The most common cause appears to be loosening of the cement used to fix the artificial socket into the hip bone. As the socket loosens progressively, it starts to rub on or abrade the bone into which it had been fixed. This constant rubbing has an affect much like a coarse sandpaper causing the dissolution of bone slowly over a period of time. Further, the rubbing or rocking process causes the generation of small particles of cement that are ingested by some of the specialised cells in the body who see the particles as foreign material. While this starts off as a defence reaction, the ingested particles leads to, through a complex mechanism, the release of powerful chemicals in an effort to defeat the "enemy" material. Unfortunately, these chemicals can lead to bone damage and further bone loss not only in the vicinity of the loose component but also at times in and around the other stable component.
Just as the cement surrounding the socket component becomes loose with time, so can the cement placed around the femoral component. The progression of the problem is as described above. Also, once the implant loosens, it can place abnormal stresses both on the bone as well on the implant itself leading to bone loss in the first instance and breakage of the component in the second.
An infection deep within the hip joint and in and around the components can be the cause of the failure. This is a particularly difficult problem to deal with both from the point of view of the surgeon as well as from the perspective of the patient. Usually though not always, the hip components are securely fixed but may have to be removed simply to allow the infection to be eradicated.
If the hip dislocates or slips out of the joint repeatedly, then that may necessitate a revision operation. A single dislocation, particularly in the first few weeks after the primary hip replacement is not necessarily a problem and often remedies itself after the hip is put back providing of course that there are no recurrences. A problem hip of this nature needs to be studies closely to establish the cause of the dislocations in the first place. It is only if the cause can be identified that there is a good chance of fixing the problem. At times, the hip may be unstable because there is not enough tension or tightness in the muscles and ligaments around the artifical replacement. This lack of tissue tension can make it easy for the hip to pop out. On the other hand the problem may lie in the orientation of the components of the hip replacement. If that is the case, then it is the orientation problem that needs to be addressed and not the soft tissues. Sometimes bony projections and outgrowths such as often occur as a natural consequence of the arthritis of the hip can cause the hip bone to impinge against these outcroppings and result in the hip joint levering out of the socket.
There are times when the surgeon during a check up may notice the development of bone defects or patches of bone loss on x-rays even though the components are securely fixed. This occurs because of the body's reaction to particles generated from wear and tear of the hip replacement bearing. The damage to the bone will continue progressively unless the bearing is renewed. Such bony damage can result is weakening of the bone and cause a fracture (a break in the bone) or cause the components to loosen. In a situation such as this, the earlier a remedial intervention is carried out the better as that may help preserve the patient's bone stock and make the redo operation technically easier.
While the problem of component breakage has now become rarer, it has not completely
disappeared. Sometimes the breakage results from a metallurgical fault during
manufacture or from weakening of the implant due to defective finishing processes
subsequent to manufacuture. On other occasions a femoral component may break
becasuse it is securely held in the bone through only a part of its length and
has loosened elsewhere. In such a situation, the stresses on the component concentrate
in a weak area and lead to its breakage.
Back to introduction to revision hip surgery
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