P H Y S I O T H E R A P Y M ARCH , 1977 Lubrication and wear of joints V. W R IG H T , M .D ., F .R .C .P . P ro fe sso r o f R h e u m a to lo g y , R h e u m a tis m R esea rch U nit, U n iv e r sity D e p a r tm e n t o f M e d ic in e , G e n era l In fir m a ry at L e e d s, E n g la n d . L ubrication aiid W ear o f Joints T h e n a tu r a l jo in t is a re m a rk a b le b earing. It is e x pected to o p e ra te in the h u m a n m a c h in e fo r a p e rio d o f a b o u t 70 y e ars w h ilst tra n s m ittin g lo ad s a n d yet a c c o m m o d a tin g a w ide ra n g e o f m o v em e n t. It n o rm a lly p e rfo rm s these fu n c tio n s w ith h ig h efficiency, a n d coefficients of frictio n as lo w as 0 .0 0 2 h a v e b een re c o rd e d . In e n g in e erin g situ a tio n s b e a rin g life is so m e tim es m ea su re d in m in u te s, o fte n in h o u rs , a n d so m e tim es in y e ars, b u t it is ra re in d ee d f o r the b e a rin g desigser to be a sked to p rovide a b e a rin g w ith a n e stim a te d life o f 70 y ears in the m ac h in e! It is f a ir to say th a t th e e n g in e e r w o u ld find it e x tre m e ly difficult t o p ro v id e a b e a rin g w hich w ould o p e ra te w ith in the e n v iro n m e n t o f th e b o d y , in the sam e space as th e n a tu r a l b e a rin g , u n d e r the sa m e lo ad s, w ith th e sam e d egree o f m o v e m e n t, w ith sim ila r frictio n a n d c o m p a ra b le m e a n life to the n a tu r a l b earing. T h e in flu e n ce o f lu b ric a tio n , fric tio n , w e a r a n d good b e a rin g d esign u p o n the efficiency a n d g e n era l eco n o m ic p e rfo rm a n c e o f the m a c h in e ry in o u r tec h n o lo g ic al society w as re co g n ise d in 1966 b y the in tro d u c tio n o f a new w o rd , ‘trib o lo g y ’, m ea n in g ‘the science a n d te c h ­ n o lo g y o f in te ra c tin g su rfa ce s in re la tiv e m o tio n ’. A t the U n iv e rsity o f L eed s we h a v e fo rm ed a B io en g in e erin g G r o u p fo r th e S tu d y o f H u m a n J o in ts w h ic h co m b in e s the e x p ertise o f rh e u m a to lo g ists, o rth o p a e d ic surgeons, trib o lo g ists, bio ch e m ists, p o ly m e r sc ien tists, m e ta llu rg is ts, ph y sic al e d u c a tio n ists a n d z o o lo g ists in an effort to u n d e rs ta n d the re m a rk a b le c h a ra c te ris tic s of h e a lth y jo in ts a n d , p e rh a p s m o re im p o rt-n itty , the re aso n s w hy som e h u m a n b e a rin g s w e ar o u t m o re ra p id ly th a n o th ers. E n g in e e rs tend to c lassify fo rm s o f lu b ric a tio n in to a few w e ll-k n o w n p h ysical a n d chem ica l m ec h an ism s, a nd a good d eal o f p ro g re s s has no w b e en m a d e to w a rd s an u n d e rs ta n d in g o f th e p e rfo rm a n c e of the n a tu r a l joint. A tte m p ts h a v e also been m ad e to d e v elo p sy nthetic lu b ric a n ts w hich m ig h t be in tro d u c e d in to jo in ts w hich sta rt to sh o w signs o f fa ilu re in the sam e w ay th a t a n e n g in e e r m ig h t p ro lo n g th e life o f a b e a rin g in a m a c h in e by in tro d u c in g an im p ro v e d lu b ric a n t. F in a lly , a n d p a r tic u la r ly in th e case o f o s te o a rth ro s is , it m ay be n e ce ssa ry to c o n sid e r jo in t re p la c e m e n t if fu n c tio n has been se verely im p a ire d a n d th e p a tie n t is su b je cted to p ain. It is im p o rta n t th a t e n d o p ro s th e s e s sh o u ld be designed o n p r o p e r e n g in e e rin g p rin c ip le s , o therw ise u n d u e w e a r o r lo o se n in g will o c cu r. In th is a rtic le a b rie f in tro d u c tio n w ill be given to stu d ies in e ac h of these fields. M od es o f L ubrication In e n g in e e rin g s itu a tio n s lu b ric a te d b e a rin g b e h a v io u r m ay 'be d e sc rib e d by o n e o f the fo llo w in g regim es: fluid-film lu b ric a tio n ; m ixed lu b ric a tio n ; b o u n d a ry lu b ric a tio n . F luid-Film L ubrication T h is o c cu rs w h e n ev e r th e b e a rin g su rfa ce s a re separated^, by a la y e r o f fluid w h ic h is th ic k e r th a n the sum of t h f ^ j su rfa c e ro u g h n e sse s. T h e re sistan c e to m o tio n a r is e y 1' e n tire ly fro m the s h e a rin g o f th e viscous layer, and in g e n era l the frictio n fo rc e can be re p re se n ted by an e q u a tio n o f the fo rm : F = A ------------- (1) w here F = fra c tio n fo rce ; U = re la tiv e sliding velo c ity ; h = lu b ric a n t film th ickness; A = effective a re a o f b e arin g ; 71 = coefficient o f viscosity. T h e coefficient o f viscosity is th e m ost im p o rta n t pro­ p e rty o f a lu b ric a n t in fluid-film b e a rin g s since it governs n o t o n ly th e re sista n c e to m o tio n b u t also th e ability of the b e a rin g to develop a d e q u a te lo a d -b e a rin g pressures in the fluid. T h e lu b ric a tio n film s in b e arin g s a re themselves q u ite th in in physical te rm s , n o rm a lly b etw een 10~5 and 10"3 in. T h is m a g n itu d e is n o rm a lly a d e q u a te to separate th e o p p o sin g b e arin g su rfa c e s w hich are, o f c o u rse , m anu­ fa c tu re d w ith a high d e g ree o f p re cisio n . S u rfa c e rough­ n esses o f typical e n g in e e rin g b e a rin g s u su a lly ra n g e fro af o ne to a few tens o f m illio n th s o f an inch. II If the a p p o sin g b e a rin g su rfa c e s c a n be p re v e n te d from to u c h in g each o th e r by in te rp o sin g a la y e r o f fluid, wear is a lm o st to ta lly p re v e n te d a n d the re sistan c e t o sliding is low. It is fo r th is re aso n th a t ‘fluid-film 1 is often de sc rib e d as the ideal m o d e o f lu b ric a tio n . Tf the load- c arry in g p re ssu re s a re g e n e ra te d by th e slid in g motion o f the su rfa ce s the m ec h an ism is k n o w n as the ‘hydro- d y n a m ic ’ lu b ric a tio n , b u t if the p re ssu re is g e nerated in a p u m p o u tsid e th e b e a rin g th e a c tio n is kno w n as ‘h y d ro s ta tic lu b ric a tio n ’. T h e th ic k n e ss o f th e fluid la y e r in a hydrodynam ic b e a rin g in w hich the v iscosity o f the fluid re m a in s con­ s ta n t in c re a s e s as the sliding speed (U ) in creases and d e c re a s e s as th e lo ad fW ) in c re a se s, o r hoc /’U’\ n \ W / ------------ (2) A n o th e r v ery im p o rta n t fe a tu re o f h y d ro d y n a m ic bear­ ing is th a t th e film th ic k n e ss d e cre ases in th e d ire c tio n of sliding— a c o n c e p t k n o w n as th e ‘p h ysical wedge a rra n g e m e n t. R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) M A A R T 1977 F I S I O T E R A P I E 3 B o u n d a ry Lubrication j t js c lear fro m e q u a tio n (2 ) ' th a t the th ic k n e ss o f the l u b r i c a n t lay e r will d ecrease a t low sp e e d s a n d high loads and in d u e c o u rse th e o p p o sin g b e arin g su rfa ce s vvill come in to c o n ta c t a t high spots o r a sp e rities. As the severity of th is c o n d itio n in cre ases the effectiveness of the lu b ric a n t la y e r dim in ish e s, fric tio n rises, a n d the b e a r in g su rfa ce s sta rt to w ear. It is fo u n d t h a t fric tio n a n d resistance to w e a r a re no longer g o v e rn ed by the v iscosity o f the b u lk fluid b u t by the p ro p e rties o f thin su rfa c e lay e rs fo rm e d b y p h ysical and chem ical a c tio n on th e su rfa ce s o f the solids; h ence the term ‘b o u n d a ry lu b ric a tio n ’. If the o p e ra tin g c o n d itio n s in m a c h in e ry a re severe, as they are in g e ar b o x e s and m a n y b e a rin g s, steps have to be tak e n to p ro v id e a d e q u a te p ro te c tio n fo r th e s u r ­ faces u n d e r c o n ditions o f b o u n d a ry lu b ric a tio n . T h is is norm ally ach iev e d in e n g in e e rin g situ a tio n s by in tr o ­ ducing an a d d itiv e in to th e b u lk lu b ric a n t. S u b sta n c es like fatty acids a re f re q u e n tly used fo r m o d e ra te ly severe conditions, w h ilst ex tre m e p re ssu re (E .P .) a d d itiv e s w h ic h t act chem ica lly w ith the b e a rin g solids a re em p lo y e d in ore e x tre m e co n d itio ss. The re sistan c e to m o tio n u n d e r b o u n d a ry -lu b ric a te d c o n d it io n s is fo u n d to o b e y sim ila r law s t o th o se of d ry friction, a lth o u g h the coefficient o f fric tio n is n o rm a lly higher in th e la tte r case. T h u s: F = | i W ---------- (3) wSnre F = fric tio n fo rce; W = a p p lie d lo ad ; P. = coefficient o f friction. It sh ould be n o ted fro m e q u a tio n s (1) a n d (3) th a t th e friction c h a ra c te ris tic s o f ‘fluid-film ’ a n d ‘b o u n d a ry - lubricated’ b e a rin g s a re q u ite different. Mixed L ubrication M any b e a rin g s o p e ra te in the in te rm e d ia te c o n d itio n between b o u n d a ry and fluid-film lu b ric a tio n . T h e s u r ­ faces are n o t c o m p le te ly se p a ra te d b y a la y e r o f fluid, although the re g io n s o f close a p p ro a c h and a sp e rity interaction o c c u r o v e r a v e ry sm all f ra c tio n o f the effective b e a rin g a re a . T h e frictio n c h a ra c te ris tic s o f th e bearing sh o w in d ic a tio n s o f b o th m ec h an ism s, a n d the j'-omplete b e h a v io u r is th u s k n o w n as ‘m ix e d lu b r ic a tio n ’.fiction U nder c o n d itio n s o f dry c o n ta c t o r b o u n d a ry lu b ric a - t.on e q u a tio n (3) gives a good d e sc rip tio n o f the re sist­ ance to sliding m o tio n . T h is e q u a tio n reflects the w ell- known law s o f fric tio n , k n ow n as A m o n to n s ’ L aw s w hich state th at: 1- the fo rce o f fric tio n is in d e p e n d e n t o f th e a p p a re n t b e arin g a re a ; 2- the force o f fric tio n is d ire c tly p r o p o rtio n a l to the a p plied lo ad . M ost su rfa c e s a re q u ite ro u g h on a m o le c u la r scale and when tw o solids c o m e to g e th e r th e y to u c h o n ly a t nigh p o in ts fo r asp e rities). A s p e rity in te ra c tio n s cause appreciable lo ca l e lastic a n d p la stic d e fo rm a tio n b u t th e real a rea o f c o n ta c t n o rm a lly re m a in s a v ery sm all frac- «on of the a p p a r e n t b e a rin g a rea . It is, o f c o u rse, the real c o n ta ct a rea w h ich g o v e rn s the re sista n c e to sliding and this e x p la in s A m o n to n s ’ first finding th a t the fo rc e w friction is in d e p e n d e n t o f the a p p a re n t b e a rin g a rea , ? >0031 sh e a r stre sse s on the a re a s o f a sp e rity i n te r ­ action a re fo u n d to be re a s o n a b ly u n ifo rm , a n d since the i n ^ a r e a , ■ is d ire c tlV P ro p o rtio n a l to to ta l uaa (w ) ; th u s e x p la in in g A m o n to n s ’ second law. Wear A lth o u g h p ra c tic a lly e v e ry th in g w h ic h an e n g in e e r m ak e s w ill u ltim a te ly w ear o u t, this a sp e c t o f trib o lo g y is the lea st e x p lo re d . T h e p h y sic al and chem ica l a ctio n s in v olved in th e w e ar pro cess a re co m p le x , a n d m u ch c u rre n t re sea rc h w o rk is d e v o te d 10 th e ir e lu c id atio n . W ell-iy jo w n form s o f w ear in c lu d e a b ra s io n , a d h e sio n , fa tig u e , e ro sio n a nd c o rro sio n . A b ra siv e w ear m a y occur w h e n a ro u g h , h a rd s u rfa c e slides o v e r a so fte r o ne (e.g. ste el over plastic), o r w hen h a rd p a rtic le s a re tra p p e d b e tw ee n ru b b in g surfaces. In th e la tte r case th e a b ra siv e m a te ria l m ay e n te r th e c o n ju n c tio n fro m th e e n v iro n m e n t, o r it m ay be d e b ris re su ltin g fro m the w e ar process. A d h e siv e w e ar, w h ic h o c c u rs as a re su lt o f local w e lding a t a sp e rity ju n c tio n s, is by fa r th e m o st co m m o n w e ar m ec h an ism . In m an y cases th e v o lu m e o f m a te ria l rem oved in th e ad h esiv e w e a r p ro c e ss satisfies the fo llo w in g re la tio n s h ip s : w here V = v o lu m e of m a te ria l w o rn a w ay ; W = a p p lie d lo ad ; x = to ta l sliding distan c e; Pm = h a rd n e s s o f so fte r m a te ria l; k = w e a r coefficient; T h is e q u a tio n e n a b le s th e b e a rin g d e s ig n e r to e s tim a te th e u se fu l life o f a b e a rin g o r e n d o p ro s th e s is u n d e r given o p e ra tin g c o n d itio n s. It also e n a b le s th e w e a r re sistan c e o f d ifferent m a te ria ls to be e v a lu a te d if th e h a rd n e ss can b e m e a su re d a n d th e d im e n sio n less w e a r coefficient e v a lu a te d . Syn ovial Joints T h e sy n o v ia l jo in t possesses m a n y o f the fe a tu re s of e n g in e erin g b e arin g s. T h e b e a rin g m a te ria l ‘a rtic u la r c a rtila g e ’ is s u p p o rte d on a re la tiv e ly h a rd b a ck in g (b o n e ) a n d lu b ric a te d by a viscous fluid (sy n o v ia l fluid). I t is n e ce ssa ry to u n d e rs ta n d th e p ro p e rtie s o f a ll th ese b e a rin g e le m e n ts a nd the c o n d itio n s e n c o u n te re d in th e jo in t if the lu b ric a tio n m ec h an ism is to be resolved. A rtic u la r c a rtila g e is a re la tiv e ly soft, p o ro u s m a te ria l w ith a s u rfa c e ro u g h n e ss a b o u t ten tim es g re a te r th a n c o n v e n tio n a l e n g in e e rin g b e a rin g su rfa ce s. Tt d e fo rm s u n d e r lo ad and th e fluid c o n ta in e d w ith in the p o ro u s m a te ria l is slow ly expelled. S y n o v ia l fluid is a d ia ly sa te o f b lo o d p lasm a to w h ich is ad d ed h y a lu ro n ic acid, m a n u fa c tu re d by th e synovial m e m b ra n e . T h e fluid is n o n -N e w to n ia n as fa r as its viscous p ro p e rtie s a re c o n ce rn ed . T h u s a t low sh e a r ra te s w h e n the jo in t is a lm o st s ta tio n a ry , sy n o v ia l fluid is u su a lly a t least as viscous as glycerine. A t the h ig h est s h e a r ra te s e n c o u n te re d in r u n ­ n in g it is o n ly a few tim es m o re viscous t h a n w a ter. T h e a d d itiv e m ain ly re sp o n sib le fo r the viscous p r o p e rty of sy n o v ia l fluid is h y a lu ro n ic a cid. N a tu r e h a s p ro v id e d a fluid w h ich gives e x cellent trib o lo g ic a l p ro p e rtie s f o r the c o n d itio n s e n c o u n te re d in th e h u m a n jo in t. S y n o v ia l jo in ts a re re ally d y n a m ic a lly lo a d e d b e arin g s. Tn the case o f the h ip a nd k nee jo in ts the lo ad s v a ry c o n sid e ra b ly . It h a s b een sh o w n th a t in the n o rm a l w a lk in g cycle lo ad s on th e hit? jo in t can rise to th re e o r fo u r tim es bo d y w e ig h t. S im ila r lo ad s a re e n c o u n te re d in the k n e e. Tn v ig o ro u s e x ercises o r a cc id e n ts m u c h g re a te r fo rce s m ay be a p p lie d to th e a r tic u la r surfaces. F ro m a k n o w le d g e o f th e o v e ra ll g e o m e try o f sy novial jo in ts, th e p ro p e rtie s o f a rtic u la r c artila g e a n d synovial fluid, the sliding speeds, a nd lo a d s e n c o u n te re d in typical a ctiv ities, it is p o ss ib le to e s tim a te the th ic k n e ss o f the R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. ) 4 P H Y S I O T H E R A P Y M A RCH , 1977 effective lu b ric a tin g layer. S ev eral im p o r ta n t resu lts e m erge fro m such in v estig a tio n s, b u t th e m a in c o n c lu ­ sions a re as fo llo w s: 1. D u rin g e xercises like w a lk in g th e effective lay e r of lu b ric a n t is th ic k e r th a n the s u rfa c e ro u g h n e s s o f h e a lth y a r tic u la r c a rtila g e (even th o u g h th e la tte r is ro u g h c o m p a re d w ith e n g in e e rin g b e a rin g surfaces): i.e. fluid-film lu b ric a tio n exists f o r p a r t o f th e cycle. 2. T h e lo c a l d e fo rm a tio n o f th e e la stic c a rtila g e e n ­ c o u ra g e s th e g e n e ra tio n o f th is e ffective la y e r o f lu b r ic a n t d u rin g sliding m o tio n a n d th e jo in t th u s e x p e rie n c e s e la s to h y d ro d y n a m ic lu b ric a tio n . N e w to n ia n b e h a v io u r) lea d s to e n h a n c e d p ro te c tio n 3. A t h e e l-strik e th e lo a d rises c o n sid e ra b ly a n d th e jo in t su rfa c e s m ove to w a rd s e a c h o th e r to give a fo rm o f fluid-film lu b ric a tio n k n o w n as squeeze-film a ctio n , p a r tic u la r ly w hen sliding v e lo c ities a re low. 4. D u rin g squeeze-film a c tio n th e u n iq u e c o m b in a tio n o f c a rtila g e p ro p e rtie s ( p a rtic u la rly ela sticity , p o r ­ o sity, a n d su rfa c e q u a lity ) w ith sy n o v ia l fluid c h a r a c ­ te ristic s (m a in ly m o le c u la r c o m p o s itio n a n d n o n - N e w to n ia n b e h a v io u r) leads to e n h a n c e d p ro te c tio n fo r th e a r tic u la r su rfa ce s. T h is h a s b een te rm e d ‘b o o ste d lu b r ic a tio n ’. 5. A fte r p ro lo n g e d lo a d in g a n d p o ss ib ly at th e tim e o f ‘to e -o ff’, w h e n the second lo a d in g p e a k is ap p lie d to th e jo in t in th e w a lk in g cycle, a rtic u la r su rfa ce s com e v e ry close to g e th e r a n d a re p ro te c te d by b o u n d a ry lu b ric a tio n actio n . It is c le a r t h a t no u n iq u e m o d e o f lu b ric a tio n g o v e rn s th e b e h a v io u r o f lo a d -b e a rin g sy n o v ia l jo in ts. In d e ed , th e jo in t a p p e a rs to e n c o u n te r all th e c o n v e n tio n a l lu b r i­ c atio n m ec h an ism s k n o w n to th e e n g in e er. In a d d itio n , the re m a rk a b le a b ility o f th e jo in t to resist squeeze-film a c tio n a p p e a rs to be u n iq u e to th e b io lo g ic a l system . Synthetic Lubricants T h e p o ssib ility o f influ e n cin g the d e v e lo p m e n t o f o s te o ­ a rth ro s is b y in tro d u c in g a sy n th e tic lu b r ic a n t in to th e cap su le o f a jo in t h a s b een e x p lo re d in re c e n t years. L u b ric a n ts w hich re se m b le sy n o v ia l fluid in flow te rm s h a v e b e e n su b je c te d to to x ic ity tests a n d a n u m b e r of c linical tria ls h a v e b een u n d e rta k e n . S ilic o n e s a n d p o ly m e r s o lu tio n s h a v e b een e v a lu a te d , a n d a lth o u g h so m e p a tie n ts d e m o n s tra te d r e m a r k a b le im p ro v e m e n t, th e re is n o t sufficient e v idence to p ro m o te c o nfidence in the view th a t lu b ric a n t c h an g e s in h u m a n j o i n t s w ill so o n be c o m m o n p la c e . E x te n siv e stu d ies o f p o te n tia l lu b r i­ c an ts lea d in g to clin ica l tria ls a re n o w in h a n d a n d it will be so m e tim e b e fo re th e fu ll p o te n tia l of this, a p p ro a c h c a n b e e v a lu a te d . T he B ehaviour o f R eplacem ent Joints T w o o f th e m a jo r p ro b le m s e n c o u n te re d in endo- p ro s th e tic d esign a re trib o lo g ic a l in n a tu re . In th e first place, th e ra te o f w e ar h a s to be sufficiently lo w to e n a b le th e jo in t to o p e ra te s a tis fa c to rily as a b e a rin g for th e re m a in in g life o f th e h u m a n m a c h in e . S e c o n d ly , the fric tio n fo rc e d e v e lo p e d be tw ee n t h s sliding surfaces m u st be m in im ised , n o t o n ly to p ro v id e fre e d o m of m o v e m e n t fo r th e p a tie n t, b u t a lso to lim it th e severity o f th e p ro b le m o f im p la n t fixation. M o s t re p la c e m e n t jo in ts a re e ith e r o f th e m etal-on- m e ta l o r m e ta l-o n -p la s tic v a rie tie s. I f tw o m e ta ls are used it is c u s to m a ry to e m p lo y like m a te ria ls to avoid c o rro s io n p ro b le m s in th e h o stile e n v iro n m e n t o f the body. S uch a c o m b in a tio n o f m e ta ls n o rm a lly leads to h igh fric tio n a n d is g e n e ra lly a v o id e d in engineering be arin g s. In th e h u m a n jo in t m a n y su c c essfu l m etal-on- m eta l b e a rin g s b ased u p o n c h ro m e -c o b a lt a llo y s h a w b een d e signed a n d used. T h e y h a v e a re la tiv e ly IcA w e a r-ra te b u t th e frictio n fo rces a n d to rq u e s a re h ig | c o m p a re d w ith m e ta l-o n -p la s tic b e arin g s. T h e main w e a r m ec h an ism s e n c o u n te re d in m e ta l-o n -m e ta l joints a re a b ra s io n a n d ad h esio n . In re c e n t y e ars d e sig n ers h a v e tu rn e d in c re a s in g ly to m e ta l-o n -p la s tic c o m b in a tio n s o f m a te ria ls . Stainless steels a n d c h ro m e -c o b a lt a llo y s h a v e b e en u sed a n d the m o st p o p u la r plastic h a s b een m e d iu m -d e n sity , high- m o le c u la r w eight p o ly e th y le n e . T h e s e c o m b in a tio n s pro ­ vide m uch lo w e r fric tio n , th u s re liev in g th e fixation p ro b le m , b u t th e plastic te n d s to w e a r a w ay by adhesion, a b ra s io n , a n d p o ssib ly fa tig u e. C a re fu l la b o r a to r y studies h a v e sh o w n th a t in lo a d -b e a rin g jo in ts such a s th e hip a n d th e k n e e th e w e a r ra te is sufficiently low to enable u se fu l lives to som e tw enty-five y e ars to be en v isag e d for th is k in d o f jo in t. T h e se a rc h fo r im p ro v e d b e a rin g m a te ria ls fo r endo- p ro s th e tic d e v e lo p m e n t c o n tin u e d a lo n g sid e fu ndam ental stu d ies o f th e frictio n a n d w e ar process. R eplacem ent jo in ts u s u a lly o p e ra te in th e p re se n c e o f a fluid similar to h e a lth y sy n o v ia l flu id , b u t it is im p o rta n t to realise th a t th o se jo in ts m ad e o f a m e ta l-o n -p la s tic com bination c f m a te ria ls c o u ld p e rfo rm a d e q u a te ly as d ry bearings. R e p la c e m e n t h u m a n b e a rin g s a re n o w a v a ila b le in a v a rie ty o f fo rm s fo r th e h ip , k n e e, s h o u ld e r, e lb o w and finger. D e v e lo p m e n t h a s b e en ra p id d u rin g th e past de ca d e a n d th e re a p p e a rs to be e v ery p ro s p e c t th r' ste a d y a n d p o ssib ly s p e c ta c u la r p ro g re s s w ill be made® th e n e x t te n years. ' A rtic le re c e iv e d A u g u s t, 1976 R ep ro du ce d by S ab in et G at ew ay u nd er li ce nc e gr an te d by th e P ub lis he r (d at ed 2 01 3. )