Introduction

Bone fracture toughness is an important parameter in resistance of bone to monotonic and fatigue failure. Earlier studies on bone fracture toughness were focused on either cortical or cancellous bone, separately [1, 2]. Reported fracture toughness values indicated that cortical bone is tougher to break as compared to cancellous bone. In order to understand complete fracture of a whole bone, the interface between cortical and cancellous bone (named as corticellous bone) might play a crucial role and is interesting topic of research. The goal of this study was to identify fracture toughness in terms of J integral and fracture mechanism of the corticellous bone.

Introduction

Radiation cross-linked UHMWPE is preferred in total hip replacements due to its wear resistance [1]. In total knees, where stresses are higher, there is concern of fatigue damage [2]. Antioxidant stabilization of radiation cross-linked UHMWPE by blending vitamin E into the polymer powder was recently introduced [3]. Vitamin E greatly hinders radiation cross-linking in UHMWPE [4]. In contrast peroxide cross-linking of UHMWPE is less sensitive to vitamin E concentration [5]. In addition, exposing UHMWPE to around 300°C, increases its toughness by inducing controlled chain scission and enhanced intergranular diffusion of chains, simultaneously [6]. We present a chemically cross-linked UHMWPE with high vitamin E content and improved toughness by high temperature melting.

Introduction. The in vivo evolution of surface material properties is important in determining the longevity of bioceramics. Fracture toughness is particularly relevant because of its role in wear resistance. Some bioceramics, such as zirconia (ZrO2) undergo in vivo phase transformation, resulting in a marked reduction in toughness and commensurate increased wear. Here, we investigated the effect of accelerated aging on the surface toughness of alumina (Al2O3), zirconia-toughened alumina (ZTA), and silicon nitride (Si3N4) femoral heads, in order to identify the optimal ceramic material for in vivo implantation and long-term durability. Materials. A newly developed Raman microprobe-assisted indentation method was applied to evaluate and compare surface fracture toughness mechanisms operative in Si3N4 (Amedica Corporation, Salt Lake City, UT, USA), Al2O3 and ZTA (BIOLOX® forte, and delta, respectively, CeramTec, GmbH, Plochingen, Germany) bioceramics. The Al2O3 and ZTA materials have long established histories in total hip arthroplasty; whereas Si3N4 has been newly developed for this purpose. The improved method proposed here consisted in coupling the “traditional” indentation technique with quantitative assessments of microscopic stress fields by confocal Raman microprobe piezo-spectroscopy. Concurrently, crack opening displacement (COD) profiles were also monitored by Raman spectroscopy. Toughness measurements were determined using both as-received and hydrothermally exposed (100–121°C for up to 300 hours) femoral heads. Results. The Raman microprobe visualized two main toughening mechanisms operative in Si3N4 and ZTA bioceramics, namely crack-face bridging by acicular Si3N4 grains and polymorphic transformation of ZrO2 dispersoids. Both mechanisms elevated the resistance to crack propagation above the brittle behavior of Al2O3, which experienced a low crack resistance of ∼1.5 MPaâ��m1/2 independent of crack length. The as-received Si3N4 showed a sharply rising R-curve, up to ∼7 MPaâ��m1/2 within a propagation distance of ∼110 µm. A rising R-curve was also observed in the as-received ZTA, although its increase was less pronounced, ∼4 MPaâ��m1/2 within ∼120 µm. After hydrothermal exposure, surface toughness values decreased by ∼5%, ∼10%, and ∼42% for Si3N4, Al2O3, and ZTA, respectively. Substantial embrittlement was particularly noted at the surface of the ZTA material, with its toughness value reduced to the level of Al2O3. At the micromechanical scale, such embrittlement is obviously related to decreased availability of transformable (metastable) tetragonal ZrO2dispersoids at the surface. In ZTA, the hydrothermal attack annihilated any rising R-curve effect; whereas this degradation mechanism was not present in either Al2O3 or Si3N4 (Fig. 1). Discussion. Empowered by the Raman microprobe, the indentation micro-fracture method was shown capable of providing reliable surface toughness measurements in dissimilar biomaterials. Different from bulk toughness, surface toughness is the most relevant parameter in designing bioceramic microstructures for use as hip arthroplasty bearing couples for improved tribological performance. It was demonstrated that the environmentally driven phase transformation in ZTA is detrimental to surface toughness. On the other hand, Si3N4 experienced surface toughness values conspicuously independent of hydrothermal attack. Conclusions. Unlike transformation toughening (which is operative in ZTA), crack-face bridging (which is the toughening mechanism in Si3N4) proved to be the most durable surface toughening effect for a biomaterial to be employed in joint arthroplasty

Objectives

There remains conflicting evidence regarding cortical bone strength following bisphosphonate therapy. As part of a study to assess the effects of bisphosphonate treatment on the healing of rat tibial fractures, the mechanical properties and radiological density of the uninjured contralateral tibia was assessed.

The long-term stability of total hip replacements (THRs) critically depends on the lasting integrity of the bond between the implant and the bone. Late failure in the absence of infection is known as ‘aseptic loosening’, a process characterised by the formation and progressive thickening of a continuous layer of fibrous tissue at the interface between the prosthesis and the bone. Aseptic loosening has been identified as the most common cause for long-term instability leading to the failure of ace-tabular cups. There is clearly a need to study the failure mechanisms in the acetabular fixation if the long-term stability of THR is to be significantly improved. The bonding strength in the presence of defects is measured using interfacial fracture toughness, and this information is not available currently.

In this work, interfacial fracture toughness of synthetic and bovine bone-cement interface has been studied using sandwiched Brazilian disk specimens. Experiments were carried out using a common bone cement, CMW, and polyurethane foam under selected loading angles from 0 to 25 degrees to achieve full loading conditions from tensile (mode I) to shear (mode II). Finite element analyses were carried out to obtain the solutions for strain energy release rate at a given phase angle (ratio of shear and tensile stress) associated with the experimental models. The effects of crack length on the measured interfacial fracture toughness were examined. Microscopic studies were also carried out to obtain the morphology of the fractured interfaces at selected loading angles.

The results show that both polyurethane foam and bovine cancellous bone seem to produce a similar type of interfacial failure of bone-cement interface, with cement pedicles being ‘pull-out’ of the pores of the foam/ bone. Damage sustained by the cement pedicles seems to increase progressively as the increase of shear loading component. The measured values of fracture toughness are a function of crack length and phase angle, and are comparable with those published in the literature on cortical bone and cement interface.

The implication of these results on the assessment of fixation in acetabular replacements is discussed, particularly in the light of results from bovine cancellous bone-cement interface.