stic cranial implants aphic scan-generated e C. van
Putten,
Jr.,
DDS,
MS,”
and
made from casts Shoki
Yamada,
MD,
computed PhDb
The Ohio State University College of Dentistry, Columbus, Ohio, and Loma Linda University, School of Medicine, Loma Linda, Calif. The complexity of cranioplasty increases with increased defect size. It is difficult to produce a symmetric, accurate implant presurgically or at the time of surgery when the defect is greater than 50 cm2. The procedure is also more difficult to perform when the defect is located in the temporal, infratemporal, or frontal areas. A new procedure generates a three-dimensional cast of the skull through computed tomography and computer-aided design reformation. This article describes the process of model generation and the production of a preprocessed cranial implant. To date, six cranial implants have been made with this technique. The whole head models are accurate and help the neurosurgeon-prosthodontist team in the creation of a symmetric, anatomically correct restoration. It is the technique of choice for large implants or where the cranial bones are thin. It is not necessary to augment or alter the implant during surgery. The technique reduces surgical time, and postsurgical complications have been minimal. (J PROSTHET DENT 1992;28:103-8)
he use of cranial implants is well documented.l-l8 The types of materials used is dictated by several factors, the most important of which are size and location of the defect.lO This article presents a new technique for prefabricating cranial implants from computed tomographic (CT) scans. This technique is particularly useful for cranial defects in esthetic areas or for defects previously considered difficult to restore.
LITERATURE
REVIEW
Currently used alloplastic materials for the reconstruction of cranial defects include stainless steel and titanium wire mesh, I-3 hydroxylapatite, 4-6 alumina ceramic,7 silicone,8 and polymethyl methacrylate (PMMA).g-13 Of the materials mentioned, PMMA is the most commonly used because of its excellent tissue compatibility, ease with which it is manipulated at surgery, strength, radiolucency, availability, low thermal and electrical conductance, and weight. Autopolymerizing PMMA is popular among neurosurgeons because the implant can be made at the time of surgery. Molding the implant at the time of surgery is safe and effective if the defect exists in the vault of the skull or is small. Small defects (5 to 15 cm2) are restored for esthetic reasons, and the implant is made by the surgeon directly on the defect. Large defects (125 cm2) require repair for protection of the brain from external injury and for prevention of cortical mantle thinning, which may cause unnecessary
aAssistant Professor, Dentistry, bProfessor,
Loma
Department of Restorative and Prosthetic The Ohio State University College of Dentistry. Department of Neurosurgery, School of Medicine, Linda University.
10/l/36758
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
neurologic symptoms.14 However, cosmetic and psychological factors are of equal importance, and defects in the frontal or temporal bones should be restored to hide the disfigurement. The complexity of cranioplasty increases with increasing defect size.15 It is difficult to produce a symmetric, accurate implant intraoperatively when the defect is 50 cm2 or larger. Restoration of large defects in the anterior temporal region, where the bones are thin or where the defect is located at the junction of the posterior portion of the sphenoid bone with the temporal bone, are difficult. Other complex areas are bony structures that have moderate curvatures or that surround vital structures (e.g., eye, temporal lobe). In addition, because the surgical site is draped, there may be a loss of the orientation and perspective needed to create the proper cranial contours. Three physical characteristics of autopolymerizing resin, the heat of polymerization, residual-free monomer, and porosity, are important to the success of the procedure. The curing process of autopolymerizing PMMA resins is exothermic. Asimacopoulos et a1.16 showed that the temp&ature of autopolymerizing PMMA applied directly to a skull defect can rise to 64” C (147’ F). This temperature was found despite continuous irrigation with cold saline solution; therefore implants made intraoperatively should be removed from the surgical site before final cure, to minimize tissue damage. Early removal could result in distortion of the implant, which would affect the stability, fit, and contour of the implant. The amount of residual methyl methacrylate monomer (MMA) in autopolymerized PMMA resins has been reported to range from 1% to 4 % immediately after curing.17-lg MMA monomer has been reported to cause contact dermatitis and allergic reactions on skin and oral mucosa.20
103
VAN
Computer
Cranial Cast
2. Computer-reformatted
CT image.
It has been reported that MMA causes acute local infections in the tissues next to cranial implants, but this response is usually short lived.21 Complications from autopolymerized acrylic resin monomer could be a factor together with other variables such as surgical time, defect size, flap thickness, and presurgical infection. Any factor or combination of factors that could lead to infection or dehiscence should be considered because implant removal may be required. Porosity and the inclusion of blood and tissue fluid reduces the strength of a prosthesis. Porosity with small
104
AND
YAMADA
Reformat
Prosthesis
Fig. 1. Cranial cast procedure includes four steps: (1) CT scan, (2) computerized mat of scan, (3) cast fabrication, and (4) prosthesis production.
Fig.
PUTTEN
Fig. 3. Initial glued together
refor-
cast is made from to form solid cast.
polycarbonate
sheets
(0.25 to 0.5 mm) bubbles can be found within most autopolymerized PMMA implants. Mason et a1.22 found that implant porosity caused erroneous evaluation of postsurgical CT scans. They stated that the bubbles mimicked cranial abscesses and stressed the importance of knowing the patient’s surgical history. Autopolymerized PMMA implants have lower tensile and flexural strengths than heat-processed resins because of the porosity, residual monomer, and fluid and soft tissue inclusions. Several authors have reported broken autopolymerized PMMA implants. 11, 14, 15, 23
JULY
1992
VOLUME
68
NUMBER
1
CRANIAL
IMPLANTS
FROM
CT SCAN-GENERATED
CASTS
Fig. 4. Lateral radiograph shows large temporal-parietal defect.
Several authors suggested the use of prefabricated heatprocessed PMMAresin implantsll, 14,15,24Heat-processed PMMA possesses all the favorable physical properties of autopolymerizing PMMA resin. In addition, heat-processing resins have low levels of residual-free monomer (0.4 % ) and are not porous when cured correctly.25 It is biocompatable, and allergic reactions are rare if the material does not contain pigments or fillers. The surgical bed is not subjected to the heat of the curing reaction or uncured PMMA and MMA. Beumer et a1.26reported 40 cranial restorations made with heat-processed PMMA resin during several years. These implants were well tolerated and postsurgical complications were minimal. An article by van GooIl reported 7-year data from 45 patients with fabricated PMMA resin implants (average size 60 cm2) and stated that postoperative and long-term complications were minimal. Prefabricated cranial implants require a working or master cast for the fabrication of the implant; therefore an impression of the area of the defect must be obtained before surgery. Several impression techniques are described in the literature.24~26~28 In addition to the working cast, careful digital palpation and mapping of the area are necessary to locate the internal and external defect margins. When the thickness of the overlying skin or muscle, or tissue edema, hinders the clinician from finding the margins, lateral and frontal cranial radiographs with or without superimposed metal grids have been suggested.2g The compiled data are used to alter the working cast to approximate a facsimile of the defect. Aquilino et a1.13believed that this process was cumbersome and suggested making the implant on a “negative” cast made from the master cast, thereby negating the need for alteration. With all these techniques the margins and contours are a product of extrapolation from the various sources of data. Most cranial implants made by these methods need adjustment or augmentation at the time of surgery to compensate for lack of marginal fit and improper contours. Martin et a1.30sug-
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 4.
5. Cranial cast showsreproduction of defect in Fig.
Fig. 6. Cast with block-out clay added to reduce cranial depth.
gestedthe useof a prefabricated heat-cured PMMA cranial implant together with autocured PMMA resinaddedto the implant marginsat the time of surgery. The autopolymerizing PMMA resin fills any voids found between the implant and bone margins. It was reported that this method evoked a lesssevereinflammatory responsewhen comparedwith other autopolymerizing PMMA systems. It is our experiencethat cranial implants made from external impressiontechniqueshave flatter contoursthan the skull. Becauseit is not possibleto make an impressionof the entire head, the external contours of the implant and the thickness of the bone edgeapproximating the implant are not known. A complex cranial region for prefabricated implants is the inferior lateral wall of the temporal fossa.The bonesin this area are thin, and attempts to create an implant from
105
VAN
Fig. 7. Final wax-up is completed directly onto cast.
Fig. 8. Process-perforated
implant.
information obtained externally are inaccurate. Because of its complexity, Beumer et a1.24suggested that replacing these delicate, thin bones was not always practical. Invariably, implants made for this region need thinning and/or augmentation of both internal and external surfaces and margins. A new procedure solves many cranial prosthetic problems. It involves the generation of a three-dimensional model of the cranial defect through CT scans and computer-aided CAD/CAM design reformation. This procedure is similar to the CT scan protocol reported by Truit et a1.31 and James et a1.32for the creation of subperiosteal implants. This article describes the process of model generation for cranial implants and for fabrication of the prosthesis. CRANIAL TECHNIQUE
CAST
FABRICATION
A written CT scan protocol is provided to the CT scan test site by the cranial cast manufacturer (Techmedia Co.,
PUTTEN
AND
YAMADA
Fig. 9. Implant is seated and sutured in place. No modification of implant is necessary.
Camarillo, Calif.) (Fig. I). The protocol provides the specific scanner settings required for the CT examination. The model is manufactured from standard image data produced by a GE CT 9800 scanner (General Electric Corp., Milwaukee, Wise.). No special software or hardware is required for the CT scan other than a small plastic motion detection rod that is taped to the head in the region of the scan. The motion detection rod is also provided by the model manufacturer. The number of slices needed to record the defect is determined by the technician. Increasing the number of CT scans improves the model accuracy because each slice is closer. CT scans made 3 mm apart are adequate for bone edges perpendicular to the slice. However, scans made of bone edges parallel to the slice need to be closer, to ensure accuracy of the model. A slice separation of 1 mm provides adequate accuracy for bone edges parallel to the CT scan beam. The completed scan can be reformatted and viewed as a solid body or in parts as defined by the technician using the image reformation functions available on the CT device (Fig. 2). The data are archived onto a magnetic tape and sent directly to the cast manufacturer. The information is translated, and resulting bone edge contours are converted to machine tool language and are used to drive a milling machine. Originally the machine milled the casts from stacks of polycarbonate slices, which were indexed and glued together (Fig. 3). Now the technique creates a life-size cast from a solid plastic resin block (Figs. 4 and 5). The model manufacturer returns the cast for implant fabrication. The cast can have a solid or hollow core depending on the size and location of the defect. IMPLANT
FABRICATION
TECHNIQUE
1. Prepare solid core casts for implant fabrication by blocking out or reducing the depth under the defect because this area cannot be finished. With hollow core casts this is not necessary because it is possible to finish directly the internal contour of the waxed prosthesis. For block-out, add modeling clay (Permoplast, AmeriJULY
1992
VOLUME
68
XUMBER
I
CRANIAL
IMPLANTS
FROM
CT SCAN-GENERATED
CASTS
Fig. 10. Oval temporal-parietal-sphenoid defect on lateral aspect of skull, approximately 50 cm2.
2.
3.
4.
5.
6.
can Arts Clay Co., Inc., Indianapolis, Ind.) (Fig. 6) to the internal aspect of the defect, reducing the depth to approximately 2 to 10 mm, depending on the thickness of the bone at the defect margin. The internal curvature of the cranial bone is created with clay and forms an internal template for wax modeling. Wax the implant with baseplate wax (Truwax, Dentsply International, York: Pa.). Finish the wax-up to create a symmetric and smooth reproduction of the lost cranial bone(s) (Fig. 7). Invest the complete wax pattern in gypsum in a suitable flask and remove the wax. Process the implant with surgical grade, heat-cured, MMA and methyl methacrylate polymer without plasticizer or fillers (Permatone, Kerr Corp., Romulus, Mich.) mixed according to the manufacturer’s specifications. Process the implant for 10 to 14 hours at 160’ F and boil for 1 hour. Boiling reduces the residual monomer to approximately 0.4 % . If the implant is large (>60 cm”) with thick sections (>7 mm), a 150” F, 15- to 20-hour cycle is used before boiling. After processing, deflask, trim, and polish the implant with rotary instruments. If the proper amount of blockout was used on the cast, minimal trimming should be needed. Perforate the implant with a No. 8 round bur to reduce the potential accumulation of fluid beneath the prosthesis after placement and to allow for ingrowth of fibrous connective tissue (Fig. 8). Countersink holes around the edges for placement of wire fixation to the skull. Package and prepare the finished implant for sterilization. Because heat would distort and discolor the implant, sterilize with ethylene oxide and aerate 2 to 3 days before surgery.
Fig. 11. Immediate nial symmetry.
postsurgical
view. Note lateral
cra-
ing two implants were placed in large frontal-parietal defects of equal or larger dimensions. There have been no postsurgical complications, and none has dehised. The fit of these implants is excellent (Fig. 9). It has not been necessary to add autopolymerizing resin to any of the implants at the time of surgery. Furthermore, the external and internal contours have not needed reduction or trimming, even in the region of the infratemporal fossa (Figs. 10 and 11).
CONCLUSION The use of CT scans to produce a cast to make cranial implants is a technologic improvement over other currently available techniques. The whole head models are accurate, and assist the neurosurgeon and prosthodontist in the creation of a symmetric, anatomically correct restoration. It is the technique of choice when implants are large or cranial bones are thin. It has not been necessary to augment or alter the implant during the surgical procedure. Furthermore, surgery time is reduced by 1 to 2 hours. Postsurgical complications have been minimal, and none of the implants has been lost to date. REFERENCES
CLINICAL
RESULTS
To date, six cranial implants have been made with this technique. Four of the implants restored large parietaltemporal-sphenoid defects larger than 60 cm2. The remainTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
1. Datti R, Cavagnargo G, Camici S. Stainless steel wire mesh cranioplasty. Ten year experience with 183 patients (100 followed up). Acta Neurochir (Wien) 1985;798:133-5. 2. Forni C, Pagni CA. An improved method for stainless steel wire mesh cranioplasty. Neurochirurgia (Stuttg) 1985;28:174-7.
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3. M&s LI. Titanium mesh and acrylic cranioplasty. Neurosurgery 1989;25:351-5. 4. Zide MF, Kent JN, Machado L. Hydroxylapatite cranioplasty directly over dura. J Oral Maxillofac Surg 1987;45(6):481-6. 5. Waite PD, Morawetz RB, Zeiger HE, Pincock JL. Reconstruction of cranial defects with porous hydroxydapatite blocks. Neurosurgery 1989;25:214-7. 6. Yamashima T. Cranioplasty with hydroxylapatite ceramic plates that can easily be trimmed during surgery: a preliminary report. Acta Neurochir (Wien) 1989;96:149-53. 7. Kobayashi S, Hara H, Okudera H, Takemae T, Sugita K. Usefulness of ceramic implants in neurosurgery. Neurosurgery 1987;21:751-5. 8. Chicarilli ZN, Ariyan S. Cranioplasty with a silicone prosthesis and split rib grafts. Head Neck Surg 1986;8:355-62. 9. Alesh F, Bauer R. Polyacryl prostheses for cranioplasty: their production in silicon rubber casts. Acta Neurochir (Wien) 1985;77:658-71. 10. Beumer J, Curtis TA, Firtell DN. Maxillofacial rehabilitation: prosthodontic and surgical considerations. St. Louis: CV Mosby, 1979379. 11. van Go01 AV. Preformed polymethylmethacrylate cranial restorations: report of 45 cases. J Maxillofac Surg 1985;13:2-8. 12. Remsen K, Lawson W, Biller HF. Acrylic frontal cranioplasty. Head Neck Surg 1986;9:32-41. 13. Aquilino SA, Jordan RD, White JT. Fabrication of an alloplastic implant for the cranial defect. J PROSTHET DENT 1988;59:68-71. 14. Foustanos AP, Anagnostopoulos D, Kotsianos G, Rapidis AD. Cranioplasty: a review of 10 cases. J Maxillofacial Surg 1983;11:83-6. 15. Cooper PR, Schlechter GB, Jacobs GB, Rubin RC, Witte RL. A pre-formed methyl methacrylate cranioplasty. Surg Nemo1 1977;8:21921. 16. Asimacopoulos TJ, Papadakis N, Mark VH. A new method of cranioplasty. J Neurosurg 1977;47:790-2. 17. Craig RG. Restorative dental materials. St Louis: CV Mosby, 1989528-g. 18. Lamb DJ, Ellis B, Priestly, D. The effects of processing variables on levels of residual monomer in autopolymerizing dental acrylic resin. J Dent 1983;11:80-8. 19. Fletcher AM, Purnaveja S, Amin WM, Ritchie GM, Moradians S, Wood AW. The level of residual monomer in self-curing denture base materials. J Dent Res 1983;62:118-20.
PUTTEN
AND
YAMADA
20. Kaaber S, Thulin H, Nielsen E. Skin sensitivity to denture base materials in burning mouth syndrome. Contact Dermatitis 1979;5:90-6. 21. Sessions RB, Wolfe SK, Moiel RH, Cheer WR. Wire mesh foundation for methyl methacrylate cranioplasty. Laryngoscope 1974;84:1020-30. 22. Mason TO, Rose BS, Goodman JH. Gas bubbles in polymethylmethacrylate cranioplasty simulating abscesses: CT appearance. AJNR 1986;7:829-831. 23. Jackson I, Hoffmann GT. Depressed cornminuted fracture of a plastic cranioplasty. J Neurosurg 1956;13:116-7. 24. Beumer J, Firtell DN, Curtis TA. Current concepts in cranioplasty. J
PROSTHET D~~~1979;42:67-77. 25. Huggett R, Brooks SC, Bates JF. The effect of different curing cycles on levels of residual monomer in acrylic resin denture base materials; Quint Dent Technol 1984;8:81-5. 26. Beumer J, Curtis TA, Firtell DN. Maxillofacial rehabilitation: prosthodontic 389-90. 27. Brown
and KE.
surgical Fabrication
considerations.
St Louis:
of an alloplastic
CV
Mosby,
cranioimplant.
1979:383, 3 PROSTHET
DENT 1970;24:213-24. 28. Jordan RD, White JT, Schupper N. Technique for cranioplasty prosthesis fabrication. J PROSTHET DENT 1978;40:230-3. 29. Firtell DN, Moore DJ, Bartlett SO. A radiographic grid for contouring cranial prosthesis. J PROSTHET DENT 1971;25:439-445. 30. Martin JW, Gantz SD, King GE, Jacob RF, Kramer DC. Cranial implant modification. J PROSTHET DENT 1984;52(3):414-6. 31. Truitt HP, James R, Altman A, Boyne P. Use of computer tomography in subperiostealimplant therapy. J PROSTHET DENT 1988;59(4):474-7. 32. James JA, Lozada JL, Truitt PH, Foust BE, Jovanovich SA. Subperiosteal implants. Calif Dent J 1988;10:4. Reprint
requests
to:
DR. MEADE C. VAN PUTTEN, JR. OHIO STATE UNIVERSITY COLLEGE POSTLE HALL 305 w.
OF DENTISTRY
12THST.
C0~~~~~~,0H43210-1241
JULY
1992
VOLUME
66
NUMBER
1
Putten,
Jr.,
DDS,
MS,”
and
made from casts Shoki
Yamada,
MD,
computed PhDb
The Ohio State University College of Dentistry, Columbus, Ohio, and Loma Linda University, School of Medicine, Loma Linda, Calif. The complexity of cranioplasty increases with increased defect size. It is difficult to produce a symmetric, accurate implant presurgically or at the time of surgery when the defect is greater than 50 cm2. The procedure is also more difficult to perform when the defect is located in the temporal, infratemporal, or frontal areas. A new procedure generates a three-dimensional cast of the skull through computed tomography and computer-aided design reformation. This article describes the process of model generation and the production of a preprocessed cranial implant. To date, six cranial implants have been made with this technique. The whole head models are accurate and help the neurosurgeon-prosthodontist team in the creation of a symmetric, anatomically correct restoration. It is the technique of choice for large implants or where the cranial bones are thin. It is not necessary to augment or alter the implant during surgery. The technique reduces surgical time, and postsurgical complications have been minimal. (J PROSTHET DENT 1992;28:103-8)
he use of cranial implants is well documented.l-l8 The types of materials used is dictated by several factors, the most important of which are size and location of the defect.lO This article presents a new technique for prefabricating cranial implants from computed tomographic (CT) scans. This technique is particularly useful for cranial defects in esthetic areas or for defects previously considered difficult to restore.
LITERATURE
REVIEW
Currently used alloplastic materials for the reconstruction of cranial defects include stainless steel and titanium wire mesh, I-3 hydroxylapatite, 4-6 alumina ceramic,7 silicone,8 and polymethyl methacrylate (PMMA).g-13 Of the materials mentioned, PMMA is the most commonly used because of its excellent tissue compatibility, ease with which it is manipulated at surgery, strength, radiolucency, availability, low thermal and electrical conductance, and weight. Autopolymerizing PMMA is popular among neurosurgeons because the implant can be made at the time of surgery. Molding the implant at the time of surgery is safe and effective if the defect exists in the vault of the skull or is small. Small defects (5 to 15 cm2) are restored for esthetic reasons, and the implant is made by the surgeon directly on the defect. Large defects (125 cm2) require repair for protection of the brain from external injury and for prevention of cortical mantle thinning, which may cause unnecessary
aAssistant Professor, Dentistry, bProfessor,
Loma
Department of Restorative and Prosthetic The Ohio State University College of Dentistry. Department of Neurosurgery, School of Medicine, Linda University.
10/l/36758
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
neurologic symptoms.14 However, cosmetic and psychological factors are of equal importance, and defects in the frontal or temporal bones should be restored to hide the disfigurement. The complexity of cranioplasty increases with increasing defect size.15 It is difficult to produce a symmetric, accurate implant intraoperatively when the defect is 50 cm2 or larger. Restoration of large defects in the anterior temporal region, where the bones are thin or where the defect is located at the junction of the posterior portion of the sphenoid bone with the temporal bone, are difficult. Other complex areas are bony structures that have moderate curvatures or that surround vital structures (e.g., eye, temporal lobe). In addition, because the surgical site is draped, there may be a loss of the orientation and perspective needed to create the proper cranial contours. Three physical characteristics of autopolymerizing resin, the heat of polymerization, residual-free monomer, and porosity, are important to the success of the procedure. The curing process of autopolymerizing PMMA resins is exothermic. Asimacopoulos et a1.16 showed that the temp&ature of autopolymerizing PMMA applied directly to a skull defect can rise to 64” C (147’ F). This temperature was found despite continuous irrigation with cold saline solution; therefore implants made intraoperatively should be removed from the surgical site before final cure, to minimize tissue damage. Early removal could result in distortion of the implant, which would affect the stability, fit, and contour of the implant. The amount of residual methyl methacrylate monomer (MMA) in autopolymerized PMMA resins has been reported to range from 1% to 4 % immediately after curing.17-lg MMA monomer has been reported to cause contact dermatitis and allergic reactions on skin and oral mucosa.20
103
VAN
Computer
Cranial Cast
2. Computer-reformatted
CT image.
It has been reported that MMA causes acute local infections in the tissues next to cranial implants, but this response is usually short lived.21 Complications from autopolymerized acrylic resin monomer could be a factor together with other variables such as surgical time, defect size, flap thickness, and presurgical infection. Any factor or combination of factors that could lead to infection or dehiscence should be considered because implant removal may be required. Porosity and the inclusion of blood and tissue fluid reduces the strength of a prosthesis. Porosity with small
104
AND
YAMADA
Reformat
Prosthesis
Fig. 1. Cranial cast procedure includes four steps: (1) CT scan, (2) computerized mat of scan, (3) cast fabrication, and (4) prosthesis production.
Fig.
PUTTEN
Fig. 3. Initial glued together
refor-
cast is made from to form solid cast.
polycarbonate
sheets
(0.25 to 0.5 mm) bubbles can be found within most autopolymerized PMMA implants. Mason et a1.22 found that implant porosity caused erroneous evaluation of postsurgical CT scans. They stated that the bubbles mimicked cranial abscesses and stressed the importance of knowing the patient’s surgical history. Autopolymerized PMMA implants have lower tensile and flexural strengths than heat-processed resins because of the porosity, residual monomer, and fluid and soft tissue inclusions. Several authors have reported broken autopolymerized PMMA implants. 11, 14, 15, 23
JULY
1992
VOLUME
68
NUMBER
1
CRANIAL
IMPLANTS
FROM
CT SCAN-GENERATED
CASTS
Fig. 4. Lateral radiograph shows large temporal-parietal defect.
Several authors suggested the use of prefabricated heatprocessed PMMAresin implantsll, 14,15,24Heat-processed PMMA possesses all the favorable physical properties of autopolymerizing PMMA resin. In addition, heat-processing resins have low levels of residual-free monomer (0.4 % ) and are not porous when cured correctly.25 It is biocompatable, and allergic reactions are rare if the material does not contain pigments or fillers. The surgical bed is not subjected to the heat of the curing reaction or uncured PMMA and MMA. Beumer et a1.26reported 40 cranial restorations made with heat-processed PMMA resin during several years. These implants were well tolerated and postsurgical complications were minimal. An article by van GooIl reported 7-year data from 45 patients with fabricated PMMA resin implants (average size 60 cm2) and stated that postoperative and long-term complications were minimal. Prefabricated cranial implants require a working or master cast for the fabrication of the implant; therefore an impression of the area of the defect must be obtained before surgery. Several impression techniques are described in the literature.24~26~28 In addition to the working cast, careful digital palpation and mapping of the area are necessary to locate the internal and external defect margins. When the thickness of the overlying skin or muscle, or tissue edema, hinders the clinician from finding the margins, lateral and frontal cranial radiographs with or without superimposed metal grids have been suggested.2g The compiled data are used to alter the working cast to approximate a facsimile of the defect. Aquilino et a1.13believed that this process was cumbersome and suggested making the implant on a “negative” cast made from the master cast, thereby negating the need for alteration. With all these techniques the margins and contours are a product of extrapolation from the various sources of data. Most cranial implants made by these methods need adjustment or augmentation at the time of surgery to compensate for lack of marginal fit and improper contours. Martin et a1.30sug-
THE
JOURNAL
OF PROSTHETIC
DENTISTRY
Fig. 4.
5. Cranial cast showsreproduction of defect in Fig.
Fig. 6. Cast with block-out clay added to reduce cranial depth.
gestedthe useof a prefabricated heat-cured PMMA cranial implant together with autocured PMMA resinaddedto the implant marginsat the time of surgery. The autopolymerizing PMMA resin fills any voids found between the implant and bone margins. It was reported that this method evoked a lesssevereinflammatory responsewhen comparedwith other autopolymerizing PMMA systems. It is our experiencethat cranial implants made from external impressiontechniqueshave flatter contoursthan the skull. Becauseit is not possibleto make an impressionof the entire head, the external contours of the implant and the thickness of the bone edgeapproximating the implant are not known. A complex cranial region for prefabricated implants is the inferior lateral wall of the temporal fossa.The bonesin this area are thin, and attempts to create an implant from
105
VAN
Fig. 7. Final wax-up is completed directly onto cast.
Fig. 8. Process-perforated
implant.
information obtained externally are inaccurate. Because of its complexity, Beumer et a1.24suggested that replacing these delicate, thin bones was not always practical. Invariably, implants made for this region need thinning and/or augmentation of both internal and external surfaces and margins. A new procedure solves many cranial prosthetic problems. It involves the generation of a three-dimensional model of the cranial defect through CT scans and computer-aided CAD/CAM design reformation. This procedure is similar to the CT scan protocol reported by Truit et a1.31 and James et a1.32for the creation of subperiosteal implants. This article describes the process of model generation for cranial implants and for fabrication of the prosthesis. CRANIAL TECHNIQUE
CAST
FABRICATION
A written CT scan protocol is provided to the CT scan test site by the cranial cast manufacturer (Techmedia Co.,
PUTTEN
AND
YAMADA
Fig. 9. Implant is seated and sutured in place. No modification of implant is necessary.
Camarillo, Calif.) (Fig. I). The protocol provides the specific scanner settings required for the CT examination. The model is manufactured from standard image data produced by a GE CT 9800 scanner (General Electric Corp., Milwaukee, Wise.). No special software or hardware is required for the CT scan other than a small plastic motion detection rod that is taped to the head in the region of the scan. The motion detection rod is also provided by the model manufacturer. The number of slices needed to record the defect is determined by the technician. Increasing the number of CT scans improves the model accuracy because each slice is closer. CT scans made 3 mm apart are adequate for bone edges perpendicular to the slice. However, scans made of bone edges parallel to the slice need to be closer, to ensure accuracy of the model. A slice separation of 1 mm provides adequate accuracy for bone edges parallel to the CT scan beam. The completed scan can be reformatted and viewed as a solid body or in parts as defined by the technician using the image reformation functions available on the CT device (Fig. 2). The data are archived onto a magnetic tape and sent directly to the cast manufacturer. The information is translated, and resulting bone edge contours are converted to machine tool language and are used to drive a milling machine. Originally the machine milled the casts from stacks of polycarbonate slices, which were indexed and glued together (Fig. 3). Now the technique creates a life-size cast from a solid plastic resin block (Figs. 4 and 5). The model manufacturer returns the cast for implant fabrication. The cast can have a solid or hollow core depending on the size and location of the defect. IMPLANT
FABRICATION
TECHNIQUE
1. Prepare solid core casts for implant fabrication by blocking out or reducing the depth under the defect because this area cannot be finished. With hollow core casts this is not necessary because it is possible to finish directly the internal contour of the waxed prosthesis. For block-out, add modeling clay (Permoplast, AmeriJULY
1992
VOLUME
68
XUMBER
I
CRANIAL
IMPLANTS
FROM
CT SCAN-GENERATED
CASTS
Fig. 10. Oval temporal-parietal-sphenoid defect on lateral aspect of skull, approximately 50 cm2.
2.
3.
4.
5.
6.
can Arts Clay Co., Inc., Indianapolis, Ind.) (Fig. 6) to the internal aspect of the defect, reducing the depth to approximately 2 to 10 mm, depending on the thickness of the bone at the defect margin. The internal curvature of the cranial bone is created with clay and forms an internal template for wax modeling. Wax the implant with baseplate wax (Truwax, Dentsply International, York: Pa.). Finish the wax-up to create a symmetric and smooth reproduction of the lost cranial bone(s) (Fig. 7). Invest the complete wax pattern in gypsum in a suitable flask and remove the wax. Process the implant with surgical grade, heat-cured, MMA and methyl methacrylate polymer without plasticizer or fillers (Permatone, Kerr Corp., Romulus, Mich.) mixed according to the manufacturer’s specifications. Process the implant for 10 to 14 hours at 160’ F and boil for 1 hour. Boiling reduces the residual monomer to approximately 0.4 % . If the implant is large (>60 cm”) with thick sections (>7 mm), a 150” F, 15- to 20-hour cycle is used before boiling. After processing, deflask, trim, and polish the implant with rotary instruments. If the proper amount of blockout was used on the cast, minimal trimming should be needed. Perforate the implant with a No. 8 round bur to reduce the potential accumulation of fluid beneath the prosthesis after placement and to allow for ingrowth of fibrous connective tissue (Fig. 8). Countersink holes around the edges for placement of wire fixation to the skull. Package and prepare the finished implant for sterilization. Because heat would distort and discolor the implant, sterilize with ethylene oxide and aerate 2 to 3 days before surgery.
Fig. 11. Immediate nial symmetry.
postsurgical
view. Note lateral
cra-
ing two implants were placed in large frontal-parietal defects of equal or larger dimensions. There have been no postsurgical complications, and none has dehised. The fit of these implants is excellent (Fig. 9). It has not been necessary to add autopolymerizing resin to any of the implants at the time of surgery. Furthermore, the external and internal contours have not needed reduction or trimming, even in the region of the infratemporal fossa (Figs. 10 and 11).
CONCLUSION The use of CT scans to produce a cast to make cranial implants is a technologic improvement over other currently available techniques. The whole head models are accurate, and assist the neurosurgeon and prosthodontist in the creation of a symmetric, anatomically correct restoration. It is the technique of choice when implants are large or cranial bones are thin. It has not been necessary to augment or alter the implant during the surgical procedure. Furthermore, surgery time is reduced by 1 to 2 hours. Postsurgical complications have been minimal, and none of the implants has been lost to date. REFERENCES
CLINICAL
RESULTS
To date, six cranial implants have been made with this technique. Four of the implants restored large parietaltemporal-sphenoid defects larger than 60 cm2. The remainTHE
JOURNAL
OF PROSTHETIC
DENTISTRY
1. Datti R, Cavagnargo G, Camici S. Stainless steel wire mesh cranioplasty. Ten year experience with 183 patients (100 followed up). Acta Neurochir (Wien) 1985;798:133-5. 2. Forni C, Pagni CA. An improved method for stainless steel wire mesh cranioplasty. Neurochirurgia (Stuttg) 1985;28:174-7.
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3. M&s LI. Titanium mesh and acrylic cranioplasty. Neurosurgery 1989;25:351-5. 4. Zide MF, Kent JN, Machado L. Hydroxylapatite cranioplasty directly over dura. J Oral Maxillofac Surg 1987;45(6):481-6. 5. Waite PD, Morawetz RB, Zeiger HE, Pincock JL. Reconstruction of cranial defects with porous hydroxydapatite blocks. Neurosurgery 1989;25:214-7. 6. Yamashima T. Cranioplasty with hydroxylapatite ceramic plates that can easily be trimmed during surgery: a preliminary report. Acta Neurochir (Wien) 1989;96:149-53. 7. Kobayashi S, Hara H, Okudera H, Takemae T, Sugita K. Usefulness of ceramic implants in neurosurgery. Neurosurgery 1987;21:751-5. 8. Chicarilli ZN, Ariyan S. Cranioplasty with a silicone prosthesis and split rib grafts. Head Neck Surg 1986;8:355-62. 9. Alesh F, Bauer R. Polyacryl prostheses for cranioplasty: their production in silicon rubber casts. Acta Neurochir (Wien) 1985;77:658-71. 10. Beumer J, Curtis TA, Firtell DN. Maxillofacial rehabilitation: prosthodontic and surgical considerations. St. Louis: CV Mosby, 1979379. 11. van Go01 AV. Preformed polymethylmethacrylate cranial restorations: report of 45 cases. J Maxillofac Surg 1985;13:2-8. 12. Remsen K, Lawson W, Biller HF. Acrylic frontal cranioplasty. Head Neck Surg 1986;9:32-41. 13. Aquilino SA, Jordan RD, White JT. Fabrication of an alloplastic implant for the cranial defect. J PROSTHET DENT 1988;59:68-71. 14. Foustanos AP, Anagnostopoulos D, Kotsianos G, Rapidis AD. Cranioplasty: a review of 10 cases. J Maxillofacial Surg 1983;11:83-6. 15. Cooper PR, Schlechter GB, Jacobs GB, Rubin RC, Witte RL. A pre-formed methyl methacrylate cranioplasty. Surg Nemo1 1977;8:21921. 16. Asimacopoulos TJ, Papadakis N, Mark VH. A new method of cranioplasty. J Neurosurg 1977;47:790-2. 17. Craig RG. Restorative dental materials. St Louis: CV Mosby, 1989528-g. 18. Lamb DJ, Ellis B, Priestly, D. The effects of processing variables on levels of residual monomer in autopolymerizing dental acrylic resin. J Dent 1983;11:80-8. 19. Fletcher AM, Purnaveja S, Amin WM, Ritchie GM, Moradians S, Wood AW. The level of residual monomer in self-curing denture base materials. J Dent Res 1983;62:118-20.
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20. Kaaber S, Thulin H, Nielsen E. Skin sensitivity to denture base materials in burning mouth syndrome. Contact Dermatitis 1979;5:90-6. 21. Sessions RB, Wolfe SK, Moiel RH, Cheer WR. Wire mesh foundation for methyl methacrylate cranioplasty. Laryngoscope 1974;84:1020-30. 22. Mason TO, Rose BS, Goodman JH. Gas bubbles in polymethylmethacrylate cranioplasty simulating abscesses: CT appearance. AJNR 1986;7:829-831. 23. Jackson I, Hoffmann GT. Depressed cornminuted fracture of a plastic cranioplasty. J Neurosurg 1956;13:116-7. 24. Beumer J, Firtell DN, Curtis TA. Current concepts in cranioplasty. J
PROSTHET D~~~1979;42:67-77. 25. Huggett R, Brooks SC, Bates JF. The effect of different curing cycles on levels of residual monomer in acrylic resin denture base materials; Quint Dent Technol 1984;8:81-5. 26. Beumer J, Curtis TA, Firtell DN. Maxillofacial rehabilitation: prosthodontic 389-90. 27. Brown
and KE.
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St Louis:
of an alloplastic
CV
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1979:383, 3 PROSTHET
DENT 1970;24:213-24. 28. Jordan RD, White JT, Schupper N. Technique for cranioplasty prosthesis fabrication. J PROSTHET DENT 1978;40:230-3. 29. Firtell DN, Moore DJ, Bartlett SO. A radiographic grid for contouring cranial prosthesis. J PROSTHET DENT 1971;25:439-445. 30. Martin JW, Gantz SD, King GE, Jacob RF, Kramer DC. Cranial implant modification. J PROSTHET DENT 1984;52(3):414-6. 31. Truitt HP, James R, Altman A, Boyne P. Use of computer tomography in subperiostealimplant therapy. J PROSTHET DENT 1988;59(4):474-7. 32. James JA, Lozada JL, Truitt PH, Foust BE, Jovanovich SA. Subperiosteal implants. Calif Dent J 1988;10:4. Reprint
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DR. MEADE C. VAN PUTTEN, JR. OHIO STATE UNIVERSITY COLLEGE POSTLE HALL 305 w.
OF DENTISTRY
12THST.
C0~~~~~~,0H43210-1241
JULY
1992
VOLUME
66
NUMBER
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