Postoperative migration at the individual osteotomy site following sagittal split ramus osteotomy: A stereometric radiographic study

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1 130 Gert Wall, DDS Consultant Department of Oral and Maxillofacial Surgery Helsingborg Hospital Helsingborg, Sweden Bo Rosenquist, DDS, PhD Associate Professor Department of Oral and Maxillofacial Surgery University Hospital Lund, Sweden Reprint requests: Gert Wall Department of Oral and Maxillofacial Surgery Helsingborg Hospital S Helsingborg Sweden Fax: helsingborgslasarett.se Postoperative migration at the individual osteotomy site following sagittal split ramus osteotomy: A stereometric radiographic study The aim of the present study was to evaluate whether migration occurs at individual osteotomy sites following sagittal split osteotomy of the mandible stabilized with rigid internal fixation, and if it occurs, how long it lasts. In 10 consecutive patients operated with bilateral sagittal split osteotomies, postoperative migration, defined as displacement of the proximal segments in relation to the distal over time, was studied 3-dimensionally by means of stereometric radiography. Follow-up was performed with stereometric radiographs obtained at intervals from 2 days until 1 year after surgery. During the 1-year observation period, migration at one or both of the osteotomy sites was found at some stage in all of the patients and in as many as 4 patients during the final 6 months. It is concluded that rigid internal fixation, as performed in the present study, does not prevent postoperative migration at the osteotomy. Furthermore, stable bone union at the osteotomy site appears to be a slower process than previously expected, thus emphasizing the importance of follow-up procedures to prevent relapse. (Int J Adult Orthod Orthognath Surg 2001;16: ) Int J Adult Orthod Orthognath Surg Vol. 16, No. 2, 2001 Relapse following bilateral sagittal split osteotomy (BSSO) of the mandible is a well-documented phenomenon. 1 5 The postoperative changes of the results achieved at surgery may be divided into short-term and long-term relapse. Early relapse seems to be caused by migration at the osteotomy site, ie, displacement of the osteotomized bone segments from the position achieved at surgery to a new resting position, 5 8 while long-term relapse has been attributed to condylar resorption. 9,10 Rigid internal fixation with bicortical screws and/or bone plates and monocortical screws has been advocated by several authors to eliminate or at least decrease relapse Other authors, however, have observed both short- and long-term relapse with the use of rigid fixation. 5,6,15 17 A possible explanation for these contradictory results is that relapse usually is measured by cephalometry. The method errors associated with cephalometry may very well influence the results of the analyses, ie, landmarks are often difficult to identify, 18,19 they may be subject to postoperative remodeling, 20 and superimposition of tracings on radiographs obtained on different occasions may affect the precision. 21,22 Furthermore, the 2-dimensional imagery in lateral cephalograms does not permit assessment of movements at the individual osteotomy sites following bilateral mandibular osteotomies.the roentgen stereophotogrammetric method developed by Selvik 23 makes it possible to study even minor displacement of bone segments in 3 dimensions. This method eliminates the errors related to cephalometry and is therefore the method of choice for analysis of postoperative displacement of the mandibular segments following BSSOs. The aims of the present study were to examine whether rigid internal fixation by bicortical screws and/or miniplates and monocortical screws prevents postoperative migration at the osteotomy site, and if not, how long this migration lasts.

2 Int J Adult Orthod Orthognath Surg Vol. 16, No. 2, Table 1 Patient data Sex Age Patient (M/F) (y + mo) Type of fixation 1 F Bilateral: miniplates and 1 bicortical screw 2 F Left side: miniplate and 1 bicortical screw; right side: bicortical screws 3 F Bilateral: bicortical screws 4 F Bilateral: bicortical screws 5 M Bilateral: bicortical screws 6 F Bilateral: bicortical screws 7 F Bilateral: bicortical screws 8 M Bilateral: bicortical screws 9 F Left side: miniplate; right side: bicortical screws 10 F Bilateral: bicortical screws Subjects and method Ten patients referred to the Department of Oral and Maxillofacial Surgery at the University Hospital of Lund and consecutively operated with BSSOs were included in the study. All osteotomies were stabilized by rigid internal fixation with bicortical screws and/or titanium miniplates and monocortical screws. No maxillomandibular fixation was used postoperatively, but in 5 patients, training elastics were inserted for a maximum of 3 weeks to guide the mandible into accurate occlusion. Thin acrylic wafers, used to achieve accurate occlusion at surgery, were kept in 4 of the patients during the same period. In Patient 2, all fixation material on the left side was removed at 3 months after surgery due to infection. In all other patients, healing was uneventful. Patient data and type of fixation are presented in Table 1. All patients consented to participate in the study, which was approved by the Ethics Committee of the Medical Faculty, University of Lund. Measurement of postoperative migration The stereometric radiographic method presented by Selvik 23 was used to calculate postoperative migration at the osteotomy site, which was defined as displacement of the proximal bone segments in relation to the distal to new resting positions between subsequent examinations. At surgery, 3 bone markers (tantalum pins, mm) were inserted into the cortex of the proximal and distal segments of the mandible (Fig 1) by means of an instrument developed by Bjork. 24 Each set of 3 bone markers formed a triangle, which constituted a rigid body model (RBM) (Fig 2) that represented the bone segment in all subsequent calculations of postoperative migration. The RBM of the distal segment of the mandible was regarded as fixed in space and served as a reference when displacement of the proximal segments was calculated. The biocompatibility of tantalum has been tested and found suitable for implantation in bone. 25,26 Stereometric radiographs were obtained with the patients heads positioned in a calibration cage as described by Rosenquist et al 20 at 2 days and at 1, 3, 6, and 12 months after surgery (Fig 3). Because of pregnancy, Patient 6 did not attend the examination at 6 months after surgery. At each examination, the patients were carefully instructed to hold their mandible in central occlusion. After the bone marker images were recorded, the positions of the RBMs were reconstructed in a laboratory 3-dimensional coordinate system defined by the calibration cage. The reference RBMs were reoriented to their original positions, as determined at the first examination at 2 days

3 132 Wall/Rosenquist Fig 1 Positions of the bone markers. Fig 2 Rigid body models (RBMs) formed by the bone markers. Focus 1 Focus 2 Fig 3 Calibration cage with films and roentgen foci. Fig 4 Rigid body model (RBM) defined by bone markers from both proximal segments. Y X Z Fig 5 Cardinal axes of the head. Positive directions of rotation and translation are indicated by arrows. after surgery, at all subsequent examinations. For a complete description of the stereo method, see Selvik. 23 In 4 of the patients it was impossible to identify all of the bone markers in each proximal bone segment. In these cases, the bone markers of both proximal segments were tested as one rigid body (Fig 4). This proved successful, and in these patients displacement at the osteotomy site was calculated with the proximal segments forming one observation segment. Migration at the osteotomy site was expressed as rotations about and translations along the cardinal axes of the head (Fig 5). The calculated values of rotation are valid for any point of the bone segment, and the values of translation are valid for the center of gravity of the RBM.

4 Int J Adult Orthod Orthognath Surg Vol. 16, No. 2, Method accuracy The accuracy of the stereometric radiographic method depends on 2 conditions: bone marker stability, ie, that the bone markers remain stable in their original positions, and bone marker configuration, ie, how the bone markers are distributed in the bone segment. The stability of the bone markers was tested at each examination and expressed as a mean error of rigid-body fitting (MERBF). A MERBF exceeding 0.2 mm indicates displacement of one or more of the bone markers. With the condition of bone marker stability fulfilled, calculated RBM displacement of 0.4 degrees and 0.2 mm is significant, ie, indicates true displacement of the bone segment. The accuracy of values of rotation is also dependent on the 3-dimensional distribution of the bone markers in each segment. The bone markers ought to be as widely separated as possible. 27 In narrow configurations with the bone markers placed in a nearly straight line, even a minor instability of one bone marker may result in a major change in the configuration of the RBM, which may be presented as a rotation of the RBM. A condition number of the bone marker configuration was therefore calculated. This number is dependent on the 3-dimensional bone marker distribution and the projection of this configuration toward the radiograpic focus, and ideally, it should not exceed 150. A higher value for this number indicates either that the bone marker configuration is too small per se or that the surface of the bone marker exposed in 1 of the 2 projections is too small. Results Stability of the bone markers (Table 2) The bone markers remained stable in their original positions in 6 of the patients. Values of MERBF exceeding 0.2 were observed in 2 of the reference RBMs (Patients 3 and 6 at the 2 final examinations, respectively) and in 3 of the observation RBMs (Patients 2, 3, and 8). Condition of the bone marker configuration (Table 3) The bone marker configuration was satisfactory in all of the reference RBMs, with no condition number exceeding 150. In the observation RBMs, numbers below 150 were found in 3 patients only (Patients 1, 8, and 9), all of which consisted of united proximal segments. However, most of the numbers exceeding 150 did not differ much from the ideal, with the exception of 2 patients (7 and 10). In these patients, numbers above 500 were observed, with a maximum of in Patient 7. Postoperative migration at the osteotomy site (Tables 4 and 5) Expressed as rotations, postoperative migration at 1 or both osteotomy sites occurred in all of the patients during the first 6 postoperative months. The number decreased to 8 of 10 patients during the final 6 months of the 1-year observation period. Expressed as translations, postoperative migration was found at some stage in all patients throughout the 1-year observation period. Although most of the migration occurred during the first 3 postoperative months, it was observed in at least 3 patients (disregarding Patient 6, who did not participate at the 6-month examination) during the 6- to 12-month interval. Discussion Bone marker stability and configuration The bone marker stability in the reference RBMs was generally excellent, with values of MERBF exceeding 0.2 mm found in only 2 patients (Patients 3 and 6) during the 2 final examination intervals, respectively. In the observation RBMs, values of MERBF exceeding 0.2 mm were found in 3 patients: in 1 segment each in Patients 2 and 3 and in the united segment in Patient 8. In RBMs with the configuration and size of the reference RBMs and with observation RBMs consisting of both proximal segments in the present study, displacement of 1 bone marker up to 1.0 mm affects the calculated value of translation

5 134 Wall/Rosenquist Table 2 Stability of bone markers* 2 days to 1 mo to 3 mo to 6 mo to Patient/segment 1 mo (mm) 3 mo (mm) 6 mo (mm) 12 mo (mm) 1 ref obs L+R ref obs L obs R ref obs L obs R ref obs L obs R ref obs L+R ref obs L obs R ref obs L obs R ref obs L+R ref obs L+R ref obs L obs R *Expressed as mean error of rigid body fitting (measured between stereometric radiographs obtained at subsequent examinations). ref = reference (distal) segment; obs L = left (proximal) segment; obs R = right (proximal) segment. Patient 6 did not attend the 6-month recall. Table 3 Condition of the bone marker configurations Segment Patient Ref Obs L+R Obs L Obs R ref = reference (distal) segment; obs L = left (proximal) observation segment; obs R = right (proximal) observation segment. with less than 0.2 mm. Analyses of individual bone marker stability in the these RBMs showed that 1 bone marker in 2 patients (2 and 3) moved more than 1.0 mm during the final 6-month observation interval. This resulted in a MERBF of 0.5 mm, and the corresponding values of translation should be regarded with caution. The bone marker configuration in all of the reference RBMs and in the united observation RBMs was ideal, with only 1 condition number slightly exceeding 150 (Patient 5). These RBMs were all large, with a wide 3- dimensional distribution of the bone markers. The bone marker configuration in the individual proximal segments was comparatively small, with a narrow distribution of the bone markers. Placed on the lateral surfaces of the mandibular rami, the bone markers were often projected more as positioned in a straight vertical line than in a triangular fashion in the anteroposterior radiographic exposures. This is of importance in the analysis of rotations, since even the minor displacement of a bone marker may be mistaken for a rotation of the RBM. In the present study, the impact of a toonarrow distribution of the bone markers was most pronounced on values of rotation about the y-axis, as can be seen in some very large values in Table 4 (Patients 3 and 7). These values are probably inaccurate. However, the condition number is of minor importance in calculations of translation,

6 Int J Adult Orthod Orthognath Surg Vol. 16, No. 2, Table 4 Migration at the osteotomy site, expressed as rotations about the cardinal axes of the head (degrees) Observation periods 2 days to 1 mo 1 mo to 3 mo 3 mo to 6 mo 6 mo to 12 mo Patient/segment x y z x y z x y z x y z 1 L+R L R L R L R L+R L R L R L+R L+R L R L = left observation (proximal) segment; R = right observation (proximal) segment. Table 5 Migration at the osteotomy site expressed as translations along the cardinal axes of the head (mm) Observation periods 2 days to 1 mo 1 mo to 3 mo 3 mo to 6 mo 6 mo to 12 mo Patient/segment x y z x y z x y z x y z 1 L+R L R L R L R L+R L R L R L+R L+R L R L = left observation (proximal) segment; R = right observation (proximal) segment.

7 136 Wall/Rosenquist which in this study proved to be a reliable tool for assessment of migration at the osteotomy site. Postoperative migration at the osteotomy site The main purpose of the present study was to investigate whether rigid internal fixation with bicortical screws and/or miniplates and monocortical screws prevents postoperative migration at the individual osteotomy site. To achieve this, accurate individual values are imperative. The stereometric radiographic method allows for accurate assessment of displacement of individual bone segments. From the result of the present study, it is obvious that the use of fixation with bicortical screws and/or miniplates and monocortical screws does not prevent postoperative migration at the osteotomy site after mandibular BSSOs. It is also evident that migration may take place as late as 6 to 12 months after surgery. Among the variables used for measurements of relapse following mandibular osteotomies are angles and distances between various cephalometric landmarks. Differences in these values between subsequent examinations are regarded as expressions of postoperative changes of skeletal relationships. Relationships between mandibular landmarks and cranial base structures, as in the SNB and mandibular plane angles, are among the most commonly used. However, as expressions of migration at the osteotomy site, these variables must be regarded with caution, since the individual osteotomy site is indistinguishable in the 2-dimensional lateral cephalogram. Displacement of the distal segment may also be caused by dental changes and/or remodeling of the temporomandibular structures, as well as by migration at the osteotomy site. Furthermore, the mandible, itself a mobile structure, may very well be displaced by involuntary movements, such as breathing or swallowing at the moment of radiographic exposure. In a study of postoperative changes at the osteotomy site, Gassmann et al 5 concluded, after comparing positional changes in anatomic landmarks within the mandible (eg, mandibular length) on lateral cephalograms, that relapse occurs, mainly at the osteotomy site and mainly within 6 weeks after surgery. In this study, relapse was also found following mandibular advancement stabilized by 2-mm bicortical screws. Caskey et al, 28 on the other hand, found excellent stability of mandibular lengthening stabilized with bicortical screw fixation, although individual variability was significant. Metal bone markers as measurement points have been suggested as a means to increase the accuracy of follow-up examinations after mandibular osteotomies Rubenstein et al 31 found no migration at the osteotomy site following mandibular advancement stabilized by rigid internal fixation. However, in that study, metal bone markers were placed only unilaterally, and postoperative analysis was performed in lateral cephalograms after superimposition. Consequently, there was no assessment of migration in more than one of the osteotomy sites in each patient. In an animal study with use of metal bone markers, Ellis et al 12 found no relapse during a 6- week observation period in animals who received rigid internal fixation after mandibular advancement. However, radiographic assessment was, again, performed on lateral cephalograms only. As in the study of Rubenstein et al, 31 no data were provided on bone marker stability. The influence of bicortical screw fixation versus miniplate and monocortical screw fixation on long-term stability after sagittal split osteotomies has been studied by Blomqvist et al. 32 They found no difference in the amount of relapse between the different techniques. This is in agreement with the findings of the present study in the respect that migration occurred at osteotomy sites stabilized with bicortical screws, as well as at those stabilized with miniplates and monocortical screws. Conclusions From the results of the present study, it is obvious that fixation of BSSOs with bicortical screws and/or miniplates and monocortical screws does not prevent postoperative migration at the osteotomy site. It is also obvious that stable bone

8 Int J Adult Orthod Orthognath Surg Vol. 16, No. 2, healing at the osteotomy site may take as long as 6 months or more, which emphasizes the importance of postoperative follow-up procedures to prevent relapse. Acknowledgments This study was generously supported by the fund for medical research of the Swedish National Board of Health and Welfare and the research funds of the Swedish Dental Association. References 1. Ive J, McNeill RW, West RA. Mandibular advancement: Skeletal and dental changes during fixation. J Oral Surg 1977;35: Kohn MW. Analysis of relapse after mandibular advancement surgery. J Oral Surg 1978;36: Schendel SA, Epker BN. Results after mandibular advancement surgery: An analysis of 87 cases. J Oral Surg 1980;38: Barer PG, Wallen TR, McNeill RW, Reitzik M. Stability of mandibular advancement osteotomy using rigid internal fixation. Am J Orthod Dentofac Orthop 1987;92: Gassmann CJ, Van Sickels JE, Thrash WJ. Causes, location and timing of relapse following rigid fixation after mandibular advancement. J Oral Maxillofac Surg 1990;48: Van Sickels JE, Larsen AJ, Thrash WJ. Relapse after rigid fixation of mandibular advancement. J Oral Maxillofac Surg 1986;44: Mayo KH, Ellis E III. Stability of the mandible after advancement and use of dental plus skeletal maxillomandibular fixation. An experimental investigation in Macaca mulatta. J Oral Maxillofac Surg 1987;45: Van Sickels JE. A comparative study of bicortical screws and suspension wires versus bicortical screws in large mandibular advancements. J Oral Maxillofac Surg 1991;49: Scheerlinck JPO, Stoelinga PJW, Blijdorp PA, Brouns JJA, Nijs MLL. Sagittal split advancement osteotomies stabilized with miniplates. A 2- to 5- year follow-up. Int J Oral Maxillofac Surg 1994; 23: Moore KE, Gooris PJJ, Stoelinga PJW. The contributing role of condylar resorption to skeletal relapse following mandibular advancement surgery: Report of five cases. J Oral Maxillofac Surg 1991;49: Van Sickels JE, Flanary CM. Stability associated with mandibular advancement treated by rigid osseous fixation. J Oral Maxillofac Surg 1985;43: Ellis E III, Reynolds S, Carlson DS. Stability of the mandible following advancement: A comparison of three postsurgical fixation techniques. Am J Orthod Dentofac Orthop 1988;94: Abeloos J, De Clercq C, Neyt L. Skeletal stability following miniplate fixation after bilateral sagittal split osteotomy for mandibular advancement. J Oral Maxillofac Surg 1993;51: Lee J, Piecuch JF. The sagittal ramus osteotomy: Stability of fixation with internal miniplates. Int J Oral Maxillofac Surg 1992;21: Van Sickels JE, Larsen AJ, Thrash JW. A retrospective study of relapse in rigidly fixated sagittal split osteotomies: Contributing factors. Am J Orthod Dentofac Orthop 1988;93: Kierl MJ, Nanda RS, Currier GF. A 3-year evaluation of skeletal stability of mandibular advancement with rigid fixation. J Oral Maxillofac Surg 1990; 48: Watzke IM, Turvey TA, Phillips C, Proffit WR. Stability of mandibular advancement after sagittal osteotomy with screw or wire fixation: A comparative study. J Oral Maxillofac Surg 1990;48: Baumrind S, Franz RC. The reliability of head film measurements. 1. Landmark identification. Am J Orthod 1971;60: Houston WJB, Maher RE, McElroy D,Sherriff M. Sources of error in measurements from cephalometric radiographs. Eur J Orthod 1986;8: Rosenquist B, Rune B, Selvik G. Displacement of the mandible during intermaxillary fixation after oblique sliding osteotomy. A stereometric and cephalometric radiographic study. J Maxillofac Surg 1985;13: Baumrind S, Miller D, Molthen R. The reliability of head film measurements. 3. Tracing superimposition. Am J Orthod 1976;70: Houston WJB, Lee RT. Accuracy of different methods of radiographic superimposition on cranial base structures. Eur J Orthod 1985;7: Selvik G. A Stereophotogrammetric Method for Study of the Kinematics of the Skeletal System [thesis]. Lund: AV-Centralen, University of Lund, Bjork A. Facial growth in man, studied with the aid of metallic implants. Acta Odontol Scand 1955;13: Alberius P. Bone reactions to tantalum markers. A scanning electron microscopic study. Acta Anat (Basel) 1983;115: Aronson A, Jonsson N, Alberius P. Tantalum markers in radiography and assessment of tissue reaction. Skeletal Radiol 1985;14: Soderkvist I, Wedin PA. Determining the movements of the skeleton using well-configured markers. J Biomech 1993;26: Caskey RT, Turpin DL, Bloomquist DS. Stability of mandibular lengthening using bicortical screw fixation. Am J Orthod Dentofac Orthop 1989; 96: Robinson M, Tanaka S. Simplified metal bonemarker method for evaluation of surgical orthodontics: Report of first use. Am J Orthod 1978; 74: Wade DB. Surgical-orthodontic stability in retrognathic patients. An implant study. Angle Orthod 1988;58: Rubenstein LK, Strauss RA, Lindauer SJ, Davidovitch M, Isaacson RJ. Tantalum implants as markers for evaluating postoperative orthognathic surgical changes. Int J Adult Orthod Orthognath Surg 1993;8: Blomqvist JE, Ahlborg G, Isaksson S, Svartz K. A comparison of skeletal stability after mandibular advancement and use of two rigid internal fixation techniques. J Oral Maxillofac Surg 1997;55: