(Chapters from history of total joints)

Modern engineer is today still much better equipped to construct a machine to withstand great loads than he is able to design a system of bearing surfaces with minimal friction and very low wear.

Total hip joints are machines and their development followed the same lines. The history of the development of total hip joints fabricated from the new strong titanium alloys illustrates well this fact. 

When the femoral shafts were succumbing to the fatigue fractures in the 1970’s, the bioengineers reacted quickly and promptly developed strong, fatigue resistant materials for manufacture of new strong total joints. Prominent among the new super strong materials were the titanium alloys.

The mechanical characteristics of the Titanium alloys were very advantageous: The alloys were twice as strong as the old alloys (stainless steel and cobalt-chrome) used for manufacture of total hip joints. The titanium alloys were 50% less stiff than the “old alloys”. Moreover, all previous experiments and observations demonstrated that titanium alloys, like the pure metal, were excellently tolerated by the body tissues.  

On the negative side was, however, the low resistance of the titanium alloys against wear. Thus, whereas the contemporary engineers were quick to develop the new strong material, they were less able to cope with the wear of the new material.

In cases where titanium alloys have been used in total hip joints for their mechanical characteristics only, there titanium alloys proved excellent material. One such example is the cementless total hip joint, where only the shaft component itself is made from the titanium alloy. The femoral ball component in these successful total hip systems (eg. Zweymuller) is manufactured from another, low wear material, such as ceramic. In cementless total hips there are no bone-cement particles to rub and abrade the surface of the stem components made from the titanium alloy.

The development of titanium-made total hips thus followed the usual way: After short laboratory testing the new product was introduced on the market. Successively the titanium made total hip joints gained general acceptance and widespread clinical use. It was first in this late stage that the problems emerged: the titanium made total hips wore off too much. And later the surgeons even began to question whether titanium particles released from total hip joints are so well tolerated by the body tissues; from the original enthusiasm to the questioning whether titanium really is so biocompatible.

 The way out of the wear problems of titanium-made total hips was to provide the titanium surfaces with a hard, stable surface that would withstand wear. Was it technically possible?

 

Long knowledge of titanium alloys 

Titanium and its alloys were used by orthopaedic surgeons since 1950’s when the surgeons began to use of titanium-made plates, screws, and pins for operative treatment of broken bones. One of the first papers about titanium, published in 1951, has the inviting title: “Titanium, a metal for surgery.” It was thus known to the orthopaedic surgeons that this metal is very well tolerated by the body’s tissues. 

There were some early warnings against the use of Titanium for manufacture of total joints too. In 1966, McKee warned for use of titanium in bearing surfaces of metal-on-metal total hips because of wear and galling when these materials articulate against each other. But McKee did not say anything about the wear characteristics of titanium when it articulated against other materials.  

Elasticity the most important characteristics 

For the majority of surgeons just then the most important characteristics of the titanium alloys was their surprising elasticity, The titanium alloys were 50% more springy than other alloys used for manufacture of total hip joints, such as cobalt-chrome alloys. Why was the elasticity so important? It was mainly the question of an even distribution of the stresses produced in the stem component by the body weight.

At every step the ball of the total hip is subjected to the hip force, a resultant of the body weight and the muscle force. The hip force produces stresses in the stem component. In an elastic stem component these stresses are distributed evenly and transported through the whole stem.

Elastic stem component would guarantee even transport and distribution of stresses in the bone cement sheath that surrounded the elastic stem.

 The even transfer of stresses through the bone cement down into the whole skeleton would keep the skeleton strong. The skeleton needs these rhythmical loads to thrive. The strong skeleton, supporting the bone-cement sheath with the stem component will thus guarantee the longevity of the implanted total hip. This argument is summarized in the following Picture: 

The Picture shows a greatly exaggerated schematic illustration how different stems transfer the load from prosthesis through bone cement to skeleton (rose in the picture).  Left picture: unloaded stem component is placed inside the bone cement (grid). The grid (inside the thighbone’s marrow hole) has regular appearance. Middle picture: The stiff femoral stem is loaded. The stem does not yield and consequently it transfers the stresses unevenly onto bone cement. The grid is distorted, most at the tip of the stem. The high stress concentration at this place may crack the bone cement with risk of future loosening of the whole stem component. Right picture: The elastic stem is loaded: the elastic stem yields and distributes the stresses evenly onto the bone cement.  The grid is minimally distorted, meaning that the stresses within the bone cement are still tolerably low. This means low risk for future loosening of the total hip joint.

Picture: The actions of the stiff and the elastic stem component Click on the icon for a full size picture.

Look again at the whole picture. As the picture stands it is an excellent argument to buy just the titanium-made total hip joint model. But  is it also a pure science? In reality, as said a known biomaterial scientist Jonathan Black, a detailed mechanical analysis of the interface between bone-cement and skeleton is almost impossible “owing to its microstructural complexity”.  

Moreover, not all surgeons in the 1970’s would subscribe to the argument that elastic stem components are so advantageous for success of total hip replacement. On the contrary, many of the surgeons believed that a rather stiff stem component protects the bone cement best.  These surgeons, such as John Charnley, would say: Look at the picture of the elastic stem component, see how much the titanium-made stem component bends. Don’t you see that this too elastic, rhythmically bending stem component must eventually damage the bone cement?

Actually, these surgeons would say that the whole above picture is an argument against the use of elastic stems! In retrospect, these latter surgeons were right, although for another reason. As the continued history demonstrated, titanium made total hips, although so springy, were not suitable for use just with the bone cement. There was John Charnley right.

This incompatibility between the bone-cement and the wear-prone titanium total hips was unknown to the surgeons in the early 1970’s; these surgeons were implanting the titanium-made total hip joints always with use of bone cement. Actually, in the USA these new titanium-made total hips were sold with the label:  “Marketed for use with bone cement in the USA”.  The hard bone cement, however, would abrade the soft, wear susceptible titanium shaft components. Paradoxically, as the later development demonstrated, the label should read: “Not to be used with bone cement”.

Picture: Label on Zimmer’s Multilock total hip

 There was another wear related problem with titanium-made total hips; the femoral ball component was made from titanium alloy too. The first titanium-made total hips had the femoral component manufactured in one piece, ball and stem together. The femoral ball component  then articulated with a cup made from polyethylene. (See the following picture.)

 Both the surgeons and the manufacturers believed that titanium-made femoral ball articulating against polyethylene cup would would produce only small quantities of wear products. The surgeons and material scientists, with few exceptions, did not dream in the 1970’s that the titanium femoral balls articulating against polyethylene would wear too much.

In the early 1970’s the theoretical grounds for the manufacture of total hip joints made from elastic titanium alloys were thus ready.  As we have seen, these grounds were not so rocky steady, they were subject to different interpretations; the general acceptance of titanium-made total hip joints by orthopaedic surgeons was based on the surgeons’ rather intuitive feeling that titanium is the metal that is best tolerated by the body. 

The first titanium-made total hip joints in the USA were developed by the Zimmer Company in collaboration with professor Augusto Sarmiento, Head of the Orthopaedic Department at the University of South California in Los Angeles. The development began in 1972 and the first titanium-made total hips were implanted in 1975. The titanium alloy’s name was Tivanium TM, basically it was the commercially used alloy of Titanium with 6% Aluminium and  4% Vanadium.  The new total hip model was named the STH system.  Note that the femoral component is a monoblock construction with the femoral ball making an integral part of the component. The stem component's form is like to Charnley's femoral component. In the  advert Zimmer Company pointed out three characteristics of the new Tivanium alloy: its high strength, its low stiffness, and its excellent biocompatibility. The wear characteristics were named only by side: “the larger head reduces contact stress…which can be expected to further reduce the already acceptably low wear rate of the ultra high molecular weight (UHMWP) material”.

Picture: The STH total hip System. Advertisement by Zimmer

Other companies, Biomet, Osteonics, Hexacell followed soon with their models of titanium-made total hip joints.

As the use of titanium- made total hip joints spread, there occurred reports that demonstrated the sensibility of the new material to wear. At revision operations of a failed titanium-made total hip the surgeons found usually black staining of the tissues around the total hip. At microscopic examination of the tissues the surgeons found small particles of titanium and some patients with failed titanium total hips had three times higher levels of titanium in the blood.  The surgeons argued about the importance of the black stain; was it an innocent “stain” or was the concentration of titanium particles in the tissues leading to osteolysis, loosening and failure of the titanium-made total hip? Worse thing would come, the scientists would discover that the so friendly titanium might not be so friendly at al.

The wear of cemented titanium total hips

The first scientists who came with warnings about high wear of titanium were the American surgeon Jorge Galante and the material scientist William Rostoker. In 1973 they published a report on “Wear in Total Hip Prostheses”. It was published in a European journal, Acta Orthopaedica Scandinavica, and consequently the report remained largely unknown for the American surgeons. The two scientists studied wear of different metals articulating with polyethylene under laboratory conditions. The titanium discs that moved against polyethylene discs were severely scored after these tests.  In their report the two scientists described this scoring of the titanium as the “more emphatic example of rubbing action damaging the passive film of the metal”. They concluded their observation with the following statement: “clearly titanium is unsuitable as the material for the femoral head in the human hip joint prosthesis”. Would this recommendation hold in the future? 

Why the titanium alloys wear so much?

Titanium and titanium alloys quickly form at their surfaces a thin layer of titanium oxide every time they come into contact with oxygen, may it be oxygen from the air or from blood in the tissues. Titanium is relatively soft, its oxide is hard; consequently the quickly formed thin oxide layer is also quickly lost. The raw surface is, however, quickly covered with a new oxide layer again. Every time the titanium material rubs against other objects and the oxide layer is removed, a new layer forms instantaneously again and again. The titanium-made femoral stem component may rub against the bone-cement sheath in which it is seated, the femoral ball component rubs against the polyethylene cup with which it articulates.

Every such movement wears off some particles of the hard titanium oxide, and every time the removed particles are quickly replaced by quick oxidation of the raw surface of the component. As long as there is available oxygen, the process is self perpetuating, producing an unending stream of titanium oxide particles.

What to do to improve the wear resistance? 

The scientists at Zimmer Company were aware of the bad wear properties of titanium alloys. Indeed it was professor Rostoker, co-author of the first study about unsuitability of titanium for total hip devices, who collaborated on the development of STH total hip models. In 1982, looking back on his studies on wear properties of titanium, he said (Zimmer , 1982, pp 28) “These early findings seemed to suggest a poor prognosis for Ti-6Al-4V (Titanium alloy) as an orthopaedic implant material. However, its excellent mechanical properties and biocompatibility encouraged further investigation. The solution to titanium alloy wear was found in a modified manufacturing process”.

Zimmer thus introduced a new polishing technique of the surfaces of titanium-made total hip devices combined with chemical passivation. This chemical process created a thicker, stronger layer of titanium oxide that resisted wear better. Indeed, laboratory measurement demonstrated that this passivation procedure reduced the wear rate of titanium alloy to the point where the wear characteristics of the passivated product “were equally good as those of conventional implant materials.”

In spite of these new improvements, the stream of the reports on failed titanium-made cemented total hips increased steadily. Sometimes, there were small epidemics of failures of titanium-made total hips caused by sudden change of manufacturing process. It needed a sleuth-like capability of the surgeons to discover such cause of total hip failure.

An illustrative case of titanium wear

Surgeons at the Wexham Park Hospital, Slough, England, were using routinely the McKee-Farrar cobalt-chrome total hip until 1985 when the manufacturer began to produce this total hip model from titanium alloy. Nothing unusual happened until December 1986, when three patients, who only had their total hip replacement performed one year ago, developed pain and signs of total hip failure. Further ten patients attended later at irregular intervals with similar signs of failure. At revision operation of all these patients the surgeons found heavy black staining of the tissues around the titanium-made total hip and also areas of skeleton destructions (Witt & Swahn 1991).

The surgeons were surprised by this haphazard occurrence of failures; they investigated the matter and discovered that the manufacturer not only changed the material for manufacture of the total hip from cobalt-chrome to titanium alloy. The manufacturer introduced also a coarse finish on these new titanium-made total hips. This change was done on the theory that bone-cement sheath would adhere better to a component with coarse surface. No previous tests were done before the total hips with the new finish were introduced on the market.

In the hospital, the boxes containing the new titanium-made total hips with coarse finish were freely  intermixed with the boxes containing the previous total hip models with smooth surface; both models were stored at the same place. At operation the surgeon then got either the coarse finish or the smooth finish total hip, completely at random. That is why the failures, observed only in the coarse finished total hips, occurred sporadically. 

Subsequent information from the manufacturer revealed that the surface of the new stem components was grinded with a stream of very hard and sharp aluminium-oxide particles. Some of them stayed loosely embedded onto the surface. 

The shaft components removed at revision operation showed signs of severe burnishing (arrows), the area where the superficial layer of the coarse surface was removed through rubbing. Illustration adapted from the paper by Witt & Swann. With every step, these remaining hard aluminium particles grinded pieces of bone-cement away and mixed with it. When mixed together with particles of bone cement the alumina particles produced excellent grinding paste. This paste grinded away large quantity of titanium alloy from the surface of the stem components leaving a burnished area on the dull surface of the shaft component. Overproduction of metallic particles then brought about inflammation around the total hip, loss of  the bone tissue, and eventually quick failure of the titanium-made total hip joint.

Picture: Femoral component with signs of severe burnishing

 Interestingly, although the English surgeons notified the remaining sharp aluminium particles on the surfaces of the failed total hips, they did not see the possibility that this manufacturing procedure was faulty and caused this unnecessary epidemic of failures. (In some way this fault reminds of the recent Sulzer's Inter-Op catastrophe).  In every case, the doctors contacted the manufacturer and the coarse shot blasting process was stopped and with it the epidemic of failures. 

Nitrogen gas implanted into Titanium alloy

Although the laboratory investigations at the University of South California showed that femoral balls fabricated from passivated titanium alloys had equally low wear as balls made from the cobalt-chrome alloy, the practice showed otherwise. The increasing stream of reports demonstrated that  when a titanium-made femoral ball component articulated with a polyethylene cup this couple produced large quantities of wear particles. Something should be done.

In 1991 appeared an advertisement on a new product from Zimmer Company. It announced “Strength and Biocompatibility”; the Zimmer Company revealed that the already excellent Tivanium alloy was improved through a unique Ti-Nidium Surface Hardening Process.  

Here, the elementary nitrogen gas ions were diffused into the prosthesis component. Inside the metal alloy the nitrogen ions binded with titanium atoms and formed hard titanium nitrides. This is a well known metallurgical procedure used for production of hard stainless steel. Would it work on titanium-made total hips? Would this process make the titanium alloys less sensitive to wear? Remember that the hard layer created by the Ti-Nidium process on the surface of the femoral ball is not a coating; it is a part of the alloy, it is strong albeit not so deep. Thus, there were all prerequisites that it will hold. The nitrogen treated titanium-made femoral balls were tested in laboratory. In these tests the nitrogen-hardened balls demonstrated lower wear rates than the untreated balls. So long so good, but. ..There was a great but. These positive reports came only from some laboratories; other laboratories reported on the contrary that nitrogen treatment had no effect; nitrogen treated femoral balls articulating against polyethylene cups still produced too much polyethylene wear, the balls successively roughened and lost their roundness.

Ti-Nidium Surface Hardening Process

These were the discordant laboratory results,  what happened in the patient's body? Reports about the clinical results of patients operated on with nitrogen treated titanium-made total hips are now occurring. The majority of these reports state that the failure rate of these total hips is too high. At examination of the failed total hips the surgeons had seen that parts of the hard surface layer on the femoral ball were abraded and small metal particles were found in the tissues around the total joint.

As always in the history of total joints, there are exceptions, there are single reports announcing excellent results with titanium-made femoral ball components. It would be interesting to ask why there are almost always such positive reports among the majority of reports presenting negative results. In what way are the patients, the methods, the surgeons, the products in these positive reports unique? What can we learn from them? Nobody was asking these questions so that we do not know the answer. 

Cautious conclusion

This history shows that material engineers still have difficulties to produce materials that would resist wear equally well as they are resisting high loads. Titanium is still an excellent material for manufacture of total hip and knee joints but within its confines; it is excellent for manufacture of uncemented shaft components, metal backings of acetabular components and like. In these situations the material is not subjected to friction, and under these conditions it functions very well.

Professor Sarmiento is a man of great personal integrity and high ethical standards. He started the development of the first titanium-made total hips in 1973; it is interesting to read what he says thirty years later:

“I hold the dubious distinction of having been the surgeon who participated in the design of the first titanium alloy total hip prosthesis implanted in the USA. Six years after the first implantation we published a preliminary report. At that time we concluded that the results from the new implant were as good as these obtained with Charnley prosthesis…..However, as the clinical follow-up extended by additional years, we noticed a gradual increase in the incidence of radiological changes which had not been apparent in the first six years….The increased rate of clinical and radiological failures eventually convinced us that the clinical use of the monoblock titanium-alloy prosthesis could no longer be justified. The implant was modified to accommodate a modular cobalt-chrome head…My own experiences have forced me to remain concerned over the use of titanium in total hip arthroplasty. The fact that there is an increase in premalignant and chromosomal changes in tissues in which metallosis forms, increases my preoccupation with titanium alloys.”

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References

Black J.: Orthopaedic Biomaterials in Research and Practice, 1988,  pp 288

Harman MK et al.: Wear Analysis of a Retrieved Hip Implant With Titanium Nitride Coating. J Arthroplasty 1997; 12: 938-45

Sarmiento A.: Correspondence: Is titanium so bad? J Bone Joint Surg-Br 2002; 84-B: 931

Witt JD,  Swann M.: Metal wear and tissue response in failed titanium alloy total hip

        replacements. J Bone Joint Surg-Br 1991; 73-B: 559-63.

Zimmer: The Use of Titanium Alloy in Orthopaedic Medicine. 1982