In vitro and in vivo study on the osseointegration of BCP-coated versus uncoated nondegradable thermoplastic polyurethane focal knee resurfacing implants

In vitro and in vivo evaluation of the osseointegration capacity of a polycarbonate-urethane zirconium-oxide composite material for application in a focal knee resurfacing implant

Currently available focal knee resurfacing implants (FKRIs) are fully or partially composed of metals, which show a large disparity in elastic modulus relative to bone and cartilage tissue. Although titanium is known for its excellent osseointegration, the application in FKRIs can lead to potential stress-shielding and metal implants can cause degeneration of the opposing articulating cartilage due to the high resulting contact stresses. Furthermore, metal implants do not allow for follow-up using magnetic resonance imaging (MRI).To overcome the drawbacks of using metal based FKRIs, a biomimetic and MRI compatible bi-layered non-resorbable thermoplastic polycarbonate-urethane (PCU)-based FKRI was developed. The objective of this preclinical study was to evaluate the mechanical properties, biocompatibility and osteoconduction of a novel Bionate® 75D - zirconium oxide (B75D-ZrO2) composite material in vitro and the osseointegration of a B75D-ZrO2 composite stem PCU implant in a caprine animal model. The tensile strength and elastic modulus of the B75D-ZrO2 composite were characterized through in vitro mechanical tests under ambient and physiological conditions. In vitro biocompatibility and osteoconductivity were evaluated by exposing human mesenchymal stem cells to the B75D-ZrO2 composite and culturing the cells under osteogenic conditions. Cell activity and mineralization were assessed and compared to Bionate® 75D (B75D) and titanium disks. The in vivo osseointegration of implants containing a B75D-ZrO2 stem was compared to implants with a B75D stem and titanium stem in a caprine large animal model. After a follow-up of 6 months, bone histomorphometry was performed to assess the bone-to-implant contact area (BIC). Mechanical testing showed that the B75D-ZrO2 composite material possesses an elastic modulus in the range of the elastic modulus reported for trabecular bone. The B75D-ZrO2 composite material facilitated cell mediated mineralization to a comparable extent as titanium. A significantly higher bone-to-implant contact (BIC) score was observed in the B75D-ZrO2 implants compared to the B75D implants. The BIC of B75D-ZrO2 implants was not significantly different compared to titanium implants. A biocompatible B75D-ZrO2 composite approximating the elastic modulus of trabecular bone was developed by compounding B75D with zirconium oxide. In vivo evaluation showed an significant increase of osseointegration for B75D-ZrO2 composite stem implants compared to B75D polymer stem PCU implants. The osseointegration of B75D-ZrO2 composite stem PCU implants was not significantly different in comparison to analogous titanium stem metal implants.

Calibration of Aseptic Loosening Simulation for Coatings Osteoinductive Effect

The risk of aseptic loosening in cementless hip stems can be reduced by improving osseointegration with osteoinductive coatings favoring long-term implant stability. Osseointegration is usually evaluated in vivo studies, which, however, do not reproduce the mechanically driven adaptation process. This study aims to develop an in silico model to predict implant osseointegration and the effect of induced micromotion on long-term stability, including a calibration of the material osteoinductivity with conventional in vivo studies. A Finite Element model of the tibia implanted with pins was generated, exploiting bone-to-implant contact measures of cylindrical titanium alloys implanted in rabbits' tibiae. The evolution of the contact status between bone and implant was modeled using a finite state machine, which updated the contact state at each iteration based on relative micromotion, shear and tensile stresses, and bone-to-implant distance. The model was calibrated with in vivo data by identifying the maximum bridgeable gap. Afterward, a push-out test was simulated to predict the axial load that caused the macroscopic mobilization of the pin. The bone-implant bridgeable gap ranged between 50 μm and 80 μm. Predicted push-out strength ranged from 19 N to 21 N (5.4 MPa-3.4 MPa) depending on final bone-to-implant contact. Push-out strength agrees with experimental measurements from a previous animal study (4 ± 1 MPa), carried out using the same implant material, coated, or uncoated. This method can partially replace in vivo studies and predict the long-term stability of cementless hip stems.

Bone ingrowth into a porous structure is achieved by preceding fibrogenesis and vascularization

Porous structures are frequently used in surgical implants to strengthen the interlocking power produced by bone ingrowth. Therefore, we aimed to elucidate the mechanism underlying bone ingrowth into a porous structure accompanied by vascularization. A nonbioactive polyetheretherketone implant with a 3D-printed porous structure was prepared and implanted in a bone hole created in the tibias of rabbits. We observed bone ingrowth in the same individual specimens immediately and at 2, 4, 8, and 12 weeks post-implantation using in-vivo computed tomography (CT). Furthermore, a detailed evaluation with blood vessels of each specimen at 2, 4, and 12 weeks was performed with ex-vivo CT and histological specimen. Additional histological evaluation was performed using thin sections of an implant made with thermoplastic polyurethane having the same structure. As a result, the bone invasion began after four weeks, when the construction of fibrous tissue and the spread of new blood vessels within the voids matured. As the bone matured in the load-bearing area, new blood vessels outside the bone matrix regressed. This longitudinal evaluation study suggests that preceding fibrogenesis and vascularization may be key in developing bone ingrowth. STATEMENT OF SIGNIFICANCE: A porous structure is an essential structure for dental and orthopedic implants because it provides strong fixation through bone invasion. Although it was known that vascularization was involved in this, the details were not known. This in vivo study revealed that in order for bone ingrowth to begin, a preparatory period of approximately 4 weeks was required to establish blood flow inside and outside the implant. Furthermore, it was confirmed that by spreading the fibrous structure in advance, it has an advantageous effect on the migration of cells involved in the formation of bones and blood vessels. We pointed out that it is necessary to consider fibrogenesis and vascularization when creating future implants.

Mechanical behaviors of high-strength fabric composite membrane designed for cardiac valve prosthesis replacement

Bovine pericardium has been commonly used as leaflets in cardiac valve prosthesis replacement for decades because of its good short-term hemocompatibility and hemodynamic performance. However, fatigue, abrasion, permanent deformation, calcification, and many other failure modes have been reported as well. The degradation of the performance will have a serious impact on the function of valve prostheses, posing a risk to the patient's health. This study aimed to introduce a flexible fabric composite with better mechanical performance such that it can be employed as a substitute material for bioprosthetic valve leaflets. This composite has a multilayered thin film structure made of ultrahigh molecular weight polyethylene (UHMWPE) fabric and thermoplastic polyurethane (TPU) membranes. The mechanical properties of three specifications with different design parameters were tested. The tensile strength, shear behavior, tear resistance, and bending stiffness of the composites were characterized and compared to those of bovine pericardium. A constitutive model was also established to describe the composites' mechanical behaviors and predict their strength. According to the results of the tests, the composite could maintain a flexible bending stiffness with high in-plane tensile strength and tear strength. Therefore, bioprosthetic valve made of this substitute material can withstand harsher loads in the blood flow environment than those made of bovine pericardium. Moreover, all these test results and constitutive models can be used in future research to evaluate hemodynamic performance and clinical applications of fabric composite valve prostheses.

Recent Advancements in Polyurethane-based Tissue Engineering

In tissue engineering, polyurethane-based implants have gained significant traction because of their high compatibility and inertness. The implants therefore show fewer side effects and lasts longer. Also, the mechanical properties can be tuned and morphed into a particular shape, owing to which polyurethanes show immense versatility. In the last 3 years, scientists have devised methods to enhance the strength of and induce dynamic properties in polyurethanes, and these developments offer an immense opportunity to use them in tissue engineering. The focus of this review is on applications of polyurethane implants for biomedical application with detailed analysis of hard tissue implants like bone tissues and soft tissues like cartilage, muscles, skeletal tissues, and blood vessels. The synthetic routes for the preparation of scaffolds have been discussed to gain a better understanding of the issues that arise regarding toxicity. The focus here is also on concerns regarding the biocompatibility of the implants, given that the precursors and byproducts are poisonous.

Anatomically and Biomechanically Relevant Monolithic Total Disc Replacement Made of 3D-Printed Thermoplastic Polyurethane

Various implant treatments, including total disc replacements, have been tried to treat lumbar intervertebral disc (IVD) degeneration, which is claimed to be the main contributor of lower back pain. The treatments, however, come with peripheral issues. This study proposes a novel approach that complies with the anatomical features of IVD, the so-called monolithic total disc replacement (MTDR). As the name suggests, the MTDR is a one-part device that consists of lattice and rigid structures to mimic the nucleus pulposus and annulus fibrosus, respectively. The MTDR can be made of two types of thermoplastic polyurethane (TPU 87A and TPU 95A) and fabricated using a 3D printing approach: fused filament fabrication. The MTDR design involves two configurations-the full lattice (FLC) and anatomy-based (ABC) configurations. The MTDR is evaluated in terms of its physical, mechanical, and cytotoxicity properties. The physical characterization includes the geometrical evaluations, wettability measurements, degradability tests, and swelling tests. The mechanical characterization comprises compressive tests of the materials, an analytical approach using the Voigt model of composite, and a finite element analysis. The cytotoxicity assays include the direct assay using hemocytometry and the indirect assay using a tetrazolium-based colorimetric (MTS) assay. The geometrical evaluation shows that the fabrication results are tolerable, and the two materials have good wettability and low degradation rates. The mechanical characterization shows that the ABC-MTDR has more similar mechanical properties to an IVD than the FLC-MTDR. The cytotoxicity assays prove that the materials are non-cytotoxic, allowing cells to grow on the surfaces of the materials.

Surface texture analysis of different focal knee resurfacing implants after 6 and 12 months in vivo in a goat model

The clinical success of osteochondral implants depends significantly on their surface properties. In vivo, an implant may roughen over time which can decrease its performance. The present study investigates whether changes in the surface texture of metal and two types of polycarbonate urethane (PCU) focal knee resurfacing implants (FKRIs) occurred after 6 and 12 months of in vivo articulation with native goat cartilage. PCU implants which differed in stem stiffness were compared to investigate whether the stem fixating the implant in the bone influences surface topography. Using optical profilometry, 19 surface texture parameters were evaluated, including spatial distribution and functional parameters obtained from the material ratio curve. For metal implants, wear during in vivo articulation occurred mainly via material removal, as shown by the significant decrease of the core-valley transition from 91.5% in unused implants to 90% and 89.6% after 6 and 12 months, respectively. Conversely, for PCU implants, the wear mechanism consisted in either filling of the valleys or flattening of the surface by dulling of sharp peaks. This was illustrated in the change in roughness skewness from negative to positive values over 12 months of in vivo articulation. Implants with a softer stem experienced the most deformation, shown by the largest change in material ratio curve parameters. We therefore showed, using a detailed surface profilometry analysis, that the surface texture of metal and two different PCU FKRIs changes in a different way after articulation against cartilage, revealing distinct wear mechanisms of different implant materials.

Translating Material Science into Bone Regenerative Medicine Applications: State-of-The Art Methods and Protocols

In the last 20 years, bone regenerative research has experienced exponential growth thanks to the discovery of new nanomaterials and improved manufacturing technologies that have emerged in the biomedical field. This revolution demands standardization of methods employed for biomaterials characterization in order to achieve comparable, interoperable, and reproducible results. The exploited methods for characterization span from biophysics and biochemical techniques, including microscopy and spectroscopy, functional assays for biological properties, and molecular profiling. This review aims to provide scholars with a rapid handbook collecting multidisciplinary methods for bone substitute R&D and validation, getting sources from an up-to-date and comprehensive examination of the scientific landscape.

Ultrasound-Based Quantification of Cartilage Damage After In Vivo Articulation With Metal Implants

OBJECTIVE

This study aims to evaluate the applicability of the ultrasound roughness index (URI) for quantitative assessment of cartilage quality ex vivo (post-mortem), after 6 months of in vivo articulation with a Focal Knee Resurfacing Implant (FKRI).

DESIGN

Goats received a metal FKRI (n = 8) or sham surgery (n = 8) in the medial femoral condyles. After 6 months animals were sacrificed, tibial plateaus were stained with Indian ink, and macroscopic scoring of the plateaus was performed based on the ink staining. The URI was calculated from high-frequency ultrasound images at several sections, covering both areas that articulated with the implant and non-articulating areas. Cartilage quality at the most damaged medial location was evaluated with a Modified Mankin Score (MMS).

RESULTS

The URI was significantly higher in the FKRI-articulating than in the sham plateaus at medial articulating sections, but not at sections that were not in direct contact with the implant, for example, under the meniscus. The mean macroscopic score and MMS were significantly higher in the FKRI-articulating group than in the sham group (P=0.035, P<0.001, respectively). Correlation coefficients between URI and macroscopic score were significant in medial areas that articulated with the implant. A significant correlation between URI and MMS was found at the most damaged medial location (ρ=0.72,P=0.0024).

CONCLUSIONS

This study demonstrates the potential of URI to evaluate cartilage roughness and altered surface morphology after in vivo articulation with a metal FKRI, rendering it a promising future tool for quantitative follow-up assessment of cartilage quality.

Applications of Artificial Intelligence in 18F-Sodium Fluoride Positron Emission Tomography/Computed Tomography:: Current State and Future Directions

This review discusses the current state of artificial intelligence (AI) in ¹⁸F-NaF-PET/CT imaging and the potential applications to come in diagnosis, prognostication, and improvement of care in patients with bone diseases, with emphasis on the role of AI algorithms in CT bone segmentation, relying on their prevalence in medical imaging and utility in the extraction of spatial information in combined PET/CT studies.

In vitro and in vivo study on the osseointegration of BCP-coated versus uncoated nondegradable thermoplastic polyurethane focal knee resurfacing implants

Focal knee resurfacing implants (FKRIs) are intended to treat cartilage defects in middle-aged patients. Most FKRIs are metal-based, which hampers follow-up of the joint using magnetic resonance imaging and potentially leads to damage of the opposing cartilage. The purpose of this study was to develop a nondegradable thermoplastic polyurethane (TPU) FKRI and investigate its osseointegration. Different surface roughness modifications and biphasic calcium phosphate (BCP) coating densities were first tested in vitro on TPU discs. The in vivo osseointegration of BCP-coated TPU implants was subsequently compared to uncoated TPU implants and the titanium bottom layer of metal control implants in a caprine model. Implants were implanted bilaterally in stifle joints and animals were followed for 12 weeks, after which the bone-to-implant contact area (BIC) was assessed. Additionally, 18F-sodium-fluoride (18F-NaF) positron emission tomography PET/CT-scans were obtained at 3 and 12 weeks to visualize the bone metabolism over time. The BIC was significantly higher for the BCP-coated TPU implants compared to the uncoated TPU implants (p = .03), and did not significantly differ from titanium (p = .68). Similar 18F-NaF tracer uptake patterns were observed between 3 and 12 weeks for the BCP-coated TPU and titanium implants, but not for the uncoated implants. TPU FKRIs with surface modifications could provide the answer to the drawbacks of metal FKRIs.