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Resorbable poly(d ,l )lactide plates and screws for osteosynthesis of condylar neck fractures in sheep
British Journal of Oral and Maxillofacial Surgery 45 (2007) 35–40
Resorbable poly(d,l)lactide plates and screws for osteosynthesis of condylar neck fractures in sheep Michael Rasse a,∗ , Doris Moser b , Christian Zahl c , Klaus Louis Gerlach c , Uwe Eckelt d , Richard Loukota e a
Department of Craniomaxillofacial and Oral Surgery, Medical University, Anichstraße 35, 6020 Innsbruck, Austria Clinic of Cranio-Maxillofacial and Oral Surgery, Medical University, Vienna, Austria c Department of Oral and Maxillofacial Surgery, Otto-von-Guericke University Magdeburg, Germany d Department of Oral and Maxillofacial Surgery, University Hospitals Carl Gustav Carus, Technical University of Dresden, Germany e Department of Oral and Maxillofacial Surgery, Leeds Teaching Hospitals NHS Trust, Leeds, UK b
Accepted 22 December 2005 Available online 17 February 2006
Abstract We made osteotomies in the condylar neck in 12 adult sheep to simulate fractures, and joined the two ends with 2 poly(d,l)lactide (PDLLA) plates and 8 PDLLA screws 2 mm in diameter. The animals were killed after 2, 6, and 12 months and bony healing was assessed macroscopically and histologically. The plates and screws remained intact and there was no displacement of the bony ends. The degrading plates, which were still visible in the specimens after 6 months, had been replaced by bone. At 12 months the PDLLA had been resorbed with no foreign body reaction and no resorption of underlying bone. The articular discs showed no signs of degeneration. © 2006 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Keywords: Mandibular condyle; Fractures; Absorbable implants; Animal experiment
Introduction Improved methods of osteosynthesis with miniplates, lag screws, and resorbable materials have provoked discussion about the treatment of fractures of the condylar neck of the mandible. When there has been gross condylar displacement conservative treatment has been abandoned because of poor functional and anatomical results. Removal of metal that has been used for osteosynthesis in the area of the condyle may put the patient at risk of complications. Metal is more often left in place than in other parts of the facial skeleton. Resorbable material is therefore of special interest in the treatment of this type of fracture. Mechanical stress in the area of the condylar process involves complex forces and vectors of the muscles of mastication, ∗
Corresponding author. E-mail address: [email protected] (M. Rasse).
and the free movement and varying load of the joint. Testing the mechanical and healing properties of resorbable material for osteosynthesis in that region in an animal model is therefore appropriate. Sheep have a similar pattern of chewing and remodelling of bone as humans, and chewing forces should be at least equal to those of humans. The temporomandibular joint undergoes adaptive changes even when there is only minor displacement of the condylar head and degenerative changes may develop earlier in adults because of their poor capacity for remodelling. We undertook this study of osteosynthesis with resorbable plates to find out whether the plates and screws were adequate to withstand the loads in the area of the condylar neck, whether there was displacement of the proximal fragment, and whether the bone healed when postoperative intermaxillary fixation could not be applied. We also wanted to find out whether there were adaptive or degenerative changes in the fossa and condyle, and how
0266-4356/$ – see front matter © 2006 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.bjoms.2005.12.014
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M. Rasse et al. / British Journal of Oral and Maxillofacial Surgery 45 (2007) 35–40
long did it take for the poly(d,l)-lactide plates and screws to be resorbed.
Material and methods We used poly-(d,l)-lactide (PDLLA) (Resorb X® , Martin Medizin-Technik GmbH, Tuttlingen, Germany) for osteosynthesis. The time for its resorption was estimated to be between 52 and 77 weeks in soft tissue and a year in bone. Its bending strength was 101.6 MPa at 4 weeks after implantation, and was similar to the initial values.1,2 Twelve Austrian mountain sheep (six male, six nonpregnant female) were operated on. They were about 2 years old and had a mean weight of 60 kg (range 56–62 kg). The study was authorized by the ethics committee of the University of Vienna. Animals were operated on under general anaesthesia and given two doses of penicillin-G 10 mega units and oxacillin 1 g as perioperative prophylaxis, and metamizole 5 mg intravenously twice a day for 3 days for postoperative analgesia. We made a preauricular incision and identified and preserved the buccal and zygomatic branches of the facial nerve. The condylar neck was divided with an oscillating saw below the insertion of the lateral pterygoid muscle, and the proximal fragment was dislocated and repositioned. The two ends were joined with 2 PDLLA-plates 2 mm thick, and 8 PDLLAscrews 2 mm in diameter, of which 4 were placed in the proximal fragment (Fig. 1). Fixation was monocortical (5 and 7 mm screws) and the insertion of the lateral pterygoid muscle was preserved. Polychromic stains were applied as follows: Oxytetracycline (Terramycin® , Pfizer Corp. Austria Ges.m.b.H, Vienna, Austria) 14 days postoperatively; calcein (Merck KGaA, Darmstadt, Germany) 28 days postoperatively; alizarin-3-methylamine N,N-diacetic acid dihydrate (Merck KGaA, Darmstadt, Germany) 56 days postoperatively; and xylenol orange tetrasodium salt (Merck KGaA, Darmstadt, Germany) 150 days postoperatively. All fluorochromes were injected subcutaneously with 2% lignocaine 2 ml (with epinephrine 1:100,000). The animals were cared for at the Center for Biomedical Research for a week and then put out to pasture. Food and water were not rationed. One sheep died from aspiration immediately after operation. Three animals were killed after 8 weeks, 5 after 6 months, and 3 after 12 months. Specimens of the operated and non-operated sides were retrieved for anatomical and histological examination. The mandibular ramus and the temporal bone with the adjacent soft tissue were harvested as a single block. Specimens were prepared for histological examination by a modified grinding-sawing technique.3 After fixation and dehydration the samples were embedded in either polymethylmethacrylate (PMMA) or Technovit® 7200 VLC
Fig. 1. Condylar neck fracture stabilised with two PDLLA-miniplates.
(Heraeus Kulzer GmbH, Wehrheim, Germany). Twelve slices of each joint were cut in a sagittal or coronal plane, ground down to a thickness of 10 m, and stained with toluidine – blue 1%. Slides were examined by epifluorescence and light microscopy.
Results Healing was uneventful in all 11 remaining cases. The animals showed no sign of deterioration in feeding or behaviour at any time. They were examined monthly and at the time of sequence staining, and showed no swelling or infection at the site of operation. Gross examination All the bony cuts had healed in an anatomically correct position without deviation of the proximal fragment. There were no pseudoarthroses. Histological examination At no stage (from 8 weeks to 12 months) were there any histological signs of damage or degeneration of the disc or fossa.
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Fig. 4. Primary bone healing, 8 weeks postoperatively.
Fig. 2. Histological section of a condylar process in sawing-grinding technique, toluidine – blue staining, 8 weeks postoperatively, healing without dislocation; two black arrows = plate 1; three black arrows = screws of second plate; white arrow = fracture line.
At 8 weeks the proximal fragments were in their anatomically correct position without displacement and were vital with signs of remodelling (Fig. 2). On the surface there were strata of up to seven layers of connective tissue in different directions with almost no cells discernible (Fig. 3). There was
Fig. 3. Histological section in sawing-grinding technique, toluidine – blue staining, 8 weeks postoperatively; surface of the condylar head.
primary bone healing within the contact zone of the fragments (Fig. 4). The callus on the lateral side of the site of the fracture was composed of fibroblasts, cartilage, and osteoblasts. It partly covered the plates (0.8–1.2 mm on the resorbable plates) (Fig. 5). One specimen showed abundant lateral callus, although the fragments were in the correct anatomical position and the fracture line was barely discernible. At 6 months the fracture line was not discernible, and there was bony union throughout all specimens. The remnants of the plates on the lateral side were partly covered by bone (1–2 mm). Alizarin was almost the only stain seen at the fracture line, and in the laterally and medially apposed bone. Occasionally, xylenol orange stains were also found in these areas. Briefly, we found that within 7–8 weeks there was increased turnover of bone in the superficial 5–6 mm of the condyle even though the anatomical position was correct. From 8 weeks to 6 months there was a slightly increased turnover, which then subsided. At 12 months we saw alizarin staining and a little xylenol orange up to the cartilage of the articulating surface. From 4.8 mm deep to the surface we also saw calcein staining.
Fig. 5. Resorbable plates covered by bone, 8 weeks postoperatively.
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Fig. 6. Screws in direct contact with bone on all surfaces, 8 weeks postoperatively. (a) 2×, Toluidine – blue staining. Original magnification 2×. (b) 4×, No connective tissue between screw and bone. Original magnification 4×. (c) 4×, All fluorescence stainings except xylenol orange. Original magnification 4×.
There was no sign of disintegration of plates and screws at 8 weeks. They were in close contact with bone on all surfaces (Fig. 6a–c). The bone adjacent to the threads and at the tip of the screws was stained with tetracycline and calcein. There was no connective tissue between the bone and the screws and no foreign body reaction adjacent to the plates and screws. By 6 months the sites of the screws were replaced by bone in all specimens and in every screw hole (Fig. 7a and b). The density of this bone was less than that of the surrounding bone and showed no sequence staining. Particles of the polymer, up to a diameter of 0.06 mm, could still be seen in the cancellous bone. These particles were surrounded by connective tissue and foreign body giant cells with vacuoles.
Fig. 7. The region of the applied screws was built through by bone in all specimens and in every screw hole (arrows), 6-month specimen. (a) 1×, Toluidine – blue staining. Original magnification 1×. (b) 1×, Calcein fluorescence staining, no staining of the newly formed bone. Original magnification 1×.
Fig. 8. Screw hole tightly filled with bone, 12-month specimen.
M. Rasse et al. / British Journal of Oral and Maxillofacial Surgery 45 (2007) 35–40
There were abundant osteoblasts. The woven bone was covered by lamellar bone and osteons. There were particles of up to 0.08 mm in the area of the plates, that were covered by, but only partly replaced by, bone. There was no sign of bony resorption adjacent to the screws or plates. At 12 months in only one section of one of the three specimens were there some foreign bodies on the lateral surface, 0.06 mm in diameter, surrounded by connective tissue. The screw holes were tightly filled with bone (Fig. 8) and the lateral surface was smooth, the masseter muscle taking origin on it. Woven bone was replaced by lamellar bone. The diameter of the condylar process remained similar on both sides.
Discussion Resorbable materials have been used clinically for osteosynthesis in various fields of maxillofacial surgery. In traumatology they are used for the treatment of zygomatic fractures4–9 and for fractures of the mandible.10–12 Although difficulties in removal of metal plates in the area of the condylar neck argue for its use in that region, little experimental work has been published.13,14 In finite element studies the stability of two titanium miniplates used for fractures of the condylar neck was doubted.15 In our experiment, however, 2 PDLLA plates and 8 PDLLA screws were sufficient to retain the proximal fragment in an anatomically correct position without the necessity for intermaxillary fixation or a soft diet. The fragments obviously showed less interdigitation than would be found in fractures, as the condyles were cut with saws. The plates were applied in a manner found best to withstand the main directions of stress in that area (two plates in a caudally open angle).16 In our study we found normal histological appearances, whereas previous authors have reported abnormal macroscopic and microscopic appearances after using resorbable or metal screws for fixation of these fractures in sheep, including bony destruction, osteophytes, flattening of the condylar head, increased thickness of cartilage, and irregularities of layers of cells and cartilage–bone junctions.14 In fractures caused by injuries, damage to the soft tissues and disc must be taken into account, but we assume that open reduction and osteosynthesis with resorbable plates has no deleterious effect. The reduction of the local pH during the period of degradation of the plates and screws, and the crystalline remnants of the material used, can cause unfavourable foreign body reactions.17–19 Foreign body reactions were, however, not found either after experimental use of amorphous PDLLA or after clinical application of the material.1,2,8 In our experiments the degraded material was still present at 6 months, the fragments having a diameter of 0.01–0.06 mm. After the use of high crystalline polymer, osteolytic changes were found around self-reinforced poly-l-lactic acid (SR-PLLA) screws in 27% of cases.10 Similar osteolytic changes have been
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reported elsewhere.20 On histological examination we found no enlargement of the diameter of the former burr-hole. Foreign body reactions as a result of lowering of the pH may be of minor importance in PDLL implants.17 The screws degraded and were replaced by dense bone by 6 months. Foreign bodies within foreign body giant cells were still found, but also many osteoblasts. We found that screw-holes were completely filled with bone, which was not reported in previous studies using other materials. High crystallinity is thought to inhibit degradation.18,21 Bulk degeneration has been reported within this material earlier than in thin layers of polymers.22,23 Even 5 years after implantation PLLA and SR-PLLA material could still be found in histological sections and on electron microscopy.19,20,24 High crystallinity polymers have been identified in remnants, and the crystallinity of the remnants was higher than in the original material.19,20 However, no adverse effects on healing were reported by these authors. At 12 months the implanted material in this study had disappeared from the area of plates and screws in all but one of 24 slices. Woven bone was transformed into lamellar bone on the lateral and medial surface and within the former screw holes. We conclude that 2 PDLLA plates with 8 screws (diameter 2 mm) are sufficient to retain and stabilise condylar neck fractures in sheep, and probably should be able to do so in humans.
References 1. Heidemann W, Jeschkeit S, Ruffieux K, et al. Degradation of poly(d,l)lactide implants with or without addition of calcium phosphates in vivo. Biomaterials 2001;22:2371–81. 2. Heidemann W, Fischer JH, Koebke J, Bussmann C, Gerlach KL. In-vivo Untersuchungen zur Degradation von Poly(d,l)Laktid undPoly(Laktidco-Glykolid)- Osteosynthesematerial. (In vivo examination of degradation of osteosynthesis material of poly(d,l) lactide and poly(lactide-coglycolide). Mund Kiefer Gesichtschir 2003;7:283–8. 3. Donath K. Die Trenn-D¨unnschliff-Technik zur Herstellung histologischer Pr¨aparate von nicht schneidbaren Geweben und Materialien. (Sawing – grinding technique for preparation of histologic slides of hard tissues and materials.) In: EXAKT-Kulzer-Druckschrift. Norderstedt, 1988. 4. Bos RRM, Boering G, Rozema FR, Leenslag JW. Resorbable poly(l)lactide plates and screws for the fixation of zygomatic fractures. J Oral Maxillofac Surg 1987;45:751–3. 5. Gerlach KL. Treatment of zygomatic fractures with biodegradable poly(d,l)lactide plates and screws. In: Heimke G, Soltesz U, Lee ACJ, editors. Clinical implant materials. Advances in biomaterials, 9. Amsterdam: Elsevier; 1990. p. 573–8. 6. Gerlach KL. In-vivo and clinical evaluations of poly(l)lactide plates and screws in maxillofacial traumatology. Clin Mater 1993;13:21–8. 7. Enislidis G, Pichorner S, Lambert F, et al. Fixation of zygomatic fractures with a new biodegradable copolymer osteosynthesis system. Preliminary results. Int J Oral Maxillofac Surg 1998;27:352–5. 8. Heidemann W, Gerlach KL. Anwendung eines resorbierbaren Osteosynthesesystems aus Poly(d,l)laktid in der Mund-, Kiefer- und Gesichtschirurgie. (Application of a resorbable system of osteosynthesis devices of poly(d,l)lactide in maxillo-facial surgery). Dtsch Zahn¨arztl Z 2002;57:50–3.
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9. Gerlach KL. Resorbierbare Polymere als Osteosynthesematerialien. (Resorbable polymers for osteosynthesis). Mund Kiefer Gesichtschir 2000;4(Suppl. 1):91–102. 10. Kallela I, Iiszuka T, Salo A, Lindqvist C. Lag screw fixation of anterior mandible fractures using biodegradable polylactide screws. A preliminary report. J Oral Maxillofac Surg 1999;57:113–8. 11. Kallela I, Laine P, Suuronen R, Ranta P, Lizuka T, Lindqvist C. Osteotomy site healing following mandibular sagittal split osteotomy and rigid fixation with polylactide resorbable screws. J Oral Maxillofac Surg 1999;28:166–70. 12. Yerit K, Enislidis G, Schopper C, et al. Fixation of mandibular fractures with biodegradable plates and screws. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2002;94:294–300. 13. Rasse M. Diacapitul¨are Frakturen der Mandibula. Eine neue Operationsmethode und erste Ergebnisse. (Capitular fractures of the mandible. A new surgical technique and first results). Z Stomatol 1993; 90:413–28. 14. Suuronen R, Vainionpaa S, Hietanen J, Vasenius J, Lindqvist C. The effect of osteotomy and osteosynthesis in the mandibular condyle. A radiologic and histologic study in sheep. Int J Oral Maxillofac Surg 1994;23:174–9. 15. Wagner A, Krach W, Schicho K, Undt G, Ploder O, Ewers R. A 3dimensional finite element analysis investigating the biomechanical behaviour of the mandible and plate osteosynthesis in cases of fractures of the condylar process. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2002;94:678–86. 16. Meyer Ch, Kahn JL, Boutemi Ph, Wilk A. Photoelastic analysis of bone deformation in the region of the mandibular condyle during mastication. J Craniomaxillofac Surg 2002;30:160–9.
17. Daniels AU, Taylor MS, Andriano KP, Heller J. Six bioabsorbable polymers: in vitro acute toxicity of accumulated degradation products. J Appl Biomater 1994;5:151–7. 18. Bergsma JE, Rozema F, Bos RR, de Bruijn WC. Foreign-body reaction to resorbable poly(l)lactide bone plates and screws used for the fixation of unstable zygomatic fractures. J Oral Maxillofac Surg 1993;51: 666–70. 19. Bergsma JE, de Bruijn WC, Rozema FR, Bos RRM, Boering G. Late degradation tissue response to poly(l)lactide bone plates and screws. Biomaterials 1995;16:25–31. 20. Suuronen R, Pohjonen T, Hietanen J, Vasenius J, Lindqvist C. A 5-year in vitro and in vivo study of the biodegradation of polylactide plates. J Oral Maxillofac Surg 1998;56:604–14. 21. Vert M, Christel P, Chabot F, Leray J. Bioresorbable plastic material for bone surgery. In: Hastings GW, Ducheyne P, editors. Macromolecular materials. Boca Raton: CRC Press; 1984. p. 119–42. 22. Li S, Garreau H, Vert M. Structure property relationships in the case of the degradation of massive poly(␣-hydroxy) acids in aqueous media. Part 1: Poly(d,l)lactic-acid. J Mater Sci Mater Med 1990;1:123– 30. 23. Li S, Garreau H, Vert M. Structure property relationships in the case of the degradation of massive poly(␣-hydroxy)acids in aqueous media. Part 2: Degradation of lactide-glycolide copolymers: PLA37.5GA25 and PLA75.GA25. J Mater Sci Mater Med 1990;1: 131–9. 24. Laitinen O, Pihlajam¨aki H, Sukura A, B¨ostman O. Transmission electron microscopic visualization of the degradation and phagocytosis of a polyl-lactide screw in cancellous bone: a long term experimental study. J Biomed Mater Res 2002;61:33–9.
Resorbable poly(d,l)lactide plates and screws for osteosynthesis of condylar neck fractures in sheep Michael Rasse a,∗ , Doris Moser b , Christian Zahl c , Klaus Louis Gerlach c , Uwe Eckelt d , Richard Loukota e a
Department of Craniomaxillofacial and Oral Surgery, Medical University, Anichstraße 35, 6020 Innsbruck, Austria Clinic of Cranio-Maxillofacial and Oral Surgery, Medical University, Vienna, Austria c Department of Oral and Maxillofacial Surgery, Otto-von-Guericke University Magdeburg, Germany d Department of Oral and Maxillofacial Surgery, University Hospitals Carl Gustav Carus, Technical University of Dresden, Germany e Department of Oral and Maxillofacial Surgery, Leeds Teaching Hospitals NHS Trust, Leeds, UK b
Accepted 22 December 2005 Available online 17 February 2006
Abstract We made osteotomies in the condylar neck in 12 adult sheep to simulate fractures, and joined the two ends with 2 poly(d,l)lactide (PDLLA) plates and 8 PDLLA screws 2 mm in diameter. The animals were killed after 2, 6, and 12 months and bony healing was assessed macroscopically and histologically. The plates and screws remained intact and there was no displacement of the bony ends. The degrading plates, which were still visible in the specimens after 6 months, had been replaced by bone. At 12 months the PDLLA had been resorbed with no foreign body reaction and no resorption of underlying bone. The articular discs showed no signs of degeneration. © 2006 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. Keywords: Mandibular condyle; Fractures; Absorbable implants; Animal experiment
Introduction Improved methods of osteosynthesis with miniplates, lag screws, and resorbable materials have provoked discussion about the treatment of fractures of the condylar neck of the mandible. When there has been gross condylar displacement conservative treatment has been abandoned because of poor functional and anatomical results. Removal of metal that has been used for osteosynthesis in the area of the condyle may put the patient at risk of complications. Metal is more often left in place than in other parts of the facial skeleton. Resorbable material is therefore of special interest in the treatment of this type of fracture. Mechanical stress in the area of the condylar process involves complex forces and vectors of the muscles of mastication, ∗
Corresponding author. E-mail address: [email protected] (M. Rasse).
and the free movement and varying load of the joint. Testing the mechanical and healing properties of resorbable material for osteosynthesis in that region in an animal model is therefore appropriate. Sheep have a similar pattern of chewing and remodelling of bone as humans, and chewing forces should be at least equal to those of humans. The temporomandibular joint undergoes adaptive changes even when there is only minor displacement of the condylar head and degenerative changes may develop earlier in adults because of their poor capacity for remodelling. We undertook this study of osteosynthesis with resorbable plates to find out whether the plates and screws were adequate to withstand the loads in the area of the condylar neck, whether there was displacement of the proximal fragment, and whether the bone healed when postoperative intermaxillary fixation could not be applied. We also wanted to find out whether there were adaptive or degenerative changes in the fossa and condyle, and how
0266-4356/$ – see front matter © 2006 The British Association of Oral and Maxillofacial Surgeons. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.bjoms.2005.12.014
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M. Rasse et al. / British Journal of Oral and Maxillofacial Surgery 45 (2007) 35–40
long did it take for the poly(d,l)-lactide plates and screws to be resorbed.
Material and methods We used poly-(d,l)-lactide (PDLLA) (Resorb X® , Martin Medizin-Technik GmbH, Tuttlingen, Germany) for osteosynthesis. The time for its resorption was estimated to be between 52 and 77 weeks in soft tissue and a year in bone. Its bending strength was 101.6 MPa at 4 weeks after implantation, and was similar to the initial values.1,2 Twelve Austrian mountain sheep (six male, six nonpregnant female) were operated on. They were about 2 years old and had a mean weight of 60 kg (range 56–62 kg). The study was authorized by the ethics committee of the University of Vienna. Animals were operated on under general anaesthesia and given two doses of penicillin-G 10 mega units and oxacillin 1 g as perioperative prophylaxis, and metamizole 5 mg intravenously twice a day for 3 days for postoperative analgesia. We made a preauricular incision and identified and preserved the buccal and zygomatic branches of the facial nerve. The condylar neck was divided with an oscillating saw below the insertion of the lateral pterygoid muscle, and the proximal fragment was dislocated and repositioned. The two ends were joined with 2 PDLLA-plates 2 mm thick, and 8 PDLLAscrews 2 mm in diameter, of which 4 were placed in the proximal fragment (Fig. 1). Fixation was monocortical (5 and 7 mm screws) and the insertion of the lateral pterygoid muscle was preserved. Polychromic stains were applied as follows: Oxytetracycline (Terramycin® , Pfizer Corp. Austria Ges.m.b.H, Vienna, Austria) 14 days postoperatively; calcein (Merck KGaA, Darmstadt, Germany) 28 days postoperatively; alizarin-3-methylamine N,N-diacetic acid dihydrate (Merck KGaA, Darmstadt, Germany) 56 days postoperatively; and xylenol orange tetrasodium salt (Merck KGaA, Darmstadt, Germany) 150 days postoperatively. All fluorochromes were injected subcutaneously with 2% lignocaine 2 ml (with epinephrine 1:100,000). The animals were cared for at the Center for Biomedical Research for a week and then put out to pasture. Food and water were not rationed. One sheep died from aspiration immediately after operation. Three animals were killed after 8 weeks, 5 after 6 months, and 3 after 12 months. Specimens of the operated and non-operated sides were retrieved for anatomical and histological examination. The mandibular ramus and the temporal bone with the adjacent soft tissue were harvested as a single block. Specimens were prepared for histological examination by a modified grinding-sawing technique.3 After fixation and dehydration the samples were embedded in either polymethylmethacrylate (PMMA) or Technovit® 7200 VLC
Fig. 1. Condylar neck fracture stabilised with two PDLLA-miniplates.
(Heraeus Kulzer GmbH, Wehrheim, Germany). Twelve slices of each joint were cut in a sagittal or coronal plane, ground down to a thickness of 10 m, and stained with toluidine – blue 1%. Slides were examined by epifluorescence and light microscopy.
Results Healing was uneventful in all 11 remaining cases. The animals showed no sign of deterioration in feeding or behaviour at any time. They were examined monthly and at the time of sequence staining, and showed no swelling or infection at the site of operation. Gross examination All the bony cuts had healed in an anatomically correct position without deviation of the proximal fragment. There were no pseudoarthroses. Histological examination At no stage (from 8 weeks to 12 months) were there any histological signs of damage or degeneration of the disc or fossa.
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Fig. 4. Primary bone healing, 8 weeks postoperatively.
Fig. 2. Histological section of a condylar process in sawing-grinding technique, toluidine – blue staining, 8 weeks postoperatively, healing without dislocation; two black arrows = plate 1; three black arrows = screws of second plate; white arrow = fracture line.
At 8 weeks the proximal fragments were in their anatomically correct position without displacement and were vital with signs of remodelling (Fig. 2). On the surface there were strata of up to seven layers of connective tissue in different directions with almost no cells discernible (Fig. 3). There was
Fig. 3. Histological section in sawing-grinding technique, toluidine – blue staining, 8 weeks postoperatively; surface of the condylar head.
primary bone healing within the contact zone of the fragments (Fig. 4). The callus on the lateral side of the site of the fracture was composed of fibroblasts, cartilage, and osteoblasts. It partly covered the plates (0.8–1.2 mm on the resorbable plates) (Fig. 5). One specimen showed abundant lateral callus, although the fragments were in the correct anatomical position and the fracture line was barely discernible. At 6 months the fracture line was not discernible, and there was bony union throughout all specimens. The remnants of the plates on the lateral side were partly covered by bone (1–2 mm). Alizarin was almost the only stain seen at the fracture line, and in the laterally and medially apposed bone. Occasionally, xylenol orange stains were also found in these areas. Briefly, we found that within 7–8 weeks there was increased turnover of bone in the superficial 5–6 mm of the condyle even though the anatomical position was correct. From 8 weeks to 6 months there was a slightly increased turnover, which then subsided. At 12 months we saw alizarin staining and a little xylenol orange up to the cartilage of the articulating surface. From 4.8 mm deep to the surface we also saw calcein staining.
Fig. 5. Resorbable plates covered by bone, 8 weeks postoperatively.
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Fig. 6. Screws in direct contact with bone on all surfaces, 8 weeks postoperatively. (a) 2×, Toluidine – blue staining. Original magnification 2×. (b) 4×, No connective tissue between screw and bone. Original magnification 4×. (c) 4×, All fluorescence stainings except xylenol orange. Original magnification 4×.
There was no sign of disintegration of plates and screws at 8 weeks. They were in close contact with bone on all surfaces (Fig. 6a–c). The bone adjacent to the threads and at the tip of the screws was stained with tetracycline and calcein. There was no connective tissue between the bone and the screws and no foreign body reaction adjacent to the plates and screws. By 6 months the sites of the screws were replaced by bone in all specimens and in every screw hole (Fig. 7a and b). The density of this bone was less than that of the surrounding bone and showed no sequence staining. Particles of the polymer, up to a diameter of 0.06 mm, could still be seen in the cancellous bone. These particles were surrounded by connective tissue and foreign body giant cells with vacuoles.
Fig. 7. The region of the applied screws was built through by bone in all specimens and in every screw hole (arrows), 6-month specimen. (a) 1×, Toluidine – blue staining. Original magnification 1×. (b) 1×, Calcein fluorescence staining, no staining of the newly formed bone. Original magnification 1×.
Fig. 8. Screw hole tightly filled with bone, 12-month specimen.
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There were abundant osteoblasts. The woven bone was covered by lamellar bone and osteons. There were particles of up to 0.08 mm in the area of the plates, that were covered by, but only partly replaced by, bone. There was no sign of bony resorption adjacent to the screws or plates. At 12 months in only one section of one of the three specimens were there some foreign bodies on the lateral surface, 0.06 mm in diameter, surrounded by connective tissue. The screw holes were tightly filled with bone (Fig. 8) and the lateral surface was smooth, the masseter muscle taking origin on it. Woven bone was replaced by lamellar bone. The diameter of the condylar process remained similar on both sides.
Discussion Resorbable materials have been used clinically for osteosynthesis in various fields of maxillofacial surgery. In traumatology they are used for the treatment of zygomatic fractures4–9 and for fractures of the mandible.10–12 Although difficulties in removal of metal plates in the area of the condylar neck argue for its use in that region, little experimental work has been published.13,14 In finite element studies the stability of two titanium miniplates used for fractures of the condylar neck was doubted.15 In our experiment, however, 2 PDLLA plates and 8 PDLLA screws were sufficient to retain the proximal fragment in an anatomically correct position without the necessity for intermaxillary fixation or a soft diet. The fragments obviously showed less interdigitation than would be found in fractures, as the condyles were cut with saws. The plates were applied in a manner found best to withstand the main directions of stress in that area (two plates in a caudally open angle).16 In our study we found normal histological appearances, whereas previous authors have reported abnormal macroscopic and microscopic appearances after using resorbable or metal screws for fixation of these fractures in sheep, including bony destruction, osteophytes, flattening of the condylar head, increased thickness of cartilage, and irregularities of layers of cells and cartilage–bone junctions.14 In fractures caused by injuries, damage to the soft tissues and disc must be taken into account, but we assume that open reduction and osteosynthesis with resorbable plates has no deleterious effect. The reduction of the local pH during the period of degradation of the plates and screws, and the crystalline remnants of the material used, can cause unfavourable foreign body reactions.17–19 Foreign body reactions were, however, not found either after experimental use of amorphous PDLLA or after clinical application of the material.1,2,8 In our experiments the degraded material was still present at 6 months, the fragments having a diameter of 0.01–0.06 mm. After the use of high crystalline polymer, osteolytic changes were found around self-reinforced poly-l-lactic acid (SR-PLLA) screws in 27% of cases.10 Similar osteolytic changes have been
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reported elsewhere.20 On histological examination we found no enlargement of the diameter of the former burr-hole. Foreign body reactions as a result of lowering of the pH may be of minor importance in PDLL implants.17 The screws degraded and were replaced by dense bone by 6 months. Foreign bodies within foreign body giant cells were still found, but also many osteoblasts. We found that screw-holes were completely filled with bone, which was not reported in previous studies using other materials. High crystallinity is thought to inhibit degradation.18,21 Bulk degeneration has been reported within this material earlier than in thin layers of polymers.22,23 Even 5 years after implantation PLLA and SR-PLLA material could still be found in histological sections and on electron microscopy.19,20,24 High crystallinity polymers have been identified in remnants, and the crystallinity of the remnants was higher than in the original material.19,20 However, no adverse effects on healing were reported by these authors. At 12 months the implanted material in this study had disappeared from the area of plates and screws in all but one of 24 slices. Woven bone was transformed into lamellar bone on the lateral and medial surface and within the former screw holes. We conclude that 2 PDLLA plates with 8 screws (diameter 2 mm) are sufficient to retain and stabilise condylar neck fractures in sheep, and probably should be able to do so in humans.
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