release of corrosion products by f-75 cobalt base alloy in the rat. iii: effects of a carbon surface...

Release of corrosion products by F-75 cobalt base alloy in the rat. 111: Effects of a carbon surface coating

J. Black, P. Oppenheimer, D. M. Morris, A.M. Peduto, and C. C. Clark McKay Laborato y of Orthopaedic Surgery Research, Departmenf of Orthopaedic Surge y, University of Pennsylvania, Philadelphia, Pennsylvania 19104

A pyrolytic carbon coating was applied to F-75 chromium-cobalt-molybdenum alloy in an effort to reduce the release of corrosion products in vivo. After intramuscular implantation in the rat, a complex pattern of serum and urine concentration elevations of chromium, cobalt, and nickel was seen. The carbon-coated implants released more chromium and cobalt than uncoated controls, as seen by significantly

elevated metal concentrations in serum and urine. Animals receiving carbon-coated implants showed a high rate of recurrent implant site inflammation. Neoplastic infiltration of the implant site occurred in 3 out of 24 animals with coated implants, but not in any of the 16 animals which received either uncoated F-75 microsphere or poly(ethy1ene) particulate implants.

INTRODUCTION

The two most important performance considerations for biomedical devices are safety and efficacy. Efficacy is principally addressed in design of function, whereas safety depends upon the selection of materials as well as upon function. One of the materials' aspects which has an important bearing on safety of metallic devices is the in vivo corrosion rate.

Cast chromium-cobalt-molybdenum alloys of the F-75 type* are generally considered to have low in vivo corrosion rates2 leading to satisfactory local host responses in animals and patients. However, these corrosion rates are non-zero and lead to systemic distribution, storage and excretion of organometallic complexes. In an effort to study the systemic and remote site aspects of in vivo corrosion, a microsphere implant model has been developed3 which permits elevation of dose rates by scaling the ratio of the implant surface area to animal body weight (SA/BW ratio).

Direct correspondence and reprint requests to: Dr. Jonathan Black, Department of Orthopaedic Surgery, University of Pennsylvania, School of Medicine, 424 Medical Edu- cation Building/GM, 36th & Hamilton Walk, Philadelphia, Pennsylvania 19104-6081.

*F-75 composition' (wt.%): 27-30 Cr, 5-7 Mo, l(max) Ni, 0.75(max) Fe, 0.35(max) C, l(max) Si, l(max) Mn, bal. Co.

Journal of Biomedical Materials Research, Vol. 21, 1213-1230(1987) 0 1987 John Wiley & Sons, Inc. CCC 0021-9304/87/101213-18$04.00

1214 BLACK ET AL.

Previous reports of the use of this model, using F-75 alloy in rats, include independent single point 10 day post-implantation4 and 30- and 120-day (Refs. 5,6) longitudinal studies. In the latter study,6 the observation of biological effects, including inhibition of weight gain and dose-related lung pathology, suggested the need to attempt to reduce the intrinsic corrosion rate of this alloy.

Impermeable coatings, such as carbon films, have been proposed as a means of achieving this goal. Coatings as thin as one micron (1 pm) have excellent tenacity on this alloy type, even when formed by vapor dep~sit ion,~ due to interfacial metal carbide formation. It has been suggested that carbon coatings, even with alloying agents, may inhibit foreign-body reactions and reduce implant encapsulation,8 presumably secondary to reducing the release of metal.

Thus an experiment was designed to compare the local and systemic responses to carbon coated F-75 alloy, to uncoated implants and to particulate nonmetallic (poly(ethy1ene)) controls. The local evaluation was histological while the systemic evaluation involved electrothermal (graphite furnace) atomic absorption spectroscopic (GFAAS) determination of chromium, cobalt and nickel concentrations in serum and urine. In addition, surface concentrations of alloy constituents of both coated and uncoated microspheres were studied semiquantitatively pre- and postimplantation by electron dispersive x-ray diffraction (EDAX).

The addition of a pyrolytic carbon coating, in thicknesses up to 1 pm, appears to increase the release rate of both chromium and cobalt radically, as judged by inflammatory and neoplastic responses at the implant site, by serum and urine concentration elevations and by post-implant differential elemental surface depletion of the microspheres.

METHODS AND MATERIALS

This study is based upon a model of accelerated corrosion established by W ~ o d m a n . ~ A standard implant surface area to body weight ratio (SA/BW) has been determined3 to be 2.9 cm2/kg. In this study, a SA/BW of lOOX or 290 cm2/kg was used, since a previous study6 had shown that this was sufficient to produce growth inhibition without mortality in the rat.

The metallic implants used* were microspheres, with a nominal 55 pm diameter, produced from an F-75 master ingot by a rotating electrode plasma arc process. These microspheres represent a sieve cut (-230 + 270) of a single production run. They were optically determined to have a mean diameter of 54.4 -t- 0.8 pm, based upon an alloy density of 8.4 g/cm3, a specific surface area of 131 cm2/g. The microspheres were implanted bare (unpassivated) or with pyrolytic carbon coatings' of the low-temperature

'Coating applied by Battelle Memorial Institute, Columbus, OH.

*All implant materials were acquired and supplied by JohnsontkJohnson Products, Orthopaedic Division, Braintree, MA.

RELEASE OF CORROSION PRODUCTS BY F-75.III 1215

isotropic (LTI) type' of 0.3, 0.5, or 1.0 pm mean thickness applied in a fluidized bed (reactants: H2/CC&, peak temp.: 840°C, 45 to 180 min). The poly(ethy1ene) powder* was a mechanically formed powder produced from ultrahigh molecular weight poly(ethy1ene). The particles had an irregular spheroidal shape, with an optically determined mean maximum dimension of 63 t 1.5 pm and, based upon a density of 0.95 g/cm3, a specific surface area of 969 cm2/g.

The materials were washed (acetone, deionized water) and oven dried ( 130"C, 3 h) before implantation. Specimen implantation weights were based upon individual animal seven day preoperative weights and polymeric or uncoated metallic implant specific surface area. The amount of metal powder implanted varied between 2.214 and 2.236 g/kg, depending upon coating thickness, while the polyethylene was implanted at 0.299 g/kg. Specimens of microspheres were reserved for metallographic examination. These were mounted, sectioned, polished and etched (HNO3/HC1/FeCI3, 15 sec).

Fifty-two male Sprague-Dawley rats, nominally 350 g weight each, were ear-punched and caged in groups of ten or eleven in modified rabbit cages. The cages were bilevel with interconnecting poly(vinylch1oride) pipe runs to allow space for exercise so as to minimize weight gain. The animals received rat chow (Rat Chow, Purina #5012) and water ad libidurn. After a 2-week acclimation period, each group was culled by eliminating the smaller animals, producing a grand mean weight of 430.4 g, and individual weights were obtained for calculation of individual implant amounts.

The five groups of animals (N = 8 each) received implants (at SA/BW = 1OOX) as follows: Group 1: poly(ethy1ene) (PE), Group 2: bare microspheres (B), Group 3: 0.3 p m coated microspheres (0.3C), Group 4: 0.5 pm coated microspheres (0.5C), and Group 5: 1.0 pm coated microspheres (1.OC). The implants were placed intramuscularly in the posterior thigh, using techniques previously described.6 After a brief recovery period, the animals were returned to their group cages.

Blood (2 mL) was obtained from each animal by retroorbital puncture, after etherization, on preoperative day 7 (POD-7) and then on postoperative day (POD) 3,10,30,60,100, and 120. In addition, blood (10 mL) was obtained by cardiac puncture at sacrifice. All blood-drawing equipment was acid- washed before use. After drawing, the specimens were allowed to clot (RT, 1 h). Clot and serum were separated by centrifugation (1200g, 30 min, 4°C) and serum was frozen for further analysis. One animal from each group was selected at random and placed in metabolic cages (E-1100, Maryland Plastics, Federalsburg, MD 21632) for 24 h prior to POD 10,30,60,100, and 120 and then killed. Urine and feces were separated and urine was frozen for later analysis. All animals were also weighed on each blood draw day and their general habits observed. All the remaining animals were killed on POD 120.

Torex Medical, Fairburn, GA (Grade: Hoechst GUR-11).

BLACK ET AL. 1216

At sacrifice (by exsanguination, post-ketamine-anesthesia, 0.1-0.14 g/Kg), a full necropsy was performed including harvesting lung tissue and any abnormally appearing tissue for histological examination. The implant sites were also harvested and frozen, unfixed, for later examination.

All GFAAS analysis of urine and serum was performed with an electrothermal spectrophotometer (PE 4000, HGA 400, Perkin-Elmer, Norwalk, CN) using analytic techniques previously r e p ~ r t e d . ~ Serum was analysed undigested after 1:2 dilution (0.03N HN03) and urine was diluted 1: 1. All reagents were ultrapure (Baker TJltrex" or equivalent). Serum protein determination was performed with 100 p L aliquots, using the Lowry spec- trophotometric method."

One implant site specimen from each group which was selected for EDAX analysis was lyophylized, mounted on a magnesium stub, and then sput- tered with gold. Analyses were performed on a scanning electron micro- scope with attached electron dispersive x-ray unit (PSEM 500, Philips, Eindhoven, Netherlands) at 18 Kev, with a 5-pm spot size, a 9" stage tilt, a 33" takeoff angle and 200 sec counting time per site. A standard location, 1/4 down the vertical microsphere diameter, was used for each study site so as to normalize dispersion due to microsphere curvature. Analyses were made for Cr, Co, Mo, and Ni, with other elements not counted. Scanning studies showed the presence of Al, Si, Au, C1, K, and Fe, as would be expected for gold-coated organic material. Unimplanted microspheres were mounted and studied in the same way. Normalization (ZAF type) procedures were applied to the counts obtained and weight percentages calculated.

RESULTS

General observations: All animals tolerated implantation well, there were no intraoperative deaths and no immediate complications were noted.

By POD 10, a connective tissue capsule had formed around the implants of each type which were examined. The wall thickness and general appearance of these capsules was the same for all specimens. However, the vol- umes of the capsules from the animals with carbon-coated implants were larger and, upon dissection, were found to contain a gray fluid, while the B and PE capsules were dry (without fluid accumuIation).

At POD 30, the incisions had healed and the hair had begun to grow back. All examined implant capsules were dry.

At POD 60, a large swelling was observed over the implant site in one of the 0.5C animals (#36). The swelling was lanced under sterile conditions and proved to be an abscess from which a small quantity of watery exudate was expressed. Smears of this fluid failed to demonstrate microorganisms. An- other animal, from the 1.OC group, experienced dyspnea following etherization and died of apparent respiratory insufficiency, despite resusci- tation efforts. All implant sites examined at this time were well encapsulated and dry.

RELEASE OF CORROSION PRODUCTS BY F-75.111 1217

At POD 100, the previously observed abscess (in animal #36) was lanced again. Animals used for metabolic studies were retained and sacrificed at POD 120.

At POD 120, all remaining animals were examined. Shortly before this time, an 0.5C animal (#40) developed a sudden, extreme swelling of the entire upper leg which contained the implant. Examination of the implant site from this animal revealed atrophied muscle tissue, interspersed with necrotic pockets and an invasive fibroplasia with neoplastic fibroblasts, consistent with fibrosarcoma. Animal #36 (with possible implant site infection, as previously mentioned) and an additional 0.5C animal (#32) showed the same histological picture at the implant site, but at an earlier state of devel- opment. The other implant sites in this and all other groups appeared dry, normal, and unremarkable, with the microspheres well contained (Fig. 1). However, all implants were surrounded by a thin (2-3 mm) layer of adipose tissue situated within apparently normal muscle. No other tissue abnormal- ities were noted at autopsy and no tissues were studied histologically, except for the lungs.

Figure 1. original magnification 40 X .

Implant site: animal #48 (1.OC). POD 120, 100 pm ground section,

1218 BLACK ET AL.

Animal weight (Table I)

All animals gained weight at a reasonable rate up to POD 100. However, between POD 100 and POD 120, the mean weight of four of the five groups decreased, suggesting a failure of animal husbandry. Thus, with the excep- tion of necropsy data, all POD 120 data has been discounted.

Evaluation of lung pathology (Table 11)

A 12-point histological scoring system was used to evaluate the lung sections. AU animals exhibited mild to moderate lung disease, with no apparent distinction between groups.

Serum metal concentrations (Tables 111-V)

Specimens were obtained for each animal in this study and analyzed individually. The 95% confidence intervals for the group means of SCrC* (Table 111), SCoC (Table IV), and SNiC (Table V), were obtained by calcu- lating the individual data point ranges from daily calibration curves for both metal concentration and protein content followed by root mean square combination.

TABLE I Animal Weights (gs)

Time (POD)

Group: -7 0 30 60 100 120

PE 441.5 472.9 519.6 +95%CI 44.1 32.5 38.2 per day 4.49 1.56 1.37 N 8 8 7

B 444.2 465.0 525.1 +95%CI 29.7 13.0 23.4 per day 2.97 2.00 0.35 N 8 8 7

0.3C 416.2 447.0 513.9 ?95%CI 39.4 18.1 37.3

N 8 8 7 0.5C 429.0 456.2 496.9

+95%CI 18.3 10.2 17.4 per day 3.89 1.36 0.95 N 8 8 7

1.oc 421.1 460.0 509.3 295%CI 50.4 9.6 15.0 per day 5.56 1.64 2.17 N 8 8 7

per day 4.40 2.23 1.25

POD -7 grand mean: 430.4 * 7.2 (N=40)

560.7 34.0

6 535.7 33.8

6 551.3 49.4

6 525.5 18.0

6 574.3 17.2

6

0.91

1.65

1.67

1.30

1.11

597.0 578.0 40.6 32.0

-0.95 5 5

601.7 602.5 21.3 34.9

0.04 6 6

618.0 593.8 66.7 74.7

-1.21 5 4

577.5 566.7 13.5 27.8

-0.54 4 3

618.8 611.2 23.6 19.5

-0.38 4 4

~

*SCrC = serum chromium concentration.


TABLE I1 Evaluation of Lung Pathology

Time (POD)

Group 10 30 60 100 120

PE 7 7 9.5 10.5,11,6,7 Mean score 7 (N = 1) 7 ( N = 1) 9.5 (N = 1) - 8.6 (N = 4)

B 9 2,4,11,6,5 Mean score - 9 (N = 1) - - 5.6 (N = 5)

0.3C 6 6 6,8,5,10,6 Mean score - 6 (N = 1) 6 (N = 1) - 7.0 (N = 5)

0.5C 7 8 6,9 9,4,6,10 Mean score 7 (N = 1) 8 (N = 1) 7.5 (N = 2) - 7.25 (N = 4)

1.oc 10 11 8,8 10,8,11,9

Histological scoring system: (max. score = 12) Lymphoid infiltration: mild = 2, moderate = 3, moderatekevere = 3.5 Hemorrhage: none = 0, diffuse = 2, focal = 3, multifocal = 4 Macrophages: none = 0, alveolarhtra-alveolar = 2, pigmented andlor multifocal = 3,

Preoperative (culled) controls: 6,9,6,5,7,5,6 6.3 (N = 7)

Mean score 10 (N = 1) 11 (N = 1) 8 (N = 2) - 9.5 (N = 4)

multifocal clumps = 4

Mean score

TABLE 111 Serum Chromium Concentration (SCrC). Protein Basis: (pg Cr/mg protein)

Time (POD)

Group -7 3 10 30 60 100

PE 9.1 14.5 +95%cI 3.5 4.2 N 10 8

B 4.8 68.8' *95%CI 1.1 19.2 N 7 8

0.3C 6.7 279* ?95%CI 1.8 63.0 N 9 8

0.5C 7.6 165* +95%CI 2.1 21.7 N 10 8

1.oc 9.2 18T *95%CI 1.5 37.7 N 12 8

POD -7 grand mean: 7.8 * 1.0 (N = 48)

13.3 5.4 8

35.1* 6.5 8

183* 34.9 8

118' 34.9 8

89.6* 13.8 8

7.2 17.8 7

14.9 12.3 7

69.1* 12.3 7

77.0* 36.8 7

38.6* 6.7 7

6.4 12.0 6 9.2 9.0 6

46.0* 18.4 6

52.4* 13.7 6

14.2 4.2 6

4.5 4.6 5 7.0 5.7 6

61.0* 22.8 5

128* 56.2 4

20.2 14.9 4

*Significantly greater than POD -7 (p < 0.05) (Student's "t"-test, grouped data). Note: N (= sample number) may be less than animal number due to specimen loss.

Urine metal concentrations (Table VI)

Specimens were obtained from each animal undergoing metabolic studies. For clarity, individual confidence intervals are not given; however, error

1220 BLACK ET AL.

estimate data for high and low concentrations of each element in urine are given.

TABLE IV Serum Cobalt Concentration (SCoC) Protein Basis: (pg Co/mg Protein)

Time (POD)

Grouu -7 3 10 30 60 100

PE 9.4 5.5 17.3 7.2 7.4 4.3 +95%CI 2.7 2.3 9.6 4.2 6.4 4.4 N 9 8 8 7 6 5

B 7.5 266* 42.1" 18.6 92.1" 5.2 t95%CI 2.4 69.2 6.0 11.0 204 4.5 N 10 8 8 6 6 6

0.3C 14.8 2330* 1120* 289* 1370* 1050* ?95%CI 5.9 523 276 260 542 271 h' 9 8 8 7 6 5

0.5C 10.6 4890' 7770" 4510* 2560" 2520" +95%CI 2.4 2560 4020 1140 322 458 N 10 8 8 7 6 4

1.oc 5.9 916* 289* 266* 230* 272* +95%CI 2.5 161 138 31.8 110 34.0

N 12 8 8 7 6 4 POD -7 grand mean: 9.4 * 1.5 (N = 48)

'' = possibly contaminated samples. *Significantly greater than POD -7 ( p < 0.05) (Student's "t"-test, grouped data). Note: N (= sample number) may be less than animal number due to specimen loss.

TABLE V Serum Nickel Concentration (SNiC). Protein Basis: pg N:/mg protein

Time (POD)

Group -7 3 10 30 60 100

PE 84.7 203" 78.8 2.9 25.8 0.8 +-95%CI 22.3 72.4 47.5 3.6 17.4 1.4 N 10 8 8 7 6 5

B 54.9 132 191* 11.7 226" 0.5 +95%CI 15.0 54.2 81.7 29.2 317 0.9 N 10 8 8 7 6 6

0.3C 81.0 154* 260* 59.2 177'' 7.6 t95%CI 27.9 26.2 84.5 32.5 222 17.2 N 9 8 8 7 6 5

0.5C 58.5 242* 190* 104 118" 38.1 +95%CI 16.8 111 63.2 71.9 171 32.2 N 10 8 8 7 5 4

1.oc 54.1 139* 128 38.7 21.0 12.7 +95%CI 17.0 55.9 98.1 43.9 19.6 14.2 N 12 8 8 7 6 4

POD -7 grand mean: 65.9 + 8.5 (N = 48)

= possibly contaminated samples. "Significantly greater than POD -7 ( p < 0.05) (Student's "t"-test, grouped data). Note: N (= sample number) may be less than animal number due to specimen loss.


TABLE VI Urine Metal Concentrations Chromium (UCrC) (ng/mL)

Time (POD)

Group 10 30 60 100

Chromium (UCrC) (ng/mL) PE 3.6 4.6 B 4.9 2.6 0.3C 28.9" 18.8" 0.5C 14.9" 10.3" 1.0c 27.5" 7.6" Error estimate: low UCrC = 3.3 * 0.2 (sd = 0.3)

high UCrC = 80.9 2 1.46 (sd = 2.0)

Cobalt (UCoC) (ng/mL) PE 14.8 17.0 B 34.2* 7.2" 0.3C 3610" 4170' 0.5C 10830" 9910% 1.0c 71 8* 181" Error estimate: low UCoC = 6.0 ? 0.1 (sd = 0.1)

high UCoC = 10970 2 560 (sd = 780)

Nickel (UNiC) (ng/mL) PE 28.2 41.1 B 32.8 15.5* 0.3C 152" 46.8 0.5C 72.2" 67.5" 1.oc 51* 18.6% Error estimate: low UNiC = 11.1 * 0.3 (sd = 0.4)

high UNiC = 121 * 7.9 (sd = 11)

1.8 m

16.4" 20.6* 7.4'

6.0 m

3370" 7470* 971"

54.5 rn

73.5" 75.8* 27.4'

2.4 10.8" 19.2' 40.9* 13.7*

15.6 14.5

2140" 9950" 1100"

31.2 27.9 78.5"

37.5 103"

Individual confidence intervals calculated from regression of calibration data, not

m = lost specimen N = 10 for error estimates " = Dif. from PE (p < 0.05) (by non-overlap of 595% confidence intervals). **N = 1 for each point.

shown for clarity.

Ratios of urine to serum metal concentrations (U/S) (Table VII)

Ratios were calculated when individual urine and serum metal concentrations where available for the same animal at the same time point and neither value fell below the single specimen detection limit. The 95% confidence intervals were derived from calibration data.

Surface concentrations of microspheres (Table VIII)

A standard site on five microspheres selected at random in each implant site or microsphere group was studied. Averages of the five sites are reported; the standard error of the mean is given as an error estimate. Results from both the unimplanted and implanted specimens are presented as obtained and normalized by Mo content.

1222

DISCUSSION

BLACK ET AL.

Application of a low-temperature, isotropic coating of carbon to an F-75 type cast cobalt-chromium alloy appears to raise rather than reduce the rate of metallic release in vim. It is strikingly apparent that the carbon-coated microspheres released chromium and cobalt at rates far above those for the bare microspheres. Tables I11 and IV show a significant post-operative elevation of both SCrC and SCoC, for all microsphere implanted groups, returning to levels indistinguishable from preoperative by POD 30 in the bare

TABLE VII Ratios of Urine to Serum Metal Concentration (U/S) Volume Basis

(Individual Animals)

Time (POD)

Group 10 30 60 100

PE

B

0.3C

0.5C

1.oc

PE

B

0.3C

0.5C

1.oc

PE

B

0.3C

0.5C

1.oc

2.0 3.2*** 2.2 2.8 3.5 0.6 1.6 0.3 3.3 0.5

5.7 2.7

7.9 15

55 16 29

24 4.0

6.0

4.1 0.8 2.7 0.3

1.0 6.6 1.4 7.8 1.9

12

Chromium (U/S) 4.3 3.2 0.8 0.4 2.4 0.2 1.1 0.1 1.6 0.2

Cobalt (U/S) 13 5.6 4.8 0.6

3.4

0.9 7.8 0.5

64

21

Nickel (U/S) *

0.5 0.1

2.9 2.5 0.4 5.0 1.8

11

4.6 1.2 rn

3.6 0.6 3.2 0.2 3.1 0.7

6.0 0.3 rn

38

29

24

2.7

1.3

2.9

*

rn

10

20 1.6

4.3 *

*

*

4.0 2.8 5.8 1.8 6.4 5.6

*

*

29

46

49

0.7

1.8

1.8

*

1.1 0.1 40 21 86 73

*

* = Serum metal concentration not different from detection limit. ** N = 1 for each point, rn = lost specimen. *** = + -95% confidence interval (by error propagation from data in Tables 111-VI).


TABLE VIII Surface Concentrations of Microspheres

Metal conc. (wt%) Std." Group+ B 0.3C 0.5C 1.oc

Mo

Cr

c o

Ni

Mo normalized

(sem)

( s -4

(sem)

Cr/Mo Co/Mo Ni/Mo

5-7

27-30

(W 2.5 (max)

3.9-6.0 9.3-13.0 0.5 (max)

Unimplanted 8.0 16.0 0.3 1.4

29.2 22.7 0.1 2.4

61.7 60.0 0.4 2.7 1.1 1.3

< O . l 0.1

3.7 1.4 7.8 3.8 0.14 0.08

Implanted (POD 120) 6.2 18.2 2.1 1.5

29.2 17.2 0.2 0.3

63.1 63.3 2.0 1.4 1.5 1.3 0.1 <0.1

4.7 0.94

0.24 0.07 10 3.5

19.4 0.9

12.9 0.7

66.2 1.2 1.5 0.1

0.67 3.4 0.08

26.2 3.1

23.9 7.5

48.6 7.4 1.3 0.1

0.91 1.9 0.05

12.5 1.4

17.3 2.1

68.9 1.5 1.3 0.1

1.4 5.5 0.11

48.3 3.4 9.9 0.7

39.5 1.8 2.3 0.3

0.21 0.82 0.05

*Standard.' **Standard error of the mean (n = 5). +Percent of total analyzed (Mo + Cr + Co + Ni). Note: No significant counts obtained from PE.

group, as previously seen in this m ~ d e l . ~ , ~ However, for the carbon-coated microsphere groups, concentrations rise to much higher levels, and remain elevated at POD 100. A similar pattern is seen in urine metal concentrations (Table VI), except that significant elevations are not seen for the bare microsphere implanted animals. Both SNiC (Table V) and UNiC (Table VI) appear sporadically elevated in all microsphere implanted groups, with no clear pattern. SNiC returns to baseline levels by POD 30 but UNiC apparently remains elevated in the carbon coated microsphere implanted groups.

Contrary to expectations, the microspheres with the intermediate thickness coating (0.5C) behaved qualitatively the worst while those with the 1.0-pm thickness were relatively the best. This can be seen most clearly in the values of SCoC and UCoC for the 0.5C group which were always at least twice those for the next highest group (0.3C). This behavior may not be related directly to processing parameters, as they were essentially constant for all three groups, except for bed residence time at peak temperature which increases monotonically with coating thickness.

1224 BLACK ET AL.

However, it is possible that the fluidized bed process does produce com- positional changes on the surface of the microspheres. Table VIII suggests that there is a surface depletion of Cr and Co, relative to Mo content, in the unimplanted, coated microspheres which is especially pronounced for the 0.5C microspheres. This apparent surface depletion is accentuated by implantations of the 0.5 and 1.OC microspheres. By comparison, the uncoated (B) microspheres, which were not post-production heat treated, apparently do not show this effect, either before or after implantation.

Kilner et al.," studied F-75 alloy disproportionation after heat treatment at temperatures over 1180"C, as might be encountered during sintering opera- tions on porous-coated devices, and concluded that rapid cooling might promote solidification of the incipient melt phases encountered after heating above 1230"C, thus trapping chemically active carbides and the sigma phase within grain boundaries. Taylor and Waterhouse'' further showed that changes in grain boundary appearance, secondary to changes in amount and type of precipitate, are possible at aging temperatures as low as 650°C. At 850°C, grain boundary changes may be seen after 3.5 min, including formation of a feathery interdendritic grain boundary precipitate while aging beyond 80 min produced "blocky" changes in grain boundary morphology. Low-temperature, short-elapsed-time carbide precipitation is further demon- strated in their studies by a 25% loss of elongation to failure after aging at 815°C for only 30 min.

In both studies, the predominant low temperature grain boundary carbide was identified as M&, with M being some mixture of Co and Cr. The sensitization and thus increased corrodability produced by such carbides is well recognized: The reduction of carbon content in surgical stainless steel was undertaken historically to reduce the carbide contribution to corrosion. Bed residence times at peak temperature (near 840°C) for the three groups of coated microspheres were 45,60, and 180 min, respectively. These times should be long enough to produce intergranular carbide precipitation and, in the 1.OC microspheres, grain boundary morphology changes.

In order to investigate this possible effect, metallographic sections were prepared. Figure 2 is an etched section of a solution heat treated master ingot,* showing the characteristic coarse grain structure with slight intergranular precipitation. 12,13 In tergranular precipitates were somew hat more prominent near the ingot periphery than at its center; there was no evidence of dendritic formation. Figure 3, prepared under the same conditions, shows the finer-grained, highly dendritic structure of the rapidly quenched and non-heat-treated microspheres. The dendrites etch strongly but the grains (10-50 ,urn) can still be seen and there is no strong evidence of precipitation. By contrast, Figure 4 shows microspheres from the 0.3C group with modified structure almost obscured by intergranular and interdendritic precipitates. In the original sections, these bear a strong resemblance to the feathery inter-

*Howmedica, Inc., Rutherford, NJ.

RELEASE OF CORROSION PRODUCTS BY F-75.III

Figure 2. etch, 15 seconds, original magnification 1000 x).

Section of cast solution heat-treated F-75 ingot (HN03/HC1/FeC13

Figure 3. original magnification 1000 x).

Section of bare microspheres (HN03/HCI/FeC13 etch, 15 secs,

Figure 4. etch, 15 secs, original magnification l000X).

Section of 0.3-pm carbon-coated microspheres (HNO3/HC1/FeCl3

1225

1226 BLACK ET AL.

dendritic precipitates observed by Taylor and WaterhouseI2 as the initial transformation during aging in this alloy. There was a range in apparent precipitation density in microspheres from any particular group; however, a rapid qualitative survey failed to reveal any significant differences in precipitate densities between groups. It should be noted that differences in grain size between those seen here and in the previous study12 may result in different kine tics.

Perhaps the increased corrosion rates are secondary to these thermal processing effects, which may be a penalty imposed inadvertently during the coating process used.

Urine/semm concentration ratios were calculated (Table VII) to determine if delayed metal release was occurring. Metals, particularly Cr[VI], if second- arily released from renal storage depots, may produce increases in this ratio. There is no observable pattern to substantiate such an effect in this experiment.

Normalized SCrC elevations were also calculated (Table IX) to provide a comparison with a previous study.6 The agreement between the bare group and previous 1OOX uncoated microsphere studies is excellent. This finding, coupled with the absence of a postoperative SCrC elevation in the PE group, apparently validates the model and points directly to the implanted alloy as the source of the observed SCrC (and thus UCrC) elevations.

The pathological findings in animals implanted with the carbon coated microspheres, with the presentation of a transient, resolvable inflammatory response, are consistent with a foreign body response to particles shed from the coatings. A similar effect has been observed in both animals’ and patients* receiving carbon coated Co-Cr implants, with a chronic response related to continued tissue-implant movement which might be expected to release additional debris. However, the generally benign local host response to micron sized carbon debrisI4 suggests that this inflammation may be mediated by carbon ”contaminants”8 such as metal carbides. Figure 5 shows a montage of implant sites, as seen by SEM. In B and PE implant sites (5a, 5b), no debris is seen, while small amounts are seen lying on the intact capsule in the 0.3C site (5c). However, in the 1.OC site shown (Sd), the capsule has been breached and the profound “orange peel” shedding may be appreciated (arrow). These fragments are apparently portions of carbon coating, since they have appropriate thicknesses and are absent in the B and PE implant sites. Although submicron carbon dust was present mixed with the coated microspheres as received, before cleaning, no such fragments were observed.

This shedding, producing physical defects in the coatings, might also contribute to accelerated corrosion, since carbon is modestly noble or cath- odic when coupled to Co-Cr alloys. Even though Buchanan et al.” concluded that such coupling to F-75, at carbon : metal area ratios of 100 or less should not produce pitting, it is possible that the heat treatment associated with coating may produce carbide sensitization at sites which would be anodic under conditions of partial exposure. Such a mechanism, producing a con- centrated crevice-type attack along the carbon-metal interface, might also


TABLE IX Comparison of Postoperative SCrC Elevations (Volume basis) (group Means

Normalized to Preoperative Values). SA/BW = lOOX

Time (POD)

Group 3 10 30 60 100 ~~

PE 1.19 1.15 < <* <* B 9.57 4.76" 2.46 1.20 < 0.3C 32.7 19.4" 11.4* 6.04" 7.72" 0.5C 15.9' 11.0* 9.56* 6.95* 12.8' 1.0c 17.6* 12.1" 4.49* 3.00 1.79 B6 7.64* 5.60' 3.83 - 4.45

*Significant elevation ( p < 0.05) (nonoverlap of &95% confidence intervals). < = Less than preoperative. <* = Significantly less than preoperative ( p < 0.05) (nonoverlap of ?95% confidence

intervals).

account for the apparent loss of adhesion of the carbon coating. The implant site muscle atropy, degeneration, and necrosis as well as the

three examples of neoplastic transformation seen in the 0.5C group were apparently associated with the considerable release of both chromium and cobalt encountered there, and perhaps accentuated by the cellular response to particulate carbon or metal carbides. There are previous reports of fibrosarcoma formation at Co-Cr particulate implant sites in ratsl6,I7 which are thought to be related to the high cobalt content of the alloys used.16 The high cobalt concentrations which must have been present in these sites, as inferred from SCoC and UCoC, may be adequate to explain the trans- formations seen, as they are similar to that observed by McNamara and Williams18 after implantation of pure cobalt discs in another rat strain (1/8, hooded Lister, POD 225 (estimated)).

The rats implanted with poly(ethy1ene) or bare metal, at the same SA/BW ratio, remained relatively healthy and exhibited no notable implant site pathology.

CONCLUSIONS

The application of a carbon coating to a cast cobalt alloy, while promising on a prospective basis, produced unsatisfactory metallurgical and biological results in this experiment. The experiment was complex with a wide variety of both controllable and uncontrollable factors. Furthermore, care should be exercised in generalizing the results to expectations accompanying the use of other carbon coatings on this alloy. Commercial coating processes vary widely in operating parameters; it may be that those whose peak temperatures do not exceed 650°C would avoid the alloy sensitization apparently seen here." However, this study should serve as a warning that the generally satisfactory local host response to F-75 type alloys may be compromised by additional thermal and chemical processing. Caution is suggested in the

1228 BLACK ET AL.

Figure 5. Implant site morphology (POD 120, SEM, original magnification 3 2 0 ~ ) (A) animal #10 (PE), (B) animal #13 (B), (C) animal #24 (0.3C), (D) animal #49 (1.OC).


further application of thermally processed protective coatings to this alloy for medical applications.

This s tudy was supported by a grant-in-aid and donated materials from Johnson&Johnson Products. The technical assistance of S. P. Richardson is gratefully acknowledged.

References 1.

2.

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(), “F 75-82: Standard specification for cast cobalt-chromium- molybdenum alloy for surgical implant applications,” 1984 Annual Book of ASTM Standards, Vol. 13.01, Medical Devices. American Society for Testing and Materials, Philadelphia, 1984, pp. 13-14. D. F. Williams, “Corrosion of orthopedic implants,” in Biocompatibilify of Orthopedic Implants, Vol. I , by D. F. Williams, (ed.), CRC Press, Boca Raton, 1982, pp. 197-230, J. L. Woodman, ”Organometallic corrosion products: An in vivo and in nitro Comparison,” Ph.D. Thesis, University of Pennsylvania, Philadel- phia, Pennsylvania, p. 172ff (1980). J. Woodman, J. Black, and S. Jimenez, ”Isolation of serum organometallic corrosion products from 316LSS and HS-21 in vitvo and in vivo, ” 1. Biomed. Mater. Xes., 18, 99-114 (1984). A. Koegel and J. Black, ”Release of corrosion products by F-75 cobalt base alloy in the rat. I: Acute serum elevations,” J. Biomed. Muter. Res., 18, 513-522 (1984). K. L. Wapner, D. M. Morris, and J. Black, ”Release of corrosion products by F-75 cobalt base alloy in the rat. 11: Morbidity apparently associated with chromium release in vivo: A 120-day rat study,” J. Biomed. Mater. Xes., 20, 219-233 (1986). H. S. Shim, N. K. Agarwall, and A. D. Haubold, ”The adhesion of thin carbon films to metallic substrates,” J. Bioeng., 1, 45-50 (1976). J. A. Hess, J. A. Molinari, and P. J. Mentag, ”Two cases of incompatibility to carbon-coated subperiosteal implants,” Oral Suvg., 54,499-505 (1982). J . N. Kent and J. C. Bokros, ”Pyrolytic carbon and carbon coated metallic dental implants,” Dent. Clin. N. A . , 24(3), 465-485 (1980).

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0. H. Lowry, N. J. Rosebrough, A. L. Farr, and R. J. Randall, "Protein measurement with the fohn phenol reagent,"]. Bid. Chem., 193,265-275 (1951). T. Kilner, R. M. Pilliar, G. C. Weatherly, and C. Allibert, "Phase identi- fication and incipient melting in a cast Co-Cr surgical implant alloy," ]. Biorned. Mater. Res., 16, 63-79 (1982). R. N. J . Taylor and R. B. Waterhouse, "A study of the aging behavior of a cobalt based implant alloy," I . Mater. Sci., 18, 3265-3280 (1983). H. S. Dobbs and J. L. M. Robertson, "Heat treatment of cast Co-Cr-Mo for orthopaedic implant use," 1. Muter. Sci., 18, 391-401 (1983). G. Helbing, D. Wolter, R. Neugebauer, and J. Coldewey, "The reaction of tissue to friction particles of LTI carbon and carbon fibers (from) reinforced carbon in the knee joint of the rat," Trans. SOC. Biomuter., 1,126 (1977). R. A. Buchanan, J. E. Lemons, C. D. Griffin, N. G. Thompson, and L. C. Lucas, "Effects of carbon coupling on the in nitro corrosion of cast surgical cobalt-base alloy," J. Biomed. Muter. Xes., 15, 611-614 (1981). R. B. Pedley, G. Meachim, and D. F. Williams, "Tumor induction by implant materials," in Fundamental Aspects of Biocompatibility, Vol. 11, by D. F. Williams, (ed), CRC Press, Boca Raton, 1981, pp. 175-202. V. A. Memoli, R. M. Urban, J. Alroy, and J. 0. Galante, "Malignant neo- plasms associated with orthopedic implant materials in rats," J. Orthop. Res., 4, 346-355 (1986). A. McNamara and D. F. Williams, "The tissue response to implants of pure cobalt in rats," Eng. Med., 10, 17-20 (1981).

Received May 1, 1987 Accepted May 13, 1987

release of corrosion products by f-75 cobalt base alloy in the rat. iii: effects of a carbon surface...

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