A comparative in vitro study on fixation of sagittal split osteotomies with Würzburg® screws, Champy® miniplates, and Biofix® (biodegradable) rods

Int..L Oral MaxiltoJim Surg. 1994, 23." 46 48 Printed in Denmark. All rights reserved

Copyright © M u n k s g a a r d 1994 [ntemationa]Joumal of

Oral &

Mccv~ofacial Sttrgery ISSN 0901-5027

A comparative in vitro study on fixation of sagittal split osteotomies with WLirzburg® screws, Champy® miniplates, and Biofix (biodegradable)rods

Joppe P. B. Bouwman ~, Dirk B. Tuinzing 2, Pieter J. Kostens& ~Department of Oral and Maxillofacial Surgery, Kennemer Gasthuis, Haarlem; 2Department of Oral and Maxillofacial Surgery, Free University Hospital Amsterdam; 3Department of Theory of Medicine, Epidemiology and Biostatistics, Medical School, Free University Amsterdam, The Netherlands

J. P. B. Bouwman, D. B. Tuinzing, P. J. Kostense." A comparative in vitro study on ,fixation o f sagittal split osteotomies with Wiirzburg ® screws, Champy ® miniplates, and Biofix ® (biodegradable) rods'. Int. Z Oral Maxillofac. Sur~g. 1994; 2 3 : 4 6 48. © Munksgaard, 1994 Abstract. A bilateral sagittal split osteotomy was performed on seven fresh cadaver mandibles. Three different systems of fixation were mechanically tested on 14 sites. Tensile diagrams were obtained in which the (offset-) yield point was measured. This resulted in mean yield stresses of 199 N for bicortical selftapping screws (n = 6), 49 N for miniplates with monocortical screws (n = 5), and 113 N for bicortical biodegradable rods (n = 3).

Multiple systems for internal fixation are now used in oral and maxillofa~cial surgery, and internal fixation to allow early mobilization and function after sagittal split osteotomy has become common over the last few years. A number of different systems have evolved since SP]ESSL6 described pretapped screws placed transcutaneously through a small stab incision. Self-tapping screws are also widely used for internal fixation, while other surgeons advocate small plates with monocortical screws 5. Some investigators suggest that this means of immobilization secures minimal postoperative displacement and therefore provides optimal stability, although a recent study by WATZKEet al. showed no difference in stability and clinical result between wire fixation and screw fixations. Elimination of intermaxillary fixation, maintenance of jaw function postsurgically, and increased patient

comfort are other advantages offered by internal screw fixation (ISF). Potential disadvantages of ISF include the possibility of a second operation to remove bone screws and plates, neurovascular bundle injury with impaired neurosensory function, exacting surgical tech-

Fig. 1. Cadaver mandible with miniplate on right side and three bicortical screws on left.

Key words: sagittal split osteotomy; rigid fixation; miniplates. Accepted for publication 30 August 1993

nique, increased incidence of temporomandibular joint disorders, scar formation from transcutaneous screw placement, increased intraoperative time, and greater expense. A second operation to remove bone screws and plates is avoided with biode-

Experimental fixation of sagittal split osteotomy g r a d a b l e m a t e r i a l s . This in vitro s t u d y compares the strength of three fixation s y s t e m s t h a t v a r y in screw size, m a t e r i a l , a n d p l a c e m e n t technique.

M a t e r i a l and m e t h o d s

Seven fresh, edentulous, cadaver mandibles were obtained from the morgue of the depart-

Fig,. 2. Applicator (top) and biodegradable rod (bottom).

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47

ment of anatomy (Medical School, Free University Amsterdam). The specimens had been refrigerated from the time of death until their use in this study. All experiments were conducted within 24 h. The head and neck regions had been clinically examined to ensure the absence of trauma, impacted third molars, or other abnormality. Each mandible was excised from the cadaver, and all overlying tissue was removed. On the disarticulated mandibles, a bilateral sagittal split osteotomy according to the method of [~UNSUCKwas performed 2. The proximal and distal segments were then repositioned and fixed. Three different systems of fixation were tested. Six hemimandibles were used to investigate the W/,irzburg titanium screws (Leibinger, Germany). Three self-tapping screws (0.9, 1.1, 1.3 cm) were positioned superiorly to the inferior alveolar nerve. All screws were bicortical, and they had a diameter of 2 mm. Five hemimandibles were then used to investigate the four-hole, stainless steel Champy ® miniplate (Martin, Germany) with monocortical 0.5- or 0.7-cm screws. Fig. 1 shows the osteotomy site in the mandible with a miniplate on the right, and on the left, three bicortical screws in place. To investigate the biodegradable Biofix ~"> rods (Bioscience, Finland), we used three hemimandibles. The Biofix device comprises three cylindric, biodegradable, composite rods, with a diameter of 1.5 mm. These are driven by an applicator (Fig. 2) in predrilled channels through the osteotomy to fix the proximal and distal fragments. Biofix biodegradable rods are constructed of patented, self-reinforced polyglycolide composite nlaterial (T6RMXLX et al.) which is coated with a thin polyglycolide layer 7. The shear strength of self-reinforced polyglycolide composite material is 180 200 MPa, which exceeds 20-30 times the strength of cancellous bone. The rods lose their strength over 30~40 days. All mechanical tests were done on a Zwick testing machine. Each specimen was then mounted in the machine. Both the condylar process and the mandibular body were fixed, and the action point of the dislocating forces was located at the coronoid process (Fig. 3). A load was then applied at a strain rate of 60 mm/min. This resulted in tensile diagrams

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Fig. 3. Top: schematic drawing of osteotomized mandible. Point of application of drawbench is located at coronoid process. Bottom: cadaver mandible with three biodegradable rods in place.

Strain

Fig. 4. Schematic drawing of stress-strain curve.

48

Bouwman et aL

tients. No postoperative complications have occurred in these patients. From a mechanical point of view, the self-tapping bicortical screws yielded the best results. This supports our clinical results in which this method has proved to be a reliable method of internal fixation.

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Acknowledgments. The authors wish to thank

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the Departments of Anatomy (Free University, Amsterdam, The Netherlands) and Dental Materials (Dental School, Amsterdam) for their contribution to this study.

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References

•

Wiirzburg

•

n=6

Champy

Biofix

n=5

n=3

Fig. 5. Individual data points of 14 experiments.

or stress/strain curves in which the (offset-) yield point was measured (Fig. 4). Results

The mean yield stresses were 199 N for the Wiirzburg experiment, 49 N for the Champy experiments, and 113 N for the Biofix experiments. Fig. 5 shows the individual data points. With the small group sizes at issue, an analysis of variance would be sensitive to deviations from the normal probability distribution. The data were therefore analyzed with the Kruskal-Wallis test 4, the nonparametric equivalent of one-way analysis of variance. The contrast among the three groups appeared to.be significant (p < 0.01). Discussion

Harvesting cadaver material for scientific experiment is difficult, and it can introduce sources of variation that are outside experimental control. This may be the reason for the variation of the results among the mandibles tested. Differences in bone quality and character would be expected to contribute to the varied data that were produced. On the bone quality, Ko 3 reported no significant change in tensile strength of human bone stored in physiologic saline at room temperature for several months. It is known that a triad configuration

of bicortical screws is stronger than a horizontal one 1. Whether healing is affected by this difference is not known. The actual amount of stability required for predictable healing in a functioning mandible is also unknown. We opted in our experiment for the horizontal configuration. The forces that produced the yield stress of the Champy plates in this study were in the range of 41-58 N. This is of an order of magnitude less than the functional forces applied to the angle of the mandible. One would expect that the loss of buttressing caused by the osteotomy requires the plate to carry the entire functional load. A period of intermaxillary fixation may therefore be advisable in these patients. RUBENS et al/ reported on this method in 20 patients, in whom they did not use I M F for longer than 5 days. The results appeared to be completely satisfactory. An important factor may be that the soft diets that were recommended to these patients caused the chewing forces to be below the range of 41-58 N. The results of the experiments of the Biofix system showed lower values than the experiments of the bicortical screws because of less retentive strength. The surface of the rods is perfectly smooth and therefore the pull-out strength is less than that of a screw. On the basis of the results of the present study, the Biofix system has been used in two pa-

1. ARDARY WC, TRACY D J, BROWNR1DGE

GW, URATA MM. Comparative evaluation of screw configuration on the stability of the sagittal split osteotomy. Oral Surg 1989: 68:125 9. 2. HUNSUCKEE. A modified intraoral sagittal splitting technique for correction of mandibular prognathism. J Oral Surg 1968: 26: 249-52. 3. Ko R. The tension test upon the compact substance of the long bones of human extremities. J Kyoto Pref Med Univ 1953: 53: 503. 4. LEHMANNEL. Nonparametrics. Statistical methods based on ranks. San Francisco, CA: Holden-Day, 1975. 5. RUBENS BC, STOELINGA PJW, BLIJDORP PA, et al. Skeletal stability following sagittal split osteotomy using monocortical miniplate internal fixation. Int J Oral Maxillofac Surg 1988: 17:371 6. 6. SPIESSL B. Rigid internal fixation after sagittal split osteotomy of the ascending ramus. In: SPIESSLB, ed.: New concepts in maxillofacial bone surgery. New York: Springer-Verlag, 1976: 115-22. 7. T(~RMALAP, ROKKANENP, LAIHOJ, TAMMINM,~KI M, VAINIONP,~A S, Inventors. Material for osteosynthesis devices. US Pat. No. 4,743,257, 10 May 1988. 8. WATZKE IM, TURVEY TA, PHILLIPS C, PROFFITWR. Stability of mandibular advancement after sagittal osteotomy with screw or wire fixation. J Oral Maxillofac Surg 1990: 48: 108-21.

Address: J. P. B. Bouwman, Kaakchirurg Kennemer Gasthu& Lokatie EG Afd. Kaakchirurgie Postbus 417 2000 A K Haarlem The Netherlands'