GB2191006A - Testing physical properties of materials e.g. cartilage - Google Patents

Abstract

An apparatus for and a method of testing the physical properties of materials such as for example cartilage. The apparatus comprises a hemispherical glass indenter 31 which in operation is vertically indented into a specimen of cartilage 90 which is displaced horizontally relative to the indenter by the action of a loading means e.g. thumbscrew 21 acting on a slideable carriage carrying a specimen tank 2 indirectly through force and displacement measuring means. The magnitude of indentation and horizontal displacement for a given load is used to determine the material properties of the cartilage, whilst the glass indenter allows inspection of the deformed cartilage surface whilst it is loaded via an optical microscope. The indenting force is applied by a pivoted beam 53 provided with a slidably adjustable weight. The indenting force and depth of the indentation are measured. The tank 2 may be filled with sinovial fluid. <IMAGE>

Description

SPECIFICATION Material property testing apparatus This invention relates to an apparatus for a method of testing the physical properties of materials and is particularly relevant to the testing of cartilage.

Arthritis, a well known illness in humans, comes in two forms, namely osteo-arthritis and rheumatoid arthritis. In both forms, a severe and often painful inflamation of the affected joint is the result of contracting the disease. Of the two, the osteo variety is the less amenable to control by drugs and is the principal field of interest of this invention.

In the affected joints of an osteo-arthritic patient, crystals of uric acid nucleate and grow within the body of the cartilage. These crystals are relatively hard and held in a tough cartilage matrix, thus forming an efficient grinding composite. This has effects which are particularly detrimental to the function of the joint. A great increase in the coefficient of friction at the sliding surfaces (perhaps of the order 100 fold) leads in turn to rapid erosion of the normal cartilage covering to the joints mating surfaces.

In order to be able to prevent or at least control the subsequent degeneration of the cartilage it is necessary to understand the microscopic physical or chemical mechanisms involved in the nucleation/growth. Unfortunately, there is very little experimental data on the microscopic properties of cartilage from which to hypothesise on these microscopic mechanisms. It is an aim of this invention to provide an apparatus for, and method of, obtain such experimental data.

In order to determine the mechanical properties of the cartilage, the apparatus should be capable of measuring the vertical load upon, and penetration of a hemispherical specimen indicator, and means should be provided to facilitate the observatior! of the area of contact, via an optical microscope. It should also be possible to apply and measure a horizontal displacement of the indent relative to the rigidly held specimen; the forces required to cause such a displacement should also be accurately determinable. Preferably, in order to provide realistic data, the specimen should be tested whilst in its normal environment, namely, sinovial fluid.

The object is to recreate in the cartilage the greatest normal stresses likely to be sustained in life, suggested as being 3MNm2. The maximum vertical movement during penetration is suggested as being in the order of 1mm: for any particular specimen the coefficient of friction may lie anywhere between 0.001 and 0. 1. Articuiar cartilage is a complex group of polymers which are known to exhibit both elastic and viscoelastic behaviour.

The present invention will now be more particularly described by way of example only with reference to the accompanying drawings in which: Figure 1 is a side elevation of the present invention.

Figure 2 is a plan view in the direction of arrow x in Fig. 1.

Figure 3 is a partial cross-sectional view in the direction of arrows A-A in Fig. 2 Figure 4 is a partial sectional view in the direction of arrows C-C in Fig. 2.

Figure 5 is a sectional view in the direction of arrows F-F in Fig. 1.

Figure 6 is a partial sectional view in the direction of arrows B-B in Fig. 1.

Figure 7 comprises a partial sectional view in the direction of arrows G-G and D-D in Fig.

1.

Figures 8 and 9 are side views and end elevations respectively of a horizontal force measuring apparatus.

Referring to the drawings in general but particularly to Figs. 1 and 2, the apparatus comprises a base portion 76 upon which is mounted a pivotable means for applying a load to the specimen 90, shown generally at 80, a means for displacing the specimen 90 relative to the loading applicator 80, shown generally at 82, means for detecting the amount of movement 40 of the specimen 90 and means for determining the amount of strain imparted into the specimen as a result of the movement shown generally at 84 in Figs. 8 and 9 and shown partially as item 22 in Fig. 2.

Referring now to Figs. 1, 2, 3 and 5, the load applicator 80 comprises a pivot block cradle 62 fixedly attached to the base 76 by screws 70, a pair of pivot block sliding bars 66 located at either end in the cradle 62 by means of bushes 65 and a pivot block 59.

through which the sliding bars 66 pass such that the pivot block 59 is slidably mounted thereupon. A thumbscrew 69 is mounted in a threaded bush 68 at one end of the cradle 62 and acts on the pivot block 59 to move it along the sliding bars 66 to a desired position. The pivot block 59 is provided with a pair of rotational bearings 58, preferably of the self aligning type, and a pivot bar 57 which is located for rotation at either end in the bearings 58.

A slider arm 53 is fixedly attached at a point along its iength to the pivot bars 57 and is provided at one end with a pivot arm block 43 which is fixedly attached to the arm 53 by a screw 44 and which links said arm 53 to a bridge plate 41. An indenter block 34 is fixedly attached at one end to the underside of the bridge plate 41 by screws 42 and is provided at its otherwise free end with an indenter 31 which is fixidly located in the bridge plate 41 by a retaining sleeve 33 which tightens the indenter 31 against a washer 32 made from a resilient material such as nylon.

A sliding weight 55 of known mass is provided on the slider arm 53 which is moved up and down said arm 53 in order to increase or decrease the load exertable by the indenter 31. A scale 54 is provided on the arm 53 to aid the positioning of the weight 55. A counter weight 60 is provided at the otherwise free end of the slider arm 53 which acts to balance the slider arms 53 and the items attached thereto when the weight 55 is in a retracted position near the pivot block 59. A first bridge comprising two uprights 71, 72 and one cross piece 73 is provided with a thumbscrew 75 which acts on the counterweight to move the slider arm 53 and items attached thereto in a vertical direction when desired.

Referring to Fig. 4, a second bridge comprising two uprights 47, 48 and a cross piece 50 is provided with a vertically acting dial gauge indicator 52 which acts on the bridge plate 41 to measure the vertical displacement of said plate 41 and the items attached thereto. Each upright 47, 48 is securely fastened to a clock block 45 by screws 49, the clock block 45 is in turn secured to the base 76 by screws 46.

Referring now to Figs. 2, 3, 6 and 7, the indenter 31, mentioned briefly above, comprises a portion of glass having a ground hemispherical end portion of radius 2.00mm which is centrally mounted in the indenter block 34 and retained therein by sleeve 33.

A specimen of cartilage 90 is held in a sleeve 1 which forms part of the specimen tank 2 such that the specimen 90 is immersed in a bath of sinovial fluid 92. Sinovial fluid is a complex, chemically active, electrocyclic solution and in order to avoid rapid corrosion/erosion of the sleeve 1 and tank 2, they should be made from stainless steel or a similar material. The tank 2 is attached to a carriage 3 by screws 6 (Fig. 7). The carriage 3 is slidably mounted on a pair of bearing bars 9 which run in linear bearings 5 to minimise frictional forces. The ends of each bar 9 are located in a sliding cradle 12 which is in turn slidably mounted on a pair of bars 13 which are positioned to allow the cradle 3 to move in the same direction as the cradle 12. The ends of each bar 13 are located in a static cradle 14.A thumbscrew 21 is provided at one end of the static cradle 14 and acts on the sliding cradle 3 to move it a long the bars 13. A spring 20 is provided to urge the cradle 12 into contact with the thumbscrew 21.

Each end of the static cradle 14 is fastened to flange members 15, 16, each of which are supported by an outer portion of a microscope stand 27, 29. The microscope stand is further provided with a mid portion 28, which fills the gap between the two outer portions 27, 29. Each portion of the stand 27, 28, 29, is securely fastened to the base plate 76 by screws 30.

Referring now to Figs. 1, 2 8 and 9 a block 22 is secured to the sliding cradle 12 by screws 23 and acts as a mounting block for a pair of cantilevers 24, 25. Each cantilever 24, 25 comprises a base portion which is screwed to the block 22 and a slender upright portion 24a 25a which extend towards the specimen carriage 3. A loading dowel 4 is located on the carriage 3 and acts between the upright portions 24a, 25c to bend them when the carriage 3 moves relative to the sliding cradle 12. A strain gauge 77a and 77b is situated on each upright 24a, 25a and acts detect the strain created in its respective upright when deflected.

Turning now to Fig. 1, a carrier arm 38 is secured at one end by screws 37 to the bridge plate 41 and is provided at its otherwise free end with a horizontally acting dial gauge indicator 40 which measures the displacement of the carriage arm 7 which is an extended portion of the carriage 3.

A microscope, not shown in the drawings, may be provided on the microscope stand 27, 28, 29 such that the specimen 90 may be viewed in detail through the indenter in the direction of arrow Z in Fig. 6.

In operation, the indenter 31 is removed from the specimen tank 2 by turning thumbscrew 75 which acts on the counterweight 60 to raise the indenter 31. A speciment 90 is inserted in the he specimen sleeve 1 and placed in the tank 2 which is then filled with sufficient sinovial fluid 92 to cover the specimen 90. The weight 55 is moved to a position along the slider arm 53 which will give a desired load on the specimen 90 when the indenter 31 is lowered. The position of the indenter 31 relative to the specimen 90 is adjusted by turning thrumbscrew 69 which acts on the pivot block 59 to move the slide arm 53 and hence the indenter 31 sideways.

The indenter 31 is lowered gradually onto the specimen 90 until all the load created by the positioning of the weight 55 is experienced by the specimen 90.

The effect of the load on the specimen 90 may be viewed through the indenter 31 with the aid of the microscope.

The depth of penetration of the indenter 31 into the specimen 90 may be determined if desired by reading the displacement of the vertically acting dial gauge indicator 50.

A sideways force may also be exerted on the specimen by turning thrumbscrew 21, this moves the sliding cradle 12 which in turn exerts a sideways force on the carriage 3 via items 24a, 25b and dowel 4. Items 24a, 25b bend as this force is exerted and cause a change in the electrical conductivity of the strain gauges 77a, 77b attached thereto. The change in resistance of the strain gauges 77a, 77b may be used to determine the force required to move the specimen 90 sideways by a predetermined amount which may be mea sured using the horizontally acting dial gauge indicator 40.

The data obtained by using the above described apparatus and method may be used to determine the material properties of a speci men of cartilage and compare it with other specimen such that the relative health of the cartilage may be determined.

Claims (1)

1. An apparatus substantially as herein de scribed with reference to Figs. 1 to 9.