Archives of Orthopaedic and Traumatic Surgery
Arch Orthop Traumat Surg 94, 191-195 (1979)
©J F Bergmann Verlag 1979
Experimental Fixation of Bone Cement and Composite Resins to Bone* J Vainio', J Kilpikari 2 , P T6rmila 2 , and P Rokkanen' i Institute of Clinical Sciences, University of Tampere and Central Hospital of Tampere, Teiskontie 35, SF-33520 Tampere 52, Finland of Materials Science, Tampere University of Technology
2 Institute
Summary The aim of this study was to ascertain whether the use of liquid acrylic resin or NeoCryl XK-53 acrylic emulsion or etching of the bone surface with phosphoric acid or the application of both these methods would improve the bonding of CMW bone cement or Concise and Silar composite resin to bone. The test materials were applied to fresh cortical bovine bone Their bonding capacity was measured by the Instron Universal Testing Machine The surfaces of the test materials and the bone surfaces were examined by means of optic microscope or scanning electron microscope The bonding strength of the bone cement and composite resins as such were found to be of the same magnitude A three to five-fold improvement was obtained with liquid acrylic intermediary material Acid etching impaired the bonding. Zusammenfassung Das Ziel dieser Untersuchung war, herauszufinden, ob der Gebrauch von flussigen zusammengesetzten Acryl-Harzen oder von Neo Cryl XK-53 Acryl-Emulsionen oder von Atzung der Knochenoberflache mit Phosphorsaure oder die Anwendung von beiden Methoden die Fixierung des Knochenzements CMW oder der zusammengesetzten Harze Concise und Silar verbessern wiirde Die untersuchten Materialien wurden an frischen corticalen Rinderknochen angebracht Die Fixierungskapazitat wurde mit dem Universal-Gerat Instron fir Materialtests gemessen Die Oberflachen des Testmaterials und des Knochens wurden mit Hilfe eines optischen oder Raster-Elektronenmikroskops untersucht Die Resultate bewiesen, daB sich der Knochenzement wie die zusammengesetzten Harze an dem Knochen fixierten Das als Zwischenstoff verwendete flissige Acryl Offprint requests to: Prof P Rokkanen (address see above) * Dedicated Professor Dr E Uehlinger on his 80th birthday
verbesserte die Haftfiahigkeit um das 3-5 fache Die Atzungen mit Saure verschlechterte die Haftfahigkeit.
Bone cement is utilized in orthopedic surgery for the fixation of different implants to bone The most frequent application of bone cement is in the fixation of artificial joints This material is also used in the osteosynthesis of severe comminuted fractures and in the filling of bone defects Bone cement anchors the bone and the implant firmly to each other After setting of the cement, the surfaces will remain in contact by the aid of physical forces Experimentally, composite resins have also been used alone as fixative in osteosynthesis in the same way as bone cement, but the fixation to the bone has been only partially successful (Vainio and Rokkanen, 1978). In the present study an attempt was made to find out whether the strength of the boundary between bone and cement can be increased by the application of liquid polymerized acrylic between the bone and the cement or by treating the bone surface by acid etching in the same way as the dental enamel is treated for the fixation of tooth-restorative material At the same time we tested whether the bis-GMA composite resins used in dentistry could be fixed to cortical bone in the same way as to dental enamel Moreover, these resins were compared with a currently used bone cement. Material and Methods We used fresh refrigerated bovine femoral or tibial bone, from which square pieces of cortical bone, measuring 2 x 2 cm, were sawed out after thawing The bone surface was ground smooth, after which the test material was applied A cylindric piece of acrylic was fixed to the test material and became chemically bonded to it during the process of polymerization.
0344-8444/79/0094/0191/$ 1 00
192
Bonding to bone
J.Vainio et al : Experimental Fixation of Bone Cement
No of specimens
Table 1 The bonding strength of the test materials to bone
Silar and Concise a
CMW Bonding strength
No of specimens
Bonding strength 42 5
1 Test material as such
8
35 0
8
2 Intermediary material Acrylic resin Neo Cryl XK-53
9 8
110 5
9
175 4
(9)b
3 Acid etching
9
02
69 2 (> 100) b
9
18 7 Silar and Concise gave same results Values given are for Silar b Loosening occurred partly at the layer between composite resin and acrylic cylinder and partly at the bone surface a
4 Acid etching and intermediary material Acrylic resin Neo Cryl XK-53
8 8
16 9 187 9
We tested CMW bone cement, without barium sulphate, and Concise and Silar composite resins As intermediary material we used Concise and Silar liquid acrylic resin or NeoCryl XK-53 acrylic emulsion A total of 130 specimens were studied. The test material was bonded to the bone in one of the following ways: 1 The test material was applied as such onto the bone surface and was pressed manually against the bone with an acrylic cylinder. 2 Liquid intermediary material was applied to the bone surface, and the test material was applied during polymerization An acrylic cylinder was pressed by hand against the test material. 3 The bone surface was treated with 37% phosphoric acid for one minute, the acid was rinsed away with water, and the bone surface was dried The test material was applied onto the etched bone surface and pressed against it with an acrylic cylinder. 4 The bone surface was treated with phosphoric acid as in point 3 Liquid intermediary material was applied to the etched surface, the test material was applied during polymerization and pressed against the bone with an acrylic cylinder. Fixation to the bone was measured using the Instron Universal Testing Machine, by the aid of which the acrylic cylinder was pulled off the bone at a rate of 1 mm/min Since loosening nearly always (Table 1) occurred at the bone surface, the results represent the bonding strength of this surface. Separate polymerized specimens of the test materials were prepared for surface studies Their surfaces were ground smooth and etched for about five minutes by means of the vapour of strong nitrous acid The surfaces were examined in optic microscope The ground bone surface and the etched bone surface were studied in scanning electron microscope.
Results The fixation of the test materials to the bone achieved by the different methods is shown in Table 1 CMW as such adhered to the bone with about the same strength than the composite resins With regard to the fixing capacity of Concise and Silar resulting from the different treatments no differences were observed (The
47 4
9 (9)b
(> 100)
b
matrix and filler components are chemically similar in the two materials) The best results were obtained when acrylic emulsion was used as intermediary material Using acrylic resin as intermediary material, the fixation was also improved, though not to the same extent as with acrylic emulsion Acid etching impaired the results significantly When both acid etching and intermediary material were applied, poorer results were obtained with acrylic resin than with acrylic emulsion. Optic microscopy of the surface of the polymerized specimens of the test materials revealed in the surface of CMW bone cement round, solid polymetric particles varying in size, the largest measuring about 100 m in diameter In addition, air bubbles were observed The space between the particles and the air bubbles was filled by a mass that appeared to be homogenous (Fig 1a) In the surface of Concise and Silar, irregularly shaped, angular particles of filled component and some air bubbles were observed, and between them a homogenous-looking mass In the surface of Silar, the largest particles of the filled component measured 120 gm (Fig lb). In the surface of the ground bone grooves caused by the grinding and some round gaps produced by blood vessels were seen in the scanning electron microscope (Fig 2 a) In the surface of the etched bone the grooves due to grinding had become levelled out and the surface was on the whole smoother than the surface of the ground bone (Fig 2b). Discussion In orthopedic surgery the question of firm and lasting fixation is of essential importance In the case of a fractured bone fixation should primarily involve the ends of the bone and secondarily the bone and the
J Vainio et al : Experimental Fixation of Bone Cement
193
Fig a Surface structure of ground polymerized CMW bone cement Optic micrograph, x 150
Fig 2a Surface structure of ground bone Scanning electron micrograph, x 300
Fig lb Surface structure of polymerized Silar Optic micrograph, x 250
Fig 2b Surface structure of ground acid-etch bone Scanning electron micrograph, x 300
means of osteosynthesis employed When fixing an artificial joint the bone and the plastic or metallic endoprosthesis have to be fastened to each other The filling of bone defects due to neoplastic growth or some other cause implies fixation of the filling material to the bone. With regard to stability, fixations are expected to answer certain demands In the case of bone that will
heal, only temporary fixation, lasting over the period of bone healing, is required Thereafter the means used for fixation may be removed On the other hand, problems may arise when an acquired or produced bone defect makes it necessary to establish a fixation serving throughout the patient's life-time. Most of the knowledge available concerning the stable fixation of implant material to bone relates to
194
the application of artificial joints During the last 20 years a hardening intermediary material, bone cement, has been used in anchoring endoprostheses to the bone This kind of fixation has also sometimes been employed in the treatment of fractures and bone defects. The weak link in the system bone-cement-implant has proved to be the boundary between bone and cement After a sufficiently long time of observation this is giving way in about 20 per cent of cases. The failure has been attributed to various causes, e g. the heat released while the cement is setting, the direct cytotoxic effect of the monomer of the cement, and trauma caused by the mechanical handling of the bone tissue. Owing to differences in chemical nature between the bone surface and the cement, the physical fixation resulting between them is too weak to bear any heavy mechanical strain Theoretically the fixation of bone cement to bone might be improved by the application of an intermediary material that forms chemical bonds with both the cement and the bone, by treating the bone surface by some technique producing better locking, or by the application of an intermediary material which is more efficiently fixed mechanically to the bone and chemically to the cement. Polymethylmethacrylate bone cement hardens by polymerization, and while this process is taking place, a liquid intermediary acrylic material may readily be added Chemical bonding to the bone is not readily achieved, although materials forming chemical bonds with bone have been developed It is known, for instance, that zinc polyacrylate cement during polymerization forms complex bonds with bone calcium (Smith, 1968 ; 1971). In dentistry, bis-GMA composite resins have been used in the cosmetically exacting restoration of anterior teeth The restorative material is fixed to the enamel by the acid etch technique, which changes the properties of the enamel surface so that the mechanical fixation is strengthened The enamel is etched by exposing the surface to the effect of phosphoric acid for about one minute The acid effect reaches a depth of about 40 tm Liquid acrylic resin applied to the surface becomes mechanically fixed to the enamel and chemically to the filling material during polymerization This method has proved useful in practice. In the present investigation an attempt was made to find out whether the fixing capacity of bone cement can be improved by the application of an intermediary material, which becomes chemically bonded to the cement, or by treating the bone surface with phosphoric acid, or by applying both methods At the same time we tested whether the acid etching method for
J.Vainio et al : Experimental Fixation of Bone Cement
enamel employed in dentistry could be adapted to cortical bone, and we compared the fixations of ordinary bone cement and composite resins achieved by these techniques In addition, to the acrylic resins commonly used as intermediary material in dentistry, we used a commercial acrylic emulsion, which was assumed to resemble in structure the methylmethacrylate monomer of bone cement A previous study (Vainio and Rokkanen, 1978) dealt with the use of Concise composite resin for the fixation of osteotomies in the rat tibia The fixation proved to be successful in about 60 per cent of cases The present results seem to suggest that the fixing capacity of Concise as such is not sufficient for successful fixation. The large particle size of the filler material of Concise appears to be in part responsible for the failure. The adhesion of bone cement and composite resin to the ground surface of cortical bone was found to be of the same order ( 35 N/cm2 and 42 5 N/cm 2 ) On inspection of the bone surfaces, grooves produced by the grinding and some hollows resulting from the structure of the bone were observed The unevenness of the bone surfaces explains that the materials adhered as such to the bone, although they do not possess any gluing properties. Acid etching reduced the fixing capacity of the test materials instead of improving it Inspection of the etched bone surfaces showed that they were levelled by the effect of the acid; hollows like those resulting in dental enamel were not observed This levelling is apparently the main cause of the weakened fixation It seems possible moreover that the effect of the acid makes the bone surface fragile. The use of a liquid acrylic intermediary material obviously led to improved fixation The fixing capacity of bone cement was increased to about three-fold by the use of acrylic resin and to about five-fold by the use of acrylic emulsion The fixing capacity of the composite resins was also clearly improved, but less so than the fixing capacity of the bone cement. As regards the fixing capacity of the filling materials applied to enamel by the method of acid etching, values of the order of 420 N/cm 2 have been reported (Phillips, 1973) When in the present investigation the same method was used in fixing the same materials to cortical bone, the values obtained were only about one-tenth that The weakness of the method when applied to bone obviously lay in the corrosion caused by the acid, which impaired the bonding On the other hand, when acrylic emulsion was utilized as intermediary material, values about one-third of the abovementioned ones were obtained When bone cement is employed in orthopedic surgery, it is usually applied to a large bone surface, and the forces operating after
J Vainio et al : Experimental Fixation of Bone Cement
fixation are of a nature different from those acting in a tooth Hence, the fixing capacity noted may nonetheless be considered quite remarkable. Examination of the surfaces showed that the test materials contained solid particles, measuring in diameter ad 100 pm in the bone cement, ad 120 gpm in Silar composite resin The particles in Concise are known to be 20 times larger than the particles in Silar. The solid particles are so large that they mechanically in part prevent contact between the fixative and the unevenness in the bone surface This may be one factor responsible for the weak fixation of the materials used This drawback is eliminated when liquid
195
acrylic intermediary material is used in fixing the cement. References Phillips, R W : Acid etching technic In: Skinner's science of dental materials, 7th ed , pp 229-230 Philadelphia-London-Toronto: W B Saunders Company 1973 Smith, D C : A new dental cement Br Dent J 125, 381 (1968) Smith, D C : A review of the zinc polycarboxylate cements J. Can Dent Assoc 37, 22 (1971) Vainio, J , Rokkanen, P : Experimental osteotomy fixed with a composite resin Arch Orthop Traumat Surg 92, 159 (1978) Received May 21, 1979
Arch Orthop Traumat Surg 94, 191-195 (1979)
©J F Bergmann Verlag 1979
Experimental Fixation of Bone Cement and Composite Resins to Bone* J Vainio', J Kilpikari 2 , P T6rmila 2 , and P Rokkanen' i Institute of Clinical Sciences, University of Tampere and Central Hospital of Tampere, Teiskontie 35, SF-33520 Tampere 52, Finland of Materials Science, Tampere University of Technology
2 Institute
Summary The aim of this study was to ascertain whether the use of liquid acrylic resin or NeoCryl XK-53 acrylic emulsion or etching of the bone surface with phosphoric acid or the application of both these methods would improve the bonding of CMW bone cement or Concise and Silar composite resin to bone. The test materials were applied to fresh cortical bovine bone Their bonding capacity was measured by the Instron Universal Testing Machine The surfaces of the test materials and the bone surfaces were examined by means of optic microscope or scanning electron microscope The bonding strength of the bone cement and composite resins as such were found to be of the same magnitude A three to five-fold improvement was obtained with liquid acrylic intermediary material Acid etching impaired the bonding. Zusammenfassung Das Ziel dieser Untersuchung war, herauszufinden, ob der Gebrauch von flussigen zusammengesetzten Acryl-Harzen oder von Neo Cryl XK-53 Acryl-Emulsionen oder von Atzung der Knochenoberflache mit Phosphorsaure oder die Anwendung von beiden Methoden die Fixierung des Knochenzements CMW oder der zusammengesetzten Harze Concise und Silar verbessern wiirde Die untersuchten Materialien wurden an frischen corticalen Rinderknochen angebracht Die Fixierungskapazitat wurde mit dem Universal-Gerat Instron fir Materialtests gemessen Die Oberflachen des Testmaterials und des Knochens wurden mit Hilfe eines optischen oder Raster-Elektronenmikroskops untersucht Die Resultate bewiesen, daB sich der Knochenzement wie die zusammengesetzten Harze an dem Knochen fixierten Das als Zwischenstoff verwendete flissige Acryl Offprint requests to: Prof P Rokkanen (address see above) * Dedicated Professor Dr E Uehlinger on his 80th birthday
verbesserte die Haftfiahigkeit um das 3-5 fache Die Atzungen mit Saure verschlechterte die Haftfahigkeit.
Bone cement is utilized in orthopedic surgery for the fixation of different implants to bone The most frequent application of bone cement is in the fixation of artificial joints This material is also used in the osteosynthesis of severe comminuted fractures and in the filling of bone defects Bone cement anchors the bone and the implant firmly to each other After setting of the cement, the surfaces will remain in contact by the aid of physical forces Experimentally, composite resins have also been used alone as fixative in osteosynthesis in the same way as bone cement, but the fixation to the bone has been only partially successful (Vainio and Rokkanen, 1978). In the present study an attempt was made to find out whether the strength of the boundary between bone and cement can be increased by the application of liquid polymerized acrylic between the bone and the cement or by treating the bone surface by acid etching in the same way as the dental enamel is treated for the fixation of tooth-restorative material At the same time we tested whether the bis-GMA composite resins used in dentistry could be fixed to cortical bone in the same way as to dental enamel Moreover, these resins were compared with a currently used bone cement. Material and Methods We used fresh refrigerated bovine femoral or tibial bone, from which square pieces of cortical bone, measuring 2 x 2 cm, were sawed out after thawing The bone surface was ground smooth, after which the test material was applied A cylindric piece of acrylic was fixed to the test material and became chemically bonded to it during the process of polymerization.
0344-8444/79/0094/0191/$ 1 00
192
Bonding to bone
J.Vainio et al : Experimental Fixation of Bone Cement
No of specimens
Table 1 The bonding strength of the test materials to bone
Silar and Concise a
CMW Bonding strength
No of specimens
Bonding strength 42 5
1 Test material as such
8
35 0
8
2 Intermediary material Acrylic resin Neo Cryl XK-53
9 8
110 5
9
175 4
(9)b
3 Acid etching
9
02
69 2 (> 100) b
9
18 7 Silar and Concise gave same results Values given are for Silar b Loosening occurred partly at the layer between composite resin and acrylic cylinder and partly at the bone surface a
4 Acid etching and intermediary material Acrylic resin Neo Cryl XK-53
8 8
16 9 187 9
We tested CMW bone cement, without barium sulphate, and Concise and Silar composite resins As intermediary material we used Concise and Silar liquid acrylic resin or NeoCryl XK-53 acrylic emulsion A total of 130 specimens were studied. The test material was bonded to the bone in one of the following ways: 1 The test material was applied as such onto the bone surface and was pressed manually against the bone with an acrylic cylinder. 2 Liquid intermediary material was applied to the bone surface, and the test material was applied during polymerization An acrylic cylinder was pressed by hand against the test material. 3 The bone surface was treated with 37% phosphoric acid for one minute, the acid was rinsed away with water, and the bone surface was dried The test material was applied onto the etched bone surface and pressed against it with an acrylic cylinder. 4 The bone surface was treated with phosphoric acid as in point 3 Liquid intermediary material was applied to the etched surface, the test material was applied during polymerization and pressed against the bone with an acrylic cylinder. Fixation to the bone was measured using the Instron Universal Testing Machine, by the aid of which the acrylic cylinder was pulled off the bone at a rate of 1 mm/min Since loosening nearly always (Table 1) occurred at the bone surface, the results represent the bonding strength of this surface. Separate polymerized specimens of the test materials were prepared for surface studies Their surfaces were ground smooth and etched for about five minutes by means of the vapour of strong nitrous acid The surfaces were examined in optic microscope The ground bone surface and the etched bone surface were studied in scanning electron microscope.
Results The fixation of the test materials to the bone achieved by the different methods is shown in Table 1 CMW as such adhered to the bone with about the same strength than the composite resins With regard to the fixing capacity of Concise and Silar resulting from the different treatments no differences were observed (The
47 4
9 (9)b
(> 100)
b
matrix and filler components are chemically similar in the two materials) The best results were obtained when acrylic emulsion was used as intermediary material Using acrylic resin as intermediary material, the fixation was also improved, though not to the same extent as with acrylic emulsion Acid etching impaired the results significantly When both acid etching and intermediary material were applied, poorer results were obtained with acrylic resin than with acrylic emulsion. Optic microscopy of the surface of the polymerized specimens of the test materials revealed in the surface of CMW bone cement round, solid polymetric particles varying in size, the largest measuring about 100 m in diameter In addition, air bubbles were observed The space between the particles and the air bubbles was filled by a mass that appeared to be homogenous (Fig 1a) In the surface of Concise and Silar, irregularly shaped, angular particles of filled component and some air bubbles were observed, and between them a homogenous-looking mass In the surface of Silar, the largest particles of the filled component measured 120 gm (Fig lb). In the surface of the ground bone grooves caused by the grinding and some round gaps produced by blood vessels were seen in the scanning electron microscope (Fig 2 a) In the surface of the etched bone the grooves due to grinding had become levelled out and the surface was on the whole smoother than the surface of the ground bone (Fig 2b). Discussion In orthopedic surgery the question of firm and lasting fixation is of essential importance In the case of a fractured bone fixation should primarily involve the ends of the bone and secondarily the bone and the
J Vainio et al : Experimental Fixation of Bone Cement
193
Fig a Surface structure of ground polymerized CMW bone cement Optic micrograph, x 150
Fig 2a Surface structure of ground bone Scanning electron micrograph, x 300
Fig lb Surface structure of polymerized Silar Optic micrograph, x 250
Fig 2b Surface structure of ground acid-etch bone Scanning electron micrograph, x 300
means of osteosynthesis employed When fixing an artificial joint the bone and the plastic or metallic endoprosthesis have to be fastened to each other The filling of bone defects due to neoplastic growth or some other cause implies fixation of the filling material to the bone. With regard to stability, fixations are expected to answer certain demands In the case of bone that will
heal, only temporary fixation, lasting over the period of bone healing, is required Thereafter the means used for fixation may be removed On the other hand, problems may arise when an acquired or produced bone defect makes it necessary to establish a fixation serving throughout the patient's life-time. Most of the knowledge available concerning the stable fixation of implant material to bone relates to
194
the application of artificial joints During the last 20 years a hardening intermediary material, bone cement, has been used in anchoring endoprostheses to the bone This kind of fixation has also sometimes been employed in the treatment of fractures and bone defects. The weak link in the system bone-cement-implant has proved to be the boundary between bone and cement After a sufficiently long time of observation this is giving way in about 20 per cent of cases. The failure has been attributed to various causes, e g. the heat released while the cement is setting, the direct cytotoxic effect of the monomer of the cement, and trauma caused by the mechanical handling of the bone tissue. Owing to differences in chemical nature between the bone surface and the cement, the physical fixation resulting between them is too weak to bear any heavy mechanical strain Theoretically the fixation of bone cement to bone might be improved by the application of an intermediary material that forms chemical bonds with both the cement and the bone, by treating the bone surface by some technique producing better locking, or by the application of an intermediary material which is more efficiently fixed mechanically to the bone and chemically to the cement. Polymethylmethacrylate bone cement hardens by polymerization, and while this process is taking place, a liquid intermediary acrylic material may readily be added Chemical bonding to the bone is not readily achieved, although materials forming chemical bonds with bone have been developed It is known, for instance, that zinc polyacrylate cement during polymerization forms complex bonds with bone calcium (Smith, 1968 ; 1971). In dentistry, bis-GMA composite resins have been used in the cosmetically exacting restoration of anterior teeth The restorative material is fixed to the enamel by the acid etch technique, which changes the properties of the enamel surface so that the mechanical fixation is strengthened The enamel is etched by exposing the surface to the effect of phosphoric acid for about one minute The acid effect reaches a depth of about 40 tm Liquid acrylic resin applied to the surface becomes mechanically fixed to the enamel and chemically to the filling material during polymerization This method has proved useful in practice. In the present investigation an attempt was made to find out whether the fixing capacity of bone cement can be improved by the application of an intermediary material, which becomes chemically bonded to the cement, or by treating the bone surface with phosphoric acid, or by applying both methods At the same time we tested whether the acid etching method for
J.Vainio et al : Experimental Fixation of Bone Cement
enamel employed in dentistry could be adapted to cortical bone, and we compared the fixations of ordinary bone cement and composite resins achieved by these techniques In addition, to the acrylic resins commonly used as intermediary material in dentistry, we used a commercial acrylic emulsion, which was assumed to resemble in structure the methylmethacrylate monomer of bone cement A previous study (Vainio and Rokkanen, 1978) dealt with the use of Concise composite resin for the fixation of osteotomies in the rat tibia The fixation proved to be successful in about 60 per cent of cases The present results seem to suggest that the fixing capacity of Concise as such is not sufficient for successful fixation. The large particle size of the filler material of Concise appears to be in part responsible for the failure. The adhesion of bone cement and composite resin to the ground surface of cortical bone was found to be of the same order ( 35 N/cm2 and 42 5 N/cm 2 ) On inspection of the bone surfaces, grooves produced by the grinding and some hollows resulting from the structure of the bone were observed The unevenness of the bone surfaces explains that the materials adhered as such to the bone, although they do not possess any gluing properties. Acid etching reduced the fixing capacity of the test materials instead of improving it Inspection of the etched bone surfaces showed that they were levelled by the effect of the acid; hollows like those resulting in dental enamel were not observed This levelling is apparently the main cause of the weakened fixation It seems possible moreover that the effect of the acid makes the bone surface fragile. The use of a liquid acrylic intermediary material obviously led to improved fixation The fixing capacity of bone cement was increased to about three-fold by the use of acrylic resin and to about five-fold by the use of acrylic emulsion The fixing capacity of the composite resins was also clearly improved, but less so than the fixing capacity of the bone cement. As regards the fixing capacity of the filling materials applied to enamel by the method of acid etching, values of the order of 420 N/cm 2 have been reported (Phillips, 1973) When in the present investigation the same method was used in fixing the same materials to cortical bone, the values obtained were only about one-tenth that The weakness of the method when applied to bone obviously lay in the corrosion caused by the acid, which impaired the bonding On the other hand, when acrylic emulsion was utilized as intermediary material, values about one-third of the abovementioned ones were obtained When bone cement is employed in orthopedic surgery, it is usually applied to a large bone surface, and the forces operating after
J Vainio et al : Experimental Fixation of Bone Cement
fixation are of a nature different from those acting in a tooth Hence, the fixing capacity noted may nonetheless be considered quite remarkable. Examination of the surfaces showed that the test materials contained solid particles, measuring in diameter ad 100 pm in the bone cement, ad 120 gpm in Silar composite resin The particles in Concise are known to be 20 times larger than the particles in Silar. The solid particles are so large that they mechanically in part prevent contact between the fixative and the unevenness in the bone surface This may be one factor responsible for the weak fixation of the materials used This drawback is eliminated when liquid
195
acrylic intermediary material is used in fixing the cement. References Phillips, R W : Acid etching technic In: Skinner's science of dental materials, 7th ed , pp 229-230 Philadelphia-London-Toronto: W B Saunders Company 1973 Smith, D C : A new dental cement Br Dent J 125, 381 (1968) Smith, D C : A review of the zinc polycarboxylate cements J. Can Dent Assoc 37, 22 (1971) Vainio, J , Rokkanen, P : Experimental osteotomy fixed with a composite resin Arch Orthop Traumat Surg 92, 159 (1978) Received May 21, 1979
