Push-Out Testing of Cementless Acetabular Components with

ORIGINAL« ARBEITEN

Push-Out Testing of Cementless Acetabular Components with

Equatorial Roughened Surface

Ausstoßversuch einer zementfreien Hüftgelenkspfanne mit äquatorialer rauer Beschichtung

Aseptic loosening due to wear particles is the most important long- term complication after total hip arthroplasty. One concept to im- prove acetabular fixation is to focus on long-term preservation of bone at the equatorial area of uncemented acetabular components.

This goal may be achieved by constructing acetabular components with an equatorial area which is rougher than the dome so that stress- shielding is less likely to occur within this area. This could potentially result in improved sealing of the implant-bone interface against wear particles and improved long-term fixation of acetabular implants. In this study we tested the primary press-fit quality of such uncemented acetabular components. With the chosen experimental setting we found that an uncemented acetabular component with a rough equa-

torial area and a smooth dome may have a press-fit quality com- parable to that of a fully roughened acetabular component This finding supports the idea to construct acetabular components with equatorial coating from the perspective of primary press-fit.

Marius v o n R n o c h ' , Robin Büscher2, A n d - reas Piotrowski3, Guido Saxler1, A l f o n s Fischer2, Franz Löer1

Schlagworte:

G e l e n k e r s a t z - P f a n n e - A u s s t o ß v e r s u c h - ä q u a t o r i a l e r a u e B e - s c h i c h t u n g

Keywords:

A r t h r o p l a s t y - Cup - P u s h - O u t T e s t i n g - E q u a t o r i a l R o u g h S u r - f a c e

Introduction

A s e p t i c l o o s e n i n g d u e to w e a r p a r t i c l e s is t h e m o s t i m p o r - t a n t l o n g - t e r m c o m p l i c a t i o n a f t e r t o t a l hip a r t h r o p l a s t y [8,

10, 13]. A n i m p o r t a n t goal of c e m e n t l e s s a c e t a b u l a r f i x a t i o n s h o u l d b e t o c o n s o l i d a t e t h e b o n d b e t w e e n i m p l a n t a n d b o n e at t h e e n t r a n c e to t h e i m p l a n t - b o n e i n t e r f a c e . By c o n - s o l i d a t i n g t h e b o n d at this l o c a t i o n a m o r e e f f e c t i v e b a r r i e r a g a i n s t i n t r u s i o n of w e a r p a r t i c l e s i n t o t h e i n t e r f a c e m a y be created. This c o n c e p t m a y lead t o n e w c e m e n t l e s s a c e t a b u l a r i m p l a n t s w i t h i m p r o v e d l o n g - t e r m results.

R e m o d e l i n g of t h e b o n e a d j a c e n t to h i p r e p l a c e m e n t s h a s b e e n described in g r e a t detail f o r t h e f e m o r a l side, b u t o n l y a f e w r e p o r t s a n d d a t a are a v a i l a b l e f o r t h e a c e t a b u l a r side [4, 6, 7, 9, 11, 12, 15]. A c c o r d i n g t o W o l f f s law [14] local b o n e is p r e s e r v e d as l o n g as stresses a r e a p p l i e d a n d is re- s o r b e d w h e n stresses a r e t r a n s f e r r e d e l s e w h e r e . This l a t t e r p h e n o m e n o n is k n o w n as s t r e s s - s h i e l d i n g . F o r c e m e n t l e s s

1 Departmenl o f Orthopaedic Surgery. University of Duisburg-Essen, Essen. Germany

2 Institute of Product Engineering / Materials Science. University o f Duisburg - Essen. Duisburg, Germany

3 gb Implantat-Technologic GmbH, Essen, Germany

a c e t a b u l a r c o m p o n e n t s it h a s b e e n d e s c r i b e d u s i n g q u a n t i t a - t i v e b o n e d e n s i t y m e a s u r e m e n t s [15]. U s i n g a f i n i t e e l e m e n t a n a l y s i s it h a s b e e n p r e d i c t e d t h a t l o n g - t e r m b o n e d e n s i t y will b e g r e a t e s t at t h e s u p e r o m e d i a l a s p e c t of t h e c u p [5].

W i t h t h i s as t h e a r e a of g r e a t e s t l o a d t r a n s f e r t h e e q u a t o r i a l area, w h i c h is m o s t i m p o r t a n t f o r p r i m a r y p r e s s - f i t fixation [3], is p r o n e t o s t r e s s - s h i e l d i n g . This i m p l i e s t h a t a c c e s s o f w e a r p a r t i c l e s to t h e i m p l a n t - b o n e i n t e r f a c e m a y be f a c i l i t a - ted as e q u a t o r i a l s t r e s s - s h i e l d i n g p r o g r e s s e s .

One c o n c e p t to i m p r o v e a c e t a b u l a r f i x a t i o n is t o f o c u s o n l o n g - t e r m p r e s e r v a t i o n of b o n e at t h e e q u a t o r i a l a r e a of u n - c e m e n t e d a c e t a b u l a r c o m p o n e n t s . This g o a l m a y b e a c h i e - v e d b y c o n s t r u c t i n g a c e t a b u l a r c o m p o n e n t s w i t h a n e q u a t o - rial a r e a w h i c h is r o u g h e r t h a n t h e d o m e so t h a t s t r e s s - s h i e l d i n g is less likely to o c c u r w i t h i n t h i s a r e a . This m a y result in i m p r o v e d s e a l i n g of t h e i m p l a n t - b o n e i n t e r f a c e a g a i n s t w e a r p a r t i c l e s a n d p r o l o n g e d l o n g - t e r m f i x a t i o n of a c e t a b u l a r i m p l a n t s .

Here w e i n v e s t i g a t e d w h e t h e r a n a c e t a b u l a r c o m p o n e n t w h i c h w a s d e s i g n e d t o p r o m o t e b e t t e r l o n g - t e r m b o n e p r e - s e r v a t i o n at t h e e q u a t o r i a l area h a s a p r e s s - f i t q u a l i t y w h i c h is c o m p a r a b l e t o a c o n v e n t i o n a l f u l l y r o u g h e n e d c e m e n t l e s s a c e t a b u l a r c o m p o n e n t . W e h y p o t h e s i z e d t h a t a n a c e t a b u l a r

~y ^ BlOmaterialien 6 (2), 2005

ORIGINAL

ARBEITEN

component with a rough equatorial area and a smooth dome has a press-fit quality comparable to that of an acetabular component which is fully roughened.

Materials and Methods

Acetabular components

We used two conventional cementless acetabular compo- nents made from titanium alloy. The cup size was 62mm.

The surface was roughened titanium plasma spraying. Five surface roughness measurements with a laser profilometer (UBM Messtechnik, Ettlingen, Germany) according to DIN 4768 were performed. The measured length was 5mm with a resolution of 2000/mm. Mean R

was 25.96 +/- 1.24 pm, mean R

was 98.96 +/- 12.6mm, and R

was 123.82 +/- 6.64mm. The two acetabular components used were of the same shape, material, surface roughness, and design features.

The only design difference was that Cup A (Fig. 1) was fully covered with a rough macrostructure whereas Cup Β was co- vered with a rough macrostructure only at the equatorial area with a smooth dome (Fig. 2). This design difference was achieved by fine turning the rough dome area of Cup A.

Pure PUR RG 80 polyurethane foam blocks sized 100mm χ 1000mm χ 70mm with a central hole of 8.5mm were ordered from the manufacturer (Gaugier Et Lutz oHG, Aalen-Ebnat, Germany). An acetabular cavity of 60mm was reamed ma- nually into these foam blocks by an experienced surgeon.

1=n> A # 0 0 1cm B # 0 0

Fig. I :Cup A was a titanium corund blasted cementless implant with a fully roughened surface.

Fig. 2:Cup Β was a titanium corund blasted cementless implant with a roughened equatorial area and a smooth dome.

Marius von Knoch: Push-Out testing of Cementless Acetabular Components

Fig. 3: The insertion device used had a diameter larger than the acetabular components tested.

This facilitated level to level inser- tion with the foam blocks.

Fig. 4: Push-out testing was per- formed perpendicular to the im- plants.

Student's t-test for unpaired data was applied to determine the significant differences in the mean maximum push-out forces for both acetabular components. A p-value of <0.05 was considered statistically significant.

Cup A

A hydraulic testing machine (RM 25, Schenck Trebel Corp., Deer Park, NY) was used to insert and to push-out the aceta- bular components. Testing was performed at room tempera- ture (22° Celsius), with a vertical advance rate of 10mm per minute. Motion and impaction force were recorded to con- trol uniform impaction throughout the experiment. The depth of insertion was determined by inserting the cup level to level with the foam block which was confirmed optically and by a rise in impaction force (Fig. 3).

Ten push-out tests were performed in with Cup A and ten with Cup Β by turning the foam block with the acetabular component upside down perpendicular to the implants.

Using the hydraulic testing machine the cups were pushed out of the foam block with a metallic pin through the 8.5mm hole (Fig. 4).

A pin with a diameter of 8mm was fixed in the crossarm to avoid any movement during push-out. Again, motion and force were recorded (Fig. 5, 6). After each test, the cups were cleaned of foam debris with compressed air and acetone so- lution.

Fig. 5: Diagram shows motion and force needed to push out Cup A the fully roughened acetabular component.

Cup A

Fig. 6: Diagram shows motion and force needed to push out Cup B, the acetabular component with the roughened equatorial area and the smooth dome.

| B I O m a t e r i a l i e n 6 (2), 2005

Marius von Knoch: Push-Out Testing of Cementless Acetabular Components

ORIGINAL· ARBEITEN Results

A f t e r b e i n g p u s h e d o u t of t h e f o a m b l o c k s all t h e c u p s w e r e c o v e r e d by p o l y u r e t h a n e d e b r i s in a u n i f o r m e q u a t o r i a l p a t - t e r n . In b o t h t y p e s of c u p m o s t d e b r i s w a s f o u n d at t h e p e r i - p h e r y w i t h less o r a l m o s t n o d e b r i s in t h e d o m e r e g i o n . T h e m e a n m a x i m u m p u s h - o u t f o r c e f o r Cup A w a s 143.7N + / - 34.2N ( r a n g e : 9 0 N t o 182N); t h e m e a n m a x i m u m p u s h - o u t f o r c e f o r Cup Β w a s 176.6N + / - 27.4N ( r a n g e : 117N t o 2ION) (Table 1). This d i f f e r e n c e w a s o f b o r d e r l i n e statistical s i g n i f i c a n c e (p=0.054).

Table I: Maximum push-out forces recorded for both types of cups.

A s c a n be seen f r o m t h e c u r v e s in F i g u r e s 5 a n d 6, t h e s l o p e of t h e c u r v e s a h e a d of t h e m a x i m u m w a s a l w a y s h i g h e r w h e n c o m p a r e d to t h e s l o p e a f t e r it. This c a n be e x p l a i n e d b y t h e d i f f e r e n c e s in s t a t i c a n d d y n a m i c f r i c t i o n . B e f o r e t h e h i g h e s t f o r c e is a t t a i n e d , t h e c u p is m o v i n g v e r t i c a l l y w i t h - o u t l o o s i n g t h e m e c h a n i c a l grip to t h e p o l y m e r (static f r i c - tion). H o w e v e r , w h e n t h e f o r c e s e x c e e d t h e v a l u e s of t h e s t a t i c friction, t h e c u p s t a r t s t o m o v e r e l a t i v e t o t h e f o a m block and d y n a m i c f r i c t i o n p r e d o m i n a n t l y o c c u r s . T h e d y - n a m i c f r i c t i o n a l f o r c e s a r e m a i n l y i n f l u e n c e d b y t h e d e g r e e of m e c h a n i c a l i n t e r l o c k i n g (i.e. s u r f a c e r o u g h n e s s ) a n d a b r a - sive w e a r of t h e p o l y u r e t h a n e , r e s p e c t i v e l y .

Discussion

Hypothesis

W i t h t h i s e x p e r i m e n t w e w e r e a b l e t o s u b s t a n t i a t e o u r h y - p o t h e s i s : a n a c e t a b u l a r c o m p o n e n t w i t h a r o u g h e q u a t o r i a l area a n d a s m o o t h d o m e h a d a p r e s s - f i t q u a l i t y w h i c h w a s c o m p a r a b l e t o a f u l l y s t r u c t u r e d a c e t a b u l a r c o m p o n e n t in o u r e x p e r i m e n t a l s e t t i n g . Of c o u r s e , it is n o t k n o w n h o w t h i s will t r a n s l a t e i n t o in v i v o c o n d i t i o n s a n d a d d i t i o n a l re- s e a r c h is n e e d e d h e r e .

Why was there a difference?

W e e x p e c t e d t h e p u s h - o u t f o r c e s f o r t h e c u p s w i t h a n e q u a - torial r o u g h e n e d s u r f a c e t o b e s i m i l a r t o t h e c o m p l e t e l y s t r u c t u r e d c u p s b e c a u s e it w a s s u s p e c t e d b y o t h e r s t h a t t h e e q u a t o r i a l area m a y p l a y a n i m p o r t a n t role in t h e p r i m a r y stability of p r e s s - f i t t e d u n c e m e n t e d a c e t a b u l a r c o m p o n e n t s [1, 2]. This r e m a i n e d t o b e p r o v e n f o r a n a l m o s t c o m p l e t e l y s m o o t h d o m e . W e w e r e s u r p r i s e d t h a t t h e p u s h - o u t f o r c e s w e r e in f a c t b o r d e r l i n e s i g n i f i c a n t l y h i g h e r f o r t h e c u p s w i t h a n e q u a t o r i a l r o u g h e n e d s u r f a c e w i t h t h e c h o s e n e x p e r i - m e n t a l s e t t i n g . This d i f f e r e n c e is d i f f i c u l t to e x p l a i n . One p o s s i b i l i t y is t h a t c u p s w i t h a f l a t t e r p o l e c o u l d b e i m p l a n t e d m o r e d e e p l y i n t o t h e f o a m b l o c k s . In fact, t h e r e m o v a l of t h e r o u g h s u r f a c e of Cup Β m a d e t h e s e c u p s at least 0 . 3 m m

f l a t t e r in t h e p o l a r r e g i o n . N o n e t h e l e s s , w e b e l i e v e t h a t d e e - p e r i m p a c t i o n of t h e c u p b e c a u s e of t h e f l a t t e r p o l e is n o t a likely e x p l a n a t i o n h e r e b e c a u s e i n s e r t i o n o f t h e c u p s w a s t e r m i n a t e d in e a c h c a s e w h e n t h e rim of t h e c u p a n d t h e f o a m b l o c k w e r e level w i t h e a c h o t h e r . This w a s e n s u r e d b y t h e p u s h i n g d e v i c e w h i c h h a d a g r e a t e r d i a m e t e r t h a n t h e c u p (Fig. 3), b y o p t i c a l c o n t r o l of i n s e r t i o n , a n d b y c o n t r o l - l i n g t h e i m p a c t i o n f o r c e .

A m o r e p r o b a b l e e x p l a n a t i o n is t h a t t h e f l a t t e r p o l e in Cup Β led to less elastic d e f o m a t i o n of t h e p o l y m e r i c m a t e r i a l w i t h i n t h e p o l a r r e g i o n d u r i n g t h e i n s e r t p r o c e d u r e . This e f f e c t w h i c h is m a i n l y d u e t o a r e d u c e d h e i g t h of t h e t y p e Β c u p m a y i n c r e a s e t h e elastic r e p u l s i o n f o r c e s a c t i n g o n t h e p o l e a n d c o n s e q u e n t l y e a s e t h e p u s h - o u t of t h e c u p .

Push-out testing

P u s h - o u t t e s t i n g of a c e t a b u l a r c o m p o n e n t s m a y n o t be r e - p r e s e n t a t i v e of t h e f o r c e s w h i c h o c c u r in v i v o . In f a c t , m e - c h a n i c a l in v i t r o t e s t i n g of i m p l a n t - r e l a t e d issues will a l - w a y s b e a s i m p l i f i c a t i o n of in v i v o f o r c e s . W e c h o s e t o m e a - s u r e s t r a i g h t p u s h - o u t f o r c e s b e c a u s e t h i s m a d e t h e p r e s s - fitted c u p s m o s t v u l n e r a b l e to l o o s e fixation.

W e w e r e n o t a i m i n g p r i m a r i l y at r e t r i e v i n g d a t a s i m i l a r t o in v i v o f o r c e s . Our a i m w a s t o c o m p a r e d i f f e r e n t c u p d e s i - g n s . T h e r e f o r e , t h e e x p e r i m e n t a l s e t - u p w i t h n o n - a n a t o m i - cal f o a m b l o c k s s e e m e d t o be s u f f i c i e n t here.

Summary

W i t h t h e e x p e r i m e n t a l s e t t i n g c h o s e n w e f o u n d t h a t a n u n - c e m e n t e d a c e t a b u l a r c o m p o n e n t w i t h a r o u g h e q u a t o r i a l area a n d a s m o o t h d o m e m a y h a v e a p r i m a r y p r e s s - f i t q u a - lity w h i c h is c o m p a r a b l e t o a f u l l y r o u g h e n e d a c e t a b u l a r c o m p o n e n t . This f i n d i n g m a y lead t o t h e d e s i g n o f n e w t y p e s o f a c e t a b u l a r c o m p o n e n t s w h i c h m a y e n h a n c e l o n g - t e r m s e a l i n g of t h e i m p l a n t - b o n e i n t e r f a c e a g a i n s t w e a r p a r - ticles a n d t h u s i m p r o v e l o n g - t e r m f i x a t i o n .

Acknowledgements

This s t u d y w a s s u p p o r t e d b y a g r a n t f r o m t h e V e r e i n d e r F r e u n d e u n d F ö r d e r e r d e s E v a n g e l i s c h e n K r a n k e n h a u s e s E s s e n - W e r d e n e.V. T h e a u t h o r s a r e g r a t e f u l to g b I m p l a n t a t T e c h n o l o g i e G m b H f o r p r o v i d i n g p r o t o t y p e a c e t a b u l a r c o m - p o n e n t s f o r t e s t i n g , D r . - I n g . G ü n t e r B e n s m a n n (CEO, g b I m - p l a n t a t T e c h n o l o g i e GmbH) f o r a d v i c e r e g a r d i n g t h e e x p e r i - m e n t a l d e s i g n , a n d t o Ms. Kaye S c h r e y e r f o r e d i t o r i a l h e l p w i t h t h e m a n u s c r i p t .

Zusammenfassung

Die a s e p t i s c h e L o c k e r u n g v o n H ü f t e n d o p r o t h e s e n a u f g r u n d v o n A b r i e b p a r t i k e l n stellt die b e d e u t e n d s t e L a n g z e i t k o m p l i - k a t i o n n a c h G e l e n k e r s a t z d a r . Es gibt d a s K o n z e p t , d a s s d e r ä q u a t o r i a l e Bereich d e r P f a n n e r a u e r k o n s t r u i e r t w i r d als d e r Pol d e r P f a n n e . H i e r d u r c h wird i n a k t i v i t ä t s b e d i n g t e r A b b a u des K n o c h e n s im Bereich des Ä q u a t o r s u n w a h r s c h e i n l i c h e r . H i e r d u r c h soll d a s I m p l a n t a t - K n o c h e n - I n t e r f a c e b e s s e r g e g e n A b r i e b p a r t i k e l a b g e s c h o t t e t w e r d e n , w a s zu e i n e r v e r - b e s s e r t e n L a n g z e i t s t a b i l i t ä t der P f a n n e f ü h r e n soll.

In d e r h i e r v o r l i e g e n d e n A r b e i t t e s t e t e n w i r die p r i m ä r e S t a - bilität e i n e r s o l c h e n P f a n n e . W i r s t e l l t e n fest, d a s s d e r p r i m ä r e P f a n n e n s i t z gleich g u t w a r f ü r e i n e h e r k ö m m l i c h e , k o m p l e t t r a u e P f a n n e u n d eine P f a n n e mit g l a t t e m Pol. Die- ses E r g e b n i s ist ein Schritt h i n z u r E n t w i c k l u n g n e u e r F i x a - t i o n s k o n z e p t e d e r a c e t a b u l ä r e n V e r a n k e r u n g . D a s Ziel ist die v e r b e s s e r t e A b s c h o t t u n g des I m p l a n t a t - K n o c h e n - I n t e r f a c e g e g e n A b r i e b p a r t i k e l z u r v e r b e s s e r t e n L a n g z e i t s t a b i l i t ä t v o n a c e t a b u l ä r e n K o m p o n e n t e n .

Cup, Group A F™<[N] Cup, Group Β Fmax [N1

A#0I 176 B#0l 185

A#02 145 B#02 210

A#03 179 B#03 186

A#04 106 B#04 196

A#05 182 B#05 180

A#06 129 B#06 200

A#07 90 B#07 180

A#08 171 B#08 148

A#09 153 B#09 117

A#I0 106 B# 10 164

BlOmaterialien 6 (2), 2005

O R I G I N A L

"ARBEITEN

Marius von Knoch, M.D.

Korrespondenzanschrift:

Department of Orthopaedic Surgery University of Duisburg-Essen Pattbergstrasse 1 - 3

45239 Essen, Germany Telefon: +49 (201) 40 89-21 61 Fax: +49 (201) 40 89-27 22 E-Mail:

mariusvonknoch@yahoo.com

Akademischer Lebenslauf:

1990 Studium Produktionstechnik Universität Bremen und Humanmedizin Universitäten Hamburg, Berlin FU, Göttingen, Lübeck

1993-1995 Dissertation „morphometrische Bestimmung der Polyethylenbeladung von Makrophagen versus Riesenzellen bei aseptischer Lockerung von Hüf- tendoprothesen.", Prof. H.-G. Willert, Göttingen 1997 Forschungsaufenthalt in USA, Anderson Ortho-

paedic Research Institute, (Dr. Charles A. Engh), Arlington, VA

1999-2001 Assistenzarzt in Endo-Klinik, Chirurgie, Hamburg, Universität Kiel, Orthopädie und Klinik und Poli- klinik für Orthopädie, Universität Duisburg-Essen, im Evangelischen Krankenhaus Essen-Werden 2002-2003 Fellowship USA, Mayo Clinic Rochester, MN, De-

partment of Orthopaedics u n d Laboratory of Or- thopaedic Research

2004 Facharzt Ärztekammer Nordrhein (Orthopädie) 2004 Oberarzt Klinik und Poliklinik für Orthopädie, Uni-

versität Duisburg-Essen, Abteilung für Orthopädie, Evangelisches Krankenhaus Essen-Werden 2004 Forschungspreis Young Scientist Award, AG Bio-

materialien NRW

2005 Habilitation, Medizinische Fakultät, Universität Duisburg-Essen

R.: Computer s i m u l a t i o n s of stress-related b o n e remodeling a r o u n d n o n c e m e n t e d a c e t a b u l a r c o m p o n e n t s . J A r t h r o p l a - sty. 1993 Dec; 8(6):595-605.

[6] Perona P. G., Lawrence J., Paprosky W. G., P a t w a r d h a n A. G., Sartori M.: A c e t a b u l a r m i c r o m o t i o n as a m e a s u r e of initial i m p l a n t stability in primary hip arthroplasty. An in vitro c o m p a r i s o n of d i f f e r e n t methods of initial a c e t a b u l a r c o m p o n e n t f i x a t i o n . J Arthroplasty. 1992 Dec; 7(4):537-47.

[7] Pitto R. P., Bohner J., Hofmeister V.: Factors a f f e c t i n g t h e p r i m a r y stability of a c e t a b u l a r c o m p o n e n t s . An in vitro study. Biomed Tech (Berl). 1997 Dec; 42(12):363-8. German.

[8] Plenk H. Jr.: 1998 P r o s t h e s i s - b o n e interface. J Biomed Mater Res 43(4):350-355.

[9] Rapperport D. J., Carter D. R„ S c h u r m a n D. J.: Contact finite e l e m e n t stress analysis of p o r o u s i n g r o w t h a c e t a b u l a r cup i m p l a n t a t i o n , ingrowth, and loosening. J Orthop Res.

1987; 5(4):548-61.

[10] Silva Μ. J., Sandell L. J.: 2002 W h a t ' s New in Orthopa- edic Research. J Bone Joint Surg A m 84-A (8): 1490-1496.

[11] Spears I. R., Pfleiderer M„ Schneider E„ Hille E„

Morlock Μ. M.: The effect of interfacial p a r a m e t e r s on c u p - b o n e relative micromotions. A finite element investigation. J Biomech. 2001 J a n ; 34(1):113-20.

[14] Spears I. R„ Pfleiderer M., Schneider E., Hille Ε., Berg- m a n n G., Morlock Μ. Μ.: Interfacial c o n d i t i o n s between a press-fit a c e t a b u l a r cup and b o n e d u r i n g daily activities:

implications for achieving b o n e i n - g r o w t h . J Biomech. 2000 Nov; 33(11): 1471-7.

[13] Willert H. G., Bertram H., Buchhorn G. H.: 1990 Osteo- lysis in alloarthroplasty of t h e hip. The role of ultra-high molecular w e i g h t p o l y e t h y l e n e w e a r particles. Clin Orthop 2 5 8 : 9 5 - 1 0 7 .

[14] Wolff J.: Das Gesetz der T r a n s f o r m a t i o n der Knochen.

Berlin 1892

[15] Wright J. M., Pellicci P. M„ Salvati Ε. Α., Ghelman B„

Roberts Μ. M., Koh J. L.: Bone density a d j a c e n t to press-fit acetabular c o m p o n e n t s . A prospective analysis with q u a n t i - tative c o m p u t e d t o m o g r a p h y . J Bone J o i n t Surg Am. 2001 Apr; 83-A(4):529-36.

[2] B e n j a m i n J., T h o m a s M., Szivek J.: The ability of v a r i o u s a c e t a b u l a r c o m p o n e n t s to resist protrusio m i g r a t i o n . Ortho- pedics. 1997 Apr; 20(4):307-10.

[13] K u h n Α., Scheller G., Schwarz Μ.: P r i m a r y stability of c e m e n t - f r e e press-fit a c e t a b u l u m cups. In vitro displacement studies. Biomed Tech (Berl). 1999 Dec; 44(12):356-9. Ger- m a n .

Literature

[1] Adler Ε., Stuchin S. Α., K u m m e r F. J.: Stability of press- fit a c e t a b u l a r cups. J Arthroplasty. 1992 Sep;7(3):295-301.

[4] Lachiewicz P. F., Suh P. B., Gilbert J. Α.: In vitro initial f i x a t i o n of p o r o u s - c o a t e d a c e t a b u l a r total hip c o m p o n e n t s . A b i o m e c h a n i c a l c o m p a r a t i v e study. J Arthroplasty. 1989 Sep; 4(3):201-5.

[5] Levenston Μ. Ε., Beaupre G. S., S c h u r m a n D. J., Carter D.

Y Q | B I O m a t e r i a l i e n 6 (2), 2005