The tribology (friction, lubrication and wear) of all-metal artificial hip joints

The tribology (friction, lubrication and wear) of all-metal artificial hip joints

Wear - - Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands 285 THE TRIBOLOGY (FRICTION, LUBRICATION AND WEAR) OF ALLMETAL ARTIFICIAL HIP ...

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Wear - - Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands




Bioengineerin9 Department, Hospital for Special Surgery, affiliated with the New York HospitaLCornell University Medical College, New York City (U.S.A.) (Received November 30, 1970)


Total replacement of the human hip joint using artificial parts has now become a widely used treatment in many cases of osteoarthrosis and similar disabling conditions. This study was restricted to the McKee-Farrar type of joint, a ball-in-socket, where the parts were made from a cobalt-chrome-molybdenum alloy. A series of joints which had been removed from patients after service times of up to fourteen months were studied in several ways. The surface finishes of worn and unworn areas were compared, and the areas of contact plotted. Wear was deduced to be mainly abrasive, from entrapped particles formed firstly during "running-in". The frictional torques, measured dynamically on a simulating machine, depended on the location of the contact area, minimum at "polar", reaching high values at "equatorial". A theory for this was proposed, and relevant clinical data was given also. INTRODUCTION

There are several conditions of the human hip joint where function is severely impaired. The reasons for the disability depend upon the disease, but usually the bearing surfaces are involved. In osteoarthrosis and rheumatoid arthritis, for example, the normally smooth layers of articular cartilage which form the load-bearing surfaces become worn, and the shape of the joints becomes distorted. Pain usually results from the sliding together of the two roughened bony surfaces. A widespread treatment in many of these cases is total joint replacement, where the femoral head is replaced by a ball on a stem, and the acetabulum by a hemispherical lining. Scales I has given a thorough review of hip prostheses. The two main types of joints used are an all-metal cobalt-chrome alloy (McKee and WatsonFarrar 1) and a metal-on-plastic, stainless steel on high molecular weight polyethylene (Charnley3). Weisman 4 has given details of the metallic materials used. Several factors influence the length of time for which these devices perform successfully, one of the most important being the risk of infection resulting from the operation (Charnley and EftekharS). However, mechanical factors are undoubtedly important, particularly in the long term. Two of these factors can be identified, one being the gradual accumulation of wear debris, and the other loosening of one or both components. The first of these represents the potential hazard of cellular reaction to particulate debris or of general systemic reaction due to debri s products; whilst Wear, 17 (1971) 285 299


P . S . WALKER. B . L . GOLD

the second, which often manifests itself as pain, will be encouraged by frictional torques of fluctuating direction imposed on the components during sliding conditions. This paper is restricted to a study of cobalt-chrome-molybdenum alloy joints of the McKee-Farrar type (Fig. 1). New joints were examined prior to implanting

Fig. 1. The McKee-Farrar total hip prosthesis, showing the skeletal parts into which it is fitted. The acetabulum (socket) is reamed to shape, and the femoral head excised to accept the stem and replacement ball.

them, and then a detailed examination was made of joints which had been removed from patients due to failure. The extent and type of the wear, and the correlation with the amount of usage were particularly studied. Using a joint simulating machine, the frictional torques of certain of the joints were measured, to show the effect of the location of the contact area. METHODS

The regions of contact between metal components under moderate loads are usually restricted to localized areas, due to the lack of compliance of the material. For all-metal hip prostheses the real area of contact at any given time will be made up from many small points of contact totalling around 2 mm 2 maximum, as calculated later. But for the full range of motion, the contact points will move around, resulting in areas within which contact has been made. These areas, which characteristically exhibited scratch marks, will be called "wear areas". A total of ten joints, which had been removed from patients from between Wear, 17 (1971) 285-299



three months to one and one-half years were studied in this series. The wear areas of both ball and socket were studied, and plotted on diagrams marked in segments. To describe the micro-morphology of the surfaces, two different methods were used. The first was by Talysurf stylus profile tracing, using a radius attachment. Replicas were made of selected areas of surface using a reliable methyl methacrylate based replicating material. The "background noise" of the method has been consistently found to be 0.025q3.05 #m c.l.a. (1-2 #in.), which was an order of magnitude less than the roughnesses which were being measured. The second method of surface measurement was by scanning electron microscopy. Again, replicas were made using cellulose acetate film moistened with acetone, pressed against the required area of the surface. This well-established method gave a resolution well within the resolving power of the scanning microscope, which is about 25-50 nm. For less detailed studies of a large number of wear areas light microscopy was used, the aim being to study the wear patterns in different positions on the ball and socket, for instance, to determine the directions of the scratch marks, and to estimate the usage of the joints in different directions. Various methods of examining worn metal surfaces were described by Scott 6. There were various areas over which contact between ball and socket took place. For instance, some joints displayed an "equatorial contact", whilst in others the "polar" regions were in contact. Theoretically, the frictional torque generated in a joint will depend on this contact area, and on other variables such as ball diameter and surface roughness. Therefore the frictional torques of a number of joints representing different types were measured, for which a joint simulating machine was used, which oscillated the femoral components of the joints to and fro through an arc of 60° at one cycle per second. A fluidics system controlled the loading pattern imposed R(t) : Joint force

applied by pneumatic- hydraulic system

beari~js, whichtransmit torqueto beam, and self-center the joint . ~





Strain gauge Measuring beam

A c r y l a t ~ X cement Socket

,,emoro,K S

head) '11.

Mountedin-~~" ~\ acrylate / / "~ cement ~

Mo~ionof femoral head -+30°aboutz-axis

Fig. 2. The arrangement of the joint in the joint simulating machine. A realistic load pattern and motions are imposed to simulate walking. Frictional torque is measured using the strain gauge and beam. Wear, 17 (1971) 285-299


P . S . WALKER, B . L . GOLD

on the joint, applied down the axis of the acetabulum. The arrangement of the joint and certain details of the machine are shown in Fig. 2. Several specimens of synovial fluid from normal human joints were pooled and used as the lubricants for the joints during the experiments. Frictional torque was measured using a strain gauge system, under full running conditions with the motion and the load pattern. RESULTS

The nature of the wear

Typical results for the roughness of the ball and the socket surfaces are shown in Fig. 3. The original surface of the ball was smooth, maximum deviations being ~





lO0?m 0.004 inch








Fig. 3. Talysurf stylus profiles of the surfaces of McKee-Farrar prostheses. (a) Original polished surface of the ball, perpendicular to flexion-extension direction; (b) and (c) wear areas of the ball, direction as for (a); (d) original machined surface of the socket, in radial direction ; (e) wear area of the socket in radial direction.

1/4/~m (1/An.), as shown on trace (a). From light microscopy it was observed that these irregularities were either fine scratches or dimples in the metal surface. This was expected from the manufacturing process which was by copy turning, grinding and then lapping, followed finally by hand polishing on a buffing wheel. The socket surface on the other hand was ground, and displayed asperities of height up to about 1/~m (40/An.), shown on trace (d). The wear areas of the ball consisted of many fine scratches, with a predominance in the flexion-extension direction. Thus stylus tracings were taken perpendicular to this direction, results showing that the center line average value (c.l.a.) was about 1/41/2 pm (10-20 pin.) (traces (b) and (c)). Peaks and troughs of up to 1 pm were not uncommon. It is interesting to compare traces (d) and (e), the surface of the acetabulum before and after use. In the former case the roughnesses are mainly asperities, whereas after use they are mainly troughs. The explanation is that the surface has "run in" by wearing off the high points, while the troughs represent scratches caused in part by wear particles formed during the running-in process. Wear, 17 (1971) 285 299



Light microscopy showed that the boundaries between the wear areas and the original polished surfaces were fairly distinct, which would be the result of small geometric macro-irregulaties on either ball or socket. Details of the scratch marks were studied by scanning electron microscopy, which showed that the scratches were in a wide variety of width, depth and direction. There was a preponderance of scratches in the flexion-extension direction, and the sizes of the scratches in this direction tended to be the greatest. Figures 4 and 5 give typical examples of surface scratches.

Fig. 4. The appearance of the edge of a wear area on the femoral head. The area is made up from many fine scratches of varying size and angle. Scanning electron micrograph of cellulose acetate replica.

The images appear as negatives of the actual surface, because of the replication method of producing the microscopic specimens. In Fig. 5 details of a few individual marks can be seen. The action of producing a scratch would normally result in a built-up edge, but there is no evidence of an edge of reasonable size in these scratches. It is likely that the edge would be readily flattened or partially knocked off under sliding action, producing wear particles. Also, wear particles would result from one scratch superimposing on another, instances of which can be seen in Fig. 5. Wear, 17 (1971) 285-299



The scratch marks on the acetabulum were of similar appearance to those of the femoral head, as shown in Figs. 6 and 7. Here, however, the scratches were suPerimposed upon the original machining marks. Again it is clear that while a given scratch is being produced it will result in wear fragments from the built-up edge, and also whenever the scratch crosses over another one.

Fig. 5. Deep wear scratches superimposed on the original polished surface. It is likely that the scratches were caused by loose abrasive particles. Scanning electron micrograph of cellulose acetate replica.

Although detailed analysis of wear particles has not yet been carried out, certain findings can be reported. The capsule around the joint forms a close contact with the metal parts, and close to the rim of the acetabulum (socket) it is typically dark in color, due to the accumulation of debris products. Study of histological sections has shown that the particles often tend to be cigar-shaped, of size I pm and less. This correlates well with the size of pieces of built-up edge which would form at the side of the scratch marks. Debris particles in synovial fluid have been measured at up to about 25 pm, but these were aggregates of much smaller particles. Wear, 17 (1971) 285-299



Areas of wear The femoral head was divided into 80 areas, as shown in Fig. 8. Thus "polar" contact between ball and socket would be in areas 1-6 and equatorial contact in areas 49-72. A typical wear pattern was as illustrated in Fig. 9, most of the wear taking

Fig. 6. Scratch marks on the acetabulum. The original machining marks from grinding are horizontal. The scratch marks are diagonal, mainly in the flexion-extension direction. place in areas 49-64. This was the most c o m m o n finding, as can be seen from Fig. 8, which shows that most joints were worn in areas 49-64, with some wear in areas 33-48 and 65-80. There were none in the observed series with "polar" contact, probably indicating that even though the nominal diameters of ball and socket would allow this to occur, small deviations from this ideal geometry were sufficient to prevent this type of contact. If a socket is assumed to be perfectly hemispherical, radius Rs, and the ball has deviations from sphericity which makes its effective radius Rb, then the relative

Wear, 17 (1971) 285-299



Fig. 7. Details at higher magnification of scratches on the acetabulum. The effect of scratches crossing over others will be to produce wear fragments.

geometry will be as drawn in Fig. 10. The base clearance e is given by: R 2 = (Rb-- e)(2Rb -- R~ + e) i.e.

e = (A 2 +2AR~) 6 - A where A = Rb-


If A is very small, then: e~(2ARs) ~ Theoretical results for a standard size of M c K e e - F a r r a r joint R~= 20.6 m m (15 in. diam.) are given in Table I. Measurement of the deviation from sphericity of the femoral head or acetaWear, 17 (1971) 285 299



4 ~ ~


Medial3 ~ 0



.-q 6° E o o~-,-,40









~20 ~b ~_.c


10 Area












20 30 40 50 60 70 80 r e f e r e n c e n u m b e q area position s h o w n on d i a g r a m

Fig. 8. Areas of the femoral heads which were worn, from a series of 10 removed McKee-Farrar hip joints. The acetabulum wear was a close image of that on the femoral head. Wear was most commonly present in 0-22½ ° included angle, areas 4%64.





Fig. 9. Area of contact and wear area of McKee-Farrar total hip prosthesis femoral head. (a) The contact area of the new joint, determined by the "engineers blue" test, (b) the observed areas of wear after removal of joint frmn the patient 4 months post-operative. Fig. 10. Relative geometry of a ball (radius Rb) and socket (radius Rs). The radii are nominally equal but irregularities on the ball make its effective radius greater than that of the socket, giving a base clearance e.

Wear, 17 (1971) 285-299


P . S. W A L K E R ,

B. L . G O L D










0.001 0.01 0.05

40 × 1 0 . 6 0.0004 0.002

0.20 0.90 2.0

0.008 0.036 0.080

bulum have shown that a figure of 0.01 m m is quite common. Thus it presumably occurs that in some positions of the joint, polar contact wili not be possible. Also, as soon as debris particles are produced which are of the order of 1/~m in size, these will cause a significant gap to occur between the joint surfaces, even under load. As for the actual area of metal in contact at any given time, this can be estimated by assuming that the metal at the contact points reaches its yield stress. Using a value of 7000 kg/cm 2 (100.000 lb./in 2) for yield stress, and a load of 150 kg on the joint, the area is about 2 m m 2. This can be contrasted to the much larger wear areas which have been defined as areas within which contact has taken place. This statement will still be valid even when the increase in real contact area due to sliding conditions is taken into consideration. Frictional torque T h e t o r q u e a c t i n g will d e p e n d u p o n the load, the coe(ficient of friction a n d the l o c a t i o n of the p o i n t s of contact. It is likely t h a t the t o r q u e for " p o l a r " c o n t a c t will be less t h a n for " e q u a t o r i a l " contact. T o o b t a i n an e s t i m a t e of the d e p e n d e n c e of t o r q u e on contact, a s s u m e a c o n t a c t s i t u a t i o n as s h o w n in Fig. 11, where the n o r m a l force b e t w e e n ball a n d s o c k e t is a r o u n d a m i n o r circle t h r o u g h P.



Fig. 11. Contact situation between ball and socket. The contact area is drawn as being symmetrical about the minor circle through P, at angle 0 to the x-z plane. R is the resultant force between ball and socket.

Wear, 17 (1971) 285 299



R is the resultant force acting along the y-axis. P is a point on the circle of contact, where the circle subtends an angle 0 at O, and the line P O projected on to the x-z-plane is at angle ~b to the z-axis. N is the normal force between ball and socket acting along an arc of length r 54). We consider the case of rotation of the ball about the z-axis, which represents flexion-extension, p is the coefficient of friction between ball and socket, 5 Q is the torque produced in the element about the z-axis, Q is the total torque. The equations for frictional torque are: For the element at P: 5Q = #SN



= # f i N x r(1 - c o s 2 0 cos 2 ~b){


Resolving vertically for the forces at the element: fiN" sin 0 -

R r . c3c~ . cos 0

2zc r cos 0



R-5¢ 27r sin 0


Substitute for 5 N in eqn. (1):

Q/Q. 8089 \ e HS (heel strike) ZZ. 3165 3/aRt




(toe off)

Z. 3165

TH?~I LL. 6652 Tg'S'~TH,[~.~R R. 2300 Fr,ct'ona

p.RI" , _




0 15° Equatoriat contact


30 °



45 °


60 °


75 °


90 ° Polar contact

Fig. 12. Frictional torque between ball and socket. #Rr is m i n i m u m torque for polar contact, where # is friction coefficient, R is load between ball and socket, and r is ball radius. The full line is the theoretical curve and the points are experimental data from Table II.

Wear, 17 (1971) 285 299



/zR" 64)" r (1 - cos 2 0 cos e q~)~ - 2~z sin ~9Total torque is given by:



Q = 2re sin~" 4


(1 - c o s / 0 cos 2 qS)~dq5


This equation can be evaluated for n/2 ~>0 ~>0, and is plotted on Fig. 12, which gives the theoretical curve for the frictional torque developed in the joint as a function of the position of the area of contact. It can be seen that for polar contact, the friction is a minimum, of value #Rr, and at first the friction rises slowly, but beyond about 60 ° from the pole the friction becomes much more sensitive to angle, until at equatorial contact the frictional torque is theoretically infinite. Comparison with this behavior calculated theoretically can be compared with the values measured experimentally. The torque measured directly on the joint simulating machine, together with details of the contact area is shown in Table II. The theoretical "polar" value for frictional torque was obtained by estimating # from the experimental results, which are marked in relation to the theoretical curve T A B L E II FRICTIONAL TORQUE MEASUREMENTS OF M C K E E - F A R R A R JOINTS~ SHOWING THE DEPENDENCE ON FRICTION OF THE TYPE OF CONTACT CONDITIONS

Joint ref no.

Contact between ball and socket

PP. 1553 1~ in. diam. RR. 2300 1~ in. diam. LL. 6625 1~ in. diam.

broad contact area 11-45 ° overall contact 30 ° mainly narrow area 11°-45 ° just above equatorial equatorial

ZZ. 3165 1~ in. diam. QQ. 8089 1~ in. diam.

Average contact angle

Frictional torque at heel strike, (load 150 ko)

(kg cm)

(ko cm)














120 est.

250 est.



Joint ref no.

Reason for removal

Time in patient (months)



Infection Infection Infection Nev~joint, not implanted Loosening of both joint components

14 10 12 --

Slight Good Slight --



1553 2300 6625 3165

QQ. 8089

Wear, 17 (1971)


Frictional torque at toe-off,

(load 200 k9)



in Fig. 12. It can be seen that the agreement in the general trend is very good; in particular, that a very large increase in frictional torque only started to take place within 30 ° of the equator. The value for frictional coefficient was calculated at around 0.08, which is a very satisfactory figure. Whilst hydrodynamic lubrication is not indicated, a very efficient boundary mechanism was presumably acting. Clinical results

Although a detailed consideration of clinical results is not to be included in this paper, it is of interest to record the results of the joints for which friction was measured. Table III records briefly the reason for failure of the prostheses, and the length of time before removal and the usage. The patients all recovered from the operation in about two weeks, and began to use the joint after that time. It can be seen that the first three joints were removed for infection; and in these cases there was no prior loosening indicated; whereas the joint which loosened had a very high frictional torque indeed. The loosening may have been partially due to the torque, but the fact that it survived 13 months may mean that the contact situation worsened with time. DISCUSSION

The findings reported here suggest several things in relation to the design of all-metal artificial hip joints. When the acetabulum was unpolished, apparently a "wearing-in" procedure took place, resulting in debris particles of significant size, which would initiate and hence maintain further wear. For this reason it would seem preferable to lap the ball and socket together following grinding, and then to polish both surfaces. However, wear particles would still seem inevitable, so to avoid them becoming trapped and causing abrasion, the provision of some means of entrapping would seem desirable. The relative geometry of the bearing surface strongly influences frictional torque, but it is very difficult to control to the accuracies involved. It is probable that for equatorial contact, because the resultant normal force between ball and socket is larger than in the polar case, the wear will be correspondingly larger and the curve for wear versus 0 will be the same shape as the frictional torque in Fig. 12. Hence, to ensure that friction and wear are minimized, the contact between ball and socket should be kept to within say 50 ° of the pole. The two lowest torque results suggest that it is advantageous if the contact area covers a broad rather than a narrow area. The explanation may be that temperatures are kept down, and wear particles do not crowd together. Also, a given area would take longer to reach an advanced state of roughness. Work is in progress to determine whether or not # increases with time due to the increased surface roughness. The effect on contact area on friction has also been raised by Scales 7 who pointed out that high torques could result from equatorial contact. Resulting partly from discussion of results of this study, the M c K e e - F a r r a r joints produced by one company now have smooth surfaces, are provided with a shallow dimple at the top of the socket, and make contact in the polar region. These features are reflected in low friction at least and represent an improvement over designs of the past. Wear, 17 (1971) 285-299



The wear behavior of the metal-on-plastic joint is somewhat different from that of the all-metal. The wear takes place almost entirely on the plastic, by a process of abrasion, where the original machining marks on the socket become worn and the surface polished, blemished only by fine abrasion scratches produced by metal asperities. The frictional torque is certainly lower than that of a polar-contact metal joint, mainly because the ball diameter is less, but the coefficients of friction between the materials may not differ appreciably in the two types of joints. Work is already in progress to investigate the friction and wear of metal-on-plastic and plastic-onplastic joints. Judging by results from the field, the performance of the two types of hip prosthesis mentioned, the McKee-Farrar and the Charnley, seems to be very satisfactory, provided the surgical technique has been successful. A high proportion of joints still perform well after several years, and in many cases they are still in good service after close to ten years. This is not to say, however, that improvements cannot be made. Wear debris is produced at a slow but steady rate, which is undesirable, and the frictional torque could be lessened if a fluid film type of lubrication could be achieved which would reduce wear. These factors are being explored by use of strong compliant materials. Poor fixation is a weakness in some cases of joints, and long-term loosening is certainly a possibility. The experience gained with hip prostheses will certainly help and stimulate the development of prostheses for other joints.


(1) Ten McKee-Farrar artificial hip joints made from cobalt-chrome alloy, which had failed in service and had been removed, were examined in various ways. (2) Wear areas were observed on the sliding components, on which there were many fine scratches. The wear areas were defined as "area within which contact had taken place". (3) The initial c.l.a, value of the polished ball surface was about 0.1 #m ( 4 #in.), and for the wear areas, up to 0.5 #m. The width and depth of the scratches were 1-5 ~tm. (4) The wear mechanism was due to abrasive particles produced during "running-in" initiated scratching and subsequent wear debris resulted from fragments of "built-up-edge" from scratch marks. (5) Small geometric irregularities as small as 1/~m on the ball or socket could radically alter the location of the contact area between ball and socket. The contact could range from "polar" to "equatorial". (6) In cases of "equatorial contact", the frictional torque, measured on a joint simulating machine, was found to be many times higher than for polar contact. If contact was within a solid angle of 60 ° of the pole, friction was only 5 0 ~ higher than the minimum at polar contact. But outside 60 °, torque rose very steeply. Friction coefficient was about 0.08, indicating a good boundary lubrication mechanism. (7) The indications were that there could be clinical significance to a high torque in a joint, in that it could precipitate loosening, and make the joint more of an effort to use. Wear, 17 (1971) 285 299




This study was made possible by close collaboration with the hip clinic of the Hospital for Special Surgery, notably with Drs P. D. Wilson Jr., E. Salvati, D. Mendes and H. C. Amstutz. The joint simulating machine was designed by Dr. B. Wolff, Civil Engineering, New York University, and commissioned in our department, with the able assistance ofR. Chandra, Columbia University. Scanning electron microscopy was by T. Buckley, Textile Physics, and Talysurf work by H. P. Jones, Mechanical Engineering, of the University of Leeds, England. We are grateful to Howmedica Inc. for helpful cooperation and for providing specimens of joints. REFERENCES 1 J.T. SCALES,Arthoplasty of the hip using foreign materials: a history, Proc, Inst. Mech Engrs., 181 (3J) (196(~67) 63. 2 G. K. MCKEE AND WATSON-FARRAR,Replacement of arthritic hips by the McKee-Farrar prostheses. J. Bone Joint Surg., 48B (2) (1966) 245 259. 3 J. CHARNLEY,Factors in the design of an artificial hip joint, Proe. Inst. Mech. Engrs., 181 (3J) (1966-67) 12. 4 S. WEISMAN,Metals for implantation in the human body, Ann. N.Y. Acad. Sci., 146 (1968) 80. 5 J. CHARNLEYAND N. EFTEKHAR,Postoperative infection in total prosthetic replacement arthoplasty of the hip, J. Bone Joint Surg., 56 (9) (1969) 641. 6 D. SCOTX,Failure diagnosis and investigation, Tribology, 3 (l) (1970) 22. 7 J. N. WILSONANDJ. T. SCALES,Loosening of total hip replacements with cement fixation, Clin. Orthop. Relat. Res., 72 (1970) 145-160. Wear, 17 (1971) 285 299