Leigh Brown

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.

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.

A metrology solution for the orthopaedic industry

Total joint replacement is one of the most common elective surgical procedures performed worldwide, with an estimate of 1.5 million operations performed annually. Currently joint replacements are expected to function for 10–15 years, however, with an increase in life expectancy, and a greater call for knee replacement due to increased activity levels, there is a requirement to improve their function to offer longer term improved quality of life for patients. The amount of wear that a joint incurs is seen as a good indicator of performance, with higher wear rates typically leading to reduced function and premature failure. New technologies and materials are pushing traditional wear assessment methods to their limits, and novel metrology solutions are required to assess wear of joints following in vivo and in vitro use. This paper presents one such measurement technique; a scanning co-ordinate metrology machine for geometrical assessment. A case study is presented to show the application of this technology to a real orthopaedic measurement problem: the wear of components in total knee replacement. This technique shows good results and provides a basis for further developing techniques for geometrical wear assessment of total joint replacements

A better understanding of the surface topography at the stem-cement interface

The long term stabilization and durability of cemented total hip replacement (THR) depends on not only the bulk properties of the components but also the interfaces through which they interact. The stem-cement interface has been consistently considered as a weak link in the stem-cement-bone system, being a transitional zone between two materials with significantly different mechanical properties. Previous research concerning this interface has been limited to investigation of interfacial shear strength through in vitro test and finite element analysis (FEA). Until now, a deep insight into the contact characteristics at this interface, especially the interaction between femoral stems with various surface finishes and bone cement, has not been established. In addition, it is still an area of debate whether a permanent fixation can be achieved by utilizing a matt femoral stem, and furthermore it is another matter of concern that a matt femoral stem would cause much more damage to the cement mantle, resulting in an acceleration of aseptic loosening of the femoral stem. This present study investigated the surface topography of stainless steel rods and Simplex P bone cement obtained from a series of pull out tests in order to gain a better understanding of the interaction at the stem-cement interface.Copyright