5.1 Bone Preparation: The Importance of Establishing the Best Bone-Cement Interface

Summary This chapter describes the interface between bone cement and bone, pointing out that the operating surgeon is responsible for establishing that interface at the time of surgery. If the interface is not well established at the start, the replacement joint has no chance of long-term function. The cement-bone interface is a mechanical interlock between the two materials that can be enhanced by the preparation of the bone surface, pressurising cement into that surface and holding the cement under pressure until its viscosity is such that bone bleeding cannot displace it. Effective pressurisation can only be obtained using suitable instruments. The effect of heating the femoral stem before insertion is described. The surgeon has to be aware of the effect of all the variables in order that the strongest possible interface is obtained at the time of surgery

Microcrack accumulation at different intervals during fatigue testing of compact bone.

Fatigue damage in bone occurs in the form of microcracks. This microdamage contributes to the formation of stress fractures and acts as a stimulus for bone remodelling. A technique has been developed, which allows microcrack growth to be monitored during the course of a fatigue test by the application of a series of fluorescent chelating agents. Specimens were taken from bovine tibiae and fatigue tested in cyclic compression at a stress range of 80MPa. The specimens were stained before testing with alizarin and up to three other chelating agents were applied during testing to label microcracks formed at different times. Microcracks initiated in interstitial bone in the early part of a specimen\u27s life. Further accumulation of microcracks is then suppressed until the period late in the specimen\u27s life. Microcracks were found to be longer in the longitudinal than in the transverse direction. Only a small proportion of cracks are actively propagating; these are longer than non-propagating cracks. These results support the concept of a microstructural barrier effect existing in bone, whereby cracks initiate easily but slow down or stop at barriers such as cement lines

Fatigue failure of osteocyte cellular processes: implications for the repair of bone.

The physical effects of fatigue failure caused by cyclic strain are important and for most materials well understood. However, nothing is known about this mode of failure in living cells. We developed a novel method that allowed us to apply controlled levels of cyclic displacement to networks of osteocytes in bone. We showed that under cyclic loading, fatigue failure takes place in the dendritic processes of osteocytes at cyclic strain levels as low as one tenth of the strain needed for instantaneous rupture. The number of cycles to failure was inversely correlated with the strain level. Further experiments demonstrated that these failures were not artefacts of our methods of sample preparation and testing, and that fatigue failure of cell processes also occurs in vivo. This work is significant as it is the first time it has been possible to conduct fatigue testing on cellular material of any kind. Many types of cells experience repetitive loading which may cause failure or damage requiring repair. It is clinically important to determine how cyclic strain affects cells and how they respond in order to gain a deeper understanding of the physiological processes stimulated in this manner. The more we understand about the natural repair process in bone the more targeted the intervention methods may become if disruption of the repair process occurred. Our results will help to understand how the osteocyte cell network is disrupted in the vicinity of matrix damage, a crucial step in bone remodelling