J . BIOMED. MATER. RES. SYMPOSIUM
NO. 6, pp. 63-66 (1975)
Mechanical Properties of Calcia Stabilized Zirconia Following in vivo and in vitro Aging G . H. KENNER, University of Illinois, Urbana, W. D. PASCO, Alfred University, Alfred, New York, J . T. FRAKES, University of Illinois Medical College, Rockford, and S. D. BROWN, University of Illinois, Urbana, Illinois 61801
Summary Aging studies were done on calcia stabilized zirconia rods of 72% theoretical density to determine the effect of actual and simulated biological environments on their strength. They were aged without stress in vitro in Ringer’s solution for I , 2 and 4 weeks or in vivo in rabbits for 12 weeks. Rods aged in vitro showed mean losses in bending strength of 16, 17 and 19% respectively after I , 2 and 4 weeks of immersion, while those aged in vivo showed a mean loss of 25%. I t was concluded that the material tested would be unsatisfactory as an orthopedic replacement because of the rapid decrease in strength which occurred when exposed to actual or simulated biological media.
INTRODU CTl ON An extensive literature has been developed over the last decade which attests to the acceptance of some ceramics by biological tissues [l-61. Somewhat less is known about the effect of the biological environment upon the essential physical properties of ceramics. In 1962, Smith [ l ] reported favorably on the resistance of Cerosium t o fatigue, abrasion and aging. Based on in vitro aging studies done in saline at elevated temperatures, he predicted that the time required for the flexural strength of cerosium to fall t o Y3 of its original value was 23,000 years. Frakes et al. [7] failed to find any statistically significant decrease in the flexural strength of alumina aged in water for 84 days although they did report a mean nonsignificant strength loss of 13%. Similar results were obtained in an unpublished study done by Avigdor, Kenner and Brown in which porous alumina bars were aged in saline for 60 days at 62°C. O n the other hand Jecmen et al. [8] did show losses of strength when alumina bars were aged in saline for 666 hr. In contrast to the stability of porous alumina shown in the above experiments were the results of in vitro static fatigue studies and investigations where specimens were implanted subcutaneously in animals. Static fatigue studies of porous alumina bars immersed in deionized water or 63
0 1975 by John Wiley & Sons, Inc.
64
KENNER ET A L
plasma-sodium citrate-phenol solution (PSCP) showed decreases in flexural strength of 10% (deioniLed water) and 15% (PSCP) over a period of 12 days. Similar nonstressed bars aged subcutaneously in rabbits showed losses of 35% after 3 months [7]. Other materials for which data has been collected on their response to real or simulated biological environments are calcium aluminates and glassy carbon. Schnittgrund et al. [9] showed losses of 40% within 100 min when calcium aluminate bars were aged under conditions of static loading and 10% when aged in vivo for 3 months. In contrast were the studies of glassy carbon which failed t o show any statistically significant losses of flexural strength in the in vivo aging studies and which were shown to be comparatively resistant to static fatigue [lo]. Zirconia was selected as a material for further investigation because it is both chemically stable and biologically inert. Also, it is comparatively inexpensive. For logistic reasons, it was decided to study the effects of simple aging under unstressed conditions both in vivo and in vitro before doing the more elaborate static fatigue experiments. MATERIAL AND METHODS The 3 in. long x '1, in. diam rods used in this study were provided by the TAM Division of National Lead. The rods as received were approximately 72% of theoretical density and had a modulus of rupture when broken in four-point loading (load rate of 75 Ib/min) of 9241 1520 psi. Nominally, they contained 4% CaO, 1% S O , , 1% A1,0, + TiO, + Fe,O,, the remainder being ZrO, containing 2% HfO,. The specimens were inspected for gross surface flaws before use. The in vivo aging was achieved by placing bars which had been sterilized at 180°C. for 2 hr in a dry atmosphere, subcutaneously in the back region using the procedures described in Frakes et al. [7]. Four rods were inserted per animal. After 3 months, the rabbits were sacrificed and the rods were removed and broken in the manner described above. The in vitro aging was performed as previously described by Schnittgrund et al. [9], by placing rods in jars containing Ringer's solution. They were allowed to soak for 1. 2, and 4 weeks after which they were removed and broken in the manner described above. RESULTS As illustrated in Table 1, the strength of zirconia bars aged in Ringer solution decreased from 9007 f 1938 psi to 7340 f. 1004 psi by the end of 4 weeks. This loss was statistically significant. Almost the entire weakening occurred within the first week. Zirconia was also found t o weaken appreciably when aged in vivo. After 12 weeks the flexural strength fell to 74% of its original value.
CALCIA STABILIZED Z I R C O N I A
65
TABLE I Flexural Strength of Zirconia Bars Following In Vivo and In Vitro Aging Duration
Sample Size
Mean
t
S.D.
-___ In V i t r o Experiment
t-value
% loss
Unaged c o n t r o l s One week
25
9007 ? 1938
-
20
7550 ? 1856
2.55
16.1
Two weeks
19
7444 ?
660
3.35
17.4
Four weeks
20
7340 ? 1004
3.47
18.5
Unaged c o n t r o l s
30
9436 ? 1046
-
Three months
22
7006 ? 2182
5.34
-I n Vivo Experiment
25.8
DISCUSSION Significant losses of strength occurred in both the in vivo and in vitro experiments. The cause is not fully understood, although stress corrosion, hydration and/or leaching at the grain boundaries where calcia accumulation is expected to have occurred are the most likely possibilities. Zirconia itself is exceptionally inert chemically but it undergoes a disruptive monoclinic to tetragonal phase transformation at about 1000-1 100°C and must be stabilized. Calcia is one material used for this purpose and the zirconia is thereby stabilized in the cubic form. However, calcia is known from previous studies to accumulate at the grain boundaries of stabilized zirconia; hence, the grain boundaries are assumed to be the location of attack. If this is so, perhaps another stabilizing compound, (e.g. yttria) would yield a zirconia more resistant to biological attack.
CONCLUSION The porous, calcia stabilized zirconia which was tested would be unsatisfactory as an orthopedic replacement because of the rapid decrease in strength which occurred when exposed to real or simulated biological media. This work was supported in part by General Research Grant PHS R R 05460 and by the Department of Ceramic Engineering.
References [ I ] L. Smith, Arch. Surg., 87, 653 (1963). [2] F. W . Rhinelander, M. Pouweyha, and J . C. Milner, J . Biomed. Muter. Res., 5 , 81 (1971).
[3] J . J . Klawitter and S. F. Hulbert, J . Biomed. Mafer. Res. Symp. No. 2 (part I), 161 (1971).
66
KENNER ET AL.
[4] L. L. Hench, R. J . Splinter, W. C. Allen, and T. K. Greenlee, J . Biomed. Muter. Res. Symp. No. 2 (part I), 117 (1971). [5] G . A . Graves, R. L. Hentrich, H. G. Stein, and P. K. Bajpai, J . Biomed. Marer. Res. Symp. No. 2 (part l), 91 (1971). [6] V . Mooney, P. K. Predecki, J . Renning, and J. G r a y , J . Biomed. Muter. Res. Symp. No. 2 (part 1) 143 (1971). [7] J. T. Frakes, S. D. Brown, and G. H . Kenner, Bull. Arner. Cerum. Soc., in press. [8] R. M . Jecmen, C. L. Eggerding, S. D. Brown, and G. D. Schnittgrund, J . Biomed. Muter. Res., 7,369 (1973). [9] G . D. Schnittgrund, G. H . Kenner, and S . D. Brown, J . Biomed. Mater. Res. Symp. No. 4 , 435 (1973). [lo] G . H. Kenner, S. D. Brown, W. D. Pasco, and J . E. Lovell, Bull. Amer. Ceram. Soc.. 52,432 (1973).
NO. 6, pp. 63-66 (1975)
Mechanical Properties of Calcia Stabilized Zirconia Following in vivo and in vitro Aging G . H. KENNER, University of Illinois, Urbana, W. D. PASCO, Alfred University, Alfred, New York, J . T. FRAKES, University of Illinois Medical College, Rockford, and S. D. BROWN, University of Illinois, Urbana, Illinois 61801
Summary Aging studies were done on calcia stabilized zirconia rods of 72% theoretical density to determine the effect of actual and simulated biological environments on their strength. They were aged without stress in vitro in Ringer’s solution for I , 2 and 4 weeks or in vivo in rabbits for 12 weeks. Rods aged in vitro showed mean losses in bending strength of 16, 17 and 19% respectively after I , 2 and 4 weeks of immersion, while those aged in vivo showed a mean loss of 25%. I t was concluded that the material tested would be unsatisfactory as an orthopedic replacement because of the rapid decrease in strength which occurred when exposed to actual or simulated biological media.
INTRODU CTl ON An extensive literature has been developed over the last decade which attests to the acceptance of some ceramics by biological tissues [l-61. Somewhat less is known about the effect of the biological environment upon the essential physical properties of ceramics. In 1962, Smith [ l ] reported favorably on the resistance of Cerosium t o fatigue, abrasion and aging. Based on in vitro aging studies done in saline at elevated temperatures, he predicted that the time required for the flexural strength of cerosium to fall t o Y3 of its original value was 23,000 years. Frakes et al. [7] failed to find any statistically significant decrease in the flexural strength of alumina aged in water for 84 days although they did report a mean nonsignificant strength loss of 13%. Similar results were obtained in an unpublished study done by Avigdor, Kenner and Brown in which porous alumina bars were aged in saline for 60 days at 62°C. O n the other hand Jecmen et al. [8] did show losses of strength when alumina bars were aged in saline for 666 hr. In contrast to the stability of porous alumina shown in the above experiments were the results of in vitro static fatigue studies and investigations where specimens were implanted subcutaneously in animals. Static fatigue studies of porous alumina bars immersed in deionized water or 63
0 1975 by John Wiley & Sons, Inc.
64
KENNER ET A L
plasma-sodium citrate-phenol solution (PSCP) showed decreases in flexural strength of 10% (deioniLed water) and 15% (PSCP) over a period of 12 days. Similar nonstressed bars aged subcutaneously in rabbits showed losses of 35% after 3 months [7]. Other materials for which data has been collected on their response to real or simulated biological environments are calcium aluminates and glassy carbon. Schnittgrund et al. [9] showed losses of 40% within 100 min when calcium aluminate bars were aged under conditions of static loading and 10% when aged in vivo for 3 months. In contrast were the studies of glassy carbon which failed t o show any statistically significant losses of flexural strength in the in vivo aging studies and which were shown to be comparatively resistant to static fatigue [lo]. Zirconia was selected as a material for further investigation because it is both chemically stable and biologically inert. Also, it is comparatively inexpensive. For logistic reasons, it was decided to study the effects of simple aging under unstressed conditions both in vivo and in vitro before doing the more elaborate static fatigue experiments. MATERIAL AND METHODS The 3 in. long x '1, in. diam rods used in this study were provided by the TAM Division of National Lead. The rods as received were approximately 72% of theoretical density and had a modulus of rupture when broken in four-point loading (load rate of 75 Ib/min) of 9241 1520 psi. Nominally, they contained 4% CaO, 1% S O , , 1% A1,0, + TiO, + Fe,O,, the remainder being ZrO, containing 2% HfO,. The specimens were inspected for gross surface flaws before use. The in vivo aging was achieved by placing bars which had been sterilized at 180°C. for 2 hr in a dry atmosphere, subcutaneously in the back region using the procedures described in Frakes et al. [7]. Four rods were inserted per animal. After 3 months, the rabbits were sacrificed and the rods were removed and broken in the manner described above. The in vitro aging was performed as previously described by Schnittgrund et al. [9], by placing rods in jars containing Ringer's solution. They were allowed to soak for 1. 2, and 4 weeks after which they were removed and broken in the manner described above. RESULTS As illustrated in Table 1, the strength of zirconia bars aged in Ringer solution decreased from 9007 f 1938 psi to 7340 f. 1004 psi by the end of 4 weeks. This loss was statistically significant. Almost the entire weakening occurred within the first week. Zirconia was also found t o weaken appreciably when aged in vivo. After 12 weeks the flexural strength fell to 74% of its original value.
CALCIA STABILIZED Z I R C O N I A
65
TABLE I Flexural Strength of Zirconia Bars Following In Vivo and In Vitro Aging Duration
Sample Size
Mean
t
S.D.
-___ In V i t r o Experiment
t-value
% loss
Unaged c o n t r o l s One week
25
9007 ? 1938
-
20
7550 ? 1856
2.55
16.1
Two weeks
19
7444 ?
660
3.35
17.4
Four weeks
20
7340 ? 1004
3.47
18.5
Unaged c o n t r o l s
30
9436 ? 1046
-
Three months
22
7006 ? 2182
5.34
-I n Vivo Experiment
25.8
DISCUSSION Significant losses of strength occurred in both the in vivo and in vitro experiments. The cause is not fully understood, although stress corrosion, hydration and/or leaching at the grain boundaries where calcia accumulation is expected to have occurred are the most likely possibilities. Zirconia itself is exceptionally inert chemically but it undergoes a disruptive monoclinic to tetragonal phase transformation at about 1000-1 100°C and must be stabilized. Calcia is one material used for this purpose and the zirconia is thereby stabilized in the cubic form. However, calcia is known from previous studies to accumulate at the grain boundaries of stabilized zirconia; hence, the grain boundaries are assumed to be the location of attack. If this is so, perhaps another stabilizing compound, (e.g. yttria) would yield a zirconia more resistant to biological attack.
CONCLUSION The porous, calcia stabilized zirconia which was tested would be unsatisfactory as an orthopedic replacement because of the rapid decrease in strength which occurred when exposed to real or simulated biological media. This work was supported in part by General Research Grant PHS R R 05460 and by the Department of Ceramic Engineering.
References [ I ] L. Smith, Arch. Surg., 87, 653 (1963). [2] F. W . Rhinelander, M. Pouweyha, and J . C. Milner, J . Biomed. Muter. Res., 5 , 81 (1971).
[3] J . J . Klawitter and S. F. Hulbert, J . Biomed. Mafer. Res. Symp. No. 2 (part I), 161 (1971).
66
KENNER ET AL.
[4] L. L. Hench, R. J . Splinter, W. C. Allen, and T. K. Greenlee, J . Biomed. Muter. Res. Symp. No. 2 (part I), 117 (1971). [5] G . A . Graves, R. L. Hentrich, H. G. Stein, and P. K. Bajpai, J . Biomed. Marer. Res. Symp. No. 2 (part l), 91 (1971). [6] V . Mooney, P. K. Predecki, J . Renning, and J. G r a y , J . Biomed. Muter. Res. Symp. No. 2 (part 1) 143 (1971). [7] J. T. Frakes, S. D. Brown, and G. H . Kenner, Bull. Arner. Cerum. Soc., in press. [8] R. M . Jecmen, C. L. Eggerding, S. D. Brown, and G. D. Schnittgrund, J . Biomed. Muter. Res., 7,369 (1973). [9] G . D. Schnittgrund, G. H . Kenner, and S . D. Brown, J . Biomed. Mater. Res. Symp. No. 4 , 435 (1973). [lo] G . H. Kenner, S. D. Brown, W. D. Pasco, and J . E. Lovell, Bull. Amer. Ceram. Soc.. 52,432 (1973).
