UNSTABLE TOTAL KNEE JOINT
The surgeon who carries out total knee arthroplasty must restore the stability of the knee joint and realign the mechanical axis of the leg. (For anatomical details see the chapter Total knee operation)
Total knee replacement may fail because the new total knee is unstable and the patient cannot use and rely on the replaced total knee joint.
If the surgeon carried out the balancing of soft tissues around the total knee joint during the primary (first) operation right, the new total knee would be reasonably stable.
As the time goes, however, the total knee joint may become unstable.
The cause of instability of a total knee joint, that was stable after the original operation, may be
Loosening with destruction of the skeleton around the total knee joint or
Destruction of soft tissues and ligaments around the total knee that keep the total knee joint stable.
Treatment of unstable total knee
Not all unstable total knee joints need revision operation. Milder instability may be treated with support braces and rehabilitation of the muscles around the total knee.
In severe cases of instability, however, revision operation is necessary to render the total knee stable again.
The type of operation depends on the cause of the instability.
The patients are often wondering whether the surgeon will put a "hinge" in their knees at revision operation. They have heard so many unfavorable things about these types of prostheses. (See also the chapter: Linked total knee).
There are, however, general rules that govern the selection of the total knee model in revision operations of the unstable total knee.
If the soft tissues are well retained and the instability of the total knee is caused by bone loss around a loose total knee, then the surgeon can exchange the loose component for a bigger one that will stretch the slack ligaments.
Picture: Revision operation of unstable total knee with well-retained soft tissues
Click on the icon for a full size picture
Think of soft tissues and ligaments around the total knee joint as an envelope or a tube that provides side stability for the implanted knee joint.
Left picture:
The total knee sways from side to side because osteolysis destructed the skeleton around the tibial component. The component sank into the shinbone and the soft tissues are no longer stretched and do not provide stability against side movement of the total knee. The soft tissues, although folded, are still well retained
Right picture:
The surgeon removed the old low tibial component and replaced it with a higher one. This exchange stretched the folded soft tissues that are again providing stability for the total knee.
The problem arises when the soft tissue envelope is deficient, damaged, or non-existent. This can happen when there were previous operations that damaged the knee joint ligaments, or when a disease (rheumatoid arthritis) destructed them.
If the soft tissues are non-existent, destructed, or weak the surgeon must use a total knee model with intrinsic stability. See more about these total knee prostheses in the chapter Linked total knees.
Suture of the damaged soft tissues and ligaments alone is usually not successful.
In more rare cases, the total knee buckles forwards and back. These patients usually have lost their posterior (back) cruciate ligament. In revision operation of these unstable total knees, the surgeon may use the posterior stabilized total knee model (see the chapter Posterior stabilized total knee) to restore the stability of the knee joint.
The surgeon decides the appropriate treatment and choice of the total knee prosthesis in cooperation with the patient after careful examination of the unstable total knee joint.
You may always ask your surgeon what caused the instability of your total knee joint
Ask your surgeon also about the benefits and risks of the proposed treatment.
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References:
McAuley JP, Engh GA: J Arthroplasty 2003; 18: 51 - 54
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