fixation of ultrahigh-molecular-weight polyethylene liners ... · nephew orthopaedics, memphis, tn)...

The Journal of Arthroplasty Vol. 12 No. 1 1997

Fixation of Ultrahigh-molecular-weight Polyethylene Liners to Metal-backed

Acetabular Cups

V i c t o r G. W i l l i a m s II, BS, L e o A. W h i t e s i d e , M D , S t e p h e n E. W h i t e , M S , a n d

D a n i e l S. M c C a r t h y

Abstract: Locking mechanisms and metal-liner interface surfaces of six modular acetabular systems were evaluated to determine their effect on micromotion and backside wear of the polyethylene liner. Rotational and axial motion between the metal shell and polyethylene liner was measured in the Duraloc (DePuy, Warsaw, IN), Harris-Galante (Zimmer, Warsaw, IN), Impact (Biomet, Warsaw, IN), Lip Loc (Biomet), Precision Osteoloc (Howmedica, Rutherford, N J), and Reflection (Smith & Nephew Orthopaedics, Memphis, TN) designs at the start of each test, and at 1 million, 5 million, and 10 million cycles. At 10 million cycles, the Lip Loc and Reflection cups had significantly lower rim micromotion than the Duraloc and Harris-Galante cups (F < .0010). The Impact, Precision Osteoloc, and Reflection cups had significantly lower rim subsidence than the Harris-Galante cup (F < .0025). The Harris-Galante cup had significantly greater rotational micromotion than the Lip Loc cup (F < .0074), and had significantly greater interface slippage than the Impact and Reflection cups (F < .0070). The Lip Loc produced significantly lower dome micromo- lion than the Harris-Galante (F < .0300). The Lip Loc and Reflection cups had significantly less backside wear than the Duraloc and Harris-Galante cups (P < .000I), the Impact cup (P < .0243), and the Precision Osteoloc (P < .0027) cup. Key words: total hip arthroplasty, implant design, acetabulum, wear, polyethylene.

Modular acetabular components with a metal shell were developed in the 1970s to limit the deformation of all-polyethylene components that appeared to cause aseptic loosening of cemented devices [1]. Crowninshield et al. reported that the metal shell provided a more uniform stress distri- but ion in the bone -cem en t [2]; however, a number of reports link polymeric and metal debris with osteolysis in total hip arthroplasty (THA) [1,3-6]. Wear of the polyethylene cup liner in the metal- backed acetabular cup has been shown to stimu-

From the Biomechanical Research Laboratory, Missouri Bone and Joint Center, St. Louis, Missouri.

Reprint requests: Leo A. Whiteside, MD, Bionrechanical Research Laboratory, 12634 Olive Boulevard, St. Louis, MO 63141.

late osteolysis [1,3-5], and, in some cases, to cause failure of the hip prosthesis. Recent reports suggest that outer wear on the surface of the polyethylene liner results from micromotion of the liner with the metal shell ]7,8].

Most acetabular metal shells depend on their locking mechanism to limit mot ion between the liner and the metal shell [8]. This study evaluated fixation of the polyethylene liner in the metal shell of six total hip systems and analyzed the effect of the locking mechanism and inner surface finish of the metal shell on the generation of backside wear of the polyethylene liner. We hypothesized that acetabular systems with peripheral locking tabs and a polished metal surface would produce the least amoun t of wear at the interface between the polyethylene liner and the metal shell.

25

26 The Journal of Arthroplasty Vol. 12 No. 1 January 1997

Materials and Methods

Rotational and axial mot ion be tween the metal shell and the polyethylene liner was measured in the Duraloc (DePuy, Warsaw, IN), Harr is-Galante (Zimmer, Warsaw, IN), Impact (Biomet, Warsaw, IN), Lip Loc (Biomet), Precision Osteoloc (How- medica, Rutherford, NJ), and Reflection (Smith & Nephew Orthopaedics, Memphis, TN). Each acetabular c o m p o n e n t has a unique locking mechan i sm that is self-engaging on loading. The Duraloc uses the sensor ring, a spring mechan i sm that locks the meta l shell into a recessed groove in the polyethylene liner (Fig. 1). The Harr is-Galante polyethylene liner is press-fit into five locking tabs that groove the polyethylene as it is inserted. The Impact c o m p o n e n t secures the polye thylene liner wi th a hemispherical metal ring and three locking tabs. The polye thylene is press-fit a round the tabs, and the locking ring prevents the cup f rom backing out. The Lip Loc c o m p o n e n t secures the polye thylene with a recessed lip and three locking tabs. The polye thylene liner of the Precision Osteoloc has 12 per ipheral tabs, an inferior recessed groove, and a circumferential ring with 12 minia ture tabs. The 12 per ipheral tabs rest on a circumferential r im in the metal shell, which is the upper ledge of a recessed ring that secures the 12 minia ture tabs. The Reflec- tion has tapered dovetail locking tabs that snap into the slots of the cup. A ridge sits on a circumferential ring that supports the tabs of the polyethylene. The inner surface of the Reflection cup is polished (Fig. 2).

Fig. 2. The Reflection metal shell has tapered dovetail peripheral locking tabs and achieves rigid peripheral locking of the polyethylene liner in the metal shell.

Three c o m p o n e n t s for each design were tested. Each ace tabular c o m p o n e n t was m o u n t e d in a se rvohydraul ic testing device (model 8501, Instron, Canton, MA) wi th a 25 ° tilt in the infe- r io r - super io r direction to s imulate in vivo place- m e n t (Fig. 3). Axial and tors ional loads were applied to the po lye thy lene liner t h rough a 28- m m femora l head glued to the po lye thy lene liner. Compress ive axial loads f rom 272 to 2720 N and in t e rna l - ex te rna l tors ional loads f rom _+ 7.5 N - m were applied using a sinusoidal w a v e f o r m at 10

Fig. 1. Photograph of the Duraloc metal shell. The flexible metal locking wire prevents pushout of the polyethylene liner, but there is no mechanism to prevent the polyethylene liner from slipping inside the metal shell.

Fig. 3. The acetabular component is mounted in an Instron servohydraulic testing device. One linearly variable differential transducer is positioned at the rim, one perpendicular to the rim, and one perpendicular to the dome. Axial and torsional load is applied through the femoral head, and micromotion is measured at the interface between the metal shell and polyethylene liner.

UHMWPE Liners in Metal-backed Cups • Williams et al. 27

Hz for 10 million cycles. This load was chosen to apply torsional load of approximate ly twice that occurring during normal activity [9] to evaluate the safety factor built into the locking mechanism. Three linearly variable differential trans- ducers (LBB-375-PA-060, Schaevitz, Pennsauken, NJ) were used to detect mot ion be tween the polye thylene liner and the metal backing. One linearly variable differential t ransducer m o u n t e d parallel to the rim measured rotational micromotion and interface slippage at the rim, one m o u n t e d perpendicular to the rim measured rim micromot ion and rim subsidence, and one m o u n t e d perpendicular to the dome of the metal shell measured dome mic romot ion and dome subsidence. Micromot ion was defined as the recoverable mot ion be tween cycles. Subsidence was defined as the nonrecoverable sinking dis- p lacement of the polyethylene into . the shell, and interface slippage was defined as the nonrecover - able rotational displacement be tween the poly- e thylene liner and the metal shell. Micromotion, subsidence, and interface slippage were measured and recorded at the start of each test and at 1 million, 5 million, and 10 million cycles. The data obtained at 10 million cycles were compared statistically. The square root of the mot ion was used to obtain a normal distribution of the data to sat- isfy model assumptions. Statistically significant differences in the square roots of scores among the groups was de termined using the F statistic associated with analysis of variance. Scheff~ post hoc tests (P < .05) were used to measure the level of significance be tween pairs. Goodness-of-fi t of the analysis of variance model was checked by careful examinat ion of the residuals.

One of the three components for each design was sputter-coated with gold-palladium before testing. These components were examined with a Wild Leitz photomacroscope at magnifications varying from 6.3× to 32x (model M420, Wild Leitz, Heerbrugg, Switzerland.) Wear on the backside of the polyethylene liner was measured at a magnifi- cation of 16× with a previously published scoring system that evaluated burnishing, scratching, abrasion, cold flow, pitting, and delamination [9]. The polyethylene liners were placed under a grid that divided the liner into eight equal sectors. Each sector was assigned a score (0-6) based on the sever- ity of burnishing, scratching, abrasion, cold flow, pitting, and delamination (Table 1).

Each component score was computed by averaging the sum of the wear scores from each sector. Analysis of variance statistical tests (P < .05) were used to establish statistical significance of differences in wear.

Table 1. Wear Score Analysis

Score Type of Wear

No evidence of wear Burnishing covered < i0% of a sector No other wear modes were present Burnishing covered < 25% of a sector Cold flow, abrasion, scratching covered

< 10% of a sector Burnishing covered 25-50% of a sector Cold flow, abrasion, scratching covered

10-25% of a sector Burnishing covered 50-75% of a sector Cold flow, abrasion, scratching covered

25-50% of a sector Burnishing covered > 75% Cold flow, abrasion, scratching covered

50-75% of a sector Cold flow, abrasion, scratching covered

> 75% of a sector

Results

Rim micromotion between the polyethylene liner and metal shell decreased as the number of cycles increased for each cup (Table 2). A significant difference appeared among the designs w h e n mot ion at the rim was analyzed for all six designs (F < .0010). At 10 million cycles, the Duraloc cup produced significantly greater rim micromotion than the Lip Loc cup with a square root mean difference of 5.3 btm (confidence limits [CL], 1.0-9.50 btm) and the Reflection cup with a square root mean difference of 5.0 btm (CL, 0.8-9.3 btm). The Harris-Galante cup produced significantly greater rim micromotion than the Lip Loc and Reflection cups. Micromotion values at 10 million cycles are listed in Table 2.

Rim subsidence increased as the number of cycles increased for each acetabular cup. A statistically significant difference occurred among the designs when rim subsidence was analyzed (F < .0025). At 10 million cycles, the Harris-Galante had significantly greater subsidence than the Impact, Precision Osteoloc, and Reflection cups. Subsidence values at 10 million cycles are listed in Table 2.

Rotational micromotion decreased as the cycles increased for every cup (Table 2). A significant difference occurred among the designs w h e n the amoun t of rotational micromot ion was analyzed (F < .0074). The Harris-Galante cup produced significantly greater micromot ion th roughout the test than did the Lip Loc cup. The rotational micromotion values at 10 million cycles are listed in Table 2.

Interface slippage did not change significantly as the number of cycles increased for each cup. Those


T a b l e 2. S u m m a r y of M o t i o n T h r o u g h 10 M i l l i o n Cycles

Rim Rim Rotational Interface Dome Dome Micromotion Subsidence Micromotion Slippage Micromotion Subsidence

(~m) (~m) (~m) (~m) (~m) (~m)

Start of Test Duraloc 82.4 _+ 30.2 72.6 _+ 40.61 73.6 _+ 29.8 276.7 ± 399.5 Harris-Galante 359.2 _+ 116.6 168.5 ,+ 181.2 529.2 _+ 280.9 470.3 _+ 286.43 Impact 51.26_+27.85 39.6_+47.7 29.8_+ 13.21 8.46_+8.70 Lip Loc 7.5 ± 7.2I 22.9_+ 15.00 8.2 ± 4.4 54.5 _+ 46.6 Precision Osteoloc 49.8 ± 24.1 39.7 _+ 14.4 27.1 _+ 13.64 66.2 ,+ 47.88 Reflection 6.26,+1.92 2.8 ± 3.27 10.86_+10.51 1.33,+2.30

1 Million Cycles Duraloc 78.3 _+ 43.7 82.7 _+ 55.3 39.8 _+ 4.22 340.4 _+ 457.3 Harris-Ga|ante 164.5 _+ 112.5 343.3 _+ 163.4 216.9 _+ 178.6 829.7 _+ 568.0 Impact 27.13_+17.47 90.20_+73.16 16.8 ± 8.07 25.06_+12.98 Lip Loc 3.35 ± 2.50 48.65 ,+ 9.26 5.2 _+ 2.I 59.5 _+ 32.1 Precision Osteoloc 22.5 ,+ 7.56 32.6 _+ 12.72 11.1 _+ 4.02 89.5 _+ 39.2 Reflection 5.46 _+ 3.72 2.66 ± 2.51 12.93 _+ 12.41 5.2 _+ 5.76

5 Million Cycles Duraloc 59.53 _+ 10.15 98.8,+ 43.52 34.3 _+ 5.28 380.1 _+ 458.83 Harris-Galante 97.13 _+ 29.68 398.83 _+ 139.90 107.26 _+ 5.80 941.5 _+ 645.9 Impact 17.8,+10.6 37.2±30.65 15.33_+6.4 18.2_+22.0 Lip Loc 3.75 _+ 1.48 50.05 _+ 6.57 3.95 _+ 0.91 143.4 ± 138.6 Precision Osteoloc 17.7 _+ 4.24 104.3 _+ 141.62 9.60 _+ 3.4 108.5 ,+ 44.5 Reflection 5.0_+ 1.41 2.2 _+ 1.31 15.1 _+ 14.0 11.9,+ 12.5

10 Million Cycles Duraloc 52.97_+11.1 I01.67 _+ 56.48 32.3_+9.3 414.86,+489.1 Harris-Galante 52.96 _+ 35.03 485.76 ± 264.38 71.87 _+ 2.82 1015.5 _+ 658.51 Impact 16.5_+9.45 45.67_+23.16 14.33,+4.96 22.6 ± 20.6 Lip Loc 3.55 ,+ 1.48 150.1 _+ 149.34 4.15 _+ 1.20 140.7_+ 143.82 Precision Osteoloc 15.03_+3.11 83.6_+ 10.75 8.6_+3.61 97.6,+31.82 Reflection 4.9_+ 1.28 12.2_+ 15.08 15.4_+ 10.02 27.5 -+9.87

F value .0010 .0025 .0074 .0070

35.9 _+ 15.93 45.0_+ 43.31 153.9_+ 66.8 136.5 +_ 100.7 64.73 _+ 50.22 9.46 _+ 14.04

2.65 _+ 1.1 0.65 _+ 0.91 60.9 _+ 10.67 35.5 _+ 10.83 19.0 _+ 7.55 24.73 _+ 41.8

33.5 _+ 14.5 37.7 _+ 53.1 81.3 _+47.09 32.6 _+ 7.92 47.8 _+ 39.4 26.4 _+ 17.74 3.85 _+ 0.07 0.75 _+ 1.06 28.8 _+ 6.94 74.0_+ 27.5 31.8 _+ 23.03 10.8 _+ 11.7

28.3 _+ 16.2 85.3 _+ 83.14 49.6 ,+ 8.76 141.7 _+ 135.86 24.8 _+ 15.4 22.73 _+ 12.20

3.6_+ 0.14 14.65 _+ 2.05 24.5 _+ 4.12 84.2 _+ 21.9 27.2 _+ 12.44 34.7 -+49.03

25.3 _+ 14.84 96.37 _+ 94.7 37.8 ± 18.9 162.4 ± 133.35

24.67± 19.0 25.33 ± 8.11 3.65 _+ 0.21 14.95 ,+ 2.05 19.7_+ 6.5 140.2 _+ 127.12 27.7 ,+ 5.87 40.1 _+ 39.41

.0314 .2

w i t h l o w i n i t i a l s l i p p a g e t e n d e d to m a i n t a i n g o o d

f i x a t i o n t h r o u g h o u t t h e tes t , a n d t h o s e w i t h h i g h

e a r l y s l i p p a g e c o n t i n u e d to sl ip t h r o u g h o u t t h e t e s t

(Tab le 2) . A s i g n i f i c a n t d i f f e r e n c e o c c u r r e d a m o n g

t h e d e s i g n s w h e n t h e a m o u n t of i n t e r f a c e s l i p p a g e

w a s a n a l y z e d (F < . 0 0 7 0 ) . A t 10 m i l l i o n cyc les , t h e

H a r r i s - G a l a n t e a n d D u r a l o c c u p s h a d t h e g r e a t e s t

i n t e r f a c e s l i p p a g e . A f t e r 10 m i l l i o n cyc les , t h e H a r -

r i s - G a l a n t e p r o d u c e d s i g n i f i c a n t l y g r e a t e r i n t e r f a c e

s l i p p a g e t h a n d i d t h e I m p a c t a n d R e f l e c t i o n cups .

D o m e p i s ton±r ig m i c r o m o t i o n a n d s u b s i d e n c e

d e c r e a s e d as t h e n u m b e r of cyc les i n c r e a s e d i n

s o m e i m p l a n t s a n d r e m a i n e d u n c h a n g e d i n o t h e r s

(Tab le 2) . A s i g n i f i c a n t d i f f e r e n c e o c c u r r e d a m o n g

t h e d e s i g n s w h e n m o t i o n a t t h e d o m e w a s a n a -

l y z e d (F < . 0 3 1 4 ) . T h e H a r r i s - G a l a n t e p r o d u c e d

s i g n i f i c a n t l y g r e a t e r d o m e p i s t o n i n g m i c r o m o t i o n

t h a n d i d t h e Lip Loc c u p . N o s i g n i f i c a n t d i f f e r e n c e s

o c c u r r e d a m o n g t h e six d e s i g n s f o r d o m e s u b s i -

d e n c e a f t e r 10 m i l l i o n cyc l e s (F < . 2 0 0 0 ) .

M a c r o s c o p i c e v a l u a t i o n of t h e l i n e r s s h o w e d sig-

n i f i c a n t w e a r o n b o t h t h e D u r a l o c a n d H a r r i s -

G a l a n t e c u p s (Figs. 4 , 5 ) . T h e D u r a l o c a n d H a r -

r i s - G a l a n t e p o l y e t h y l e n e l i n e r s h a d e v i d e n c e of

c o l d f low, s c r a t c h i n g , a n d a b r a s i o n of p o l y e t h y l e n e

i n t h e d o m e r e g i o n (Fig. 4 ) . M e t a l d e b r i s w a s

e m b e d d e d i n 4 0 % of o n e of t h e D u r a l o c p o l y e t h y -

l e n e l i n e r s a n d t h e r e w a s e v i d e n c e of s e v e r e

d e l a m i n a t i o n (Fig. 5). T h e d i f f e r e n c e s in b a c k s i d e

w e a r a m o n g t h e f o u r c o m p o n e n t s w i t h a r i g id

p e r i p h e r a l l o c k i n g m e c h a n i s m w e r e m i n i m a l , a n d

c o n s i s t e d of f l a t t e n i n g of m a c h i n e m a r k s a r o u n d

t h e e d g e s of t h e s c r e w h o l e s . N o a b r a s i o n o r c o l d

f l o w w a s s e e n o n a n y of t h e p o l y e t h y l e n e c o m p o -

n e n t s t h a t h a d r i g id p e r i p h e r a l l o c k i n g m e c h a -

n i s m s (Fig. 6) .

T h e m e a n w e a r s c o r e s a t 10 m i l l i o n cyc l e s w e r e

3 .0 -+ 0 . I 2 5 fo r t h e D u r a l o c , 3 .6 ± 0 . 4 fo r t h e H a r -

r i s - G a l a n t e , 1.0 + 0 . 0 7 fo r t h e P r e c i s i o n O s t e o l o c ,

0 . 8 ± 0 .2 fo r t h e h n p a c t , 0 . 1 2 5 ± 0 . 1 2 5 fo r t h e

R e f l e c t i o n , a n d 0 . 0 8 3 _+ 0 . 1 4 4 fo r t h e Lip Loc. T h e

Lip Loc, R e f l e c t i o n , I m p a c t , a n d P r e c i s i o n O s t e o l o c

c u p s p r o d u c e d s i g n i f i c a n t l y l o w e r w e a r s c o r e s t h a n

d id t h e D u r a l o c (P < . 0 0 0 1 ) a n d t h e H a r r i s - G a l a n t e

(P < . 0001) cups . T h e Lip Loc p r o d u c e d s i g n i f i c a n t l y

l o w e r w e a r s c o r e s t h a n d i d t h e I m p a c t (P < . 0 1 6 3 )

a n d P r e c i s i o n O s t e o l o c (P < . 0 0 1 9 ) c u p s . T h e Re -

f l e c t i o n p r o d u c e d s i g n i f i c a n t l y l o w e r w e a r s c o r e s

t h a n d id t h e I m p a c t (P < . 0 2 4 3 ) a n d P r e c i s i o n

O s t e o l o c (P < . 0 0 2 7 ) c u p s (Fig. 7) .

Fig. 4. Gold-palladium-coated specimen of the Har- ris-Galante polyethylene liner after 10 million cycles. Severe abrasion surrounds the screw holes and extends across most of the surface of the component. Gouged- out defects mark the position of the peripheral tabs.


Discussion

i

Osteolytic destruct ion of bone is a major compli- cation of both cemented and cementless hip ar throplas ty [10]. Particulate wear debris, pr imari ly f rom ul t rahigh-molecular -weight polyethylene, induces osteolysis via in f l ammatory mediators [1,3,4-71. Recent reports suggest that backside wear of the polye thylene liner m a y be a significant source of po lye thylene wear debris. Zhu et al. reported that mic romot ion be tween the liner and the shell is caused most ly by rotational forces [8]. To ensure that a reasonable safety margin was tested, a value for torsional load approx imate ly twice physiologic load [111 was chosen for this study. The locking m e c h a n i s m is the feature of the modu la r acetabular c o m p o n e n t that controls the mot ion be tween the polyethylene liner and the metal shell. The results of this s tudy demons t ra te

that rigid per ipheral capture of the polye thylene liner could effectively control mic romot ion and minimize wear.

The cups that did not employ a tight peripheral snap-fit mechan i sm to lock the polyethylene liner directly to the metal shell provided minimal rotational stability. The ring on the Duraloc shell initially secured the liner in place, but lacked a mechan ism to limit rotational motion. This inability to resist rotational mot ion generated polyethylene debris at the locking springs. The flexible tab locking mechanism on the Harris-Galante cup did not adequately resist rotational forces and, therefore, allowed a large amoun t of rotational micromotion. Seating of the polyethylene liner in the metal shell produced large polyethylene slivers as the fingers grooved the polyethylene. Peripheral capture mechanisms that locked the polyethylene liner directly to the metal shell effectively limited rotational motion. The Impact, Precision Osteoloc, and Reflection implants have oversized tabs on the metal shell and slightly undersized slots in the polyethylene liners that facil- itate tight fit at the tab interface.

Most studies that have examined liner conformity have repor ted gaps be tween the liner and the meta l shell [7,12]. The ability of the polye thylene liners to conform with the shell depends on several mechanisms. Both Fehring et al. and Huk et al. found that some implants require load to create contact area be tween the liner and the shell, while others depend on cold flow to conform the liner to the shell [7,121. Conformity of the polyethylene liner to the meta l -backed shell can also de te rmine the a m o u n t of mot ion that is seen at the interface of the two surfaces. Zhu et al. repor ted that poor conformi ty be tween the polye thylene liner and the meta l shell can cause excessive mic romot ion at the

Fig. 5. Mating surfaces of a Duraloc acetabular component. Metal debris particles are embedded in most of the superior region of the polyethylene liner. The metal surfaces are also worn.

Fig. 6. Gold-palladium-coated specimen of the Lip Loc polyethylene component after 10 million cycles. No signs of abrasion are present. The machining marks are flattened around the periphery of the screw holes.


4 ¸

Fig. 7. Wear scores were sta- 3.5 tistically significantly different

between acetabular compo- 3 nents with rigid peripheral locking mechanisms and those ~ 2.5- with minimal peripheral cap- ~ ture. The differences in back- ~ 2) side wear among the four ~ 1.5) components with a rigid peripheral locking mechanism were minimal and consisted of flattening of machining marks around the edge of the screw holes.

After 10,000,000 cycles

1 / Y, T

0.5 -:, T 0-:

Lip Loc Reflection Impact Precision Osteoloc

l T

Duraloc Harris/Galante

interface of surfaces [8]. During the 1-million-cycle test performed in this study, however, progres- sively increasing conformity did not improve fixation, al though it did result in cold flow of the polyethylene into the screw holes.

Backside wear of a polyethylene liner seems to be directly related to motion between the shell and liner. The more axial and rotational motion that occurred between the liner and the shell, the more cold flow, abrasion, pitting, and scratching of the backside surface were apparent. Cups that had significant rim and rotational micromotion such as the Duraloc and Harris-Galante cups had the most backside wear and the highest wear scores in the group. The Impact, Precision Osteoloc, and Reflection cups had the least backside wear and micromotion.

Bobyn et al. reported that a smoother finish diminishes polyethylene debris from liner shell mot ion [131. The Duraloc and Harris-Galante cups did not have a highly polished inner surface, and this probably contributed to the severe wear on the outer surface of the polyethylene liners seen in this study. The implant with a polished inner metal shell did not have significantly less wear than that of the other implants with rigid peripheral locking mechanisms, so it is likely that the most important feature to prevent backside wear is the locking mechanism and not the surface finish of the metal shell. As there was no difference in wear scores among implants that had effective locking of the polyethylene, it is not likely that minor polishing of the inner surface will produce significant wear reduction over that achieved by rigid capture of the liner.

Recent reports of severe wear of the outer surface of the polyethylene liner support the findings of this study [7,14]. It is clear that ineffective locking of the polyethylene liner in the metal shell can

cause osteolysis and early failure of the implant [7,14]. Acetabular components with sound peripheral locking mechanisms between the metal and polyethylene, and snug fit of the liner in the shell, are likely to have superior clinical performance.

Acknowledgments

The authors thank Diane Morton, BA, for edito- rial assistance in the preparation of the manuscript, and Jane Fornoff, DPhil, Division of Biostatistics, Washington University, for statistical assistance.

References

l. Collier P, Mayor MB, Supprenant BA et al: The biomechanical problems of polyethylene as a bear- ing surface. Clin Orthop 261:107, 1990

2. Crowninshield RD, Pedersen DR, Brand RA, John- ston RC: Analytical support for acetabular component metal backing. In Hungerford DS (ed): The hip: Proceedings of the 1 lth Open Scientific Meeting of the Hip Society. Mosby, St. Louis, 1983

3. Amstutz HC, Campbell P, Kossovsky N, Clarke IC: Mechanisms and clinical significance of wear debris- induced osteolysis. Clin Orthop 276:7, 1992

4. Langkamer VG, Case CP, Heap P et ah Systemic distribution of wear debris after hip replacement. J Bone Joint Surg 74B:831, 1992

5. Schmalzried TP, Kwong LM, Jasty M e t al: The mechanism of loosening of cemented acetabular components in total hip arthroplasty. Clin Orthop 274:60, 1992

6. Willert HG, Bertram H, Buchhorn GH: Osteolysis in arthroplasty of the hip. Clin Orthop 258:95, 1990

7. Huk OL, Bansal M, Bens F et al: Polyethylene and metal debris generated by non-articulating surfaces of modular acetabular components. J--Bone Joint Surg 76B:568, 1994


8. Zhu M, Christie M J, Spengler DM: Micromotion of acetabular polyethylene liner in metal shell. Pre- sented at the 20th Annual Meeting of the Society for Biomaterials, Boston, MA, April 1994

9. Davidson JA, Schwartz G, Lynch G, Gir S: Wear, creep, and frictional heating of femoral implant articulating surfaces and the effect on long-term performance. Part II. Friction, heating, and torque. J Biomed Mater Res 22:69, 1988

10. Tanner MG, Whiteside LA, White SE: Effect of polyethylene quality on wear in total knee arthroplasty. Clin Orthop 317:83, 1995

11. Goldberg BA, Noble PC: The effect of cup design on particulate penetrat ion of the acetabular interface.

Presented at the 20th Annual Meeting of the Society for Biomaterials, Boston, MA, April 1994

12. Fehring TK, Hurley PT, Braun E et ah Modular acetabular components: are they really metal backed? Presented at the 59th Annual Meeting of the Ameri- can Academy of Orthopaedic Surgeons, Washington, DC, February 1992

13. Bobyn JD, Tanzer M, Krygier JJ et ah Concerns with modulari ty in total hip arthroplasty. Clin Orthop 298:27, i994

14. Berry DJ, Barnes CL, Scott RD et ah Catastrophic failure of the polyethylene liner of uncemented acetabular components. J Bone Joint Surg 76B:575, 1994

fixation of ultrahigh-molecular-weight polyethylene liners ... · nephew orthopaedics, memphis, tn)...

Documents