Diagnostic methods used in total knee surgery
To arrive at the right diagnosis of the knee joint disease may be easy in some cases and it may involve several diagnostic procedures in other cases. Here follows a short description of some diagnostic methods:
X-ray (roentgen) picture
corresponds to a negative photographic picture. The tissues with much calcium, such as bone, are opaque for X-rays and appear as white on a X-ray picture. Tissues with much water, as joint cartilage, are pervious for X-rays, and appear as black on X-ray picture.
The X-ray picture of a healthy knee joint show the layer of the joint cartilage that separates the two joint surfaces as a dark line in the middle of the knee joint – the joint line.
The layer of joint cartilage on the surfaces of the bony ends of the knee joints is pervious for X-rays. It appears as a dark band some 4-5 mm thick, called “joint line”. This joint line, clearly visible, divides the white “shadow” of the femoral condyles from the shadow of the tibial condyles.
In many knee joint diseases, such as arthritis and osteoarthritis, the joint cartilage is damaged, the thickness of the cartilage cushion successively diminishes, and the cartilage may disappear altogether.
On a X-ray picture, the disappearance of joint cartilage is represented by disappearance of the “joint line”. The disappearance of “joint line” is thus a diagnostic sign of arthritis or osteoarthritis of the knee joint.
Note that in an arthritic knee joint the femoral and tibial condyles on the inside of the knee joint are in close contact (upper picture), because there is no cartilage cushion between them.
On X-rays the disappearance of the joint cartilage is represented by disappearance of the “joint line” (lower picture) on the inside of the knee joint; the shadows of the femoral and tibial condyles on the inside of the knee joint are in close contact, the joint line is lost. Note also that the condyles are deformed and enlarged by spurs.
The sign of lost joint line is, however, not absolute. In some patients with OA of the knee joint, the area of the destructed joint cartilage may be small or localized so that it does not appear on plain X-ray pictures. On X-ray pictures of these patients, the joint line is still preserved.
In these patients it is necessary to take special X-ray picture from different angles of view, or use other diagnostic methods such as MRI or arthroscopy.
A special X-ray technique that divides the investigated organ into many very thin “slices”. A computer then composes the definitive picture from these small “spot” slices. The surgeon can, for example, see a three-dimensional picture of the knee joint with many details not seen on conventional X-ray pictures.
The CAT shows for the surgeon a really three-dimensional picture of the distorted anatomy of the knee joint, how much skeleton is still preserved, how large is the marrow cavity, etc. These pictures are necessary for fabrication of custom made total knee prostheses because the manufacturer needs to know the dimensions of the skeleton.
MRI – Magnetic Resonance Imaging. It is a special technique that measures the amount of water (hydrogen) atoms in the tissues. No X-rays are used, only very strong magnets. This technique shows a very detailed picture of changes in the soft tissues, or areas in bone with changes water content. (Avascular necrosis, e.g.).
The technique must not be used in patients with pacemakers. Also patients with other objects inside body, which are manufactured from magnetic metallic alloys, are forbidden to have MRI examination. All artificial joints are made from non-magnetic metal alloys, and thus the MRI examination may be done in patients with artificial joints. By use of new MARS technique even bone and other soft tissues around artificial knee joints may by investigated with MRI.
The MRI is useful, for example, for assessing the state of the cruciate ligaments and the joint cartilage before the total knee joint operation. For diagnosis of avascular necrosis it is a very efficient method that shows the changes in the skeleton before they even become apparent on the X-rays.
The precision of the MRI may be increased when one injects the rare metal Gadolinium together with MRI. Gadolinium concentrates in areas of scar tissues that may be distinguished by this technique.
There are situations, when patients with artificial joints need MRI of other body organs (abdominal or chest cavity or extremities).
These patients may be anxious that their total joints may be damaged by the MRI examination.
Basically, MRI or rather its strong magnetic field produces two effects on the metallic implants / total joints in the body.
- If the implant is fabricated from ferromagnetic material, the MRI apparatus attracts or deflects the object. Patients with such implants must not be examined with MRI.
Modern orthopedic devices are fabricated of NON FERROMAGNETIC IMPLANTS and experiments demonstrated that their position is not influenced by the electromagnetic field produced by the MRI apparatus. (Clerc 1997)
- The strong magnetic field might produce electric (eddy) currents in the metallic implant. These currents might heat up the material. Experiments showed that heating of the metallic implants ranged from 0 to 0,5 degrees Celsius, depending on the form and material of the implant and the length of the examination. Thus the heat effect is totally negligible. (Buchli 1988, Ho 2001)
Some metals when injected into circulation will concentrate in areas with high turnover of newly formed bone tissue, such as in areas of bone fracture or bone infection. The isotopes of these metals emit a radiation and when taken up in the tissues, the radiation may be detected with special scanning techniques. Such metals are Technetium, Gallium, Indium. The Technetium bone scan can reveal hair-line bone fractures, which do not show on conventional X-rays, or detect bone tumors.
The Indium scan is used to identify the areas of infection, for example the “indolent” infections of total hips. This test is conducted in two steps. In the first step, the laboratory removes a sample of your blood, isolates your white blood cells and labels them with Indium-isotope. This will take some time. You will return some days later, and the labeled white cells will be injected back into your blood circulation. These labeled white cells will travel in the circulation, seek the spots in the body where infection is ongoing and be taken up there. The white cells labeled with Indium will thus label the infection spot.
The radiation from the isotopic agents is weak, the amount of radiation is generally not much more than the radiation from a single X-ray picture.
Computerized bone scanning shows a three-dimensional picture of the places with high uptake of isotopes.
Doctors often use bone scan in patients with pain in the total knee. Studies demonstrated that (Technetium) bone scan is not reliable during the first year after the total knee surgery. Later on, a patient with a negative bone scan is unlikely to have loosening of the total knee, although about 5% of patients with loose total knees have had negative bone scans.
Erythrocyte sedimentation rate (ESR), more familiarly “sed”, measures the speed at which clumps of red blood cells falls through blood serum to the bottom of a long thin tube. If the cells fall faster than normal (usually the normal speed limit is up to 20 to 28 mm/hour), then the test is interpreted as a sign of active inflammation. This test is non specific, the “sed” may be higher both in an innocent flu and in a widespread tumor. Very high “sed” usually is found in severe infections and in some blood tumors.
CRP – C reactive protein. This test measures the concentration of a substance called C reactive protein in blood. With high concentrations the test is considered positive. The test is positive in infections. It follows the intensity of infection more quickly than “sed”.
White blood cells. The concentration of white blood cells in the blood is increased in infections. The upper limit for normalcy varies in different laboratories. It is, however, known that the late, slowly developing infections of the total knee prosthesis seldom have high counts of white blood cells.
Rheumatoid Factor (RF) is a compound (antibody) secreted by certain tissue cells (B cells). It is found in the blood of up to 85 % people with R.A. (Rheumatoid Arthritis). Its occurrence is associated with the more progressive and aggressive form of R.A. The diagnostic value of this test is, however, diminished by the fact that about 7% of people aged 70+ also bear this factor although they have no signs of R.A.
Genetic markers typing. This is as yet not a standard procedure. Researchers have been able to correlate fairly accurately the different types of R.A. with variable sub-types of genetic markers, such as HLA-DR1. Such genetic information allows the rheumatologist to gauge the severity of the inflammation and fine-tune the treatment.
The surgeon uses a rigid arthroscope, a tube of 5 – 6 mm in diameter with fiber optic to examine the knee joint. The artroscope is introduced into the knee joint through very small incisions. Usually, the arthroscope is coupled to a vide TV screen. The whole examination may be also recorded on a video band. The examination is done in anesthesia.
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- Clerc CO et al. J Biomed Mater Res 1997; 38: 229-34
- Eustace S et al. Orthop Clin North Am 1998; 29: 67-84
- Ho WS J Magn Reson Imaging 2001; 14: 472-7
- Puri L et al. J Bone Joint Surg-Am, 2002;84-A: 609 -14
- White L et al Radiology 2000; 215: 254 -62