TOTAL KNEE LOOSENING
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What is an aseptic loosening?
All manmade objects wear out and decay with time. So do the artificial joints too. With time, the once firm bond between the skeleton and the artificial joint, created at the surgery, will be gone. The artificial joint will then lay loose in the skeleton.
As with loose teeth, loose total joints do not function properly, are painful and stiff.
There are many causes why the once stable total joint becomes loose. For example, the deep infection is a well known cause of loosening of total joints.
When the surgeons speak about loosening of a total joint, however, they always mean an aseptic loosening, that is a loosening of the total joint without involvement of bacteria.
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Aseptic loosening of the total hip and total knee joint Click on the icon for a full size image Stable total joint: The whole outer surface of a stable total hip and / or knee joint prosthesis is in close contact with the skeleton. In porous coated cementless total joints the bone growths into the porous surface. A stable well anchored total joint is a pain-free joint. Loose total joint: Loose total hip and knee joint lies loosely in a cavity that forms around the loose total joint. This cavity is padded with loose connective tissue. The total joint moves, i.e. it changes its position, within this cavity. This motion may be apparent on successive X-rays and it may be the first sign that the total joint is gone loose. The movement causes pain and stiffness. The shaft of the total hip sinks deeper in the femoral bone, the cup migrates The total knee components rotate, the tibial component sinks deeper in the tibial marrow cavity. |
What causes aseptic loosening of a total knee joint?
Aseptic loosening of the total knee prosthesis from its bond with the skeleton is, after the kneecap complications, the second most common late complication of artificial joint surgery.
There is not one, but several factors that together break the once stable interlock between the total knee joint and the skeleton.
Mechanical factors:
Repeated cyclic stresses imposed by everyday activities on the bond between the prosthesis and the skeleton. Total kne joints cannot adapt themselves to these stresses as the healthy bone tissue can. The total knee joints don't have the feedback system that protects the natural joints against overload.
Remember that even with leisure walk, the total knee joint sustains stresses at least two times the body weight, much more with speedy walk or running.
Biological factors: (Archibeck, 2000)
The gliding surfaces of the artificial joints generate continuously submicroscopic wear particles. The number of these tiny particles, most of them so small that they cannot be seen in a light microscope, is enormous.
These particles spread into the tissues around the artificial joint and provoke an inflammation reaction there. The inflammation reaction triggers osteolysis.
Osteolysis, as mechanism is called, dissolves the skeleton around the total prosthesis.
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Osteolysis Click on the icon for a full size picture. The prevailing theory that explains how the osteolysis dissolves the skeleton around the once stable total knee joint is following: The surfaces of an artificial joint produce many very small particles when gliding against each other. It is the softer material that wears away, in a metal-on-polyethylene total joint the particles consist of polyethylene. Rhythmic movements of the total joint pressurize the joint fluid containing these particles in the tissues around the total joint. In the tissues around the total joint there are "garbage cells", called macrophages, whose function is to clean the tissues from all rests of dead cells and other "garbage". When macrophages digest the fine polyethylene particles, transported with the joint fluid, they change their benign nature and begin to digest the healthy bone tissue. The "bone dissolving disease" or osteolysis is thus established. In some patients this "dissolving" of the bone stock may be so widespread that large parts of the skeleton are being completely destructed. But certainly there are special biological factors, that predestine one person to develop osteloysis easier than other patients. This is a large research field for the "molecular biology" in the future. (Puzas 2002) The picture of the loose artificial joint is always the same. The layer of bone tissue that once adhered close to the artificial joint is replaced by a layer of loose connective tissue that now separates the artificial joint from the bone. This layer of loose tissue is transparent for X-rays whereas the bone tissue and the metallic prosthesis are impermeable for the X-rays. On X-ray pictures this transparent layer of loose tissue forms a dark line interposed between the radioopaque contours of the skeleton and the contours of the artificial joint . Although these lines are dark, the surgeons speak about "radiolucent" lines
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The scientist also discuss other factors that might be involved in the development of osteolysis around an artificial joint. One such factor is the alleged hypersensitivity of some patients to the materials from which the artificial joint is fabricated.
How often does aseptic loosening occur?
The rates of aseptic loosening in published studies
- depend on the selection of patients, their age, their hip diseases
- on the type of the prosthesis, cemented, cementless, etc.
- and on the surgeon who published the results.
If you read such statistics, take always notice how big the original group of patients was, what was the diagnosis for operation, how long the follow-up lasted, and how many patients were lost to follow-up. Even famous surgeons have been publishing embellished reports on the operation results with new total hip prostheses.
There is also much "hidden pain" in many reports. Many patients (according to some studies about 25 % of all patients) have had pain in their total joint although they were not operated on second time. These patients do not appear in statistics.
Risk factors for aseptic loosening:
- Previous operation of the joint, especially if it was a total joint operation.
- How well the surgery was done. Studies demonstrated that the rate of loosening is higher in hospitals that perform only small amounts of total joint operations. Ask your surgeon about his experience and his personal results with the artificial joint he/she is recommending to you.
- Your physical activity. Young active patients have higher rates of aseptic loosening. "Neither surgeons nor engineers will ever make an artificial joint which will last 30 years and at the same time enable the patient to play football." (Charnley 1979 ). As yet, Sir John Charnley has proven right.
- See also the chapter Young for total knee?
- The design of the artificial joint. Some total joints models loosen more often then others. Often, these total joint models disappear from the market without further notice. The scientists occasionally discover some reasons why these new designs failed so often - unfortunately, the discovery comes always too late. Some hospitals may still use these prostheses if there is large local inventory of them. Thus, look always at the performance records of the artificial joint that the surgeon recommends for you.
- The quality of your bones. In theory, the harder your bones are and the more of the bone substance (bone stock) is there, the stronger the interlock will be and the longer the prosthesis will last.
Strong bone stock is usually found in obese people.
Deficient bone stock is found in older people and in patients with rheumatoid arthritis. The deficient bone stock in these patients is, however, counterbalanced by the low physical activity of these patients.
- Excessive weight. This is a controversial issue. Yet, you should be aware, that every kilogram of your body weight loads three to seven more kilograms of stress on the interlock between the total hip joint and your skeleton. Thus, keep your weight down.
To prevent aseptic loosening of your artificial joint you should follow this simple advice: do not overload your artificial joint.
I should also mention the ongoing drug trials to prevent the development of prosthetic loosening: NSAID drugs and biphosphonate drugs.
The NSAID drugs are supposed to mitigate the inflammatory reaction caused by wear particles. Some studies indicate, however, that NSAID accelerate the development of osteolysis.
The biphosphonate drugs are supposed to make the bone substance more resistant to destruction by inflammatory cells. (Archibeck 2000). In one such rare study, the patients on biphosphopnate drugs have had lesser bone loss than other total hip patients (sorry, no total knee studies published as yet) .
Treatment of aseptic loosening .
Not every total knee joint that appears to be loose on an X-ray picture evokes pain and reduction of joint function.
Many patients live happily with their well functioning artificial joints while the X-rays of their artificial joints show the picture of " a loose artificial joint". Remember that the X-ray picture of your artificial joint and your personal comfort might not be correlated.
If the patients experience discomfort or even pain from their total knee joints and the X-ray pictures show signs of incipient aseptic loosening, the first step usually ordered by the surgeon is a restricted weight bearing regime. Often this may by all that is needed. The loose prosthesis may find a new stable position, the discomfort and pain disappears, and the radiolucent lines seen on the X-rays do not progress.
If the radiolucent lines on X-ray pictures widen and if the pain and other discomfort from the artificial joint increases then a revision operation becomes necessary.
The surgeons are discussing whether regular controls with X-ray pictures of the total joint are necessary for all patients operated on with a total joint prosthesis to discover these silent skeletal destructions.
References:
Archibeck, J Bone Joint Surg-Am, 2000, 81-A, 1485
Britton Ae et al J Bone Joint Surg 1997-Br;79-B, 93-8
McKellop Clin Orthop 1996; 311: 3 -20
Puzas JE et al J Bone Joint Surg-Am, 2002, 84-A, 133-141
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