K.E. Tanner

Effect of filler content on mechanical and dynamic mechanical properties of particulate biphasic calcium phosphate--polylactide composites.

A bioabsorbable self-reinforced polylactide/biphasic calcium phosphate (BCP) composite is being developed for fracture fixation plates. One manufacturing route is to produce preimpregnated sheets by pulling polylactide (PLA) fibres through a suspension of BCP filler in a PLA solution and compression moulding the prepreg to the desired shape. To aid understanding of the process, interactions between the matrix and filler were investigated. Composite films containing 0-0.25 volume fraction filler, produced by solvent casting, were analysed using SEM, tensile testing and dynamic mechanical analysis (DMA). Homogeneous films could be made, although some particle agglomeration was seen at higher filler volume fractions. As the filler content increased, the failure strain decreased due to a reduction in the amount of ductile polymer present and the ultimate tensile strength (UTS) decreased because of agglomeration and void formation at higher filler content. The matrix glass transition temperature increased due to polymer chain adsorption and immobilization onto the BCP particles. Complex damping mechanisms, such as particle-particle agglomeration, may exist at the higher BCP volume fractions.

Stress and strain distribution within the intact femur: compression or bending?

The aim of this research was to test the hypothesis that the intact femur is loaded predominately in compression. The study was composed of two parts: a finite element analysis of the intact femur to assess if a compressive stress distribution could be achieved in the diaphyseal region of the femur using physiological muscle and joint contact forces; a simple radiological study to assess the in vivo deflections of the femur during one legged stance. The results of this investigation strongly support the hypothesis that the femur is loaded primarily in compression, and not bending as previously thought. The finite element analysis demonstrated that a compressive stress distribution in the diaphyseal femur can be achieved, producing a stress distribution which appears to be consistent with the femoral cross-sectional geometry. The finite element analysis also predicted that for a compressive load case there would be negligible deflections of the femoral head. The radiological study confirmed this, with no measurable in vivo deflection of the femur occurring during one legged stance.

Contribution, development and morphology of microcracking in cortical bone during crack propagation.

A fracture mechanics study of cortical bone is presented to investigate the contribution, development morphology of microcracking in cortical bone during crack propagation. Post-hoc analyses of microcrack orientation, crack propagation velocity and fracture surface roughness were conducted on previously tested human and bovine bone compact tension specimens. It was found that, consistent with its higher toughness, bovine bone formed significantly more longitudinal, transverse and inclined microcracks than human bone. However, in human bone more of the microcracks that formed were longitudinal than transverse or inclined, a feature that would optimise bones toughness. Crack propagation velocity in human and bovine bone displayed the same characteristic pattern with crack extension, where an increase in velocity is followed by a consequent decrease and vice versa. On the basis of this pattern, a model or crack propagation has been proposed. It provides a detailed account of mocrocrack formation and contribution towards the propagation of a fracture crack. Analyses of fracture surfaces indicated that, consistent with its higher toughness, bovine bone displays a rougher surface than human bone but they both have the same basic fractured element, i.e. a mineralised collagen fibril.

Experimental validation of a microcracking-based toughening mechanism for cortical bone

It has been proposed that cortical bone derives its toughness by forming microcracks during the process of crack propagation (J. Biomech. 30 (1997) 763; J. Biomech. 33 (2000) 1169). The purpose of this study was to experimentally validate the previously proposed microcrack-based toughening mechanism in cortical bone. Crack initiation and propagation tests were conducted on cortical bone compact tension specimens obtained from the antlers of red deer. For these tests, the main fracture crack was either propagated to a predetermined crack length or was stopped immediately after initiating from the notch. The microcracks produced in both groups of specimens were counted in the same surface area of interest around and below the notch, and crack growth resistance and crack propagation velocity were analyzed. There were more microcracks in the surface area of interest in the propagation than in initiation specimens showing that the formation of microcracks continued after the initiation of a fracture crack. Crack growth resistance increased with crack extension, and crack propagation velocity vs. crack extension curves demonstrated the characteristic jump increase and decrease pattern associated with the formation of microcracks. The scanning electron micrographs of crack initiation and propagation displayed the formation of a frontal process zone and a wake, respectively. These results support the microcrack-based toughening mechanism in cortical bone. Bone toughness is, therefore, determined by its ability to form microcracks during fracture.

Quantification of bone ingrowth within bone-derived porous hydroxyapatite implants of varying density

Hydroxyapatite has been investigated for use in the osseous environment for over 20 years and the biocompatibility of the ceramic and its osseoconductive behavior is well established. Therefore, the use of porous hydroxyapatite for the repair of osseous defects seems promising with potential for complete penetration of osseous tissue and restoration of vascularity throughout the repair site. However, there have been few systematic studies of the effects of physical properties such as macropore size and pore connectivity on the rate and quality of bone integration within porous hydroxyapatite implants. This paper quantifies the early biological response to a well-characterized series of implants with uniform microstructure and phase composition, but differing macrostructures and demonstrates the dependence of the rate of osseointegration on the apparent density of porous hydroxyapatite as a function of pore connectivity. Furthermore, compression testing established that bony ingrowth has a strong reinforcing effect on porous hydroxyapatite implants, which is more pronounced in the lower density implants, as a result of a greater relative volume of bone ingrowth.

In vitro mechanical and biological assessment of hydroxyapatite-reinforced polyethylene composite

In vitro performance of hydroxyapatite (HA)-reinforced polyethylene (PE) composite (HAPEX) has been characterized from both mechanical and biological aspects. The mechanical properties of HAPEX, such as tensile strength and Youngs modulus, showed little change after immersion in a physiological solution at 37 and 70 degrees C for various periods. In addition, the biological response of primary human osteoblast-like (HOB) cells in vitro on HAPEX was assessed by measuring DNA synthesis and osteoblast phenotype expression. Cell proliferation rate on HAPEX was demonstrated by an increase in DNA content with time. A high tritiated thymidine ([3H]-TdR) incorporation/DNA rate was observed on day 1 for HAPEX, indicating a stimulatory effect on cell proliferation. The alkaline phosphatase (ALP) activity was expressed earlier on HAPEX than on unfilled PE and increased with time, indicating that HOB cells had commenced differentiation. Furthermore, it was found that the HA particles in the composite provided favourable sites for cell attachment. It appears that the presence of HA particles in HAPEX may have the advantage of acting as microanchors for bone bonding in vivo.

Interfaces in analogue biomaterials

Abstract Bone is a nanoscale composite with mechanical properties which cannot be duplicated by monolithic materials. Composite systems for bone replacement have been developed using various bioactive ceramic or glass reinforcements in a bioinert polymer matrix. The mechanical behaviour of these composites is dependent upon the interface between the composite components, which in turn affect the interface with the biological system.

Finite element analysis of the implanted proximal tibia: A relationship between the initial cancellous bone stresses and implant migration

The cancellous bone stresses within the implanted proximal tibia were examined using a three-dimensional anatomical finite element model. Three versions of a proximal tibial prosthesis were examined: an all polyethylene press-fit design; a metal backed, stemmed press-fit design and a (horizontally) cemented metal backed, stemmed design. All three designs had published migration and survivorship data. The objectives of the study were (i) to compare the stresses generated by each of the tibial components, (ii) examine the influence of the resected surface morphology and (iii) compare the initial cancellous bone stresses with the published migration and survivorship data. The all polyethylene prosthesis generated the highest cancellous bone stresses. Addition of a metal backing and a stem reduced the stresses, but the cemented device produced the lowest cancellous bone stresses. The surface morphology had a significant effect on the cancellous bone stresses generated by press-fit prostheses. As the bone-prosthesis contact area decreased, the peak cancellous bone stresses increased by as much as 243%. The surface morphology had no effect on the cancellous bone stresses generated by the cemented implant. Good correlation was found between the predicted cancellous bone stresses and the migration and survivorship data, with the implant generating the highest cancellous bone stresses migrating the most and having the poorest survival rates at 5 year. The results support the hypothesis that the progressive failure of cancellous bone is a mechanism of implant migration regardless of the method of fixation and the implantation site.

Hydroxyapatite-polyethylene composites: Effect of grafting and surface treatment of hydroxyapatite

The mechanical properties of hydroxyapatite-polyethylene composites have been improved by increasing the interfacial bond between the hydroxyapatite reinforcing particles and the polyethylene. Two types of surface treatment have been investigated: both use silane coupling of the hydroxyapatite, either with or without grafting of the polyethylene. These treatments have lead to the presence of a silicon-containing interphase promoting chemical adhesion and penetration of the polymer into cavities in the ceramic particles. The resultant bond has improved the mechanical properties and fracture behaviour of the composites.

Selective laser sintering of hydroxyapatite reinforced polyethylene composites for bioactive implants and tissue scaffold development

Abstract Selective laser sintering (SLS) has been investigated for the production of bioactive implants and tissue scaffolds using composites of high-density polyethylene (HDPE) reinforced with hydroxyapatite (HA) with the aim of achieving the rapid manufacturing of customized implants. Single-layer and multilayer block specimens made of HA-HDPE composites with 30 and 40 vol % HA were sintered successfully using a CO2 laser sintering system. Laser power and scanning speed had a significant effect on the sintering behaviour. The degree of particle fusion and porosity were influenced by the laser processing parameters, hence control can be attained by varying these parameters. Moreover, the SLS processing allowed exposure of HA particles on the surface of the composites and thereby should provide bioactive products. Pores existed in the SLS-fabricated composite parts and at certain processing parameters a significant fraction of the pores were within the optimal sizes for tissue regeneration. The results indicate that the SLS technique has the potential not only to fabricate HA-HDPE composite products but also to produce appropriate features for their application as bioactive implants and tissue scaffolds.