METAL - ON - METAL TOTAL HIPS
CONTENTS
WHAT IS IT
The metal-on-metal total hip joints have both bearing surfaces (the ball and the cup component) made of metal.
Modern metal-on-metal total hip prostheses have adequate space between the femoral ball and the cup surface which allow lubrication of the surfaces with "joint fluid".
The larger the diameter of the ball component, the better the lubrication. The metal on metal total joints have usually ball components with ball diameter 28 -32 mm.
Concern for shock propagation through hard metals
Walking produces repeated blows when the leg comes in contact with the ground (this moment is called "heel strike"). These blows propagate to the artificial hip joint and to its interface with the skeleton. Some scientists fears that unprotected total hip models consisting only of materials much harder than bone and cartilage will propagate these shock waves directly on bone and damage the coupling between the total hip and the skeleton.
Direct metal-on-metal coupling in an artificial hip has no soft cushion that might dampen the repeated shocks that ensue from the contact of the leg with the floor during walking.
In the polyethylene-on-metal hip systems it is supposed that the soft polyethylene cup plays a shock absorber.
Construction of a metallic cup with a sandwiched polyethylene layer Click on the icon for a full size picture.
The manufacturers thus introduced polyethylene shock absorbers into the modern metal-on metal total hips. The cup component consists of three layers: the outer and inner layer are made of metal, and between these two layers there is sandwiched a polyethylene layer. This is a complicated construction with possible risks of disintegration of the complicated cup component. There are, however, no published reports of such damage.
(Protek)
After the introduction of surface hip replacements the engineers constructed metal-on-metal total hip devices with "extra-large" bearing components (diameter 52 -54mm). These can be used to replace the failed surface replacement device.
Picture: Extra large diameter bearing components (Exchange of failed surface replacement hip device to total hip device). Click on the icon for a full size picture
If the ball component of a surface replacement hip device fails, whereas the cup component remains steadily fixed in the pelvis socket, the surgeon removes only the failed ball component with an revision operation. The cup component is left in place.
The surgeon then opens the marrow cavity of the thigh bone and puts in a femoral shaft component with metallic ball specially designed for this replacement operation. The failed surface replacement hip is thus changed into metal-on metal total hip
The ball component of the new total total hip has equally large ball diameter as was the diameter of the removed surface ball. Thus the new ball component would fit perfectly in the cup that was left in place. The change of the surface replacement into a total hip replacement is now finished. The operation trauma by this revision operation is low because the surgeon did not need to touch the cup component.
(Corin's Cormet & Optimom)
Another advantage of the super-large ball diameter hip models is their resistance to dislocation. Thus, these models are also used for revision operation of patients whose total hips dislocate often.
The friction resistance of modern metal-on-metal bearing couples, which was the main problem of the "old metal-on-metal" total hips, is equally low as the friction resistance of other bearing couples. (Ceramic or polyethylene-on-metal bearings).
The main characteristics
The metallic hip joint surfaces are hard and scratch resistant
The occasional scratches on the metallic surfaces disappear with the continuous motion of the surfaces - the metal surfaces "polish" the scratches.
The metal-on-metal total hips produce (in laboratory experiments) about 50 times less wear particles than the polyethylene-on-.metal total hips. (www.metasul.com)
Metal-on-metal total hips are not completely biologically inert, they produce soluble metallic salts that enter the body's fluids (blood and urine). Moreover, small metallic particles are placed in the tissues around the total joints.
The serum levels of metals entering the alloys used for the manufacture of the total hips, mainly Nickel, Cobalt, and Chrome, are elevated in the blood of patients with metal-on-metal total hips as compared to the levels found in patients with metal-on-polyethylene total hips.
Nickel is usually eliminated quickly from the body by urine, whereas Cobalt and Chrome stay longer in the body, Chromium is even retained in the body's tissues. (Brodner 2000, Schaffer 1999).
WHY ? (The advantages)
The current theory states that osteolysis (bone dissolving disease) around a total hip is caused by the reaction of the body’s immune system against the wear particles. The greater the wear particle production the higher the risk of osteolysis and subsequent failure of the prosthesis.
The production of wear particles is large only during the first two postoperative years, later one the production diminishes ("wear in" state)
In theory, metal-on-metal total hip joints, which produce much less of the wear particles, will thus have much fewer failures.
Moreover, some laboratory experiments indicate that metallic particles are less irritating than the polyethylene particles. Thus, the tissues around the metal-on-metal total hip would tolerate greater concentrations of metallic particles.
Laboratory studies also demonstrate that metal / metal total hip prostheses, which have large diameters of femoral head and cup components have very low friction.
Remember, however, that the laboratory conditions may not apply in the human body.
Concerns
High serum level of metals that constitute the metallic alloys from which the metal-on-metal total hips are manufactured is, however, cause for concern, because the serum levels do not drop after time. Thus, these patients are supposed to live with these high blood levels of metal many years. The risks of this situation, if any, are as yet not known.
The "old" metal on metal total hips (McKee) have been in place for several years, yet these patients still have elevated levels of trace metals, from which these prostheses were manufactured, in their blood and urine as the Table shows:
Metal levels Elevated (compared with normal controls)
Chromium in blood 9-fold Chromium in urine 35-fold Cobalt in blood 3-fold Obviously, the serum & urine levels of these trace metals remain high also when the prosthesis have been in place for several years. (Jacobs 1996)
Even patients operated on with the new metal on metal total hips have elevated levels of trace metals in blood and urine, compared with patients operated on with metal on polyethylene total hips of the same model.
The Table shows how much increased the levels of trace metals in the blood and urine after the total hip operation in two patients groups.
Note that both groups have had their total hip prosthesis in two years, that both prostheses models were identical except for that one had a metallic cup and the other had an polyethylene cup.
METAL Increase in Metal on Metal total hips Increase in Metal on Polyethylene total hips Cobalt in blood 24 fold no change Chromium in blood 2 fold 2 fold Cobalt in urine 103 fold no change Chromium in urine 29 fold no change
(McDonald 2002)
Moreover, the new metal-metal total hip models have both the cup devices and the femoral ball devices very large, to diminish the alleged friction.
Remember: Large diameter of bearing components makes for a large surface. The larger the surface, the more it wears, and the metal ions enter the blood circulation. On hand of some laboratory tests a group of surgeons maintains on the contrary that the friction and wear is lower in very large metallic bearing components (diameter >32 mm) (Daniel & McMinn, 2006)
Some scientist argue that laboratory experiments show that the wear in metal-metal total hips diminish substantially after the initial period of about two years ("wear- in" period). In spite of this laboratory evidence, the values of metal ions in the circulation remain high also after this "wear-in" period. -see the data above.
FOR WHOM
Metal-on-metal total hips are suitable for young patients because these patients are more active, taking more and quicker steps.
The young patients have excellent chance to live many more years
Thus, the young patients need a total hip prosthesis that will last long and will produce low quantity of wear particles. Metal on metal bearing couples produce lower volume of wear particles than metal-on-polyethylene bearing couples.
There are, however, two problems:
One is the increased level of trace metals in the blood of the patients. Because the metals are eliminated from the body with the urine, the patients with impaired kidney function have very high blood levels of these metals. It is thus wise to examine the kidney function in all patients who are potential candidates for metal-on-metal total hips.
Second, two coupling systems are recommended to young patients, because both produce low quantities of wear products:
the ceramic total hips and
the metal-on-metal total hips.
Then there are also metallic surface shells for surface replacement hip prostheses
Which of these systems should you choose?
At present there is no statistics that would indicate which system is producing better results. You should discuss these problems with your surgeon carefully.
The Operation and the Results
The metal on metal (m/m) total hips were in use before the polyethylene on metal total hips came into use. There were many manufacturers of these old m/m total hips and the accuracy of some products was not good. This may explain why besides many reports demonstrating rather high rates of these old m/m total hips, there are appearing reports on the very good results of these old m/m total hips. One recent report (Brown 2002) revealed that during 20 years of follow up, 84% of the old McKee-Farrar total hips survived.
There is a long row of reports on the laboratory behavior of metal-on-metal total hips, whereas the reports on the efficacy of these total hips in living patients are extremely scarce. Whereas one manufacturer claims that more than 100 000 metal-on-metal total hips have been sold, the two available reports describe the results of totally 300 operations. ( Dorr 2000 , Weber 1992)
The operation with the metal on metal total hip is not different from the operation with a conventional metal on polyethylene total hip. The published results show that the usual postoperative complications (nerve damage, dislocation of the new hip) occur also after the operation with the metal on metal total hip.
The studies show that the surgeons use both cemented and cementless fixation of the metal on metal total hip prostheses. The postoperative mobilization is similar to the mobilization after the metal-on-polyethylene total hips.
The published results (encompassing 47 patients in the last report, Dorr 2000) showed very good results. The majority of the patients experienced relief of pain (89 points on 100 points scale) and improved walking.
The patient self-assessment of the result 5 years after the operation is shown in the Table
| RESULT | Per cent of 47 patients |
| Excellent & very good | 89 % |
| Good | 7 % |
| Fair & poor | 4 % |
These results are equally good as the results achieved with the conventional metal on polyethylene total hip joints.
The patients in the last published report (Dorr 2002) have had a mean age of 70 years - not a specially young population. The mechanical failure rate and the rate of all revision operations during five years is shown in the Table
| Type of total hip | Rate of all revision operations | Rate of revision operations for loosening |
| metal-on-metal | 7,1 % | 2 % |
For comparison the five year results of conventional polyethylene on metal total hips in equally old population operated on for 25 years ago (Berry 2002)
| polyethylene-on-metal | 2,6 % | 1,6 % |
Risks
There are two main long term concerns with metal-on-metal total hip prostheses
Immune reaction against metallic particles.
The skin sensitivity against metal is well known. It was theorized, that similar sensitivity may be the cause of loosening of the prostheses. However, as yet it was not shown unequivocally that metal sensitivity and immune response of the body to the metals is responsible for loosening and failures of total joints.
The skin sensitivity tests show that up to 15% of people have skin sensitivity against Nickel and 8% against Chrome (contact dermatitis). Skin tests hypersensitivity against metals may not have any correlation with deep tissues immune response to the metallic parts of a total hip prosthesis.
In every case, if the surgeons have observed 15% failures of total hip joins associated with metal hypersensitivity, they would certainly be very concerned with the problem. The survey of the published reports show that they are not. See also the chapter Metal allergy details
(Archibeck 2000, Merrit 1996)
Cancer caused by metal particles/salts
Metals composing the alloys for manufacture of total hip joints are normally present in our bodies as trace elements necessary for function of hormone and enzyme systems in our bodies. In higher concentrations, however, these metals may cause harm. In laboratory experiments higher levels of these metals in the blood of laboratory animals produced cancer. The available industrial norms state threshold levels of these metals in the blood of the workers dealing with these metals.
In the 1990's, there appeared two reports claiming that the old metal-on-metal total hips were associated with increased risk of some forms of cancer. A third report, however, claimed that the observed increased risk of cancer in patients with a metal on metal total hip was caused by other factors than the presence of the metal on metal hip prosthesis (Visuri 1996).
Since then there were published many other reports in which the risk of cancer was either not found or was very low. In these later reports, however, the metal-on-metal total hips were not studied separately.
One manufacturer of the metal-on-metal total hips summarized these reports in the following way
"Various studies with a total over 54 000 total joint replacement patients showed a lower standardized cancer incidence rate than controls."
One recent report surveying about 40 000 total hip patients (not metal-on metal hips only) stated that
" the small but statistically significant increase in kidney and prostate cancers and decrease in gastric (stomach) cancer deserve further study" ( Nyreen 1995).
For more information see also the chapters: Life with the total hip and Total hip and cancer
Materials for MoM hip prostheses
All metal on metal total and surface replacement hip joints are made from the cobalt-chrome alloys. There are several reason for this choice of metallic alloy.
The cobalt chrome alloys are strong.
High strength of the alloy is basically due to the cobalt, itself a hard, strong metallic element. The alloy may be considered a solid solution of mainly cobalt, chrome, and molybdenum, other elements are present in lesser quantities. One important element, making only about 0,1 to 0,3 per cent of volume, is carbon. Carbon forms compounds called carbides with other metals in the alloy. The quantity of carbides in the alloy decide upon the strength of the alloy and about the wear resistance of the surface of the alloy.
The cobalt chrome alloys are resistant against wear.
The more carbide crystals are present in the alloy the higher is the wear resistance. Alloys with small amount of small carbide crystals have usually low wear resistance.
The cobalt chrome alloys are resistant against corrosion.
Resistance to corrosion is mainly due to the chromium component.
The main disadvantage of Cobalt-Chrome alloys is that it work-hardens very readily. The work-hardening is so high that, in fact, the alloys cannot be machined to shape. It is for this reason that the devices made from this material must be cast.
This poses a lot of problem for the manufacture of the devices made from cobalt-chrome alloys.
One way to avoid the problem with casting of cobalt-chromium alloys is to use the sintering process.
The component is shaped from a fine metallic powder of the alloy. The component is then subjected to high pressure of at least 1000 atmospheres at temperatures of at least 1100 degrees C, but below the melting point of the alloy, in an oxygen free atmosphere, such as argon. This is actually a Hot Isostatic Pressing procedure applied on the metallic powder.
The process produces plastic flow of the alloy, thereby collapsing voids and cavities between the small powder particles, creating a solid , strong device.
On the other hand, application of hiping (Hot Isostatic Pressing) procedure on the already cast cobalt chrome device depletes the device of carbide crystals. This may have drastic consequences for the wear resistance of the device.
It was shown that hiping of the cast metallic total hip surface made from cobalt chrome alloys increases the production of wear particles up to 60 times as compared with the "as cast" components..
As yet, the wear resistance of metal on metal joint surfaces have been tested in the so called hip simulators. The best such machines imitate the motions of the normal human hip joint very closely. Unfortunately, the results achieved with these simulators vary, so that it may be difficult to use these results to predict the wear behavior of the tested total hip joint when it will be used in living patients.
Size of the femoral ball component
The size does matter
The size of the diameter of the femoral ball component influences the properties of the total joint prosthesis. The size is then influenced by the characteristics of the materials used for manufacture of the total hip prosthesis.
1) Wear : The larger the radius of the femoral ball the larger the distance traveled by the femoral head on the femoral cup surface with every step. Because the wear from the joint surfaces is proportional to this distance, the femoral ball components with small diameter will produce less wear particles than femoral balls with large diameter.
This is true for metal-on-polyethylene bearing surfaces. The Charnley's metal-on-polyethylene total hip has a 22 mm large femoral ball and in laboratory experiments it produces lower wear rates than models with femoral balls of larger diameter.
Some scientist argue that for bearings composed of two hard surfaces, this relation may not be true because these bearing have another type of lubrication of their joint surfaces. Especially during rapid movement, such bearings are lubricated by a small layer of (joint) fluid that squeezes between the hard surfaces. The larger the diameter of these (congruent) joint surfaces, and the speedier the movement, the thicker the layer of the lubricating fluid and thus the smaller production of wear particles.
For the metal-on-metal total hip surfaces this implies that the larger the diameter of the cup and ball components the better their lubrication. With this mode of lubrication, the metal on metal surfaces will produce only small quantities of metallic wear particles.
Remember, however, that these laboratory studies apply only for relatively rapid movements in the total hip joint during rather speedy walking, not for movements in the total hip during slow walking.
2) Stability. The larger the diameter of the femoral ball component the larger the range of motion in the artificial hip joint, in general. (There are also other factors that influence the range of motion in the total hip joint). With the increase of the range of motion there is also increase of the stability of the artificial total hip joint. Calculation showed that increase of the the ball diameter from 22 to 28 millimeters increased the range of motion (to the impingement limit) with 8 degrees.
For hip surface replacement devices the range of motion, theoretically, will be limited only by the anatomy of the patient's hip joint.
The size of the femoral ball diameter is limited actually by the thickness of the cup component's walls.
Picture: Size of the cup component is dependent of the material
Cup component made from metal alloy (upper picture) usually can have walls 5 millimeters thick and still it will not deform. Thus for a femoral head with 55 millimeters diameter the outer size (diameter) of the cup will be 60 millimeters. Such size will readily accommodate into the socket of the hip joint in patients with "normal size of the skeleton.
Ceramic cup component (lower picture), on the other hand, must have thick walls. The thickness of the ceramic walls should be >6 millimeters, and the metallic back-up further 4 - 5 millimeters. Thus, the outer diameter of ceramic cup to accommodate 55 millimeters ceramic ball would be > 65 millimeters. The surgeon will be forced to sacrifice too much of the pelvic bone to accommodate such large ceramic-on-ceramic total hip into the patient's pelvic bones.
For this reason, total and superficial hip devices with large diameters of femoral balls (>54 millimeters) are manufactured as yet with metal-on-metal bearing surfaces. If the cross-linked polyethylene really will prove to be very wear resistant it will be possible to produce polyethylene cup components with thin walls (about 5 millimeters). This will make it possible to produce metal-on-polyethylene total hip devices with large femoral balls (>50 millimeters)
References:
Archibeck MDI, Clin Orthop 2000; 379: 12-21
Brown SR et al.: Clin Orthop 2002; 402:157-63
Dorr LD: J Bone Joint Surg- Am, 2000, 82-A; 789-98,
Brodner A et al: Z Orthop Ihre Grenzgeb 2000; 138: 425-9
Nyren et al: J Natl Cancer Inst 1995; 87: 28-33
Merritt Clin Orthop 1996; 329 Suppl: S233 -34
Schaffer J et al : J Toxicol Clin Toxicol 1999; 37: 839 - 44
Weber B: Z Orthop Ihre Grenzgeb 1992; 130: 306-9
McDonald et al. Metal on Metal..., Meeting of the Hip Society, Dallas, Texas, February 2002
Jacobs et al. Clin Orthop 1996;329 Suppl: S 256-63
Visuri et al. Clin Orthop 1996;329 Suppl: S 280-9
Daniel & McMinn et al. J Bone Joint Surg-Br, 2006, 88-B:443-48
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