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PSEUDOCYSTS OF THE MANDIBULAR CONDYLE IN CHILDREN
PSEUDOCYSTS OFTHE MANDIBULAR CONDYLE IN CHILDREN TIMOTHY E. COLLINS, D.D.S.; DANIEL M. LASKIN, D.D.S., M.S.; FRANK H. FARRINGTON, D.D.S., M.S.; NAVIN S. SHETTY, D.D.S., M.S.; ARTHUR MOURINO, D.D.S., M.S.
Qseudocysts of the mandibular condyle are well-circumscribed radiolucencies in the anterior aspect of the condyle. Researchers believe they are anatomic variants that are accentuated and distorted during panoramic radiography. The distortion occurs because the central X-ray beam does not pass through the condyle at a right angle; rather, it passes obliquely in the horizontal plane and superiorly in the vertical plane. The beam's angle causes the medial ridge of the mandibular neck to be projected posterosuperiorly and the lateral ridge to be projected anteroinferiorly. These ridges then appear radiographically as corticated margins that envelop the radiolucent cystlike pterygoid fovea. Computerized tomography, or CT, scans confirm that the radiolucent portion of the pseudocyst is caused by cupping of the pterygoid fovea, whereas the radiopaque borders are due to the dense medial and lateral ridges of the condyle. Friedlander and colleagues' published a study of pseudocysts of the mandibular condyle that was based on an adult population. The panoramic radiographs of 9 of 507 consecutive
The authors evaluated the panoramic radiographs of 1,193
consecutively treated patients 18 years old and younger for the presence of asymptomatic radiolucencies in the mandibular
condyles. The 27 radiographs initially selected were examined by four independent reviewers. Of
the 1,193 patients, 18 (1.5 percent [10 female and eight
male])
met the criteria for condylar
pseudocysts. Follow-up radiographs showed minimal or no change in size of the radiolucencles. Clinicians need to be aware of these anatomic variations so that patients are not subjected to inappropriate treatment.
adult patients (1.8 percent) exhibited the pseudocysts. Although pseudocysts of the mandibular condyle have been documented in the adult population, there is only a single case report of a pseudocyst in a child, a 5-year-old who was examined after an automobile accident.2 A
Towne projection showed a pseudocyst in the child's right condyle. This finding leads to the question of whether pseudocysts actually are rare in children. Also, because there are remodeling changes in the growing condyle owing to alterations in mandibular function and occlusion,3-5 are there differences in the frequency of such cysts in children vs. the frequency in adults? The purpose of this study was to determine the prevalence of pseudocysts in children by examining the panoramic radiographs of a large series of young patients and to compare the findings with those reported previously in an adult population. SUBJECTS AND METHODS
We reviewed the panoramic radiographs of 1,193 consecutive patients up to 18 years of age who were treated at the pediatric and orthodontic clinics at the Virginia Commonwealth University School of Dentistry. All radiographs were made on either a Versaview Panoramic X-Ray system (J. Morita), a Siemens Orthopos system (DSiemens, Germany), a GE 200 system (Gendex Corp.) or an JADA, Vol. 128, June 1997 747
-CLINICAL PRACTICE
Figure 1. A composite of four panoramic radiographs showing pseudocysts. A. Right condyle of a white boy, 14 years, 8 months old. B. Right condyle of a white girl, 11 years, 4 months old. C. Left condyle of a white girl, 14 years, 3 months old. D. Left condyle of a white boy, 15 years, 4 months old.
S.S. White system (S.S. White Dental Products). The radiographs were taken at a range of 7 to 16 milliamperes, using 66 to 90 kilovoltage peak, or kVp (depending on the patient's jaw size and on which machine was used). The radiographs were taken with KODAK Dental Film (KODAK Panaramic TMG/RA15) and KODAK X-OMATIC regular intensifying screens (Eastman Kodak Co.). All films were developed on either a KODAK M3SA X-OMATIC Processor or on a Konica Medical Film Processor QX-70 (Koniac Corp., Japan), according to manufacturer's directions. The same diagnostic criteria that Friedlander and colleagues' used to describe mandibular condylar pseudocysts in adults were used in this 748 JADA, Vol. 128, June 1997
study. Radiolucencies had to meet the following criteria: - be located in the anterior aspect of the condyle (as seen on the panoramic radiograph); - measure at least 0.5 centimeters x 0.4 cm; - be either completely or partially (more than 80 percent) circumscribed by a discrete sclerotic margin (Figure 1). Age and race data. The sample population consisted of 599 girls and 594 boys. The age range for the total group was from 3.3 to 18.7 years. The mean age for the group was 11.3 years and the mode was 11.8 years. The girls' ages ranged from 4.3 to 18 years, with a mean of 11.4 years and a mode of 11.8 years. The boys' ages ranged from 3.3 to 18.7 years, with a mean of 11.3 years
and a mode of 11.8 years. Race and ethnicity data for the group were as follows: 883 whites (74 percent), 269 AfricanAmericans (22.5 percent), 25 Asians (2.1 percent), eight Hispanics (0.7 percent) and eight of unknown or unspecified race/ethnicity (0.7 percent). This last group comprised patients who did not answer the race/ethnicity question on the history form and for whom a photograph was not available. The race/ethnicity breakdown by gender was as follows: girls-446 whites (74 percent), 137 African-Americans (23 percent), 10 Asians (2 percent), four Hispanics (0.7 percent) and two unknown (0.3 percent); boys-437 whites (73.5 percent), 132 Afiican-Americans (22.2 percent), 15 Asians (2.5 percent), four Hispanics (0.7 percent) and six unknown (1 percent). One reviewer (T.C.) examined the radiographs, using the selection criteria. Those radiographs that the reviewer considered to be positive were randomly mixed with 13 negative radiographs; they then were examined independently by four dental faculty members who were familiar with the previously mentioned criteria for a pseudocyst. A condyle was considered positive if at least three of the four independent reviewers agreed it was. RESULTS
In his initial review of the 1,193 panoramic radiographs, the reviewer selected all those that he considered to show a radiolucent area in the anterior condyle. The table shows that 27 radiographs were found to be positive and that two of them showed bilateral radiolucencies. When the four independent reviewers examined these 27 positive radio-
CLINICAL PRACTICE TABLE
graphs plus the 13 negative ones, they unanimously agreed that all of the negative radiographs were negative. The four independent reviewers agreed unanimously on 14 of the 27 positive radiographs (including the two with bilateral radiolucencies) and three of the four agreed on four of the other radiographs. Only two of the reviewers agreed that five of the remaining nine radiographs were positive and at least three of the four reviewers agreed that four were negative. Of the final 18 patients selected, there were nine white girls, eight white boys and one African-American girl. The mean age of these patients was 14.1 years, and the age ranged from 8.3 to 18.3 years. Of these patients, five were in the mixeddentition stage and 13 were in the permanent-dentition stage. None of the patients had any current serious medical problems, although three of them re-
ported having allergies or hay fever, one reported having an innocent heart murmur and another reported having been hospitalized for meningitis as an infant. On average, the radiolucencies measured 10.5 millimeters in height and 5.2 mm in width. Height ranged from 8 to 14 mm; width ranged from 4 to 7 mm. We were able to obtain followup panoramic radiographs,
taken 1 to 3 years after initial presentation, for 11 of the 18 patients. Minimal change was noted in the size of the radiolucencies over time. The average change in height was 0.1 centimeter and the average change in width was -0.03 cm.
I Some clinicians believe that deepening of e fovea area represents remodeling in response to afterations in occlusion and mandibular function. Our finding indicates that the deepening of the fovea may be detennined more by genetics than by
environmental stimulL In this study, 1.5 percent of the patients (18 of 1,193) were found to have met the pseudocyst criteria vs. 1.8 percent of the patients (nine of 507) in the Friedlander study. Pseudocysts were found in 17 of 18 white patients in the present study, whereas all nine patients who met the criteria in the Firiedlander study were white. DISCUSSION
In the study by Friedlander and
colleagues,' CT scans and tomograms showed that the radiolucencies noted on the panoramic radiographs were anatomic variants. The CT scans revealed a marked cupping of the pterygoid fovea and dense cortical bone on the medial and lateral ridges of the condyle (Figure 2). The pterygoid fovea is the site of insertion of both the superior and inferior heads of the lateral pterygoid muscle."6 Some clinicians believe that deepening of the fovea area represents remodeling in response to alterations in occlusion and mandibular function,"3-5 which is more likely to occur in older people. Our results, however, showed a prevalence of pseudocysts in children 18 years and under that was fairly similar to that in the adult population reported by Friedlander and colleagues.' This finding indicates that the deepening of the fovea may be determined more by genetics than by environmental stimuli. Conditions to consider. Practitioners need to consider a number of conditions when they are confronted with a condylar radiolucency in a child. Although benign and malignant neoplasms in this area are not common, the differential diagnosis should include Langerhans' cell histiocytosis (eosinophilic granuloma), cenJADA, Vol. 128, June 1997 749
-C[INICAL PRACTICE
However, it usually is multilocular and there often is a history of trauma. CONCLUSION
Figure 2. Mandible of a child showing the medial and lateral ridges of the neck of the condyle and the cupped pterygoid fovea.
tral giant cell granuloma, simple bone cyst, chondroma and aneurysmal bone cyst.7 These all may appear as radiolucent lesions of the mandible, although they rarely are confined to the anterior half of the condyle and are infrequently corticated. Eosinophilic granuloma occurs in older children, but does not have a sclerotic border. Central giant cell granuloma rarely involves the condyle and its radiographic appearance generally is multilocular rather than
Dr. Collins Is a for-
mer resident, pediatric dentistry, Virginia Commonwealth University, Medlcal College of
Virginia, School of Dentistry, Richmond, Va. He Is now a pediatric dentist, Virginia Department of
Health, Washington County, Va.
unilocular. Also, the lesion typically produces expansion of the jaw. Simple bone cysts are quite uncommon in the mandible and usually are not rimmed. Moreover, in one study, 80 percent of the cases were preceded by trauma.7 Chondromas sometimes occur in the mandible, but they are relatively rare in the condyle. The lesion commonly presents as a painless, slowly progressive mass; most often it appears radiographically as an irregular radiolucency, which may contain foci of calcification. Aneurysmal bone cyst occurs in young patients.
Dr. Laskin is professor and chairman, Department of Oral and Maxillofacial
Surgery, Virginia Commonwealth University, Medical College of Virginia, School of Dentistry, 521 N. 11th St.,
Richmond, Va. 23298. Please address reprint requests to Dr. Laskin.
750 JADA, Vol. 128, June 1997
Patients who present with asymptomatic radiolucencies that appear to be similar to the ones described in this study and that of Friedlander and colleagues' generally do not need to have further imaging or exploratory surgery before a diagnosis can be made. However, we advise practitioners to make a panoramic radiograph 4 to 6 months after the patient's initial presentation so as to reconfirm the diagnosis of a pseudocyst. Radiolucent images that do not conform to the criteria set forth by Friedlander and colleagues' should be investigated with ancillary imaging studies and histologic evaluation, when appropriate. o 1. Friedlander AH, Monson ML, Friedlander MD, Esquerra AC. Pseudocysts ofthe mandibular condyle. J Oral Maxillofac Surg 1992;50(8):821-4. 2. Al-Quattan MM, Clarke HM. Pseudocyst of the mandibular condyle in a child. J Oral Maxillofac Surg 1992;52(10):1072. 3. Takenoshita Y. Development with age of the human mandibular condyle. Part I: Gross anatomy and inner architecture. Dentomaxillofac Radiol 1982;11(1):5-15. 4. Takenoshita Y. Development with age of the human mandibular condyle. Part II. Assessment of the mineral content and microradiographic study. Dentomaxillofac Radiol 1982;11(1):17-24. 5. Takenoshita Y. Development with age of the human mandibular condyle: histiological study. J Craniomandibular Pract 1987;5(4):317-23. 6. Carpentier P, Yung JP, MarguellesBonnet R, Meunissier M. Insertions of the
lateral pterygoid
I
muscle: an anatomic study of the human
Dr. Mourino is an asDr. Farrington Is associete professor and chairman, Ds
pertbinent of Pediatric Dentisry, Virginia Commonwealth University, Medical College of Virginia, School of Dentistry,
Richmond, Vs.
Dr.
Shetty Is assis-
-tent professor, Depertinent of Pediatric Dentistry, Virginia Commonwealth University, Medical College of Virginia, School of Dentistry, Richmond, Va.
societe professor
temporomandibular
and director, Advanced Education
joint. J Oral Maxillofac Surg
In Pedlatric
1988;46(6):477-82. 7. Regezi JA,
Dentiatry, virginia Commonwealth
C
Sciubba JJ. Oral
University, Medical Coiiege of Virginia,
pathology. 2nd ed.
School of Dentistry, Richmond, Va.
422
Philadelphia:
Saunders; 1993:350-
Qseudocysts of the mandibular condyle are well-circumscribed radiolucencies in the anterior aspect of the condyle. Researchers believe they are anatomic variants that are accentuated and distorted during panoramic radiography. The distortion occurs because the central X-ray beam does not pass through the condyle at a right angle; rather, it passes obliquely in the horizontal plane and superiorly in the vertical plane. The beam's angle causes the medial ridge of the mandibular neck to be projected posterosuperiorly and the lateral ridge to be projected anteroinferiorly. These ridges then appear radiographically as corticated margins that envelop the radiolucent cystlike pterygoid fovea. Computerized tomography, or CT, scans confirm that the radiolucent portion of the pseudocyst is caused by cupping of the pterygoid fovea, whereas the radiopaque borders are due to the dense medial and lateral ridges of the condyle. Friedlander and colleagues' published a study of pseudocysts of the mandibular condyle that was based on an adult population. The panoramic radiographs of 9 of 507 consecutive
The authors evaluated the panoramic radiographs of 1,193
consecutively treated patients 18 years old and younger for the presence of asymptomatic radiolucencies in the mandibular
condyles. The 27 radiographs initially selected were examined by four independent reviewers. Of
the 1,193 patients, 18 (1.5 percent [10 female and eight
male])
met the criteria for condylar
pseudocysts. Follow-up radiographs showed minimal or no change in size of the radiolucencles. Clinicians need to be aware of these anatomic variations so that patients are not subjected to inappropriate treatment.
adult patients (1.8 percent) exhibited the pseudocysts. Although pseudocysts of the mandibular condyle have been documented in the adult population, there is only a single case report of a pseudocyst in a child, a 5-year-old who was examined after an automobile accident.2 A
Towne projection showed a pseudocyst in the child's right condyle. This finding leads to the question of whether pseudocysts actually are rare in children. Also, because there are remodeling changes in the growing condyle owing to alterations in mandibular function and occlusion,3-5 are there differences in the frequency of such cysts in children vs. the frequency in adults? The purpose of this study was to determine the prevalence of pseudocysts in children by examining the panoramic radiographs of a large series of young patients and to compare the findings with those reported previously in an adult population. SUBJECTS AND METHODS
We reviewed the panoramic radiographs of 1,193 consecutive patients up to 18 years of age who were treated at the pediatric and orthodontic clinics at the Virginia Commonwealth University School of Dentistry. All radiographs were made on either a Versaview Panoramic X-Ray system (J. Morita), a Siemens Orthopos system (DSiemens, Germany), a GE 200 system (Gendex Corp.) or an JADA, Vol. 128, June 1997 747
-CLINICAL PRACTICE
Figure 1. A composite of four panoramic radiographs showing pseudocysts. A. Right condyle of a white boy, 14 years, 8 months old. B. Right condyle of a white girl, 11 years, 4 months old. C. Left condyle of a white girl, 14 years, 3 months old. D. Left condyle of a white boy, 15 years, 4 months old.
S.S. White system (S.S. White Dental Products). The radiographs were taken at a range of 7 to 16 milliamperes, using 66 to 90 kilovoltage peak, or kVp (depending on the patient's jaw size and on which machine was used). The radiographs were taken with KODAK Dental Film (KODAK Panaramic TMG/RA15) and KODAK X-OMATIC regular intensifying screens (Eastman Kodak Co.). All films were developed on either a KODAK M3SA X-OMATIC Processor or on a Konica Medical Film Processor QX-70 (Koniac Corp., Japan), according to manufacturer's directions. The same diagnostic criteria that Friedlander and colleagues' used to describe mandibular condylar pseudocysts in adults were used in this 748 JADA, Vol. 128, June 1997
study. Radiolucencies had to meet the following criteria: - be located in the anterior aspect of the condyle (as seen on the panoramic radiograph); - measure at least 0.5 centimeters x 0.4 cm; - be either completely or partially (more than 80 percent) circumscribed by a discrete sclerotic margin (Figure 1). Age and race data. The sample population consisted of 599 girls and 594 boys. The age range for the total group was from 3.3 to 18.7 years. The mean age for the group was 11.3 years and the mode was 11.8 years. The girls' ages ranged from 4.3 to 18 years, with a mean of 11.4 years and a mode of 11.8 years. The boys' ages ranged from 3.3 to 18.7 years, with a mean of 11.3 years
and a mode of 11.8 years. Race and ethnicity data for the group were as follows: 883 whites (74 percent), 269 AfricanAmericans (22.5 percent), 25 Asians (2.1 percent), eight Hispanics (0.7 percent) and eight of unknown or unspecified race/ethnicity (0.7 percent). This last group comprised patients who did not answer the race/ethnicity question on the history form and for whom a photograph was not available. The race/ethnicity breakdown by gender was as follows: girls-446 whites (74 percent), 137 African-Americans (23 percent), 10 Asians (2 percent), four Hispanics (0.7 percent) and two unknown (0.3 percent); boys-437 whites (73.5 percent), 132 Afiican-Americans (22.2 percent), 15 Asians (2.5 percent), four Hispanics (0.7 percent) and six unknown (1 percent). One reviewer (T.C.) examined the radiographs, using the selection criteria. Those radiographs that the reviewer considered to be positive were randomly mixed with 13 negative radiographs; they then were examined independently by four dental faculty members who were familiar with the previously mentioned criteria for a pseudocyst. A condyle was considered positive if at least three of the four independent reviewers agreed it was. RESULTS
In his initial review of the 1,193 panoramic radiographs, the reviewer selected all those that he considered to show a radiolucent area in the anterior condyle. The table shows that 27 radiographs were found to be positive and that two of them showed bilateral radiolucencies. When the four independent reviewers examined these 27 positive radio-
CLINICAL PRACTICE TABLE
graphs plus the 13 negative ones, they unanimously agreed that all of the negative radiographs were negative. The four independent reviewers agreed unanimously on 14 of the 27 positive radiographs (including the two with bilateral radiolucencies) and three of the four agreed on four of the other radiographs. Only two of the reviewers agreed that five of the remaining nine radiographs were positive and at least three of the four reviewers agreed that four were negative. Of the final 18 patients selected, there were nine white girls, eight white boys and one African-American girl. The mean age of these patients was 14.1 years, and the age ranged from 8.3 to 18.3 years. Of these patients, five were in the mixeddentition stage and 13 were in the permanent-dentition stage. None of the patients had any current serious medical problems, although three of them re-
ported having allergies or hay fever, one reported having an innocent heart murmur and another reported having been hospitalized for meningitis as an infant. On average, the radiolucencies measured 10.5 millimeters in height and 5.2 mm in width. Height ranged from 8 to 14 mm; width ranged from 4 to 7 mm. We were able to obtain followup panoramic radiographs,
taken 1 to 3 years after initial presentation, for 11 of the 18 patients. Minimal change was noted in the size of the radiolucencies over time. The average change in height was 0.1 centimeter and the average change in width was -0.03 cm.
I Some clinicians believe that deepening of e fovea area represents remodeling in response to afterations in occlusion and mandibular function. Our finding indicates that the deepening of the fovea may be detennined more by genetics than by
environmental stimulL In this study, 1.5 percent of the patients (18 of 1,193) were found to have met the pseudocyst criteria vs. 1.8 percent of the patients (nine of 507) in the Friedlander study. Pseudocysts were found in 17 of 18 white patients in the present study, whereas all nine patients who met the criteria in the Firiedlander study were white. DISCUSSION
In the study by Friedlander and
colleagues,' CT scans and tomograms showed that the radiolucencies noted on the panoramic radiographs were anatomic variants. The CT scans revealed a marked cupping of the pterygoid fovea and dense cortical bone on the medial and lateral ridges of the condyle (Figure 2). The pterygoid fovea is the site of insertion of both the superior and inferior heads of the lateral pterygoid muscle."6 Some clinicians believe that deepening of the fovea area represents remodeling in response to alterations in occlusion and mandibular function,"3-5 which is more likely to occur in older people. Our results, however, showed a prevalence of pseudocysts in children 18 years and under that was fairly similar to that in the adult population reported by Friedlander and colleagues.' This finding indicates that the deepening of the fovea may be determined more by genetics than by environmental stimuli. Conditions to consider. Practitioners need to consider a number of conditions when they are confronted with a condylar radiolucency in a child. Although benign and malignant neoplasms in this area are not common, the differential diagnosis should include Langerhans' cell histiocytosis (eosinophilic granuloma), cenJADA, Vol. 128, June 1997 749
-C[INICAL PRACTICE
However, it usually is multilocular and there often is a history of trauma. CONCLUSION
Figure 2. Mandible of a child showing the medial and lateral ridges of the neck of the condyle and the cupped pterygoid fovea.
tral giant cell granuloma, simple bone cyst, chondroma and aneurysmal bone cyst.7 These all may appear as radiolucent lesions of the mandible, although they rarely are confined to the anterior half of the condyle and are infrequently corticated. Eosinophilic granuloma occurs in older children, but does not have a sclerotic border. Central giant cell granuloma rarely involves the condyle and its radiographic appearance generally is multilocular rather than
Dr. Collins Is a for-
mer resident, pediatric dentistry, Virginia Commonwealth University, Medlcal College of
Virginia, School of Dentistry, Richmond, Va. He Is now a pediatric dentist, Virginia Department of
Health, Washington County, Va.
unilocular. Also, the lesion typically produces expansion of the jaw. Simple bone cysts are quite uncommon in the mandible and usually are not rimmed. Moreover, in one study, 80 percent of the cases were preceded by trauma.7 Chondromas sometimes occur in the mandible, but they are relatively rare in the condyle. The lesion commonly presents as a painless, slowly progressive mass; most often it appears radiographically as an irregular radiolucency, which may contain foci of calcification. Aneurysmal bone cyst occurs in young patients.
Dr. Laskin is professor and chairman, Department of Oral and Maxillofacial
Surgery, Virginia Commonwealth University, Medical College of Virginia, School of Dentistry, 521 N. 11th St.,
Richmond, Va. 23298. Please address reprint requests to Dr. Laskin.
750 JADA, Vol. 128, June 1997
Patients who present with asymptomatic radiolucencies that appear to be similar to the ones described in this study and that of Friedlander and colleagues' generally do not need to have further imaging or exploratory surgery before a diagnosis can be made. However, we advise practitioners to make a panoramic radiograph 4 to 6 months after the patient's initial presentation so as to reconfirm the diagnosis of a pseudocyst. Radiolucent images that do not conform to the criteria set forth by Friedlander and colleagues' should be investigated with ancillary imaging studies and histologic evaluation, when appropriate. o 1. Friedlander AH, Monson ML, Friedlander MD, Esquerra AC. Pseudocysts ofthe mandibular condyle. J Oral Maxillofac Surg 1992;50(8):821-4. 2. Al-Quattan MM, Clarke HM. Pseudocyst of the mandibular condyle in a child. J Oral Maxillofac Surg 1992;52(10):1072. 3. Takenoshita Y. Development with age of the human mandibular condyle. Part I: Gross anatomy and inner architecture. Dentomaxillofac Radiol 1982;11(1):5-15. 4. Takenoshita Y. Development with age of the human mandibular condyle. Part II. Assessment of the mineral content and microradiographic study. Dentomaxillofac Radiol 1982;11(1):17-24. 5. Takenoshita Y. Development with age of the human mandibular condyle: histiological study. J Craniomandibular Pract 1987;5(4):317-23. 6. Carpentier P, Yung JP, MarguellesBonnet R, Meunissier M. Insertions of the
lateral pterygoid
I
muscle: an anatomic study of the human
Dr. Mourino is an asDr. Farrington Is associete professor and chairman, Ds
pertbinent of Pediatric Dentisry, Virginia Commonwealth University, Medical College of Virginia, School of Dentistry,
Richmond, Vs.
Dr.
Shetty Is assis-
-tent professor, Depertinent of Pediatric Dentistry, Virginia Commonwealth University, Medical College of Virginia, School of Dentistry, Richmond, Va.
societe professor
temporomandibular
and director, Advanced Education
joint. J Oral Maxillofac Surg
In Pedlatric
1988;46(6):477-82. 7. Regezi JA,
Dentiatry, virginia Commonwealth
C
Sciubba JJ. Oral
University, Medical Coiiege of Virginia,
pathology. 2nd ed.
School of Dentistry, Richmond, Va.
422
Philadelphia:
Saunders; 1993:350-