An evaluation of bone growth into porous high density polyethylene.

The purpose of this investigation was to study bone growth into porous polyethylene rods as a function of time and pore structure. Previous studies have indicated the biocompatibility of solid polyethylene materials which are currently being used clinically. Porous polyethylene rods were implanted in the femurs of mongrel dogs which are sacrificed four, eight, and 16 weeks postoperatively. The implants were then sectioned and examined histologically and microadiographically. Quantitative techniques were employed to determine the amount of bone ingrowth as a function of time and pore size. The pore structures of the materials were evaluated using optical microscopy and mercury intrusion porosimetry. The results of this investigation have demonstrated that porous polyethylene is capable of accepting bone growth into pores as small as 40 mum. The optimum rate of bone ingrowth was observed in pore sizes of approximately 100 to 135 mum, with no increase in the rate of bone ingrowth observed in samples possessing larger pore sizes. No adverse tissue response was found at implant times up to 16 weeks in pore sizes of 100 mum or larger.     

Characteristics of tissue growth into Proplast and porous polyethylene implants in bone.

(1) Bone does not form within internal pores of undistorted Proplast implants because of the small interconnecting pore size of the material; (2) the nonosseous, fibrous tissue which exists in the pores of Proplast implants in bone is not attached to the surrounding bone (i.e., Sharpey's fibers are not present). The load-bearing support which can be afforded by Proplast implants is limited by the incomplete bone ingrowth along the margins of the material and the tensile strength of Proplast.     

The rate of bone ingrowth into porous metal.

Experiments have been devised to study the rate of ingrowth of bone into porous metal with pore sizes up to 100 mu and to study the significance of a gap between the porous metal surface and bone. When the porous coat was in direct apposition with bone, the implant was firmly locked in place after a three week period and the plateau value of implant-tissue shear strength was reached at four weeks. A gap of 1.5 mm between the bone and the implant was bridged by new bone within four weeks.     

Preliminary observations of bone ingrowth into porous materials.

A preliminary investigation has been performed (a) to determine the kinetics of bone ingrowth into porous materials and to determine if this ingrowth could be catalyzed by the presence of a foreign substrate; and (b) to measure the bonding capability of bone with a porous-surfaced metallic implant. Tests on porous-surfaced implants corroborate the work of other investigators in showing that bony tissue will grow into a porous substance that has pores large enough to support tissue nourishment. The shear strength of the bone-implant interface appears to increase with pore size and time of healing. Furthermore, it may be possible to catalyze this tissue ingrowth by the introduction into the fracture site of a foreign substance; in this experiment, glass beads 200-290mu in diameter were used.     

Quantification of bone ingrowth into porous block hydroxyapatite in humans.

This study sought to quantify bone ingrowth from a single bone-implant surface into porous block hydroxyapatite used in maxillofacial applications. Seventeen maxillary hydroxyapatite implants (implant time of 4-138 months, 39-month mean) were harvested for analysis from 14 patients. The implants had been placed into the lateral maxillary wall during orthognathic surgery, juxtapositioned to the maxillary sinus. Ingrowth was measured in 100-microm increments from a bone-implant interface to a depth of 1500 microm. Bone ingrowth averaged over the 14 patients (0-1100 microm depth) is described by the equation % ingrowth - 20% * (depth in millimeters) + 41.25% (R2 = 0.98, n = 10 incremental depths). Beyond 1100 microm, the average ingrowth remained constant at 15.0 +/- 0.7%. The duration of implantation also showed as affect on the percent ingrowth into the implants at the incremental depths, and the percent ingrowth asymptotically approached a maximum. Overall, the composite average data from all depths is best described by the logarithmic function % ingrowth = 15% * ln(implantation time in months) - 24.0% (R2 = 0.71, n = 14 patients). Several factors may come into play in determining bone ingrowth including the mechanical environment, the osteoconductivity of the implant material, and the osteogenic capability of the tissues in the pore spaces. Measurements of bone ingrowth are most influenced by the depth into the implant and the time the implant was in the body; the age of the patient had little affect on bone ingrowth.     

Assessment of bone ingrowth into porous biomaterials using MICRO-CT

The three-dimensional (3D) structure and architecture of biomaterial scaffolds play a critical role in bone formation as they affect the functionality of the tissue-engineered constructs. Assessment techniques for scaffold design and their efficacy in bone ingrowth studies require an ability to accurately quantify the 3D structure of the scaffold and an ability to visualize the bone regenerative processes within the scaffold structure. In this paper, a 3D micro-CT imaging and analysis study of bone ingrowth into tissue-engineered scaffold materials is described. Seven specimens are studied in this paper; a set of three specimens with a cellular structure, varying pore size and implant material, and a set of four scaffolds with two different scaffold designs investigated at early (4 weeks) and late (12 weeks) explantation times. The difficulty in accurately phase separating the multiple phases within a scaffold undergoing bone regeneration is first highlighted. A sophisticated three-phase segmentation approach is implemented to develop high-quality phase separation with minimal artifacts. A number of structural characteristics and bone ingrowth characteristics of the scaffolds are quantitatively measured on the phase separated images. Porosity, pore size distributions, pore constriction sizes, and pore topology are measured on the original pore phase of the scaffold volumes. The distribution of bone ingrowth into the scaffold pore volume is also measured. For early explanted specimens we observe that bone ingrowth occurs primarily at the periphery of the scaffold with a constant decrease in bone mineralization into the scaffold volume. Pore size distributions defined by both the local pore geometry and by the largest accessible pore show distinctly different behavior. The accessible pore size is strongly correlated to bone ingrowth. In the specimens studied a strong enhancement of bone ingrowth is observed for pore diameters>100 microm. Little difference in bone ingrowth is measured with different scaffold design. This result illustrates the benefits of microtomography for analyzing the 3D structure of scaffolds and the resultant bone ingrowth.     

A novel bioactive porous CaSiO3 scaffold for bone tissue engineering

The aim of this study was to fabricate bioactive porous CaSiO3 scaffolds and examine their effects on proliferation and differentiation of osteoblast-like cells. In this study, porous CaSiO3 scaffolds were obtained by sintering a ceramic slip-coated polymer foam at 1350 degrees C. X-ray diffraction (XRD) of the scaffolds indicated that the products were essentially pure alpha-CaSiO3. The obtained scaffolds had a well-interconnected porous structure with pore sizes ranging from several micrometers to more than 100 microm and porosities of 88.5 +/- 2.8%. The in vitro bioactivity of the scaffolds was investigated by soaking them in simulated body fluid (SBF) for 7 days and then characterizing them by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) analysis. The results indicated that hydroxyapatite (HAp) was formed on the surface of the scaffolds. In addition, the scaffolds were incubated in Ringer's solution at 37 degrees C to study the in vitro degradation by measurement of weight loss after incubation, which showed that the CaSiO3 scaffolds were degradable. The cellular responses to the scaffolds were assessed in terms of cell proliferation and differentiation. Osteoblast-like cells were seeded into the CaSiO3 scaffolds. SEM observations showed that there was significant cell adhesion, as the cells spread and grew in the scaffolds. In addition, the proliferation rate and alkaline phosphatase (ALP) activity of the cells in the scaffolds were improved as compared to the controls. These studies demonstrate initial in vitro cell compatibility and their potential application to bone tissue engineering.     

Bone growth into porous carbon, polyethylene, and polypropylene prostheses.

Using rats as a model, porous discs of RPG carbon and polypropylene and polyethylene were localized subperiosteally and supraperiosteally in the skull. Bone and blood vessels grew into the discs, which had adequate pore size, when placed in direct contact with bone. No bone was generated from the periosteum. Both plastic materials were estimated to be better than carbon for use in osseous reconstructive work. More long term material-tissue stability and reaction studies should be performed.     

Electrical stimulation therapy and rotary jet-spinning scaffold to treat bone defects

The combination of electrical stimulation (ES) and bone tissue engineering (BTE) has been successful in treatments of bone regeneration. This study evaluated the effects of ES combined with PCL + β-TCP 5% scaffolds obtained by rotary jet spinning (RJS) in the regeneration of bone defects in the calvaria of Wistar rats. We used 120 animals with induced bone defects divided into 4 groups (n = 30): (C) without treatment; (S) with PCL+ β-TCP 5% scaffold; (ES) treated with ES (10 μA/5 min); (ES + S) with PCL + β-TCP 5% scaffold. The ES occurred twice a week during the entire experimental period. Cell viability (in vitro: Days 3 and 7) and histomorphometric, histochemical, and immunohistochemical (in vivo; Days 30, 60, and 90) analysis were performed. In vitro, ES + S increased cell viability after Day 7 of incubation. In vivo, it was observed modulation of inflammatory cells in ES therapy, which also promoted blood vessels proliferation, and increase of collagen. Moreover, ES therapy played a role in osteogenesis by decreasing ligand kappa B nuclear factor-TNFSF11 (RANKL), increasing alkaline phosphatase (ALP), and decreasing the tartarate-resistant acid phosphatase. The combination of ES with RJS scaffolds may be a promising strategy for bone defects regeneration, since the therapy controlled inflammation, favored blood vessels proliferation, and osteogenesis, which are important processes in bone remodeling.     

Effects of growth factors in vivo. I. Cell ingrowth into porous subcutaneous chambers.

Growth factors secreted by platelets and macrophages may play roles in atherogenesis and in wound repair. The multiple biologic effects of these factors are being studied extensively in vitro, but their roles in vivo are relatively unexplored. The cellular responses to platelet-derived growth factor (PDGF), transforming growth factor beta (TGF beta), basic fibroblast growth factor (bFGF), and epidermal growth factor (EGF) were examined in a wound chamber model in rats. Growth factors were emulsified in bovine dermal collagen suspensions, placed in 1 X 30-mm porous polytetrafluoroethylene tubes, inserted subcutaneously, and removed after 10 days. The presence of PDGF (400 ng), TGF beta (200 ng), or bFGF (100 ng) increased the DNA content of the chambers two- to sixfold, compared with controls. Regardless of dose, EGF (100-800 ng) did not affect the DNA content. The increases in DNA observed for PDGF, TGF beta, or bFGF resulted from accumulations of varying numbers of fibroblasts, capillaries, macrophages, and leukocytes in 10-day chambers. The addition of 250 micrograms/ml heparin to the collagen suspension potentiated the response to PDGF and bFGF, but not to TGF beta or EGF. The clearance of 125I-labeled growth factors from the chambers was biphasic. After an initial rapid phase, the remaining growth factor was slowly cleared. The half-life of the initial phase was rapid for PDGF (12 hours) and bFGF (9 hours) and somewhat slower for TGF beta (22 hours). There was no difference in the rate of clearance between collagen and collagen/heparin matrices for any of the growth factors examined. These studies demonstrate that PDGF, bFGF, and TGF beta can induce granulation tissue development in normal animals. The similarity in cellular responses to three peptides with differing in vitro actions suggests that the responses observed at 10 days reflect a secondary process, possibly mediated by effector cells such as macrophages, lymphocytes, or granulocytes that are attracted into the chamber by each growth factor, rather than a direct effect of the factors themselves.     

Effect of biological/physical stimulation on guided bone regeneration through asymmetrically porous membrane

Asymmetrically porous polycaprolactone (PCL)/Pluronic F127 guided bone regeneration (GBR) membranes were fabricated. The top surface of the membrane had nanosize pores (～10 nm) which can effectively prevent invasion by fibrous connective tissue but permeate nutrients, whereas the bottom surface had microsize pores (～200 μm) which can enhance the adhesiveness with bone tissue. Ultrasound was applied to a bone morphogenetic protein (BMP-2)-immobilized PCL/F127 GBR membrane to investigate the feasibility of using dual biological (BMP-2) and physical (ultrasound) stimulation for enhancing bone regeneration through the membrane. In an animal study using SD rats (cranial defect model), the bone regeneration behavior that occurred when using BMP-2-loaded GBR membranes with ultrasound treatment (GBR/BMP-2/US) was much faster than when the same GBR membrane was used without the ultrasound treatment (GBR/BMP-2), as well as when GBR membranes were used without stimulations (GBR). The enhanced bone regeneration of the GBR/BMP-2/US group can be interpreted as resulting from the synergistic or additive effect of the asymmetrically porous PCL/F127 membrane with unique properties (selective permeability, hydrophilicity, and osteoconductivity) and the stimulatory effects of BMP-2 and ultrasound (osteoinductivity). The asymmetrically porous GBR membrane with dual BMP-2 and ultrasound stimulation may be promising for the clinical treatment of delayed and insufficient bone healing.     

Craniofacial growth under the influence of the blood supply. 12. Regulation of bone remodeling

Growth is controlled by phylogenetic and ontogenetic factors. Regulation of bone growth can be regarded as taking place at 3 levels, a high, general level at which genetics and hormones represent the main regulators, a middle, regional level where the vascular influence predominates, and a low, local level dominated by mechanical factors. Stresses within the bone, which under physiological conditions are in a state of dynamic equilibrium, play a significant role in the shaping of bony structures. The apposition or resorption of bony material is accelerated if such stresses exceed or drop below certain limits. Biophysically, bone is comparable to technical materials consisting mainly of hydroxyl apatite and collagen. Deformation of the hydroxyl apatite is associated with piezoelectric effects which induce changes in bone structure.     

Enhanced bone ingrowth into hydroxyapatite with interconnected pores by Electrical Polarization

Hydroxyapatite (HA) ceramics are used as implants to repair damaged/removed bone, and negative or positive electrical polarization enhances osteoblast and decreases osteoclast activity, respectively, in vivo. We compared the ability of electrically polarized and non-polarized HA with interconnected pores (IPHA) implants to promote bone growth. Polarized or non-treated IPHAs were implanted into the right or left femoral condyle of rabbits (N = 10 in each group), and we performed histological examination, including enzymatic staining for osteoblasts and osteoclasts, 3 and 6 weeks after implantation. We observed improved bone ingrowth and increased osteoblast activity in polarized implants with complete bone penetration into polarized implants occurring as early as 3 weeks after surgery. In contrast, non-polarized implants were not fully ossified at 6 weeks after surgery. Furthermore, positively charged implant regions had decreased osteoclast activity compared to negatively charged or uncharged regions. We propose two different models to explain these observations.     

Early tissue infiltrate in porous polyethylene implants into bone: a scanning electron microscope study.

The results of the present study demonstrate the utility of the scanning electron microscope for characterizing the ultrastructure of the initial tissue infiltrate in porous polyethylene implants. Shortly after implantation a thin noncellular fibrous-like coating was observed to form on the pore surface. The cells observed in the polyethylene pellets 3 days after implantation were generally consistent with what one would expect to see in a hematoma. As early as 14 days after implantation much of the blood clot was replaced by newly formed bone spicules. Tissue shrinkage accompanying dehydration of the specimen for scanning electron microscope study although a disadvantage occasionally proved useful in that it provided the opportunity to study the internal surface of the fibrous coating when separated from the surface of the implant. Less shrinkage was observed in implants whose pores were filled with bone spicules.     

Electrolytic A12O3 coating on co-cr-mo implant alloys of hip prosthesis

The ceramic films over metallic implant surfaces have the potential to improve implant performance with respect to implant fixation, wear, or corrosion. In this study, the electrolytic Al2O3 coatings on F-1537 Co-Cr-Mo alloy were conducted in an aqueous solution of Al(NO3)3. Through the cycle polarization test in Hank's solution, it was found that the corrosion potential and protection potential of the alumina-coated were higher than that of the uncoated, and the corrosion current density was lower. The phase transformation of A12O3 film on Co-Cr-Mo alloy annealed at 800 K revealed todlite (5Al2O3 . H2O) and thetaAl2O3 (113) preferred orientation for 20 min, and thetaAl2O3 (200) preferred orientation with eta phase for 80 min. Scanning electron microscopy/energy dispersive spectroscopy observations after the scratch tests showed that the adhesion of the alumina films on Co-Cr-Mo alloy can load a stress over the yield strength (450 MPa) of Co-Cr-Mo alloy. The wear loss of ultra-high molecular-weight polyethylene to the Al2O3-coated specimen was eight times less than that to the uncoated. It is concluded that such Al2O3-coated films on Co-Cr-Mo implant alloy exhibit excellent quality in corrosion, adhesion, and wear for the application of hip prosthesis.     

Electrical stimulation of cardiac myocytes

The influence of nonuniform cell shape and field orientation on the field stimulation thresholds of cardiac myocytes was studied both experimentally and computationally. The percent change in excitation threshold, which was studied with patch clamp technique, was found to be 182±83.1% (mean ±SD) higher when the electric field (EF) was parallel to the transverse cell axisversus the longitudinal axis (p<0.0006). On reversing the polarity of the applied EF, the percentage change in threshold was observed to increase by 98.9±71.0% (p<0.0002), implying asymmetry of the stimulation threshold of isolated myocytes. Finite element models were made to investigate the distribution of the transmembrane potential of these experimentally studied myocytes. A typical cell model showed that the maximum transmembrane potential induced on opposite ends of the cell was 39.1 mV and −46.5 mV for longitudinal field (aligned with the long axis of the cell), but was 40.5 mV and −44.8 mV for transverse field (aligned with the short axis of the cell). More significantly, it was found that the maximum transmembrane potential occurred at discrete points or “hot spots” on the cell membrane. It is hypothesized that the depolarization of the cell initiates at the hot spot and then spreads over the entire cell. The different excitation thresholds for different polarities of the applied EF can be explained by the different maximum induced at the opposite ends of the cell.