Introduction

Dual mobility (DM) total hip arthroplasty (THA) prostheses are designed to increase stability. In the setting of primary and revision THA, DM THA are used most frequently for dysplasia and instability diagnoses, respectively. As the use of DM THA continues to increase, with 8,031 cases logged in the American Joint Replacement Registry from 2012–2018, characterizing in vivo damage and clinical failure modes are important to report.

Introduction

Pin-tract infections are a common problem in orthopaedic surgery, which limits the time an external fixator or Taylor spatial frame can be applied to a patient. The purpose of our study is to evaluate the ability of a novel implant surface coating — cationic steroid antibiotic (CSA)-44 — to delay or prevent the onset of these infections. This coating mimics endogenous antimicrobial peptides of the innate immune system and has been shown to effectively eradicate biofilms as well as prevent infection and stimulate healing of open, contaminated fractures.

Prior work in the setting of MRSA (clinical isolate), showed that enhancement of Ti6Al4V with anodized nanotubes apparently disrupts the formation and adhesion of MRSA biofilm. The greater amount of cultured MRSA using effluent released from in vitro nanotube surfaces by sonication, compared with thermal plasma sprayed (TPS), indicated probable disruption of biofilm formation and adhesion. The use of nanosilver nanotubes in vivo in a rabbit model showed that after 1 week of infection followed by 1 week of vancomycin treatment, the nanotube MRSA level was 30% that of TPS, and the nanosilver nanotube MRSA level was only 5% of TPS. The implementation of the technology will enhance the remodeled bone locking ability of rough TPS, with surface nanotubes that provide antibacterial properties and increased bone adhesion.

Lap shear tests of the nanotubes were performed according to ASTM F1044. In multiple tests, circular adhesive films bonded Ti6Al4V bars containing nanotubes with plain Ti6Al4V. The assemblies were suitably arranged in a tensile tester and pulled to shear failure. There were three modes of failure; shear failure within the adhesive, failure of the adhesive from the plain titanium, and shear failure of the nanotubes from the bar. Tests determined the shear strength of the adhesive and its bonding strength to bare titanium. ImageJ software determined the area of each of the three failure modes. From this analysis, the shear strength of the nanotubes of each sample was calculated.

The analyses showed the shear strength of the nanotubes to be as high as 65MPa (9,500psi) with a more typical shear strength of 55MPa (8,000 psi), and several surfaces with 45MPa (6,000 psi). The literature presents models predicting the shear stress in bonded hip stems. Assuming the TPS with nanotubes performs similar to a bonded hip stem, owing to the locking of the bone with the TPS, a typical shear stress prediction for physiological loads is approximately 10 MPa. The nanotube shear strengths were 4–6 times higher than the expected stress during use.

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Recent clinical data suggest improvement in the fixation of tibia trays for total knee arthroplasty when the trays are additive manufactured with highly porous bone ingrowth structures. Currently, press-fit TKA is less common than press-fit THA. This is partly because the loads on the relatively flat, porous, bony apposition area of a tibial tray are more demanding than those same porous materials surrounding a hip stem. Even the most advanced additive manufactured (AM) highly porous structures have bone ingrowth limitations clinically as aseptic loosening still remains more common in press-fit TKA vs. THA implants.

Osseointegration and antibacterial properties have been shown in vitro and in vivo to improve when implants have modified surfaces that have biomimetic nanostructures designed to mimic and interact with biological structures on the nano-scale. Pre-clinical evaluations show that TiO₂ nanotubes (TNT), produced by anodization, on Ti6Al4V surfaces positively enhance the rate at which osseointegration occurs and TNT nano-texturization enhances the antibacterial properties of the implant surface.²

In this in vivo sheep study, identical Direct Metal laser Sintered (DMLS) highly porous Ti6Al4V specimens with and without TNT surface treatment are compared to sintered bead specimens with plasma sprayed hydroxyapatite-coated surface treatment. Identical DMLS specimens made from CoCrMo were also implanted in sheep tibia bi-cortically (3 per tibia) and in the cancellous bone of the distal femur and proximal tibia (1 per site). Animals were injected with fluorochrome labels at weeks 1, 2 and 3 after surgery to assess the rate of bone integration. The cortical specimens were mechanically tested and processed for PMMA histology and histomorphometry after 4 or 12 weeks. The cancellous samples were also processed for PMMA histology and histomorphometry. The three types of bone labels were visualized under UV light to examine the rate of new bony integration.

At 4 weeks, a 42% increase in average pull-out shear strength between nanotube treated specimens and non-nanotube treated specimens was shown. A 21% increase in average pull-out shear strength between nanotube treated specimens and hydroxyapatite-coated specimens was shown. At 12 weeks, all specimens had statistically similar pull-out values. Bone labels demonstrated new bone formation into the porous domains on the materials as early as 2 weeks.

A separate in vivo study on 8 rabbits infected with methicillin-resistant Staphylococcus aureus showed bacterial colonization reduction on the surface of the implants treated with TNT. In vitro and in vivo evidence suggests that nanoscale surfaces have an antibacterial effect due to surface energy changes that reduce the ability of bacteria to adhere.

These in vivo studies show that TNT on highly porous AM specimens made from Ti6Al4V enhances new bone integration and also reduce microbial attachment.

Periprosthetic infection remains a clinical challenge that may lead to revision surgeries, increased spending, disability, and mortality. The cost for treating hip and knee total joint infections is anticipated to be $1.62 billion by 2020. There is a need for implant surface modifications that simultaneously resist bacterial biofilm formation and adhesion, while promoting periprosthetic bone formation and osseointegration.

In vitro research has shown that nanotextured titanium promotes osteoblast differentiation, and upregulates metabolic markers of osteoblast activity and osteoblast proliferation. In vivo rat studies confirmed increased bone-implant contact area, enhanced de novo bone formation on and adjacent to the implant, and higher pull-out forces compared to non-textured titanium. The authors have advanced a benign electrochemical anodization process based on ammonium fluoride that creates a nanotube surface in as little as 10 minutes (Fig. 1), which can also integrate antibacterial nanosilver (Fig. 2).

The work reported here summarizes in vitro post-inoculation and in vivo post-implantation studies, showing inherent inhibition of methicillin-resistant Staphylococcus aureus (MRSA) by titanium surfaces with nanotubes (TiNT), nanotubes with nanosilver (TiNT+Ag), plain (Ti), and thermal plasma sprayed (TPS) titanium. Ti6Al4V was the base material for all surfaces. In vitro studies evaluated Ti, TPS, four TiNT groups with varying nanotube diameters (60nm, 80nm, 110nm, 150nm), and TiNT+Ag. After seeding with MRSA (10⁵, 10⁶, and 10⁸ CFU/mL), the 110nm diameter nanotubes showed MRSA inhibition up to three-orders of magnitude lower than the Ti and TPS surfaces at 2, 6, and 48 hours.

Following on the in vitro results, New Zealand White rabbits underwent a bilateral implantation of intramedullary tibial implants of the four material groups (4 mm outside diameter; 110nm NT diameter on TiNT and TiNT+Ag implants). One intramedullary canal was inoculated with clinically-derived MRSA (10⁵ CFU in broth) at the time of implantation; one canal had only culture media introduced (control). At a 2-week endpoint, limbs were harvested for analysis, including implant sonication with sonicant bacterial cultured, histology, and microcomputed chromatography. In the sonicant analysis cohort, TPS showed the lowest average MRSA count, while TiNT and TiNT+Ag were the highest. There was one sample each of TPS, TiNT and TiNT+Ag that showed no MRSA. After an additional 24-hour implant incubation, the TiNT and TiNT+Ag samples had no bacteria, but the TPS grew bacteria; therefore, the authors hypothesize that MRSA more readily releases from the TiNT and TiNT+Ag implants during sonication, indicating weaker biofilm adhesion and development. Histologic analysis is currently underway. In a therapeutic experiment, rabbits underwent bilateral implantation, followed by 1 week of infection development, and then 1 week of vancomycin treatment. At the endpoint, implants were sonicated and bacteria was quantified from the sonicant. TiNT showed viable MRSA at only 30% that of TPS-coated levels, while TiNT+Ag implants showed viable MRSA at only 5% that of TPS-coated levels (Fig. 3). These early results indicate that the TiNT and TiNT+Ag surfaces have some inherent antibacterial activity against MRSA, which may increase the efficacy of systemic antibiotic treatments in the setting of periprosthetic joint infections.

Titanium knee, shoulder and hip implants are typically grit-blasted, thermal plasma spray coated, or sintered to provide ingrowth surface features having texture with pore sizes on the order of hundreds of micrometers. This provides macro and micro-mechanical locking upon bone remodeling. However, at the nanoscale and cellular level, these surfaces appear smooth. In vitro and in vivo research shows surfaces with nanoscale features result in enhanced osseointegration, greater bone-implant contact area and pullout force, and the potential to be bactericidal via a simple hybrid anodization surface modification process. Prior processes for creating nanotube nano-textured surfaces via electrochemical anodization relied on hydrofluoric acid electrolyte and platinum cathodes. This novel process uses ammonium fluoride electrolytes and graphite cathodes which are more cost effective and easier to handle during processing. Hybrid electrolytes with differing concentrations of ethylene glycol, water, and ammonium fluoride provide a variety of nanotube morphologies and sizes. Nano-tubular surfaces on knee tibial and femoral implants, hip stems and acetabular cups, bone screws and other 3D printed parts have been enhanced by this method of nano-texturing in as little as 30 minutes.

In vivo work in a Sprague Dawley rat model showed bone-implant contact area up to 2.9-times greater, and uniaxial pullout forces up to 6.9-times greater, than implanted smooth titanium controls at 4 and 12-week time points. In these tests, 1.25mm Kirschner wires were implanted in the rat femora to simulate an intramedullary nail. Histomorphometry in the mid-shaft and distal regions showed greater trabecular thickness and bone tissue mineral density than controls. Axial pullout tests often resulted in bone failure before the bone-implant interface.

In vitro evidence suggests that nanoscale surfaces may have an antibacterial effect due to surface energy changes that reduce the ability of bacteria to adhere. However, it is recognized that silver is highly antibacterial in appropriate concentrations. It is also recognized that nanosilver, approximately 10–20nm, is especially effective. Ammonium fluoride anodization is modified using a hybrid electrolyte that includes silver fluoride. By substituting some of the ammonium fluoride with silver fluoride, to maintain a constant total fluorine mass, nanosilver is integrated within and among the nanotubes in the same single process that forms the nanotubes.

This hybrid process in nano-texturing titanium implants can be integrated into current manufacturing production at low cost.

Purpose:

Acetabular bone loss during revision total hip arthroplasty (THA) poses a challenge for reconstruction as segmental and extensive cavitary defects require structural support to achieve prosthesis stability. Trabecular metal (TM) acetabular augments structurally support hemispherical cups. Positive short-term results have been encouraging, but mid- to long-term results are largely unknown. The purpose of this study was to determine the continued efficacy of TM augments in THA revisions with significant pelvic bone loss.

Introduction:

Unicompartmental knee arthroplasty (UKA) has been used in the past decades to treat progressive cartilage degeneration in a single compartment. Concern has been raised over the rate of revision procedures for polyethylene wear and osteoarthritic progression into the adjacent compartment. Few studies have examined the pathology of cartilage degeneration in the setting of UKA. This study aims to investigate the viability of knee chondrocytes introduced to high and low concentrations of orthopaedic wear debris particulate.