The implants used for the revision operation are broadly similiar to the ones used for first time sugery. They consist of a cup section inserted into the natural socket of the pelvis and a stem with a ball at its top aspect that is inserted into the femur bone. When the two are put together, a joint results much as is the case in nature where the ball at the top end of the thigh bone forms a joint with the hollow socket of the pelvis bone.
In many instances however, the implants required for revision surgery are more
complicated and have to be selected for each individual patient depending upon
the prevailing clinical situation.
The socket component increasingly used for revision surgery nowadays is of the type that is implanted without cement. It has to be said however, that some surgeons do continue to report good results from revisions carried out with cemented socket components. The implants used without cement are of a range of sizes and the most appropriate one for a particular patient can be chosen at the time of surgery and inserted after a trial fit. On occasion, even the largest sized component may not be satisfactory and this is particularly the case when the bone loss is significant and also where it has occured only in one sector of the natural socket as opposed to uniformly all round the socket. In such situations, the surgeon may used specialised components that have "add ons" to allow better filling of any defects. These "add on" modules are assembled to the artificial socket prior to implantation into the patient.
There are instances when the bone defects are so great that there is insufficient bone to anchor satisfactorily, a socket without using cement. In such a situation, the surgeon has the option of using socket reinforcing steel cages that are first implanted into the damaged socket much like a scaffold for a building. A socket can then be cemented within the scaffold. If the bone loss is even greater, say in a situation when damage from the failed replacement has caused a large hole in the natural socket of the pelvis, a large shield with an integral metal shell is first placed on the bone. This is held by means of screws placed via the shield into relatively intact bone. The shield achieves stability by overlapping the margins of the socket. Once this has been fixed in place, the plastic socket may be cemented into it's cavity. Larger and more complex metal components and replacement parts are available to deal with bigger bone losses.
With regards to the femoral components, there are two broad trends in the more complex cases. One is to rebuild the damage in the femur (thigh bone) with bone graft. This technique is called impaction grafting. The femoral component which is much like the one used in primary surgery, is then cemented into the grafted bone. The other broad trend is to use modular components without cement. Surgeons proficient with both techniques have obtained good results though the impaction grafting procedure has been around for longer than the modern modular components for use without cement (the cementless components). As the risks of disease transmission with bank transplant material have become well known and as the legislation for running the banks and using donated bone has become more complex, surgeons are not using impaction grafting as frequently as was the case only a few years ago. The components used without cement are supplied in a "knocked down" state which is very handy as it allows the surgeon to "make" an implant by putting together the right sized components at the time of surgery. This mean that the implant can be "fabricated" in the operation theatre taking into account the best fit and suitability for the patient. Often the surgeon will have a wide choice of components to put together in each section of the implant for maximum flexibility.
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