The characteristics of orthopaedic alloys
The total hip prosthesis is not made from pure metals, but from orthopaedic metal alloys, specially produced for fabrication of artificial joints.
The demands on the orthopaedic alloys are hard. The alloy must be very strong - they must not break or even bend permanently under heavy load not too stiff - a too stiff device will "shield" the skeleton too much from the body weight (see also Function of THP / stress shileding) biocompatible - must be well tolerated by the bone tissue.
ORTHOPAEDIC METAL ALLOYS
Every big manufacturer of artificial hip joints has developed one or more metal alloys to meet this requirement for different types of artificial joints he produces. New metal alloys appear continually on the market and the old alloys are withdrawn. According to the base metals in their composition there are :
the base metals are cobalt (> 34%) and chrome (>19%) mixed with smaller quantities of other metals, even nickel.
the base metal is titanium, commercially are used alloys with (ca 4%) alluminium
Stainless Steel alloys
the base metal is iron (> 58%), mixed with larger quantities of chrome and nickel and some other metals
All orthopaedic alloys, although produced under different trade names, have similar composition and meet the same technical standards. The mechanical performance of modern alloys used in total hip joint manufacture is very satisfactory and there are no absolute "winners" in this "race of alloys".
Trabecular metal - porous tantalum or Spongy metal
|A new form of porous coating is the "spongy" or trabecular metal coating. This is made from tantalum metal which has a "spongy" structure and is also called trabecular metal because of its likeness with the trabecular bone (spongious (sponge) bone). The microscopic picture of the trabecular metal shows that it is composed of microscopic beams of pure tantalum metal which look like the microscopic beams of the trabecular bone. This structure allows much more ingrowth of bone tissue than the usual metal coating with microscopic balls or net made from titanium|
The engineers can control the thickness of the beams and thus the rigidity of the resulting product.
(Click on the icon for a full size picture)
The trabecular metal has one big advantage: its mechanical characteristics come very close to the mechanical characteristics of the spongious bone itself. It is thus used mainly in reconstructive procedures where it replaces the lost bone.
So the bioengineers often use trabecular metal to make parts of skeleton, for example parts of a destructed pelvis bone, from this material.
Whereas the traditional porous coatings allows ingrowth of bone tissue some tenths of a milimeter, the trabecular metal allows much greater ingrowth of bone tissue.
|This picture shows how the "traditional" surface coating made from pure titanium beads (the lower row picture) is different from the bone structure (the upper row picture). That is why the titanium surface coating fails often because the two structures (bone tissue and sintered metal beads are too dissimilar.|
It also shows again how the trabecular metal's architecture (the middle row picture) is like the architecture of the spongy bone. This explains why the spongy metal is so excellent replacement for the bone tissue - it has similar mechanical characteristics as the spongy bone itsels
Mechanical characteristics of orthopaedic alloys
(The scale is relative)
Biocompatibility - ( means well tolerated by body's tissues )
All modern alloys are well tolerated by bone tissue - in bulk form. The best tolerated is Titanium in pure form. For this extreme biocompatibility, pure Titanium is often used as porous coating for the surfaces of total hip prostheses.
In dust form, as wear particles, all these alloys, even a pure Titanium, may, however, trigger osteolysis if they land in the tissues around the total hip prosthesis. Metallic wear particles in the soft tissues paint the tissues black, this is called metallosis.
Metal allergy in patients with total joints
The metallic alloys used for fabrication of artificial joints undergo corrosion and release metallic ions into the patient's body.
The metallic ions of these metals (Cobalt, Chromium, Nickel, but also the relatively inert Titanium) may combine with patient's proteins and trigger allergic immune response. One such allergy reaction is skin rash observed (very seldom) in patients with orthopaedic metal devices implanted in their bodies.
Allergy to metal is tested by skin patch test; the scientist speak about skin sensitivity. It is not clear whether the sensitivity of skin to metal is related to sensitivity to metal in deep tissues such as join capsule and soft tissues around the joint.
The frequency of skin sensitivity to metals in patients with artificial joints is substantially higher than that in the general population. (Hallab, 2001)
| ||Percent Metal Sensitive|
|General population||10 %|
|Patients with stable total joints||25 %|
|Patients with loose total joints||60 %|
At present (Hallab 2001), the risk to patients to develop such skin reaction after implantation of artificial joints may be considered minimal.
The relieable diagnosis of metal sensitivity is still difficult because of lack of reliable tests.
Two questions arise:
Is the sensitivity against metal one of the causes of failure of total hip replacement?
In 2001 (Hallab 2001), there was no convincing proof of this idea in the available data. But in 2006 there are papers suggesting that the sensitivity to metals may be a cause of metal on metal artificial hip failure (see the chapter Metal allergy details).
Can patients with known skin hypersensitivity against any of the "orthopaedic metals" (Chromium, Cobalt, and Nickel) have a successful total hip replacement operation?
At this time, there is no evidence that there is an increased risk of a reaction to an implanted artificial joint in patients who have skin sensitivity (proven by skin patch method). (Hallab, 2001)
Statistics demonstrate that many patient with a positive cutaneous (skin) test against some of the "orthopaedic metals" have a well functioning total hip prosthesis. Your surgeon, however, should be informed if you are allergic against any of these metals and he will also decide about the necessary preoperative tests and about the type of prosthesis.
If you wish to know more about allergic reaction against orthopaedic metal alloys see also the chapter Metal allergy details
Fatigue fracture of total hip prosthesis
The everyday life puts astounding demands on the materials of the total hip joint. For example, a sixty-year-old patient who weighs 75 kilograms and will live further 17 years ( a quite common characteristics of a "normal" TH patient), will expose the shaft of his total hip for thirty-four millions blows, each blow with a force of 200 kilograms if he goes slowly, and 600 kilograms if he is running.
The shaft of the modern total hip prosthesis will sustain such large loads, if they occur occasionally; the shaft may fail, however, even for lower loads, if they occur very often. The metal alloy will succumb to the so- called fatigue failure and break. There is a limit, how much repetitive loads the prosthesis will eventually sustain. This limit is specific for every form of the total hip prosthesis and for the metal alloy used for manufacture. Above this limit, the prosthetic shaft will sustain the fatique fracture.
All modern alloys used for manufacture of the total hip prosthesis are strong enough to resist fatigue fractures from these repeating stresses in average, not extremely heavy patients. There have been reported, however, very occasional cases of fatigue fractures of the modern prosthetic shafts. Closer examination of these cases revealed that the fractures occurred in heavy patients, often after an accident. The examination of the broken shafts often revealed metallurgical defects in the metal of the shaft, such as scratches on the surface, defects that occured during casting, etc. .
Many manufactures have also developed bulky models of artificial joints with larger dimensions for heavy-weight patients. Usually patients >100kg body weight are considered heavy-weight.
Stress shielding - a too stiff shaft
The prosthetic shaft takes off a part of the stress that walking and other everyday activities put on the upper part of the thigh bone holding the prosthesis. A too stiff shaft of a total hip prosthesis "stress shields" the upper part of the thigh bone to much. This is so because the alloys used for fabrication of the shaft are much stiffer than the skeleton of the thigh bone. The shielded bone does not thrive, loses its substance, and becomes weak. The total hip joint has weak anchorage in a weak skeleton and may fail. See more in the chapter Function of a THP
Titanium alloy has the lowest stiffness of all orthopaedic alloys and therefore shafts of cementless total hips are often made from Titanium alloys
The stiffness of the shaft component is also dependent of the form of the shaft - of its cross-section area. The smaller this area the less stiff is the shaft.
Thus, when contemplating to produce a shaft component with stiffness close to the stiffness of the bone, the engineer must consider not only the characteristics of the material but also its structure and its form.
The latest technique for production of less stiff total joint prostheses is the Trabecular Metal Technology. A metallic sponge made from Tungsten has about the same stiffness as bone. When a layer of the metallic sponge is placed on the surface of the total hip prosthesis, it will make a smooth transition from the stiff metal to the weak bone. The scientists hope that this technology will diminish the stress shielding effect of the too stiff total hip and knee prostheses (See also the chapter Cemented and cementless TH).
Metallic surfaces in contact with body's fluids corrode. Their surface dissolves and the dissolved metals enter the circulation. The concentration of the metals (Cobalt, Chromium, or Titan) in the blood increases. The orthopaedic alloys are very resistant to this corrosion. Yet, the corrosion occurs when
- two dissimilar metals are in contact - this happens in modular total hip stems, at the junction of the ball component with the taper of the shaft component . When both the ball and the stem are made from Cobalt - Chrome alloy, slight corrosion is observed in about 6 % of the components. When the ball is from Cobalt-Chrome and the stem is from Titanium alloy, the corrosion is observed in 33 % of the components. (Collier 1995)
- In metal-on-metal total hip joints, there appears wear of the joint surfaces, with production of many small particles of metal alloys. These small particles together have enormous area, they corrode and dissolve in the body fluids.
As a result of these processes, the concentrations of Titanium, Chromium, and Cobalt in the blood and urine of patients with these prostheses are elevated. (Jacobs). The trace-metals Cobalt and the Chromium are a part of body's enzyme system, but these metals have caused cancer in workers exposed to large concentrations of these metals. As yet, however, there is no proof that elevated serum levels of Cobalt, Chrome, or Titanium produce pathological changes or incite cancer in patients with these total hip prostheses.
Blood levels of Aluminum, a metal which is a part of the Titanium alloys, are not elevated in patients with total hip prostheses manufactured from Titanium alloys.
The question of the long-term effects of orthopaedic metals (Cobalt, Chromium, and Titan) on patients with total hip replacement is still not decided. (Jacobs)
Chromium and Cobalt are excreted by kidneys; in patients with impaired renal function and corroding total hip prosthesis, the blood concentrations of these metals are very high. (Brodner) Thus, patients with impaired renal function should have total hip prostheses with do not produce elevated levels of these metals in the blood of these patients.
Corrosion resistant orthopaedic steel alloys and other orthopaedic metal alloys are not ferro-magnetic. Thus, patients with these prostheses can be examined with MRI.
Technical details :
Hot Isostatic Pressing
All metal alloys used for manufacture of orthopaedic implants are solidified solutions of crystals. All, at some stage, are melted and allowed to cool in a mould. During cooling the metal alloy crystallizes and contracts. The crystals are known as grains and may vary greatly in size.
Very large grains, as can occur in cast cobalt chrome alloy, can lead to catastrophic failure of implants.
During cooling, as the material shrinks, there appear voids in the structure of the cold alloy.
For optimum mechanical properties of the metallic orthopaedic device, the crystal size in the alloy must be uniform, the structure must be free of voids, and the alloy should not contain any impurities.
Because it is virtually impossible to prevent some of these defects to occur in cast materials, the manufacturers use mechanical working of the cast alloys to close the voids between individual crystals and to expel the impurities.
One such method is called Hot Isostatic Pressing (HIP or "hiping") of cast materials. In this process the components are subject 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. The hiping is particularly suitable to improve mechanical properties of cast cobalt-chrome components. The process produces plastic flow of the alloy thereby collapsing voids and cavities in the material that might have acted as initiators of device fracture.
Hiped alloy is stronger than "as cast" alloy, but hiping also changes the microstructure of the alloy. The carbides present in the solid solution of the alloy are driven out of the finished product. This process may drastically change some important characteristics of the product, such as wear resistance.
From Black, Orthopaedic Biomaterials, 1988, and 1995
Collier JP et al. The tradeoffs associated with modular hip prostheses. Clin Orthop, 1995; 311: 91-101.
Hallab N. et al. Metal sensitivity in patients with orthopaedic implants. J Bone Joint Surg-Am, 2001, 83-A:428 -33.
Jacobs JJ. Metal Release in patients who have had a primary total hip arthroplasty. J Bone Joint Surg-Am, 1998; 80-A: 1447-58
Brodner W et al. Serum cobalt and serum chromium level in 2 patients with chronic renal failure... Z Orthop Ihre Grenzgeb 2000; 138: 425-9