Archives of Orthopaedic and Traumatic Surgery
Arch Orthop Traumat Surg 92, 19-30 (1978)
© J F Bergmann Verlag 1978
The Shear Strength of Trabecular Bone from the Femur, and some Factors Affecting the Shear Strength of the Cement-Bone Interface* M Halawa 2 , A J C Lee', R S M Ling2 , and S S Vangala' 'Department of Engineering Science, University of Exeter, Exeter, Devon, England 2 Princess Elizabeth Orthopaedic Hospital, Wonford Road, Exeter, Devon, England
Summary The shear strength of trabecular bone from the femur has been studied In general, the strongest trabecular bone is found close to the cortico-cancellous junction, though its shear strength depends also on the relationship of the trabeculae to the plane of shear. Some factors affecting the shear strength of the cementbone interface have been investigated In vitro, maximal cement-bone interface shear strength is obtained by exposing and thoroughly cleaning strong trabecular bone, and then forcing onto it under pressure low viscosity cement.
cepted as a basis for implant fixation, especially at the hip P M M A has no inherent adhesive or bonding properties, but relies solely on mechanical interlocking with the host bone for fixation. The aim of this study is the investigation of some of the variables affecting the mechanical interlocking that occurs between P M M A and trabecular bone, and hence the fixation of the cement to bone.
Zusammenfassung Die Scherkrafte der Knochentrabekel des Femur wurden untersucht Im allgemeinen wird der stirkste travikulare Knochen nahe des corticospongiosen Uberganges gefunden, wobeijedoch die Scherkraft zusatzlich von dem Verhaltnis der Knochentrabekel zur Ebene, in der die Scherkrifte wirken, abhangt Einige Faktoren, die die Scherkraft an der Zementknochengrenze beeinflussen, wurden untersucht In vitro wird die gr 613 te Scherkraft an der Zementknochengrenze erreicht durch Freilegen und grundliches Subern des starken travikularen Knochens und anschlie Bend durch Einpressen von Zement mit niedriger Viskositat.
The Shear Strength of Trabecular Bone
After almost two decades in clinical use, polymethylmethacrylate (P M M A ) bone cement is widely ac* This investigation was supported by Howmedica International Ltd , North Hill Plastics Division, 622 Western Avenue, Park Royal, London, W 3 OTF, and by The Northcott Devon Medical Foundation, Exeter Offprint requests to: R S M Ling (address see above)
PartI
Although a number of workers (Behrens et al , 1974; Ducheyne et al , 1977; Galante et al , 1970 ; Pugh et al , 1973) have investigated the mechanical properties of trabecular bone from the femur, their studies have mainly been confined to its compressive strength and elastic modulus In the context of the use of acrylic cement for the fixation of implants to the femur, especially those having intramedullary stems, the shear strength of trabecular bone must be regarded as of more importance than its compressive strength, since mechanical loosening of the implant, if it takes place, is likely to occur through the failure in shear of the cement-bone interface or of the trabecular bone itself. Relatively little attention has been devoted to the shear strength of trabecular bone from the femur (Greenwald and Wilde, 1976; Kolbel et al , 1977), and even less to its regional variations within the femur. This part of the study is therefore devoted to the determination of the shear strength of trabecular bone at varying distances from the cortex along the entire length of the femur For purposes of comparison, the 0344-8444/78/0092/0019/$ 2 40
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
20
shear strength of bone from normal femoral heads and from femoral heads obtained at replacement arthroplasty for osteoarthritis was determined.
Materials and Methods Six pairs of femora were obtained at post mortem and x-rayed to exclude obvious pathological abnormalities Nine femoral heads were obtained at replacement arthroplasty for osteoarthritis of
the hip Paired femora were necessary to enable one member of each pair to act as control for the other Immediately after removal and x-ray, the femora were placed in polythene bags which were stored in a deep freezer at -300 C Storage time was kept to a minimum, and in no case did it exceed 48 h. Specimens from the femora were machined in the semifrozen state to minimise tissue damage from the heat of machining (Walker et al , 1976) Machining was carried out using a crown drill of 8 73 mm inside diameter revolving at 1500 r p m. (Fig 1) Specimens were removed from 24 selected sites in the coronal plane from each femur (Fig 2) These were selected in order to assess regional variations in shear strength After machining, specimens were placed in dry, labelled test-tubes. Testing was carried out in cancellous bone, cancellous bone within 3 mm of the cortico-cancellous junction (which henceforth will be referred to as cortico-cancellous bone) and at the subarticular regions, in subchondral cortico-cancellous bone (Fig 3). For the shear tests, a Mayes Universal Testing Machine' (Fig 4) having a shear test attachment (Fig 5) was employed. The specimen holder diameter corresponded exactly to the inner diameter of the crown drill A total of 360 samples was tested in this part of the investigation One femur from each pair yielded specimens for cancellous bone shear tests whilst specimens from the other were used to determine the shear strength of the corticocancellous bone Longer samples were cut in half after being tested Sample No 1 (Fig 2) enabled the shear strength profile of
Fig 1 Crown drill used to extract cylindrical specimens from the femur
Selection of sit the
coronal p
Fig 3 Cylindrical specimen shewing test location for cancellous bone, A and cancellous bone within 3 mm of the cortico-cancellous junction B
10
211
Fig 4 Mayes Universal Testing Machine Fig 2 Selection of sites in the coronal plane of the femur
I Manufactured by Mayes Testing Machines Ltd , Windsor, England
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
PLANE EWHOLDOER
HEAD
21
shear stress since, owing to the irregular surface of trabecular bone, the area of failure is not a circle of diameter 8 73 mm though this was assumed in the computations This assumption is necessary in order that the results can be compared with those of others. The standard deviations are indicated. The variation of shear strength across the coronal plane at the base of the femoral neck is depicted in Figure 7 Figure 8 represents the shear strength of bone from different sites within the femoral head and compares normal femoral heads with those retrieved at replacement arthroplasty for osteoarthritis.
Discussion
SHEAR TSTj APPARAIUS
I I 6 'FINER'
RTRING HEAD
Fig 5 Shear test and push-out test apparatus
the cancellous bone and the cortico-cancellous junction across the coronal plane to be determined Samples machined from the 9 femoral heads recovered at replacement arthroplasty for osteoarthritis were also tested in the shear mode Most specimens were tested at a deflection rate of 5 mm min', i e a strain rate of 0.01 S though some were tested at the higher strain rate of 0.1 s l The load deflection curve was automatically displayed by an XY plotter 8 samples in which shear strengths were found over an order of magnitude greater than the mean were rejected as indicating that the plane of shear was within cortical bone.
Results The results are shown in graphic form in Figure 6 The abscissa represents the sample location as in Figure 2 and the ordinate the "shear stress" This is not the true
Referring to Figure 6, the major feature to emerge is the consistently higher shear strength of the corticocancellous bone (within 3 mm of the cortex) in comparison with that of cancellous bone 5 mm or more away from the cortex This is not an unexpected finding, and probably reflects the increasing density of cancellous bone as it approaches its cortical attachment This has implications for intramedullary fixation. Trabecular orientation in relation to the plane of shear is clearly a matter of considerable importance with respect to shear strength, and will vary with the position of the sample. The laws of bone architecture determine the orientation of the trabeculae and thus in some regions, the plane of shear is parallel to the trabeculae, whilst in others, orthogonal Subchondral bone from the femoral condyles is stronger when tested perpendicular to the trabeculae (specimen Nos 11 and 21) than when tested parallel to them (specimen Nos 10, 12, 20 and 22) The shear strength of specimen 23 is maximal at site b, i e in the middle of the head, reflecting the intersection in this region of what Singh et al ( 1973) term the primary compressive trabeculae with the primary tensile trabeculae (Fig 9) The variation of shear strength across the coronal plane at the base of the neck (Fig 7) shows a relatively high level medially where part at least of the primary compressive trabeculae are being sheared close to their cortical attachment The very low strength in the middle of the specimen reflects the fact that this site lies in the area between the two trabecular systems, whilst on the lateral side the socalled primary tensile trabeculae are being sheared, again close to their cortical attachment The plane of shear at the medial and lateral ends of this specimen (No 1) bears a roughly constant angular relationship to the primary compressive and primary tensile trabecular systems respectively, the former clearly being stronger. The relationship between plane of shear, trabecular
22
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
SHEAR PLANE IN 13
•
12
CANCELLOUS BONE
STRAIN
SHEAR PLANE IN
RATE
CORTICO-CANCELLOUS
O 101 s'
JUNCTION
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PLANE IN
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12
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Fig 6 Results of sl lear strength tests of bone at selected sites in the femora. Average of 5 samples per test standard deviation Shear -2
DEl
5-
Stress(N/mnm )
Bars indicate
OSTEOARTHRITIC
-
HEADS
NORMAL HEADS
8 4SHEAR
6
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3.
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Lateral Aspect
Sample 1
Fig 7 Variations in shear strength across the coronal plane at the base of the femoral neck
Inferior head (23)
l
Cenre of head (23 cb
Weight bearing area (23)
"' Posterior 124)
Il Anterior (241
Fig 8 Comparison of shear strength of trabecular bone from femoral heads of normal and osteoarthritic hips
orientation and shear strength may have implications for the correct positioning of the prosthetic components of surface replacements at both the knee and the hip. The data from Figure 8 indicate a surprising reduction in the shear properties of specimens from sites 23 and 24 in femoral heads recovered from operations for osteoarthritis, in comparison with normal femoral heads Moreover, the standard deviations in the former
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
23
Conclusions i The shear strength of cancellous bone from the femur increases as the cortico-cancellous junction is approached. ii The shear strength of cancellous bone from the femur depends upon its site in the femur and the relationship of the trabeculae to the plane of shear. iii The shear strength of cancellous bone in the femoral head is lower and exhibits greater variation in femoral heads from osteoarthritic hips than from normal hips. iv Cancellous bone exhibits viscoelastic properties. PartII Some FactorsAffecting the Strength of the Cement-bone Interface
Fig 9 Coronal section of femur shewing primary compressive trabeculae, A and primary tensile trabeculae B
are much higher, suggesting inconsistent mechanical behaviour This is a reflection of the variable and sometimes gross destructive changes in these femoral heads, with cystic changes, trabecular fatigue microfractures (Freeman, 1972) and boney collapse It indicates the need for caution in the selection of cases for surface replacement at the hip The findings in the femoral heads from osteoarthritic hips are somewhat at variance with those of Greenwald and Wilde (1974) who found an average shear strength of 4 6 N/mm, again with a high S D The method of testing was different however and gross abnormalities were only noted in 2 specimens out of 9, whereas these were observed in 7 out of 9 in the present investigation. The data in Figure 6 from the relatively small number of samples tested at the higher strain rate suggest that cancellous bone does exhibit viscoelastic properties, a matter sometimes in dispute (Freeman and Swanson, 1966 ; Pugh et al , 1973) Moreover, the persistence of viscoelastic behaviour in spite of the considerable interval between subject death and specimen testing indicates, according to Fitzgerald ( 1975, 1977) that the viscoelastic behaviour in life was probably more marked than these results suggest This takes no account of the possible hydraulic influence of circulating blood and tissue fluids. Finally, comment should be made on the absence of results at specimen sites 6 and 16 At these middiaphyseal areas, there is for practical purposes no endosteal trabecular bone and thus the shear strength cannot be tested.
A major factor affecting the shear strength of the cement-bone interface must clearly be the shear strength of the bone itself, the determination of which was established in the first part of this investigation. The aim of the second part of this study was to determine the influence of the following variables on the shear strength of the cement-bone interface: i thickness of cancellous bone adjacent to the cortex. ii methods of preparation of the medullary canal. iii methods of preparation and introduction of the cement. Materials and Methods The acquisition and machining of cylindrical samples was again as described in Part I In addition a second type of sample for
'pushout' testing was prepared by sawing each femur into slices 20 mm thick perpendicular to its long axis The slices were numbered and were taken from exactly similar positions in each pair of femora (Fig 10) For both of these types of specimen, the medullary canals of the femora were prepared in different ways prior to the machining and sawing (vid inf ) Fourteen pairs of femora were used for this, phase of the study and were x-rayed and stored as in Part I prior to the preparation of the medullary canals and machining The interval between machining and cementing was not longer than 3 h The shear strength of the cement-bone interface was determined using the two types of specimen as follows:
a) Cylindrical Specimens The effects of two variables were studied. i Thickness of cancellous bone within the cortex. ii The method of preparing the medullary canal. i Thickness of CancellousBone within the Cortex The medullary canal of one femur of a pair was prepared using a rasp and curette to leave > 5 mm of cancellous bone within the cortex, and the canal of the other femur was prepared to leave between 2-3 mm
24
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
Selection sites for Out test
Fig 11 Bone-cement composite cylindrical specimen iii Time of cement insertion. iv Pressurisation of cement.
Fig 10 Selection of sites for 'push-out' test samples within the cortex Both canals were then washed with a water jet at a pressure of 0 21 N/mm 2 and cylindrical specimens were obtained from both femora by machining with the crown drill at positions I to 22 (Fig 2) The specimens were placed in aluminium moulds of the same diameter as the inner diameter of the crown drill Simplex P RO 2 bone cement dough was prepared by adding the monomer to the polymer in a P V C mixing bowl and mixing for 2/2 min at a beating frequency of 1 Hz The ambient temperature was maintained at 21 °C and the humidity at 50 + 10 % Half a minute after the cessation of mixing, the cement dough was inserted under thumb pressure into the moulds on top of the cylindrical bone specimens and thus applied to the end surfaces of the specimens After polymerisation the cement bone composite specimens (Fig 11) were tapped out of their moulds and allowed to cool in air at 21°C They were next tested in the shear fixture in the Mayes machine, being so positioned that the plane of shear was at the cement-bone interface Specimens having oblique interfaces were rejected.
i Thickness of CancellousBone within the Cortex Three pairs of femora were employed The medullary canal of one femur from each pair was prepared leaving approximately 2 mm of cancellous bone in tact within the cortex, whilst the other was prepared so as to leave not less than 5 mm of cancellous bone intact within the cortex. The medullary canals were then washed using a water jet under a pressure of approximately 0 21 N/mm 2 and the femora were sliced as described above Simplex P R O2 cement dough was prepared as above and inserted into the medullary canals of the slices under palm pressure Continuous monitoring of the pressure at the cement-bone interface was carried out using a Piezo electric pressure transducer 3 located at the cement-bone interface, and in no case was it allowed to exceed 0 15 N/mm2 . After polymerisation, the specimens were allowed to cool in air at 210 C and then tested in the 'pushout' fixture in the Mayes machine (Fig 5). ii Methodfor PreparingMedullary Canal Two pairs of femora were employed The medullary canal from one femur ofeach pair was prepared using a rasp and curette whilst the other was prepared in similar fashion and then washed using a water jet under a pressure of approximately 0 21 N/mm 2 The thickness of cancellous bone was kept as near constant as possible at 2-3 mm The specimens were sliced, cemented and tested as in i. above, again with continuous cement-bone interface pressure monitoring.
i Thickness of cancellous bone within the cortex. ii Method of preparing the medullary canal.
iii Time of Insertion of Cement Three pairs of femora were used. The medullary canals of all 3 pairs of femora were prepared using a rasp and curette, maintaining a constant thickness of cancellous bone of 2-3 mm within the cortices The canals were then washed using the water jet and the femora sliced The slices from one femur of each pair were cemented 3 min after the beginning of mixing, whilst the slices from the other femur of each pair were cemented 6 min after the beginning of mixing. Cement-bone interface pressure monitoring was continuous in each specimen as in i above After polymerisation and cooling, the specimens were tested in the 'pushout' fixture. iv Pressurisation Two pairs of femora were employed The medullary canals of both pairs were prepared as in iii above, including washing, and the femora were then sliced The slices obtained from one femur from each pair were cemented at 3 min after the commencement of the mixing of the cement components and pressurised for a minimum of 30 sec at 0 15 N/mm 2 recorded
2 Manufactured by North Hill Plastics Division of Howmedica International
3 Vibrometer 12 QP250 ck Manufactured by Vibro-meter Corporation, Fribourg, Switzerland
ii Method of Preparing the Medullary Canal The medullary canals of two pairs of femora were prepared using rasp and curette so as to leave a 2-3 mm layer of cancellous bone within the cortices. The canal of 1 femur from each pair was then washed thoroughly using a water jet under 0 21 N/mm2 pressure. Cylindrical specimens were obtained, cemented and tested as above. b) 20 mm Thick Femoral Slices Eleven pairs of femora were employed for these tests The effects of 4 variables were studied:
M Halawa et al : Shear Strength of Trabecular Bone from the Femur at the cement-bone interface The slices from the other femur of each pair were dealt with similarly, except that pressurisation was maintained at 0 3 N/mm 2 at the cement-bone interface The specimens were allowed to cool in air at 210 C after polymerisation and tested in the 'pushout' fixture. In one pair of femora the variables were combined as follows: One femur was prepared leaving 5 mm of cancellous bone within the cortex, washing was not performed and after slicing the cement was inserted 6 min after the commencement of mixing under a pressure of 0 15 N/mm2 (worst case). In the other femur, 2-3 mm of cancellous bone was left within the cortex, the canal was washed with a water jet, and after slicing, cement was inserted 3 min after the commencement of mixing under a pressure of 0 3 N/mm2 After polymerisation and cooling, the specimens were tested in the 'pushout' fixture (best case).
25
the latter actually failed as they were being tapped out of the moulds prior to testing. b) Femoral Slices The results are shown in Table 1 and are expressed as the load at failure in Newtons 154 specimens were tested. A correction for the increased bone-cement interface area produced by reducing the thickness of the cancellous rim from 5 mm to 2-3 mm was applied in tabulating the results for the first variable, and was derived as follows: An extra pair of femora was sliced as in the experimental femora and the canal of one member of the pair was prepared leaving a 2 3 mm layer of cancellous bone, whereas in the other member of the pair a layer of cancellous bone 5 mm thick was left. The slices were then sectioned coronally and the included angles at the medial and lateral surfaces of the canal of each slice measured The coronal and sagittal diameters of the canal in each slice were measured, and from these figures, the surface areas of the ellipsoids representing the area of bone in contact with cement were calculated These areas of the corresponding slices from each pair were expressed as a ratio which proved to be relatively constant for all slices except 4 and 5 (Fig 10) This ratio was used for correcting the results.
Results a) CylindricalSpecimens The results obtained in testing the cylindrical bonecement composite specimens are depicted in the histogram (Fig 12) 120 specimens were tested The abscissa represents the sample location as in Figure 2 and the ordinate the cement-bone interface shear strength in N/mm 2 The influence of washing was so marked that the study of unwashed specimens was abandoned after approximately 15 such specimens had been tested 3 of
7
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Fig 12 Results of shear strength of bone-cement composite cylindrical specimens Average of 3 samples per test result Strain rate 0 01 s'
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
26
Table 1 Results of push-out specimen tests: load at failure in Newtons 154 specimens tested Statistical Comment (Dr J Gowers): Within each pair of femora, the effect of each variable is significant at the 0 1 % level An analysis of variance was performed on each variable in order to estimate whether the variables are significant over the whole population from which the specimen cadavers are a sample Cortico-cancellous bone, washing, and early insertion of cement (i e low viscosity cement) are all significant at the 5 % level Pressurisation fails to be significant This is probably an accident of small numbers, in view of the uncharacteristic results in cadaver 10 There is no interaction between the variables and position of testing Age
Distal femur
Proximal femur
Sex
Cadaver number
6
7
3971 (2906)
1872 (620)
1328 (287)
72
F
1
6178 (6198)
5348 (4436)
3904 (2770)
3460 (1609)
30
M
2
5601 (3590)
2640 (374)
2022 (1911)
4360 (1885)
8104 (3524)
16
M
3
2276 (1490)
2812 (1350)
3218 (3167)
3664 (3001)
2990 (1464)
2187 (721)
78
M
4
3439 (2800)
2574 (358)
3460 (1864)
4813 (51)
2466 (242)
1916 (940)
3926 (2600)
38
M
5
2124 (1152)
1014 (967)
2169 (1789)
3962 (1760)
2067 (511)
719 (239)
4196 (2267)
84
M
6
2770 (2300)
2421 (1919)
2297 (1129)
928 (577)
1741 (1158)
1098 (973)
3078 (1855)
90
M
7
(6 min insertion)
3634 (1962)
3422 (873)
2962 (1011)
4738 (623)
6465 (3994)
4523 (2226)
4316 (3713)
78
F
8
0.3 N/mm2 pressure
2664 (2009)
2412 (1806)
3280 (2067)
4130 (498)
2676 (955)
5272 (1329)
77
F
9
(0.15 N/mm2 pressure)
3879 (2756)
3101 (1436)
2232 (1867)
4109 (4080)
3123 (2571)
5744 (4801)
4491 (2860)
57
F
10
Best Case (Worst Case)
7533 (1263)
6586 (932)
10285 (1849)
11348 (3229)
9973 (1864)
4638 (210)
4949 (488)
54
F
11
4
2
3
1478 (1067)
2240 (984)
2371 (384)
3619 (3439)
6203 (2576)
2863 (1552)
7627 (6368)
(> 5 mm cancellous retained)
3816 (2929)
4985 (2695)
Curettage & washing
4370 (2050)
(Curettage only) 3 min insertion
Sample number
1
5
Variable 2-3 mm rim cancellous retained
From Table 1, the following become apparent: i With the retention of a 2-3 mm layer of cancellous bone, the load at failure on average was 100 % higher than with the retention of 5 mm rim of cancellous bone. ii The load at failure with a clean bone surface was on average 200 % higher than with a surface which was not cleaned before cementation. iii The load at failure when cement was inserted 3 min after the beginning of mixing was on average 60% higher than when it was inserted 6 min after the beginning of mixing. iv The load at failure when the cement was exposed to 0 3 N/mm 2 pressure before polymerisation was on average 100 % higher than when it was exposed to 0.15 N/mm2. v In the pair of femora in which a 'best' and 'worst' case was developed, the difference in load at failure was on average 800 %.
3674 61860)
In this type of test, because each slice from one femur of a pair is compared with the corresponding slice from the other member of the pair, the overall effect of the taper of the medullary canal is eliminated. In vivo, the latter may by itself make a substantial contribution to fixation, to an extent which is at present under study.
Discussion The results of testing both types of specimen show a direct relationship between the shear strength of the trabecular bone at the interface and the shear strength of the interface itself, a finding at variance with those of Greenwald and Wilde ( 1974) With interfacial trabecular bone of a given shear strength, the other variables considered in this study are clearly all capable of exerting a substantial influence upon the shear strength
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
27
Amstutz (1976) showed that a combination of early insertion and high applied pressure produced maximum penetration of cement dough into holes drilled into aluminium plugs, and clearly the advantage of early insertion in their experiments, as in those reported here, lies in the low viscosity of the cement when inserted early.
Fig 13 A Specimen prepared by rasping and curettage only, B Specimen prepared by rasping, curettage and washing
of the cement-bone interface This is what might be expected, since acrylic cement achieves fixation by mechanical interlocking and the variables under study are all concerned with the capacity of the cement dough to establish contact with the bone surface and then penetrate the inter-trabecular spaces Subsequent polymerisation of the cement within these spaces then 'locks' the cement to the bone Thus, washing the bone surface thoroughly helps to remove shattered trabeculae, blood and fat which would otherwise block the trabecular spaces and prevent the inflow of the cement dough Figure 13 shows a comparison between a surface which has simply been prepared by rasping and curettage and one which has been prepared by rasping, curettage and washing. It is obvious that penetration of the inter-trabecular spaces by cement dough is much more likely to be achieved in the latter Figure 14 shows a similar comparison between the appearance of the lower end of the femur sectioned with an oscillating saw during knee replacement before and after washing with a pulsating saline jet from a bone lavage instrument 4. Once again it is self evident that penetration of inter-trabecular spaces by cement dough will be more effective in the latter situation Once the bone surface is cleared of blood and debris, and the trabecular spaces open, the capacity of the cement to penetrate these spaces will depend upon the viscosity of the dough and the pressure applied to it The lower the viscosity and the higher the pressure, the greater the penetration. Walker and Bienenstock (1970) investigated the relationship between the time from the beginning of mixing of the monomer and polymer and the viscosity of the cement dough They confirmed experimentally what all surgeons using acrylic cement know from experience, i.e with increasing time after mixing, the viscosity of the cement dough increases, slowly at first and then rapidly as polymerisation approaches Markolf and 4
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Lee and his colleagues (1978), amongst others, have pointed out that the mechanical strength of acrylic cement is improved by the exclusion of blood and tissue debris (achieved by thorough washing of the bone surface) and the use of low viscosity cement dough, as well as by the application of pressure to the dough before polymerisation The present work suggests that such measures are also highly favourable to the strength of the fixation between the cement and bone. The inferences to be drawn from this in vitro study are self evident In the femur, the strongest trabecular bone lies close to the cortex The maximum shear strength of the cement-bone interface is achieved by exposing and thorougly cleaning this layer of bone and then inserting cement of low viscosity under pressure. Direct extrapolation, however, of this technique into the in vivo situation at operation raises a number of questions which probably only time will answer. Does the removal of all but 2-3 mm of trabecular bone from within the medullary canal interfere with revascularisation of the endosteal bone, and if it does, is this likely to be of importance? Clinical experience to date suggests that it is not. Does the bone washing and cleaning technique add significantly to the damage to the endosteal bone inflicted by the surgical preparation of the medullary canal? Once again, if it does, is this of importance? Two years clinical experience with a bone lavage instrument5 employing a pulsating saline jet under a pressure of 0.275 N/mm2 has shown no indication that it is harmful. Is it damaging to insert cement early, and thus at low viscosity, rather than late? On theoretical grounds, the early insertion of low viscosity cement might be disadvantageous in two ways: a) With early insertion the tissues are exposed to the cytotoxic action of the monomer in the cement dough for a longer period before polymerisation, and the concentration of the monomer in the dough may be slightly higher than with late insertion The evidence now suggests that the major damage to bone with the use of acrylic cement is produced by the mechanical and vascular injury occasioned by the preparation of the bone for the reception of the cement rather than by any chemical cytotoxic action of the cement per se (Jefferiss et al , 1975 ; Linder, 1977) Moreover, in 5
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28
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
Fig 14 A Surface of femur prepared by oscillating saw only, B and by oscillating saw and bone lavage instrument: operation photographs
recent experimental work reported by Miller and his colleagues (Miller et al , 1978) the injection under pressure of low viscosity cement dough into the intertrabecular spaces in the distal femoral metaphysis of the dog did not lead to the death of the surrounding trabecular bone.
b) Central absorption of monomer from the cement surface might be increased It was at one time thought that the hypotensive episodes sometimes seen following the insertion of cement in total hip replacement arthroplasty were due to the central action of absorbed monomer (Homsy et al , 1972) More recent work
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
(Modig, 1975 ; Corkill et al , 1978) has suggested that this hypothesis is probably untrue and fears that the early insertion of cement might be dangerous from this standpoint appear to be unfounded. Can the application of pressure to the cement be harmful? On the contrary, not only does the present work show that it promotes contact between cement and bone, but the regular clinical use of a pressurising technique in the acetabular cavity in replacement arthroplasty over the past 6 years has proved eminently successful (Lee et al , 1974). One other point calls for comment: The influence of the techniques described in this paper upon the tissue response to acrylic cement It is possible that the fibrous tissue layers around the cement reported by several authors (Homsy et al , 1969; Willert, 1973 ; Willert et al , 1974; Vernon Roberts and Freeman, 1976) are at least in part a consequence of the use of techniques of bone surface preparation and cement insertion which did not include serious and effective attempts to clean the bone surface, followed by pressurisation of low viscosity cement dough onto and into the bone It is perfectly clear from Homsy's description of his experimental work in dogs ( 1969), in which fibrous tissue sequestration membranes up to 3 mm in thickness were invariably found surrounding the acrylic implants, that no such manoeuvres were undertaken Fixation was often grossly defective in these specimens In experiments in dogs, Miller and his colleagues (1977) have shown how failure to clean scrupulously the endosteal layer of the medullary canal of the femur before cement insertion may lead to the persistence of large irregular areas of blood clot between cement and bone These were clearly seen in histological sections It would not be surprising if these were later absorbed and replaced by fibrous tissue. Such a sequence was seen in the studies of Homsy (1969) In conventional techniques of replacement arthroplasty, scant attention has usually been given to meticulous cleaning and preparation of the bone cavities Thus histological studies of specimens obtained from such operations may well lead to the impression that layers of fibrous tissue are an inevitable feature between bone and cement The whole process of remodelling (Willert and Semlitsch, 1974) of bone surrounding the cement may be substantially influenced by the technique of bone preparation and insertion. The work of Charnley (1970) and of Guy et al (1975) lends further support to this view, though the latter authors did not emphasise the point themselves. Whatever bone remodelling does occur around the cement in vivo is not likely to increase the shear strength of the cement-bone interface, which is probably at its maximum immediately after implantation. The reduction in cement-bone interface shear strength
29
(if any) produced by remodelling is less likely to be a late embarassment, in clinical terms, when the immediate post implantation cement-bone interface shear strength is high than when it is low It is as well to emphasise at this point that if sound cement bone contact is not achieved at the time of operation it is not likely to be achieved subsequently Thus the use, in vivo, of the methods shown in this study to promote the shear strength of the cement-bone interface in vitro, is in the opinion of the authors justifiable Furthermore, although the work reported here was carried out with the femur as the experimental model, the general conclusions are likely to be applicable to other situations in which acrylic cement is being used to achieve implant fixation. Conclusions In vitro, maximal cement-bone interface shear strength is obtained by exposing and thoroughly cleaning strong trabecular bone, and then forcing onto it under pressure low viscosity bone cement Similar techniques are likely to be effective in vivo and are probably generally applicable wherever acrylic cement is being used for implant fixation. The authors acknowledge with gratitude the expert assistance of
Dr J Gowers of the Institute of Biometry and Community Medicine, Exeter, with the statistical treatment of Table 1.
References Behrens, J C , Walker, P S , Shoji, H : Variations in strength and structure of cancellous bone at the knee J Biomechanics 7, 201 (1974) Charnley, J : The reaction of bone to self curing acrylic cement. J Bone Jt Surg 52 B, 340 (1970) Corkhill, J A , Croute, D G , James, M L , Ling, R S M : Methylmethacrylate metabolism in man The hydrolysis of methylmethacrylate to methacrylic acid during total hip replacement (in press, 1978) Ducheyne, P , Heymans, L , Martens, M , Aernoudt, E , de Meester, P , Mulier, J C : The mechanical behaviour of intra-condylar cancellous bone of the femur at different loading rates J Biomechanics 10, 747 (1977) Fitzgerald, E R : Dynamic mechanical measurements during the life to death transition in animal tissues Biorheology 12, 397-408 (1975) Fitzgerald, E R : Postmortem transition in the dynamic mechanical properties of the bone Med Physics 4, 49-53 (1977) Freeman, M A R : The pathogenesis of primary osteoarthritis. In: Modern Trends in Orthopaedics, Vol 6, A G Apley, ed. London: Butterworth's 1972 Galante, J , Rostoker, W , Ray, R D : The physical properties of trabecular bone Calc Tissue Res 5, 236 (1970) Greenwald, A S , Wilde, A H : Some observations on the interface strength of bone cement Biomechanics Laboratory Research Report, 002-74, The Cleveland Clinic Foundation 1974
30
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
Guy, J G , Jamieson-Evans, D C , Park, W M , Rannie, I , Charnley, J : A long-term micro-focal, radiological and histological study of the reaction of bone to acrylic cement. J Bone Jt Surg 57B, 113 (1975) Homsy, C A : Prosthesis seating compounds of rapid cure acrylic polymer Paper read at the National Academy of Science and American Academy of Orthopaedic Surgeons Joint Workshop On Total Hip Replacement and Skeletal Attachment 1969 Homsy, C A , Tullos, H S , Anderson, M S , Differante, N M , King, J W : Some physiological aspects of prosthesis stabilisation with acrylic polymer Clin Orthop 83, 317 (1972) Jefferiss, C D , Lee, A J C , Ling, R S M : Thermal aspects of self-curing polymethylmethacrylate J Bone Jt Surg. 57 B, 511 (1975) Kolbel, R , Bergmann, G , Rohlmann, A : Dynamic properties of the bone-cement bond Paper read at 3rd Annual Meeting of the Society for Biomaterials New Orleans 1977 Lee, A J C , Ling, R S M : A device to improve the extrusion of bone cement into the bone of the acetabulum in the replacement of the hip joint Biomed Engineering 9, 1 (1974) Linder, L : Reaction of bone to the acute chemical trauma of bone cement J Bone Jt Surg 59 A, 82 (1977) Markolf, K C , Amstutz, H C : Penetration and flow of acrylic bone cement Clin Orthop 121, 99 (1976) Miller, J , Burke, D L , Stachiewicz, J , Ahmed, A , Kelebay, L : Loosening of arthroplastic components as a result of blood clot interposed between P M M A and bone at the time of surgery Paper read at the 23rd Annual Meeting of The Orthopaedic Research Society, Las Vegas 1977 Miller, J , Tremblay, G R , Burke, D L , Ahmed, A , Kelebay, L C : The injection of acrylic cement into cancellous bone
as a method for the prevention of loosening of arthroplasty components Paper read at the 24th Annual Meeting of the Orthopaedic Research Society, Dallas 1978 Modig, J : Studies of the Aetiology and Nature of the Pulmonary and Circulatory Reactions during Total Hip Replacement. Doctoral Thesis at Uppsala University 1975 Pugh, J W , Rose, R M , Rodin, E L : Elastic and Viscoelastic properties of trabecular bone; dependance on structure J Biomechanics 6, 475 (1973) Singh, M , Riggs, B L , Beabout, J W , Jowsey, J : Femoral Trabecular pattern index for Evaluation of Spinal Osteoporosis Mayo Clinic Proceedings 48, 184 (1973) Swanson, S A V , Freeman, M A R : Is bone hydraulically strengthened? Med Biol Enging 4, 433 (1966) Vernon-Roberts, B , Freeman, M A R : Morphological and Analytical Studies of the Tissues adjacent to joint prosthesis: Investigations into the causes of loosening of prosthesis. In: Advances in Artificial Hip and Knee Joint Technology, M Schaldach, D Hohmann, eds Berlin-Heidelberg-New York: Springer 1976 Walker, P S , Bienenstock, M : Fixation Properties of acrylic cement Rev Hosp Spec Surg 1, 27 (1970) Walker, T W , Graham, J D , Mills, R H : Changes in the Mechanical Behaviour of the Human Femoral Head Associated with Arthritic Pathology J Biomech 9, 615 (1976) Willert, J -G , Ludwig, J , Semlisch, M : Reaction of Bone to methacrylate after hip arthroplasty J Bone Jt Surg 56 A, 1368 (1974)
Received March 3, 1978
Arch Orthop Traumat Surg 92, 19-30 (1978)
© J F Bergmann Verlag 1978
The Shear Strength of Trabecular Bone from the Femur, and some Factors Affecting the Shear Strength of the Cement-Bone Interface* M Halawa 2 , A J C Lee', R S M Ling2 , and S S Vangala' 'Department of Engineering Science, University of Exeter, Exeter, Devon, England 2 Princess Elizabeth Orthopaedic Hospital, Wonford Road, Exeter, Devon, England
Summary The shear strength of trabecular bone from the femur has been studied In general, the strongest trabecular bone is found close to the cortico-cancellous junction, though its shear strength depends also on the relationship of the trabeculae to the plane of shear. Some factors affecting the shear strength of the cementbone interface have been investigated In vitro, maximal cement-bone interface shear strength is obtained by exposing and thoroughly cleaning strong trabecular bone, and then forcing onto it under pressure low viscosity cement.
cepted as a basis for implant fixation, especially at the hip P M M A has no inherent adhesive or bonding properties, but relies solely on mechanical interlocking with the host bone for fixation. The aim of this study is the investigation of some of the variables affecting the mechanical interlocking that occurs between P M M A and trabecular bone, and hence the fixation of the cement to bone.
Zusammenfassung Die Scherkrafte der Knochentrabekel des Femur wurden untersucht Im allgemeinen wird der stirkste travikulare Knochen nahe des corticospongiosen Uberganges gefunden, wobeijedoch die Scherkraft zusatzlich von dem Verhaltnis der Knochentrabekel zur Ebene, in der die Scherkrifte wirken, abhangt Einige Faktoren, die die Scherkraft an der Zementknochengrenze beeinflussen, wurden untersucht In vitro wird die gr 613 te Scherkraft an der Zementknochengrenze erreicht durch Freilegen und grundliches Subern des starken travikularen Knochens und anschlie Bend durch Einpressen von Zement mit niedriger Viskositat.
The Shear Strength of Trabecular Bone
After almost two decades in clinical use, polymethylmethacrylate (P M M A ) bone cement is widely ac* This investigation was supported by Howmedica International Ltd , North Hill Plastics Division, 622 Western Avenue, Park Royal, London, W 3 OTF, and by The Northcott Devon Medical Foundation, Exeter Offprint requests to: R S M Ling (address see above)
PartI
Although a number of workers (Behrens et al , 1974; Ducheyne et al , 1977; Galante et al , 1970 ; Pugh et al , 1973) have investigated the mechanical properties of trabecular bone from the femur, their studies have mainly been confined to its compressive strength and elastic modulus In the context of the use of acrylic cement for the fixation of implants to the femur, especially those having intramedullary stems, the shear strength of trabecular bone must be regarded as of more importance than its compressive strength, since mechanical loosening of the implant, if it takes place, is likely to occur through the failure in shear of the cement-bone interface or of the trabecular bone itself. Relatively little attention has been devoted to the shear strength of trabecular bone from the femur (Greenwald and Wilde, 1976; Kolbel et al , 1977), and even less to its regional variations within the femur. This part of the study is therefore devoted to the determination of the shear strength of trabecular bone at varying distances from the cortex along the entire length of the femur For purposes of comparison, the 0344-8444/78/0092/0019/$ 2 40
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
20
shear strength of bone from normal femoral heads and from femoral heads obtained at replacement arthroplasty for osteoarthritis was determined.
Materials and Methods Six pairs of femora were obtained at post mortem and x-rayed to exclude obvious pathological abnormalities Nine femoral heads were obtained at replacement arthroplasty for osteoarthritis of
the hip Paired femora were necessary to enable one member of each pair to act as control for the other Immediately after removal and x-ray, the femora were placed in polythene bags which were stored in a deep freezer at -300 C Storage time was kept to a minimum, and in no case did it exceed 48 h. Specimens from the femora were machined in the semifrozen state to minimise tissue damage from the heat of machining (Walker et al , 1976) Machining was carried out using a crown drill of 8 73 mm inside diameter revolving at 1500 r p m. (Fig 1) Specimens were removed from 24 selected sites in the coronal plane from each femur (Fig 2) These were selected in order to assess regional variations in shear strength After machining, specimens were placed in dry, labelled test-tubes. Testing was carried out in cancellous bone, cancellous bone within 3 mm of the cortico-cancellous junction (which henceforth will be referred to as cortico-cancellous bone) and at the subarticular regions, in subchondral cortico-cancellous bone (Fig 3). For the shear tests, a Mayes Universal Testing Machine' (Fig 4) having a shear test attachment (Fig 5) was employed. The specimen holder diameter corresponded exactly to the inner diameter of the crown drill A total of 360 samples was tested in this part of the investigation One femur from each pair yielded specimens for cancellous bone shear tests whilst specimens from the other were used to determine the shear strength of the corticocancellous bone Longer samples were cut in half after being tested Sample No 1 (Fig 2) enabled the shear strength profile of
Fig 1 Crown drill used to extract cylindrical specimens from the femur
Selection of sit the
coronal p
Fig 3 Cylindrical specimen shewing test location for cancellous bone, A and cancellous bone within 3 mm of the cortico-cancellous junction B
10
211
Fig 4 Mayes Universal Testing Machine Fig 2 Selection of sites in the coronal plane of the femur
I Manufactured by Mayes Testing Machines Ltd , Windsor, England
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
PLANE EWHOLDOER
HEAD
21
shear stress since, owing to the irregular surface of trabecular bone, the area of failure is not a circle of diameter 8 73 mm though this was assumed in the computations This assumption is necessary in order that the results can be compared with those of others. The standard deviations are indicated. The variation of shear strength across the coronal plane at the base of the femoral neck is depicted in Figure 7 Figure 8 represents the shear strength of bone from different sites within the femoral head and compares normal femoral heads with those retrieved at replacement arthroplasty for osteoarthritis.
Discussion
SHEAR TSTj APPARAIUS
I I 6 'FINER'
RTRING HEAD
Fig 5 Shear test and push-out test apparatus
the cancellous bone and the cortico-cancellous junction across the coronal plane to be determined Samples machined from the 9 femoral heads recovered at replacement arthroplasty for osteoarthritis were also tested in the shear mode Most specimens were tested at a deflection rate of 5 mm min', i e a strain rate of 0.01 S though some were tested at the higher strain rate of 0.1 s l The load deflection curve was automatically displayed by an XY plotter 8 samples in which shear strengths were found over an order of magnitude greater than the mean were rejected as indicating that the plane of shear was within cortical bone.
Results The results are shown in graphic form in Figure 6 The abscissa represents the sample location as in Figure 2 and the ordinate the "shear stress" This is not the true
Referring to Figure 6, the major feature to emerge is the consistently higher shear strength of the corticocancellous bone (within 3 mm of the cortex) in comparison with that of cancellous bone 5 mm or more away from the cortex This is not an unexpected finding, and probably reflects the increasing density of cancellous bone as it approaches its cortical attachment This has implications for intramedullary fixation. Trabecular orientation in relation to the plane of shear is clearly a matter of considerable importance with respect to shear strength, and will vary with the position of the sample. The laws of bone architecture determine the orientation of the trabeculae and thus in some regions, the plane of shear is parallel to the trabeculae, whilst in others, orthogonal Subchondral bone from the femoral condyles is stronger when tested perpendicular to the trabeculae (specimen Nos 11 and 21) than when tested parallel to them (specimen Nos 10, 12, 20 and 22) The shear strength of specimen 23 is maximal at site b, i e in the middle of the head, reflecting the intersection in this region of what Singh et al ( 1973) term the primary compressive trabeculae with the primary tensile trabeculae (Fig 9) The variation of shear strength across the coronal plane at the base of the neck (Fig 7) shows a relatively high level medially where part at least of the primary compressive trabeculae are being sheared close to their cortical attachment The very low strength in the middle of the specimen reflects the fact that this site lies in the area between the two trabecular systems, whilst on the lateral side the socalled primary tensile trabeculae are being sheared, again close to their cortical attachment The plane of shear at the medial and lateral ends of this specimen (No 1) bears a roughly constant angular relationship to the primary compressive and primary tensile trabecular systems respectively, the former clearly being stronger. The relationship between plane of shear, trabecular
22
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
SHEAR PLANE IN 13
•
12
CANCELLOUS BONE
STRAIN
SHEAR PLANE IN
RATE
CORTICO-CANCELLOUS
O 101 s'
JUNCTION
E
11
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12
.
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17
-6
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Fig 6 Results of sl lear strength tests of bone at selected sites in the femora. Average of 5 samples per test standard deviation Shear -2
DEl
5-
Stress(N/mnm )
Bars indicate
OSTEOARTHRITIC
-
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8 4SHEAR
6
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t
t 1
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Fig 7 Variations in shear strength across the coronal plane at the base of the femoral neck
Inferior head (23)
l
Cenre of head (23 cb
Weight bearing area (23)
"' Posterior 124)
Il Anterior (241
Fig 8 Comparison of shear strength of trabecular bone from femoral heads of normal and osteoarthritic hips
orientation and shear strength may have implications for the correct positioning of the prosthetic components of surface replacements at both the knee and the hip. The data from Figure 8 indicate a surprising reduction in the shear properties of specimens from sites 23 and 24 in femoral heads recovered from operations for osteoarthritis, in comparison with normal femoral heads Moreover, the standard deviations in the former
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
23
Conclusions i The shear strength of cancellous bone from the femur increases as the cortico-cancellous junction is approached. ii The shear strength of cancellous bone from the femur depends upon its site in the femur and the relationship of the trabeculae to the plane of shear. iii The shear strength of cancellous bone in the femoral head is lower and exhibits greater variation in femoral heads from osteoarthritic hips than from normal hips. iv Cancellous bone exhibits viscoelastic properties. PartII Some FactorsAffecting the Strength of the Cement-bone Interface
Fig 9 Coronal section of femur shewing primary compressive trabeculae, A and primary tensile trabeculae B
are much higher, suggesting inconsistent mechanical behaviour This is a reflection of the variable and sometimes gross destructive changes in these femoral heads, with cystic changes, trabecular fatigue microfractures (Freeman, 1972) and boney collapse It indicates the need for caution in the selection of cases for surface replacement at the hip The findings in the femoral heads from osteoarthritic hips are somewhat at variance with those of Greenwald and Wilde (1974) who found an average shear strength of 4 6 N/mm, again with a high S D The method of testing was different however and gross abnormalities were only noted in 2 specimens out of 9, whereas these were observed in 7 out of 9 in the present investigation. The data in Figure 6 from the relatively small number of samples tested at the higher strain rate suggest that cancellous bone does exhibit viscoelastic properties, a matter sometimes in dispute (Freeman and Swanson, 1966 ; Pugh et al , 1973) Moreover, the persistence of viscoelastic behaviour in spite of the considerable interval between subject death and specimen testing indicates, according to Fitzgerald ( 1975, 1977) that the viscoelastic behaviour in life was probably more marked than these results suggest This takes no account of the possible hydraulic influence of circulating blood and tissue fluids. Finally, comment should be made on the absence of results at specimen sites 6 and 16 At these middiaphyseal areas, there is for practical purposes no endosteal trabecular bone and thus the shear strength cannot be tested.
A major factor affecting the shear strength of the cement-bone interface must clearly be the shear strength of the bone itself, the determination of which was established in the first part of this investigation. The aim of the second part of this study was to determine the influence of the following variables on the shear strength of the cement-bone interface: i thickness of cancellous bone adjacent to the cortex. ii methods of preparation of the medullary canal. iii methods of preparation and introduction of the cement. Materials and Methods The acquisition and machining of cylindrical samples was again as described in Part I In addition a second type of sample for
'pushout' testing was prepared by sawing each femur into slices 20 mm thick perpendicular to its long axis The slices were numbered and were taken from exactly similar positions in each pair of femora (Fig 10) For both of these types of specimen, the medullary canals of the femora were prepared in different ways prior to the machining and sawing (vid inf ) Fourteen pairs of femora were used for this, phase of the study and were x-rayed and stored as in Part I prior to the preparation of the medullary canals and machining The interval between machining and cementing was not longer than 3 h The shear strength of the cement-bone interface was determined using the two types of specimen as follows:
a) Cylindrical Specimens The effects of two variables were studied. i Thickness of cancellous bone within the cortex. ii The method of preparing the medullary canal. i Thickness of CancellousBone within the Cortex The medullary canal of one femur of a pair was prepared using a rasp and curette to leave > 5 mm of cancellous bone within the cortex, and the canal of the other femur was prepared to leave between 2-3 mm
24
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
Selection sites for Out test
Fig 11 Bone-cement composite cylindrical specimen iii Time of cement insertion. iv Pressurisation of cement.
Fig 10 Selection of sites for 'push-out' test samples within the cortex Both canals were then washed with a water jet at a pressure of 0 21 N/mm 2 and cylindrical specimens were obtained from both femora by machining with the crown drill at positions I to 22 (Fig 2) The specimens were placed in aluminium moulds of the same diameter as the inner diameter of the crown drill Simplex P RO 2 bone cement dough was prepared by adding the monomer to the polymer in a P V C mixing bowl and mixing for 2/2 min at a beating frequency of 1 Hz The ambient temperature was maintained at 21 °C and the humidity at 50 + 10 % Half a minute after the cessation of mixing, the cement dough was inserted under thumb pressure into the moulds on top of the cylindrical bone specimens and thus applied to the end surfaces of the specimens After polymerisation the cement bone composite specimens (Fig 11) were tapped out of their moulds and allowed to cool in air at 21°C They were next tested in the shear fixture in the Mayes machine, being so positioned that the plane of shear was at the cement-bone interface Specimens having oblique interfaces were rejected.
i Thickness of CancellousBone within the Cortex Three pairs of femora were employed The medullary canal of one femur from each pair was prepared leaving approximately 2 mm of cancellous bone in tact within the cortex, whilst the other was prepared so as to leave not less than 5 mm of cancellous bone intact within the cortex. The medullary canals were then washed using a water jet under a pressure of approximately 0 21 N/mm 2 and the femora were sliced as described above Simplex P R O2 cement dough was prepared as above and inserted into the medullary canals of the slices under palm pressure Continuous monitoring of the pressure at the cement-bone interface was carried out using a Piezo electric pressure transducer 3 located at the cement-bone interface, and in no case was it allowed to exceed 0 15 N/mm2 . After polymerisation, the specimens were allowed to cool in air at 210 C and then tested in the 'pushout' fixture in the Mayes machine (Fig 5). ii Methodfor PreparingMedullary Canal Two pairs of femora were employed The medullary canal from one femur ofeach pair was prepared using a rasp and curette whilst the other was prepared in similar fashion and then washed using a water jet under a pressure of approximately 0 21 N/mm 2 The thickness of cancellous bone was kept as near constant as possible at 2-3 mm The specimens were sliced, cemented and tested as in i. above, again with continuous cement-bone interface pressure monitoring.
i Thickness of cancellous bone within the cortex. ii Method of preparing the medullary canal.
iii Time of Insertion of Cement Three pairs of femora were used. The medullary canals of all 3 pairs of femora were prepared using a rasp and curette, maintaining a constant thickness of cancellous bone of 2-3 mm within the cortices The canals were then washed using the water jet and the femora sliced The slices from one femur of each pair were cemented 3 min after the beginning of mixing, whilst the slices from the other femur of each pair were cemented 6 min after the beginning of mixing. Cement-bone interface pressure monitoring was continuous in each specimen as in i above After polymerisation and cooling, the specimens were tested in the 'pushout' fixture. iv Pressurisation Two pairs of femora were employed The medullary canals of both pairs were prepared as in iii above, including washing, and the femora were then sliced The slices obtained from one femur from each pair were cemented at 3 min after the commencement of the mixing of the cement components and pressurised for a minimum of 30 sec at 0 15 N/mm 2 recorded
2 Manufactured by North Hill Plastics Division of Howmedica International
3 Vibrometer 12 QP250 ck Manufactured by Vibro-meter Corporation, Fribourg, Switzerland
ii Method of Preparing the Medullary Canal The medullary canals of two pairs of femora were prepared using rasp and curette so as to leave a 2-3 mm layer of cancellous bone within the cortices. The canal of 1 femur from each pair was then washed thoroughly using a water jet under 0 21 N/mm2 pressure. Cylindrical specimens were obtained, cemented and tested as above. b) 20 mm Thick Femoral Slices Eleven pairs of femora were employed for these tests The effects of 4 variables were studied:
M Halawa et al : Shear Strength of Trabecular Bone from the Femur at the cement-bone interface The slices from the other femur of each pair were dealt with similarly, except that pressurisation was maintained at 0 3 N/mm 2 at the cement-bone interface The specimens were allowed to cool in air at 210 C after polymerisation and tested in the 'pushout' fixture. In one pair of femora the variables were combined as follows: One femur was prepared leaving 5 mm of cancellous bone within the cortex, washing was not performed and after slicing the cement was inserted 6 min after the commencement of mixing under a pressure of 0 15 N/mm2 (worst case). In the other femur, 2-3 mm of cancellous bone was left within the cortex, the canal was washed with a water jet, and after slicing, cement was inserted 3 min after the commencement of mixing under a pressure of 0 3 N/mm2 After polymerisation and cooling, the specimens were tested in the 'pushout' fixture (best case).
25
the latter actually failed as they were being tapped out of the moulds prior to testing. b) Femoral Slices The results are shown in Table 1 and are expressed as the load at failure in Newtons 154 specimens were tested. A correction for the increased bone-cement interface area produced by reducing the thickness of the cancellous rim from 5 mm to 2-3 mm was applied in tabulating the results for the first variable, and was derived as follows: An extra pair of femora was sliced as in the experimental femora and the canal of one member of the pair was prepared leaving a 2 3 mm layer of cancellous bone, whereas in the other member of the pair a layer of cancellous bone 5 mm thick was left. The slices were then sectioned coronally and the included angles at the medial and lateral surfaces of the canal of each slice measured The coronal and sagittal diameters of the canal in each slice were measured, and from these figures, the surface areas of the ellipsoids representing the area of bone in contact with cement were calculated These areas of the corresponding slices from each pair were expressed as a ratio which proved to be relatively constant for all slices except 4 and 5 (Fig 10) This ratio was used for correcting the results.
Results a) CylindricalSpecimens The results obtained in testing the cylindrical bonecement composite specimens are depicted in the histogram (Fig 12) 120 specimens were tested The abscissa represents the sample location as in Figure 2 and the ordinate the cement-bone interface shear strength in N/mm 2 The influence of washing was so marked that the study of unwashed specimens was abandoned after approximately 15 such specimens had been tested 3 of
7
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Fig 12 Results of shear strength of bone-cement composite cylindrical specimens Average of 3 samples per test result Strain rate 0 01 s'
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
26
Table 1 Results of push-out specimen tests: load at failure in Newtons 154 specimens tested Statistical Comment (Dr J Gowers): Within each pair of femora, the effect of each variable is significant at the 0 1 % level An analysis of variance was performed on each variable in order to estimate whether the variables are significant over the whole population from which the specimen cadavers are a sample Cortico-cancellous bone, washing, and early insertion of cement (i e low viscosity cement) are all significant at the 5 % level Pressurisation fails to be significant This is probably an accident of small numbers, in view of the uncharacteristic results in cadaver 10 There is no interaction between the variables and position of testing Age
Distal femur
Proximal femur
Sex
Cadaver number
6
7
3971 (2906)
1872 (620)
1328 (287)
72
F
1
6178 (6198)
5348 (4436)
3904 (2770)
3460 (1609)
30
M
2
5601 (3590)
2640 (374)
2022 (1911)
4360 (1885)
8104 (3524)
16
M
3
2276 (1490)
2812 (1350)
3218 (3167)
3664 (3001)
2990 (1464)
2187 (721)
78
M
4
3439 (2800)
2574 (358)
3460 (1864)
4813 (51)
2466 (242)
1916 (940)
3926 (2600)
38
M
5
2124 (1152)
1014 (967)
2169 (1789)
3962 (1760)
2067 (511)
719 (239)
4196 (2267)
84
M
6
2770 (2300)
2421 (1919)
2297 (1129)
928 (577)
1741 (1158)
1098 (973)
3078 (1855)
90
M
7
(6 min insertion)
3634 (1962)
3422 (873)
2962 (1011)
4738 (623)
6465 (3994)
4523 (2226)
4316 (3713)
78
F
8
0.3 N/mm2 pressure
2664 (2009)
2412 (1806)
3280 (2067)
4130 (498)
2676 (955)
5272 (1329)
77
F
9
(0.15 N/mm2 pressure)
3879 (2756)
3101 (1436)
2232 (1867)
4109 (4080)
3123 (2571)
5744 (4801)
4491 (2860)
57
F
10
Best Case (Worst Case)
7533 (1263)
6586 (932)
10285 (1849)
11348 (3229)
9973 (1864)
4638 (210)
4949 (488)
54
F
11
4
2
3
1478 (1067)
2240 (984)
2371 (384)
3619 (3439)
6203 (2576)
2863 (1552)
7627 (6368)
(> 5 mm cancellous retained)
3816 (2929)
4985 (2695)
Curettage & washing
4370 (2050)
(Curettage only) 3 min insertion
Sample number
1
5
Variable 2-3 mm rim cancellous retained
From Table 1, the following become apparent: i With the retention of a 2-3 mm layer of cancellous bone, the load at failure on average was 100 % higher than with the retention of 5 mm rim of cancellous bone. ii The load at failure with a clean bone surface was on average 200 % higher than with a surface which was not cleaned before cementation. iii The load at failure when cement was inserted 3 min after the beginning of mixing was on average 60% higher than when it was inserted 6 min after the beginning of mixing. iv The load at failure when the cement was exposed to 0 3 N/mm 2 pressure before polymerisation was on average 100 % higher than when it was exposed to 0.15 N/mm2. v In the pair of femora in which a 'best' and 'worst' case was developed, the difference in load at failure was on average 800 %.
3674 61860)
In this type of test, because each slice from one femur of a pair is compared with the corresponding slice from the other member of the pair, the overall effect of the taper of the medullary canal is eliminated. In vivo, the latter may by itself make a substantial contribution to fixation, to an extent which is at present under study.
Discussion The results of testing both types of specimen show a direct relationship between the shear strength of the trabecular bone at the interface and the shear strength of the interface itself, a finding at variance with those of Greenwald and Wilde ( 1974) With interfacial trabecular bone of a given shear strength, the other variables considered in this study are clearly all capable of exerting a substantial influence upon the shear strength
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
27
Amstutz (1976) showed that a combination of early insertion and high applied pressure produced maximum penetration of cement dough into holes drilled into aluminium plugs, and clearly the advantage of early insertion in their experiments, as in those reported here, lies in the low viscosity of the cement when inserted early.
Fig 13 A Specimen prepared by rasping and curettage only, B Specimen prepared by rasping, curettage and washing
of the cement-bone interface This is what might be expected, since acrylic cement achieves fixation by mechanical interlocking and the variables under study are all concerned with the capacity of the cement dough to establish contact with the bone surface and then penetrate the inter-trabecular spaces Subsequent polymerisation of the cement within these spaces then 'locks' the cement to the bone Thus, washing the bone surface thoroughly helps to remove shattered trabeculae, blood and fat which would otherwise block the trabecular spaces and prevent the inflow of the cement dough Figure 13 shows a comparison between a surface which has simply been prepared by rasping and curettage and one which has been prepared by rasping, curettage and washing. It is obvious that penetration of the inter-trabecular spaces by cement dough is much more likely to be achieved in the latter Figure 14 shows a similar comparison between the appearance of the lower end of the femur sectioned with an oscillating saw during knee replacement before and after washing with a pulsating saline jet from a bone lavage instrument 4. Once again it is self evident that penetration of inter-trabecular spaces by cement dough will be more effective in the latter situation Once the bone surface is cleared of blood and debris, and the trabecular spaces open, the capacity of the cement to penetrate these spaces will depend upon the viscosity of the dough and the pressure applied to it The lower the viscosity and the higher the pressure, the greater the penetration. Walker and Bienenstock (1970) investigated the relationship between the time from the beginning of mixing of the monomer and polymer and the viscosity of the cement dough They confirmed experimentally what all surgeons using acrylic cement know from experience, i.e with increasing time after mixing, the viscosity of the cement dough increases, slowly at first and then rapidly as polymerisation approaches Markolf and 4
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Lee and his colleagues (1978), amongst others, have pointed out that the mechanical strength of acrylic cement is improved by the exclusion of blood and tissue debris (achieved by thorough washing of the bone surface) and the use of low viscosity cement dough, as well as by the application of pressure to the dough before polymerisation The present work suggests that such measures are also highly favourable to the strength of the fixation between the cement and bone. The inferences to be drawn from this in vitro study are self evident In the femur, the strongest trabecular bone lies close to the cortex The maximum shear strength of the cement-bone interface is achieved by exposing and thorougly cleaning this layer of bone and then inserting cement of low viscosity under pressure. Direct extrapolation, however, of this technique into the in vivo situation at operation raises a number of questions which probably only time will answer. Does the removal of all but 2-3 mm of trabecular bone from within the medullary canal interfere with revascularisation of the endosteal bone, and if it does, is this likely to be of importance? Clinical experience to date suggests that it is not. Does the bone washing and cleaning technique add significantly to the damage to the endosteal bone inflicted by the surgical preparation of the medullary canal? Once again, if it does, is this of importance? Two years clinical experience with a bone lavage instrument5 employing a pulsating saline jet under a pressure of 0.275 N/mm2 has shown no indication that it is harmful. Is it damaging to insert cement early, and thus at low viscosity, rather than late? On theoretical grounds, the early insertion of low viscosity cement might be disadvantageous in two ways: a) With early insertion the tissues are exposed to the cytotoxic action of the monomer in the cement dough for a longer period before polymerisation, and the concentration of the monomer in the dough may be slightly higher than with late insertion The evidence now suggests that the major damage to bone with the use of acrylic cement is produced by the mechanical and vascular injury occasioned by the preparation of the bone for the reception of the cement rather than by any chemical cytotoxic action of the cement per se (Jefferiss et al , 1975 ; Linder, 1977) Moreover, in 5
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28
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
Fig 14 A Surface of femur prepared by oscillating saw only, B and by oscillating saw and bone lavage instrument: operation photographs
recent experimental work reported by Miller and his colleagues (Miller et al , 1978) the injection under pressure of low viscosity cement dough into the intertrabecular spaces in the distal femoral metaphysis of the dog did not lead to the death of the surrounding trabecular bone.
b) Central absorption of monomer from the cement surface might be increased It was at one time thought that the hypotensive episodes sometimes seen following the insertion of cement in total hip replacement arthroplasty were due to the central action of absorbed monomer (Homsy et al , 1972) More recent work
M Halawa et al : Shear Strength of Trabecular Bone from the Femur
(Modig, 1975 ; Corkill et al , 1978) has suggested that this hypothesis is probably untrue and fears that the early insertion of cement might be dangerous from this standpoint appear to be unfounded. Can the application of pressure to the cement be harmful? On the contrary, not only does the present work show that it promotes contact between cement and bone, but the regular clinical use of a pressurising technique in the acetabular cavity in replacement arthroplasty over the past 6 years has proved eminently successful (Lee et al , 1974). One other point calls for comment: The influence of the techniques described in this paper upon the tissue response to acrylic cement It is possible that the fibrous tissue layers around the cement reported by several authors (Homsy et al , 1969; Willert, 1973 ; Willert et al , 1974; Vernon Roberts and Freeman, 1976) are at least in part a consequence of the use of techniques of bone surface preparation and cement insertion which did not include serious and effective attempts to clean the bone surface, followed by pressurisation of low viscosity cement dough onto and into the bone It is perfectly clear from Homsy's description of his experimental work in dogs ( 1969), in which fibrous tissue sequestration membranes up to 3 mm in thickness were invariably found surrounding the acrylic implants, that no such manoeuvres were undertaken Fixation was often grossly defective in these specimens In experiments in dogs, Miller and his colleagues (1977) have shown how failure to clean scrupulously the endosteal layer of the medullary canal of the femur before cement insertion may lead to the persistence of large irregular areas of blood clot between cement and bone These were clearly seen in histological sections It would not be surprising if these were later absorbed and replaced by fibrous tissue. Such a sequence was seen in the studies of Homsy (1969) In conventional techniques of replacement arthroplasty, scant attention has usually been given to meticulous cleaning and preparation of the bone cavities Thus histological studies of specimens obtained from such operations may well lead to the impression that layers of fibrous tissue are an inevitable feature between bone and cement The whole process of remodelling (Willert and Semlitsch, 1974) of bone surrounding the cement may be substantially influenced by the technique of bone preparation and insertion. The work of Charnley (1970) and of Guy et al (1975) lends further support to this view, though the latter authors did not emphasise the point themselves. Whatever bone remodelling does occur around the cement in vivo is not likely to increase the shear strength of the cement-bone interface, which is probably at its maximum immediately after implantation. The reduction in cement-bone interface shear strength
29
(if any) produced by remodelling is less likely to be a late embarassment, in clinical terms, when the immediate post implantation cement-bone interface shear strength is high than when it is low It is as well to emphasise at this point that if sound cement bone contact is not achieved at the time of operation it is not likely to be achieved subsequently Thus the use, in vivo, of the methods shown in this study to promote the shear strength of the cement-bone interface in vitro, is in the opinion of the authors justifiable Furthermore, although the work reported here was carried out with the femur as the experimental model, the general conclusions are likely to be applicable to other situations in which acrylic cement is being used to achieve implant fixation. Conclusions In vitro, maximal cement-bone interface shear strength is obtained by exposing and thoroughly cleaning strong trabecular bone, and then forcing onto it under pressure low viscosity bone cement Similar techniques are likely to be effective in vivo and are probably generally applicable wherever acrylic cement is being used for implant fixation. The authors acknowledge with gratitude the expert assistance of
Dr J Gowers of the Institute of Biometry and Community Medicine, Exeter, with the statistical treatment of Table 1.
References Behrens, J C , Walker, P S , Shoji, H : Variations in strength and structure of cancellous bone at the knee J Biomechanics 7, 201 (1974) Charnley, J : The reaction of bone to self curing acrylic cement. J Bone Jt Surg 52 B, 340 (1970) Corkhill, J A , Croute, D G , James, M L , Ling, R S M : Methylmethacrylate metabolism in man The hydrolysis of methylmethacrylate to methacrylic acid during total hip replacement (in press, 1978) Ducheyne, P , Heymans, L , Martens, M , Aernoudt, E , de Meester, P , Mulier, J C : The mechanical behaviour of intra-condylar cancellous bone of the femur at different loading rates J Biomechanics 10, 747 (1977) Fitzgerald, E R : Dynamic mechanical measurements during the life to death transition in animal tissues Biorheology 12, 397-408 (1975) Fitzgerald, E R : Postmortem transition in the dynamic mechanical properties of the bone Med Physics 4, 49-53 (1977) Freeman, M A R : The pathogenesis of primary osteoarthritis. In: Modern Trends in Orthopaedics, Vol 6, A G Apley, ed. London: Butterworth's 1972 Galante, J , Rostoker, W , Ray, R D : The physical properties of trabecular bone Calc Tissue Res 5, 236 (1970) Greenwald, A S , Wilde, A H : Some observations on the interface strength of bone cement Biomechanics Laboratory Research Report, 002-74, The Cleveland Clinic Foundation 1974
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M Halawa et al : Shear Strength of Trabecular Bone from the Femur
Guy, J G , Jamieson-Evans, D C , Park, W M , Rannie, I , Charnley, J : A long-term micro-focal, radiological and histological study of the reaction of bone to acrylic cement. J Bone Jt Surg 57B, 113 (1975) Homsy, C A : Prosthesis seating compounds of rapid cure acrylic polymer Paper read at the National Academy of Science and American Academy of Orthopaedic Surgeons Joint Workshop On Total Hip Replacement and Skeletal Attachment 1969 Homsy, C A , Tullos, H S , Anderson, M S , Differante, N M , King, J W : Some physiological aspects of prosthesis stabilisation with acrylic polymer Clin Orthop 83, 317 (1972) Jefferiss, C D , Lee, A J C , Ling, R S M : Thermal aspects of self-curing polymethylmethacrylate J Bone Jt Surg. 57 B, 511 (1975) Kolbel, R , Bergmann, G , Rohlmann, A : Dynamic properties of the bone-cement bond Paper read at 3rd Annual Meeting of the Society for Biomaterials New Orleans 1977 Lee, A J C , Ling, R S M : A device to improve the extrusion of bone cement into the bone of the acetabulum in the replacement of the hip joint Biomed Engineering 9, 1 (1974) Linder, L : Reaction of bone to the acute chemical trauma of bone cement J Bone Jt Surg 59 A, 82 (1977) Markolf, K C , Amstutz, H C : Penetration and flow of acrylic bone cement Clin Orthop 121, 99 (1976) Miller, J , Burke, D L , Stachiewicz, J , Ahmed, A , Kelebay, L : Loosening of arthroplastic components as a result of blood clot interposed between P M M A and bone at the time of surgery Paper read at the 23rd Annual Meeting of The Orthopaedic Research Society, Las Vegas 1977 Miller, J , Tremblay, G R , Burke, D L , Ahmed, A , Kelebay, L C : The injection of acrylic cement into cancellous bone
as a method for the prevention of loosening of arthroplasty components Paper read at the 24th Annual Meeting of the Orthopaedic Research Society, Dallas 1978 Modig, J : Studies of the Aetiology and Nature of the Pulmonary and Circulatory Reactions during Total Hip Replacement. Doctoral Thesis at Uppsala University 1975 Pugh, J W , Rose, R M , Rodin, E L : Elastic and Viscoelastic properties of trabecular bone; dependance on structure J Biomechanics 6, 475 (1973) Singh, M , Riggs, B L , Beabout, J W , Jowsey, J : Femoral Trabecular pattern index for Evaluation of Spinal Osteoporosis Mayo Clinic Proceedings 48, 184 (1973) Swanson, S A V , Freeman, M A R : Is bone hydraulically strengthened? Med Biol Enging 4, 433 (1966) Vernon-Roberts, B , Freeman, M A R : Morphological and Analytical Studies of the Tissues adjacent to joint prosthesis: Investigations into the causes of loosening of prosthesis. In: Advances in Artificial Hip and Knee Joint Technology, M Schaldach, D Hohmann, eds Berlin-Heidelberg-New York: Springer 1976 Walker, P S , Bienenstock, M : Fixation Properties of acrylic cement Rev Hosp Spec Surg 1, 27 (1970) Walker, T W , Graham, J D , Mills, R H : Changes in the Mechanical Behaviour of the Human Femoral Head Associated with Arthritic Pathology J Biomech 9, 615 (1976) Willert, J -G , Ludwig, J , Semlisch, M : Reaction of Bone to methacrylate after hip arthroplasty J Bone Jt Surg 56 A, 1368 (1974)
Received March 3, 1978
