This study aims to establish a mandible fracture model, and to review fracture healing following fixation with a locking miniplate system. Eighteen 2-year-old sheep were divided into three groups of six. Each animal had a single fracture that was anatomically reduced and internally fixed by a single 4-hole plate with two monocortical screws each side of the fracture. The fractures were internally fixed with poorly contoured conventional miniplates or poorly contoured mini-locking plate or well contoured conventional miniplates. Two sheep in each of the three groups were killed at 2, 4 and 8 weeks after surgery. The mandibles were radiographed then decalcified specimens were reviewed microscopically. No clinical difference was observed between the groups. All fractures were at an advanced stage of bony union by 4 weeks. Fracture union appeared radiographically more advanced with the locking plate system. This study established a protocol for simulating a fracture model for the study of fracture healing. A more advanced stage of union was seen for fractures internally fixed with locking plates/screws than with a conventional system. The observations suggest the purported biological benefits of locking miniplate system do exist.
Locking miniplate systems have been advocated for craniomaxillofacial fixation of fractures, osteotomies and reconstructions. The purported advantages of greater stability, higher retention, faster application, prevention of screw loosening and less interference with bone circulation are similar to those of the heavier duty locking reconstruction plates which have been in clinical use for over 20 years. Although the locking screw–plate principle has been researched and substantiated by numerous publications with respect to the locking reconstruction plates, the new locking miniplate is a smaller, thinner and less rigid system that has not been fully investigated despite having been introduced into clinical use. The aim of this experimental animal study was to establish a sheep mandible fracture model to simulate a true fracture line, and to obtain preliminary data on the biology of fracture healing and peri-implant bone response following fixation with the locking miniplate system for mandibular angle fractures. Sheep were used as their mandible resembles that of humans in shape, size and growth.
A variety of mini and micro plates are available commercially. Clinical and mechanical studies have provided evidence to support the efficacy of semi-rigid fixation with mini non-compression plates following Champy’s principles. One group of injuries with consistently higher treatment complication rates is mandibular angle fractures. Internal fixation techniques applied to mandibular angle fractures, compared to other sites, result in higher rates of infection and fixation failure. Conventional non-compression miniplates rely on good contact between bone ends and accurate plate adaptation to the bone surface to provide stability. In the angle region of the mandible where transoral access is limited and anatomically more difficult to contour a plate, the torque forces generated by a poorly adapted plate when screws are tightened are transferred to the bone–plate and bone–screw interface which may lead to resorption under the plate and around the screws, resulting in their loosening. A poorly contoured plate can also affect fixation stability and the forces generated may result in fracture displacement and consequent malocclusion.
An internal fixation system in which the plate is not required to contact bone but provides the same or greater stability, such as the locking miniplate, would therefore be an advantage. The concept of locking the screw on to the plate was first developed in the early 1980s with the introduction of the titanium coated hollow-screw reconstruction plate system (THORP) for bridging of mandibular defects. Experimental studies demonstrated that such a locking plate/screw system offered greater stability and allowed use of fewer screws per osseous fragment. Another advantage of the locking reconstruction plate was that, unlike conventional systems, it did not rely on intimate contouring to the surface of the underlying bone to provide stable fixation. In trying to achieve stability by tightening the screws to compress the plate against bone surface, conventional plates have been shown to cause subimplant ischaemia, necrosis, cortical porosis and resorption. Once subimplant resorption occurs, stability is lost. Locking of the screw within the plate prevents compression of the plate against the bone, resulting in much less resorption and reducing the likelihood of screw loosening with consequent inflammation and infection. The locking screw/plate system thus effectively combines the advantages of an external fixation device with internal plate osteosynthesis. The original THORP system achieved locking by insertion of an expansion screw into the head of the bone screw to expand the flanges laterally and thus lock the screw head against the hole of the plate. This has now been replaced by a simplified system whereby the screw head and hole of the plate are threaded to engage each other when the screw is inserted. These locking systems have since been subject to many biomechanical investigations which demonstrated these locking reconstruction plates to be reliable, with few complications.
The recently introduced smaller 2.0 mm locking miniplate and screw system has also undergone laboratory and in vivo studies that demonstrate its mechanical properties to be superior to conventional systems. To the authors’ knowledge, only one clinical trial has been published and data are lacking to substantiate the biological response of bone and fracture healing to this type of non-contact internal locking mini-fixation system. A sheep model was deemed the most akin to human mandible, to be granted ethical approval, to be studied with this methodology.
Materials and methods
A preliminary investigation was performed to determine the optimal method to create a natural mandible fracture which is consistent and reproducible. Previously described methods of creating a ‘fracture’ by osteotomy are not satisfactory as loss of bone occurs if a bur or saw is used. After trials of various surgical instruments, the technique described by Rozema et al., was favoured to provide the most controlled, consistent and predictable fracture line. A device was constructed from a vice like two leg mechanical puller (ARX-2MP4; Trax-Kawasaki Pneumatics Australia Pty Ltd.) which was modified to provide three point application ( Fig. 1 a). On application of the device, controlled force is applied by turning the central arm against the buccal surface, thus tightening the clamp on the mandible ( Fig. 1 b). To guide propagation of a linear fracture pattern to deliberately maximise fracture instability, slots were placed with a No. 1 round bur in the thick cortex of the upper and lower borders of the mandible in the angle region. In six cadaveric sheep mandibles, a natural fracture line was found to propagate consistently at the desired site without causing damage to adjacent bone or soft tissue. The fractures could then be reduced anatomically.
Eighteen 2-year-old Merino sheep of approximately 40 kg, divided into three study groups of six animals each, were used for this study. The study protocol was approved by the animal ethics committee at the Howard Florey Institute of Experimental Physiology and Medicine, The University of Melbourne (AEC 01-043). All surgery was carried out under general anaesthesia using 5% thiopentone sodium (20 mg/kg, Thiobarbl ® , Jurox, Australia) intravenously as the induction agent and isoflurane (Isoflo ® , Abbot, Australia) to maintain anaesthesia. The angle and lower border of the mandible was shaved, prepared with povidine iodine (Betadine ® , Johnson and Johnson, Australia) and isolated with sterile drapes. A single dose of procaine penicillin (Norocillin ® , Heriot Agvert P/L, Australia) was given intramuscularly. Local anaesthetic solution, bupivicaine 0.5% with adrenaline 1:200,000 (Marcain ® , Astra Pharmaceuticals P/L, Australia), was infiltrated at the surgical site. A curvilinear submandibular incision was made, 10 mm below the inferior border of the mandible centred over the angle region. The masseter muscle buccally, and medial pterygoid muscle lingually, were reflected from bone to expose the angle of the mandible.
Each animal had a single fracture created with the fracture device at the angle of the right mandible. The fracture was anatomically reduced and internally fixed by a single 4 hole plate (conventional miniplate or the mini-locking plate), placed on the upper lateral surface of the mandible, at the junction of the upper and middle thirds of the mandible. This was secured with two monocortical screws of 2 mm diameter on each side of the fracture, placed at right angles to the plate. Six sheep had the fracture internally fixed with poorly contoured conventional 2 mm titanium plates and screws.
To simulate poor adaptation, these plates were shaped according to a template to have a concave profile so that the ends of the plate had a 3 mm offset from the bone surface when applied. Another six sheep had their fracture internally fixed with the 2 mm titanium locking mini-fixation system, which were also applied poorly contoured to bone surface. In the third group, six sheep had well contoured and adapted conventional 2 mm plates and screws applied for fixation of the fracture. One sheep in each group also had both type of plates applied poorly-contoured on intact bone at the angle on the contralateral (left) side of the mandible. Both the conventional and locking miniplate/screws were supplied by Synthes ® (Mathys Medical Ltd., Switzerland). Dimensions of the locking miniplate system are similar to that of the conventional 2 mm miniplate system.
At all stages, care was taken to ensure no intra-oral communication occurred. Closure was performed in layers and no maxillomandibular fixation was applied. After surgery, animals were observed to resume full balance in metabolism cages for 1 week before transferring back to the normal pens. They were fed on an unrestricted diet of lucerne and oats. For evaluation of bone regeneration, sequential labelling was performed at 2, 4 and 8 weeks with intravenous tetracycline hydrochloride (20 mg/kg, Aldrich Chemical Co., USA), calcein (30 mg/kg, Sigma Chemical Co., USA) and xylenol orange (30 mg/kg, Sigma Chemical Co., USA), respectively. Two sheep in each of the three groups were killed at 2, 4 and 8 weeks after surgery. At time of death, the animals were injected intravenously with a vascular marker, 250 ml 87% (w/w) barium sulphate. The right and left mandibles were removed en bloc and fixed in 10% neutral buffered formaldehyde solution (Formalin ® Asia Pacific Speciality Chemicals Ltd., Australia).
The mandibles were radiographed before 100 μm thick sections were cut through the plate and fracture site for fluorescence microscopy. The specimen was decalcified and stained using haematoxylin–eosin for light microscopic examination. Histomorphometric assessment was performed with a computerised image analyser (Qwin ® version 2.3, Leica micro system Imaging Solutions Ltd.) to measure subimplant cortical width, viable osteocyte count as well as assessing the bone–screw interface. No statistical analysis was performed, as sample sizes were small and duration relatively short.
Surgery was performed without complications and postoperative recovery was uneventful for all animals. One animal in the locking plate group had a reduced appetite for the first 36 h and another in the contoured conventional plate group developed slightly more prominent swelling at the surgical site. Both animals progressed satisfactory with no complications. In all three treatment groups, the animals initially lost some weight during the first week but regained and increased weight over the following weeks ( Table 1 ). There were no wound complications at any stage and at the time of mandible harvest, there was no evidence of plate fracture, screw loosening or screw fracture in any of the animals.
No clinical difference during postoperative recovery was observed between the three treatment groups. Irrespective of fixation type, fractures were at an advanced stage of bony union by 4 weeks, demonstrating functional stability at the fracture sites that were barely visible except for the tetracycline stain along the fracture line. By the eighth week, fracture sites in the three groups had fully remodelled, demonstrating anatomical alignment. Occlusion could be only subjectively assessed, but there was no obvious deformity or malocclusion. There was also minimal visible callus formation, suggesting that adequate primary stability was achieved with all three fixation techniques. An interesting observation, noted in all three groups, was moderate resorption and blunting of the angle of the mandible ( Fig. 2 ), which was most likely related to the surgical approach where stripping of the masseter muscle was required. Radiographs confirmed anatomical reduction of all fractures and satisfactory positioning of fixation plates. Integrity of the inferior alveolar artery was evident by the perfusion of barium across the site of the fracture and continuing into the anterior mandible. The fracture union appeared radiographically more advanced in the animals where the locking plate/screw system was used, with the fracture line being less distinct than that in animals fixed with the conventional system. Figure 3 shows (a) poorly contoured plate at 2 weeks sheep M15, 4 weeks sheep M10, 8 weeks sheep M8 (b) locking plate at 2 weeks sheep M13, 4 weeks sheep M5, 8 weeks sheep M2 (c) contoured conventional plate at 2 weeks sheep M16, 4 weeks sheep M11 and 8 weeks sheep M9.
Microscopic examination of haematoxylin-eosin sections confirmed the absence of any peri-implant soft tissue inflammation or infection. Presence of a localised fibro-fatty marrow with macrophages and giant cells was a common finding in all specimens and considered consistent with response to trauma associated with the fracture. Mandibles internally stabilised with contoured conventional plates demonstrated woven bone at the fracture site but minimal callus during the healing period. There was a thin layer of new bone on the cortical surface with local pockets of minor resorption. The amount of bone surrounding the screws showed an increase from 4 to 8 weeks.
In comparison, where poorly contoured conventional plates were used, a number of different features were observed. There was more callus at the fracture site, associated with resorption of fracture ends and a larger fracture gap. Chondroid tissue was present at the fracture site ( Fig. 4 ). There were areas of moderate resorption as well as non-vital bone deep to areas where the plate contacted bone, but new bone formation was also present where the plate had lifted off the bone surface. New bone was seen progressively surrounding the screws with time. An interesting finding was the presence of patchy resorption of the opposite lingual cortex.
The locking plate group demonstrated less resorption of bone ends and greater bone formation in the fracture gap as compared with poorly contoured conventional plates. There was noticeably more new bone formation on the cortical surface, deep to the plate. Of interest however, and in contrast to that seen with conventional plates, was the presence of new bone deposition on the opposite, lingual cortex ( Fig. 5 ). Bone was also observed to develop around the locking screws over the 8 weeks. Fluorescent microscopy supported these histological findings, in showing minimal callus but good bone formation at the fracture sites, and confirming that the most active phase of bone regeneration was during the initial 2 weeks ( Fig. 6 ).