Polymer Wear and Its Control - American Chemical Society

15 Penetration of Metallic Femoral Components into Polymeric Tibial Components Observed in a Knee Joint Simulator Downloaded via UNIV OF TEXAS AT EL PASO on October 21, 2018 at 18:10:21 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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D.Dowson ,B. J. Gillis , and J. R. Atkinson 1

Department of Mechanical Engineering, Institute of Tribology, The University of Leeds, Leeds LS2 9JT, United Kingdom Adelaide Brighton Cement Ltd., Birkenhead, South Australia Department of Metallurgy, The University of Leeds, Leeds LS2 9JT, United Kingdom

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The major factors limiting the effective l i f e of a total replacement synovial joint are loosening of one or more of the prosthetic components, a variety of medical problems (e.g. infection) and wear. Most internal prostheses now consist of metallic and polymeric components and i t is the penetration of the former into the latter which is the subject of this paper. A knee joint simulator designed to evaluate the mechanical and tribological characteristics of total replacement knee joints will be described. Measurements of penetrations of metallic femoral components into the polymeric tibial components by means of dual index holographic contouring will be presented and the findings compared with observations from detailed studies of wear in well-controlled laboratory machines and the limited evidence of in-vivo performance of replacement knee joints.

The development of satisfactory total replacement synovial joints is sometimes described as the major advance in orthopaedic surgery in recent times. The hip joint was the first major load bearing joint to respond to the efforts of surgeons, engineers and materials scientists in the past two decades to develop satisfactory replacements, but attention is now focussed upon the more complex knee joint.

0097-6156/85/0287-0215S06.00/0 ©1985 American Chemical Society

Lee; Polymer Wear and Its Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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P O L Y M E R W E A R A N D ITS C O N T R O L

Most t o t a l replacement joints consist of a metallic and a polymeric component, although alternative materials such as ceramics and carbon reinforced materials are currently being examined f o r this r o l e . The l i f e of a prosthesis i s thus d i r e c t l y affected by the rate of penetration of the metallic component into the polymeric component and this has prompted considerable interest i n the subject of wear of polymers i n the h o s t i l e environment of the body. When polytetrafluoroethylene (PTFE) was introduced i n the early I960's the l i f e of t o t a l replacement hip joints was limited to about three years by the poor wear c h a r a c t e r i s t i c s of the polymer. When ultra-high molecular weight polyethylene (UHMWPE) replaced PTFE, the rate of penetration of the m e t a l l i c component into the polymeric component was reduced to such an extent that loosening emerged as a major aspect of prosthetic l i f e . There i s , nevertheless, a need to pursue studies of the wear of prosthetic materials to f a c i l i t a t e the development of s a t i s f a c t o r y materials which can be used with confidence i n l o n g - l i f e prostheses. A knee joint simulator was designed and b u i l t i n the b i o engineering laboratory at Leeds i n the l a t e 1970 s to enable the mechanical and t r i b o l o g i c a l c h a r a c t e r i s t i c s of current and projected knee joint replacements to be assessed. The simulator has been described by Dowson et a l ^ ' and i t i s shown i n Figure 1. R e a l i s t i c load and motion cycles are applied to the knee joints to simulate any desired a c t i v i t y , but usually walking, and the penetration of the metallic femoral components into the polymeric t i b i a l components a f t e r a large number of cycles can then be used to assess the l i f e of the j o i n t s . Furthermore, the penetrations recorded, which include both wear and creep, can be related to results of laboratory wear studies of the behaviour of UHMWPE and to the in-vivo performance of t o t a l replacement knee j o i n t s . The simulator i s therefore an important machine i n both the pragmatic and fundamental aspects of t o t a l replacement knee j o i n development. Dual index holographic contouring has been used to record the nature and the extent of femoral penetration into worn polymeric t i b i a l components tested on the knee j o i n t simulator. The system i s b r i e f l y described and results are presented for two different forms of prostheses. Wear factors based upon at least 10 cycles have been derived from the o v e r a l l penetrations recorded i n the simulator and compared with laboratory wear studies and a single result from in-vivo conditions. ,

Apparatus and Experimental Technique (2) The Morrison^ ' load cycles were applied to selected prostheses i n the simulator shown i n Figure 1. Cyclic loading patterns were applied i n the T i b i a l Axis and A-P directions, the hydraulic mechanisms being controlled by a waveform synthesiser supplied with some 200 b i t s of information. The primary joint motion i n flexion-extension was achieved by means of a cam driven rack-andpinlon system, while a l o w - f r i c t i o n s e l f - a l i g n i n g bearing arrangement on the upper, femoral, specimen holder allowed rotation about the A-P a x i s .


DOWSON ET AL.

Penetration of Metallic into Polymeric Components

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Figure l a Knee Joint Simulator

Key

1 2

Tape Reader Peekel Strain measurement device

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Waveform Synthesiser

4

Joint Carriage and Holder

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5

Tibial Axis Servo Valve

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Tibial Axis Switching Box

6

A - P Axis Servo Valve

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A - P Axis Carrier Frequency

7

A - P Axis Loading Cylinder

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80L Oil Reservoir

8

Tibial Axis Loading Cylinder

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Drive Shaft

9

Drive Motor

20

Pressurised OH Reservoir

10

Temperature Measurement Meters

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A - P Axis Power Amplifier

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2

13 1

4

Cathode Ray Oscilloscope Filter Tibial

A

x

j

s

carrier Frequency Amplifier

Tibial Axis Power Amplifier and Power Pack Amplifier

( A - P constant

force cylinder) 21

Pump Mousing


P O L Y M E R W E A R A N D ITS

Figure 1(c)

CONTROL

'Freeman-Swanson* Prosthesis i n Simulator.

Figure 1(d)

'Leeds' Prosthesis i n Simulator.


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DOWSON ET AL.

Reflective dual-index holography was used to measure the penetration of the m e t a l l i c femoral component into the polymeric t i b i a l component at the end of each t e s t . The procedure produced contour maps of the images of the wear specimens before and a f t e r the wear process. The general arrangement of the measuring system i s shown i n Figure 2 and the authors g r a t e f u l l y acknowledge the advice and assistance made available by Dr. M.J. Lalor and Mr. J.T. Atkinson of Liverpool Polytechnic when the f a c i l i t y was established i n Leeds. A 5-mW laser (L) and a rotational beam s p l i t t e r (BS) were used, together with standard o p t i c a l mirrors (M), s p a t i a l f i l t e r s (S) and a convex collimating lens (C). The l i q u i d c e l l (T) was f i t t e d with an o p t i c a l glass front and a p e r i s t a l t i c pump was used to change the immersion l i q u i d s . The two l i q u i d s used were ethanol or methanol or mixtures of both, depending upon the contour depth (A h) required. The contour depth i s given by,

where, Ah « contour depth increment X wavelength (laser beam) l 2 ~ f a c t i v e indices of immersion l i q u i d s 35

n

, n

r e

r

The polymeric t i b i a l components were coated with pure aluminium, some thick In a vacuum evaporation unit to improve the quality of the holographs. This f i l m was removed i n isopropyl alcohol before each wear test. Holographs of the 'unworn' and 'worn' t i b i a l components were assessed and the volume of material removed (V) i n a known period was estimated by simple geometrical procedures. The loading cycle enabled ( PdX) to be evaluated and an equivalent wear factor (k) was calculated from the relationship,

V

=

kN / PdX

- where (J*PdX) refers to one cycle and (N) i s the number of cycles.

Results Experiments were carried out i n the simulator on an early form of 'Freeman-Swanson' Knee Joint and on a 'Leeds' Knee Joint. D i s t i l l e d water was allowed to drip onto the prosthesis to wet the interface, the flow rate being adjusted to maintain temperature of the t i b i a l component at 37°C. Contour diagrams based upon holography are recorded f o r the i n i t i a l and worn t i b i a l components of each joint i n Figures 3 and 4 and schematic representations of the wear scars are shown i n Figures 5 and 6. The operating conditions and the derived wear factors are recorded i n Table I •


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POLYMER WEAR AND ITS CONTROL

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DOWSON ET AL.

Figure 3(a)

Figure 3(b)


Contours of 'Freeman-Swanson' Knee Prosthesis T i b i a l Component ( I n i t i a l Design). Prior to Test (Ah - 200 ym).

Contours of 'Freeman-Swanson' Knee Prosthesis T i b i a l Component ( I n i t i a l Design). After Test i n Knee Joint Simulator (Ah » 140 ym)


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P O L Y M E R W E A R A N D ITS C O N T R O L

Figure 4a. Contours of 'Leeds' Knee Prosthesis T i b i a l Component p r i o r to test i n knee j o i n t simulator (Ah = 1.75 ^im).

Figure 4b. Contours of 'Leeds* Knee Prosthesis T i b i a l Component after test i n knee j o i n t simulator (Ah - 350 pim). Lee; Polymer Wear and Its Control ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

DOWSON ET AL.


in 6 CD

true length = 13.4/cos15° = 13.87 A h = 30 pm t>

represents node points

Figure 5

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Representation of Wear Scar i n 'Freeman-Swanson Knee Joint. (Simulator t e s t ) .


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P O L Y M E R W E A R A N D ITS

Figure 6

CONTROL

Representation of Wear Scar i n 'Leeds' Knee J o i n t . (Simulator Test).


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DOWSON

ET AL.


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KNEE PROSTHESIS

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•FREEMAN-SWANSON ( i n i t i a l design)

/WdX

(Nm/cycle)

Number of Cycles V

'FREEMAN-SWANSON' (in-vivo)

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'LEEDS

47.7

67.1

1,030,563

1,008,574

Polymer Wear and Its Control - American Chemical Society

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