimed at improving implant design and material. For preclinical testing of new TKR systems standardized wear tests are required. However, these wear tests do not reproduce the entire in vivo situation, since the pattern and amount of wear and subsequent implant failure are underestimated. Therefore, daily activity, kinematics, implant aging and position, third-body-wear and surface properties have to be considered to estimate the wear of implant components in vivo. Hence, severe test conditions are in demand for a better reproduction of the in vivo situation of TKR. In the present article an overview of different experimental wear test scenarios considering clinically relevant polyethylene wear situations using severe test conditions is presented.

In vivo roughening of retrieved total knee arthroplasty femoral components

BACKGROUND

Joint registry data highlights the higher rates of cumulative revision for younger patients undergoing TKR. One of the reasons associated with this higher revision rate may be due to the wear of the UHMWPE leading to loosening. Alternate bearing surfaces have been developed to address this problem; however, roughening of the metal bearing surface has not been demonstrated in vivo.

METHOD

We recorded roughness measurements of retrieved femoral components.

RESULTS

Average lateral condyle roughness was 0.032 μm, compared to control values of 0.020 μm, p=0.002; average medial condyle roughness was 0.028 μm, compared to a control value of 0.019, p<0.001.

CONCLUSION

There was a small but statistically significant roughening of femoral components in vivo compared to controls. This may have important implications for aseptic loosening of knee arthroplasty components and the decision to use scratch resistant components.

Total knee prosthesis polyethylene wear reduction by a new metal part finishing method

PURPOSE

The purpose of this study was to assess a new metal component finishing designed to improve total knee prosthesis durability. Wear of ultrahigh molecular-weight polyethylene (UHMWPE), with generation of submicrometer- and micrometer-sized particles, has been associated with osteolysis and artificial joint failure. Wear extent is influenced by several factors, some of which are related to manufacturing.

METHODS

UHMWPE wear was assessed in metal prosthesis components finished with the Microloy® technology and in traditionally finished components by wear simulation experiments (pin on disk and knee simulator tests) and analysis of wear debris.

RESULTS

Microloy®-finished prosthesis showed a 48.5% reduction in UHMWPE total weight loss compared with traditional components (P=0.002). A statistically significant (P<0.05) reduction of UHMWPE debris were detected from the Microloy®-finished compared with the traditionally finished components.

CONCLUSIONS

These findings suggest the Microloy® metal finishing may enhance the long-term performance of knee prostheses.

Encoding scratch and scrape features for wear modeling of total joint replacements

Damage to hard bearing surfaces of total joint replacement components typically includes both thin discrete scratches and broader areas of more diffuse scraping. Traditional surface metrology parameters such as average roughness (R_a) or peak asperity height (R_p) are not well suited to quantifying those counterface damage features in a manner allowing their incorporation into models predictive of polyethylene wear. A diffused lighting technique, which had been previously developed to visualize these microscopic damage features on a global implant level, also allows damaged regions to be automatically segmented. These global-level segmentations in turn provide a basis for performing high-resolution optical profilometry (OP) areal scans, to quantify the microscopic-level damage features. Algorithms are here reported by means of which those imaged damage features can be encoded for input into finite element (FE) wear simulations. A series of retrieved clinically failed implant femoral heads analyzed in this manner exhibited a wide range of numbers and severity of damage features. Illustrative results from corresponding polyethylene wear computations are also presented.

Wear-pattern analysis in retrieved tibial inserts of mobile-bearing and fixed-bearing total knee prostheses

Components from 73 failed knee replacements (TKRs) consisting of rotating-platform, mobile-bearing and fixed-bearing implants were examined to assess the patterns of wear. The patterns were divided into low-grade (burnishing, abrasion and cold flow) and high-grade (scratching, pitting/metal embedding and delamination) to assess the severity of the wear of polyethylene. The rotating-platform group had a higher incidence of low-grade wear on the upper surface compared with the fixed-bearing group. By contrast, high-grade wear comprising scratching, pitting and third-body embedding was seen on the lower surface. Linear regression analysis showed a significant correlation of the wear scores between the upper and lower surfaces of the tibial insert (R(2) = 0.29, p = 0.04) for the rotating-platform group, but no significant correlation was found for the fixed-bearing counterpart. This suggests that high-grade wear patterns on the upper surface are reduced with the rotating-platform design. However, the incidence of burnishing, pitting/third-body embedding and scratching wear patterns on the lower surface was higher compared with that in the fixed-bearing knee.

Analysis of retrieved ultra-high-molecular-weight polyethylene tibial components from rotating-platform total knee arthroplasty

Mobile-bearing total knee Arthroplasties (TKAs) were designed to increase conformity, decrease contact stresses, and decrease polyethylene damage. Our objective was to evaluate the performance of retrieved mobile-bearing TKAs with respect to wear damage of the polyethylene in a series of components obtained at revision surgery. Tibial component polyethylene superior and inferior surface damage and radiographic radiolucency analysis was conducted on 40 retrieved mobile-bearing TKAs. Higher levels of superior articulating surface damage were found to be associated with higher levels of inferior surface damage in this retrieval study. Greater levels of damage were present on both surfaces in components with greater radiographic radiolucency scores and mechanically loose components. The mobile-bearing TKA remains vulnerable to polyethylene wear damage at the superior surface and introduces an independent inferior surface also vulnerable to wear damage.

Mobile-bearing knees reduce rotational asymmetric wear

Polyethylene wear of bearing components is the most common long-term complication in total knee arthroplasty. One would anticipate differing kinematics would generate different wear patterns (including wear type, degree, and symmetry) on the articulating surface of mobile-bearing and fixed-bearing inserts. Because mobile-bearing designs facilitate movement of the insert relative to the tray when the knee rotates, we hypothesized mobile-bearing designs would reduce the incidence of rotational asymmetric wear. We examined 51 worn tibial inserts, including 15 from mobile-bearing rotating-platform posterior-cruciate-sacrificing dished prostheses and 36 from fixed-bearing posterior-cruciate-retaining flat prostheses, which were retrieved at revision surgery with an average implantation time of 115 months. We divided wear types into low-grade wear (burnishing, abrasion, and cold flow) and high-grade wear (scratching, pitting, metal embedding, and delamination) to assess wear degree of polyethylene. To assess symmetry of wear, the insert surface was divided into medial and lateral sides and each side was further divided into three equal zones along the anteroposterior direction. Low-grade wear was more common in mobile-bearing knees, whereas high-grade wear was more common in fixed-bearing knees. We identified no internal/external rotational asymmetric wear or anteroposterior asymmetric wear in mobile-bearing knees.

Quantification of polyethylene degradation in mobile bearing knees: a retrieval analysis of the Anterior-Posterior-Glide (APG) and Rotating Platform (RP) Low Contact Stress (LCS) knee

BACKGROUND

Routine qualitative observations of more than 850 polyethylene fixed and mobile bearings at our institution have noted minimal wear of mobile bearings. The APG mobile bearing is the most recent design variant of the LCS knee, allows multi-directional movement at the tibiofemoral articulation, and is posterior cruciate sparing. Even though it is difficult to perform, quantitative wear measurement is important in determining the likely longevity of new arthroplasty devices, and is especially relevant because of increasing numbers of new mobile bearing designs.

PATIENTS AND METHODS

We analyzed 10 retrieved APG and 7 retrieved RP tibial bearings (De Puy) with a mean implantation period of 33 (9-70) months. We used coordinate-measuring techniques to quantitatively determine linear penetration, and optical and scanning electron microscopy to assess wear mechanisms qualitatively.

RESULTS

The mean total volume loss (superior and inferior articulations) of the APG and RP designs was 85 mm((3))/year and 77 mm((3))/year, respectively. Burnishing was the predominant wear mechanism, and to a lesser extent scratching, abrasion and pitting. Multidirectional scratching and abrasion were noted on the APG inserts inferiorly, whereas there was circumferential scratching on the RP inserts.

INTERPRETATION

Our short-term results for the APG and RP mobile bearing designs are similar and compare more than favorably with reported values for fixed-bearing designs. However, increased backside wear due to multidirectional movement may predispose the APG design to greater wear in the long term.

UHMWPE and vitamin E bioactivity: an emerging perspective

Ultra-high molecular weight polyethylene (UHMWPE) is a semicrystalline biomaterial widely used in the components for articular prosthesis for its excellent mechanical qualities. Two major problems limit the UHMWPE prosthesis life-wearing and delamination, both phenomena being mainly due to chemical oxidation of polymer. Wearing causes the release of generated particulate that triggers a macrophage reaction leading to chronic inflammation and osteolysis, while delamination, due to the mechanical stress, macroscopically alters the surfaces. The most diffused method to reduce wearing is UHMWPE molecular cross-linking by high-energy irradiation followed by melting that also reduces polymer fatigue strength. For this reason, the use of vitamin E (alpha-tocopherol), as an anti-oxidative and biocompatible additive for normal and cross-linked UHMWPE, has been suggested as an alternative method to improve polymer wearing resistance without altering its mechanical strength. This paper describes briefly the rationale of vitamin E as UHMWPE additive and its possible use as an emerging perspective in the orthopaedic field.

Surface engineering of prosthetic knee components

Conventional total and uni-compartmental knee replacement designs usually incorporate metal femoral and tibial components with an ultra high molecular weight polyethylene (UHMWPE) bearing surface. The tibial components can be modular or monobloc and are of the fixed bearing type in the majority of cases. Mobile bearings are also in common use with a modular meniscal insert with either rotation only or a combination of rotation and translation. Wear of the UHMWPE components remains of primary concern in these prosthetic devices. Catastrophic wear and delamination has been largely addressed by improvements in UHMWPE quality and manufacturing methods, however, abrasive and adhesive wear of the UHMWPE components remains a concern for long-term survivorship of total knee replacement. This review very briefly covers published long-term survivorship of primary knee arthroplasties, primary wear mechanisms present in knee replacements and the potential for wear reduction by surface engineering of the metal wear counterfaces. There are several methods and materials available, which offer the potential for significantly reduced wear.

Surface roughness of the proximal and distal bearing surface of mobile bearing total knee prostheses

Proximal and distal articulations surface roughness measurements were done on 8 mobile bearing knee designs immediately after removal from sterile packaging. Roughness parameters Ra and Rp based on ISO 97, representing mean deviation from the smooth surface line and mean peak to smooth surface line, were recorded using a contact surface profilometer at 10 random sites in the anteroposterior and mediolateral direction on the femoral and tibial metal components and the proximal and distal surface of the ultra-high molecular weight polyethylene (PE) inserts. No differences were found for surface roughness values for the metal components. Surface roughness values were greater for the distal PE bearing surfaces compared with the proximal PE bearing surfaces for each design tested. The roughness values for the PE inserts showed a directional dependence. Complex kinematics of mobile bearing knees coupled with this rougher distal interface could influence the rate of generation of wear particles and total volume of particles produced especially in the early postoperative period.

In vitro kinematic patterns are similar for a fixed platform and a mobile bearing prosthesis

In vitro dynamic simulation of knee flexion was performed to quantify knee kinematics for a mobile bearing prosthesis that allows the tibial insert to translate and rotate with respect to the baseplate. Six cadaver knees were tested in the intact state, after implanting a fixed platform prosthesis, and after implanting a mobile bearing prosthesis. The mobile bearing prosthesis significantly increased the tibial internal rotation and medial shift compared with the intact knee, near 90 degrees of flexion. Both prostheses increased the patellar medial shift near 90 degrees of flexion. The patellar flexion was significantly larger for the mobile bearing prosthesis than for the fixed platform prosthesis for most of flexion. Motion of the insert with respect to the baseplate may have contributed to the variations in tibiofemoral kinematics, whereas tibiofemoral kinematic changes influenced the patellofemoral kinematics. Although the kinematics were similar for the 2 types of prosthesis, the possibility of complications related to increased patellar flexion and backside wear of the tibial insert should be considered.