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ter, West Los Angeles; Adjunct Assistant Professor of Removable Prosthodontics, UCLA School of Dentistry Los Angeles, Calif.; R a l p h P. F e l l e r , D M D , MS, M P H , Chief Dental Service, Jerry L. Pettis Memorial Veterans Hospital, Loma Linda; Professor of Prosthodontics, Loma Linda University, School of Dentistry, Loma Linda, Calif. Executive Committee: K r i s h a n K. K a p u t , D M D , MS, Chairman; H a r r y B l e c h m a n , DDS; G e o r g e W. C a r r o l l , DDS; R o b e r t H. D e u p r e e , P h D ; P a u l E h r lich, D D S ; R o b e r t H. S p r i g g , D D S . Data Monitoring Board: G e r a l d S h k l a r , D M D , MS,
Biostatistical Staff: R o b e r t H. D e u p r e e , P h D , B a r bara Komenda,
Margaret Lee, Claire Paradise, Tenya Economou, Patricia Danner
Research Staff: A n g e l i n e B r a d y , Allene Dillon, A r men Melikian, Elizabeth Villetto
Lillia Villnaueva, Alicia Moran, S a m b r a n o , Bill S l a g l e , a n d B a r b a r a
Participating Centers: VA Medical Centers at Loma Linda, San Diego, Sepulveda, and West Los Angeles, and VA Outpatient Clinic at Los Angeles
Chairman; P o t t e r C h a n g , P h D ; R i c h a r d T o p a z i a n , DDS.
Tissue compatibility of methylmethacrylate in cranial prostheses: A preliminary investigation J. J. G a r y , D D S , a D. L. M i t c h e l l , D D S , M S , b S. M. S t e i f e l , D V M , e a n d M, L. H a l e , P h D d
Naval Dental School, National Naval Dental Center, and Armed Forces Radiobiological Research Institute, Bethesda, Md. An in vivo study using 48 d i s e a s e - f r e e male L e w i s rats w a s conducted to determine the histologic difference b e t w e e n an alloplastic cranial prosthesis m a d e with a m o n o m e r directly from the manufacturer and a triple-distilled monomer. The histologic difference in the tissue reaction b e t w e e n a cranial prosthesis sterilized with ethylene oxide and one sterilized with cobalt-60 irradiation w a s also evaluated. Histologic tissue biopsies of the cranium and brain t i s s u e s w e r e obtained at 1, 3, 6, and 12 w e e k s . Tissue biopsies after the third w e e k s h o w e d minimal inflammation and the microscopic findings w e r e consistent with the reparative s t a g e of wound healing. The findings also s u g g e s t that distillation of the m o n o m e r in h e a t - p o l y m e r i z e d m e t h y l - m e t h a c r y l a t e is u n n e c e s s a r y for cranial prostheses. Cobalt-60 irradiation w a s found to be an effective alternative method of sterilization for such prostheses. (J PROSTHET DENT 1991;66:530-6.)
The opinions or assertions contained in this article are the private ones of the authors and are not to be construed as official or as reflecting the views of the Department of the Army, the Department of the Navy, or the Defense Nuclear Agency. Supported by the Armed Forces Radiobiology Research Institute, Defense Nuclear Agency, under work unit 00094, and by NMRDC work unit M0095.06.3014. Research was conducted according to the principles enunciated in the "Guide for the Care and Use of Laboratory Animals" prepared by the Institute of Laboratory Animal Resources, National Research Council. aColonel (DC) USA; resident, Maxillofacial Prosthetics Division, Prosthodontics Department, Naval Dental School. bCaptain (DC) USN; Chief, Maxillofacial Prosthetics Division, Prosthodontics Department, Naval Dental School. CMajor (VC) USA; Chief, Comparative Pathology Division, Veterinary Medicine Department, Armed Forces Radiobiology Research Institute. dRadiation Biochemistry Department, Armed Forces Radiobiological Research Institute. 10/1/24202
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V a r i o u s alloplastic repair procedures have been advocated for restoring cranial defects. 1"6 Alloplastic materials for cranial repair decrease surgical time, provide protection to exposed structures, and improve the prognosis for a predictable and esthetic result. 4 Alloplastic cranial prostheses are made of either heatpolymerizing or autopolymerizing methylmethacrylate. 1 Grunewald 7 favored a distilled monomer in heat-polymerized methylmethacrylate. He opposed autopolymerizing methylmethacrylate because the tertiary amines and other additives may produce unfavorable tissue reactions such as edema, hyperemia, fibroblastic proliferation, and lymphocytic and eosinophilic infiltration. Brown s advocated a heat-polymerized methylmethacrylate in a pure resin form that is free of coloring agents or adulterants. Other authors 1, 3, 9 recommended that the monomer be triple dis-
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VOLUME 66
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TISSUE COMPATIBILITY OF METHYLMETHACRYLATE
T a b l e I. P r o p r i e t a r y information for acrylic materials tested Permatone Polymer
Polymethyl methacrylate Benzoyl peroxide Titanium dioxide Methyl methacrylate Methyl acrylate Hydroquinone Dimethyl-p-toluidine Ethylene dimethacrylate
Cranioplastic
Manufacturer's monomer
Distilled monomer
99.85% 0.10% 0.05 %
Polymer
Monomer
99.00-99.60 % 0.40-1.00% 97.90 %
90.80 % 8.90% 3 ppm
25-50 ppm
95.05 % 20 _+ 5 ppm 0.66 % 4.28%
2.00%
T a b l e II. Disk fabrication and sterilization method Group*
1 2 3 4 5 6 (control)
Methyl methacrylate
Monomer
Sterilization method
Heat polymerized Distilled Heat polymerized Distilled Heat polymerized Manufacturer Heat polymerized Manufacturer Autopolymerized Manufacturer C1 - Defect created, no disk placed (4 rats) C2 - No defect (4 rats)
Ethylene oxide Cobalt-60 Ethylene oxide Cobalt-60 Cobalt-60
*N ffi48 (8 rats per group).
tilled to remove the inhibitors, plasticizers, and coloring agents t h a t could irritate the tissues. Some 2, 6,10 have advocated the use of autopolymerizing methylmethacrylate m a r k e t e d specifically for restoring cranial defects. This resin is presterilized and contains an inhibitor, a tertiary amine, and other additives. The principal disadvantages of the direct application of autopolymerizing methylmethacrylate are the heat of polymerization and the presence of free monomer, i, n Residual liquid monomer arising from incomplete polymerization has been suggested as the cause of local tissue reactions, 2, 6,12-14 b u t apparently the toxicity is of short duration. 2 Mixing the materials in a polyethylene bag reduces monomer contamination of the dura. The heat of polymerization can be controlled with sterile water irrigation. No studies have been reported t h a t specifically state t h a t a monomer m u s t be triple distilled before its use for a cranial prosthesis. Inhibitors, plasticizers, and other additives have been suggested as the possible cause of local tissue reactions. However, the surgical procedure, the toxicity of residual liquid monomer, or the heat of polymerization may be the true etiology. This article describes an in vivo study made to determine (1) any histologic difference between an alloplastic cranial prosthesis made from a monomer directly from the manufacturer and one t h a t has been triple distilled and (2) any histologic difference in the tissue reaction between a cranial prosthesis sterilized with ethylene oxide and one sterilized with cobalt-60 irradiation.
THE JOURNAL OF PROSTHETIC DENTISTRY
T a b l e III. Results of histologic grading with different
disk types Week Group
1 2 3 4 5 6 C-1 C-2
Disk type
DM/EO DM/CO MM/EO MM/CO AP
1
3
6
12
3/2* 3/2
1/1 1/1 1/1 1/1 1/1 1 1
1/1 1/1 1/1 1/1 1/1 1 1
1/1 1/1 1/1 1/1 1/1 1 1
3/2 3/3 2/2 3 1
*There were two rats in each experimentalgroup. Each number represents graded responsefor one rat. Results represent averageof four sections:MM, manufacturer's monomer; DM, distilled monomer; AP, autopolymerized methyl methacrylate;EO, sterilizedby ethylene oxide;CO, sterilized by cobalt-60 irradiation; C1, control group with surgicaldefect only; C2, control group with no defect.
MATERIAL
AND
METHODS
Forty-eight male Lewis VAF/plus rats (Rattus norvegicus, Charles River Laboratories, Raleigh, N.C.), 6 months of age and weighing 350 gm, were used in this study. Rats were quarantined on arrival and screened for evidence of disease before used. The rats were m a i n t a i n e d in a facility accredited by the American Association for Accreditation of Laboratory Animals. T h e y were kept in plastic microisolater cages on hardwood chip contact bedding, and they
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Fig. 1, Location of cranial disk (CD) in rat's cranium.
were provided commercial rodent chow and acidified tap water ad libitum. Animal holding rooms were maintained at 70 ° ± 2 ° F with 50% ± 10% relative humidity with at least 10 air changes of 100 % conditioned fresh air per hour. The rats were on a 12-hour light/dark full-spectrum lighting cycle with no twilight. Previous investigations have shown that the rat calvarial wound model has many similarities to the human maxillofacial region. The development of rat calvaria from a membrane precursor resembles the membranous bone development of the human skull. In addition, the rat has been used by other investigators in studying maxillofacial development.15,16 To ensure that the development of the calvaria was complete, 6-month-old rats were used. For surgical procedures, rats were anesthetized by intraperitoneal injection of ketamine hydrochloride (12 mg/ 100 gm body weight) supplemented with the muscle relaxant acepromazine maleate (0.44 mg/100 gm body weight). Surgical procedures were performed under aseptic conditions, and the rats were monitored until recovery from anesthesia. They were checked daily thereafter. At the end of the experimental period, all of the rats were killed by carbon dioxide inhalation in a charged euthanasia chamber. This study included an assessment of two different methods of sterilization: cobalt-60 irradiation or ethylene oxide sterilization. Thirty-two disks 8 mm in diameter and 0.5 m m thick were made of clear heat-polymerized methylmethacrylate (Permatone, Sybron-Kerr, Romulus, Mich.). The disks were processed at 165 ° F for 11/2 hours and then immersed for 30 minutes in boiling water. The distilled monomer was obtained from a triple distillation of the manufacturer's monomer. A chemical analysis (Crippen Laboratories, Inc., New Castle, Del.) of the distilled
5 3 '~
monomer determined the remaining amount of hydroquinone. The proprietary information for the acrylic materials is shown in Table I. All of the disks had a slight lip to prevent inferior displacement and two perforations to allow fluid exchange. Eight disks processed with the distilled monomer and eight disks processed with the manufacturer's monomer were sterilized with ethylene oxide for 13A hours at 130 ° F and then aerated for 24 hours in an ethylene oxide gas sterilizer (Amsco, American Sterilizer, Co., Erie, Pa.). In addition, eight disks processed with the distilled monomer and eight disks processed with the manufacturer's monomer were sterilized with cobalt-60 irradiation at a dose level of 1.8 Mrad to provide a level of confidence of sterility. This dose level was used because Cranioplastic material (L. D. Caulk, Milford, Del.) is steril:-zed at 1.8 to 2.2 Mrad. In addition to preformed polymerized material, an autopolymerizing methylmethacrylate (Codman Crainoplastic, L.D. Caulk) marketed specifically for repairing cranial defects was tested. At the time of surgery, this resin was mixed according to the manufacturer's directions, placed in the cranial defect in a dough-like mass, and irrigated with sterile saline. The manufacturer had sterilized the polymer by cobalt-60 irradiation and the monomer by filtration. The rats were divided into six groups of eight as delineated in Table II. Four of the groups were tested with heat-polymerized methylmethacrylate disks made with either the distilled monomer or the manufacturer's monomer. These four groups were further divided by the method of sterilization so that both the distilled monomer and the manufacturer's monomer were assessed in terms of sterilization by ethylene oxide or cobalt-60 irradiation. An additional group of eight rats was used to test the effects of the autopolymerizing methylmethacrylate. For controls, four rats underwent surgical procedures similar to those on the experimental rats, but no disks were placed in the surgically created defect (C1), and another group of four rats was subjected to nonsurgical manipulations (C2). After induction of anesthesia with ketamine hydrochloride, the skin overlying the coronal .portion of the skull was shaved and scrubbed with gauze soaked with isopropyl alcohol. A midline incision to bone was followed by reflection of the pericranial tissues laterally to expose the frontalparietal bone complex. A surgical trephine (outer cutting diameter of 8 mm and adjustable external ring collar to limit the depth of the cut) was used to create a singular trephinated defect at the junction of the frontal, parietal, and occipital bones. During the procedure, the surgical field was flooded with copious amounts of sterile saline to irrigate the surgical field and minimize heat generated by the trephine procedure. Hemostasis was achieved through firm pressure with a sterile gauze, and the cranial prosthesis was placed (Fig. 1). The wound was closed in layers with a 5-0 monofilament subcut followed by skin closure
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T I S S U E C O M P A T I B I L I T Y OF M E T H Y L M E T H A C R Y L A T E
Fig. 2. Normal cranial tissue of control where no surgical defect was created. (Original magnification xl0.) Note intact periosteum (P).
with 9 mm clips (Mikron, Clay-Adams Co., Parsippany, N.J.). At the end of 1, 3, 6, and 12 weeks, two rats in each category and one rat in each control group were killed by carbon dioxide (C02) gas inhalation in a charged C02 chamber. The head of each rat was removed, skinned, placed in a coded specimen bottle containing 10% formalin, and submitted to the veterinary pathology laboratory for histologic processing and evaluation. After 2 days in a decalcification solution (Calex, Fischer Scientific Products, Rockville, Md.), the brain and cranial bone were cut on sagittal and transverse planes intersecting at the bone-disk junction, yielding four surfaces for histologic evaluation. The cut sections were then embedded in paraffin, cut with a microtome, mounted on a glass slide, and stained with hematoxylin and eosin. Evaluations by a veterinary pathologist were conducted in such a manner that only the principal investigator knew which type of disk in each group was being evaluated. Grading was subjective and based on tissue reactivity to the implanted disk. The factors evaluated in determining the assigned grade were (1) influx of inflammatory cells into the defect, (2) fibrous connective tissue response in the periosteum and the tissue covering the defect, and (3) periosteal bone proliferation. The final grade given was an average reading of all four slide sections. Grading from 1 to 3 was assigned according to the following criteria. Grade 1
a. No or minimal inflammatory cell response b. Minimal periosteal reaction of the bone surrounding the defect c. No macrophage infiltration d. Compact, mature fibrous connective tissue covering the defect e. No meningeal reaction f. Essentially normal tissue
THE JOURNALOF PROSTHETICDENTISTRY
Fig. 3. Grade 3 response from disk made from distilled monomer sterilized by cobalt 60-irradiation. Note marked fibroblastic proliferation of periosteum (P). (Original magnification x10.)
Grade 2
a. Mild-to-moderate response and a slight increase in inflammatory cells (occasional lymphocytes present) b. Moderate fibroblastic proliferation/reaction of periosteum distal to defect
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GARY ET AL
Fig. 4. Grade 2 response from disk made from autopolymerized methylmethacrylate. Note fibroblastic proliferation of periosteum (P), trephined bone (B), and outline shape of disk (D). (Original magnification ×10.)
Fig. 5. Grade I response from disk made from manufacturer's monomer and sterilized by cobalt 60-irradiation; rat sacrificed at 3 weeks. Note height of periosteum (P) and multifocal areas of new bone (B). (Original magnification ×10.)
c. Minimal number of infiltrating macrophages d. Mild-to-moderate new periosteal bone formation observed distal to defect e. Minimal-to-mild meningeal reaction Grade 3 a. Marked increase of inflammatory cell infiltration (lymphocytes with occasional neutrophils) b. Moderate-to-severe fibroblastic/fibrocytic periosteal reaction/response c. Moderate-to-marked numbers of infiltrating macrophages d. Moderate-to-marked new periosteal bone proliferation distal to defect e. Mild-to-moderate meningeal reaction
534
RESULTS The results of histologie grading for the implanted disks and both controls are reported in Table III. Representative examples of normal tissue and the various grades are seen in Figs. 2 through 5. The rats that were killed and evaluated 1 week after surgery showed the most pronounced tissue reactivity (grade 3). Marked fibroblastic proliferation, with moderate-tomarked numbers of infiltrating lymphocytes and macrophages, was observed in the periosteum adjacent to the defect and in the connective tissue above it. There was mod-
OCTOBER 1991 VOLUME 66 NUMBER 4
TISSUE COMPATIBILITY OF METHYLMETHACRYLATE
erate osteoblastic proliferation of the cranial bone within the fibrous periosteum distal to the defect. Meninges were minimally thickened by increased connective tissue, and they contained multifocal areas of mild, acute hemorrhage. By increasing the time between surgery and death of the rats, tissue responses in all groups decreased to grade 1. Tissues of all groups observed after the third week exhibited minimal inflammatory response and were consistent with the reparative phase of wound healing. Many of the tissue sections observed at this time exhibited osteoblastic activity within the fibrous connective tissue proliferation attempting to bridge the defect created by the trephine. Increased new bone formation was also observed in the fibrous periosteum along the edges of the trephined cranial bone, giving a lipped appearance to the cut cranium. The rats that were trephined but received no implant exhibited the greatest amount of new bone proliferation. Multifocal regions of granulomatous inflammation were observed surrounding pieces of sequestered bone or bone dust within the meninges and periosteum adjacent to the defect. The hemorrhage and granulomatous tissue reaction seen histologically were a result of surgical technique rather than tissue response to the composition of the disks. The histologic results suggested that the manner of sterilization and the composition of the disk had no apparent effect on overall tissue reaction. The grades assigned to the disks may be based more on a reparative response of normal tissue to surgery rather than on a response to the methylmethacrylate cranial disk. DISCUSSION The local tissue reaction to the use of methylmethacrylate for cranial prostheses may be caused by the heat of polymerization, the chemical components of the heatpolymerized or autopolymerized methylmethacrylate, and/ or the surgical procedure. 17 In the present study, the thermal effects of the autopolymerized methylmethacrylate were minimized by the use of a small amount of the resin and irrigation with sterile saline during the polymerization stage. The effects of surgical trauma were minimized by careful surgical technique and by irrigation of the surgical field with sterile saline during trephination. In spite of these precautions, it was impossible to avoid a tissue reaction to the surgical procedure, as evidenced by the grade 3 reaction observed in the control rat killed at I week. If the chemical composition of the cranial disks is considered a possible cause of tissue irritancy, then the components of methylmethacrylate should be considered. Because of the possibility that all the components offer the potential for chemical irritation to the tissues, the pure inert form of methylmethacrylate was recommended in the past. To obtain a purer form of methylmethacrylate, the monomer was triple distilled or washed with a caustic soda. Inhibitors are best removed from methylmethacrylate by distillation. However, the process of distillation is costly
THE JOURNAL OF PROSTHETIC DENTISTRY
and time-consuming, and distilled methylmethacrylate is more hazardous to handle. When a chemical analysis of the distilled monomer was done in this study, the amount of hydroquinone was reduced from 25 to 50 ppm to 3 ppm, and the cross-linking agent was eliminated. CONCLUSIONS No histologic difference in tissue response was noted between the distilled monomer and the manufacturer's monomer in a heat-polymerized methylmethacrylate disk implanted in rat calvaria. It is suggested that neither the use of distilled or nondistilled monomer in heat-polymerized methylmethacrylate disks nor the use of autopolymerized methylmethacrylate disks leads to serious effects on the regenerative processes in rats. At the 3-week postsurgery period, all tissue sections exhibited a grade 1 tissue response. The finding that the additives in methylmethacrylate had no tendency for tissue irritancy is also supported by other authors, is, 19 The residues and breakdown products of benzoyl peroxide, which include phenylbenzoate, benzoic acid, and carbon dioxide, are harmless. Hydroquinone and its monoethyl ester are washed out at the time of polymerization and are essentially eliminated from the polymerized product. The remaining additives have no reputation for irritancy. When a heat-polymerized methylmethacrylate cranial prosthesis is made, it must be sterilized before use. Ethylene oxide gas is most commonly used in the hospital environment. To remove residual ethylene oxide and its secondary products (ethylene glycol and ethylene chlorohydrin) from the sterilized cranial prosthesis, the prosthesis must be aerated. If the residual concentration of ethylene oxide is high, these residues are likely to cause hemolytic activity and dermal or mucous membrane irritability in the tissues. In this study, the cranial disks were sterilized with ethylene oxide for 13/4 hours at 130 ° F. To provide a level of confidence that all residual products of ethylene oxide had been removed, the disks were aerated for 24 hours at 120 ° F. The total time of sterilization was approximately 26 hours. Cobalt-60 irradiation has been recognized as the preferred method of sterilization for medical devices and opthalmic ointment. In many parts of Europe, products irradiated at 2.5 Mrad can be released for marketing as sterile without final testing. The high penetrating power of gamma radiation from a cobalt-60 irradiation source makes it particularly suitable for sterilization of medical devices.20,21 An added benefit is that it takes approximately 2 hours for the item to reach a dose level of 1.8 Mrad. Experimental test results from L.D. Caulk* have shown that, for the bioburden of Cranioplastic material, a dose level of 0.84 to 1.0 Mrad yielded a probability for sterility *Grosso JR. Director of Quality, L.D. Caulk. Personal Communication.
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of a p p r o x i m a t e l y 10 -6. H o w e v e r , L. D. C a u l k e s t a b l i s h e d a p r o c e s s t h a t specified a dose level of 1.8 t o 2.2 M r a d t o prov i d e a level of c o n f i d e n c e i n sterility. T h i s s t u d y f o u n d n o h i s t o l o g i c d i f f e r e n c e s i n t i s s u e react i o n b e t w e e n d i s k s s t e r i l i z e d b y e t h y l e n e oxide or c o b a l t 60 i r r a d i a t i o n . C o b a l t - 6 0 i r r a d i a t i o n c o u l d b e a n a l t e r n a t i v e m e t h o d of s t e r i l i z a t i o n , b u t f u r t h e r r e s e a r c h is n e e d e d t o d e t e r m i n e t h e effective M r a d d o s e level of s t e r i l i z a t i o n . T h e p o s s i b l e c h a n g e s in s t r e n g t h c h a r a c t e r i s t i c s of m e t h y l m e t h a c r y l a t e c a u s e d b y i r r a d i a t i o n s h o u l d also b e s t u d i e d .
REFERENCES 1. Beumer J, Firtell DN, Curtis TA. Current concepts in cranioplasty. J PROSTHET DENT 1979;42:67-77. 2. Sessions RB, Wolfe SK, Moiel RH, Creek WR. Wire mesh foundation for methyl methacrylate cranioplasty. Laryngoscope 1974;84:1020-30. 3. Firtell DN, Moore DJ, Bartlett SO. A radiographic grid for contouring cranial prostheses. J PROSTHETDENT 1971;25:439-45. 4. Firtell DN, Grisius RJ. Cranioplasty of the difficult frontal region. J PROSTHETDENT 1981;46:425-9. 5. Sabin H. Cranial implant problems. J PROSTHETDENT 1975;34'.659-65. 6. Cabanela ME, Coventry MB, MacCarty CS, Miller WE. The fate of patients with methylmethacrylate cranioplasty. J Bone Joint Surg 1972;54A:278-81. 7. Grunewald AH. The prosthodontist's role in cranioplasty. J PROSTHET DENT 1955;5:235-43. 8. Brown KE. Fabrication of an alloplastic cranioimplant. J PROSTHET DENT 1970;24:213-24. 9. Bartlett SO, Moore DJ. Ocular prosthesis: a physiologic system. J PROSTHETDENT 1973;29:450-9.
10. Spence WT. Form-fitting plastic cranioplasty. J Neurosurg 1954;11:21925. 11. Martin JW, Ganz SD, King GE, Jacob RF, Kramer DC. Cranial implant modification. J PROSTHETDENT 1984;52:414-6. 12. McCabe JF, BaNker RM. Tissue sensitivity to acrylic resin. Br Dent J 1976;140:34%50. 13. Grasso P. Long term effects of plastics. Food Cosmet Toxicol 1972; 10:567-83. 14. Smith DC, Bains MED. The detection and estimation of residual monomer in polymethylmethacrylate. J Dent Res 1956;35:16-24. 15. Takagi K, Urist MR. The reaction of the dura to bone morphogenetic protein (BMP) in repair of skull defects. Ann Surg 1982;196:100-9. 16. Schmitz JP, Hollinger JO. The critical size defect as an experimental model for craniomandibulofacial nonunions. Clin Orthop 1986;205:299308. 17. Feith R. Side-effects of acrylic cement implanted into bone. Acta Orthop Scand 1975;(Suppl No. 161). 18. Galin MA, Turkish L, Chowchuvich E. Detection removal and effect of unpolymerized methylmethacrylate in intraocular lenses. Am J Ophthalmol 1977;84:153-9. 19. Smith DC, Bains MED. Residual methyl methacrylate in the denture base and its relation to denture sore mouth. Br Dent J 1955;98:55-8. 20. Tsuji K, Rahn PD, Steindler KA. 6°Co irradiation as an alternate method for sterilization of penicillin G, neomycin, novobiocin, and dihydrostreptomycin. J Pharm Sci 1983;72(1):23-6. 21. Nash RA. Radiosterilized tetracycline ophthalmic ointment. Bull Parent Drug Assoc 1974;28(4):181-7. Reprint requests to:
DR. JOHNJ. GARY USA DENTAC FORT BLISS,TX 79920-5000
Tarnish and corrosion with the use of intraoral magnets C a r l J. D r a g o , D D S , M S Gundersen Clinic, Ltd., La Crosse, WiN. Intraoral m a g n e t s are s m a l l enough and provide sufficient r e t e n t i v e forces to be u s e d in various prosthodontic procedures. This study d e s c r i b e d 25 p a t i e n t s w i t h 60 m a g n e t i c attachments. All i m p l a n t a b u t m e n t s g a v e e v i d e n c e of tarnish, as did 68% of the m a g n e t i c k e e p e r s in p a t i e n t s w i t h natural teeth. Corrosion w a s found in 41.7% of the m a g n e t i c k e e p e r s . Although 88% of the patients w e r e satisfied at 3 months, only 28% w e r e satisfied at 2 years. R a r e earth m a g n e t s probably can be u s e d for retention o f r e m o v a b l e prostheses. H o w e v e r , the m a g n e t s are l i k e l y to tarnish and corrode, m a k i n g t h e m u n a c c e p t a b l e for use intraorally at this time. (J PROSTHET DENT 1991;66:536-40.)
T h e use of m a g n e t s in p r o s t h o d o n t i c s h a s b e e n inc r e a s i n g over t h e p a s t s e v e r a l years. 1 W i n k l e r 2 u s e d m a g n e t s as a n aid t o i m p r o v e d e n t u r e f u n c t i o n b y h a v i n g like m a g n e t i c poles c o n t a c t e a c h o t h e r in o p p o s i n g d e n t u r e bases. B e c a u s e like poles r e p e l e a c h o t h e r , h e p r o p o s e d t h i s
Presented to the Minnesota section of the American Association for Dental Research, St. Paul, Minn. Supported by the Gundersen Medical Foundation, La Crosse, WiN. 10/1/23139
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a r r a n g e m e n t to p r o m o t e d e n t u r e s t a b i l i t y . O t h e r s h a v e u s e d t h e p r i n c i p l e t h a t d i s s i m i l a r poles a t t r a c t o n e a n o t h e r a n d h a v e r e p o r t e d i m p r o v e d results. 3~5 Magnets made from cobalt/samarium and neodymium/ i r o n a n d b o r o n h a v e p r o v e n t o b e c a p a b l e of r e t e n t i o n e q u i v a l e n t to m e c h a n i c a l a t t a c h m e n t s u s e d in p r o s t h o dontics. 5 T a r n i s h a n d corrosion of m a g n e t s h a v e b e e n discussed. 4-6 H o w e v e r , few l o n g - t e r m s t u d i e s are a v a i l a b l e for r e v i e w reg a r d i n g t h e i n t r a o r a l t a r n i s h a n d c o r r o s i o n r e s i s t a n c e of
OCTOBER 1991 VOLUME 66 NUMBER 4
ter, West Los Angeles; Adjunct Assistant Professor of Removable Prosthodontics, UCLA School of Dentistry Los Angeles, Calif.; R a l p h P. F e l l e r , D M D , MS, M P H , Chief Dental Service, Jerry L. Pettis Memorial Veterans Hospital, Loma Linda; Professor of Prosthodontics, Loma Linda University, School of Dentistry, Loma Linda, Calif. Executive Committee: K r i s h a n K. K a p u t , D M D , MS, Chairman; H a r r y B l e c h m a n , DDS; G e o r g e W. C a r r o l l , DDS; R o b e r t H. D e u p r e e , P h D ; P a u l E h r lich, D D S ; R o b e r t H. S p r i g g , D D S . Data Monitoring Board: G e r a l d S h k l a r , D M D , MS,
Biostatistical Staff: R o b e r t H. D e u p r e e , P h D , B a r bara Komenda,
Margaret Lee, Claire Paradise, Tenya Economou, Patricia Danner
Research Staff: A n g e l i n e B r a d y , Allene Dillon, A r men Melikian, Elizabeth Villetto
Lillia Villnaueva, Alicia Moran, S a m b r a n o , Bill S l a g l e , a n d B a r b a r a
Participating Centers: VA Medical Centers at Loma Linda, San Diego, Sepulveda, and West Los Angeles, and VA Outpatient Clinic at Los Angeles
Chairman; P o t t e r C h a n g , P h D ; R i c h a r d T o p a z i a n , DDS.
Tissue compatibility of methylmethacrylate in cranial prostheses: A preliminary investigation J. J. G a r y , D D S , a D. L. M i t c h e l l , D D S , M S , b S. M. S t e i f e l , D V M , e a n d M, L. H a l e , P h D d
Naval Dental School, National Naval Dental Center, and Armed Forces Radiobiological Research Institute, Bethesda, Md. An in vivo study using 48 d i s e a s e - f r e e male L e w i s rats w a s conducted to determine the histologic difference b e t w e e n an alloplastic cranial prosthesis m a d e with a m o n o m e r directly from the manufacturer and a triple-distilled monomer. The histologic difference in the tissue reaction b e t w e e n a cranial prosthesis sterilized with ethylene oxide and one sterilized with cobalt-60 irradiation w a s also evaluated. Histologic tissue biopsies of the cranium and brain t i s s u e s w e r e obtained at 1, 3, 6, and 12 w e e k s . Tissue biopsies after the third w e e k s h o w e d minimal inflammation and the microscopic findings w e r e consistent with the reparative s t a g e of wound healing. The findings also s u g g e s t that distillation of the m o n o m e r in h e a t - p o l y m e r i z e d m e t h y l - m e t h a c r y l a t e is u n n e c e s s a r y for cranial prostheses. Cobalt-60 irradiation w a s found to be an effective alternative method of sterilization for such prostheses. (J PROSTHET DENT 1991;66:530-6.)
The opinions or assertions contained in this article are the private ones of the authors and are not to be construed as official or as reflecting the views of the Department of the Army, the Department of the Navy, or the Defense Nuclear Agency. Supported by the Armed Forces Radiobiology Research Institute, Defense Nuclear Agency, under work unit 00094, and by NMRDC work unit M0095.06.3014. Research was conducted according to the principles enunciated in the "Guide for the Care and Use of Laboratory Animals" prepared by the Institute of Laboratory Animal Resources, National Research Council. aColonel (DC) USA; resident, Maxillofacial Prosthetics Division, Prosthodontics Department, Naval Dental School. bCaptain (DC) USN; Chief, Maxillofacial Prosthetics Division, Prosthodontics Department, Naval Dental School. CMajor (VC) USA; Chief, Comparative Pathology Division, Veterinary Medicine Department, Armed Forces Radiobiology Research Institute. dRadiation Biochemistry Department, Armed Forces Radiobiological Research Institute. 10/1/24202
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V a r i o u s alloplastic repair procedures have been advocated for restoring cranial defects. 1"6 Alloplastic materials for cranial repair decrease surgical time, provide protection to exposed structures, and improve the prognosis for a predictable and esthetic result. 4 Alloplastic cranial prostheses are made of either heatpolymerizing or autopolymerizing methylmethacrylate. 1 Grunewald 7 favored a distilled monomer in heat-polymerized methylmethacrylate. He opposed autopolymerizing methylmethacrylate because the tertiary amines and other additives may produce unfavorable tissue reactions such as edema, hyperemia, fibroblastic proliferation, and lymphocytic and eosinophilic infiltration. Brown s advocated a heat-polymerized methylmethacrylate in a pure resin form that is free of coloring agents or adulterants. Other authors 1, 3, 9 recommended that the monomer be triple dis-
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TISSUE COMPATIBILITY OF METHYLMETHACRYLATE
T a b l e I. P r o p r i e t a r y information for acrylic materials tested Permatone Polymer
Polymethyl methacrylate Benzoyl peroxide Titanium dioxide Methyl methacrylate Methyl acrylate Hydroquinone Dimethyl-p-toluidine Ethylene dimethacrylate
Cranioplastic
Manufacturer's monomer
Distilled monomer
99.85% 0.10% 0.05 %
Polymer
Monomer
99.00-99.60 % 0.40-1.00% 97.90 %
90.80 % 8.90% 3 ppm
25-50 ppm
95.05 % 20 _+ 5 ppm 0.66 % 4.28%
2.00%
T a b l e II. Disk fabrication and sterilization method Group*
1 2 3 4 5 6 (control)
Methyl methacrylate
Monomer
Sterilization method
Heat polymerized Distilled Heat polymerized Distilled Heat polymerized Manufacturer Heat polymerized Manufacturer Autopolymerized Manufacturer C1 - Defect created, no disk placed (4 rats) C2 - No defect (4 rats)
Ethylene oxide Cobalt-60 Ethylene oxide Cobalt-60 Cobalt-60
*N ffi48 (8 rats per group).
tilled to remove the inhibitors, plasticizers, and coloring agents t h a t could irritate the tissues. Some 2, 6,10 have advocated the use of autopolymerizing methylmethacrylate m a r k e t e d specifically for restoring cranial defects. This resin is presterilized and contains an inhibitor, a tertiary amine, and other additives. The principal disadvantages of the direct application of autopolymerizing methylmethacrylate are the heat of polymerization and the presence of free monomer, i, n Residual liquid monomer arising from incomplete polymerization has been suggested as the cause of local tissue reactions, 2, 6,12-14 b u t apparently the toxicity is of short duration. 2 Mixing the materials in a polyethylene bag reduces monomer contamination of the dura. The heat of polymerization can be controlled with sterile water irrigation. No studies have been reported t h a t specifically state t h a t a monomer m u s t be triple distilled before its use for a cranial prosthesis. Inhibitors, plasticizers, and other additives have been suggested as the possible cause of local tissue reactions. However, the surgical procedure, the toxicity of residual liquid monomer, or the heat of polymerization may be the true etiology. This article describes an in vivo study made to determine (1) any histologic difference between an alloplastic cranial prosthesis made from a monomer directly from the manufacturer and one t h a t has been triple distilled and (2) any histologic difference in the tissue reaction between a cranial prosthesis sterilized with ethylene oxide and one sterilized with cobalt-60 irradiation.
THE JOURNAL OF PROSTHETIC DENTISTRY
T a b l e III. Results of histologic grading with different
disk types Week Group
1 2 3 4 5 6 C-1 C-2
Disk type
DM/EO DM/CO MM/EO MM/CO AP
1
3
6
12
3/2* 3/2
1/1 1/1 1/1 1/1 1/1 1 1
1/1 1/1 1/1 1/1 1/1 1 1
1/1 1/1 1/1 1/1 1/1 1 1
3/2 3/3 2/2 3 1
*There were two rats in each experimentalgroup. Each number represents graded responsefor one rat. Results represent averageof four sections:MM, manufacturer's monomer; DM, distilled monomer; AP, autopolymerized methyl methacrylate;EO, sterilizedby ethylene oxide;CO, sterilized by cobalt-60 irradiation; C1, control group with surgicaldefect only; C2, control group with no defect.
MATERIAL
AND
METHODS
Forty-eight male Lewis VAF/plus rats (Rattus norvegicus, Charles River Laboratories, Raleigh, N.C.), 6 months of age and weighing 350 gm, were used in this study. Rats were quarantined on arrival and screened for evidence of disease before used. The rats were m a i n t a i n e d in a facility accredited by the American Association for Accreditation of Laboratory Animals. T h e y were kept in plastic microisolater cages on hardwood chip contact bedding, and they
53]-
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Fig. 1, Location of cranial disk (CD) in rat's cranium.
were provided commercial rodent chow and acidified tap water ad libitum. Animal holding rooms were maintained at 70 ° ± 2 ° F with 50% ± 10% relative humidity with at least 10 air changes of 100 % conditioned fresh air per hour. The rats were on a 12-hour light/dark full-spectrum lighting cycle with no twilight. Previous investigations have shown that the rat calvarial wound model has many similarities to the human maxillofacial region. The development of rat calvaria from a membrane precursor resembles the membranous bone development of the human skull. In addition, the rat has been used by other investigators in studying maxillofacial development.15,16 To ensure that the development of the calvaria was complete, 6-month-old rats were used. For surgical procedures, rats were anesthetized by intraperitoneal injection of ketamine hydrochloride (12 mg/ 100 gm body weight) supplemented with the muscle relaxant acepromazine maleate (0.44 mg/100 gm body weight). Surgical procedures were performed under aseptic conditions, and the rats were monitored until recovery from anesthesia. They were checked daily thereafter. At the end of the experimental period, all of the rats were killed by carbon dioxide inhalation in a charged euthanasia chamber. This study included an assessment of two different methods of sterilization: cobalt-60 irradiation or ethylene oxide sterilization. Thirty-two disks 8 mm in diameter and 0.5 m m thick were made of clear heat-polymerized methylmethacrylate (Permatone, Sybron-Kerr, Romulus, Mich.). The disks were processed at 165 ° F for 11/2 hours and then immersed for 30 minutes in boiling water. The distilled monomer was obtained from a triple distillation of the manufacturer's monomer. A chemical analysis (Crippen Laboratories, Inc., New Castle, Del.) of the distilled
5 3 '~
monomer determined the remaining amount of hydroquinone. The proprietary information for the acrylic materials is shown in Table I. All of the disks had a slight lip to prevent inferior displacement and two perforations to allow fluid exchange. Eight disks processed with the distilled monomer and eight disks processed with the manufacturer's monomer were sterilized with ethylene oxide for 13A hours at 130 ° F and then aerated for 24 hours in an ethylene oxide gas sterilizer (Amsco, American Sterilizer, Co., Erie, Pa.). In addition, eight disks processed with the distilled monomer and eight disks processed with the manufacturer's monomer were sterilized with cobalt-60 irradiation at a dose level of 1.8 Mrad to provide a level of confidence of sterility. This dose level was used because Cranioplastic material (L. D. Caulk, Milford, Del.) is steril:-zed at 1.8 to 2.2 Mrad. In addition to preformed polymerized material, an autopolymerizing methylmethacrylate (Codman Crainoplastic, L.D. Caulk) marketed specifically for repairing cranial defects was tested. At the time of surgery, this resin was mixed according to the manufacturer's directions, placed in the cranial defect in a dough-like mass, and irrigated with sterile saline. The manufacturer had sterilized the polymer by cobalt-60 irradiation and the monomer by filtration. The rats were divided into six groups of eight as delineated in Table II. Four of the groups were tested with heat-polymerized methylmethacrylate disks made with either the distilled monomer or the manufacturer's monomer. These four groups were further divided by the method of sterilization so that both the distilled monomer and the manufacturer's monomer were assessed in terms of sterilization by ethylene oxide or cobalt-60 irradiation. An additional group of eight rats was used to test the effects of the autopolymerizing methylmethacrylate. For controls, four rats underwent surgical procedures similar to those on the experimental rats, but no disks were placed in the surgically created defect (C1), and another group of four rats was subjected to nonsurgical manipulations (C2). After induction of anesthesia with ketamine hydrochloride, the skin overlying the coronal .portion of the skull was shaved and scrubbed with gauze soaked with isopropyl alcohol. A midline incision to bone was followed by reflection of the pericranial tissues laterally to expose the frontalparietal bone complex. A surgical trephine (outer cutting diameter of 8 mm and adjustable external ring collar to limit the depth of the cut) was used to create a singular trephinated defect at the junction of the frontal, parietal, and occipital bones. During the procedure, the surgical field was flooded with copious amounts of sterile saline to irrigate the surgical field and minimize heat generated by the trephine procedure. Hemostasis was achieved through firm pressure with a sterile gauze, and the cranial prosthesis was placed (Fig. 1). The wound was closed in layers with a 5-0 monofilament subcut followed by skin closure
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T I S S U E C O M P A T I B I L I T Y OF M E T H Y L M E T H A C R Y L A T E
Fig. 2. Normal cranial tissue of control where no surgical defect was created. (Original magnification xl0.) Note intact periosteum (P).
with 9 mm clips (Mikron, Clay-Adams Co., Parsippany, N.J.). At the end of 1, 3, 6, and 12 weeks, two rats in each category and one rat in each control group were killed by carbon dioxide (C02) gas inhalation in a charged C02 chamber. The head of each rat was removed, skinned, placed in a coded specimen bottle containing 10% formalin, and submitted to the veterinary pathology laboratory for histologic processing and evaluation. After 2 days in a decalcification solution (Calex, Fischer Scientific Products, Rockville, Md.), the brain and cranial bone were cut on sagittal and transverse planes intersecting at the bone-disk junction, yielding four surfaces for histologic evaluation. The cut sections were then embedded in paraffin, cut with a microtome, mounted on a glass slide, and stained with hematoxylin and eosin. Evaluations by a veterinary pathologist were conducted in such a manner that only the principal investigator knew which type of disk in each group was being evaluated. Grading was subjective and based on tissue reactivity to the implanted disk. The factors evaluated in determining the assigned grade were (1) influx of inflammatory cells into the defect, (2) fibrous connective tissue response in the periosteum and the tissue covering the defect, and (3) periosteal bone proliferation. The final grade given was an average reading of all four slide sections. Grading from 1 to 3 was assigned according to the following criteria. Grade 1
a. No or minimal inflammatory cell response b. Minimal periosteal reaction of the bone surrounding the defect c. No macrophage infiltration d. Compact, mature fibrous connective tissue covering the defect e. No meningeal reaction f. Essentially normal tissue
THE JOURNALOF PROSTHETICDENTISTRY
Fig. 3. Grade 3 response from disk made from distilled monomer sterilized by cobalt 60-irradiation. Note marked fibroblastic proliferation of periosteum (P). (Original magnification x10.)
Grade 2
a. Mild-to-moderate response and a slight increase in inflammatory cells (occasional lymphocytes present) b. Moderate fibroblastic proliferation/reaction of periosteum distal to defect
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GARY ET AL
Fig. 4. Grade 2 response from disk made from autopolymerized methylmethacrylate. Note fibroblastic proliferation of periosteum (P), trephined bone (B), and outline shape of disk (D). (Original magnification ×10.)
Fig. 5. Grade I response from disk made from manufacturer's monomer and sterilized by cobalt 60-irradiation; rat sacrificed at 3 weeks. Note height of periosteum (P) and multifocal areas of new bone (B). (Original magnification ×10.)
c. Minimal number of infiltrating macrophages d. Mild-to-moderate new periosteal bone formation observed distal to defect e. Minimal-to-mild meningeal reaction Grade 3 a. Marked increase of inflammatory cell infiltration (lymphocytes with occasional neutrophils) b. Moderate-to-severe fibroblastic/fibrocytic periosteal reaction/response c. Moderate-to-marked numbers of infiltrating macrophages d. Moderate-to-marked new periosteal bone proliferation distal to defect e. Mild-to-moderate meningeal reaction
534
RESULTS The results of histologie grading for the implanted disks and both controls are reported in Table III. Representative examples of normal tissue and the various grades are seen in Figs. 2 through 5. The rats that were killed and evaluated 1 week after surgery showed the most pronounced tissue reactivity (grade 3). Marked fibroblastic proliferation, with moderate-tomarked numbers of infiltrating lymphocytes and macrophages, was observed in the periosteum adjacent to the defect and in the connective tissue above it. There was mod-
OCTOBER 1991 VOLUME 66 NUMBER 4
TISSUE COMPATIBILITY OF METHYLMETHACRYLATE
erate osteoblastic proliferation of the cranial bone within the fibrous periosteum distal to the defect. Meninges were minimally thickened by increased connective tissue, and they contained multifocal areas of mild, acute hemorrhage. By increasing the time between surgery and death of the rats, tissue responses in all groups decreased to grade 1. Tissues of all groups observed after the third week exhibited minimal inflammatory response and were consistent with the reparative phase of wound healing. Many of the tissue sections observed at this time exhibited osteoblastic activity within the fibrous connective tissue proliferation attempting to bridge the defect created by the trephine. Increased new bone formation was also observed in the fibrous periosteum along the edges of the trephined cranial bone, giving a lipped appearance to the cut cranium. The rats that were trephined but received no implant exhibited the greatest amount of new bone proliferation. Multifocal regions of granulomatous inflammation were observed surrounding pieces of sequestered bone or bone dust within the meninges and periosteum adjacent to the defect. The hemorrhage and granulomatous tissue reaction seen histologically were a result of surgical technique rather than tissue response to the composition of the disks. The histologic results suggested that the manner of sterilization and the composition of the disk had no apparent effect on overall tissue reaction. The grades assigned to the disks may be based more on a reparative response of normal tissue to surgery rather than on a response to the methylmethacrylate cranial disk. DISCUSSION The local tissue reaction to the use of methylmethacrylate for cranial prostheses may be caused by the heat of polymerization, the chemical components of the heatpolymerized or autopolymerized methylmethacrylate, and/ or the surgical procedure. 17 In the present study, the thermal effects of the autopolymerized methylmethacrylate were minimized by the use of a small amount of the resin and irrigation with sterile saline during the polymerization stage. The effects of surgical trauma were minimized by careful surgical technique and by irrigation of the surgical field with sterile saline during trephination. In spite of these precautions, it was impossible to avoid a tissue reaction to the surgical procedure, as evidenced by the grade 3 reaction observed in the control rat killed at I week. If the chemical composition of the cranial disks is considered a possible cause of tissue irritancy, then the components of methylmethacrylate should be considered. Because of the possibility that all the components offer the potential for chemical irritation to the tissues, the pure inert form of methylmethacrylate was recommended in the past. To obtain a purer form of methylmethacrylate, the monomer was triple distilled or washed with a caustic soda. Inhibitors are best removed from methylmethacrylate by distillation. However, the process of distillation is costly
THE JOURNAL OF PROSTHETIC DENTISTRY
and time-consuming, and distilled methylmethacrylate is more hazardous to handle. When a chemical analysis of the distilled monomer was done in this study, the amount of hydroquinone was reduced from 25 to 50 ppm to 3 ppm, and the cross-linking agent was eliminated. CONCLUSIONS No histologic difference in tissue response was noted between the distilled monomer and the manufacturer's monomer in a heat-polymerized methylmethacrylate disk implanted in rat calvaria. It is suggested that neither the use of distilled or nondistilled monomer in heat-polymerized methylmethacrylate disks nor the use of autopolymerized methylmethacrylate disks leads to serious effects on the regenerative processes in rats. At the 3-week postsurgery period, all tissue sections exhibited a grade 1 tissue response. The finding that the additives in methylmethacrylate had no tendency for tissue irritancy is also supported by other authors, is, 19 The residues and breakdown products of benzoyl peroxide, which include phenylbenzoate, benzoic acid, and carbon dioxide, are harmless. Hydroquinone and its monoethyl ester are washed out at the time of polymerization and are essentially eliminated from the polymerized product. The remaining additives have no reputation for irritancy. When a heat-polymerized methylmethacrylate cranial prosthesis is made, it must be sterilized before use. Ethylene oxide gas is most commonly used in the hospital environment. To remove residual ethylene oxide and its secondary products (ethylene glycol and ethylene chlorohydrin) from the sterilized cranial prosthesis, the prosthesis must be aerated. If the residual concentration of ethylene oxide is high, these residues are likely to cause hemolytic activity and dermal or mucous membrane irritability in the tissues. In this study, the cranial disks were sterilized with ethylene oxide for 13/4 hours at 130 ° F. To provide a level of confidence that all residual products of ethylene oxide had been removed, the disks were aerated for 24 hours at 120 ° F. The total time of sterilization was approximately 26 hours. Cobalt-60 irradiation has been recognized as the preferred method of sterilization for medical devices and opthalmic ointment. In many parts of Europe, products irradiated at 2.5 Mrad can be released for marketing as sterile without final testing. The high penetrating power of gamma radiation from a cobalt-60 irradiation source makes it particularly suitable for sterilization of medical devices.20,21 An added benefit is that it takes approximately 2 hours for the item to reach a dose level of 1.8 Mrad. Experimental test results from L.D. Caulk* have shown that, for the bioburden of Cranioplastic material, a dose level of 0.84 to 1.0 Mrad yielded a probability for sterility *Grosso JR. Director of Quality, L.D. Caulk. Personal Communication.
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of a p p r o x i m a t e l y 10 -6. H o w e v e r , L. D. C a u l k e s t a b l i s h e d a p r o c e s s t h a t specified a dose level of 1.8 t o 2.2 M r a d t o prov i d e a level of c o n f i d e n c e i n sterility. T h i s s t u d y f o u n d n o h i s t o l o g i c d i f f e r e n c e s i n t i s s u e react i o n b e t w e e n d i s k s s t e r i l i z e d b y e t h y l e n e oxide or c o b a l t 60 i r r a d i a t i o n . C o b a l t - 6 0 i r r a d i a t i o n c o u l d b e a n a l t e r n a t i v e m e t h o d of s t e r i l i z a t i o n , b u t f u r t h e r r e s e a r c h is n e e d e d t o d e t e r m i n e t h e effective M r a d d o s e level of s t e r i l i z a t i o n . T h e p o s s i b l e c h a n g e s in s t r e n g t h c h a r a c t e r i s t i c s of m e t h y l m e t h a c r y l a t e c a u s e d b y i r r a d i a t i o n s h o u l d also b e s t u d i e d .
REFERENCES 1. Beumer J, Firtell DN, Curtis TA. Current concepts in cranioplasty. J PROSTHET DENT 1979;42:67-77. 2. Sessions RB, Wolfe SK, Moiel RH, Creek WR. Wire mesh foundation for methyl methacrylate cranioplasty. Laryngoscope 1974;84:1020-30. 3. Firtell DN, Moore DJ, Bartlett SO. A radiographic grid for contouring cranial prostheses. J PROSTHETDENT 1971;25:439-45. 4. Firtell DN, Grisius RJ. Cranioplasty of the difficult frontal region. J PROSTHETDENT 1981;46:425-9. 5. Sabin H. Cranial implant problems. J PROSTHETDENT 1975;34'.659-65. 6. Cabanela ME, Coventry MB, MacCarty CS, Miller WE. The fate of patients with methylmethacrylate cranioplasty. J Bone Joint Surg 1972;54A:278-81. 7. Grunewald AH. The prosthodontist's role in cranioplasty. J PROSTHET DENT 1955;5:235-43. 8. Brown KE. Fabrication of an alloplastic cranioimplant. J PROSTHET DENT 1970;24:213-24. 9. Bartlett SO, Moore DJ. Ocular prosthesis: a physiologic system. J PROSTHETDENT 1973;29:450-9.
10. Spence WT. Form-fitting plastic cranioplasty. J Neurosurg 1954;11:21925. 11. Martin JW, Ganz SD, King GE, Jacob RF, Kramer DC. Cranial implant modification. J PROSTHETDENT 1984;52:414-6. 12. McCabe JF, BaNker RM. Tissue sensitivity to acrylic resin. Br Dent J 1976;140:34%50. 13. Grasso P. Long term effects of plastics. Food Cosmet Toxicol 1972; 10:567-83. 14. Smith DC, Bains MED. The detection and estimation of residual monomer in polymethylmethacrylate. J Dent Res 1956;35:16-24. 15. Takagi K, Urist MR. The reaction of the dura to bone morphogenetic protein (BMP) in repair of skull defects. Ann Surg 1982;196:100-9. 16. Schmitz JP, Hollinger JO. The critical size defect as an experimental model for craniomandibulofacial nonunions. Clin Orthop 1986;205:299308. 17. Feith R. Side-effects of acrylic cement implanted into bone. Acta Orthop Scand 1975;(Suppl No. 161). 18. Galin MA, Turkish L, Chowchuvich E. Detection removal and effect of unpolymerized methylmethacrylate in intraocular lenses. Am J Ophthalmol 1977;84:153-9. 19. Smith DC, Bains MED. Residual methyl methacrylate in the denture base and its relation to denture sore mouth. Br Dent J 1955;98:55-8. 20. Tsuji K, Rahn PD, Steindler KA. 6°Co irradiation as an alternate method for sterilization of penicillin G, neomycin, novobiocin, and dihydrostreptomycin. J Pharm Sci 1983;72(1):23-6. 21. Nash RA. Radiosterilized tetracycline ophthalmic ointment. Bull Parent Drug Assoc 1974;28(4):181-7. Reprint requests to:
DR. JOHNJ. GARY USA DENTAC FORT BLISS,TX 79920-5000
Tarnish and corrosion with the use of intraoral magnets C a r l J. D r a g o , D D S , M S Gundersen Clinic, Ltd., La Crosse, WiN. Intraoral m a g n e t s are s m a l l enough and provide sufficient r e t e n t i v e forces to be u s e d in various prosthodontic procedures. This study d e s c r i b e d 25 p a t i e n t s w i t h 60 m a g n e t i c attachments. All i m p l a n t a b u t m e n t s g a v e e v i d e n c e of tarnish, as did 68% of the m a g n e t i c k e e p e r s in p a t i e n t s w i t h natural teeth. Corrosion w a s found in 41.7% of the m a g n e t i c k e e p e r s . Although 88% of the patients w e r e satisfied at 3 months, only 28% w e r e satisfied at 2 years. R a r e earth m a g n e t s probably can be u s e d for retention o f r e m o v a b l e prostheses. H o w e v e r , the m a g n e t s are l i k e l y to tarnish and corrode, m a k i n g t h e m u n a c c e p t a b l e for use intraorally at this time. (J PROSTHET DENT 1991;66:536-40.)
T h e use of m a g n e t s in p r o s t h o d o n t i c s h a s b e e n inc r e a s i n g over t h e p a s t s e v e r a l years. 1 W i n k l e r 2 u s e d m a g n e t s as a n aid t o i m p r o v e d e n t u r e f u n c t i o n b y h a v i n g like m a g n e t i c poles c o n t a c t e a c h o t h e r in o p p o s i n g d e n t u r e bases. B e c a u s e like poles r e p e l e a c h o t h e r , h e p r o p o s e d t h i s
Presented to the Minnesota section of the American Association for Dental Research, St. Paul, Minn. Supported by the Gundersen Medical Foundation, La Crosse, WiN. 10/1/23139
536
a r r a n g e m e n t to p r o m o t e d e n t u r e s t a b i l i t y . O t h e r s h a v e u s e d t h e p r i n c i p l e t h a t d i s s i m i l a r poles a t t r a c t o n e a n o t h e r a n d h a v e r e p o r t e d i m p r o v e d results. 3~5 Magnets made from cobalt/samarium and neodymium/ i r o n a n d b o r o n h a v e p r o v e n t o b e c a p a b l e of r e t e n t i o n e q u i v a l e n t to m e c h a n i c a l a t t a c h m e n t s u s e d in p r o s t h o dontics. 5 T a r n i s h a n d corrosion of m a g n e t s h a v e b e e n discussed. 4-6 H o w e v e r , few l o n g - t e r m s t u d i e s are a v a i l a b l e for r e v i e w reg a r d i n g t h e i n t r a o r a l t a r n i s h a n d c o r r o s i o n r e s i s t a n c e of
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