Despite the best intentions a hip replacement will wear out, and surgery to reconstruct or replace the present hip replacement will become necessary. Subsequent surgery is referred to as revision surgery. Revision surgery is generally more complex than primary surgery because of scarring, bone loss, increased bleeding, and problems encountered with implant removal. As mentioned previously, revision surgery may become necessary for a variety of reasons. Infection, bone loss, and most commonly implant loosening. The focus of this discussion will be on the process of implant loosening referred to as aseptic loosening (meaning loosening not related to an infectious process).
Many of the early total hip replacements were fixed to the bone with a grouting material known as methylmetharylate more commonly referred to as bone cement. Over time this bone cement can weaken leading to cracks within the cement, and eventual implant loosening. Another common cause of ascetic loosening is the processes of bone resorption due to the inflammatory processes set up by the body's immune responses to plastic wear particles. Aseptic loosening is more common in young heavy adults, who put increased stress across their hip joint. It is this increased force that leads to more rapid plastic wear, particle formation, and inflammatory bone loss. Another important mechanism leading to implant failure is improper implant position. Malposition of implants at primary surgery can increase the forces across the hip joint and eventually lead to failure.
It is important to remember that the process of failure can go on for many years without causing any pain. When enough bone loss has occurred, and the implants become loose, the patient will begin to feel pain. Generally if the acetabulum is loose the patient will have pain in the groin or buttock, and if the femoral component becomes loose the patient will experience pain radiating down the thigh. Often by the time symptoms have manifested there is extensive bone loss making revision surgery difficult. Again this scenario underscores the importance of close annual clinical and radiographic follow up. Conservative therapy is generally reserved for the patient who is asymptotic, has radiographic evidence of extensive plastic wear, little or nor bone loss, and stable components. If, however, follow-up reveals progressive bone loss a liner change and debridement of the bone loss areas is recommended. This early surgery on the asymptomatic patient removes the particle producing plastic liner and settles down the inflammatory response causing the bone loss. Another option for the asymptomatic patient with early bone loss and stable components is Fosamax. This medication is an inhibitor of Osteoporosis. Although it can be associated with GI discomfort early results suggest that it may prevent the progression of bone loss associated with the process of aseptic loosening.
In the unfortunate situation where there has been extensive bone loss and implant loosening, revision surgery becomes much more challenging. The revision surgeon needs to be well equipped with many surgical techniques allowing for successful reconstruction. A bone bank may be necessary for reconstruction. This bone is used as structural support for the new implants that are placed. Allograft is only used in the most severe of circumstances where the patients own bone is so badly destroyed that it is unable to support the new implants. Ninety percent of the time there is enough bone present that will allow a reconstruction without the need for allograft.
Revision surgery is only undertaken after thorough medical evaluation. Revision surgery compared to primary surgery is longer, requires more extensive exposure, is associated with more bleeding, and is also associated with higher infection and dislocation rates. Because of this the results of revision surgery in regards to patient satisfaction and pain is not as good as that associated with primary surgery. Postoperative the patient can expect to have more pain and have a longer rehabilitation period, the specifics of which depends on the extent and complexity of the surgical procedure. For example, if allograft has been used, healing between the host bone and allograft can take many months. This may necessitate an extended period of protected weight bearing.