- Email: [email protected]
Wear properties of articular cartilage in vitro
1. Biomcchatics.Vol. 4. pp. 319-389.
Perpmoa Press, 1971. Printedin Circa Britk~~
WEAR PROPERTIES
OF ARTICULAR IN VITRO*
CARTILAGE
WILLIAMH. SIMON* Section on Rheumatic Diseases, Laboratory of Experimental Pathology, National institute of Arthritis and Metabolic Diseases, National Institutes of Health, Bethesda, Maryland, U.S.A. Abstract-The
wear of articular cartilage (human pateilar and canine humeral head) was studied
in vitro with a rotating steel abrader. With saline lubrication at 37°C. the cartiiage was more
resistant to abrasion than neoprene rubber (Shore durometer 50). oak and t&a wood. but ~SS than a series of plastics (nylon, high and low density polyethylenes, polycarbonate and polo_ tetraftuorocrhyiene). Synovial fluid had a distinct wear-protecting effect; this was eliminated by digestion of the m&n with trypsin but not by testicular hyaluronidase. The viscosity of the synovial fluid therefore did not account for its protective ei%ct. FibrUated cartikge was abraded more readily than intact tissue. but age otherwise he no effect. It is concluded that surface friction as well as intrinsic properties of the cartilage (strength and stifhess) arc major determinants of its wear properties: _
INTROlXJCTION DEGENERATIVE joint disease is characterized
in large part by mechanical abrasion of the cartilaginous-bearing surface of movable joints. Although there have been a number of valuable studies of the lubrication mechanism of joints, little is known of the material properties of the cartilage which governs its resistance to wear (Sokoloff. 1969). The following studies were carried out in uitru to give information on this subject and to allow comparison with certain non-cartilaginous materials. When cartilage is rubbed against another cartilage friction is very low, and abrasion is prohibitively slow for purposes of testing. For this reason a metallic abrader was used. The abrader obviously was unphysiologic, but it provided a constant experimental condition which permitted responses toseveral variables that are of biological consequence. METHODS AND MATERIALS
Wear-testing
apparatus
The device, designed and built by C. W. McCutchen, was a modified Unimat lathe-
milling machine (Americal Edelstall Co., N.Y.). The cartilage-covered bone was carried on a two-blade flexure (Fig. 1) which permitted it to move parallel with the bed of the machine with minimal friction. A deadweight loading system, which included another flexure. pushed the specimen against a rotating abrader. A linear variable differential transformer-stip chart recorder system. accurate to O-25pm, plotted the position of the specimen and hence the depth of penetration as time and wear proceeded. The entire carriage was mounted on the Unimat’s work table and could be shifted both along and across the bed, in order to place the spot on the cartilage into register with the abrader. The abrader was a 3/8 in. dia fine rotary tile (Simonds Saw and Steel Co., Fitchburg, Mass.) of high speed steel. The abrader was run backwards at 120 rev/mm. Running forward, the file cut through the cartilage in a matter of seconds. Run backward, the negative rake of the cutting edges prevented clean cutting. The debris particles were much smaller and the process seemed to be a dis-
.‘Receiced 15 Febnrqv 197 1. *Dr. Simon is a guest worker from The National Institute of Child Health and Human Devtlopment. 379
WILLlAM H. SIMON
RECORDCR
Fig. 1. D&am of wear machime: A, abrader; S. Specimen; F, Alter holder; M, removable mount; Fl,. double flexurc; Fl,, single Uexure; T, linear variable differential transformer; W, weight; U, Unimat assembly.
orderly wearing instead of an orderly cutting. The grooves in the flle allowed for the circulation of lubricant between the cartilage and the abrader. A variable-speed laboratory stirrer was used to turn the Unimat spindle by means of a belt drive. The spindle could be shifted in its carrier parallel to its own axis. and the spindle carrier moved up, down and around the vertical rod on the Unirnat. The abrasion wab done in an atmosphere of wet, body-temperature (37°C) air which was made by mixing steam and air. A plastic housing over the specimen maintained the humidity of the immediate environment of the specimen. A slow, steady drip of physiological saline or synovial fluid was directed onto the cartilageabrader interface. From here it dripped onto a preweighed Millipore filter (47 mm dia. 14~ pore size. Millipore Filter Co.. Bedford, Mass.) under suction which collected the abraded debris for eventual weighing. The filter was washed with distilled water to remove salt before weighing: and in the case of synovia-lubricated specimens, with 0.2%
testicular hyaloronidase to prevent the pores from being plugged by hyaluronate. Wear could thus be measured in terms of the distance the abrader had moved into the cartilage or of the weight of the accumulated debris. The specimen was afIixed to a holder by two screws. The holder was carried on the flexure system by a mount which held it securely under load yet allowed it to be lifted off to change the sample. Specimen material 65 human patellas and 76 canine humeral heads (38 matched pairs) were obtained at necropsy and stored frozen until used. The human specimens were from patients 7-82 years of age. and many had areas of osteoarthritic fibrillation. In all, 34 tests were carried out on fibrillated and 86 on intact areas of patellar cartilage. The dogs ranged from 2-4 years of age and the cartilages were grossly normal. The specimens were thawed and immersed in physiological saline solution prior to use (1 hr for human, and 30 min for
PROPERTIES OF ARTICULAR
material). To remove as much as possible of adherent synovial mucin the cartilage was swirled in a 0.2% solution of testicular hyaiuronidase (Worthington Biochemical Corp., Freehold, NJ., 540 ulmg) for 1 mm. The specimens were then again rinsed and bathed m-the saline for 15 min. . The non-cartilaginous materials tested ,included neoprene rubber (Shore durometer 50), balsa and oak wood, Teflon (polytetrafluoroethylene). nylon-66, high and low density poiyethylenes. and polycarbonate. These materials were made into discs 1a9cm in dia. and bonded to a brass plate with Eastman 9 10 (alpha-2-cyanoacrylate) or doublebacked adhesive (Minnesota Mining and Manufacturing Co., St. Paul, Minn.). The plastic and oak specimens were 3 mm thick. The rubber and balsa discs were 2,4 or 10 mm thick. canine
Experimental procedure
To establish a baseline condition in which the indentation caused by the application of load could be discriminated from the defonnation caused by the abrasion, a polished grooved stainless steel bail. 318 in. in dia., was first rotated against the cartilages for the same period and under the same load as for the abrader ball. A 15 min value was recorded to allow comparisons of indentation between variously treated cartilages (A indentation). The periods of testing were: human cartilage and plastics. 1 hr; canine cartilage, rubber and wood, 30 min. The load used for human and inert specimens was 200 g. Because the dog cartilages were so thin (0.85 mm average) a lower load was employed (50 g). The area of contact between the abrader ball and the specimens was initialiy approximately 1 mm* and the estimated pressures 200 and 50 g/mm2 or 284 and 71 lb/in.2 (psi). These rapidly fell off with deformation and abrasion so that at the conclusion of the tests average stress values were between 5 and 10 psi. The thinness of the canine cartilage precluded collection and weighing of the abraded debris, and
CARTILAGE
381
only the depth of penetration was recorded. For all the other specimens, both the depth and weight of the debris were measured. The volume of the abraded material was calculated as z (hZR - h3/3) where h was the depth of the abrasion and R the radius of the abrader (3/16 in.. 5 mm). The Milhpore filter and collected debris were dried to constant weight overnight in a vacuum dessicator over anhydrous CaSO,, and the weight of the titer was subtracted to obtain dry weight of debris. 24 areas were abraded on each patella. l/2 on the medial and l/2 on the iateral facets. The joint specimens were then fixed in 10% buffered formalin, photographed and sectioned through the center of the wear crater. The thickness of the cartilage on either side of the crater was measured with a measuring magnifier. All tests (including those on the inert materials) were carried out under conditions of lubrication. For synovial lubrication of the dog cartilages, fluid from bovine hocks was employed; for human cartilage. human synoviai fluid. Tests on the wood specimens were carried out against the side rather than the end of the grain. Special tests
’ The following parameters were examined. 1. The effect of synovial fluid vis-Lvis saline as a lubricant was tested on 4 human patellas and 12 dog shoulders as well as on specimens of wood and rubber. In addition, the effect of hyaiuronidase and trypsin on the synovial fluid as lubricant also was tested. The last menGoned tests were done with synovial mucin rather than whole synovial fluid so that trypsin inhibitors could be removed. This was accomplished by concentrating the mucin under suction in a. 0.008 I-( pore size collodion bag apparatus (Schleicher and Schuell, Inc., Keene, N.H.) and reconstituting it to the original volume in 0.155 M veronate buffer (pH 7.2). Following the tryptic digestion, soy bean inhibitor was added to
382
WILLIAM
H. SIMON
the reconstituted synovial fluid to stop the digestion. 2. The effect of osteoarthritic fibrillation was investigated vis-8-vis intact human patellar cartilage. 3. The effect of age, as distinguished from fibrillation, was determined in areas of cartilage devoid of fibrillary degeneration. 4. The effects of various agents that might alter the physical texture of the cartilage were examined in one or both species: 10% neutral buffered formalin; 0.2% collagenase (142 ulmg, Worthington Biochemical Corp., Freehold, NJ., CLS OHC); 0.2% trypsin (209 u/mg, Worthington TRL 8GA); polyvalent cations (O-085M La& FeCl,); and distilled water. The duration of the immersion of the species in these solutions prior to abrasion and the lubricant employed are listed in Tables 1 and 2. 10,000 u penicillin and 10,000 mcg streptomycin were added to the trypsin, collagenase and the corresponding controls during the incubation.
intact patellas lubricated with saline. Two points were measured on each of these. All sites tested were greater than 2 mm thick. The data from both sides were entirely comparable (Table 1). No statistical difference was found in data obtained on the medial and lateral facets of eleven intact patellas. The data f?om paired dog cartilages also were reproducible (Table 2). No relationship between wear and thickness of non-fibrillated cartilage was found. In general, the depth of abrasion and the weight of the dried abraded debris paralleled each other. The apparent ‘dry density’ of the abraded material computed from Fig. 2 was I.372 0.159 (standard error) for nonfibrillated and 1*39a0,194 for fibrillated cartilage. There was considerable variation in dry density between specimens indicated by the large standard error (see discussion). Dr. Thomas 0. Haver, University of Maryland, kindly measured the amount of iron transferred from the abrader to the Millipore filter by atomic absorption spcctrophotometry. The largest quantity of iron in 4 specimens tested was 15 kg, an amount of less than 1 per cent of the weight of the abraded material. Histologically the defects were sharpedged; there was no depletion of metachro-
RESULTS
The reproducibility of the method was tested by comparing both the weight of wear debris
and depth
of abrasion
on 4 pairs of
Table 1. Abrasionof human patellar cartilage* Abrasion
Expeliment
Right vs. left Collagenase lO%Formalin 0.085 M FeCI, Distilled HI0 Intact-e Fibrillated cartilage
No. tests
8 6 8 4 4 2 2
Experimental
Controlt Duration of immcrsioo (hr)
>
1
> 12 >43 > 12 > 12 > 1
>
1
Experimental lubxicant O-15 M NaCl O-15M NaCl
0.15 M NaCl 0.085 M FeCI, Distilled HI0 Synovia synovia
Depth (mm) 0.72kO.138 0.82 k 0.094 0.81ltO.113 0.74 z 0.208 o-3220.135 0.79 z!z0.070 1~54~0-360
Weight (mp) 5.4r0.61 7.0-c 1.32 9*1*0*89 14.3 f 4.58 g-222.15 5.2*0*50 23.5 2 1.31
Depth (-1
0.72zkO.138 1*12=0*055 1.21=0*083 1.4 20.125 0*55=0*180 0*34*0*030 0.58 ,cO.o60
Weight Cm@
6.52 1.38 16.5*2*49 22.2z6.39 32.425.20 8*2=2.00 3*9_coQ5 26.5 2 5.32
A Indentation (%I
+59.3 -30-2 - 2.8 - 12.0
*AI1 cartilage areas tested were normal grossly except in the group designated fibrillated. Values are mean-C standard error. t’fhe control for each experiment was the contralateral patella of the same individual. equilibrated and lubricated with saline. Two or four tests were performed on each pateila
1
O*I5MNaCl 2 2 2 3 2 3
2
I
4 2 I
tests
No.
(t, 3.3) Synovial mucin (I) 3.3) 0.15 M NaCl 0.15 M NsCl O-15 M NaCI 0.15 M NaCl 0.15 M NaCl 0.D M NaCl
Syoovial mucin
0.15 M NaCl 0.15 M NaCl 0.15 M NaCl
Control
O-26+- 0.035 0.28kO.020 0.34
(mm)
Control*
0.21 -tO.OSS 0.26 * 0.055 0.25 -t 0.023 0.3Ort-0.020 0.29 f O-030 0~21*0969
(V 2.7) 0.15 M NaCl O-15 M NaCl 0.15 M NaCl 0.085 M LaCI, Oa5 M FeCI, Dirtilkd H,O
0,27-+0.050 0.57 k 0.030 0.3 I zt 04BO 0~12+-OW4 0.45 -c O@JO 0~35-c0~030
0.31 -tO.tJOS
0.20
0.27 k 0.030 O-14-r-0.055 0.12
b-d
of the sameanimal. 1).Relative viscosity,37°C.
0.17-tO.025
(7) 1-l) Synovial mucin + ttypsin
-
Experimental
Depth of Abrasion
(111.1) Synovial mucin + hyaluronidase 0.29
0, I5 M N&l Synovia (q 3.7) Synovia + hyaluronidase
Experimental
Lubricant
*Thecontrolfor each experiment was the contmluteral joint
>I2 >I2 > 48 > 2 > 2 > 2
1
O.ISMNaCl
Co(lagenaee Trypsin lO%Formalin 0.085M LaCI, 09g5IU FeCI, Distilled ItO
4 1 4
hr
0+15MNaCl 0.15 M NaCl O*ll,MNaCI
Solution
Cartilage pre-treatment
Tnble 2. Abrasion of canine cartilage
--_.-
+ I2 +I8 -14 +I8 -17 -39
(W
A
indentation
384
WILLIAM H. SIMON
Fig. 2. Variations of depth of excavation (upper) and weight of abraded cxmilage flower) with age. Fibrillated cmiiage shows higher values at ail ages.
matic matrix in the vicinity of the defect, nor alteration of the collagen as gauged by polarization microscopy. Among the non-cartilaginous materials, the neoprene rubber, oak and balsa wood had greater degrees of abrasion than articular cartilage, but the several plastics wore less than the cartilage (Table 3). The balsa wore so rapidly that the procedure for measuring it had to be modified: the time required to abrade to a depth of 2 mm was recorded rather than the depth over a 30 or 60 min period. The wear curve of articular cartilage showed a lag period of 1 or more minutes before measurable abrasion appeared (Fig. 3). This lag was not observed with the inert materials. Synovial lubrication
Synovial fluid lubrication protected against
wear of canine, and human cartilage, rubber and balsa wood (Tables l-3). For the inert materials tested the protective effect of synovial fluid was less than that for cartilage (Table 3). Hyaluronidase treatment of synovial fluid and its concomitant reduction in viscosity failed to reduce its effectiveness in protecting cartilage against wear. By contrast. tryptic digestion did not appreciably alter the viscosity, but greatly reduced the wear-protective properties of the synovial mucin (Table 2). Osteoarthriticjbrillation
Fibrillated cartilage showed much more wear both in terms of size of defect and weight of collected material (Fig. 2). Unlike intact cartilage, no lag period was observed (Fig. 3). On fibrillated cartilage, the effect of synovial lubrication was inconsistent. It was apparently
Nylun 66 High density polyethylene Pulycdunute Polytluurutetraethylene Low density polyethylene Oak
Pkwlic
*Minutes to abrade 2 mm.
Balsa
BitlSti
Neoprene (Shore Durometer 50)
Kubber
Wd
l‘YP
Materii~l Deplh (mm) 1*63~0~010 2.01 -tO.ISO 1.70+-0.3tKt odJ9 i- ow3 0.25 2 0.026 0~321r.OMO 0.58 + o-066 0.61& 0.075 I.44 ? 0.036 I .lS* 4.0’
2 4 IO 3 3 3 3 3 3 3 IO
4 4 4 3 3 3 3 3 8 2 2
(me)
Weight
I.51 iO.05S 1.47-t0-315 1~27~O~lSO
Depth (mm)
Weigh1 (mr)
--
27. I f
090
64.3-t I.50 64.4224.I5 47.9~3.70
Synovia
_______l_.__
I .40 i 0.050 14.0’ 7.0*
-
Abrwiou
76.9 4 15.29 90.62 I.59 78.3 -+ 17% 2.7 20.33 9.02 O-61 9.32 0.19 14*0-+ 0.36 7.5* I.91 24.5& I.64
Saline Thickness (mm)
_____I_.
tests
NO.
Table 3. Abrasion of inert malerials --
O-13 0. I6 0. I6 0.12 0.20 oa7 0.32 0.32
0.42
indenlation Imm)
(1 set)
Ins~anlaneous
-
386
WILLIAM H. SIMON
protective in terms of defect size, but not in reducing the weight of the debris (Table 1). Age eflect The amount of fibrillation increased with
age of the specimen. However, when areas of fibrillation were excluded no general increase in abrasion depth could be correlated with age (Fig. 2). There was a slight increase in the weight of collected material from persons 20 years and above compared with younger individuals.
tissue, i.e. the cartilage was water-logged and therefore less dense. Introduction of ferric ions increased the wear, both in terms of volume (size of the defect) and weight of material abraded in both species (Tables 1 and 2). LaCl, decreased the wear and also decreased the stiffness of the tissue. DiSCU!JSION
Technical considerations.
It was necessary to employ the two methods for measuring the wear of cartilage the size of the excavation and the weight of 1.4 the abraded debris-for several reasons. The weight of the debris would have certain advantages over the size data because the meas1.2 urements of the latter would be complicated by compressive indentation of cartilage, in addition to the depth of penetration of the I ‘.Oabrader. On the other hand, the cartilage of 5 2 0.8 the dog joints, unlike the human, were too thin to permit accurate weighing. The two : g 0,s measures each had a cenain reproducibiity E as indicated by the relatively small standard t no.4 error considering the small number of tests. Nevertheless, the ‘dry density’, i.e. the weight of the dried tissue debris/volume of the cavity computed from the depth by the formula indicated above, was inordinately high in all Of 0 IO 20 30 40 so 60 cases. The density of wet, intact articular TIME (minutu) cartilage is 1.076 (Linn and Sokoloff, 1965) Fig. 3. Wear curves of non-fibrillated and fibrillated human and the water content is 78 per cent. The dry patelIar cartilage. A small 4ag period is suggested in the former. The vertical line through each point represents f 1 density should therefore be about 0,237, but standarderror. as calculated from the ‘depth of abrasion’, was 1.37. One can conceive of several Chemical alteration of cartilage technical explanations for the disparity. It Formalin fixation increased the wear in seems most likely that the volume of the both human and canine cartilage (Tables 1 and excavation was underestimated here because 2). Collagenase and trypsin treatment also the depth was measured from a baseline posiincreased wear of the tissue (Tables 1 and 2). tion in compression. The compressed, and Unlike formalin, the latter substances softened therefore consolidated material in turn was rather than hardened the cartilage (A indenta- itself abraded and did not appear in the depth tion). Washing out ions with distilled water of the ‘excavation’. When the depth of the increased the turgor of the tissue; the volume indentation was added to the depth of the ‘abof abraded material was perhaps slightly rasion’. the dry densities more closely approxincreased, but the dry weight was reduced imated the theoretical values. The possibility presumably because water was imbibed by the that the weight of the debris was exaggerated
PROPERTlES
OF ARTICULAR
by particles from the abrader, or by leaching out of matrix components from the margins of the excavation seems unlikely because of the minuscule quantities of iron measured, and the histologic appearance of the abraded tissue. In any event, data obtained by each method of measurement should be compared to itself and cannot be directly extrapolated to the other. Fortunately, the findings obtained by both methods were in general agreement. A similar phenomenon occurred in the non-cartilaginous materials. Nylon, for example, had a creep deformation of 0.23 mm in 1 hr at 37°C while the ‘depth of abrasion’ was only 0.09 mm. General wear characteristics
of cartilage
Under saline lubrication, intact articular cartilage was more resistant to wear than rubber or wood, but less so than the several plastic materials tested. Indeed the superiority of the plastics probably is greater than the values indicated in the tables, because their baseline indentations were less than in the case of the cartilages. The brevity of the lag period prior to the measurable wear casts some doubt on the idea that the tangential or surface layer of collagen constitutes a protective ‘armor coat’ for the cartilage (Chrisman, 1969). Furthermore, shaving off the surface of the cartilage failed to accelerate the abrasion. The wear properties of viscoelastic materials vary with the conditions of testing such as speed. temperature, lubrication and surface finish, but two principal determinants arc the stiffness and strength of the material. Increasing the tensile strength raises the resistance to tearing, while decreasing sti.Rness, i.e. increasing creep deformation, allows the abrader to indent the material and thereby decrease the abrasive stress. In the present experiments, attempts were made to increase and decrease the stiffness and strength of the cartilage. Stiffness was increased, i.e. A indentation was negative. by fixation with formalin and immersion in distilled water. Formalin increased the wear both in the
CARTILAGE
387
human and canine tissue. Formaldehyde increases cross-links in collagen (Nold er al. 1970; Veis and Drake, 1963) which is the principal source of tensile strength in cartilage. It also precipitates and hardens other proteins of the matrix. The literature on the effect of formalin treatment on the tensile properties of collagenous tissues is inconsistent, but several studies indicate that their strength is diminished thereby (Viidik and Lewin, 1966). Both these changes might account for the increased wear observed here. The results with the distilled water were in the same direction as with the formalin, but were less marked. Washing out electrolytes increases the stifhress of cartilage (McCutchen, 1962; SokolofT, 1963), but presumably has little effect on the tensile properties of the collagen. Treatment with collagenase and trypsin resulted in a marked softening of the cartilage and greatly increased the wear. These are proteolytic enzymes which split matrix proteins. The former acts on the collagen fibrils. the latter on the interflbrillar proteins which interact with the collagen to provide the cartilage with its specific material properties (Sokoloff, 1969). The present observations indicate that the loss of strength is the dominant effect of these enzymes on the cartilage. LaCl, resulted in increased indentability of the cartilage; the wear was decreased. FeCl, increased the stiffness and there was increased wear. Once again wear is related to stiffness in the direction given by the theory of wear proposed above. We are unable to account for the fact that these two cations did not act in the same direction. Cations react primarily with the fixed sulfate groups of cartilage mucopolysaccharides. There are data in the literature indicating that polyvalent cations, even when isotonic, as well as an excess of monovalent cations cause articular cartilage to soften (Sokoloff, 1963). Inerr materials
Wear
of
non-cartilaginous
materials
388
WlLLIAMH.SlMON
generally followed the same principles. Balsa wood, which has the lowest tensile strength. 130 psi (personal communication. J. T. Drow, Chief, Branch of Forest Products Utilization Research. Forest Service. U.S. Department of Agriculture) wore most rapidly; while nylon, with a tensile strength of 9-12 X 101 psi (Bolz and Tuve. 1970), was most resistant. The particular wear-resisting properties of nylon under water lubrication, compared with other plastics, also is related to its wettability (Bowers et al. 1954). Articular cartilage. which has a tensile strength of approximately 3 X IV psi (Kempson et al. 1969) wore less than oak wood, O-8x lo3 psi (Forest Products Laboratory. 1955) and more than teflon. 2-4.5 X 103 psi (Bolz and Tuve, 1970). A high water content distinguishes articular cartilage from the inert materials studied. This may facilitate resistance to wear by allowing more creep deformation than any of the inert materials tested, by dissipating frictional heat, and by lubrication. The lubrication is both external. i.e. through ‘weeping’ against the abrader, and internal. separating matrix fibrils from each other. Synovial lubrication It is known that wear of joints in vitro is increased when lubricant fluid is not present (Jones, 1934). Synovial fluid afforded an additional protection against abrasion of cartilage and rubber. Synovial mucin has been shown repeatedly to reduce the coefficient of friction of joints (McCutchen, 1962; Linn, 1967) and rubber (McCutchen, 1966). It is not a lubricant for stiff materials. The greater effectiveness with balsa than with oak wood is in accord with this principle. The mode of action of synovial mucin in reducing friction is the subject of considerable controversy. Traditional views that it results from viscosity have been shaken by work showing that the friction-lowering effect of synovia is not reduced by depolymerization with testicular hyaluronidase (McCutchen. 1966; Linn and
Radin, 1968). Tryptic digestion. by contrast, which does not reduce viscosity, does impair the friction-reducing effect (Wilkins, 1968; Linn and Radin. 1968). Indeed. it has recently been proposed that it is a synovial glycoprotein rather than hyaluronate which is the effective agent (Radin et al. 1970). The present observations with enzymatic treatment confirm and extend these previous observations on friction to the wear of articular cartilage. Fibrillation and aging Osteoarthritic fibrillation is a pathologic state in which there is a partial disruption of the collagen of the cartilage. It is therefore not surprising that fibrillated cartilage was abraded more readily than intact tissue. Fibrillation typically is an age-related phenomenon. but when fibrillated specimens were excluded, no effect of age on wear-resistance of intact, non-fibrillated cartilage was found. This observation is consistent with the lack of age changes in several other chemical and physical properties of articular cartilage: elasticity (Sokoloff. 1966), water (Linn and Sokoloff, 1965) protein (Miller et al. 1969) and hexosamine content (Sokoloff. 1969). The present studies have shown that both surface friction and intrinsic properties of the tissue (strength and stiffness) are major determinants of the wear resistance of articular cartilage. Synovial lubrication has a distinct protective effect. Although there have been many studies of the hyaluronate content and rheologic properties of synovial fluid in relation to aging and degenerative joint disease. the lubricating ability has not yet been studied in this regard. This sort of information should be obtained before introduction of artificial lubricants into joints for the treatment of osteoarthritis is justified. The greater susceptibility of fibrillated cartilage to abrasion also is evident from the present data, but they have not identified events that precede osteoarthritic degeneration. Although the wear resistance of articular
PROPERTIES
OF ARTICULAR
cartilage in vitro is less than several plastic materials tested. in tliuo there is an important difference between this tissue and inert materials: many components of the cartilage are replenished metabolically and hence provide an opportunity to prevent material fatigue. Acknowiedgemenrs-The author Sokoloff and C. W. McCutchen
thanks Drs. Leon for their invaluable advice throughout this study: Messrs. Ernest E. Beiie and James H. Donachy for the preparation of the inert specimens; Mrs. Genrude Nicholson for the illustration. and Mrs. Lee Ratner for typing the manuscript. REFERENCES Bolz. R. E. and Tuve. G. L. (Editors) (1970) Handbook of Tables for Applied &ngineerinp Science, pp. 105-122. Chemical Rubber, Cleveland. Ohio. Bowers, R. C.. Clinton. W. C. and Zisman. W. A. (1954) Friction and lubrication of nylon. Ind. Engng. Chem. ind. Edn. 46.24
16-24 19.
Chrisman. 0. D. (19691 Biochemical aspects of degenerative joint disease. Clin. Onhop. 64.77-86. Forest Products Laboratory ( 1955) The Wood Handbook. U.S.D.A. Handbook 72, U.S. Government Printing Office. Washington. D.C. Jones. E. S. ( 1934) Joint lubrication. Lancer 1, 1426-1427. Kempson. C. E.. Freeman. M. and Swanson. S. A. V. f 1969) Tensile properties of atticular cartilage. Notrcre. Lond. 220.1127-l 128. Linn. F. C. (1967) Lubrication of animal joints. 1. The arthrotripsometer. J. Bone It Surg. 49A. 1079- 1098. Linn. F. C. and Radin. E. 1. ( 19681 Lubrication of animal joints. III. The effect of certain chemical alterations of
CARTILAGE
389
the cartilage and lubricant. Arrhriris Rheum. 11. 674-682. Linn. F. C. and Sokoloff. L. (1965) Movement and composition of interstitial fluid of cartilage. Arrhritis Rheum. 8.48 I-494.
McCutchen. C. W. t.1962) The frictional properties of animal joints. Wear 5. 1- 17. McCutchen. C. W. ( 1966) Boundary lubrication by synovial fluid: Demonstration and possible osmotic explanation. Fedn Proc. Fedn Am. Sots exp. Biol. 25.106 l-68. Miller. E. J., van der Korst. J. K. and Sokoloff. L. ( 1969) Collagen of human articular and costal cartilage. Arrh&s
Rheum.
12.2 l-29.
Nold. J. G.. Kant. A. H. and Gross. J. f 19701 Collanen molecules: Distribution of alpha chains. Science. .\? Y. 170. 1096-1098. Radin. E. 1.. Swarm. D. A. and Weisser. P. A. ( 1970) Separation of a hyaluronate-free lubricating fraction from synovial fluid. Nature. Lond. 228.377-378. Sokoloff. L. ( 1963) Elasticity of articular cartilage: Effect of ions and viscous solutions. Science, ,V.Y. 141. 1055-1057. Sokoioff. L. (19661 Elasticity of aging cartilage. Fedn Proc. Fedn Am. Sots exe. Biol. 25.1089-1095. Sokoloff. L. t 1969) The ‘Biology of Degenerarive Joint Disease. University of Chicago Press. Chicago. Illinois. Veis. A. and Drake, M. P. (1963) The introduction of intramolecular covalent cross-lies into ichthyocol tropocollagen with monofunctional aldehydes. J. biol. Chem. 238.2003-20 11. Viidik. A. and Lewin. T. (1966) Changes in tensile strength characteristics and histology of rabbit ligaments induced by daferent modes of postmortal storage. Acia. orrhop. stand. 32. 141- 155. Wilkins. J. F. (1968) Proteolytic destruction of synovial boundary lubrication. Narure, Lond. US, 1050- 105 1.
Perpmoa Press, 1971. Printedin Circa Britk~~
WEAR PROPERTIES
OF ARTICULAR IN VITRO*
CARTILAGE
WILLIAMH. SIMON* Section on Rheumatic Diseases, Laboratory of Experimental Pathology, National institute of Arthritis and Metabolic Diseases, National Institutes of Health, Bethesda, Maryland, U.S.A. Abstract-The
wear of articular cartilage (human pateilar and canine humeral head) was studied
in vitro with a rotating steel abrader. With saline lubrication at 37°C. the cartiiage was more
resistant to abrasion than neoprene rubber (Shore durometer 50). oak and t&a wood. but ~SS than a series of plastics (nylon, high and low density polyethylenes, polycarbonate and polo_ tetraftuorocrhyiene). Synovial fluid had a distinct wear-protecting effect; this was eliminated by digestion of the m&n with trypsin but not by testicular hyaluronidase. The viscosity of the synovial fluid therefore did not account for its protective ei%ct. FibrUated cartikge was abraded more readily than intact tissue. but age otherwise he no effect. It is concluded that surface friction as well as intrinsic properties of the cartilage (strength and stifhess) arc major determinants of its wear properties: _
INTROlXJCTION DEGENERATIVE joint disease is characterized
in large part by mechanical abrasion of the cartilaginous-bearing surface of movable joints. Although there have been a number of valuable studies of the lubrication mechanism of joints, little is known of the material properties of the cartilage which governs its resistance to wear (Sokoloff. 1969). The following studies were carried out in uitru to give information on this subject and to allow comparison with certain non-cartilaginous materials. When cartilage is rubbed against another cartilage friction is very low, and abrasion is prohibitively slow for purposes of testing. For this reason a metallic abrader was used. The abrader obviously was unphysiologic, but it provided a constant experimental condition which permitted responses toseveral variables that are of biological consequence. METHODS AND MATERIALS
Wear-testing
apparatus
The device, designed and built by C. W. McCutchen, was a modified Unimat lathe-
milling machine (Americal Edelstall Co., N.Y.). The cartilage-covered bone was carried on a two-blade flexure (Fig. 1) which permitted it to move parallel with the bed of the machine with minimal friction. A deadweight loading system, which included another flexure. pushed the specimen against a rotating abrader. A linear variable differential transformer-stip chart recorder system. accurate to O-25pm, plotted the position of the specimen and hence the depth of penetration as time and wear proceeded. The entire carriage was mounted on the Unimat’s work table and could be shifted both along and across the bed, in order to place the spot on the cartilage into register with the abrader. The abrader was a 3/8 in. dia fine rotary tile (Simonds Saw and Steel Co., Fitchburg, Mass.) of high speed steel. The abrader was run backwards at 120 rev/mm. Running forward, the file cut through the cartilage in a matter of seconds. Run backward, the negative rake of the cutting edges prevented clean cutting. The debris particles were much smaller and the process seemed to be a dis-
.‘Receiced 15 Febnrqv 197 1. *Dr. Simon is a guest worker from The National Institute of Child Health and Human Devtlopment. 379
WILLlAM H. SIMON
RECORDCR
Fig. 1. D&am of wear machime: A, abrader; S. Specimen; F, Alter holder; M, removable mount; Fl,. double flexurc; Fl,, single Uexure; T, linear variable differential transformer; W, weight; U, Unimat assembly.
orderly wearing instead of an orderly cutting. The grooves in the flle allowed for the circulation of lubricant between the cartilage and the abrader. A variable-speed laboratory stirrer was used to turn the Unimat spindle by means of a belt drive. The spindle could be shifted in its carrier parallel to its own axis. and the spindle carrier moved up, down and around the vertical rod on the Unirnat. The abrasion wab done in an atmosphere of wet, body-temperature (37°C) air which was made by mixing steam and air. A plastic housing over the specimen maintained the humidity of the immediate environment of the specimen. A slow, steady drip of physiological saline or synovial fluid was directed onto the cartilageabrader interface. From here it dripped onto a preweighed Millipore filter (47 mm dia. 14~ pore size. Millipore Filter Co.. Bedford, Mass.) under suction which collected the abraded debris for eventual weighing. The filter was washed with distilled water to remove salt before weighing: and in the case of synovia-lubricated specimens, with 0.2%
testicular hyaloronidase to prevent the pores from being plugged by hyaluronate. Wear could thus be measured in terms of the distance the abrader had moved into the cartilage or of the weight of the accumulated debris. The specimen was afIixed to a holder by two screws. The holder was carried on the flexure system by a mount which held it securely under load yet allowed it to be lifted off to change the sample. Specimen material 65 human patellas and 76 canine humeral heads (38 matched pairs) were obtained at necropsy and stored frozen until used. The human specimens were from patients 7-82 years of age. and many had areas of osteoarthritic fibrillation. In all, 34 tests were carried out on fibrillated and 86 on intact areas of patellar cartilage. The dogs ranged from 2-4 years of age and the cartilages were grossly normal. The specimens were thawed and immersed in physiological saline solution prior to use (1 hr for human, and 30 min for
PROPERTIES OF ARTICULAR
material). To remove as much as possible of adherent synovial mucin the cartilage was swirled in a 0.2% solution of testicular hyaiuronidase (Worthington Biochemical Corp., Freehold, NJ., 540 ulmg) for 1 mm. The specimens were then again rinsed and bathed m-the saline for 15 min. . The non-cartilaginous materials tested ,included neoprene rubber (Shore durometer 50), balsa and oak wood, Teflon (polytetrafluoroethylene). nylon-66, high and low density poiyethylenes. and polycarbonate. These materials were made into discs 1a9cm in dia. and bonded to a brass plate with Eastman 9 10 (alpha-2-cyanoacrylate) or doublebacked adhesive (Minnesota Mining and Manufacturing Co., St. Paul, Minn.). The plastic and oak specimens were 3 mm thick. The rubber and balsa discs were 2,4 or 10 mm thick. canine
Experimental procedure
To establish a baseline condition in which the indentation caused by the application of load could be discriminated from the defonnation caused by the abrasion, a polished grooved stainless steel bail. 318 in. in dia., was first rotated against the cartilages for the same period and under the same load as for the abrader ball. A 15 min value was recorded to allow comparisons of indentation between variously treated cartilages (A indentation). The periods of testing were: human cartilage and plastics. 1 hr; canine cartilage, rubber and wood, 30 min. The load used for human and inert specimens was 200 g. Because the dog cartilages were so thin (0.85 mm average) a lower load was employed (50 g). The area of contact between the abrader ball and the specimens was initialiy approximately 1 mm* and the estimated pressures 200 and 50 g/mm2 or 284 and 71 lb/in.2 (psi). These rapidly fell off with deformation and abrasion so that at the conclusion of the tests average stress values were between 5 and 10 psi. The thinness of the canine cartilage precluded collection and weighing of the abraded debris, and
CARTILAGE
381
only the depth of penetration was recorded. For all the other specimens, both the depth and weight of the debris were measured. The volume of the abraded material was calculated as z (hZR - h3/3) where h was the depth of the abrasion and R the radius of the abrader (3/16 in.. 5 mm). The Milhpore filter and collected debris were dried to constant weight overnight in a vacuum dessicator over anhydrous CaSO,, and the weight of the titer was subtracted to obtain dry weight of debris. 24 areas were abraded on each patella. l/2 on the medial and l/2 on the iateral facets. The joint specimens were then fixed in 10% buffered formalin, photographed and sectioned through the center of the wear crater. The thickness of the cartilage on either side of the crater was measured with a measuring magnifier. All tests (including those on the inert materials) were carried out under conditions of lubrication. For synovial lubrication of the dog cartilages, fluid from bovine hocks was employed; for human cartilage. human synoviai fluid. Tests on the wood specimens were carried out against the side rather than the end of the grain. Special tests
’ The following parameters were examined. 1. The effect of synovial fluid vis-Lvis saline as a lubricant was tested on 4 human patellas and 12 dog shoulders as well as on specimens of wood and rubber. In addition, the effect of hyaiuronidase and trypsin on the synovial fluid as lubricant also was tested. The last menGoned tests were done with synovial mucin rather than whole synovial fluid so that trypsin inhibitors could be removed. This was accomplished by concentrating the mucin under suction in a. 0.008 I-( pore size collodion bag apparatus (Schleicher and Schuell, Inc., Keene, N.H.) and reconstituting it to the original volume in 0.155 M veronate buffer (pH 7.2). Following the tryptic digestion, soy bean inhibitor was added to
382
WILLIAM
H. SIMON
the reconstituted synovial fluid to stop the digestion. 2. The effect of osteoarthritic fibrillation was investigated vis-8-vis intact human patellar cartilage. 3. The effect of age, as distinguished from fibrillation, was determined in areas of cartilage devoid of fibrillary degeneration. 4. The effects of various agents that might alter the physical texture of the cartilage were examined in one or both species: 10% neutral buffered formalin; 0.2% collagenase (142 ulmg, Worthington Biochemical Corp., Freehold, NJ., CLS OHC); 0.2% trypsin (209 u/mg, Worthington TRL 8GA); polyvalent cations (O-085M La& FeCl,); and distilled water. The duration of the immersion of the species in these solutions prior to abrasion and the lubricant employed are listed in Tables 1 and 2. 10,000 u penicillin and 10,000 mcg streptomycin were added to the trypsin, collagenase and the corresponding controls during the incubation.
intact patellas lubricated with saline. Two points were measured on each of these. All sites tested were greater than 2 mm thick. The data from both sides were entirely comparable (Table 1). No statistical difference was found in data obtained on the medial and lateral facets of eleven intact patellas. The data f?om paired dog cartilages also were reproducible (Table 2). No relationship between wear and thickness of non-fibrillated cartilage was found. In general, the depth of abrasion and the weight of the dried abraded debris paralleled each other. The apparent ‘dry density’ of the abraded material computed from Fig. 2 was I.372 0.159 (standard error) for nonfibrillated and 1*39a0,194 for fibrillated cartilage. There was considerable variation in dry density between specimens indicated by the large standard error (see discussion). Dr. Thomas 0. Haver, University of Maryland, kindly measured the amount of iron transferred from the abrader to the Millipore filter by atomic absorption spcctrophotometry. The largest quantity of iron in 4 specimens tested was 15 kg, an amount of less than 1 per cent of the weight of the abraded material. Histologically the defects were sharpedged; there was no depletion of metachro-
RESULTS
The reproducibility of the method was tested by comparing both the weight of wear debris
and depth
of abrasion
on 4 pairs of
Table 1. Abrasionof human patellar cartilage* Abrasion
Expeliment
Right vs. left Collagenase lO%Formalin 0.085 M FeCI, Distilled HI0 Intact-e Fibrillated cartilage
No. tests
8 6 8 4 4 2 2
Experimental
Controlt Duration of immcrsioo (hr)
>
1
> 12 >43 > 12 > 12 > 1
>
1
Experimental lubxicant O-15 M NaCl O-15M NaCl
0.15 M NaCl 0.085 M FeCI, Distilled HI0 Synovia synovia
Depth (mm) 0.72kO.138 0.82 k 0.094 0.81ltO.113 0.74 z 0.208 o-3220.135 0.79 z!z0.070 1~54~0-360
Weight (mp) 5.4r0.61 7.0-c 1.32 9*1*0*89 14.3 f 4.58 g-222.15 5.2*0*50 23.5 2 1.31
Depth (-1
0.72zkO.138 1*12=0*055 1.21=0*083 1.4 20.125 0*55=0*180 0*34*0*030 0.58 ,cO.o60
Weight Cm@
6.52 1.38 16.5*2*49 22.2z6.39 32.425.20 8*2=2.00 3*9_coQ5 26.5 2 5.32
A Indentation (%I
+59.3 -30-2 - 2.8 - 12.0
*AI1 cartilage areas tested were normal grossly except in the group designated fibrillated. Values are mean-C standard error. t’fhe control for each experiment was the contralateral patella of the same individual. equilibrated and lubricated with saline. Two or four tests were performed on each pateila
1
O*I5MNaCl 2 2 2 3 2 3
2
I
4 2 I
tests
No.
(t, 3.3) Synovial mucin (I) 3.3) 0.15 M NaCl 0.15 M NsCl O-15 M NaCI 0.15 M NaCl 0.15 M NaCl 0.D M NaCl
Syoovial mucin
0.15 M NaCl 0.15 M NaCl 0.15 M NaCl
Control
O-26+- 0.035 0.28kO.020 0.34
(mm)
Control*
0.21 -tO.OSS 0.26 * 0.055 0.25 -t 0.023 0.3Ort-0.020 0.29 f O-030 0~21*0969
(V 2.7) 0.15 M NaCl O-15 M NaCl 0.15 M NaCl 0.085 M LaCI, Oa5 M FeCI, Dirtilkd H,O
0,27-+0.050 0.57 k 0.030 0.3 I zt 04BO 0~12+-OW4 0.45 -c O@JO 0~35-c0~030
0.31 -tO.tJOS
0.20
0.27 k 0.030 O-14-r-0.055 0.12
b-d
of the sameanimal. 1).Relative viscosity,37°C.
0.17-tO.025
(7) 1-l) Synovial mucin + ttypsin
-
Experimental
Depth of Abrasion
(111.1) Synovial mucin + hyaluronidase 0.29
0, I5 M N&l Synovia (q 3.7) Synovia + hyaluronidase
Experimental
Lubricant
*Thecontrolfor each experiment was the contmluteral joint
>I2 >I2 > 48 > 2 > 2 > 2
1
O.ISMNaCl
Co(lagenaee Trypsin lO%Formalin 0.085M LaCI, 09g5IU FeCI, Distilled ItO
4 1 4
hr
0+15MNaCl 0.15 M NaCl O*ll,MNaCI
Solution
Cartilage pre-treatment
Tnble 2. Abrasion of canine cartilage
--_.-
+ I2 +I8 -14 +I8 -17 -39
(W
A
indentation
384
WILLIAM H. SIMON
Fig. 2. Variations of depth of excavation (upper) and weight of abraded cxmilage flower) with age. Fibrillated cmiiage shows higher values at ail ages.
matic matrix in the vicinity of the defect, nor alteration of the collagen as gauged by polarization microscopy. Among the non-cartilaginous materials, the neoprene rubber, oak and balsa wood had greater degrees of abrasion than articular cartilage, but the several plastics wore less than the cartilage (Table 3). The balsa wore so rapidly that the procedure for measuring it had to be modified: the time required to abrade to a depth of 2 mm was recorded rather than the depth over a 30 or 60 min period. The wear curve of articular cartilage showed a lag period of 1 or more minutes before measurable abrasion appeared (Fig. 3). This lag was not observed with the inert materials. Synovial lubrication
Synovial fluid lubrication protected against
wear of canine, and human cartilage, rubber and balsa wood (Tables l-3). For the inert materials tested the protective effect of synovial fluid was less than that for cartilage (Table 3). Hyaluronidase treatment of synovial fluid and its concomitant reduction in viscosity failed to reduce its effectiveness in protecting cartilage against wear. By contrast. tryptic digestion did not appreciably alter the viscosity, but greatly reduced the wear-protective properties of the synovial mucin (Table 2). Osteoarthriticjbrillation
Fibrillated cartilage showed much more wear both in terms of size of defect and weight of collected material (Fig. 2). Unlike intact cartilage, no lag period was observed (Fig. 3). On fibrillated cartilage, the effect of synovial lubrication was inconsistent. It was apparently
Nylun 66 High density polyethylene Pulycdunute Polytluurutetraethylene Low density polyethylene Oak
Pkwlic
*Minutes to abrade 2 mm.
Balsa
BitlSti
Neoprene (Shore Durometer 50)
Kubber
Wd
l‘YP
Materii~l Deplh (mm) 1*63~0~010 2.01 -tO.ISO 1.70+-0.3tKt odJ9 i- ow3 0.25 2 0.026 0~321r.OMO 0.58 + o-066 0.61& 0.075 I.44 ? 0.036 I .lS* 4.0’
2 4 IO 3 3 3 3 3 3 3 IO
4 4 4 3 3 3 3 3 8 2 2
(me)
Weight
I.51 iO.05S 1.47-t0-315 1~27~O~lSO
Depth (mm)
Weigh1 (mr)
--
27. I f
090
64.3-t I.50 64.4224.I5 47.9~3.70
Synovia
_______l_.__
I .40 i 0.050 14.0’ 7.0*
-
Abrwiou
76.9 4 15.29 90.62 I.59 78.3 -+ 17% 2.7 20.33 9.02 O-61 9.32 0.19 14*0-+ 0.36 7.5* I.91 24.5& I.64
Saline Thickness (mm)
_____I_.
tests
NO.
Table 3. Abrasion of inert malerials --
O-13 0. I6 0. I6 0.12 0.20 oa7 0.32 0.32
0.42
indenlation Imm)
(1 set)
Ins~anlaneous
-
386
WILLIAM H. SIMON
protective in terms of defect size, but not in reducing the weight of the debris (Table 1). Age eflect The amount of fibrillation increased with
age of the specimen. However, when areas of fibrillation were excluded no general increase in abrasion depth could be correlated with age (Fig. 2). There was a slight increase in the weight of collected material from persons 20 years and above compared with younger individuals.
tissue, i.e. the cartilage was water-logged and therefore less dense. Introduction of ferric ions increased the wear, both in terms of volume (size of the defect) and weight of material abraded in both species (Tables 1 and 2). LaCl, decreased the wear and also decreased the stiffness of the tissue. DiSCU!JSION
Technical considerations.
It was necessary to employ the two methods for measuring the wear of cartilage the size of the excavation and the weight of 1.4 the abraded debris-for several reasons. The weight of the debris would have certain advantages over the size data because the meas1.2 urements of the latter would be complicated by compressive indentation of cartilage, in addition to the depth of penetration of the I ‘.Oabrader. On the other hand, the cartilage of 5 2 0.8 the dog joints, unlike the human, were too thin to permit accurate weighing. The two : g 0,s measures each had a cenain reproducibiity E as indicated by the relatively small standard t no.4 error considering the small number of tests. Nevertheless, the ‘dry density’, i.e. the weight of the dried tissue debris/volume of the cavity computed from the depth by the formula indicated above, was inordinately high in all Of 0 IO 20 30 40 so 60 cases. The density of wet, intact articular TIME (minutu) cartilage is 1.076 (Linn and Sokoloff, 1965) Fig. 3. Wear curves of non-fibrillated and fibrillated human and the water content is 78 per cent. The dry patelIar cartilage. A small 4ag period is suggested in the former. The vertical line through each point represents f 1 density should therefore be about 0,237, but standarderror. as calculated from the ‘depth of abrasion’, was 1.37. One can conceive of several Chemical alteration of cartilage technical explanations for the disparity. It Formalin fixation increased the wear in seems most likely that the volume of the both human and canine cartilage (Tables 1 and excavation was underestimated here because 2). Collagenase and trypsin treatment also the depth was measured from a baseline posiincreased wear of the tissue (Tables 1 and 2). tion in compression. The compressed, and Unlike formalin, the latter substances softened therefore consolidated material in turn was rather than hardened the cartilage (A indenta- itself abraded and did not appear in the depth tion). Washing out ions with distilled water of the ‘excavation’. When the depth of the increased the turgor of the tissue; the volume indentation was added to the depth of the ‘abof abraded material was perhaps slightly rasion’. the dry densities more closely approxincreased, but the dry weight was reduced imated the theoretical values. The possibility presumably because water was imbibed by the that the weight of the debris was exaggerated
PROPERTlES
OF ARTICULAR
by particles from the abrader, or by leaching out of matrix components from the margins of the excavation seems unlikely because of the minuscule quantities of iron measured, and the histologic appearance of the abraded tissue. In any event, data obtained by each method of measurement should be compared to itself and cannot be directly extrapolated to the other. Fortunately, the findings obtained by both methods were in general agreement. A similar phenomenon occurred in the non-cartilaginous materials. Nylon, for example, had a creep deformation of 0.23 mm in 1 hr at 37°C while the ‘depth of abrasion’ was only 0.09 mm. General wear characteristics
of cartilage
Under saline lubrication, intact articular cartilage was more resistant to wear than rubber or wood, but less so than the several plastic materials tested. Indeed the superiority of the plastics probably is greater than the values indicated in the tables, because their baseline indentations were less than in the case of the cartilages. The brevity of the lag period prior to the measurable wear casts some doubt on the idea that the tangential or surface layer of collagen constitutes a protective ‘armor coat’ for the cartilage (Chrisman, 1969). Furthermore, shaving off the surface of the cartilage failed to accelerate the abrasion. The wear properties of viscoelastic materials vary with the conditions of testing such as speed. temperature, lubrication and surface finish, but two principal determinants arc the stiffness and strength of the material. Increasing the tensile strength raises the resistance to tearing, while decreasing sti.Rness, i.e. increasing creep deformation, allows the abrader to indent the material and thereby decrease the abrasive stress. In the present experiments, attempts were made to increase and decrease the stiffness and strength of the cartilage. Stiffness was increased, i.e. A indentation was negative. by fixation with formalin and immersion in distilled water. Formalin increased the wear both in the
CARTILAGE
387
human and canine tissue. Formaldehyde increases cross-links in collagen (Nold er al. 1970; Veis and Drake, 1963) which is the principal source of tensile strength in cartilage. It also precipitates and hardens other proteins of the matrix. The literature on the effect of formalin treatment on the tensile properties of collagenous tissues is inconsistent, but several studies indicate that their strength is diminished thereby (Viidik and Lewin, 1966). Both these changes might account for the increased wear observed here. The results with the distilled water were in the same direction as with the formalin, but were less marked. Washing out electrolytes increases the stifhress of cartilage (McCutchen, 1962; SokolofT, 1963), but presumably has little effect on the tensile properties of the collagen. Treatment with collagenase and trypsin resulted in a marked softening of the cartilage and greatly increased the wear. These are proteolytic enzymes which split matrix proteins. The former acts on the collagen fibrils. the latter on the interflbrillar proteins which interact with the collagen to provide the cartilage with its specific material properties (Sokoloff, 1969). The present observations indicate that the loss of strength is the dominant effect of these enzymes on the cartilage. LaCl, resulted in increased indentability of the cartilage; the wear was decreased. FeCl, increased the stiffness and there was increased wear. Once again wear is related to stiffness in the direction given by the theory of wear proposed above. We are unable to account for the fact that these two cations did not act in the same direction. Cations react primarily with the fixed sulfate groups of cartilage mucopolysaccharides. There are data in the literature indicating that polyvalent cations, even when isotonic, as well as an excess of monovalent cations cause articular cartilage to soften (Sokoloff, 1963). Inerr materials
Wear
of
non-cartilaginous
materials
388
WlLLIAMH.SlMON
generally followed the same principles. Balsa wood, which has the lowest tensile strength. 130 psi (personal communication. J. T. Drow, Chief, Branch of Forest Products Utilization Research. Forest Service. U.S. Department of Agriculture) wore most rapidly; while nylon, with a tensile strength of 9-12 X 101 psi (Bolz and Tuve. 1970), was most resistant. The particular wear-resisting properties of nylon under water lubrication, compared with other plastics, also is related to its wettability (Bowers et al. 1954). Articular cartilage. which has a tensile strength of approximately 3 X IV psi (Kempson et al. 1969) wore less than oak wood, O-8x lo3 psi (Forest Products Laboratory. 1955) and more than teflon. 2-4.5 X 103 psi (Bolz and Tuve, 1970). A high water content distinguishes articular cartilage from the inert materials studied. This may facilitate resistance to wear by allowing more creep deformation than any of the inert materials tested, by dissipating frictional heat, and by lubrication. The lubrication is both external. i.e. through ‘weeping’ against the abrader, and internal. separating matrix fibrils from each other. Synovial lubrication It is known that wear of joints in vitro is increased when lubricant fluid is not present (Jones, 1934). Synovial fluid afforded an additional protection against abrasion of cartilage and rubber. Synovial mucin has been shown repeatedly to reduce the coefficient of friction of joints (McCutchen, 1962; Linn, 1967) and rubber (McCutchen, 1966). It is not a lubricant for stiff materials. The greater effectiveness with balsa than with oak wood is in accord with this principle. The mode of action of synovial mucin in reducing friction is the subject of considerable controversy. Traditional views that it results from viscosity have been shaken by work showing that the friction-lowering effect of synovia is not reduced by depolymerization with testicular hyaluronidase (McCutchen. 1966; Linn and
Radin, 1968). Tryptic digestion. by contrast, which does not reduce viscosity, does impair the friction-reducing effect (Wilkins, 1968; Linn and Radin. 1968). Indeed. it has recently been proposed that it is a synovial glycoprotein rather than hyaluronate which is the effective agent (Radin et al. 1970). The present observations with enzymatic treatment confirm and extend these previous observations on friction to the wear of articular cartilage. Fibrillation and aging Osteoarthritic fibrillation is a pathologic state in which there is a partial disruption of the collagen of the cartilage. It is therefore not surprising that fibrillated cartilage was abraded more readily than intact tissue. Fibrillation typically is an age-related phenomenon. but when fibrillated specimens were excluded, no effect of age on wear-resistance of intact, non-fibrillated cartilage was found. This observation is consistent with the lack of age changes in several other chemical and physical properties of articular cartilage: elasticity (Sokoloff. 1966), water (Linn and Sokoloff, 1965) protein (Miller et al. 1969) and hexosamine content (Sokoloff. 1969). The present studies have shown that both surface friction and intrinsic properties of the tissue (strength and stiffness) are major determinants of the wear resistance of articular cartilage. Synovial lubrication has a distinct protective effect. Although there have been many studies of the hyaluronate content and rheologic properties of synovial fluid in relation to aging and degenerative joint disease. the lubricating ability has not yet been studied in this regard. This sort of information should be obtained before introduction of artificial lubricants into joints for the treatment of osteoarthritis is justified. The greater susceptibility of fibrillated cartilage to abrasion also is evident from the present data, but they have not identified events that precede osteoarthritic degeneration. Although the wear resistance of articular
PROPERTIES
OF ARTICULAR
cartilage in vitro is less than several plastic materials tested. in tliuo there is an important difference between this tissue and inert materials: many components of the cartilage are replenished metabolically and hence provide an opportunity to prevent material fatigue. Acknowiedgemenrs-The author Sokoloff and C. W. McCutchen
thanks Drs. Leon for their invaluable advice throughout this study: Messrs. Ernest E. Beiie and James H. Donachy for the preparation of the inert specimens; Mrs. Genrude Nicholson for the illustration. and Mrs. Lee Ratner for typing the manuscript. REFERENCES Bolz. R. E. and Tuve. G. L. (Editors) (1970) Handbook of Tables for Applied &ngineerinp Science, pp. 105-122. Chemical Rubber, Cleveland. Ohio. Bowers, R. C.. Clinton. W. C. and Zisman. W. A. (1954) Friction and lubrication of nylon. Ind. Engng. Chem. ind. Edn. 46.24
16-24 19.
Chrisman. 0. D. (19691 Biochemical aspects of degenerative joint disease. Clin. Onhop. 64.77-86. Forest Products Laboratory ( 1955) The Wood Handbook. U.S.D.A. Handbook 72, U.S. Government Printing Office. Washington. D.C. Jones. E. S. ( 1934) Joint lubrication. Lancer 1, 1426-1427. Kempson. C. E.. Freeman. M. and Swanson. S. A. V. f 1969) Tensile properties of atticular cartilage. Notrcre. Lond. 220.1127-l 128. Linn. F. C. (1967) Lubrication of animal joints. 1. The arthrotripsometer. J. Bone It Surg. 49A. 1079- 1098. Linn. F. C. and Radin. E. 1. ( 19681 Lubrication of animal joints. III. The effect of certain chemical alterations of
CARTILAGE
389
the cartilage and lubricant. Arrhriris Rheum. 11. 674-682. Linn. F. C. and Sokoloff. L. (1965) Movement and composition of interstitial fluid of cartilage. Arrhritis Rheum. 8.48 I-494.
McCutchen. C. W. t.1962) The frictional properties of animal joints. Wear 5. 1- 17. McCutchen. C. W. ( 1966) Boundary lubrication by synovial fluid: Demonstration and possible osmotic explanation. Fedn Proc. Fedn Am. Sots exp. Biol. 25.106 l-68. Miller. E. J., van der Korst. J. K. and Sokoloff. L. ( 1969) Collagen of human articular and costal cartilage. Arrh&s
Rheum.
12.2 l-29.
Nold. J. G.. Kant. A. H. and Gross. J. f 19701 Collanen molecules: Distribution of alpha chains. Science. .\? Y. 170. 1096-1098. Radin. E. 1.. Swarm. D. A. and Weisser. P. A. ( 1970) Separation of a hyaluronate-free lubricating fraction from synovial fluid. Nature. Lond. 228.377-378. Sokoloff. L. ( 1963) Elasticity of articular cartilage: Effect of ions and viscous solutions. Science, ,V.Y. 141. 1055-1057. Sokoioff. L. (19661 Elasticity of aging cartilage. Fedn Proc. Fedn Am. Sots exe. Biol. 25.1089-1095. Sokoloff. L. t 1969) The ‘Biology of Degenerarive Joint Disease. University of Chicago Press. Chicago. Illinois. Veis. A. and Drake, M. P. (1963) The introduction of intramolecular covalent cross-lies into ichthyocol tropocollagen with monofunctional aldehydes. J. biol. Chem. 238.2003-20 11. Viidik. A. and Lewin. T. (1966) Changes in tensile strength characteristics and histology of rabbit ligaments induced by daferent modes of postmortal storage. Acia. orrhop. stand. 32. 141- 155. Wilkins. J. F. (1968) Proteolytic destruction of synovial boundary lubrication. Narure, Lond. US, 1050- 105 1.