Clinical safety and wear resistance of the phospholipid polymer-grafted highly cross-linked polyethylene liner

Unicondylar knee arthroplasty demonstrating a significant increased risk for aseptic revisions compared to unconstrained and constrained total knee arthroplasty: An analysis of aseptic revisions after unicondylar and primary total knee arthroplasty of the German Arthroplasty Registry

PURPOSE

Due to ageing population, the implantation rate of total knee arthroplasties (TKAs) is continuously growing. Aseptic revisions in primary knee arthroplasty are a major cause of revision. The aim of the following study was to determinate the incidence and reasons of aseptic revisions in constrained and unconstrained TKA, as well as in unicondylar knee arthroplasties (UKAs).

METHODS

Data collection was performed using the German Arthroplasty Registry. Reasons for aseptic revisions were calculated. Incidence and comparison of aseptic revisions were analysed using Kaplan-Meier estimates. A multiple χ2 test with Holm's method was used to detect group differences in ligament ruptures.

RESULTS

Overall, 300,998 cases of knee arthroplasty with 254,144 (84.4%) unconstrained TKA, 9993 (3.3%) constrained TKA and 36,861 (12.3%) UKA were analysed. Aseptic revision rate in UKA was significantly increased compared to unconstrained and constrained TKA (p < 0.0001). In constrained TKA, a 2.0% revision rate for aseptic reasons were reported after 1 year, while in unconstrained TKA 1.1% and in UKA, 2.7% of revisions were identified. After 7 years in constrained TKA 3.3%, in unconstrained TKA 2.8%, and in UKA 7.8% sustained aseptic revision. Ligament instability was the leading cause of aseptic revision accounting for 13.7% in unconstrained TKA. In constrained TKA, 2.8% resulted in a revision due to ligament instability. In the UKA, the most frequent cause of revisions was tibial loosening, accounting for 14.6% of cases, while progression of osteoarthritis accounted for 7.9% of revisions. Ligament instability was observed in 14.1% of males compared to 15.9% of females in unconstrained TKA and in 4.6% in both genders in UKA.

CONCLUSION

In patients with UKA, aseptic revision rates are significantly higher compared to unconstrained and constrained TKA. Ligament instability was the leading cause of aseptic revision in unconstrained TKA. In UKA, the most frequent cause of revisions was tibial loosening, while progression of osteoarthritis was the second most frequent cause of revisions. Comparable levels of ligament instability were observed in both sexes.

LEVEL OF EVIDENCE

Level III, cohort study.

Minimum 7-year results of cementless total hip arthroplasty with vitamin E-diffused and 2-methacryloyloxyethyl phosphorylcholine-grafted highly cross-linked polyethylene

The purpose of this study was to evaluate the mid-term clinical results and polyethylene wear of vitamin E-diffused highly cross-linked polyethylene (HXLPE) and 2-methacryloyloxyethyl phosphorylcholine (MPC)-grafted HXLPE in cementless total hip arthroplasty (THA). Thirty-four THAs with vitamin E-diffused HXLPE (VEPE) and 32-mm cobalt-chromium head, and 116 THAs with MPC-grafted HXLPE and 32-mm alumina head were evaluated. The Merle d'Aubigné and Postel scores were administered. Kaplan-Meier survivorship was analyzed. Annual radiographs were analyzed using computerized method and linear steady-state wear rate was measured. The mean duration of follow-up was 9 years (range, 7-11 years) in VEPE group and 8 years (range, 7-10 years) in MPC group. The mean Merle d'Aubigné and Postel scores improved postoperatively in both groups. Kaplan-Meier survivorship with endpoint of revision was 100% (95% confidence interval, 100%-100%) in VEPE group and 98.3% (95% confidence interval, 93.4%-99.6%) in MPC group at 10 years (P = .44). The mean steady-state wear rate was 0.007 mm/year in VEPE group and 0.006 mm/year in MPC group (P = .60). The clinical results of both groups were good and wear rates of both liners were very low.

Photoassisted Surface Modification with Zwitterionic Phosphorylcholine Polymers for the Fabrication of Ideal Biointerfaces

Surface modification using zwitterionic 2-methacryloyloxyethyl phosphorylcholine (MPC) polymers is commonly performed to fabricate interfaces that reduce nonspecific fouling by biomolecules and cells. Accordingly, several clinically used devices, such as guide wires, stents, oxygenators, left ventricular assist devices, and microcatheters have been modified using MPC polymers. The specific types of surface modifications vary across substrates and applications. Recently, photoreactions have garnered attention for surface modification due to their stability and tunability. This review highlights various studies that employed photoreactions to modify surfaces using MPC polymers, especially photoinduced graft polymerization of MPC. In addition to antifouling materials, several micromanipulated, long-lasting hydrophilic, and super antiwear surfaces are summarized. Furthermore, several photoreactive MPC polymers that can be used to control interactions between biomolecules and materials are presented along with their potential to form selective recognition surfaces that target biomolecules for biosensors and diagnostic devices.

Updates on Biomaterials Used in Total Hip Arthroplasty (THA)

One of the most popular and effective orthopedic surgical interventions for treating a variety of hip diseases is total hip arthroplasty. Despite being a radical procedure that involves replacing bone and cartilaginous surfaces with biomaterials, it produces excellent outcomes that significantly increase the patient's quality of life. Patient factors and surgical technique, as well as biomaterials, play a role in prosthetic survival, with aseptic loosening (one of the most common causes of total hip arthroplasty failure) being linked to the quality of biomaterials utilized. Over the years, various biomaterials have been developed to limit the amount of wear particles generated over time by friction between the prosthetic head (metal alloys or ceramic) and the insert fixed in the acetabular component (polyethylene or ceramic). An ideal biomaterial must be biocompatible, have a low coefficient of friction, be corrosion resistant, and have great mechanical power. Comprehensive knowledge regarding what causes hip arthroplasty failure, as well as improvements in biomaterial quality and surgical technique, will influence the survivability of the prosthetic implant. The purpose of this article was to assess the benefits and drawbacks of various biomaterial and friction couples used in total hip arthroplasties by reviewing the scientific literature published over the last 10 years.

Ultra-High-Molecular-Weight Polyethylene in Hip and Knee Arthroplasties

Ultra-high-molecular-weight polyethylene (UHMWPE) wear and particle-induced osteolysis contribute to the failure of total hip arthroplasty (THA) and total knee arthroplasty (TKA). Highly crosslinked polyethylene (HXLPE) was developed in the late 1990s to reduce wear and has shown lower wear rates and loosening than conventional UHMWPE in THA. The irradiation dose for crosslinking is up to 100 kGy. However, during crosslinking, free radical formation induces oxidation. Using HXLPE in THA, the cumulative revision rate was determined to be significantly lower (6.2%) than that with conventional UHMWPE (11.7%) at a mean follow-up of 16 years, according to the Australian Orthopaedic Association National Joint Replacement Registry. However, HXLPE does not confer to TKA the same advantages it confers to THA. Several alternatives have been developed to prevent the release of free radicals and improve polymer mechanical properties, such as thermal treatment, phospholipid polymer 2-methacryloyloxyethyl phosphorylcholine grafting, remelting, and vitamin E addition. Among these options, vitamin E addition has reported good clinical results and wear resistance similar to that of HXLPE without vitamin E, as shown by short-term clinical studies of THA and TKA. This review aims to provide a comprehensive overview of the development and performance of UHMWPE in THA and TKA.

Biomimetic materials based on zwitterionic polymers toward human-friendly medical devices

This review summarizes recent research on the design of polymer material systems based on biomimetic concepts and reports on the medical devices that implement these systems. Biomolecules such as proteins, nucleic acids, and phospholipids, present in living organisms, play important roles in biological activities. These molecules are characterized by heterogenic nature with hydrophilicity and hydrophobicity, and a balance of positive and negative charges, which provide unique reaction fields, interfaces, and functionality. Incorporating these molecules into artificial systems is expected to advance material science considerably. This approach to material design is exceptionally practical for medical devices that are in contact with living organisms. Here, it is focused on zwitterionic polymers with intramolecularly balanced charges and introduce examples of their applications in medical devices. Their unique properties make these polymers potential surface modification materials to enhance the performance and safety of conventional medical devices. This review discusses these devices; moreover, new surface technologies have been summarized for developing human-friendly medical devices using zwitterionic polymers in the cardiovascular, cerebrovascular, orthopedic, and ophthalmology fields.

Efficacy of hydrated phospholipid polymer interfaces between all-polymer bearings for total hip arthroplasty

Measurements of wear resistance and metal ion release are important for designing bearing couples or interfaces in total hip arthroplasty (THA). In this study, we investigated wear resistance and metal ion release of surface-modified metal-free all-polymer hip bearings, such as poly(ether-ether-ketone), (PEEK) on cross-linked polyethylene (PEEK-on-CLPE), with a hydrated gel-like surface layer, to propose an improved alternative to the conventional materials used to design THA bearings. The PEEK surface resulted in less metal ion release than the cobalt-chromium-molybdenum (Co-Cr-Mo) alloy surface owing to the lack of metal. The PEEK-on-CLPE bearing (6.33 mg/10⁶  cycles) had lower wear (rate) than the bearing with Co-Cr-Mo alloy-on-CLPE (10.47 mg/10⁶  cycles) under controlled laboratory conditions; the wear performance of the all-polymer hip bearings was further improved with hemi- or both-surface modified with a hydrated poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC) layer (3.74 and 3.06 mg/10⁶  cycles, respectively). The PMPC-grafted interface of PEEK-on-CLPE will be especially suitable for THA candidates. This study is of key importance for the design of lifelong THA and a better understanding of the limitations resulting from using PEEK. Further studies are necessary to evaluate the possibility of using this material in artificial hips.

Long-lasting hydrophilic surface generated on poly(dimethyl siloxane) with photoreactive zwitterionic polymers

Poly(dimethylsiloxane) (PDMS) is known as one of the most established polymers for making elastomers. Therefore, it is commonly used for the fabrication of biomedical devices. Many PDMS surface modification processes have been proposed recently to increase PDMS reliability in medical fields. However, the modified surface's long-term stability is still limited. Hydrophobic recovery of PDMS is widely recognized as a factor that reduces the efficacy of PDMS surface modification. The photoreactive zwitterionic polymer effectively suppresses the hydrophobic recovery of PDMS, according to the current analysis. The photoreactive zwitterionic monomer, 2-[2-(Methacryloyloxy)ethyldimethylanmmonium] ethyl benzophenoxy phosphate (MBPP) was polymerized by conventional radical polymerization and coated on O₂-plasma-treated PDMS specimens. The specimens were immersed in an aqueous solution of 2-methacryloyloxyethyl phosphorylcholine (MPC) and exposed under ultraviolet (UV) radiation for 3 h. Instead, of poly(MBPP) (PMBPP), benzophenone (BP) was also used as a conventional photoinitiator. The time-dependent change in the wettability and elemental composition of the specimen surface was monitored for nine weeks after photo-grafting of poly[2-methacryloyloxyethyl phosphorylcholine (MPC)] (PMPC). The advancing and receding contact angles (θ_A/θ_R) of the pristine PDMS specimen were 112°/71° and significantly decreased immediately after the grafting of PMPC regardless of types of photoinitiator. However, the hydrophobicity of the surface gradually recovered, and θ_A was changed from 12° to 81° for nine weeks of storage under air atmosphere when BP was used as a photoinitiator for graft polymerization of MPC. However, surface hydrophilicity (θ_A ≅ 20°) of the surface grafted with PMPC with PMBPP as an initiator was effectively preserved for nine weeks. This surface also showed excellent lubricity and non-fouling properties regardless of the storage periods. Therefore, zwitterionic photoreactive polymer, PMBPP, is then used as a macrophotoinitiator for the surface modification of PDMS.

Controlled biointerfaces with biomimetic phosphorus-containing polymers

Phosphorus is a ubiquitous and one of the most common elements found in living organisms. Almost all molecules containing phosphorus in our body exist as analogs of phosphate salts or phosphoesters. Their functions are versatile and important, being responsible for forming the genetic code, cell membrane, and mineral components of hard tissue. Several materials inspired from these phosphorus-containing biomolecules have been recently developed. These materials have shown unique properties at the biointerface, such as nonfouling ability, blood compatibility, lubricity, mineralization induction capability, and bone affinity. Several unfavorable events occur at the interface of materials and living organisms because most of these materials have not been designed while taking host responses into account. These unfavorable events are directly linked to reducing functions and shorten the usable periods of medical devices. Biomimetic phosphorus-containing polymers can improve the reliability of materials in biological systems. In addition, phosphorus-containing biomimetic polymers are useful not only for improving the biocompatibility of material surfaces but also for adding new functions due to the flexibility in molecular design. In this review, we describe the recent advances in the control of biointerfacial phenomena with phosphorus-containing polymers. We especially focus on zwitterioninc phosphorylcholine polymers and polyphosphoesters.

Short-term clinical and radiographic outcomes of total hip arthroplasty with PMPC-grafted highly cross-linked polyethylene liners against 32-mm femoral heads

We evaluated the clinical and radiographic outcomes of femoral head penetration and total hip arthroplasties with untreated and poly (2-methacryloyloxyethyl phosphorylcholine) (PMPC)-grafted highly cross-linked polyethylene (HXLPE) acetabular liners against 26-, 28-, and 32-mm cobalt-chromium alloy femoral heads 3 years after the index surgery. Three combinations of the articulating surfaces were evaluated in the present study: untreated or PMPC-grafted HXLPE liner against 26- or 28-mm femoral heads (n = 16, 24) [control (26 or 28 mm) and PMPC (26 or 28 mm)] and PMPC-grafted HXLPE liner against 32-mm femoral heads (n = 64) [PMPC (32 mm)]. The clinical outcomes improved at 3 years postoperatively for the groups. No periprosthetic osteolysis or acetabular component migration was detected, and no revision surgery was performed among the groups. The steady-state wear rate of the PMPC (26 or 28 mm) group (0.021 mm/year) was lower than that of the control (26 or 28 mm) group (- 0.015 mm/year); the steady-state wear was under the clinical threshold. In contrast, the steady-state wear rate of the PMPC (32 mm) group (-0.006 mm/year) showed no significant difference when compared to that of the PMPC (26 or 28 mm) group (p < 0.01). The results obtained in the present study clearly demonstrate that PMPC-grafting onto an HXLPE surface improved the wear resistance of acetabular liners, even when coupled with larger femoral heads. Although further follow-up evaluations are required, PMPC-grafted HXLPE acetabular liners may be a promising approach to extend the longevity of artificial joints.

2-Methacryloyloxyethyl Phosphorylcholine Polymer Coating Inhibits Bacterial Adhesion and Biofilm Formation on a Suture: An In Vitro and In Vivo Study

Initial bacterial adhesion to medical devices and subsequent biofilm formation are known as the leading causes of surgical site infection (SSI). Therefore, inhibition of bacterial adhesion and biofilm formation on the surface of medical devices can reduce the risk of SSIs. In this study, a highly hydrophilic, antibiofouling surface was prepared by coating the bioabsorbable suture surface with poly(2-methacryloyloxyethyl phosphorylcholine (MPC)-co-n-butyl methacrylate) (PMB). The PMB-coated and noncoated sutures exhibited similar mechanical strength and surface morphology. The effectiveness of the PMB coating on the suture to suppress adhesion and biofilm formation of methicillin-resistant Staphylococcus aureus and methicillin-susceptible Staphylococcus aureus was investigated both in vitro and in vivo. The bacterial adhesion test revealed that PMB coating significantly reduced the number of adherent bacteria, with no difference in the number of planktonic bacteria. Moreover, fluorescence microscopy and scanning electron microscopy observations of adherent bacteria on the suture surface after contact with bacterial suspension confirmed PMB coating-mediated inhibition of biofilm formation. Additionally, we found that the PMB-coated sutures exhibited significant antibiofouling effects in vivo. In conclusion, PMB-coated sutures demonstrated bacteriostatic effects associated with a highly hydrophilic, antibiofouling surface and inhibited bacterial adhesion and biofilm formation. Therefore, PMB-coated sutures could be a new alternative to reduce the risk of SSIs.

Effects of a roughened femoral head and the locus of grafting on the wear resistance of the phospholipid polymer-grafted acetabular liner

Although laboratory tests and mid-term clinical outcomes show the clinical safety and remarkable wear resistance of the highly cross-linked polyethylene (HXLPE) acetabular liner with a nanometer-scaled graft layer of poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC), the wear resistance of the layer under severe abrasive conditions is concerning. We evaluated the effects of a roughened femoral head and the grafting locus on the wear resistance of the PMPC-grafted HXLPE liner and the effect of PMPC grafting on wear resistance of the HXLPE substrate by removing the PMPC-grafted layer using a severely roughened femoral head. Against a moderately roughened femoral head, the PMPC-grafted HXLPE liner showed negative wear, although an untreated HXLPE liner increased the wear by 154.1% compared with wear against a polished femoral head, confirming that PMPC grafts were unaffected. Against a severely roughened femoral head, the PMPC-grafted layer of the head contact area might be removed under severe conditions. However, the wear rate was reduced by 52.5% compared to that of untreated HXLPE liners. Moreover, the head non-contact area-modified PMPC-grafted HXLPE liner against a polished femoral head reduced the wear by 76.8% compared with untreated HXLPE liner; thus, this area may be also important in the development of fluid-film lubrication. STATEMENT OF SIGNIFICANCE: Here we describe effects of a roughened femoral head and the locus of grafting on the wear-resistance of the phospholipid polymer grafted highly cross-linked polyethylene (PMPC-HXLPE) liner. Against a moderately roughened femoral head, the PMPC-HXLPE liner showed negative wear, confirming that PMPC grafts were unaffected. After removing the PMPC layer of the head contact area using a severely roughened femoral head, the wear rate not only exceeded that of untreated HXLPE liners, but was reduced by 52.5%, confirming that PMPC grafting does not affect the wear-resistance of the HXLPE substrate. In addition, the head non-contact area-modified PMPC-HXLPE liner reduced the wear by 76.8%. Thus, this area may also may be important in the development of fluid-film lubrication.

Hydrated Phospholipid Polymer Gel-Like Layer for Increased Durability of Orthopedic Bearing Surfaces

Recently, traditional strategies for manipulating orthopedic bearing substrates have attempted to improve their wear resistance by adjusting polyethylene substrate through cross-linking and antioxidant blending. However, further research is required on the substrate, as well as the surface focused on the structure and role of articular cartilage. We therefore develop an orthopedic bearing surface comprising a nanometer-scale hydrated gel-like layer by grafting highly hydrophilic poly(2-methacryloyloxyethyl phosphorylcholine), with the aim of mimicking the lubrication mechanism of articular cartilage, and investigate its surface characteristics, bulk characteristics, and behavior under load bearing conditions upon accelerated aging. Neither the hydrophilicity nor lubricity of the gel-like surface was influenced by accelerated aging; instead, high stability was revealed, even under strong oxidation conditions. The characteristics of the hydrated gel-like surface potentiated the wear resistance of the cross-linked polyethylene liner, irrespective of accelerated aging. These results suggest that the hydrated gel-like surface enhances the longevity of cross-linked polyethylene bearings even under load-bearing conditions. Furthermore, the inflection point on the time series of wear can be a suitable indicator of the durability of the life-long protectant. In conclusion, the hydrated gel-like surface can positively increase orthopedic implant durability.

Romantic Surfaces: A Systematic Overview of Stable, Biospecific, and Antifouling Zwitterionic Surfaces

This Feature Article focuses on recent advances in the bioconjugation of surface-bound zwitterionic polymers for biospecific antifouling surfaces. Various approaches for the functionalization of antifouling zwitterionic polymers are systematically investigated, such as chain-end and side-chain functionalization. Side-chain functionalization methods can be further classified as those that are achieved through homopolymerization of custom-synthesized zwitterionic monomers equipped with reactive groups, or those that are achieved via synthesis of random or block copolymers combining different monomers with antifouling functionality and others with reactive groups. Several of the pros and cons of these approaches are outlined and discussed. Finally, some perspective and future directions of research are presented toward long-term stable, generically repelling surfaces that strongly and specifically adhere to a single component in a complex mixture.

A phospholipid polymer graft layer affords high resistance for wear and oxidation under load bearing conditions

Manipulating the surface and substrate of cross-linked polyethylene (CLPE) is an essential approach for obtaining life-long orthopedic bearings. We therefore proposed a bearing material comprised of an antioxidative substrate generated by vitamin E blending (HD-CLPE[VE]) with a poly(2-methacryloyloxyethyl phosphorylcholine) (PMPC)-grafted surface, and investigated its wear resistance and oxidative stability under accelerated aging and load bearing conditions. Neither the hydration nor friction kinetics of the molecular network structure of the PMPC-grafted surface or the HD-CLPE(VE) substrate were influenced by accelerated aging but rather exhibited high stability even under high oxidation conditions. The characteristics of the PMPC-grafted surface improved the wear and impact fatigue resistance of the HD-CLPE(VE) liner regardless of accelerated aging. Notably, the PMPC-grafted surface was found to affect the potential oxidative stability at the rim part of the acetabular liner. PMPC chains serve several important functions on the surface regardless of load bearing, such as high lubricity or low lipophilicity attributed to phosphorylcholine groups and/or surrounding water-fluid film, and suppression of lipid diffusion attributed to methacrylate main chains on the surface. Together, these results provide preliminary evidence that the PMPC graft layer and vitamin E-blended substrate might positively affect the extent of orthopedic implant durability.