Unbreakable total hips?

	Sulzer Company backed its metal on metal total hip model with “a Limited Lifetime Warranty on Metasul components sold in the United States”. Metasul TM was the trade mark name of the “ubreakable” super alloy developed by Sulzer Company for their total joints. Click on the icons for a full size picture
Sulzer’s Lifetime warranty on Metasul alloy

 Next to “heat-proof” I suppose that “unbreakable” is one of the most useful words in advertising. The message sinks in so that one still meets people who believe that there are unbreakable objects or, if there aren’t , then there ought to be. Nothing is, however, unbreakable not even the total hip joints, the only question is “how soon”?  

In the early 1970’s, with the increasing duration of use of total hip replacements, appeared increasing numbers of fatigue fractures of stem components in total hips. Although the heavy, younger, and more active patients were at higher risk, the problem was not absent for patients who were small, frail, elderly, or limited in activity. The revision operation of the broken stem was a complex and potentially dangerous procedure. So the stem breakages were the problem. 

The mechanism that caused the breakage of the stem components is called fatigue fracture. The technical side of the fatigue fracture involves much technological and engineering stuff, with use of sophisticated terms such as Young modulus, Wöhler diagram and like. I will obviate all these highly technical issues.

I will rather describe how different surgeons reacted on this unexpected complication and what measures they took to get “unbreakable” total hip joints. Although you would expect that the first measure would be to ask the engineers for stronger  materials for manufacture of total hips, it was not always the case. One leading surgeon for example proclaimed that “stainless steel is tending to be derogated, perhaps unfairly, in favour of expensive alloys containing high proportions of relatively rare metals (i.e. 35 % nickel). It would be a pity if this trend should go unchallenged…especially in view of the possibility of future world shortage of rare metallic elements.”

 

The stem breakages in 1970’s – why so late? 

The first reports on breakages of stem components of total hips appeared seemingly out of the blue in the 1973. Before that year I could not find any report on stem breakages. In the short period from 1973 through to 1976, however, there appeared twenty six papers reporting observations and statistics of breakage of the stem components of total hips. 

The reason why the publications describing the fractured stems did not appear earlier is still obscure. The total hip replacement went into general use in the late 1960’s. Thackrays began selling Charnley’s design in 1963 (in limited numbers first), Muller design was introduced in 1965. We know that in 1970 Thackrays, the manufacturer of Charnley’s total hip was producing about 9 000 total hips annually. We also know that Charnley admonished them to be prepared to increase their production ten times, to 100 000 total hip devices annually. Other manufacturers were producing and selling equally many or even more total hip devices, so that about 1970 there were accumulating several thousands of patients at risk for fatigue fracture of their stem components. 

The existence of fatigue fractures breakages must have been known to the surgeons and manufacturers already before 1973. Sulzer introduced a new metallic alloy in 1972 “to avoid fracture of loosened stems”.  Their total hip model, which was developed together with the Swiss surgeon Maurice Muller, had the highest rates of fatigue fractures. Thus, Sulzer must have reports about the alarmingly high rates of fatigue fractures of their product in good time before 1973. The development of a new metallic alloys takes namely about 3 – 5 years. 

 I may offer one (totally unscientific) explanation: The statistics were not published earlier because it did not suit the manufacturers to have these reports published earlier. The wave of publications on fatigue fractures that started 1973 created demand for “unbreakable” total hips. As the Sulzer’s example shows, about 1973  the manufacturers finished the development of new “unbreakable” total hips and were ready to satisfy the demand for the new “unbreakable” total hip devices. The reports on the fatigue fractures were thus encouraged in subtle ways since this date.

 

Symptoms of broken stems

The breakage affected only the anchoring stem of the total hip joint whereas the rest of the total hip joint, its ball and cup components were left intact. The breakage occurred usually in the lover and middle parts of the stem where the stem was slimmer and thus less resistant to fatigue fracture. The breakages in the upper part of the stem were less frequent because the stem was there more robust and resistant to fatigue.

Photo of a broken stem

 

	Roentgenograms of stems broken in their upper parts showed dramatic pictures of a spectacular catastrophe as on this Picture. Both broken parts of the stem lay apart; this is a picture of a catastrophe much like the pictures of broken bridges and collapsed buildings. Patients with such breakage of their stems usually presented with sudden, excruciating pain in the operated on total hip.
Roentgenogram of a broken stem

For the majority of patients with stem breakages located in the middle and lower parts of the stem, however, the pain in the replaced total hip was less severe but increased insidiously. Sometimes the surgeon even suspected a loose total hip initially (which it in reality was). The roentgenograms of these patients’ total hips showed less dramatic pictures. The breakage of the stem looked like a rather fine black line through the stem component. On roentgenograms of some patients the breakage line was almost invisible so that an inexperienced eye could miss it.  

For the unhappy patients with such “invisible lines” the pain in their hips could continue for months and even years. These patients were told repeatedly the usual phrase “your x-ray pictures show nothing wrong”. Eventually an experienced radiologist scrutinized carefully the whole series of the x-ray pictures. And there it was, the cause of the nagging pain: the hair-fine fracture through the stem.

 

Stem breakages – was it an epidemic? 

The reported incidence of fatigue fractures of the stem component ranged from 0.26% to 10.7%. This is a considerable difference, the highest incidence, 10.7%, is forty times higher than the lowest incidence, 0.26%,  but what caused it? The contemporary surgeons did not contemplate about why the incidence varied so much. But certainly, for the surgeon who encountered 140 broken stems among 1500 patients this was a real epidemic. 

 One reason for the differences in the incidence of broken stems was the statistical method itself.

The number of the observed breakages depends namely on the length of the observation period; the longer the surgeon follows up his patients the more stem breakages he discovers. So that the surgeon who follows up his patients for, say, five years will observe much higher numbers of broken stems than his colleague who follows up the patients for, say, two years only although both surgeons use identical total hip models. Obviously, the simple incidence figures of stem breakages were unreliable, comparison of different reports was misleading, but the surgeons were not much concerned.

Eventually, the British material scientist HS Dobbs demonstrated for orthopaedic surgeons the right statistical method; instead of calculating the percentages of patients whose hips succumbed to a fatigue fracture Dobbs taught the orthopaedic surgeons how to calculate the percentages of patients who survived without fatigue fracture. People throwing dice will understand the difference in the methods.

His paper written in 1979 has the spectacular title “The fracture of the femoral components – an Act of God?” In spite of the spectacular title the paper presents the first scientific analysis of the stem breakage problem. The term “Act of God” used in the title of the paper draws a little mystique over the whole issue, but it is actually an old statistical term denoting unpredictable happenings and has nothing religious in itself.  

  The majority of the papers on stem breakages reported failures of two total hip models, the John Charnley’s and the Maurice Mueller’s total hip devices. These two prostheses were different in all: the design, the materials for fabrication, and the frequency of r stem breakages. Perhaps most important:  their designers and manufacturers took completely different steps to solve the problem of stem breakages. 

	Maurice Mueller’s total hip, the model with the highest rates of stem breakages, was fabricated from cast cobalt chrome alloy. Note that the stem component is curved, rather like a banana. The mean rate of stem breakages of this model was the staggering 3.4%; but there were surgeons who observed 9% and 10.7% of stem breakages in operations with these total hip devices!
Mueller’s total hip

	The Charnley’s total hip was fabricated from annealed stainless steel and had a straight and rather small stem component. The mean frequency of stem breakages of this model was 1.3%. One report, however, written by the by the designer himself, John Charnley, gave the frequency of stem breakages as 0.26%. This was a statistics based according to the author “on some 6 500 operations”.
Charnley’s total hip

 

Fatigue fracture –the cause of breakages.

 The stem breakages were caused by a mechanism known to material scientists as fatigue fracture. (Fatigue fracture is an event by which the metallic structure breaks after many repeated cycles of loading and unloading. The important thing is that the loads that cause this fatigue fracture are small, lower than the metal’s nominal strength. Often the loads causing fatigue fracture make only 25 to 50% of the loads that the metal will sustain without breakage by one-time loading). The fatigue fracture of stem components resulted from interplay of many factors. For the case of simplicity I reduced them to three groups:

 

Material & Design	Fixation to     skeleton	Patient characteristics
↓	↓	↓
	Fatigue  fracture

 

Flaws in Material & Design 

The early total hips were manufactured from two materials, from the annealed soft stainless steel and from the cast cobalt chrome alloys. Both materials are well known for their low resistance to fatigue fracture. The surgeons chose these materials because of previous experience with them in other fields of medicine.

Stainless steel was used for manufacturing of plates, nails, pins and screws for operative treatment of broken bones. So the biocompatibility of this material was well proven by its long use in fracture surgery; the stainless steel was easy to work with, and there was a long experience with manufacturing devices from it. Besides, it was cheap. All these characteristics made it an ideal material for fabrication of total hips for the manufacturer with small means.

Instead of turning a fracture plate the manufacturer just turned a shaft component from the stainless steel rod and believed that it all will work well. No expensive tests and inspections were necessary.

Unfortunately, the stainless steel is not much resistant against fatigue fracture. This was widely known. The official publication of fracture surgeons (ASIF) warned: “The ASIF (fracture) implants are not prostheses. The load should be transmitted not only through the plate but through the bone as well. If, for any reason, this is not the case, and the implant alone assumes all bending stresses over a longer time, then an implant fracture becomes possible”.  But total hips devices were prostheses and they “assumed all bending stresses over a longer time” – so it is no wonder that total hips made from stainless steel eventually broke. One is rather wondering why the fatigue fractures did not occur more often.

The cast cobalt chrome alloys (Austenal, later named Vitallium) were in use for manufacture of devices for replacement of broken femoral heads and necks since 1950. So there was enough experience that showed that this material is excellently tolerated by the body.

But this material has other disadvantages. For one, it is more expensive than the annealed stainless steel. Second, the cobalt chrome alloys are hard and cannot be turned on a lathe, so the devices must be cast. This is a technique with great risk of producing faults in the finished product such as small bubbles of gas trapped inside the material, local cracks and other small irregularities formed during the casting process.

The fatigue fracture starts on places with such material faults as a small crack and develops eventually in a full-blown fracture. Therefore, devices with such defects must be disclosed at the inspection of the finished product and discarded, which is not always so easy because these defects are usually very small.

Even if the ready product, the stem component, was free from material faults, the material itself, the cast cobalt chrome alloy (Vitallium) was not particularly resistant against fatigue fracture. Actually, the soft annealed stainless steel and the hard cast cobalt chrome alloy have about the same fatigue strength (resistance against the fatigue fracture).

 

Faulty fixation of the stem – bone cement. 

All reports on broken stems described breakage of cemented stem components. It is important to realize this fact, because the broken stems were a part of a composite structure. First, there was the skeleton, the thighbone with its marrow hole. Inside this marrow hole there was placed the stem component, well cushioned in an even bone cement mantle. If all these three components kept together the stem component was really “unbreakable” –or almost so. Look at the Picture.

 

The upper illustration shows the forces acting on the stem component: the body weight and the muscle pull. Together they form a hip force. Please note that the hip force is greater than the body weight and that it bends the stem downwards and rotates it. The bending motions in the stem are very small if the stem component is firmly fixated to the skeleton with the bone cement. The hip force puts repeated peak loads on the stem component with every step, rising from chair, or managing the stair.   Illustration A in the lower row shows the ideal state: the blue stem component is enshrouded in a thick and even cement mantle (grey) and both are firmly seated in the marrow hole of the thighbone (orange). The uppermost part of the thighbone is well retained and gives good support to the cement-stem composite. (The circle) Even a weak stem component which is so well supported will sustain repeated big loads without fatigue fracture. Illustration B in the lower row shows what really happen to total hips in the body: The illustration shows that the uppermost part of the thighbone vanished (encircled). This actually occurs regularly, as the x-ray pictures show it, in 30 to 80% of all total hip replacements. The disappearance of bone takes time but it is established after some two years. Scientists and surgeons still argue why the rest of the femoral neck (the uppermost part of the sawed off thighbone) disappears and if it is important or not for the outcome of the surgery.  The illustration shows that the upper part of the cement mantle is gone too (encircled). Why? Perhaps the surgeon did not succeed to produce a perfect cement mantle at the operation, perhaps there was not enough room inside the marrow hole for the cement mantle, or the pressure from a too heavy and active patient destructed the mantle. In every case, weak or absent upper part of cement mantle is observed on x-ray pictures of about 30 to 50% of all cemented total hips. When the support for the stem on the inside of the thigh (the circle) disappears, the stem is transformed into a cantilever: the tip of it is still firmly fixated whereas the upper part of the stem is swaying free, offer for the bending hip force. If the hip force is too strong and the stem is too weak then the result is a fatigue fracture.

Forces acting on the stem (upper row); the cantilever mechanism (the lower row).

 

What to do?

Develop stronger metal alloys or improve the cementing technique? 

That was the question in the 1970’s. It is curious that the question was asked at all. Yet it is even more curious that the answer on this question depended on who examined the cases of broken stems: The surgeons with access to x-ray pictures of broken stems only had only one answer- the wrong cementing technique,  the surgeons with access to the metallurgical expertise saw weak materials with manufacturing defects in the first place.

The first group of surgeons satisfied themselves with the study of the x-rays of the broken total hips only. This was the easiest and cheapest way of inquiry. X-ray pictures were available for all patients; the surgeons himself assessed them and draw his conclusion from them. No need for expensive metallurgical examinations of the broken stems, no need for protracted discussions with metallurgists about the quality of material and its influence on the incidence of fatigue fracture.

On the x-ray pictures of the broken stems the surgeons saw a mixture of white and black shadows. The most distinctive white shadow was the stem component itself. Then followed  shades of grey. These were the shadows of  the opaque bone cement and of the skeleton. Sometimes the whole x-ray picture looked like a woolly lamb in the mist on a dark evening. (To convince yourselves look again at the roentgenogram of the broken stem). Yet the surgeons could in some miraculous way  always distinguish the quality and integrity of the cement mantle around the stem component on these roentgenogram. So it is not surprising that this group of surgeons proclaimed that the bad cementing technique was the sole culprit for the fatigue fractures.

John Charnley, whose manufacturing company Thackrays moreover faced litigation from patients whose weak “flat back” stems broke, wrote in a letter to the manufacturer  in 1974 “ the failure (of the total hip) in the body is the result more of the surgical manner in which it is inserted than the metallurgy of the implant…”.  

In his paper 1975 Charnley continued his defence of the weak annealed stainless steel used for fabrication of his total hips: “The patients especially at risk are males weighing over 170 lb (75.5 kg) and from these is an obvious need for a heavier design of prosthesis. Whereas the overall fracture rate is only 0.23 per cent, the rate for males over 196 lb (88 kg) is 6.0 per cent. It is believed that defective surgical technique, in failing to provide adequate support by cement to the concavity of the upper levels of the prosthesis is probably the main cause of fracture and reasonably good cement technique explains why the fracture rate is not higher in the present series. Indications for improving cement technique are outlined.” All blame is elegantly put on the clumsy surgeon who failed to provide “adequate support by cement”, the possibility that weak material might be the culprit is not even mentioned.  

Following his conviction  Charnley introduced in 1975 a new so called Cobra stem component. It had a “3 mm of metal added to the lateral border of the stem (which) enhances strength by more than 10%... A system of cold coining is expected to raise significantly the tensile strength over the annealed metal and bring it in an improvement in fatigue resistance”. There are no figures to show how great this “improvement” was in reality, the author does not present any results of laboratory tests; I am not sure whether such tests were  carried out. How worked the new Cobra model produced from the old material?

In 1984 two renowned American surgeons described stem fractures of “extra-heavy, large Charnley serrated Cobra femoral components”. The conclusion of their report says: “Making a prosthesis thicker by using the same material does not solve the problem of fatigue fracture if the material, design, or cement interface is at fault.”

Ingenious total joint surgeons were seldom ingenious material scientists too. Most of them sought therefore the advice from material scientists. John Charnley’s biographer William Waugh, however, noted “These were complicated metallurgical matters (fatigue fracture) but Charnley did not hesitate to express his opinions, which sometimes caused a little irritation”. Sometimes these opinions were, however, in disagreement with basic laws of material science which was worse. For example in discussing the advantages of stronger materials Charnley proclaimed “The message for the orthopaedic surgeon therefore is that for a given size and shape of prosthesis the stiffness cannot be increased merely by changing the alloy.”  

The statement is wrong. Different materials, inclusive of metals, do have different stiffness. Using stiffer metal alloy for fabrication would indeed make the total hip more stiff; but to what purpose? On the contrary, many surgeons were striving to produce less stiff total hip joint, with stiffness closer to the more elastic skeleton.  Did the author perhaps confused “strength” with ”stiffness”? According to J.E. Gordon, professor of material science, the confusing of these two completely different material characteristics is typical for lay people. Not probable. But even if Charnley meant “strength”  the statement is wrong: Total hips fabricated from stronger, more fatigue resistant alloys would be stronger and resist fatigue fractures better.

Very probably John Charnley  simply wished to have stiff stems in his total hip models. He was convinced that a stiff stem component would protect the bone cement from damage. Therefore he wished to have a stiff stem component, not caring for people who were concerned for so called stress shielding in total hip models with stiff stem components.

The future development of Charnleys total hip model demonstrated it. In the 1980, one year after the publication of Charnley's book, Thackrays changed the alloy for manufacture of Charnley’s total hip models using a new stainless steel super alloy with twice the fatigue strength of the old annealed material.

The conviction that bad cementing technique is the main culprit of the fatigue fractures had, however, a hard life. In the 1984 wrote two renowned British scientists (one surgeons and one material scientist) “All in all, the available evidence indicates that a stem exemplified by the Charnley “flat back”, manufactured from either 316 stainless steel or cast cobalt chrome …will rarely break when well supported”.

When I read such statement, I always ask:  Was it realistic to expect perfect bone cement support of stem component in all patients operated on? Do all surgeons succeed to produce an evenly thick cement mantle round the stem?  Do all patients have a marrow hole large enough to accommodate a thick cement mantle? Should  only frail, thin, immobile patients benefit from total hip surgery? Should the operation be denied to younger, active patients who have the whole life before them only because the material of the total hips is weak?

 

Weak materials were the culprit – new alloys were needed

 There was the other way how to get rid of the fatigue fractures. This was a laborious and also more expensive way: To study carefully the broken stem components, find why the stems broke and then develop new stronger materials and new designs that would make the stems “unbreakable” even with imperfect cement mantle. Such "unbreakable" total hips could be inserted also in younger patients with active lives before them.                          

The scientists studying the fatigue fractures estimated first the stresses that occurred in the stem component by the action of the hip force (body weight). They then compared these stresses with the strength of the material used for fabrication of the stem. If the stresses in the stem exceeded the fatigue strength of the material from which the stem was manufactured then there was a increased risk that the stem will fail. 

These studies demonstrated that stem components fabricated from annealed stainless steel and cast cobalt chrome alloys were really weak. Weight of 80 kilograms put on the Charnley’s stem component in the laboratory produced stresses in the stems that exceeded the fatigue strength of the material. Charnley’s observation was right: patients with total hip weighting > 75 kilograms  were at increased risk for fatigue fracture.

 Not only were the materials weak, they were also full of flaws. Concise metallurgical inspections of the broken stems in the 1970’s demonstrated metallurgical defects in practically all broken stems. All studies of the broken stems pointed out that these defects were the places where the fatigue fractures started.

This is so because such material flaws, even if they are almost microscopic in size are places where the stresses concentrate. The concentration of the stresses cause that the stresses increase enormously at these places and cause a break that continues a little through the material with every repeated load. It is only a question of time until a complete fatigue fracture ensues. The scientists call these small defects “stress risers”.  Look on the Picture

On the right side is the picture of the femoral component loaded with body weight (hip force). The hip force creates tensile stresses in the shaft. The shaft component has a very small crack in the middle (circle).   The left picture shows the stresses around the crack – it is a magnified picture, in reality the crack may be only a tenth of millimeter wide. You see that stresses (the red lines) flow evenly through the intact material of the stem. There is enough space between the stress lines demonstrating that the stress in those areas is low.  In the crack region, especially at the tip of the crack, the stress lines become very crowded demonstrating that the stress (i.e. the load per area unit) is increased. The high local stress breaks the material at the tip of the crack so that the crack propagates. A small defect becomes larger, the crack spreads a little with every new load and it is only a question of time before the whole stem will break apart.

Stress concentration in the crack region of the femoral stem component

 Why were these material defects so frequent in the 1970’s when they have so deleterious effect on the function of the total hips? One reason was that the knowledge about the deleterious effects of these material defects did not transpire to all material scientists and manufacturers. It is possible that some material engineers in the early 1970’s still did not believe that these minuscule material defects, the small cracks on the surface of the total hips were so important. But they were. Scrupulous examination of broken stems discovered for example that a serial number of the device etched on the stem created a very small defect on the surface of the stem that was the starting place for the future fatigue fracture.

Introduction of rigorous quality control of finished products played decisive roll for improvement of performance of modern total joint implants. ___________________________ Quality Assurance in the 1960s' at a big implant manufacturer.   Upper picture shows a certificate for an individual hip prosthesis, which includes data on chemical analysis of the material, mechanical properties, and radiographic test of the device Middle picture shows a radiogram of the Moore hip prosthesis. The radiogram would discover possible flaws within the material, such as cracks and gas bubbles. Lower picture shows the mechanical testing. The prosthesis is subjected to loads that exceed the loads that will act on the prosthesis in the body. Such tests were done on all hip prostheses put onto market by this manufacturer (Zimmer).

Inspection and Quality Assurance of Moore Prosthesis (Zimmer, 1969)

 It is also probable that many smaller manufacturers in the 1970’s did not have sufficient equipment to discover the material flaws in the ready product; careful inspection and quality assurance of the finished products were (and still are) expensive. The quality assurance process required (and still requires) well well equipped laboratory and  well educated people, which was and still is very expensive. Many small manufacturers thus left successively the market because they could not compete in the quality of their products. On the other hand, in the 1960's and 1970's there were even unscrupulous manufacturers offering cheep copies of “big name prostheses”. ( About as you may buy a cheap “original” Rolex watch on some markets). The manufacturing quality of these products was exceptionally bad. These were 1970’s, there was no authority controlling the quality of total joints. 

In conclusion: the fatigue fractures of stem components in the early 1970’s were caused mainly by weak materials with many fabrication flaws that were used for manufacture of the early total hips. The inspection of the quality of total joints was practically absent what made the situation still worse.  Other factors contributing to the development of fatigue fractures were bad fixation of the weak stem components and overweight /activity of the patients. 

Especially the last factors, the patient's weight and activity were often discussed as culprits.  In the 1970's namely, the prevailing opinions among surgeons was that the patients themselves were to be blamed.  First the patients destructed their hip joints because they were too active, this opinion said,  and then they destructed their new total hip joints because they became pain-free and too much active again. Should the patients ever enjoy their newly won good motion?

 The known English  hip surgeon R.S. Ling wrote in discussing the causes of failures of total hips: "The surgeon must not forget that patients suffering from osteoarthritis of the hip have already succeeded in destroying their own hip joint". In this climate it seemed natural that some surgeons  recommended "appropriate selection of patients and sensible control of activity levels on the part of the patient" as an alternative to the development of super alloys for strong total hip devices to prevent the fatigue fractures of the total hips.

 These surgeons even threatened with dire consequences if the manufacturers would introduce strong materials for manufacture of total hips. John Charnley wrote in 1979 "To avoid fatigue fractures of femoral prostheses many surgeons imagine that all that is needed is for prostheses to be made out out of some alloy possessing exceptionally high resistance to fatigue. But, even if this were possible, there would then almost certainly be an increased rate of re-operation for late loosening...". Was this prophecy correct? The author of this prophecy evidently changed his opinion quickly; already one year later, Thackrays, the manufacturer of Charnley's total hip model began to use a super strong stainless steel Ortron for the manufacture of the Charnley's total hips. Neither did I find any proof in the published statistics that these new strong total hips had high rates of loosening (except for one lone paper from South Africa).

 

Race for “the best alloy”. 

The concept of offering total hip replacement only to small, immobile and frail patients was, of course, untenable in the length. The common sense said to the surgeons in the early 1970’s that there should be total hip models strong enough to be offered to younger, active patients without risking fatigue fracture.

This concept needed collaboration with material engineers and manufacturers. This was also an expensive way.

According to Dr Semlitsch, Head Consultant to Sulzer Medicinaltechnik, for laboratory testing of a new material’s characteristics one needs a charge of 2 to 10 kilograms of the studied alloy. This is an expensive procedure. For the first industrial tests one need somehow between 20 and 200 kilograms of the new alloy, still an expensive procedure. But for the commercially effective production the industry will produce between 5 and 50 tons of the material annually. The whole process takes about 3 to 5 years, the costs were in the 1980’s some “millions dollars”. If the product is successful, the expenses will return in about 10 years. All these figures are data from the  1980’s.

Obviously only large companies with solid economic ground could participate in the development of new “unbreakable” alloys. But it was also the only way to improve the safety of total hip replacement surgery. The race for “unbreakable” super alloys started.

	Race for super alloys. Although all big manufacturers successively developed their own metal alloys for their new total joints models, it is assuring to know what the leading material scientists said: “There are no winners in this race; all alloys follow the international standards and are best for their special purpose.”
Zimmer Company advertises its new super alloy “Zimaloy”

The first company to introduce a new “unbreakable” alloy was probably the Sulzer Company, the company who manufactured the original Muller prosthesis that broke so often. The company introduced in 1972 the high-strength cobalt-nickel-chromium-molybdenum alloy to “avoid fracture of loosened stems”. What a long name but an effective product! According to the manufacturer “Since then, stem fracture is unknown for these prostheses” This example demonstrates nicely that big manufacturers learned from the previous failures. On the other hand the Sulzer Company is a large industrial complex with huge metallurgical knowledge; the medical branch of the company could exploit all this experience.

Also Charnley’s total hip model benefited from Sulzer’s experience. In the 1980’s Thackrays in collaboration with Sulzer introduced the high-strength steel containing such “rare metals” as Nickel and Niobium for manufacture of the Charnley hip model. The strength of the steel was moreover increased by adding a gas – Nitrogen - to it.

The manufacture of cobalt-chrome alloys was also changed. The strength of these materials was increased by forging of the cast product.

In general the fatigue strength of the new “super alloys” is today about twice as high as the fatigue strength of the old alloys from 1970’s. Moreover, in the mid 1970’s the manufacturers began to use a new material with new exciting characteristic, Titanium alloys, for manufacture of total hips. But that is another history.

The control of the quality of finished products and registration of material failures of total joints improved enormously since the early 1970’s. The process started in the USA where the Federal  organ FDA created special database for registration of failed medical devices. The European countries followed with creation of a certifying authority for certification of new total joint devices. 

 

Are the modern total hips really unbreakable?

The answer is: Unfortunately not! The fatigue fractures of modern total hips became rare, but did not disappear entirely. In Sweden today (1999 -2003), the revision operation for “Implant fracture” still makes about 1.5% of all revision operations.

The mean incidence of fatigue fractures of the stem

in the papers published between 1973 – 1975  was 2.6%.

in Swedish National Hip Register twenty five years later (1999-2003)  was 0.15%,

almost twenty times lover. These figures show that it was worth to bet on the development of new strong alloys even if they still are not totally “unbreakable”.

The modern total hips are often used without cement fixation but the abandonment of cement fixation did not remove the fatigue fractures entirely. On the contrary, especially in the beginning of their use, some cementless total hip models (Lord, Judet) produced true epidemic of fatigue stem failures. Again, this is another history.

The sudden occurrence of the fatigue failures of total hips in the early 1970’s had positive effects on the development of total joints.

First, the surgeons recognized that they must collaborate with material scientists.

Second, the fatigue fracture of the two commonly used alloys (stainless steel and cobalt chrome alloys) helped in the introduction of a new material – the Titanium Aluminium alloy.

Third, the study of the incidence of fatigue fractures helped in the introduction of a new statistical method – the survival statistic.