Simon Barrans

The design of aerostatic bearings for application to nanometre resolution manufacturing machine systems

This paper addresses the design of aerostatic bearings for use within machines requiring nanometre precise translation or rotation. The major design parameters which affect aerostatic bearing performance are discussed. The supply pressure ratio and hence, the bearing clearance and the type and specific size of inlet flow control device are shown to require special consideration. Specific information on the tolerance of each of the parameters which have a direct effect on the nano-precision location of these bearings is given. In addition the paper addresses the manufacturing tolerances and manufacturing variations which can occur in practice and places limits on the magnitude that these sources of errors can contribute to the permissible manufacturing variations.

Femoral stem wear in cemented total hip replacement

The great success of cemented total hip replacement to treat patients with end-stage osteoarthritis and osteonecrosis has been well documented. However, its long-term survivorship has been compromised by progressive development of aseptic loosening, and few hip prostheses could survive beyond 25 years. Aseptic loosening is mainly attributed to bone resorption which is activated by an in-vivo macrophage response to particulate debris generated by wear of the hip prosthesis. Theoretically, wear can occur not only at the articulating head—cup interface but also at other load-bearing surfaces, such as the stem—cement interface. Recently, great progress has been made in reducing wear at the head—cup interface through the introduction of new materials and improved manufacture; consequently femoral stem wear is considered to be playing an increasingly significant role in the overall wear of cemented total hip replacement. In this review article, the clinical incidences of femoral stem wear are comprehensively introduced, and its significance is highlighted as a source of generation of wear debris and corrosion products. Additionally, the relationship between femoral stem surface finish and femoral stem wear is discussed and the primary attempts to reproduce femoral stem wear through in-vitro wear testing are summarized. Furthermore, the initiation and propagation processes of femoral stem wear are also proposed and a better understanding of the issue is considered to be essential to reduce femoral stem wear and to improve the functionality of cemented total hip replacement.

Influence of femoral stem surface finish on the apparent static shear strength at the stem–cement interface

The stem-cement interface has long been implicated in failure of cemented total hip replacement. Much research has been performed to study the factors affecting the bond strength between the femoral stem and the bone cement. The present study aims to further investigate the influence of femoral stem surface finish on the apparent static shear strength at the stem-cement interface through a series of pull out tests, where stainless steel rods are employed to represent the femoral stem. The results demonstrated that there was a general tendency for the apparent static shear strength to be increased with the rise of surface roughness. The polished and glass bead-blasted rods illustrated a slip-stick-slip failure whereas the shot-blasted and grit-blasted rods displayed gross interface failure. Following pull out test, cement transfer films were detected on the polished rods, and there was cement debris adhered to the surface of the grit-blasted rods. Micropores, typically 120 mum in diameter, were prevalent in the cement surface interfaced with the polished rods, and the cement surfaces in contact with the shot-blasted and grit-blasted rods were greatly damaged. There was also evidence of metal debris embedding within the cement mantle originating from the tests of the grit-blasted rods, indicating an extremely strong mechanical interlocking at the interface. In summary, this present research demonstrated that the grit-blasted rods with the highest surface roughness were the best in terms of apparent static shear strength. However, it seemed to be most applicable only to the stem designs in which mechanical interlocking of the stem in the initial fixed position was essential.

The influence of bone cement type on production of fretting wear on the femoral stem surface: A preliminary study

BACKGROUND It has been reported that bone cement correlates with survivorship of cemented total hip replacement. However, little research has been published to investigate the influence of bone cement type on production of fretting wear on the femoral stem. METHODS In the present study, we performed six in vitro wear simulations using the same type of femoral stem (polished Exeter V40™) and three different bone cements (Simplex P, Palacos R, and CMW 3). FINDINGS Fretting wear was consistently reproduced on the stem surface and the wear locations compared well with the results of retrieval studies. Selected 3D surface parameters were utilised to quantitatively evaluate fretting wear and no significant difference was identified in terms of fretting wear severity between these simulations. The bone cements were all badly damaged in those sites contacting the fretting wear areas on the femoral stem. Additionally, there were plenty of wear debris present on the cement surface, and the energy dispersive X-ray analysis confirmed that it was just cement particles for Simplex P bone cement, whilst it included metallic particles for Palacos R and CMW 3 bone cements. INTERPRETATION This preliminary study shed some light on the influence of bone cement type on production of fretting wear on the femoral stem surface but further research is needed to gain a better understanding on this issue.

Investigation of relative micromotion at the stem-cement interface in total hip replacement

Abstract Cemented total hip replacement has become a standard surgical technique to treat patients with osteoarthritis and osteonecrosis. The stem—cement interface experiences fretting wear in vivo due to low-amplitude oscillatory micromotion under physiological loading, and this wear is currently becoming important as a potential mechanism for the overall wear of cemented total hip replacements. However, the relative micromotion at the stem—cement interface has not been widely reported. In the present study, a new micromotion sensor is developed that is based on the deformation of a strain gauge, and this sensor is used to probe the migration of a polished Exeter stem within a Simplex P cement mantle through an in vitro wear simulation. It is demonstrated that the stem migration value generally increases with an increase in the number of loading cycles, with a gradual decrease of migration rate. Additionally, fretting wear is successfully replicated on the stem surface, and the micropores in the cement surface are considered to contribute to initiation and propagation of the fretting damage on the stem. This is confirmed by the observation that no evidence of fretting wear is detected on the stem where the surface is in contact with the pore-free areas on the cement. This study allows a deep insight into the micromotion at the stem—cement interface, and provides evidence highlighting the significance of the micropores in the cement surface in the generation of fretting wear on a polished femoral stem.

The significance of the micropores at the stem-cement interface in total hip replacement.

Cemented total hip replacement has been performed worldwide to treat patients with osteoarthritis and osteonecrosis, with aseptic loosening as its primary reason for revision. It has been indicated that the stem–cement interfacial porosity may contribute to the early loosening of cemented hip prosthesis. In addition, it is generally accepted that the micropores in bone cement surface and in the bulk material are detrimental to the mechanical integrity of bone cement and act as stress concentrators, resulting in generation of fatigue cracks in the cement mantle. Furthermore, it was demonstrated that the micropores also play an important part in initiation and propagation of fretting wear on polished femoral stems. Taking this into consideration, a detailed review of the potential significance of the micropores in bone cement and the methods that could be employed to reduce porosity is given in this article. It was considered that modern cementing techniques are clinically beneficial and should be applied in surgery to further improve the survivorship of cemented total hip replacement.

Reproduction of fretting wear at the stem—cement interface in total hip replacement

Abstract The stem-cement interface experiences fretting wear in vivo due to low-amplitude oscillatory micromotion under physiological loading, as a consequence it is considered to play an important part in the overall wear of cemented total hip replacement. Despite its potential significance, in-vitro simulation to reproduce fretting wear has seldom been attempted and even then with only limited success. In the present study, fretting wear was successfully reproduced at the stem-cement interface through an in-vitro wear simulation, which was performed in part with reference to ISO 7206-4: 2002. The wear locations compared well with the results of retrieval studies. There was no evidence of bone cement transfer films on the stem surface and no fatigue cracks in the cement mantle. The cement surface was severely damaged in those areas in contact with the fretting zones on the stem surface, with retention of cement debris in the micropores. Furthermore, it was suggested that these micropores contributed to initiation and propagation of fretting wear. This study gave scope for further comparative study of the influence of stem geometry, stem surface finish, and bone cement brand on generation of fretting wear.

Stress in V-section band clamps

Abstract This paper presents an analysis of the stresses in V-section band clamps by examining the correlation between experimental work and theoretical models. Theoretical models incorporating traditional beam-bending theories and allowing for friction were developed to calculate the stress distribution and displacements within the clamps. The theoretical models demonstrated that the normal manufacturing tolerances associated with this type of component, combined with the uncontrolled operating parameters, will produce a wide variation in working stresses. These theoretical models were validated using strain and displacement measurements from a test with a V-section band clamp positioned around rigid flanges. The experimental results all fell within the range of stresses predicted by the theoretical models. The paper provides a knowledge base for the rational design of V-section band clamps.

Axial Load Capacity of V-Section Band Clamp Joints

In this paper a method of predicting the axial load generated by V-section band clamps, taking into account both circumferential and transverse friction has been proposed. An experimental method of determining this axial load is also demonstrated and the theoretical predictions are shown to be accurate. The effect of this axial load on the turbocharger components making up the V-band joint has been investigated with the aid of finite element analysis. It has been demonstrated that this load can make a significant contribution to the level of stress in these components.

Finite element prediction of the ultimate axial load capacity of V-section band clamps

Band clamps with a flat bottomed V-section are used to connect a pair of circular flanges to provide a joint with significant axial strength. Despite the wide application of V- band clamps, their behaviour is not fully understood and the ultimate axial strength is currently only available from physical testing. This physical testing has indicated that the ultimate strength is determined by two different types of structural deformation, an elastic deformation mode and a plastic deformation mode. Initial finite element analysis work has demonstrated that analysis of this class of problem is not straightforward. This paper discusses the difficulties encountered when simulating this type of component interaction where contact is highly localised and contact pressures are high.