J Oral Maxillofac 48:817-922.
Surg
1990
Assessment of the Stability of Mandibular Setback Procedures With Rigid Fixation CHRISTOPHER
A. SOROKOLIT,
DDS, MS,* AND RAM S. NANDA, DDS, MS, PHDt
The postsurgical changes associated with sagittal ramus osteotomy and mandibular setback stabilized with rigid fixation were evaluated. Lateral cephalometric radiographs of 25 individuals (11 males and 14 females) with a mean age of 23.4 years were evaluated presurgery, immediate postsurgery, and after a follow-up period of 7 to 42 months. The mean amount of surgical setback was 5.1 + 3.0 mm and the mean amount of postsurgical anterior movement was 0.51 + 1.04 mm, representing a 10% relapse of the original surgical correction. The postsurgical relapse of the mandible was not found to be related to the amount of surgical movement. Sixteen of the 18 cases that demonstrated anterior relapse moved forward between 0.5 and 1.5 mm. The amount of relapse was consistent and statistically significant (P < .05), but small enough that it was not considered to be clinically significant. The findings of this study indicate that mandibular setback with a sagittal ramus osteotomy and its stabilization with rigid fixation appears to be a stable clinical procedure.
masticatory muscles to adapt to the new environment. Recent studies’,” have reported relapse of 43% and 50% of the surgical correction. Relapse was attributed to the pull of the pterygomasseteric sling and distal rotation of the proximal segment at the time of surgery. However, causes of relapse are still not clear, and more quantitative data are needed to clearly establish the amount of postsurgical movement associated with surgical correction of mandibular prognathism via sagittal ramus osteotomy and rigid fixation.
The sagittal ramus osteotomy has been used extensively for the correction of mandibular prognathism since it was first described by Trauner and Obwegeser.’ Over the years, many modifications have been proposed to increase the stability of the surgical result.2-6 However, postsurgical relapse has remained a problem with the correction of mandibular prognathism.7 The use of rigid fixation, introduced by Spiessl,* has been used in an attempt to improve the skeletal stability. Relapse has been linked to instability at the osteotomy site, distal rotation of the proximal segment, postsurgical pull of the pterygomasseteric sling, and failure of other
Materials and Methods This retrospective study was based on the examination of 25 individuals (11 males and 14 females) with an age range of 14 to 36 years (mean, 23.4). There were 5 patients (2 males) in the age range 14 to 19 years. A verification of completion of their mandibular growth was accomplished by superimposition of their serial cephalometric radiograph tracings. All subjects received surgical correction of mandibular prognathism by a sagittal ramus osteotomy using a procedure similar to the modified technique described by Epker’ and Jeter et a1,6 as well as rigid
Received from the Department of Orthodontics, University of Oklahoma, College of Dentistry, Oklahoma City. * Former Graduate Resident; presently in private practice, Fort Worth, TX. t Professor and Chairman. Based on a thesis submitted to the faculty of graduate studies, University of Oklahoma, College of Dentistry, in partial fulfillment of the requirements for the degree of Master of Science. Address correspondence and reprint requests to Dr Nanda: Department of Orthodontics, College of Dentistry, PO Box 26901, Oklahoma City, OK 73190. 0 1990 geons
American
Association
of Oral
and Maxillofacial
Sur-
0278-2391/90/4808-0009$3.00/O
817
818
fixation for postoperative stabilization of the mandible. In the surgical technique, the cut was made through the cortex medially just above the lingula extending just posterior to the inferior alveolar neurovascular bundle. By visualization of the bundle in the medial dissection, injury to the nerve in this area was minimized. The muscle attachments were left intact to aid in maintaining blood supply. The bony splits were made at the midpoint of the inferior border in order to maximize the amount of bone available for the rigid fixation. If the inferior alveolar foramen was in a low position, the medial horizontal osteotomy was performed at a higher level. The sagittal osteotomy provided the broadest area of bone contact and care was taken for the overlapping bony fragments to be closely adapted. The fragments were rigidly stabilized bilaterally with three 2-mm screws without compression. In most cases two screws were placed above and one below the mandibular canal. The surgery was performed by three surgeons. None of the subjects had any maxillary surgical procedure. Five patients had a reduction genioplasty and two patients had chin augmentation with porous hydroxylapatite. Serial lateral cephalometric radiographs were taken within 2 weeks presurgery (Tl), within 1 week postsurgery (T2), and at a follow-up period (T3) that ranged from 7 to 42 months (mean, 15.3) postsurgery. All radiographs were traced and digitized by the same investigator. A surgical splint was present in 20 of the immediate postsurgical radiographs. To account for the mandibular opening due to the splint, an acetate template of the mandible was made and autorotated around the midcondylar point until the dentition occluded. The following cephalometric landmarks were identified: sella (S), nasion (N), anterior nasal spine (ANS), posterior nasal spine (PNS), points A and B, pogonion (Pog), gnathion (Gn), menton (Me), the center of the mandibular symphysis (D), and the most posterior point on the lingual surface of the mandibular symphysis-symphysis posterior (SymP). The palatal plane, drawn from ANS to PNS, was used as a primary reference plane. This plane was chosen because it was easily reproducible and nearly parallel to the Frankfort horizontal in the majority of subjects.” Also, the palatal plane is located close to the lower part of the face, which allowed an accurate presurgical and postsurgical comparison of the area of interest. Perpendicular lines were projected from points N, SymP, Me, B, D, and Pog, to the palatal plane to locate the corresponding points NP, SymP, MeP, BP, DP, and Pogp on the palatal plane (Fig 1). The sagittal position of the mandible was assessed by the linear distance between the points BP to NP, DP to
STABILITY OF MANDIBULAR
SETBACK PROCEDURES
Ma
FIGURE 1. Cephalometric linear measurements used to show surgical and postsurgical sagittal and vertical changes. Sagittal distances: S-l, BP to Np; S-2, SymPP to NP. Vertical distances: V-l, B to BP; V-2, Me to MeP; V-3, SymP to SymPP.
NP, Pogp to NP, and SymPP to NP. The vertical position of the mandible was assessed by the linear distance between Me and MeP, SymP and SymPP, and B to BP. Rotational changes were assessed from measurements of angles sella nasion B point (SNB) and sella nasion symphysis posterior (SNSymP). A standard Steiner12-type analysis was used to evaluate dental changes. The error of tracing and digitization was assessed for each cephalometric measurement. Twenty-four radiographs were chosen at random and traced and digitized. These cephalometric radiographs were again traced and digitized 2 weeks after the first measurements. The absolute mean difference between the 24 pairs of cephalometric measurements and the standard deviation of the differences were calculated (Table 1). Point B was used to evaluate sagittal changes because it was not affected by a genioplasty, was an accepted landmark, and was found to have the most reliable reproducibility. The error for each measurement was small. Three additional sagittal measurements were used to verify the stability of B point, because it has a potential for bony remodeling. The mean sag&al value was calculated using DP to NP, Pogp to NP, and SymPP to NP. If the change as measured from BP to NP was more than two SD from the mean of the three other measurements, B point was considered to have remodeled. Two-tailed t tests were used to determine the sig-
819
SOROKOLIT AND NANDA
Table 1. Results of Estimation of Error in Measurement of 24 Pairs of Cephalometric Radiographs, Each Traced and Digitized Twice Measurement
Mean Absolute Error
SD of Error
0.395 0.358 0.247 0.349
0.310 0.300 0.192 0.270
0.475 0.511
0.364 0.380
0.244 0.763 0.846
0.165 0.271 0.759
0.398 0.647 0.878
0.260 0.531 0.578
Angular SNA SNB ANB SNSymP Sagittal (mm) BP to N” SymPP to NP Vertical (mm) Me to MeP B to BP SymP to SymPP Dental l/ to NA (mm) l/ to SN /l to mand plane
Abbreviations: SNA, sella nasion A point; SNB, sella nasion B point; ANB, A point nasion B point; SNSymP, sella nasion symphysis posterior; BP, point perpendicular from B point; NP, point perpendicular from nasion; SymPP, point perpendicular from symphysis posterior; Me, menton; MeP, point perpendicular from menton; B, B point; SymP, symphysis posterior; l/, maxillary incisor; /l, mandibular incisor; NA, nasion A point; SN, sella nasion; mand, mandibular.
nificance of the mean change. An analysis of covariance was performed to examine the difference between surgeons, and a regression analysis was used to determine the predictive capability of presurgical and surgical variables.
Results
The mean amounts of surgical and postsurgical change were calculated and tabulated (Table 2). The mean amount of surgical movement of the mandible in the posterior sagittal direction from B to N on the palatal plane was 5.1 + 3 .O mm. This amount of anterior relapse represented approximately 10% of the original surgical movement. This postsurgical sagittal change was statistically significant (P = .0224). Similar results were obtained for the postsurgical sagittal changes as measured by DP to NP and SymPP to NP, which indicated mean anterior mandibular change of 13% and 14% of the mean setback, respectively. Anterior sagittal relapse was noted for 72% of the sample, 20% experienced no relapse, and 8% showed additional posterior movement. Among the individuals who did show anterior relapse, the mean change was 21% of the amount of surgical correction. A clinically small but statistically significant change was noted in the angular measures of mandibular position. The SNB, ANB, and SNSymP all demonstrated a mean relapse of approximately 0.3 + 0.7 degrees. The vertical changes in the position of the mandible and the dental changes were minimal and not significant. The differences in the amounts of sagittal and vertical relapse between the three different surgeons was analyzed by an analysis of covariance. There was no significant difference in the amount of
Table 2. Means of Change (mm) in Cephalometric Parameters Immediately After Surgery (T2 to Tl ) and Postsurgical FollowUp (T3 to T2) Surgical Change Measurement Angular SNA SNB ANB SNSymP Sagittal S to N BP to NP SymPP to Np Vertical Me to MeP B to BP Dental li to NA l/ to SN 11 to mandibular plane
Mean + SD
Postsurgical Change P Value
Mean 2 SD
* k 2 2
P Value
+- 0.70 f 1.87 If: 1.67 f 1.69
.2323 .0001* .0001* .0001*
-0.03 0.28 -0.31 0.32
0.70 0.64 0.67 0.54
.8315 .0382* .0310* .0065*
-0.18 2 0.75 -5.14 f 3.05 -4.70 k 3.64
.2329 .0001* .0001*
-0.06 f 0.29 0.51 2 1.04 0.67 +- 1.17
.3 151 .0224* .0089*
-0.88 2 1.92 -0.77 rf- 3.03
.0337* .2176
-0.14 * 1.44 0.05 f 1.97
.6400 .9019
0.03 * 1.00 1.08 k 2.67 -0.43 2 2.54
.8788 .0537 .4094
a.20 5 1.19 -0.78 f 2.79 1.46 % 3.90
.3978 .1765 .0726
-0.17 -2.81 2.64 -2.68
Abbreviations: SNA, sella nasion A point; SNB, sella nasion B point; ANB, A point nasion B point; SNSymP, sella nasion symphysis posterior; S, sella; N, nasion; BP, point perpendicular from B point; N’, point perpendicular from nasion; SymPP, point perpendicular from symphysis posterior; Me, menton; MeP, point perpendicular from menton; B, B point; l/, maxillary incisor; /l, mandibular incisor; NA, nasion A point; SN, sella nasion. * P < .05 by two-tailed t test.
STABILITY OF MANDIBULAR
sagittal or vertical postsurgical skeletal change between patients of the different surgeons. Individual surgical and postsurgical sagittal movements of the mandible were rank ordered based on the amount of surgical sagittal setback as measured BP to NP (Fig 2). In most cases, there was a small and somewhat uniform amount of anterior postsurgical movement. There were three cases in which postsurgical change was greater than 1.5 mm; two of these were in the anterior direction, and one was in the posterior direction. A stepwise regression analysis indicated that the only variable that accounted for the sagittal relapse was the length of time elapsed after the surgery (P < .05). Even the length of time was not found to be a good predictor of the amount of relapse. Only 17% of the variability of the sagittal relapse was explained by the length of time postsurgically. The amount of individual surgical and postsurgi-
SETBACK PROCEDURES
cal vertical change, as measured from Me to MeP, indicated that the vertical change was variable. Some of the cases had postsurgical vertical change in the same direction as the surgical correction; other patients had postsurgical relapse in the opposite direction. There were nine cases that demonstrated vertical postsurgical change that was greater than the error of measurement; five of these were superior and four of these were inferior movements. Discussion Postsurgical stability of the correction of mandibular prognathism has been a problem. Relapse has been linked to instability at the osteotomy site, distal rotation of the proximal segment, postsurgical pull of the pterygomasseteric sling, and failure of other masticatory muscles to adapt to the new environment .
25 24 23 22 21 M 19 18 17 16 -
9 5
i 5 z
FIGURE 2. Surgical mandibular setback and the amount of movement of the mandible as measured by BP to NP during the follow-up period, which is shown as the black bar. The changes are rank ordered from the largest to the smallest surgical sagittal movement.
l5 14 -
I3 12 11 10 9816 5 4 3 2
w
-
Movement during rurglul
cOrrWtiOn
m
-
Movement during follow-UP WrlOd
i
1
I
14
I
13
I
12
I
11
I
10
I
9
I
8
I
7
I
6
I
5
I
4
I
1
I
3
2
1
821
SOROKOLIT AND NANDA
The findings of this study indicated that the mean sagittal relapse was 0.51 mm (P < .05), representing 10% of the surgical correction. This is less than the recently reported amount of relapse, which ranged from 43%” to 50%.9 Individual data indicated that there was one case in which the mandible moved farther posterior and two cases where the mandible relapsed forward more than 1.5 mm. The case that moved farther posterior exhibited a postsurgical vertical increase that rotated the symphysis down and posterior. Both cases that demonstrated anterior relapse greater than 1.5 mm were males less than 20 years old at the time of surgery; a possibility of late mandibular growth cannot be ruled out. The observed mean sagittal stability was most likely due to a combination of proper presurgical orthodontic setup and careful surgical technique in which the muscles were minimally altered, the bony interface was well prepared for a close union, and control of the proximal segments was maintained in order to minimize any distal rotation. The observed mean vertical changes were not statistically significant (P > .05), but individual data indicated there was considerable variability of the postsurgical vertical changes. Similar variability has been reported.” Changes in the vertical dimension may be due to occlusal settling during the completion of orthodontic treatment, or error introduced during the cephalometric autorotation used to compensate for the surgical splint. The mean postsurgical dental changes were not significant (P > .05). Individual cases illustrated that in cases with more skeletal movement, there was more dental movement. All cases that demonstrated dental movement were under orthodontic treatment, and hence the dental changes appeared to be compensatory. In contrast to a previous report,‘3 there was no incidence of a return to a negative ovetjet. An analysis of covariance indicated that there was no significant difference in the amount of postsurgical movement between the different surgeons. Previous reports comparing several surgeons have reported both a relationship with relapseI and no relationship with the amount of relapse.” None of the demographic or morphological attributes were found to have any predictive value for the degree of relapse noted in this study. The amount of sagittal relapse has been reported to be linearly related to the amount of surgical movement for both mandibular advancements and mandibular setbacks.9q’5,16 The relationship between the amount of setback and the amount of relapse has been linked to an increased amount of distal rotation of the proximal segment.” However,
Frank0 et al9 reported that proximal segment rotation was not a factor in the consideration of relapse. This study confirms the observations of Komori et al’* and Law et all9 that there is no relationship between the amount of surgical movement and the amount of relapse. The high degree of stability observed in this sample is most likely due to the rigidly held large medullary surface area resulting from the sagittal split technique. Presurgical orthodontics allows good intercuspation after surgery, and hence is also an important factor in the prevention of relapse. The procedure for surgical correction of mandibular prognathism is exceedingly variable between patients as well as between surgeons. A clinical procedure may not be the same in each patient. The surgical cuts and bone trimming to allow a good fit of the proximal and distal segments are as much an art as a science. Therefore, the surgical procedure itself could account for the amount of observed variability. The amount of relapse was statistically significant but was small enough to be of little clinical importance. The findings of this study indicate that mandibular setback with a sagittal split ramus osteotomy and rigid fixation appears to be a fairly stable clinical procedure. Acknowledgment The authors wish to thank Dr N.R. Markowitz, Chairman of Oral and Maxillofacial Surgery, College of Dentistry, University of Oklahoma and Dr Larry Wolford, Dallas, TX for providing cases used in this study.
References 1. Trauner R, Obwegeser H: The surgical correction of mandibular prognathism and retrognathia and consideration of genioplasty. Surgical procedures to correct mandibular prognathism and reshaping of the chin. Oral Surg Oral Med Oral Path01 10:677, 1957 2. Dal Pont G: Retromolar osteotomy for correction nathism. J Oral Surg 19:42, 1961
of prog-
3. Hunsuck EE: A modified intraoral sagittal splitting technique for correction of mandibular prognathism. J Oral Surg 26:250, 1%8 4. Wang JH, Waite DE: Vertical osteotomy vs sagittal split osteotomy of the mandibular ramus: Comparison of operative and postoperative factors. J Oral Surg 33596, I%8 5. Epker BN: Modifications in the sagittal osteotomy of the mandible. J Oral Surg 35:157, 1977 6. Jeter TS, Van Sickels JE, Dilwick MF: Modified techniques for internal fLwation of sagittal ramus osteotomies. J Oral Maxillofac Surg 42:270, 1984 8. Spiessl B: Rigid internal fixation after sagittal split osteotomy of the ascending ramus, in Spiessl B (ed): New Concepts in Maxillofacial Bone Surgery (ed 1). New York, NY, Springer-Verlag, 1976, chap 6, pp 115-122
822
STABILITY OF MANDIBULAR
9. Frank0 JE, Van Sickles JE, Thrash WJ: Factors contributing to relapse in rigidly fixed mandibular setbacks. J Oral Maxillofac Surg 47:451, 1989 10. Phillips C, Zayyoun HS, Thomas PM, et al: Skeletal alterations followina TOVRO or BSSRO nrocedures. Int J Adult Orthod &thognath Surg 1:203, 1986 11. Rio10 ML, Moyers RE, McNamara JA, et al: An atlas of craniofacial growth: Cephalometric standards from the university school growth study, The University of Michigan. Monograph 2. Craniofacial growth series. Ann Arbor, MI, Center for Human Growth and Development, University of Michigan, 1974 12. Steiner CC: Cephalometrics for you and me. Am J Orthod 39~729, 1953 13. Paulus GW, Steinhauser EW: A comparative study of wire osteosynthesis versus bone screws in the treatment of mandibular prognathism. Oral Surg 54:2, 1982
14. Schendel SA, Epker BN: Results after mandibular advancement surgery: An analysis of 87 cases. J Oral Surg 38:265, 1980 15. Reitzik M: Skeletal and dental changes after surgical correction of mandibular prognathism. J Oral Surg 38:109, 1980 16. Kobayashi T, Watanabe I, Ueda K, et al: Stability of the mandible after sagittal ramus osteotomy for correction of nrognathism. J Oral Maxillofac Sum 44:693, 1986 17. Fish LC, Epker BN: Prevention of-relapse in surgicalorthodontic treatment. I. Mandibular procedures. J Clin Orthod 20:826, 1986 18. Komori E, Aigase K, Sugisaki M, et al: Cause of early skeletal relapse after mandibular setback. Am J Orthod Dentofac Orthop 95:29, 1989 19. Law JH, Rotshoff KS, Smith RJ: Stability following combined maxillary and mandibular osteotomies treated with rigid fixation. J Oral Maxillofac Surg 47: 128, 1989
SETBACK PROCEDURES
Surg
1990
Assessment of the Stability of Mandibular Setback Procedures With Rigid Fixation CHRISTOPHER
A. SOROKOLIT,
DDS, MS,* AND RAM S. NANDA, DDS, MS, PHDt
The postsurgical changes associated with sagittal ramus osteotomy and mandibular setback stabilized with rigid fixation were evaluated. Lateral cephalometric radiographs of 25 individuals (11 males and 14 females) with a mean age of 23.4 years were evaluated presurgery, immediate postsurgery, and after a follow-up period of 7 to 42 months. The mean amount of surgical setback was 5.1 + 3.0 mm and the mean amount of postsurgical anterior movement was 0.51 + 1.04 mm, representing a 10% relapse of the original surgical correction. The postsurgical relapse of the mandible was not found to be related to the amount of surgical movement. Sixteen of the 18 cases that demonstrated anterior relapse moved forward between 0.5 and 1.5 mm. The amount of relapse was consistent and statistically significant (P < .05), but small enough that it was not considered to be clinically significant. The findings of this study indicate that mandibular setback with a sagittal ramus osteotomy and its stabilization with rigid fixation appears to be a stable clinical procedure.
masticatory muscles to adapt to the new environment. Recent studies’,” have reported relapse of 43% and 50% of the surgical correction. Relapse was attributed to the pull of the pterygomasseteric sling and distal rotation of the proximal segment at the time of surgery. However, causes of relapse are still not clear, and more quantitative data are needed to clearly establish the amount of postsurgical movement associated with surgical correction of mandibular prognathism via sagittal ramus osteotomy and rigid fixation.
The sagittal ramus osteotomy has been used extensively for the correction of mandibular prognathism since it was first described by Trauner and Obwegeser.’ Over the years, many modifications have been proposed to increase the stability of the surgical result.2-6 However, postsurgical relapse has remained a problem with the correction of mandibular prognathism.7 The use of rigid fixation, introduced by Spiessl,* has been used in an attempt to improve the skeletal stability. Relapse has been linked to instability at the osteotomy site, distal rotation of the proximal segment, postsurgical pull of the pterygomasseteric sling, and failure of other
Materials and Methods This retrospective study was based on the examination of 25 individuals (11 males and 14 females) with an age range of 14 to 36 years (mean, 23.4). There were 5 patients (2 males) in the age range 14 to 19 years. A verification of completion of their mandibular growth was accomplished by superimposition of their serial cephalometric radiograph tracings. All subjects received surgical correction of mandibular prognathism by a sagittal ramus osteotomy using a procedure similar to the modified technique described by Epker’ and Jeter et a1,6 as well as rigid
Received from the Department of Orthodontics, University of Oklahoma, College of Dentistry, Oklahoma City. * Former Graduate Resident; presently in private practice, Fort Worth, TX. t Professor and Chairman. Based on a thesis submitted to the faculty of graduate studies, University of Oklahoma, College of Dentistry, in partial fulfillment of the requirements for the degree of Master of Science. Address correspondence and reprint requests to Dr Nanda: Department of Orthodontics, College of Dentistry, PO Box 26901, Oklahoma City, OK 73190. 0 1990 geons
American
Association
of Oral
and Maxillofacial
Sur-
0278-2391/90/4808-0009$3.00/O
817
818
fixation for postoperative stabilization of the mandible. In the surgical technique, the cut was made through the cortex medially just above the lingula extending just posterior to the inferior alveolar neurovascular bundle. By visualization of the bundle in the medial dissection, injury to the nerve in this area was minimized. The muscle attachments were left intact to aid in maintaining blood supply. The bony splits were made at the midpoint of the inferior border in order to maximize the amount of bone available for the rigid fixation. If the inferior alveolar foramen was in a low position, the medial horizontal osteotomy was performed at a higher level. The sagittal osteotomy provided the broadest area of bone contact and care was taken for the overlapping bony fragments to be closely adapted. The fragments were rigidly stabilized bilaterally with three 2-mm screws without compression. In most cases two screws were placed above and one below the mandibular canal. The surgery was performed by three surgeons. None of the subjects had any maxillary surgical procedure. Five patients had a reduction genioplasty and two patients had chin augmentation with porous hydroxylapatite. Serial lateral cephalometric radiographs were taken within 2 weeks presurgery (Tl), within 1 week postsurgery (T2), and at a follow-up period (T3) that ranged from 7 to 42 months (mean, 15.3) postsurgery. All radiographs were traced and digitized by the same investigator. A surgical splint was present in 20 of the immediate postsurgical radiographs. To account for the mandibular opening due to the splint, an acetate template of the mandible was made and autorotated around the midcondylar point until the dentition occluded. The following cephalometric landmarks were identified: sella (S), nasion (N), anterior nasal spine (ANS), posterior nasal spine (PNS), points A and B, pogonion (Pog), gnathion (Gn), menton (Me), the center of the mandibular symphysis (D), and the most posterior point on the lingual surface of the mandibular symphysis-symphysis posterior (SymP). The palatal plane, drawn from ANS to PNS, was used as a primary reference plane. This plane was chosen because it was easily reproducible and nearly parallel to the Frankfort horizontal in the majority of subjects.” Also, the palatal plane is located close to the lower part of the face, which allowed an accurate presurgical and postsurgical comparison of the area of interest. Perpendicular lines were projected from points N, SymP, Me, B, D, and Pog, to the palatal plane to locate the corresponding points NP, SymP, MeP, BP, DP, and Pogp on the palatal plane (Fig 1). The sagittal position of the mandible was assessed by the linear distance between the points BP to NP, DP to
STABILITY OF MANDIBULAR
SETBACK PROCEDURES
Ma
FIGURE 1. Cephalometric linear measurements used to show surgical and postsurgical sagittal and vertical changes. Sagittal distances: S-l, BP to Np; S-2, SymPP to NP. Vertical distances: V-l, B to BP; V-2, Me to MeP; V-3, SymP to SymPP.
NP, Pogp to NP, and SymPP to NP. The vertical position of the mandible was assessed by the linear distance between Me and MeP, SymP and SymPP, and B to BP. Rotational changes were assessed from measurements of angles sella nasion B point (SNB) and sella nasion symphysis posterior (SNSymP). A standard Steiner12-type analysis was used to evaluate dental changes. The error of tracing and digitization was assessed for each cephalometric measurement. Twenty-four radiographs were chosen at random and traced and digitized. These cephalometric radiographs were again traced and digitized 2 weeks after the first measurements. The absolute mean difference between the 24 pairs of cephalometric measurements and the standard deviation of the differences were calculated (Table 1). Point B was used to evaluate sagittal changes because it was not affected by a genioplasty, was an accepted landmark, and was found to have the most reliable reproducibility. The error for each measurement was small. Three additional sagittal measurements were used to verify the stability of B point, because it has a potential for bony remodeling. The mean sag&al value was calculated using DP to NP, Pogp to NP, and SymPP to NP. If the change as measured from BP to NP was more than two SD from the mean of the three other measurements, B point was considered to have remodeled. Two-tailed t tests were used to determine the sig-
819
SOROKOLIT AND NANDA
Table 1. Results of Estimation of Error in Measurement of 24 Pairs of Cephalometric Radiographs, Each Traced and Digitized Twice Measurement
Mean Absolute Error
SD of Error
0.395 0.358 0.247 0.349
0.310 0.300 0.192 0.270
0.475 0.511
0.364 0.380
0.244 0.763 0.846
0.165 0.271 0.759
0.398 0.647 0.878
0.260 0.531 0.578
Angular SNA SNB ANB SNSymP Sagittal (mm) BP to N” SymPP to NP Vertical (mm) Me to MeP B to BP SymP to SymPP Dental l/ to NA (mm) l/ to SN /l to mand plane
Abbreviations: SNA, sella nasion A point; SNB, sella nasion B point; ANB, A point nasion B point; SNSymP, sella nasion symphysis posterior; BP, point perpendicular from B point; NP, point perpendicular from nasion; SymPP, point perpendicular from symphysis posterior; Me, menton; MeP, point perpendicular from menton; B, B point; SymP, symphysis posterior; l/, maxillary incisor; /l, mandibular incisor; NA, nasion A point; SN, sella nasion; mand, mandibular.
nificance of the mean change. An analysis of covariance was performed to examine the difference between surgeons, and a regression analysis was used to determine the predictive capability of presurgical and surgical variables.
Results
The mean amounts of surgical and postsurgical change were calculated and tabulated (Table 2). The mean amount of surgical movement of the mandible in the posterior sagittal direction from B to N on the palatal plane was 5.1 + 3 .O mm. This amount of anterior relapse represented approximately 10% of the original surgical movement. This postsurgical sagittal change was statistically significant (P = .0224). Similar results were obtained for the postsurgical sagittal changes as measured by DP to NP and SymPP to NP, which indicated mean anterior mandibular change of 13% and 14% of the mean setback, respectively. Anterior sagittal relapse was noted for 72% of the sample, 20% experienced no relapse, and 8% showed additional posterior movement. Among the individuals who did show anterior relapse, the mean change was 21% of the amount of surgical correction. A clinically small but statistically significant change was noted in the angular measures of mandibular position. The SNB, ANB, and SNSymP all demonstrated a mean relapse of approximately 0.3 + 0.7 degrees. The vertical changes in the position of the mandible and the dental changes were minimal and not significant. The differences in the amounts of sagittal and vertical relapse between the three different surgeons was analyzed by an analysis of covariance. There was no significant difference in the amount of
Table 2. Means of Change (mm) in Cephalometric Parameters Immediately After Surgery (T2 to Tl ) and Postsurgical FollowUp (T3 to T2) Surgical Change Measurement Angular SNA SNB ANB SNSymP Sagittal S to N BP to NP SymPP to Np Vertical Me to MeP B to BP Dental li to NA l/ to SN 11 to mandibular plane
Mean + SD
Postsurgical Change P Value
Mean 2 SD
* k 2 2
P Value
+- 0.70 f 1.87 If: 1.67 f 1.69
.2323 .0001* .0001* .0001*
-0.03 0.28 -0.31 0.32
0.70 0.64 0.67 0.54
.8315 .0382* .0310* .0065*
-0.18 2 0.75 -5.14 f 3.05 -4.70 k 3.64
.2329 .0001* .0001*
-0.06 f 0.29 0.51 2 1.04 0.67 +- 1.17
.3 151 .0224* .0089*
-0.88 2 1.92 -0.77 rf- 3.03
.0337* .2176
-0.14 * 1.44 0.05 f 1.97
.6400 .9019
0.03 * 1.00 1.08 k 2.67 -0.43 2 2.54
.8788 .0537 .4094
a.20 5 1.19 -0.78 f 2.79 1.46 % 3.90
.3978 .1765 .0726
-0.17 -2.81 2.64 -2.68
Abbreviations: SNA, sella nasion A point; SNB, sella nasion B point; ANB, A point nasion B point; SNSymP, sella nasion symphysis posterior; S, sella; N, nasion; BP, point perpendicular from B point; N’, point perpendicular from nasion; SymPP, point perpendicular from symphysis posterior; Me, menton; MeP, point perpendicular from menton; B, B point; l/, maxillary incisor; /l, mandibular incisor; NA, nasion A point; SN, sella nasion. * P < .05 by two-tailed t test.
STABILITY OF MANDIBULAR
sagittal or vertical postsurgical skeletal change between patients of the different surgeons. Individual surgical and postsurgical sagittal movements of the mandible were rank ordered based on the amount of surgical sagittal setback as measured BP to NP (Fig 2). In most cases, there was a small and somewhat uniform amount of anterior postsurgical movement. There were three cases in which postsurgical change was greater than 1.5 mm; two of these were in the anterior direction, and one was in the posterior direction. A stepwise regression analysis indicated that the only variable that accounted for the sagittal relapse was the length of time elapsed after the surgery (P < .05). Even the length of time was not found to be a good predictor of the amount of relapse. Only 17% of the variability of the sagittal relapse was explained by the length of time postsurgically. The amount of individual surgical and postsurgi-
SETBACK PROCEDURES
cal vertical change, as measured from Me to MeP, indicated that the vertical change was variable. Some of the cases had postsurgical vertical change in the same direction as the surgical correction; other patients had postsurgical relapse in the opposite direction. There were nine cases that demonstrated vertical postsurgical change that was greater than the error of measurement; five of these were superior and four of these were inferior movements. Discussion Postsurgical stability of the correction of mandibular prognathism has been a problem. Relapse has been linked to instability at the osteotomy site, distal rotation of the proximal segment, postsurgical pull of the pterygomasseteric sling, and failure of other masticatory muscles to adapt to the new environment .
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FIGURE 2. Surgical mandibular setback and the amount of movement of the mandible as measured by BP to NP during the follow-up period, which is shown as the black bar. The changes are rank ordered from the largest to the smallest surgical sagittal movement.
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SOROKOLIT AND NANDA
The findings of this study indicated that the mean sagittal relapse was 0.51 mm (P < .05), representing 10% of the surgical correction. This is less than the recently reported amount of relapse, which ranged from 43%” to 50%.9 Individual data indicated that there was one case in which the mandible moved farther posterior and two cases where the mandible relapsed forward more than 1.5 mm. The case that moved farther posterior exhibited a postsurgical vertical increase that rotated the symphysis down and posterior. Both cases that demonstrated anterior relapse greater than 1.5 mm were males less than 20 years old at the time of surgery; a possibility of late mandibular growth cannot be ruled out. The observed mean sagittal stability was most likely due to a combination of proper presurgical orthodontic setup and careful surgical technique in which the muscles were minimally altered, the bony interface was well prepared for a close union, and control of the proximal segments was maintained in order to minimize any distal rotation. The observed mean vertical changes were not statistically significant (P > .05), but individual data indicated there was considerable variability of the postsurgical vertical changes. Similar variability has been reported.” Changes in the vertical dimension may be due to occlusal settling during the completion of orthodontic treatment, or error introduced during the cephalometric autorotation used to compensate for the surgical splint. The mean postsurgical dental changes were not significant (P > .05). Individual cases illustrated that in cases with more skeletal movement, there was more dental movement. All cases that demonstrated dental movement were under orthodontic treatment, and hence the dental changes appeared to be compensatory. In contrast to a previous report,‘3 there was no incidence of a return to a negative ovetjet. An analysis of covariance indicated that there was no significant difference in the amount of postsurgical movement between the different surgeons. Previous reports comparing several surgeons have reported both a relationship with relapseI and no relationship with the amount of relapse.” None of the demographic or morphological attributes were found to have any predictive value for the degree of relapse noted in this study. The amount of sagittal relapse has been reported to be linearly related to the amount of surgical movement for both mandibular advancements and mandibular setbacks.9q’5,16 The relationship between the amount of setback and the amount of relapse has been linked to an increased amount of distal rotation of the proximal segment.” However,
Frank0 et al9 reported that proximal segment rotation was not a factor in the consideration of relapse. This study confirms the observations of Komori et al’* and Law et all9 that there is no relationship between the amount of surgical movement and the amount of relapse. The high degree of stability observed in this sample is most likely due to the rigidly held large medullary surface area resulting from the sagittal split technique. Presurgical orthodontics allows good intercuspation after surgery, and hence is also an important factor in the prevention of relapse. The procedure for surgical correction of mandibular prognathism is exceedingly variable between patients as well as between surgeons. A clinical procedure may not be the same in each patient. The surgical cuts and bone trimming to allow a good fit of the proximal and distal segments are as much an art as a science. Therefore, the surgical procedure itself could account for the amount of observed variability. The amount of relapse was statistically significant but was small enough to be of little clinical importance. The findings of this study indicate that mandibular setback with a sagittal split ramus osteotomy and rigid fixation appears to be a fairly stable clinical procedure. Acknowledgment The authors wish to thank Dr N.R. Markowitz, Chairman of Oral and Maxillofacial Surgery, College of Dentistry, University of Oklahoma and Dr Larry Wolford, Dallas, TX for providing cases used in this study.
References 1. Trauner R, Obwegeser H: The surgical correction of mandibular prognathism and retrognathia and consideration of genioplasty. Surgical procedures to correct mandibular prognathism and reshaping of the chin. Oral Surg Oral Med Oral Path01 10:677, 1957 2. Dal Pont G: Retromolar osteotomy for correction nathism. J Oral Surg 19:42, 1961
of prog-
3. Hunsuck EE: A modified intraoral sagittal splitting technique for correction of mandibular prognathism. J Oral Surg 26:250, 1%8 4. Wang JH, Waite DE: Vertical osteotomy vs sagittal split osteotomy of the mandibular ramus: Comparison of operative and postoperative factors. J Oral Surg 33596, I%8 5. Epker BN: Modifications in the sagittal osteotomy of the mandible. J Oral Surg 35:157, 1977 6. Jeter TS, Van Sickels JE, Dilwick MF: Modified techniques for internal fLwation of sagittal ramus osteotomies. J Oral Maxillofac Surg 42:270, 1984 8. Spiessl B: Rigid internal fixation after sagittal split osteotomy of the ascending ramus, in Spiessl B (ed): New Concepts in Maxillofacial Bone Surgery (ed 1). New York, NY, Springer-Verlag, 1976, chap 6, pp 115-122
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STABILITY OF MANDIBULAR
9. Frank0 JE, Van Sickles JE, Thrash WJ: Factors contributing to relapse in rigidly fixed mandibular setbacks. J Oral Maxillofac Surg 47:451, 1989 10. Phillips C, Zayyoun HS, Thomas PM, et al: Skeletal alterations followina TOVRO or BSSRO nrocedures. Int J Adult Orthod &thognath Surg 1:203, 1986 11. Rio10 ML, Moyers RE, McNamara JA, et al: An atlas of craniofacial growth: Cephalometric standards from the university school growth study, The University of Michigan. Monograph 2. Craniofacial growth series. Ann Arbor, MI, Center for Human Growth and Development, University of Michigan, 1974 12. Steiner CC: Cephalometrics for you and me. Am J Orthod 39~729, 1953 13. Paulus GW, Steinhauser EW: A comparative study of wire osteosynthesis versus bone screws in the treatment of mandibular prognathism. Oral Surg 54:2, 1982
14. Schendel SA, Epker BN: Results after mandibular advancement surgery: An analysis of 87 cases. J Oral Surg 38:265, 1980 15. Reitzik M: Skeletal and dental changes after surgical correction of mandibular prognathism. J Oral Surg 38:109, 1980 16. Kobayashi T, Watanabe I, Ueda K, et al: Stability of the mandible after sagittal ramus osteotomy for correction of nrognathism. J Oral Maxillofac Sum 44:693, 1986 17. Fish LC, Epker BN: Prevention of-relapse in surgicalorthodontic treatment. I. Mandibular procedures. J Clin Orthod 20:826, 1986 18. Komori E, Aigase K, Sugisaki M, et al: Cause of early skeletal relapse after mandibular setback. Am J Orthod Dentofac Orthop 95:29, 1989 19. Law JH, Rotshoff KS, Smith RJ: Stability following combined maxillary and mandibular osteotomies treated with rigid fixation. J Oral Maxillofac Surg 47: 128, 1989
SETBACK PROCEDURES
