Early and late mechanical stability of the cementless bone-implant interface in total joint arthroplasty
Pegg EC., Mellon SJ., Gill HS.
© 2014 Springer-Verlag London. All rights are reserved.Successful cementless implant fixation is essential for the survivorship and good function of a joint replacement. Fixation is often split into two events: stability of the joint in the initial stages (primary stability) and biological growth towards the surface of the implant resulting in full fixation (secondary stability). Good primary stability of the joint can be achieved by ensuring a press fit between the bone and the implant surface. For this to be possible, it is important that there is a good match between the shape of the implant and the reamed bone. The roughness of the implant surface can also aid primary stability by causing a "scratch fit" into the bone. Many authors support the theory that a certain amount of micromotion of the implant within the bone is acceptable but that if this exceeds the threshold of 150 μm, then fibrous tissue will surround the implant and primary stability will not be possible. The patient activity immediately after surgery is also of great importance and should be minimized to ensure the implant remains fixed. As has been outlined, achieving primary stability is only part of the story. For full fixation of an implant within the bone, bone growth needs to occur to fill the gap between the bone and the implant surface. This secondary fixation relies upon the correct biological signals to be produced to stimulate the osteoblasts to produce mineralized bone. These signals can be influenced by many factors including surface chemistry and roughness. Most commercially available coatings incorporate a rough metallic coating underlayer and a hydroxyapatite top coating; these are applied to the surfaces of implants in the region where bone fixation is desired. More recently, metallic foams manufactured from titanium and tantalum alloys have been introduced which provide a highly porous surface for bone ingrowth and can be applied as coatings or used as solid materials. These innovations in cementless component design have enabled current cementless components to be a viable alternative to cemented components, with comparable survivorship and outcome. New designs are constantly helping to increase our understanding of what causes an implant to become well fixed and how we can improve the function of these components further.