Abstract
The aim of this study was the biomechanical validation of three-dimensional finite element analysis (FEA) with a servo-hydraulic testing unit (STU) for a resorbable fixation system (RFS) in a rabbit model. Bilateral mandibular vertical body osteotomies (BMVBO) were performed in 15 female New Zealand rabbits. The animals were divided into three groups. The STU and FEA tests were done immediately after surgery in group 1 (1 day), at the first postoperative month in group 2, and at the third postoperative month in group 3. Both stress tests were carried out by applying vertical forces at the lower incisal edge, loading from 0 N force and increasing this until breakage occurred at the bone. The maximum forces that the hemimandibles could stand and the amount of deformation were recorded and analysed with the FEA and STU tests. We found the STU and FEA test results to be similar and that they could be used interchangeably for groups 1 and 3. However, the FEA results differed most from the real STU values in group 2 because of callus formation that had not ossified at the osteotomy line.
There are several studies describing the use of osteosynthesis systems for the treatment of jaw fractures and during facial osteotomies. Nowadays, the tendency towards the usage of resorbable osteosynthesis systems is gradually increasing. Various engineering techniques, such as tension and photoelastic measurement studies, as well as analytical and finite element-based approaches, have been used for biomechanical studies of the mandible. A test model fixated at two points may be used for this purpose.
Servo-hydraulic tests (STU) have been used to mimic strain gauges at the bone surface of autopsy mandibles and to register the direction and magnitude of the principal strains under loading. In these studies, certain necessary assumptions have been made for the points of application and the magnitudes and directions of the forces acting on the jaw. In addition, these analyses have been limited to static situations, and information is obtained only on the strains in the superficial bone layer of the jaw.
Another approach is to use mathematical models for the mandible. Models in which the mandible has been treated as a straight or curved beam can provide an insight into the biomechanics of the mandible. This can be achieved by use of finite element analysis (FEA), in which the mandible is subdivided into a large number of small elements of finite dimensions. FEA is an analytical tool that allows virtual modelling of complex problems, estimation of regional stresses, and determination of structural stresses in a highly reliable way and enables computers to solve problems. It is an important tool to investigate the pattern of tensile forces in complex biological structures. The range and distribution of stress and strain along a mandible can be determined for a given set of muscle, bite, and joint forces with a three-dimensional (3D) FEA model. This enables the analysis of the effect on stress and strain patterns, such as changes in loading, mandibular geometry, bone distribution, tooth loss, orthodontic forces, and implants.
Validation is the process of ensuring that a computational model accurately represents the physics of the real-world system. While some consider validation of natural systems to be impossible, the engineering viewpoint suggests the ‘truth’ about the system is a statistically meaningful prediction that can be made for a specific set of boundary conditions. This does not suggest that in vitro experimental validation (in a controlled laboratory environment) represents the in vivo case (within the living system) since the boundary conditions are likely impossible to mimic. It means that if a simplified model cannot predict the outcome of a basic experiment, it is probably not suited to simulate a more complex system.
The aim of this study was to perform a biomechanical validation of 3D FEA with STU for a resorbable fixation system (RFS) in bilateral mandibular vertical body osteotomies (BMVBO) in a rabbit model.
Materials and methods
The study protocol was reviewed and approved by the institutional animal care and use committee and was carried out in accordance with the Declaration of Helsinki on medical protocol and ethics.
Fifteen healthy female adult New Zealand rabbits, ranging in age from 6 to 8 months and weighing from 3 to 3.5 kg, were used as subjects. The animals were divided into three experimental groups, each including five animals (groups 1, 2, and 3). A total of 30 hemimandibles of 15 rabbits were used. The biomechanical analyses of rabbit hemimandibles were performed immediately after the operations in group 1, at the first postoperative month in group 2, and at the third postoperative month in group 3.
The resorbable fixation system (RFS) and surgery
The RFS system consisted of 2.0-mm profile four-hole straight miniplates and 7-mm long screws from Inion (Inion CPS Biodegradable Fixation System; Inion Oy, Tampere, Finland). The resorbable plate was made of amorphous poly- l -lactide (PLLA), poly- d , l -lactide (PDLLA), and trimethylene carbonate (TMC) copolymers.
The most appropriate area to apply the RFS was determined to be between the mental foramen and masseter muscle attachment at the posterior mandible. The animals were sedated with 35 mg/kg ketamine HCl (Ketalar; Parke–Davis, Detroit, MI, USA) intramuscular (i.m.) and 10 mg/kg xylazine (Rompun; Bayer, Germany). Ketamine HCl 17.5 mg/kg and xylazine 5 mg/kg were administered i.m. as maintenance dose. Four percent of articaine HCl with 1/100,000 epinephrine HCl (Ultracaine D-S Forte, Hoechst) was used as local anaesthesia.
The operation areas were disinfected with 1% povidone–iodine (Batticon, ADEKA, Turkey). After administration of the local anaesthetic, a 2-cm long submandibular incision and blunt dissection were performed to skeletonize the mandibular bodies. The osteotomies of all the hemimandibles were performed between the first premolar teeth and 1 mm distal to the mental nerve. One four-hole straight miniplate and four screws were placed in a perpendicular fashion.
The osteotomies were performed using oscillating sagittal saws (NSK, Japan) ( Fig. 1 ). After completion of the cuts, the biodegradable miniplate/screws were placed by drilling and tapping procedures. The plates were biomechanically stable in situ in all of the models. Once fixations had been completed bilaterally, the wounds were rinsed with sterile saline and incisions were closed with 4/0 polyglactin 910 sutures (Vicryl, Ethicon, Johnson & Johnson, USA).
