J
Oral Maxillofac Surg
49:112-116, 1931
Mandibular Ramus Anatomy as It Relates to the Medial Osteotomy of the Sagittal Split Ramus Osteotomy BRIAN R. SMITH, DDS, MS,* JEFFREY L. RAJCHEL, DDS, MS,t DANIEL E. WAITE, DDS, MS,+ AND LANCE READ, DDS$ The sagittal split ramus osteotomy is probably the most frequently used procedure for correction of mandibular skeletal dentofacial deformities. Despite numerous improvements in the technique in the 30 years since the procedure was introduced, a number of troublesome complications still occur. These include unfavorable fracture during surgery, paresthesia, and relapse. The purpose of this study was to determine where fusion of the buccal and lingual cortical plates occurs in the upper mandibular ramus, as it is thought that placement of the horizontal medial osteotomy above the point of fusion (without any intervening medullary bone) may lead to unfavorable fracture during splitting. Forty-nine human mandibles were sectioned vertically at three locations perpendicular to the surface of the ramus and the occlusal plane. Measurements were made to locate vertically the point of fusion of the buccal and lingual cortical plates relative to the lingula and to the depth of the sigmoid notch. The point of fusion occurred between 7.5 and 13.3 mm above the lingula. Only 2% of mandibles had fusion at or below the level of the lingula in the anterior ramus, whereas in the posterior ramus, 6.1% of mandibles were fused at that level. At a level halfway from the lingula to the sigmoid notch, 20% of mandibles were fused in the anterior ramus, whereas in the mid- to posterior ramus, the incidence was as high as 39%. The location of the medial horizontal osteotomy should be at or just above the tip of the lingula. A higher level of cut may be associated with an increased difficulty in splitting or incidence of unfavorable fracture.
The sagittal split ramus osteotomy (SSRO) of the mandible has been used for over 30 years and has undergone numerous modifications and improvements. Despite these improvements, a number of troublesome complications still occur. Although infrequent in the hands of an experienced operator, unfavorable fractures (“bad splits”) occasionally develop and can lead to intraoperative difficulties as well as relapse. A number of authors have proposed potential causes of unfavorable fractures in the sagittal osteotomy. Frequently cited reasons for unfavorable fracture have included incomplete osteotomies,’ use of osteotomes that are too large,2 attempting to split the segments too rapidly3 or not rapidly
* Assistant Professor, Department of Oral and Maxillofacial Surgery, University of Texas Dental Branch at Houston. t In private practice of oral and maxillofacial surgery, Asheville, NC. $ Professor and Chairman, Baylor College of Dentistry, Dallas. !$Resident, Department of Oral and Maxillofacial Surgery, Baylor College of Dentistry, Dallas. Presented at the 71st Annual AAOMS Meeting Oral Abstract Session, San Francisco, September 1989. Address correspondence and reprint requests to Dr Smith: Department of Oral and Maxillofacial Surgery, UTHSCH, Dental Branch, 6516 John Freeman Ave, Suite 2.059, Houston, TX 77030. 0 1991 American
Association
of Oral and Maxillofacial
Sur-
geons 0278-2391/91/4902-0002$3.00/O
112
113
SMITH ET AL,
enough,4 presence of impacted third molars,5 misdirecting the medial osteotomy upwards toward the condyle,6 and placement of the medial osteotomy too far superior to the lingula.397 Placement of the medial osteotomy too high is thought to lead to unfavorable fracture due to fusion of the medial and lateral cortical plates; cancellous bone between the two plates allows a plane in which a favorable fracture can take place. No investigations have been done to determine where the cortical plates fuse in the ramus. The purpose of this study was to determine where fusion of the medial and lateral cortical plates occurs in the upper mandibular ramus (superior to the lingula). The findings of this investigation should present the surgeon with meaningful information regarding the osteotomy design for the sagittal split mandibular procedure. Materials and Methods This study was conducted on 50 dried intact adult Asian mandibles of unknown sex. All of the specimens had a complete or nearly complete dentition. Two phases of investigation were performed: 1) bilateral gross measurements, and 2) evaluation of one half of the mandibles (selected randomly) sectioned mediolaterally. Each mandible was assigned a number from 1 to 50. They were hemisected in the midline and labeled on each side just anterior to the mental foramen (lL, IR, etc). Each mandible was carefully inspected to see if it lit the basic criteria (adult and with a complete or nearly complete dentition). One mandible was excluded because it was severely decomposed, with little of its internal structure intact. The remainder of the specimens were deemed to be worthy of inclusion in the study. Prior to initiating the cuts, all of the teeth were removed from the mandibles. Because of the expectation of bilateral symmetry,8 the study was only carried out on one side of each mandible. These were randomly selected by coin toss. Each hemimandible was cut in cross section in six specific locations perpendicular to the sagittal plane of the ramus and perpendicular to the occlusal plane. The sections were made with a 4-in diameter, l-mm thick Carborundum disc on a highspeed dental lathe. The positions of the sections were defined as follows (Fig I): section A, one half of the distance between section B and the anterior border of the mandibular ramus (coronoid notch); section B, a vertical line through the depth of the sigmoid notch perpendicular to the occlusal plane and to the sagittal plane of the ramus; section C, one half of the distance between section B and the
FIGURE I.
Position of sections through the ramus.
posterior border of the ramus at the level of the coronoid notch. Prior to sectioning, a mark was made with an indelible marker parallel to the occlusal plane through the tip of the lingula from the anterior border of the ramus to the posterior border of the mandible. This indelible line was drawn on both the medial and lateral surfaces of the ramus. Another line was drawn parallel to the occlusal plane along the depth of the sigmoid notch. These lines served to maintain the orientation of the pieces after they had been cut and to provide a place to measure from in mandibles in which the lingula was not present on the segment to be measured. In each case, the occlusal plane was estimated by drawing a line parallel to the crest of bone in the incisor region and the second/third molar region. This was used for orientation only, not as an absolute indication of height of the occlusal plane. The following measurements were made of each section with a microcaliper capable of measuring to the nearest 0.1 mm (Helios Microcaliper Inoxyd, Precision Gage Co, Chicago, IL): 1) the vertical distance from the tip of the lingula to the point at which the medial and lateral cortical plates became fused without any intervening medullary bone, measured perpendicular to the occlusal plane; 2) the vertical distance from the depth of the sigmoid notch to the point at which the medial and lateral cortical plates became fused; 3) the thickness of the ramus at the level of the lingula; 4) the thickness of the ramus at a level one-half the distance between the lingula and the depth of the sigmoid notch. If the buccal and lingual cortical plates were fused at the level of the
114
RAMUS ANATOMY RE MEDIAL CUT OF SAGITTAL SPLIT
Table 1. Vertical Distance in Millimeters From Lingula to Point of Fusion of Medial and Lateral Cortical Plates (n = 49)
Section
Minimum
Maximum
Mean
Variance
SD
No. of Specimens With Fusion at or Below Lingula
A B C
0.9 -5.7 - 14.8
25.4 12.6 14.1
13.3 7.5 7.9
40.0 15.3 26.6
6.3 3.9 5.2
1 2 3
Positive numbers indicate distance above lingula to point of fusion.
lingula or halfway to the sigmoid notch, this was recorded. Results
Section A was consistently located just posterior to the tip of the coronoid process and anterior to the lingula. Section B was through the depth of the sigmoid notch and was consistently located a few millimeters posterior to the lingula. Section C was located near the posterior border of the mandible. The vertical distances measured perpendicular to the occlusal plane from the tip of the lingula to the point in the ramus at which the medial and lateral cortical plate fuse are presented in Table 1. The mean levels above the lingula at which fusion occurred in sections A, B, and C were 13.3 mm, 7.5 mm, and 7.9 mm, respectively (Fig 2). In section A, there was one case where fusion occurred below the level of the lingula, and in sections B and C there were two and three such cases, respectively. The distances measured perpendicular to the occlusal plane, from the depth of the sigmoid notch to the point of fusion of the medial and lateral cortical plates, are presented in Table 2. The mean distance from the depth of the sigmoid notch to the point of
13.3
fusion in section A, B, and C, was 2.0 mm, 7.0 mm, and 6.9 mm, respectively. Thus, in cuts B and C, fusion occurred, on the average, roughly halfway from the lingula to the sigmoid notch, whereas more anteriorly in section A fusion occurred more superiorly, nearer the sigmoid notch. Table 3 presents the measurements of thickness of the ramus at the level of the lingula and half of the distance from the lingula to the sigmoid notch. The ramus was found to be thickest posteriorly in section C, and at the level of the lingula became thinner superiorly and anteriorly. It is important to note that although a few of the mandibles had fusion of the buccal and lingual cortical plates at or below the level of the lingula (one, two, and three mandibles for sections A, B, and C, respectively), a much higher number of mandibles had fusion at or below a level one-half the distance from the lingula to the sigmoid notch. At that level in section A, 10 specimens were fused, whereas in sections B and C, 19 and 16 specimens were fused, respectively. Another way of looking at the incidence of fusion is in terms of cumulative percentage of mandibles fused at progressive levels above the tip of the lingula. Figure 3 presents the data in such a fashion,
FIGURE 2. Millimeters above lingular tip where fusion of buccal and lingual cortical plates occur in the mandibular ramus.
115
SMITH ET AL
Table 2. Vertical Distance in Millimeters From Depth of Sigmoid Notch to Point of Fusion of Medial and Lateral Cortical Plates (n = 49) Section .4 B C
Minimum
Maximum
Mean
Variance
SD
19.6 17.9 26.5
2.0 7.0 6.9
39.4 13.6 19.6
6.3 3.7 4.4
-8.5 1.0 2.0
Positive numbers indicate distance below, depth of sigmoid notch to point of fusion.
and clearly demonstrates that fusion of the buccal and lingual plates rapidly becomes more prevaIent as the medial osteotomy is elevated above the tip of the lingula. Figure 3 also demonstrates the much higher incidence of cortical fusion at a given vertical level in the mid- to posterior ramus (sections B and C) than in the anterior ramus (section A). Discussion
As originally proposed by Trauner and Obwegeser, the horizontal medial osteotomy should be made high, “just below the mandibular notch” (sigmoid notch).* More recent articles have generally advocated placing the medial osteotomy lower, just above the lingula.7~‘0Y1’Wolford et al stated that the medial osteotomy should be placed “through the superior aspect of the lingula [which] may facilitate a less problematic split. Generally, the distance between the buccal and lingual cortical plates increases at more inferior levels in this area of the ramus because of greater interposed medullary bone. At more superior levels, the buccal and lingual cortical plates may be fused together, increasing the difficulty of completing the split.“7 Carleton, Schow, and Peterson have pointed out the importance of the angle of the medial ramus osteotomy, suggesting that it be directed inferior to the occlusai plane in cases in which the occlusal plane to posterior border angulation is less than 70”.6 Wolford et al suggest that the osteotomy be made perpendicular to the anterior surface of the
ascending ramus rather than parallel to the occlusal plane, which directs the posterior end of the osteotomy slightly inferiorly.’ These suggestions are based primarily on clinical observation and experience rather than anatomic studies. The results of this investigation support placement of the medial osteotomy at the level of the tip of the lingula. Only a few mandibles had fusion of the buccal and lingual cortical plates below the level of the lingula (one hemimandible at section A [anterior to the lingula, 2.0%], two hemimandibles at section B Ljust posterior to the lingula. 4.1%], and three hemimandibles at section C [posterior ramus, 6.1%]). In contrast, if the osteotomy is made at a more superior level, for example, halfway between the lingula and the depth of the sigmoid notch, a much higher incidence of difftcult or unfavorable splitting can be expected. In this study, 20.4% of mandibles had fusion of the buccal and lingual cortical plates in the anterior ramus at section A. A much higher incidence of fusion at or below that level was found in the ramus posterior to the lingula (38.8% at section B and 32.6% at section C). The results of this study do not support the extension of the medial osteotomy to the posterior border, as suggested originally by Trauner and Obwegeser’ and later supported by Mercier.‘* Mertier found that the ramus became progressively more cortical and less medullary as one proceeded posteriorly. He felt that the medial osteotomy should be carried to the posterior border of the
Table 3. Ramus Thickness in Millimeters at Linguia and One-Half the Distance from Tip of Lingula to Depth of Sigmoid Notch (n = 49) No. of Specimens With Fusion of Plates at or Below That Level
Section/Level
Minimum
Maximum
Mean
Variance
SD
A-Lingula IL! to SN
1.7 2.3
12.0 9.1
3.3 1.9
3.3 1.9
1.8 1.4
10
B-Lingula l/2 to SN
2.5 1.7
10.6 9.1
3.8 2.3
3.8 2.3
1.9 1.5
2 19
C-Lingula t/2 to SN
2.7 2.2
7.7 6.7
4.9 4.6
1.1 1.1
1.1 I.0
3 16
Abbreviation:
SN, sigmoid notch.
1
116
RAMUS ANATOMY RE MEDIAL CUT OF SAGI-ITAL SPLIT
:
‘S
I
0
Section A
m Soctirn g
mm above
Lingula
FIGURE 3. Incidence of fusion of cortical plates in the mandibular ramus as a function of level in mm above the tip of the lingula. Note the higher incidence of fusion of the plates above the lingula in the posterior ramus (sections B and C) than in the anterior ramus (section A).
mandible to take advantage of a “neutral zone” of cancellous bone that was found frequently (but not always) in the preangular area superior and anterior to the angle. Our results suggest that, based on considerations of fusion, there is no rationale to extend the medial osteotomy to the posterior border (a mean distance of 3 1.5 mm from the coronoid notch) because the incidence of fusion of the cortices increases posterior to the lingula, increasing the potential for an unfavorable fracture. Instead, these results support the work of Dal Pant,” Hunsuck,13 Epker, l1 and Jonsson,14 who all suggested extending the medial osteotomy only as far posteriorly as the lingual fossa. This is more easily achieved, with less periosteal stripping and less chance for hemorrhage. The mean length of the medial osteotomy would, therefore, be about 18 mm. References 1. Epker BN, Fish LC: Dentofacial Deformities, Integrated Orthodontic and Surgical Correction, vol 1. St Louis, MO, Mosby, 1986, p 171-172 2. Simpson W: The short lingual cut in the sagittal osteotomy. J Oral Surg 30:811, 1972 3. Guernsey LH, DeChamplain RW: Sequellae and complications of the intraoral sag&al osteotomy in the mandibular rami. Oral Surg Oral Med Oral Path01 22:176, 1971
4. Macintosh RB: Experience with the sagittal osteotomy of the mandibular ramus: A 13-year review. J Max-Fat Surg 9:151, 1981 5. Epker BN, Fish LC: Dentofacial Deformities, Integrated Orthodontic and Surgical Correction, vol 1. St Louis, MO, Mosby, 1986, p 234 6. Carleton AS, Schow SR, Peterson LJ: Prevention of the misdirected sag&al split. J Oral Maxillofac Surg 44:81, 1986 7. Wolford LM, Bennett MA, Rafferty CG: Modification of the mandibular ramus sagittal split osteotomy. Oral Surg Oral Med Oral Path01 64:146, 1987 8. Rajchel J, Ellis E, Fonseca RJ: The anatomical location of the mandibular canal: Its relationship to the sag&al ramus osteotomy. Int J Adult Orthod Orthognathic Surg 1:37, 1986 9. Trauner R, Obwegeser H: The surgical correction of mandibular prognathism and retrognathia with consideration of genioplasty. Part I. Surgical procedures to correct mandibular prognathism and reshaping of the chin. Oral Surg Oral Med Oral Path01 10:677, 1957 10. Dal Pont G: Retromolar osteotomy for the correction of prognathism. J Oral Surg Anesth Hosp Dent Service 19:42, 1961 11. Epker BN: Modifications in the sagittal osteotomy of the mandible. J Oral Surg 35: 157, 1977 12. Mercier P: The inner osseous architecture and sagittal splitting of the ascending ramus of the mandible. J Max-Fat Surg 1:171, 1973 13. Hunsuck EE: A modified intraoral sagittal splitting technique for correction of mandibular prognathism. J Oral Surg 26:250, 1%8 14. Jonsson E, Svartz K, Welander U: Sagittal split technique I. Immediate postoperative conditions: A radiographic follow up study. Int J Oral Surg 8:75, 1979
Oral Maxillofac Surg
49:112-116, 1931
Mandibular Ramus Anatomy as It Relates to the Medial Osteotomy of the Sagittal Split Ramus Osteotomy BRIAN R. SMITH, DDS, MS,* JEFFREY L. RAJCHEL, DDS, MS,t DANIEL E. WAITE, DDS, MS,+ AND LANCE READ, DDS$ The sagittal split ramus osteotomy is probably the most frequently used procedure for correction of mandibular skeletal dentofacial deformities. Despite numerous improvements in the technique in the 30 years since the procedure was introduced, a number of troublesome complications still occur. These include unfavorable fracture during surgery, paresthesia, and relapse. The purpose of this study was to determine where fusion of the buccal and lingual cortical plates occurs in the upper mandibular ramus, as it is thought that placement of the horizontal medial osteotomy above the point of fusion (without any intervening medullary bone) may lead to unfavorable fracture during splitting. Forty-nine human mandibles were sectioned vertically at three locations perpendicular to the surface of the ramus and the occlusal plane. Measurements were made to locate vertically the point of fusion of the buccal and lingual cortical plates relative to the lingula and to the depth of the sigmoid notch. The point of fusion occurred between 7.5 and 13.3 mm above the lingula. Only 2% of mandibles had fusion at or below the level of the lingula in the anterior ramus, whereas in the posterior ramus, 6.1% of mandibles were fused at that level. At a level halfway from the lingula to the sigmoid notch, 20% of mandibles were fused in the anterior ramus, whereas in the mid- to posterior ramus, the incidence was as high as 39%. The location of the medial horizontal osteotomy should be at or just above the tip of the lingula. A higher level of cut may be associated with an increased difficulty in splitting or incidence of unfavorable fracture.
The sagittal split ramus osteotomy (SSRO) of the mandible has been used for over 30 years and has undergone numerous modifications and improvements. Despite these improvements, a number of troublesome complications still occur. Although infrequent in the hands of an experienced operator, unfavorable fractures (“bad splits”) occasionally develop and can lead to intraoperative difficulties as well as relapse. A number of authors have proposed potential causes of unfavorable fractures in the sagittal osteotomy. Frequently cited reasons for unfavorable fracture have included incomplete osteotomies,’ use of osteotomes that are too large,2 attempting to split the segments too rapidly3 or not rapidly
* Assistant Professor, Department of Oral and Maxillofacial Surgery, University of Texas Dental Branch at Houston. t In private practice of oral and maxillofacial surgery, Asheville, NC. $ Professor and Chairman, Baylor College of Dentistry, Dallas. !$Resident, Department of Oral and Maxillofacial Surgery, Baylor College of Dentistry, Dallas. Presented at the 71st Annual AAOMS Meeting Oral Abstract Session, San Francisco, September 1989. Address correspondence and reprint requests to Dr Smith: Department of Oral and Maxillofacial Surgery, UTHSCH, Dental Branch, 6516 John Freeman Ave, Suite 2.059, Houston, TX 77030. 0 1991 American
Association
of Oral and Maxillofacial
Sur-
geons 0278-2391/91/4902-0002$3.00/O
112
113
SMITH ET AL,
enough,4 presence of impacted third molars,5 misdirecting the medial osteotomy upwards toward the condyle,6 and placement of the medial osteotomy too far superior to the lingula.397 Placement of the medial osteotomy too high is thought to lead to unfavorable fracture due to fusion of the medial and lateral cortical plates; cancellous bone between the two plates allows a plane in which a favorable fracture can take place. No investigations have been done to determine where the cortical plates fuse in the ramus. The purpose of this study was to determine where fusion of the medial and lateral cortical plates occurs in the upper mandibular ramus (superior to the lingula). The findings of this investigation should present the surgeon with meaningful information regarding the osteotomy design for the sagittal split mandibular procedure. Materials and Methods This study was conducted on 50 dried intact adult Asian mandibles of unknown sex. All of the specimens had a complete or nearly complete dentition. Two phases of investigation were performed: 1) bilateral gross measurements, and 2) evaluation of one half of the mandibles (selected randomly) sectioned mediolaterally. Each mandible was assigned a number from 1 to 50. They were hemisected in the midline and labeled on each side just anterior to the mental foramen (lL, IR, etc). Each mandible was carefully inspected to see if it lit the basic criteria (adult and with a complete or nearly complete dentition). One mandible was excluded because it was severely decomposed, with little of its internal structure intact. The remainder of the specimens were deemed to be worthy of inclusion in the study. Prior to initiating the cuts, all of the teeth were removed from the mandibles. Because of the expectation of bilateral symmetry,8 the study was only carried out on one side of each mandible. These were randomly selected by coin toss. Each hemimandible was cut in cross section in six specific locations perpendicular to the sagittal plane of the ramus and perpendicular to the occlusal plane. The sections were made with a 4-in diameter, l-mm thick Carborundum disc on a highspeed dental lathe. The positions of the sections were defined as follows (Fig I): section A, one half of the distance between section B and the anterior border of the mandibular ramus (coronoid notch); section B, a vertical line through the depth of the sigmoid notch perpendicular to the occlusal plane and to the sagittal plane of the ramus; section C, one half of the distance between section B and the
FIGURE I.
Position of sections through the ramus.
posterior border of the ramus at the level of the coronoid notch. Prior to sectioning, a mark was made with an indelible marker parallel to the occlusal plane through the tip of the lingula from the anterior border of the ramus to the posterior border of the mandible. This indelible line was drawn on both the medial and lateral surfaces of the ramus. Another line was drawn parallel to the occlusal plane along the depth of the sigmoid notch. These lines served to maintain the orientation of the pieces after they had been cut and to provide a place to measure from in mandibles in which the lingula was not present on the segment to be measured. In each case, the occlusal plane was estimated by drawing a line parallel to the crest of bone in the incisor region and the second/third molar region. This was used for orientation only, not as an absolute indication of height of the occlusal plane. The following measurements were made of each section with a microcaliper capable of measuring to the nearest 0.1 mm (Helios Microcaliper Inoxyd, Precision Gage Co, Chicago, IL): 1) the vertical distance from the tip of the lingula to the point at which the medial and lateral cortical plates became fused without any intervening medullary bone, measured perpendicular to the occlusal plane; 2) the vertical distance from the depth of the sigmoid notch to the point at which the medial and lateral cortical plates became fused; 3) the thickness of the ramus at the level of the lingula; 4) the thickness of the ramus at a level one-half the distance between the lingula and the depth of the sigmoid notch. If the buccal and lingual cortical plates were fused at the level of the
114
RAMUS ANATOMY RE MEDIAL CUT OF SAGITTAL SPLIT
Table 1. Vertical Distance in Millimeters From Lingula to Point of Fusion of Medial and Lateral Cortical Plates (n = 49)
Section
Minimum
Maximum
Mean
Variance
SD
No. of Specimens With Fusion at or Below Lingula
A B C
0.9 -5.7 - 14.8
25.4 12.6 14.1
13.3 7.5 7.9
40.0 15.3 26.6
6.3 3.9 5.2
1 2 3
Positive numbers indicate distance above lingula to point of fusion.
lingula or halfway to the sigmoid notch, this was recorded. Results
Section A was consistently located just posterior to the tip of the coronoid process and anterior to the lingula. Section B was through the depth of the sigmoid notch and was consistently located a few millimeters posterior to the lingula. Section C was located near the posterior border of the mandible. The vertical distances measured perpendicular to the occlusal plane from the tip of the lingula to the point in the ramus at which the medial and lateral cortical plate fuse are presented in Table 1. The mean levels above the lingula at which fusion occurred in sections A, B, and C were 13.3 mm, 7.5 mm, and 7.9 mm, respectively (Fig 2). In section A, there was one case where fusion occurred below the level of the lingula, and in sections B and C there were two and three such cases, respectively. The distances measured perpendicular to the occlusal plane, from the depth of the sigmoid notch to the point of fusion of the medial and lateral cortical plates, are presented in Table 2. The mean distance from the depth of the sigmoid notch to the point of
13.3
fusion in section A, B, and C, was 2.0 mm, 7.0 mm, and 6.9 mm, respectively. Thus, in cuts B and C, fusion occurred, on the average, roughly halfway from the lingula to the sigmoid notch, whereas more anteriorly in section A fusion occurred more superiorly, nearer the sigmoid notch. Table 3 presents the measurements of thickness of the ramus at the level of the lingula and half of the distance from the lingula to the sigmoid notch. The ramus was found to be thickest posteriorly in section C, and at the level of the lingula became thinner superiorly and anteriorly. It is important to note that although a few of the mandibles had fusion of the buccal and lingual cortical plates at or below the level of the lingula (one, two, and three mandibles for sections A, B, and C, respectively), a much higher number of mandibles had fusion at or below a level one-half the distance from the lingula to the sigmoid notch. At that level in section A, 10 specimens were fused, whereas in sections B and C, 19 and 16 specimens were fused, respectively. Another way of looking at the incidence of fusion is in terms of cumulative percentage of mandibles fused at progressive levels above the tip of the lingula. Figure 3 presents the data in such a fashion,
FIGURE 2. Millimeters above lingular tip where fusion of buccal and lingual cortical plates occur in the mandibular ramus.
115
SMITH ET AL
Table 2. Vertical Distance in Millimeters From Depth of Sigmoid Notch to Point of Fusion of Medial and Lateral Cortical Plates (n = 49) Section .4 B C
Minimum
Maximum
Mean
Variance
SD
19.6 17.9 26.5
2.0 7.0 6.9
39.4 13.6 19.6
6.3 3.7 4.4
-8.5 1.0 2.0
Positive numbers indicate distance below, depth of sigmoid notch to point of fusion.
and clearly demonstrates that fusion of the buccal and lingual plates rapidly becomes more prevaIent as the medial osteotomy is elevated above the tip of the lingula. Figure 3 also demonstrates the much higher incidence of cortical fusion at a given vertical level in the mid- to posterior ramus (sections B and C) than in the anterior ramus (section A). Discussion
As originally proposed by Trauner and Obwegeser, the horizontal medial osteotomy should be made high, “just below the mandibular notch” (sigmoid notch).* More recent articles have generally advocated placing the medial osteotomy lower, just above the lingula.7~‘0Y1’Wolford et al stated that the medial osteotomy should be placed “through the superior aspect of the lingula [which] may facilitate a less problematic split. Generally, the distance between the buccal and lingual cortical plates increases at more inferior levels in this area of the ramus because of greater interposed medullary bone. At more superior levels, the buccal and lingual cortical plates may be fused together, increasing the difficulty of completing the split.“7 Carleton, Schow, and Peterson have pointed out the importance of the angle of the medial ramus osteotomy, suggesting that it be directed inferior to the occlusai plane in cases in which the occlusal plane to posterior border angulation is less than 70”.6 Wolford et al suggest that the osteotomy be made perpendicular to the anterior surface of the
ascending ramus rather than parallel to the occlusal plane, which directs the posterior end of the osteotomy slightly inferiorly.’ These suggestions are based primarily on clinical observation and experience rather than anatomic studies. The results of this investigation support placement of the medial osteotomy at the level of the tip of the lingula. Only a few mandibles had fusion of the buccal and lingual cortical plates below the level of the lingula (one hemimandible at section A [anterior to the lingula, 2.0%], two hemimandibles at section B Ljust posterior to the lingula. 4.1%], and three hemimandibles at section C [posterior ramus, 6.1%]). In contrast, if the osteotomy is made at a more superior level, for example, halfway between the lingula and the depth of the sigmoid notch, a much higher incidence of difftcult or unfavorable splitting can be expected. In this study, 20.4% of mandibles had fusion of the buccal and lingual cortical plates in the anterior ramus at section A. A much higher incidence of fusion at or below that level was found in the ramus posterior to the lingula (38.8% at section B and 32.6% at section C). The results of this study do not support the extension of the medial osteotomy to the posterior border, as suggested originally by Trauner and Obwegeser’ and later supported by Mercier.‘* Mertier found that the ramus became progressively more cortical and less medullary as one proceeded posteriorly. He felt that the medial osteotomy should be carried to the posterior border of the
Table 3. Ramus Thickness in Millimeters at Linguia and One-Half the Distance from Tip of Lingula to Depth of Sigmoid Notch (n = 49) No. of Specimens With Fusion of Plates at or Below That Level
Section/Level
Minimum
Maximum
Mean
Variance
SD
A-Lingula IL! to SN
1.7 2.3
12.0 9.1
3.3 1.9
3.3 1.9
1.8 1.4
10
B-Lingula l/2 to SN
2.5 1.7
10.6 9.1
3.8 2.3
3.8 2.3
1.9 1.5
2 19
C-Lingula t/2 to SN
2.7 2.2
7.7 6.7
4.9 4.6
1.1 1.1
1.1 I.0
3 16
Abbreviation:
SN, sigmoid notch.
1
116
RAMUS ANATOMY RE MEDIAL CUT OF SAGI-ITAL SPLIT
:
‘S
I
0
Section A
m Soctirn g
mm above
Lingula
FIGURE 3. Incidence of fusion of cortical plates in the mandibular ramus as a function of level in mm above the tip of the lingula. Note the higher incidence of fusion of the plates above the lingula in the posterior ramus (sections B and C) than in the anterior ramus (section A).
mandible to take advantage of a “neutral zone” of cancellous bone that was found frequently (but not always) in the preangular area superior and anterior to the angle. Our results suggest that, based on considerations of fusion, there is no rationale to extend the medial osteotomy to the posterior border (a mean distance of 3 1.5 mm from the coronoid notch) because the incidence of fusion of the cortices increases posterior to the lingula, increasing the potential for an unfavorable fracture. Instead, these results support the work of Dal Pant,” Hunsuck,13 Epker, l1 and Jonsson,14 who all suggested extending the medial osteotomy only as far posteriorly as the lingual fossa. This is more easily achieved, with less periosteal stripping and less chance for hemorrhage. The mean length of the medial osteotomy would, therefore, be about 18 mm. References 1. Epker BN, Fish LC: Dentofacial Deformities, Integrated Orthodontic and Surgical Correction, vol 1. St Louis, MO, Mosby, 1986, p 171-172 2. Simpson W: The short lingual cut in the sagittal osteotomy. J Oral Surg 30:811, 1972 3. Guernsey LH, DeChamplain RW: Sequellae and complications of the intraoral sag&al osteotomy in the mandibular rami. Oral Surg Oral Med Oral Path01 22:176, 1971
4. Macintosh RB: Experience with the sagittal osteotomy of the mandibular ramus: A 13-year review. J Max-Fat Surg 9:151, 1981 5. Epker BN, Fish LC: Dentofacial Deformities, Integrated Orthodontic and Surgical Correction, vol 1. St Louis, MO, Mosby, 1986, p 234 6. Carleton AS, Schow SR, Peterson LJ: Prevention of the misdirected sag&al split. J Oral Maxillofac Surg 44:81, 1986 7. Wolford LM, Bennett MA, Rafferty CG: Modification of the mandibular ramus sagittal split osteotomy. Oral Surg Oral Med Oral Path01 64:146, 1987 8. Rajchel J, Ellis E, Fonseca RJ: The anatomical location of the mandibular canal: Its relationship to the sag&al ramus osteotomy. Int J Adult Orthod Orthognathic Surg 1:37, 1986 9. Trauner R, Obwegeser H: The surgical correction of mandibular prognathism and retrognathia with consideration of genioplasty. Part I. Surgical procedures to correct mandibular prognathism and reshaping of the chin. Oral Surg Oral Med Oral Path01 10:677, 1957 10. Dal Pont G: Retromolar osteotomy for the correction of prognathism. J Oral Surg Anesth Hosp Dent Service 19:42, 1961 11. Epker BN: Modifications in the sagittal osteotomy of the mandible. J Oral Surg 35: 157, 1977 12. Mercier P: The inner osseous architecture and sagittal splitting of the ascending ramus of the mandible. J Max-Fat Surg 1:171, 1973 13. Hunsuck EE: A modified intraoral sagittal splitting technique for correction of mandibular prognathism. J Oral Surg 26:250, 1%8 14. Jonsson E, Svartz K, Welander U: Sagittal split technique I. Immediate postoperative conditions: A radiographic follow up study. Int J Oral Surg 8:75, 1979
