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Skeletal stability following mandibular advancement and rigid fixation with polylactide biodegradable screws
Int. Z Oral Maxillofae. Surg. 1998; 27:3 8 Printed in Denmark. All rights reserved
Copyright 9 Munksgaard 1998 internationalJournalof
Oral& Max~ofacial Surgery ISSN 0901-5027
Aesthetic and reconstructivesurgery
Skeletal stability following mandibular advancement and rigid fixation with polylactide biodegradable screws
Ilkka Kallela 1, Pekka Laine 1, Riitta Suuronen 1, Tateyki lizuka 1, Sinikka Pirinen 2, Christian Lindqvist ~ 1Department of Oral and Maxillofaciat Surgery, Helsinki University Central Hospital; 2Department of Orthodontics and Pedodontics, University of Helsinki, Helsinki, Finland
L Kallela, P. Laine, R. Suuronen, T. Iizuka, S. Pirinen, C. Lindqvist." Skeletal stability following mandibular advancement and rigid fixation with polylactide biodegradable screws. Int. J. Oral Maxillofac. Surg. 1998; 27." 3-8.
9 Munksgaard, 1998 Abstract. Skeletal stability during the first year after mandibular advancement
surgery and fixation using biodegradable selfreinforced poly-l-lactide (SR-PLLA) screws, without postoperative intermaxillary fixation, was studied in 25 patients by means of cephalometric measurements. The magnitude of advancement was on average, 3.88 mm at pogonion (PG) (range 1.25-6.5 mm) and 4.57 mm at Bpoint (range 2.75-7.5 ram). After one year a mean relapse backwards of 0.59 mm at the P G (15%) and 0.78 mm at the B-point (17%) was observed. Nineteen patients (76%) and 17 patients (68%) were stable at P G and B-point, respectively. SR-PLLA screws are considered to be comparable to other forms of rigid internal fixation for fixation of bilateral splitting osteotomies after mandibular advancement, as far as skeletal stability is concerned.
Bilateral sagittal splitting osteotomy (BSSO) 2,24 is now a popular surgical procedure for correction of mandibular hypoplasia, prognathism and asymmetries. A major concern of surgical-orthodontic teams has been that after treating patients with this procedure or other orthognathic procedures, repositioned bone fragments tend to move from positions achieved during surgery. This has been attributed largely to increased Soft tissue and muscular tensions and/or alterations in the condylearticular fossa relationship occurring during the operative procedures 6. Several factors affecting the tendency to relapse after treatment of mandibular hypoplasia with BSSO and mandibular advancement have been identified or suggested. There would seem to be a correlation between the amount of
advancement and the extent of relapse 1,7,17,23. Anticlockwise rotation of the mandible is known to increase risk of relapse 14. Condylar resorption can result in significant relapse 16. Appropriate pre- and postoperative orthodontic therapy is believed to decrease the tendency to relapse. The factor affecting relapse that has been most studied is the means by which the fragments are stabilized relative to one another. With the advent of BSSO, various kinds of transosseous wires 24 together with several weeks of intermaxillary fixation (IMF) have been used. Rigid internal fixation (RIF) without I M F was introduced by SPIESSL 19 in the 1970s. His method involved the use of three lag-screws at the osteotomy site, two above the neurovascular bundle and one below, to stabilize the bony fragments. Since then, many
Key words: mandibular advancement; skeletal stability; polylactide screws. Accepted for publication 1 July 1997
modifications of the screw osteosynthesis principle have been used, varying in relation to numbers, placement patterns, sites and sizes of screws 9,17,22. Miniplates were introduced for fixation in BSSOs, without IMF, by RUBENS et al. 15 in the late 1980s. These R I F methods are reported to have two advantages over transosseous wiring and IMF. Firstly, there is the obvious benefit that the patient avoids IMF. Secondly, better skeletal and dental stability can be achieved s,l~ 12,16, although conflicting findings have been reported 25. There are few clinical reports about use of biodegradable osteosynthesis material for fixation in BSSO. DU~BACH 3'4 used poly-p-diaxanon screws and rods. There are no reports of longterm results. OBWEGESER 13 used screws of allogenic cortical bone and claimed good clinical results. Since 1991, selfre-
4
Kallela et aL
Fig. 1. Photograph of equipment and screws used in this study. From left to right: Biofix| SR-PLLA 3.5 mm sagittal osteotomy screw, screwdriver, countersink, 3.5 mm tap, 2.7 mm drill bit and transbuccal trocar (Bioscience, Tampere, Finland). The first screws had a hemispherical head, but were later replaced by the model shown in the picture, to reduce the diameter of the transbuccal trocar. After placement, the excess screw head was removed with a drill.
inforced poly-Mactide (SR-PLLA) screws have also been used to stabilize BSSOs in the D e p a r t m e n t o f Oral and Maxillofacial Surgery o f Helsinki University Central Hospital. They have allowed predictable b o n y consolidation at osteotomy sites 22. We report here on a study o f skeletal stability following m a n d i b u l a r advancem e n t after BSSO fixed with biodegradable S R - P L L A screws.
haemostat clamp and gently pushed upwards and backwards into the temporomandibular joint fossae. Fragments were fixed using two pretapped positional SR-PLLA screws (Bioscience, Tampere, Finland) placed over the neurovascular bundle on both sides. The core diameter of the screws was 2.7 mm and the thread diameter 3.5 mm. The molecular weight of the material was 710 000. The initial shear force needed to break the screw was 110 N (Fig. 1). Postoperatively, no IMF was used. Most patients were equipped with light guiding
elastics. Occlusal splints were removed at five weeks and orthodontic treatment was restarted after consolidation. During follow up, specific attention was paid to clinical signs of relapse. Postoperative clinical follow up involved weekly appointments for approx. six weeks. Thereafter, patients were seen at three and six months, and at one year. Lateral cephalometric radiographs and panoramic mandibular views were obtained one month before operation (TO radiographs), postoperatively during the first three days in the Department (T1), and at 3, 6 and 12 months (T2, T3 and T4, respectively). TO, T1 and T4 radiographs were considered essential to the study. Patients for whom these radiographs were missing, were excluded. T2 lateral cephalometric radiographs were missing in three, and T3 radiographs in six patients. A standard cephalometric radiographic technique was used with a magnifying factor of 1.1 for the midsagittal structures. Magnification was corrected for in relation to the linear parameters used in this study. Panoramic mandibular views were analysed in particular for resorptive changes in the condylar region. For cephalometric analysis (Fig. 2), an acetate paper tracing was prepared from the TO lateral cephalometric radiograph of each patient by tracing contours of the anterior cranial base and maxilla. Four bony cephalometric landmarks were marked (anterior nasal spine (ANS), nasion (N), posterior nasal spine (PNS) and sella (S)). Using the technique of structural superimposition these points were transferred to tracings from subsequent radiographs. The mandibular cephalometric points articulare (AR), supramentale (B), gonion (GO), menton (ME) and
Material and methods
Between 1991 and 1995, 33 patients underwent a BSSO for mandibular hypoplasia fixed with biodegradable SR-PLLA screws in the Department of Oral and Maxillofacial Surgery of Helsinki University Central Hospital. Twenty-five patients met the requirements for inclusion, follow up and radiological documentation as described below. SR-PLLA screws were used in all patients in whom the planned advancement was symmetrically bilateral, and less than 7 mm. Patients undergoing other maxillary or mandibular ostetomies simultaneously with BSSO were excluded from the study. All patients invited to receive SR-PLLA fixation agreed to take part in the study. The mean age of the patients was 27 years (range 18 38 years). Twenty were women, five men. All patients underwent pre- and postoperative orthodontic treatment. Surgery was performed under general anesthesia via nasotracheal intubation. The OBWEGESER-DAL PONT2'24 osteotomy design was used. After osteotomy the distal part was positioned with the help of a prefabricated occlusal splint and IMF was accomplished. Proximal segments were grasped using a
H'el
q
Fig. 2. Cephalometric landmarks used in this study.
Stability following biodegradable fixation pogonion (PG) were traced individually for all radiographs. When a double contour was detected (GO, AR), the midpoint was taken. To ensure that points in the chin region were correctly located, a separate acetate template was prepared to allow transfer of the ME, PG and B points to subsequent tracings for each patient. All patients were wearing occlusal splints when the T1 radiograph was taken. To minimize the effect of the splint, we used the technique described earlier8 and prepared a separate mandibular acetate template from the T1 tracing. This template was then placed on the T1 tracing and the mandible autorotated around the condylion (CO) point until the edge of the lower incisors met the maxillary dentition. Cephalometric analysis involved four linear and three angular measurements. To construct a reference line for horizontal positioning of the PG and B points, a line was drawn perpendicular to the line ANS-PNS, through point N. Linear measurements were then made from PG and B perpendicular to this reference line. The anterior facial height (AFH =linear measurement from N to the horizontal line through ME, parallel to ANS PNS) and the posterior facial height (PFH=S GO) were measured to allow assessment of vertical changes. The angular measurements were the mandibular plane angle (MPA=angle between the lines (S-N) and (ME-GO)), the goniai angle (ARGO-ME) and the position of the ascending ramus (S AR-GO). All tracings and measurements were made manually by the same person. Measurements were registered in half millimetres and in half degrees. All tracings and measurements were made twice, with at least two weeks' interval. The two sets of records for all patients were used to analyse error. Methodological error (e) resulting from superimposition, tracing and measuring was calculated from the formula e =,/27d2/2n where d was the difference between measurements and n the number of duplicate determinations. Methodological errors are listed in Table 1. For the statistical analysis, all data recorded were stored in an SAS computer. Means of duplicate values were used. Changes at each time point and between T1 and T4 were analysed. A paired Student's ttest was used to determine statistical significance. Covariance analysis was used to deter-
mine whether there was any correlation between cephalometric variables at TO, sex and age, and postsurgical (T1-T4) changes in PG, B and AFH. Covariance analysis was also used to determine the correlation between surgical (TO T1) changes and postoperative (TI-T4) changes in PG, B and AFH.
Results Changes over the various time periods are recorded in Tables 2, 3 and 4. A n t e r i o r positioning o f the chin (3.88 m m at PG, SD 1.38), range 1.25-6.5 mm; a n d 4.57 m m at B, SD 1.33, range 2.75-7.5 m m ) was accomplished during surgery by moving the distal fragment anteriorly. Clockwise rotation o f the anterior f r a g m e n t (movement at point B was greater t h a n at point P G in horizontal direction) was observed in all cases. A F H increased by 3.93 mm. Gonial angles moved anteriorly (the angle S - A R - G O decreased). Posterior facial height ( P F H ) declined slightly.
Linear variables (mm) PG B AFH PFH
0.56 0.63 0.56 1.06
Angular variables (~ MPA S-AR-GO AR-GO ME
0,51 0.98 1.15
M a n d i b u l a r plane angle (MPA) increased by 2.42 ~ the gonial angle ( A R G O - M E ) by 6.25 ~. A F H decreased by 0.89 m m between T1 a n d T2. The relapse was 0.31 m m at P G a n d 0.39 m m at B. Skeletal changes between T2 a n d T3 were minimal. A F H decreased by 0.64 m m between T3 and T4. The relapse was 0.30 m m at P G and 0.22 m m at B. A f t e r one year, T 1 T 4 , the m e a n relapse at P G was 0.59 m m (15%), and at B, 0.78 m m (17%). The m e a n decrease in A F H was 1.65 m m (42% relapse), a n d in P F H , 1.05 mm. There were no angular changes relating to the position o f the b o d y and ascending ramus o f the mandible. D u r i n g clinical follow up the examiners saw n o signs o f relapse o f the facial skeleton or dentition. The m e a n overjet was 2.04 m m (range 1 3 mm). All patients h a d Angle I first m o l a r relationship. N o signs o f condylar resorp-
Table 2. Changes from TO to TI (surgical changes) and from T1 to T2 TO to T1 (n=25) Linear variables (mm) PG B AFH PFH
T1 to T2 (n=21)
d
SD
d
SD
-3.88 -4.57 -3.93 -0.65
1.38a 1.33~ 2.07 a 2.61
0.31 0.39 0.89 0.49
1.01
-2.42 2.87 -6.25
2.14 a 2.34 a 3.06 a
0.10 0.58 -0.01
0.83 b
1.20c 1.50
Angular variables (~ MPA S-AR-GO AR-GO-ME
0.75 1.18b 1.75
T0=before surgery, Tl=immediately after surgery, T2=3 months after surgery, d=mean difference, n=sample size, SD=standard deviation. a P<0.001, b P<0.05, ~ P<0.01. Negative value indicates advancement, positive value posterior relapse in PG and B. Otherwise a negative value indicates an increase and a positive value a decrease in the variable.
Table 3. Changes from T2 to T3 and from T3 to T4 T2 to T3 (n=18) Linear variables (mm)
Table 1. Methodological errors in cephalometric analysis
5
PG B AFH PFH
d
SD
T3 to T4 (n=21) d
-0.11 0.13 0.03 0.72
0.79 0.77 0.92 1.92
0.30 0.22 0.64 -0.19
-0.03 0.13 -0.03
0.63 1.00 1.31
0.32 0.25 0.31
SD 0.90 0.61 0.79 a 1.84
Angular variables (~ MPA S-AR-GO AR-GO-ME
1.08 0.80 1.28
T2=3 months after surgery, T3=6 months after surgery, T4=l year after surgery, d=mean difference, n=sample size, SD=standard deviation. a P<0.01. Negative value indicates advancement, positive value posterior relapse in PG and B. Otherwise a negative value indicates an increase and a positive value a decrease in the variable.
6
Kallela et aL
Table 4. Changes from T1 to T4 (overall
changes)
mm
8.0
[] A D V A N C E M E N T
(n=25) Linear variables (ram) PG B AFH PFH
d
SD
0.59
1.17~'
0.78
0.98 b
1.65 1.05
1.49b 1.98a
0.24 0.24 0.07
0.99 2.01 1.62
[]
7.0
RELAPSE
6.0 5.0 4.0
Angular variables (~ MPA S AR-GO AR GO-ME
Tl=immediately after surgery, T4=I year after surgery, d=mean difference, n=sample size, SD=standard deviation. a P<0.05, b P<0.001. Negative value indicates advancement, positive value posterior relapse in PG and B. Otherwise a negative value indicates an increase and a positive value a decrease in the variable.
3.0 2,0
1.0 0
N
|
- 1.0 - 2.0 - 3.0 9 4.0
1 2
3
4
5 6
7 8
9 10111213141516171819202122232425
Fig. 3. Mandibular advancement and skeletal relapse at pogonion (PG). Directional changes:
tion were seen in panoramic radiographs of the mandible. Figs. 3 and 4 indicate changes relating to PG and B in different patients. Nineteen patients (76%) were stable at PG (relapse TI-+I ram) (Fig. 5). One patient (4%) had relapse anteriorly of > 1-2 mm, two (8%) had relapse posteriorly of > 1 - 2 mm and three (12%) had relapse posteriorly of > 2 - 4 mm. Seventeen patients (68%) were stable at B (relapse TI-+I mm). Five patients (20%) had relapse posteriorly of >1-2 mm and three (12%) had relapse posteriorly of > 2 - 4 mm. No statistically significant (P<0.05) correlation was found between preoperative characteristics and postoperative changes. There was a statistifically significant negative correlation between surgical and postoperative changes in relation to AFH. There was no statistically significant correlation between surgical and postoperative changes in relation to PG and B.
(-) denotes posterior movement; (+) denotes anterior movement.
/rim
8.0
[] A D V A N C E M E N ~v 9 RELAPSE
7.0 6.0 5.0 4.0 3.0
It~l I/lil
IN
2.0
N
1.0 0 - 1.0 - 2.0
-3.0 - 4.0
1 2
3
4
5
6
7 8 9 10111213141516171819202122232425
Fig. 4. Mandibular advancement and skeletal relapse at B-point. Directional changes: (-)
Discussion
denotes posterior movement; (+) denotes anterior movement.
In most of our patients relapse was within 1 ram. Considering the methodological error and the clinical setting, this is of no clinical significance. There were, however, eight patients who had more relapse. Four of these patients had only moderate advancements. In previous studies, relapse after RIF with screws or miniplates without IMF after mandibular advancement, varied from 5.2% to 26% i's'9'15 17,23. In some
studies, the direction of the net relapse was backwards, in others forwards. Comparisons between different studies are difficult because of differences in patient populations, cephalometric analyses, magnitudes of advancements, use of occlusal splints in postoperative radiographs and follow-up periods. In the present study, the net backward re-
lapse of 15% at point PG and of 17% at point B are well within the abovementioned range. Variability in relapse after mandibular advancement has also been shown after rigid fixation using metallic screws and miniplates7,~6. Theoretically, relapse after rigid fixation is caused by movement at the osteotomy site and/or
Stability following biodegradable fixation
7
S R - P L L A screws. The results, with regard to postoperative skeletal stability, are comparable to those obtained using other forms of R I F in patients needing advancements of up to 6-7 mm.
Acknowledgment. This study was supported by a grant from the Helsinki University Central Hospital Research Funds. References 1. BLOMQVIST JE, ISAKSSONS. Skeletal sta-
bility after mandibular advancement: a comparison of two rigid internal fixation techniques. J Oral Maxillofac Surg 1994: 52: 1133-7. 2. DAL PONT G. Retromolar osteotomy for
Fig. 5. Cephalometric radiographs of a 24-year-old woman taken a) immediately after mandibular advancement surgery and fixation with biodegradable screws, b) after one year showing stable results.
repositioning in the condyle area. Changes in condylar position after sagittal splitting osteotomy and rigid fixation are frequent 2~ Studies in animals 5 have shown that adaptive remodelling takes place in the condylar region after mandibular orthognathic surgery. Such alterations in condylar position and anatomical structures, together with technical errors, could explain the great variability in relapse between patients with equal advancements and similar fixation. This could also explain why some patients have forward relapse. H a l f of the relapse in our patients developed in the second half of the first year after operation. It has been shown 2I that the strength of an osteotomy site fixed with S R - P L L A after BSSO in sheep was greater after 14 weeks than at unoperated sites. This indicates that there must also have been other factors leading to relapse in our patients, apart from movement at the osteotomy site. The incidence of progressive condylar resorption (PCR) can be as high as 7.7% 16 after sagittal splitting osteotomy and miniplate fixation. It usually manifests itself during the second half of the year after the procedure, and can a m o u n t to total relapse 16. Radiological follow up revealed no P C R in our patients. However, the number of patients might have been too small and the follow up too short to show the real incidence of P C R in these patients. An essential factor in relation to the cause of relapse after mandibular advancement surgery, is considered to be
the stretching of paramandibular soft tissue as a result of surgical intervention 6,9. The greater the advancement, the greater the stretching of soft tissues. M a n y investigators have found a correlation between extent of advancement and relapse 1,7,17,23, but this was not obvious from the present study. However, the four patients with the greatest advancements were all in the relapse group. Advancements of more than 6 or 7 m m 9,17 have been shown to be more prone to relapse after metallic screw fixation. Some caution is warranted when S R - P L L A screws are used after mandibular advancement of more than 6 mm. Short I M F for one week with skeletal suspension is has been shown to result in stability after metallic screw fixation of large advancements, and might also be used with SRP L L A screw fixation. Clinical results in relation to occlusion were good in all patients despite occasional moderate skeletal relapse. This might have been due to postoperative orthodontic therapy, correcting any developing horizontal discrepancy. Orthodontic therapy, by settling the occlusion, might also explain some of the relapse in A F H . Other possible causes are rotation of the mandible and resorptional changes in the chin. Anterior vertical relapse has been reported by many other investigators, and a correlation with magnitude of advancement has been noted 16,23. The results of our study show that bilateral splitting osteotomy of the mandible can be fixed using biodegradable
the correction of prognathism. J Oral Surg 1961: 19: 42-7. 3. DUMBACHJ. Zugschraubenosteosynthese nach Ramusosteotomie mit resorbieren Osteosyntheseschrauben aus Polydioxanon (PDS)- erste Ergebnisse. Dtsch Z Mund Kiefer GesichtsChir 1984: 8: 1458. 4. DUMBACHJ. Osteosynthese mit resorbieren PDS-Stiften nach sagittaler Spaltung und Rtickversetzung des Unterkiefers. Dtsch Zahnfirzt Z 1987: 8: 145. 5. ELLIS E, HINTONRJ. Histologic examination of the temporomandibular joint after mandibular advancement with and without rigid fixation: an experimental investigation in adult macaca mulatta. J Oral Maxillofac Surg 1991: 49:1316 27. 6. EPKER BN, WESSBEROGA. Mechanisms of early skeletal relapse following surgical advancement of the mandible. Br J Oral Maxitlofac Surg 1982: 20: 175-82. 7. KIERL M J, NANDA RS, CURRIER G E A
3-year evalution of skeletal stability of mandibular advancement with rigid fixation. J Oral Maxiltofac Surg 1990: 48: 587-92. 8. KIRKPATRICK TB, WOODS MG, SWIFT JQ, MARKOWlTZ NR. Skeletal stability following mandibular advancement and rigid fixation. J Oral Maxillofac Surg 1987: 45: 572-6. 9. KNAUV CA, WALLEN TR, BLOOMQUIST DS. Linear and rotational changes in large mandibular advancements using three or four fixation screws. Int J Adult Orthod Orthognath Surg 1993: 8: 24563. 10. LAKE SL, McNEILL RW, LITTLE RM, WEST RA. Surgical mandibular advancement: a cephalometric analysis of treatment response. Am J Orthod 1981: 80: 37644. 11. MOENNINGJE, BUSSARDDA, LAPP TH, GARRISON BT. A comparison of relapse in bilateral sagittal split osteotomies for mandibular advancement: rigid internal fixation (screws) versus inferior border wiring with anterior skeletal fixation. Int J Adult Orthodont Orthognath Surg 1990: 5! 175 82.
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Kallela et al.
12. MOMMAERTSMY. Lag screw versus wire osteosynthesis in mandibular advancement. Int J Adult Orthodont Orthognath Surg 1991: 6:153 60. 13. OBWEGESER JA. Bioconvertible screws made of allogenic cortical bone for osteosyntheis following sagittal split osteotomy without postoperative immobilisation. J Cranio-Max-Fac-Surg 1994: 22: 63-75. 14. PROFFIT WR. Treatment planning: the search for wisdom. In: PROFFIT WR, WroTE RE Surgical-orthodontic treatment. St. Louis: Mosby, 1991: 142-90. 15. RUBENS Be, STOELINGA PJW, BL1JDORP PA, SCHOENAERS JHA, POLmS C. Skeletal stability following sagittal split ostetomy using monocortical miniplate internal fixation. Int J Oral Maxillofac Surg 1988: 17: 371-6. 16. SCHEERLINCK JPO, STOELINGA PJW, BLIJDORP PA, BROUNS JJA, NIJS MLL. Sagittal split advancement ostetomies stabilized with miniplates: a ~5-year follow-up, int J Oral Maxillofac Surg 1994: 23:127 31. 17. VAN NICKELSJE, LARSEN AJ, TRASH WJ. Relapse after fixation of mandibular ad-
vancement. J Oral Maxillofac Surg 1986: 44: 698-702. 18. VAN SrCKELSJE. A comparative study of bicortical screws and suspension wires versus bicortical screws in large mandibular advancements. J Oral Maxillofax Surg 1991: 49: 1293-6. 19. SPtESSL B. Osteosynthese bei sagittalen Osteotomie nach Obwegeser/Dal Pont. Fortschr Kiefer- und Gesicbtschirurgie. 1974: 18:145 8. 20. STROSTER TG, PANGRAZIO-KULBERSHV. Assessment of condylar position following bilateral sagittal split ramus osteotomy with wire fixation or rigid fixation. Int J Adult Orthod Orthognath Surg 1994: 9:55 63. 21. SUURONEN R, LAINE P, SARKIALA E, POHJONEN T, LINDQVIST C. Sagittal split osteotomy fixed with biodegradable, selfreinforced poly-l-lactide screws. A pilot study in sheep. Int J Oral Maxillofac Surg 1992: 21: 303-8. 22. SUURONEN R, LAINE P, POHJONEN T, LINDQVIST C. Sagittal ramus osteotomies fixed with biodegradable screws: a preliminary report. J Oral MaxiIlofac Surg 1994: 52: 715-20.
23. THUER U, INGERVALL B, VUILLEMIN T. Stability and effect on the soft tissue profile of mandibular advancement with sagittal split osteotomy and rigid internal fixation. Int J Adult Orthod Orthognath Surg 1994: 9: 175-85. 24. TRAUN~RR, OBWEGESERH. The surgical correction of mandibular prognathism and retrognathia with consideration of genioplasty. Surgical procedures to correct mandibular prognathism and reshaping of the chin. Oral Surg 1957: 10: 67789. 25. WATZKE IM, TURVEY TA, PHtLLIPS C, PROFFIT WR. Stability of mandibular advancement after sagittal osteotomy with screw or wire fixation: a comparative study. J Oral Maxillofac Surg 1990: 48: 108-121. Address: Dr Ilkka Kallela Department of Oral and Maxillofacial Surgery Helsinki University Central Hospital Kasarmikatu 11-13 SF-O0130 Helsinki Finland
Copyright 9 Munksgaard 1998 internationalJournalof
Oral& Max~ofacial Surgery ISSN 0901-5027
Aesthetic and reconstructivesurgery
Skeletal stability following mandibular advancement and rigid fixation with polylactide biodegradable screws
Ilkka Kallela 1, Pekka Laine 1, Riitta Suuronen 1, Tateyki lizuka 1, Sinikka Pirinen 2, Christian Lindqvist ~ 1Department of Oral and Maxillofaciat Surgery, Helsinki University Central Hospital; 2Department of Orthodontics and Pedodontics, University of Helsinki, Helsinki, Finland
L Kallela, P. Laine, R. Suuronen, T. Iizuka, S. Pirinen, C. Lindqvist." Skeletal stability following mandibular advancement and rigid fixation with polylactide biodegradable screws. Int. J. Oral Maxillofac. Surg. 1998; 27." 3-8.
9 Munksgaard, 1998 Abstract. Skeletal stability during the first year after mandibular advancement
surgery and fixation using biodegradable selfreinforced poly-l-lactide (SR-PLLA) screws, without postoperative intermaxillary fixation, was studied in 25 patients by means of cephalometric measurements. The magnitude of advancement was on average, 3.88 mm at pogonion (PG) (range 1.25-6.5 mm) and 4.57 mm at Bpoint (range 2.75-7.5 ram). After one year a mean relapse backwards of 0.59 mm at the P G (15%) and 0.78 mm at the B-point (17%) was observed. Nineteen patients (76%) and 17 patients (68%) were stable at P G and B-point, respectively. SR-PLLA screws are considered to be comparable to other forms of rigid internal fixation for fixation of bilateral splitting osteotomies after mandibular advancement, as far as skeletal stability is concerned.
Bilateral sagittal splitting osteotomy (BSSO) 2,24 is now a popular surgical procedure for correction of mandibular hypoplasia, prognathism and asymmetries. A major concern of surgical-orthodontic teams has been that after treating patients with this procedure or other orthognathic procedures, repositioned bone fragments tend to move from positions achieved during surgery. This has been attributed largely to increased Soft tissue and muscular tensions and/or alterations in the condylearticular fossa relationship occurring during the operative procedures 6. Several factors affecting the tendency to relapse after treatment of mandibular hypoplasia with BSSO and mandibular advancement have been identified or suggested. There would seem to be a correlation between the amount of
advancement and the extent of relapse 1,7,17,23. Anticlockwise rotation of the mandible is known to increase risk of relapse 14. Condylar resorption can result in significant relapse 16. Appropriate pre- and postoperative orthodontic therapy is believed to decrease the tendency to relapse. The factor affecting relapse that has been most studied is the means by which the fragments are stabilized relative to one another. With the advent of BSSO, various kinds of transosseous wires 24 together with several weeks of intermaxillary fixation (IMF) have been used. Rigid internal fixation (RIF) without I M F was introduced by SPIESSL 19 in the 1970s. His method involved the use of three lag-screws at the osteotomy site, two above the neurovascular bundle and one below, to stabilize the bony fragments. Since then, many
Key words: mandibular advancement; skeletal stability; polylactide screws. Accepted for publication 1 July 1997
modifications of the screw osteosynthesis principle have been used, varying in relation to numbers, placement patterns, sites and sizes of screws 9,17,22. Miniplates were introduced for fixation in BSSOs, without IMF, by RUBENS et al. 15 in the late 1980s. These R I F methods are reported to have two advantages over transosseous wiring and IMF. Firstly, there is the obvious benefit that the patient avoids IMF. Secondly, better skeletal and dental stability can be achieved s,l~ 12,16, although conflicting findings have been reported 25. There are few clinical reports about use of biodegradable osteosynthesis material for fixation in BSSO. DU~BACH 3'4 used poly-p-diaxanon screws and rods. There are no reports of longterm results. OBWEGESER 13 used screws of allogenic cortical bone and claimed good clinical results. Since 1991, selfre-
4
Kallela et aL
Fig. 1. Photograph of equipment and screws used in this study. From left to right: Biofix| SR-PLLA 3.5 mm sagittal osteotomy screw, screwdriver, countersink, 3.5 mm tap, 2.7 mm drill bit and transbuccal trocar (Bioscience, Tampere, Finland). The first screws had a hemispherical head, but were later replaced by the model shown in the picture, to reduce the diameter of the transbuccal trocar. After placement, the excess screw head was removed with a drill.
inforced poly-Mactide (SR-PLLA) screws have also been used to stabilize BSSOs in the D e p a r t m e n t o f Oral and Maxillofacial Surgery o f Helsinki University Central Hospital. They have allowed predictable b o n y consolidation at osteotomy sites 22. We report here on a study o f skeletal stability following m a n d i b u l a r advancem e n t after BSSO fixed with biodegradable S R - P L L A screws.
haemostat clamp and gently pushed upwards and backwards into the temporomandibular joint fossae. Fragments were fixed using two pretapped positional SR-PLLA screws (Bioscience, Tampere, Finland) placed over the neurovascular bundle on both sides. The core diameter of the screws was 2.7 mm and the thread diameter 3.5 mm. The molecular weight of the material was 710 000. The initial shear force needed to break the screw was 110 N (Fig. 1). Postoperatively, no IMF was used. Most patients were equipped with light guiding
elastics. Occlusal splints were removed at five weeks and orthodontic treatment was restarted after consolidation. During follow up, specific attention was paid to clinical signs of relapse. Postoperative clinical follow up involved weekly appointments for approx. six weeks. Thereafter, patients were seen at three and six months, and at one year. Lateral cephalometric radiographs and panoramic mandibular views were obtained one month before operation (TO radiographs), postoperatively during the first three days in the Department (T1), and at 3, 6 and 12 months (T2, T3 and T4, respectively). TO, T1 and T4 radiographs were considered essential to the study. Patients for whom these radiographs were missing, were excluded. T2 lateral cephalometric radiographs were missing in three, and T3 radiographs in six patients. A standard cephalometric radiographic technique was used with a magnifying factor of 1.1 for the midsagittal structures. Magnification was corrected for in relation to the linear parameters used in this study. Panoramic mandibular views were analysed in particular for resorptive changes in the condylar region. For cephalometric analysis (Fig. 2), an acetate paper tracing was prepared from the TO lateral cephalometric radiograph of each patient by tracing contours of the anterior cranial base and maxilla. Four bony cephalometric landmarks were marked (anterior nasal spine (ANS), nasion (N), posterior nasal spine (PNS) and sella (S)). Using the technique of structural superimposition these points were transferred to tracings from subsequent radiographs. The mandibular cephalometric points articulare (AR), supramentale (B), gonion (GO), menton (ME) and
Material and methods
Between 1991 and 1995, 33 patients underwent a BSSO for mandibular hypoplasia fixed with biodegradable SR-PLLA screws in the Department of Oral and Maxillofacial Surgery of Helsinki University Central Hospital. Twenty-five patients met the requirements for inclusion, follow up and radiological documentation as described below. SR-PLLA screws were used in all patients in whom the planned advancement was symmetrically bilateral, and less than 7 mm. Patients undergoing other maxillary or mandibular ostetomies simultaneously with BSSO were excluded from the study. All patients invited to receive SR-PLLA fixation agreed to take part in the study. The mean age of the patients was 27 years (range 18 38 years). Twenty were women, five men. All patients underwent pre- and postoperative orthodontic treatment. Surgery was performed under general anesthesia via nasotracheal intubation. The OBWEGESER-DAL PONT2'24 osteotomy design was used. After osteotomy the distal part was positioned with the help of a prefabricated occlusal splint and IMF was accomplished. Proximal segments were grasped using a
H'el
q
Fig. 2. Cephalometric landmarks used in this study.
Stability following biodegradable fixation pogonion (PG) were traced individually for all radiographs. When a double contour was detected (GO, AR), the midpoint was taken. To ensure that points in the chin region were correctly located, a separate acetate template was prepared to allow transfer of the ME, PG and B points to subsequent tracings for each patient. All patients were wearing occlusal splints when the T1 radiograph was taken. To minimize the effect of the splint, we used the technique described earlier8 and prepared a separate mandibular acetate template from the T1 tracing. This template was then placed on the T1 tracing and the mandible autorotated around the condylion (CO) point until the edge of the lower incisors met the maxillary dentition. Cephalometric analysis involved four linear and three angular measurements. To construct a reference line for horizontal positioning of the PG and B points, a line was drawn perpendicular to the line ANS-PNS, through point N. Linear measurements were then made from PG and B perpendicular to this reference line. The anterior facial height (AFH =linear measurement from N to the horizontal line through ME, parallel to ANS PNS) and the posterior facial height (PFH=S GO) were measured to allow assessment of vertical changes. The angular measurements were the mandibular plane angle (MPA=angle between the lines (S-N) and (ME-GO)), the goniai angle (ARGO-ME) and the position of the ascending ramus (S AR-GO). All tracings and measurements were made manually by the same person. Measurements were registered in half millimetres and in half degrees. All tracings and measurements were made twice, with at least two weeks' interval. The two sets of records for all patients were used to analyse error. Methodological error (e) resulting from superimposition, tracing and measuring was calculated from the formula e =,/27d2/2n where d was the difference between measurements and n the number of duplicate determinations. Methodological errors are listed in Table 1. For the statistical analysis, all data recorded were stored in an SAS computer. Means of duplicate values were used. Changes at each time point and between T1 and T4 were analysed. A paired Student's ttest was used to determine statistical significance. Covariance analysis was used to deter-
mine whether there was any correlation between cephalometric variables at TO, sex and age, and postsurgical (T1-T4) changes in PG, B and AFH. Covariance analysis was also used to determine the correlation between surgical (TO T1) changes and postoperative (TI-T4) changes in PG, B and AFH.
Results Changes over the various time periods are recorded in Tables 2, 3 and 4. A n t e r i o r positioning o f the chin (3.88 m m at PG, SD 1.38), range 1.25-6.5 mm; a n d 4.57 m m at B, SD 1.33, range 2.75-7.5 m m ) was accomplished during surgery by moving the distal fragment anteriorly. Clockwise rotation o f the anterior f r a g m e n t (movement at point B was greater t h a n at point P G in horizontal direction) was observed in all cases. A F H increased by 3.93 mm. Gonial angles moved anteriorly (the angle S - A R - G O decreased). Posterior facial height ( P F H ) declined slightly.
Linear variables (mm) PG B AFH PFH
0.56 0.63 0.56 1.06
Angular variables (~ MPA S-AR-GO AR-GO ME
0,51 0.98 1.15
M a n d i b u l a r plane angle (MPA) increased by 2.42 ~ the gonial angle ( A R G O - M E ) by 6.25 ~. A F H decreased by 0.89 m m between T1 a n d T2. The relapse was 0.31 m m at P G a n d 0.39 m m at B. Skeletal changes between T2 a n d T3 were minimal. A F H decreased by 0.64 m m between T3 and T4. The relapse was 0.30 m m at P G and 0.22 m m at B. A f t e r one year, T 1 T 4 , the m e a n relapse at P G was 0.59 m m (15%), and at B, 0.78 m m (17%). The m e a n decrease in A F H was 1.65 m m (42% relapse), a n d in P F H , 1.05 mm. There were no angular changes relating to the position o f the b o d y and ascending ramus o f the mandible. D u r i n g clinical follow up the examiners saw n o signs o f relapse o f the facial skeleton or dentition. The m e a n overjet was 2.04 m m (range 1 3 mm). All patients h a d Angle I first m o l a r relationship. N o signs o f condylar resorp-
Table 2. Changes from TO to TI (surgical changes) and from T1 to T2 TO to T1 (n=25) Linear variables (mm) PG B AFH PFH
T1 to T2 (n=21)
d
SD
d
SD
-3.88 -4.57 -3.93 -0.65
1.38a 1.33~ 2.07 a 2.61
0.31 0.39 0.89 0.49
1.01
-2.42 2.87 -6.25
2.14 a 2.34 a 3.06 a
0.10 0.58 -0.01
0.83 b
1.20c 1.50
Angular variables (~ MPA S-AR-GO AR-GO-ME
0.75 1.18b 1.75
T0=before surgery, Tl=immediately after surgery, T2=3 months after surgery, d=mean difference, n=sample size, SD=standard deviation. a P<0.001, b P<0.05, ~ P<0.01. Negative value indicates advancement, positive value posterior relapse in PG and B. Otherwise a negative value indicates an increase and a positive value a decrease in the variable.
Table 3. Changes from T2 to T3 and from T3 to T4 T2 to T3 (n=18) Linear variables (mm)
Table 1. Methodological errors in cephalometric analysis
5
PG B AFH PFH
d
SD
T3 to T4 (n=21) d
-0.11 0.13 0.03 0.72
0.79 0.77 0.92 1.92
0.30 0.22 0.64 -0.19
-0.03 0.13 -0.03
0.63 1.00 1.31
0.32 0.25 0.31
SD 0.90 0.61 0.79 a 1.84
Angular variables (~ MPA S-AR-GO AR-GO-ME
1.08 0.80 1.28
T2=3 months after surgery, T3=6 months after surgery, T4=l year after surgery, d=mean difference, n=sample size, SD=standard deviation. a P<0.01. Negative value indicates advancement, positive value posterior relapse in PG and B. Otherwise a negative value indicates an increase and a positive value a decrease in the variable.
6
Kallela et aL
Table 4. Changes from T1 to T4 (overall
changes)
mm
8.0
[] A D V A N C E M E N T
(n=25) Linear variables (ram) PG B AFH PFH
d
SD
0.59
1.17~'
0.78
0.98 b
1.65 1.05
1.49b 1.98a
0.24 0.24 0.07
0.99 2.01 1.62
[]
7.0
RELAPSE
6.0 5.0 4.0
Angular variables (~ MPA S AR-GO AR GO-ME
Tl=immediately after surgery, T4=I year after surgery, d=mean difference, n=sample size, SD=standard deviation. a P<0.05, b P<0.001. Negative value indicates advancement, positive value posterior relapse in PG and B. Otherwise a negative value indicates an increase and a positive value a decrease in the variable.
3.0 2,0
1.0 0
N
|
- 1.0 - 2.0 - 3.0 9 4.0
1 2
3
4
5 6
7 8
9 10111213141516171819202122232425
Fig. 3. Mandibular advancement and skeletal relapse at pogonion (PG). Directional changes:
tion were seen in panoramic radiographs of the mandible. Figs. 3 and 4 indicate changes relating to PG and B in different patients. Nineteen patients (76%) were stable at PG (relapse TI-+I ram) (Fig. 5). One patient (4%) had relapse anteriorly of > 1-2 mm, two (8%) had relapse posteriorly of > 1 - 2 mm and three (12%) had relapse posteriorly of > 2 - 4 mm. Seventeen patients (68%) were stable at B (relapse TI-+I mm). Five patients (20%) had relapse posteriorly of >1-2 mm and three (12%) had relapse posteriorly of > 2 - 4 mm. No statistically significant (P<0.05) correlation was found between preoperative characteristics and postoperative changes. There was a statistifically significant negative correlation between surgical and postoperative changes in relation to AFH. There was no statistically significant correlation between surgical and postoperative changes in relation to PG and B.
(-) denotes posterior movement; (+) denotes anterior movement.
/rim
8.0
[] A D V A N C E M E N ~v 9 RELAPSE
7.0 6.0 5.0 4.0 3.0
It~l I/lil
IN
2.0
N
1.0 0 - 1.0 - 2.0
-3.0 - 4.0
1 2
3
4
5
6
7 8 9 10111213141516171819202122232425
Fig. 4. Mandibular advancement and skeletal relapse at B-point. Directional changes: (-)
Discussion
denotes posterior movement; (+) denotes anterior movement.
In most of our patients relapse was within 1 ram. Considering the methodological error and the clinical setting, this is of no clinical significance. There were, however, eight patients who had more relapse. Four of these patients had only moderate advancements. In previous studies, relapse after RIF with screws or miniplates without IMF after mandibular advancement, varied from 5.2% to 26% i's'9'15 17,23. In some
studies, the direction of the net relapse was backwards, in others forwards. Comparisons between different studies are difficult because of differences in patient populations, cephalometric analyses, magnitudes of advancements, use of occlusal splints in postoperative radiographs and follow-up periods. In the present study, the net backward re-
lapse of 15% at point PG and of 17% at point B are well within the abovementioned range. Variability in relapse after mandibular advancement has also been shown after rigid fixation using metallic screws and miniplates7,~6. Theoretically, relapse after rigid fixation is caused by movement at the osteotomy site and/or
Stability following biodegradable fixation
7
S R - P L L A screws. The results, with regard to postoperative skeletal stability, are comparable to those obtained using other forms of R I F in patients needing advancements of up to 6-7 mm.
Acknowledgment. This study was supported by a grant from the Helsinki University Central Hospital Research Funds. References 1. BLOMQVIST JE, ISAKSSONS. Skeletal sta-
bility after mandibular advancement: a comparison of two rigid internal fixation techniques. J Oral Maxillofac Surg 1994: 52: 1133-7. 2. DAL PONT G. Retromolar osteotomy for
Fig. 5. Cephalometric radiographs of a 24-year-old woman taken a) immediately after mandibular advancement surgery and fixation with biodegradable screws, b) after one year showing stable results.
repositioning in the condyle area. Changes in condylar position after sagittal splitting osteotomy and rigid fixation are frequent 2~ Studies in animals 5 have shown that adaptive remodelling takes place in the condylar region after mandibular orthognathic surgery. Such alterations in condylar position and anatomical structures, together with technical errors, could explain the great variability in relapse between patients with equal advancements and similar fixation. This could also explain why some patients have forward relapse. H a l f of the relapse in our patients developed in the second half of the first year after operation. It has been shown 2I that the strength of an osteotomy site fixed with S R - P L L A after BSSO in sheep was greater after 14 weeks than at unoperated sites. This indicates that there must also have been other factors leading to relapse in our patients, apart from movement at the osteotomy site. The incidence of progressive condylar resorption (PCR) can be as high as 7.7% 16 after sagittal splitting osteotomy and miniplate fixation. It usually manifests itself during the second half of the year after the procedure, and can a m o u n t to total relapse 16. Radiological follow up revealed no P C R in our patients. However, the number of patients might have been too small and the follow up too short to show the real incidence of P C R in these patients. An essential factor in relation to the cause of relapse after mandibular advancement surgery, is considered to be
the stretching of paramandibular soft tissue as a result of surgical intervention 6,9. The greater the advancement, the greater the stretching of soft tissues. M a n y investigators have found a correlation between extent of advancement and relapse 1,7,17,23, but this was not obvious from the present study. However, the four patients with the greatest advancements were all in the relapse group. Advancements of more than 6 or 7 m m 9,17 have been shown to be more prone to relapse after metallic screw fixation. Some caution is warranted when S R - P L L A screws are used after mandibular advancement of more than 6 mm. Short I M F for one week with skeletal suspension is has been shown to result in stability after metallic screw fixation of large advancements, and might also be used with SRP L L A screw fixation. Clinical results in relation to occlusion were good in all patients despite occasional moderate skeletal relapse. This might have been due to postoperative orthodontic therapy, correcting any developing horizontal discrepancy. Orthodontic therapy, by settling the occlusion, might also explain some of the relapse in A F H . Other possible causes are rotation of the mandible and resorptional changes in the chin. Anterior vertical relapse has been reported by many other investigators, and a correlation with magnitude of advancement has been noted 16,23. The results of our study show that bilateral splitting osteotomy of the mandible can be fixed using biodegradable
the correction of prognathism. J Oral Surg 1961: 19: 42-7. 3. DUMBACHJ. Zugschraubenosteosynthese nach Ramusosteotomie mit resorbieren Osteosyntheseschrauben aus Polydioxanon (PDS)- erste Ergebnisse. Dtsch Z Mund Kiefer GesichtsChir 1984: 8: 1458. 4. DUMBACHJ. Osteosynthese mit resorbieren PDS-Stiften nach sagittaler Spaltung und Rtickversetzung des Unterkiefers. Dtsch Zahnfirzt Z 1987: 8: 145. 5. ELLIS E, HINTONRJ. Histologic examination of the temporomandibular joint after mandibular advancement with and without rigid fixation: an experimental investigation in adult macaca mulatta. J Oral Maxillofac Surg 1991: 49:1316 27. 6. EPKER BN, WESSBEROGA. Mechanisms of early skeletal relapse following surgical advancement of the mandible. Br J Oral Maxitlofac Surg 1982: 20: 175-82. 7. KIERL M J, NANDA RS, CURRIER G E A
3-year evalution of skeletal stability of mandibular advancement with rigid fixation. J Oral Maxiltofac Surg 1990: 48: 587-92. 8. KIRKPATRICK TB, WOODS MG, SWIFT JQ, MARKOWlTZ NR. Skeletal stability following mandibular advancement and rigid fixation. J Oral Maxillofac Surg 1987: 45: 572-6. 9. KNAUV CA, WALLEN TR, BLOOMQUIST DS. Linear and rotational changes in large mandibular advancements using three or four fixation screws. Int J Adult Orthod Orthognath Surg 1993: 8: 24563. 10. LAKE SL, McNEILL RW, LITTLE RM, WEST RA. Surgical mandibular advancement: a cephalometric analysis of treatment response. Am J Orthod 1981: 80: 37644. 11. MOENNINGJE, BUSSARDDA, LAPP TH, GARRISON BT. A comparison of relapse in bilateral sagittal split osteotomies for mandibular advancement: rigid internal fixation (screws) versus inferior border wiring with anterior skeletal fixation. Int J Adult Orthodont Orthognath Surg 1990: 5! 175 82.
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12. MOMMAERTSMY. Lag screw versus wire osteosynthesis in mandibular advancement. Int J Adult Orthodont Orthognath Surg 1991: 6:153 60. 13. OBWEGESER JA. Bioconvertible screws made of allogenic cortical bone for osteosyntheis following sagittal split osteotomy without postoperative immobilisation. J Cranio-Max-Fac-Surg 1994: 22: 63-75. 14. PROFFIT WR. Treatment planning: the search for wisdom. In: PROFFIT WR, WroTE RE Surgical-orthodontic treatment. St. Louis: Mosby, 1991: 142-90. 15. RUBENS Be, STOELINGA PJW, BL1JDORP PA, SCHOENAERS JHA, POLmS C. Skeletal stability following sagittal split ostetomy using monocortical miniplate internal fixation. Int J Oral Maxillofac Surg 1988: 17: 371-6. 16. SCHEERLINCK JPO, STOELINGA PJW, BLIJDORP PA, BROUNS JJA, NIJS MLL. Sagittal split advancement ostetomies stabilized with miniplates: a ~5-year follow-up, int J Oral Maxillofac Surg 1994: 23:127 31. 17. VAN NICKELSJE, LARSEN AJ, TRASH WJ. Relapse after fixation of mandibular ad-
vancement. J Oral Maxillofac Surg 1986: 44: 698-702. 18. VAN SrCKELSJE. A comparative study of bicortical screws and suspension wires versus bicortical screws in large mandibular advancements. J Oral Maxillofax Surg 1991: 49: 1293-6. 19. SPtESSL B. Osteosynthese bei sagittalen Osteotomie nach Obwegeser/Dal Pont. Fortschr Kiefer- und Gesicbtschirurgie. 1974: 18:145 8. 20. STROSTER TG, PANGRAZIO-KULBERSHV. Assessment of condylar position following bilateral sagittal split ramus osteotomy with wire fixation or rigid fixation. Int J Adult Orthod Orthognath Surg 1994: 9:55 63. 21. SUURONEN R, LAINE P, SARKIALA E, POHJONEN T, LINDQVIST C. Sagittal split osteotomy fixed with biodegradable, selfreinforced poly-l-lactide screws. A pilot study in sheep. Int J Oral Maxillofac Surg 1992: 21: 303-8. 22. SUURONEN R, LAINE P, POHJONEN T, LINDQVIST C. Sagittal ramus osteotomies fixed with biodegradable screws: a preliminary report. J Oral MaxiIlofac Surg 1994: 52: 715-20.
23. THUER U, INGERVALL B, VUILLEMIN T. Stability and effect on the soft tissue profile of mandibular advancement with sagittal split osteotomy and rigid internal fixation. Int J Adult Orthod Orthognath Surg 1994: 9: 175-85. 24. TRAUN~RR, OBWEGESERH. The surgical correction of mandibular prognathism and retrognathia with consideration of genioplasty. Surgical procedures to correct mandibular prognathism and reshaping of the chin. Oral Surg 1957: 10: 67789. 25. WATZKE IM, TURVEY TA, PHtLLIPS C, PROFFIT WR. Stability of mandibular advancement after sagittal osteotomy with screw or wire fixation: a comparative study. J Oral Maxillofac Surg 1990: 48: 108-121. Address: Dr Ilkka Kallela Department of Oral and Maxillofacial Surgery Helsinki University Central Hospital Kasarmikatu 11-13 SF-O0130 Helsinki Finland