MORE DETAILS ON ASEPTIC LOOSENING OF TOTAL HIPS
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 cavity. |
What causes aseptic loosening of a total hip joint?
There is not one, but several factors that together cause that the once stable interlock between the surface of the artificial joint and the skeleton breaks.
And there are two theories that explain why the total hip prosthesis eventually loses its stable fixation to the skeleton:
The mechanical theory
The biological theory
The mechanical theory
Repeated cyclic stresses are imposed on the interlock between the total hip prosthesis and the patient's skeleton by everyday activities. But the total hip joints cannot adapt themselves to these stresses as the healthy joints can.
Some facts to consider
1) The frictional resistance of the healthy normal hip joint is 70 times lower than that of the best currently available total hip prostheses. This means that the interface (the area where the bone attaches to the total hip) of the total hip prosthesis will have to sustain 70 times higher stresses than the healthy hip joint even with moderate activity.
The frictional resistance increases with the load on the hip. Thus every patient activity that increases this load, such as jogging, jumping, and like increases the frictional resistance and the stress on the joint interface even more.
2) In the extreme movements, the neck of the total hip joint impinges on the rim of the cup component, creating extra stresses in the cup component’s interface with the skeleton. In a healthy hip joint there are several anatomical mechanisms that prevent the development of such peak stresses.
3) Total hip replacement implies an almost total denervation of the hip joint. The total hip prosthesis and the tissues around it are deprived of the protective function of the nerve supply of the healthy hip joint.
Remember that even with leisure walk, the total hip joint sustains stresses of two and half times the body weight, much more with speedy walk or running.
Why are high stresses on the newly implanted total hip detrimental?
Picture: High stress destructs the dead bone layer
(Click on the icon for a full size image)
Directly after the operation, the bone tissue in direct contact with the total joint consists of dead bone. This is the result of cutting, chiseling, and drilling activities during the preparation of a bed for the total hip prosthesis.
This layer of dead bone tissue is thin and it is able to sustain a certain stress imposed on it, so that under favorable circumstances it will last until the body replaces the dead bone layer with a layer of living bone.
If the stresses on the dead bone will be high, the body will have no chance replace it with a living bone tissue. Instead, the dead bone layer will be replaced by a layer of soft fibrous tissue.
On X-ray pictures, this soft tissue layer is apparent as dark "radiolucent line" around the total joint component. In this picture the "radiolucent line" surrounds the cup.
The fibrous tissue is, of course, a bad support for a total hip prosthesis so that with new stresses the bone under it will be abraded, grated off, producing yet more soft connective tissue.
An important part in this scenario plays the joint fluid, that will be pumped with every step into the soft tissue layer, increasing the pressures in this layer.
A vicious cycle will be thus established until the whole prosthesis will be enclosed with loose fibrous tissue.
According to this mechanical model the patient’s activity and the surgeon’s careful operation technique are the most important factors for a long-lasting fixation of the total hip prosthesis.
To exhort the young active patients to abstain from the use of their new pain-free hip joints is difficult and makes no sense to these patients.
No wonder that this teaching never became popular.
The biological theory
When the surgeons began to study the soft tissues around the loose total hips, they discovered that these tissues produced many enzymes and other substances that supported the dissolution of bone tissue. The surgeons also also discovered that this tissue contained many tiny wear particles of different origin: some particles were coming from polyethylene components, from ceramic and metallic joint surfaces, and from bone cement if the total hip was cemented.
Successively evolved a biological theory of total hip loosening based on these observations. (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: with each step, around half a million submicroscopic particles are created from the polyethylene bearing surface of a total hip joint. (McKellop,, 1996. www. durasul.com)
These particles spread into the tissues around the artificial joint and provoke an inflammation reaction there. The soft tissue created by this reaction has osteolytic properties, it dissolves bone. This bone dissolving process is called osteolysis.
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Osteolysis (Click on the icon for a full size picture.) How the dissolving of the bone tissue proceeds. 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 into the small crevices around the as yet stable total joint. In these crevices there is usually some connective tissue. In these tissue 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 soft tissue expands around the surface of the joint prosthesis and dissolves yet more bone tissue. In the new space such created every new step pumps more joint fluid with more wear particles. These particles activate yet more garbage cells that "eat" yet more bone tissue, etc. A vicious cycle of 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 osteolysis easier than other patients. This is a large research field for the "molecular biology" in the future. (Puzas 2002) The anatomical image 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 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.
This theory has one big advantage: it suits the manufacturer of the total hip prostheses. The manufacturer can examine new materials with lower and lower wear rates, produce an unending stream of new models of total hip prostheses ( more and more expensive) with lower and lower rates of wear in laboratory conditions.
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.
See the figures in the Table (Malchau 2002)
| TYPE OF OPERATION | % ASEPTIC LOOSENING |
| Primary total hip surgery | 5 % |
| Revision of a previous total hip surgery | 15 % |
- 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 age for total hip
- 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. 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). As yet, the results of these trials were not published.
Treatment of aseptic loosening .
Not every total hip 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 hip 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.
Silent osteolysis- against the theories?
There is one special form of osteolysis called "silent osteolysis". Patients with this form of osteolysis do not have pain because this insidious form of osteolysis destructs central parts of the skeleton under the prosthetic component, whereas the periphery of the component remains attached at the periphery.
This form of osteolysis is frequent in two categories of total joints:
1) in young patients with cementless total hips. It develops 4-6 years after the primary total hip operation
2) in patients of all ages with cementless total knees. It develops some years after the primary total knee operation.
The use of cementless total hips and total knees was started because the use of bone cement was said to promote and sustain osteolysis, although of another form.
As yet nobody commented on these contradictory facts.
The surgeons are discussing whether regular controls with X-ray pictures of the total joints are necessary for young patients operated on with a cementless total hip prosthesis and uncemented total knees to discover the silent skeletal destructions.
References:
Archibeck, J Bone Joint Surg-Am, 2000, 81-A, 1485
Britton A et al J Bone Joint Surg-Br 1997;79-B, 93-8
Ling Complications of total hip replacement, 1984
Malchau et al. Prognosis of total hip replacement, 69. Annual Meeting of AAOS, 2002
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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