Electrically active bioceramics: a review of interfacial responses

Electrical potentials in mechanically loaded bone have been implicated as signals in the bone remodeling cycle. Recently, interest has grown in exploiting this phenomenon to develop electrically active ceramics for implantation in hard tissue which may induce improved biological responses. Both polarized hydroxyapatite (HA), whose surface charge is not dependent on loading, and piezoelectric ceramics, which produce electrical potentials under stress, have been studied in order to determine the possible benefits of using electrically active bioceramics as implant materials. The polarization of HA has a positive influence on interfacial responses to the ceramic. In vivo studies of polarized HA have shown polarized samples to induce improvements in bone ingrowth. The majority of piezoelectric ceramics proposed for implant use contain barium titanate (BaTiO(3)). In vivo and in vitro investigations have indicated that such ceramics are biocompatible and, under appropriate mechanical loading, induce improved bone formation around implants. The mechanism by which electrical activity influences biological responses is yet to be clearly defined, but is likely to result from preferential adsorption of proteins and ions onto the polarized surface. Further investigation is warranted into the use of electrically active ceramics as the indications are that they have benefits over existing implant materials.

Mechanical properties of three different compositions of calcium phosphate bioceramic following immersion in Ringer's solution and distilled water

Dissolution tests were carried out to compare the mechanical properties of calcium phosphate based bioceramics with different compositions, before and after ageing for various time periods in Ringer's solution (pH 7.2) or distilled water (pH 7.2 and 4.0) at 37 degrees C. The results indicate that the sample composition seems to have more of an effect on the mechanical properties than does the storage environment. No obvious decrease in mechanical properties was found after samples had been aged in the various solutions during the different time periods. This indicates that these samples could be of significant clinical interest as their good structural properties were retained.

Mechanical characterization of dense calcium phosphate bioceramics with interconnected porosity

Porous hydroxyapatite/tricalcium phosphate (HA/TCP) bioceramics were fabricated by a novel technique of vacuum impregnation of reticulated polymeric foams with ceramic slip. The samples had approximately 5-10% interconnected porosity and controlled pore sizes appropriate to allow bone ingrowth, combined with good mechanical properties. A range of polyurethane foams with 20, 30 and 45 pores per inch (ppi) were used as templates to produce samples for testing. The foams were inpregnated with solid loadings in the range of 60-140 wt%. The results indicated that the average apparent density of the HA/TCP samples was 2.48 g/cm(3), the four-point bending strength averaged 16.98 MPa, the work of fracture averaged 15.46 J/m(2) and the average compressive strength was 105.56 MPa. A range of mechanical properties resulted from the various combinations of different grades of PU foam and the solid loading of slips. The results indicated that it is possible to manufacture open pore HA/TCP bioceramics, with compressive strengths comparable to human bone, which could be of significant clinical interest.

Fabrication of porous bioceramics with porosity gradients similar to the bimodal structure of cortical and cancellous bone

The aim of this study was to fabricate porous implant materials with graded pore structures similar to the bimodal structure of cortical and cancellous bone. Porous hydroxyapatite/tricalcium phosphate (HA/TCP) bioceramics with interconnected porosity and controlled pore sizes required to simulate natural bone tissue morphology were fabricated by a novel technique of vacuum impregnation of reticulated polymeric foams with ceramic slip. Functionally gradient materials (FGMs) with porosity gradients were made by joining different pore per inch (ppi) foams together by either stitching or pressfitting to form templates. Post production, no defects could be seen at the interface between the two different porosity sections. The macropore sizes of the HA/TCP bioceramics were larger than 100 mum which is appropriate for bone ingrowth. A sample with a graded porous structure which is close to the human bone morphology was also developed. The two component structures were conspicuously different but joined together firmly. Four point bend testing of FGM samples showed them to have similar mechanical properties to homogeneous ceramics based on foam templates with uniform pore sizes, with no evidence of interfacial weakness. Many potential biomedical applications could be developed utilising graded porous structures. The ease of processing will make it possible to fabricate a range of complex shapes for different applications.

Fabrication and mechanical testing of porous calcium phosphate bioceramic granules

Porous hydroxyapatite/tricalcium phosphate (HA/TCP) granules were fabricated by a novel technique of vacuum impregnation of reticulated polyurethane (PU) foams with ceramic slip. The resultant granules had 5-10% interconnected porosity with controlled pore sizes necessary to allow bone ingrowth combined with good mechanical properties. Using PU foams with a different number of pores per inch (ppi), porous HA/TCP granules in the size range of 2-8 mm were successfully manufactured. Dieplunger tests were used to compare the compression and relaxation properties of the granules with those of a commercially available bone graft product, BoneSave. The results of the die-plunger testing showed that the experimental granules were stiffer than the BoneSave materials and had less of a tendency to crumble to powder after testing. This therefore suggests that these experimental granules would be useful for impaction grafting and space filling applications.

Characterisation of the internal and external surfaces of four types of Foley catheter using SEM and profilometry

Unfortunately the use of Foley catheters for long-term catheterisation is frequently associated with complications such as infection and encrustation. This study investigated whether a link could exist between the surface properties of the catheters and the problems that can develop. The internal and external surfaces of four different types of urinary catheter were examined. Three latex devices coated with either PTFE or hydrogel or surface treated with silicone were investigated. In addition, an all-silicone device was examined. The surfaces of the all-silicone catheters were relatively smooth and featureless. In contrast, the external surfaces of each of the latex devices were 'paved' in nature. The internal surfaces of latex based devices produced by different manufacturers showed distinct differences with evidence of inorganic inclusions on the internal surfaces of two of the catheter types. These findings may be significant in the context of catheter infection and encrustation.

Kink, flow and retention properties of urinary catheters part 1: conventional foley catheters

The treatment for urinary incontinence, a common condition affecting a considerable number of older and disabled members of society, involves the use of a Foley catheter for drainage of the bladder. The basic design of the catheter has remained the same for over seventy years. Despite modifications to the materials used there has been very little research directly comparing the physical properties of the different types of catheter. This study developed a range of tests to enable comparison of the resistance to kinking, flow rate properties and the retention forces of both latex-based and all-silicone catheters. The results indicated that the all-silicone device had superior resistance to kinking and better flow properties when compared to the latex-based catheters. However, greater retention forces were recorded for the all-silicone device, in both the inflated and deflated condition, indicating that much more force would be required to remove the this type of catheter.

Kink, flow and retention properties of urinary catheters part 2: novel design

Research has shown that all the current designs of urinary catheters based on natural latex or silicone rubber are susceptible to the major problems associated with their use--infection, encrustation and blockage. Research, to date, has focussed on the biological aspects of these complications; little research has been directed towards the contribution of the design or materials used in the manufacture of the catheters. The aim of the current study was to evaluate a totally new concept in catheter design based on nylon braid encased in a polyurethane matrix. Novel prototypes were tested using a range of established protocols to assess their flow properties, resistance to kinking and retention properties. The results were compared to those for the conventional latex and silicone based catheters currently in clinical use. The indications were that the new designs had superior flow properties and equivalent retention properties to the all-silicone catheters. Following further modification they also had superior resistance to kinking.

Materials for urinary catheters: a review of their history and development in the UK

The Foley catheter, introduced in the mid-1930s and originally manufactured from latex, is still the most commonly used device for the management of urinary incontinence (UI). Despite the passage of time, there are still problems associated with the use of these devices. It is currently estimated that the management and treatment of UI costs the UK National Health Service (NHS) in the order of 500 million pound per annum. Faced with the known demographic changes in the adult population these costs will continue to rise for the foreseeable future. This review examines the range of materials currently used to manufacture Foley catheters from both latex and silicone. It outlines the common problems associated with their clinical use-infection, encrustation and blockage. The main changes that have been made to the materials employed in response to these problems are analysed. In the first instance the use of controlled release glass and slow release polymers to introduce disinfectants and antibacterial agents is considered. Attempts to alter surface properties by using coatings based on silver, polytetrafluoroethylene (PTFE), hydrogels and silicone are then described. It can be seen that despite these approaches, problems remain with the design and materials currently used to manufacture catheters. The review concludes that changes to the materials currently used for the manufacture of commercially available catheters could potentially alleviate many of the existing problems. However, standards need to be developed in order to enable direct comparison of the mechanical and physical properties of existing and potential catheter designs to ensure their effective function in-service.

Optimizing a hydroxyapatite/tricalcium-phosphate ceramic as a bone graft extender for impaction grafting

The mechanical properties of morsellized bone allografts and synthetic hydroxapatite/tricalcium-phosphate (HA/TCP) ceramic extender materials for the use in impaction grafting revision hip surgery were investigated using two test methods: a basic compression test and an endurance test in an in-vitro model of an impaction grafted femur. Formalin fixed ovine bone graft was identified as mechanically similar to fresh human bone and thus suitable as an experimental material for in-vitro testing. For 1 : 1 volumetric mixes of bone allograft and synthetic extender, the granular ceramic's properties were varied in porosity, chemical composition, sintering temperature and particle size. Initial mechanical stability, a crucial prerequisite for clinical success in impaction grafting, was increased for all bone/extender mixes. A high porosity, tricalcium-phosphate rich ceramic of medium particle size and sintered at high temperatures was recognized as an optimized extender material for impaction grafting balancing the mechanical and biological demands. Using the extender without bone graft as a pure replacement is not recommended.

In vitro fatigue behaviour of vacuum plasma and detonation gun sprayed hydroxyapatite coatings

The fatigue behaviour of vacuum plasma sprayed (VPS) and detonation gun sprayed (DGUN) hydroxyapatite coatings on titanium substrates has been compared in air and in buffered Ringer's solution. There was an increase in the surface microcracking and bulk porosity of both types of coating tested in air. After 1 million cycles in Ringer's solution the VPS coatings had completely delaminated from their substrates. In contrast the DGUN coatings retained their integrity when tested up to 10 million cycles but were beginning to show signs of delamination at the interface.

In vitro dissolution behaviour of two morphologically different thermally sprayed hydroxyapatite coatings

Two types of hydroxyapatite coating on titanium substrates were produced by vacuum plasma spraying and detonation gun spraying. The response of the coatings to immersion in Ringer's solution at pH 7.2 and held at 37 degrees C for periods of one, two, four and eight weeks was compared. X-ray diffraction was used to measure changes in crystallinity and residual stresses. Porosity and coating thickness were measured using optical microscopy combined with image analysis. Surface morphology was characterised using scanning electron microscopy and Talysurf measurements. The DGUN coatings were found initially to have a lower crystallinity and porosity but higher surface roughness and residual stress level than the VPS coatings. This resulted in a higher rate of dissolution compared to the more stable VPS coatings.

Direct morphological comparison of vacuum plasma sprayed and detonation gun sprayed hydroxyapatite coatings for orthopaedic applications

Hydroxyapatite coatings on titanium substrates were produced using two thermal spray techniques vacuum plasma spraying and detonation gun spraying. X-ray diffraction was used to compare crystallinity and residual stresses in the coatings. Porosity was measured using optical microscopy in conjunction with an image analysis system. Scanning electron microscopy and surface roughness measurements were used to characterise the surface morphologies of the coatings. The vacuum plasma sprayed coatings were found to have a lower residual stress, a higher crystallinity and a higher level of porosity than the detonation gun coatings. It is concluded that consideration needs to be given to the significance of such variations within the clinical context.

An in vitro study of the effect of environment and storage time on the fracture properties of bone cement

Changes either within the bone cement or at the cement-bone interface are known to contribute to loosening and hence failure of many cemented joint replacements. This study examines the in vitro changes in the fracture properties of bone cement as a result of storage, at both 21 and 37 degrees C, in air, water, Ringer's solution and lipid over a period of 2 years. Specimens stored in the fluid media were found to behave in a more ductile manner than those stored in air. Samples stored at 37 degrees C behaved in a more brittle manner than those stored at 21 degrees C. Although the work of fracture values measured for the samples stored in the water-based media increased during the first 18 months, this was followed by a decrease in the subsequent 6 months.

Comparison of bone screw holding strength in healthy bovine and osteoporotic human cancellous bone

Pushout tests were carried out on two different diameter, self-tapping, cancellous bone screws (4 mm and 6.4 mm) inserted in bovine cancellous bone sections taken perpendicular and at 45 degrees to the long axis of the bone. Comparison was made with the same type of screws inserted into sections taken from osteoporotic and/or osteoarthritic human femoral heads. The results showed no significant difference in the holding strength of different diameter screws in bovine bone sections of given orientation but significant differences when the trabecular orientation of the section was changed. By comparison, the holding strengths of the same screws in diseased femoral heads were found to be both significantly different from each other and significantly lower than for the same screws inserted in healthy bovine bone. The screw diameter, trabecular orientation, mineral content and the health of the bone all need to be considered in order to optimise the holding strength of the screws in cancellous bone.

Comparative analysis of four types of synthetic anterior cruciate ligament replacement in the goat: in vivo histological and mechanical findings

The in vivo biological and mechanical performance of four artificial anterior cruciate ligament replacements, available for clinical use, was examined in the goat over a period of 2 yr. The prostheses used in the study were manufactured from polytetrafluoroethylene, Dacron or a composite of Dacron and carbon fibres. Histologically, no device was found to perform adequately both intraosseously and intra-articularly. The breaking load for each device immediately after insertion and after 12 month was found to be lower than that of the normal anterior cruciate ligament. All devices were found to have partially or completely ruptured intra-articularly after 2 yr.

Prosthetic anterior cruciate ligaments in the rabbit. A comparison of four types of replacement

Four types of prosthetic replacement for the anterior cruciate ligament (carbon fibre, carbon fibre and Dacron composite, Dacron alone and bovine xenograft) were assessed at three, six and 12 months after implantation in the knees of New Zealand white rabbits. The synovium and both intra-articular and intra-osseous portions of the ligaments were examined macroscopically, by light microscopy and by scanning electron microscopy. All the knees showed mild synovitis, and there was no significant growth into the intra-articular part of any ligament. Carbon fibre and xenograft did not appear to be suitable materials in this animal model. The composite ligament showed short-term ingrowth of fibrous tissue only into the periphery of the sheath in its intra-osseous portion, whereas the Dacron ligament showed progressive fibrous tissue ingrowth with some bony incorporation of its outer fibres.

The spatial arrangement of bone mineral as revealed by ion bombardment

Ion etching and thinning using ionized argon beams have been used to prepare surfaces and thin sections of human and animal compact cortical bone for examination in both scanning and transmission modes of the electron microscope. Using these techniques the mineral component in bone is revealed as a continuous, vermiform microskeleton composed of spheroidal segments approximately 0.1 micrometer in diameter. Electron diffraction and micro-analysis have confirmed that the mineral segments contain calcium and phosphate ions arranged in a poorly crystallised form of hydroxyapatite.

Two phases of the bone mineral as revealed by the high resolution scanning electron microscope on ion-etched bone surfaces and as seen on surfaces untreated and chemically etched

Recently reported uses of the technique of high resolution scanning microscopy of polished and Argon ion-etched bone surfaces have revealed that, at the ultrastructural level, the bone mineral is spatially arranged in a network of twisted, closely packed segments containing globular and cylindrical components. The ion-etching technique, which preferentially removes organic and less dense material from the bone surface, has been subsequently used by the present authors for detailed screening of a quantity of human cortical and trabecular bone of different age and maturity during which it has been found that apart from this "structured bone' containing twisted segments, the mineral is also organised in another regular form, the "lining bone', which has the appearance of solid, smooth and dense slabs or sheets lining active bone surfaces and bone cell lacunae and canaliculi. In the present study, in order to exclude the possibility that these two newly described phases of the bone mineral microskeleton are results of an etching artifact, their SEM appearance in 14 Argon ion-etched human bone specimens from individuals aged 11 post-natal days to 79 years was compared in various compartments to that seen on surfaces which were untreated, just polished or etched by hot NaOCl. The results have shown that both the structured and lining bone are genuine features since although the best results for their clear and reliable distinction were achieved by the ion-etching technique, these images could also be recognised on bone surfaces which were untreated or treated in a different way.