Three-dimensional titanium miniplates for fixation of subcondylar mandibular fractures: Comparison of five designs using patient-specific finite element analysis
Introduction
The mandible is the most vulnerable bone to fractures in the maxillofacial complex. Mandibular fractures amounted to 42% of all maxillofacial fractures in a recent prevalence study in a European population (Boffano et al., 2015). One type of mandibular fractures is condyle fractures, which can be classified according to the anatomical position of fracture into three subgroups: intracapsular condylar head fracture, extracapsular condylar neck fracture, and extracapsular subcondylar (condylar base) fractures (Loukota et al., 2005, Wermker, 2009). Condyle extra-capsular fractures comprised 26% of mandibular fractures in the same previously mentioned European study, ranking first among all types of mandibular fractures. In another study using the data from one hospital in Texas over a 17-year period of time (Morris et al., 2015), condyle and subcondylar fractures accounted for 18.4% of maxillofacial fractures, ranking third after angle and symphysis fractures. These figures seem to be largely dependent on the geographical areas and populations. Interesting trends for mandibular fractures were recently reported by Zhou et al. (2016), who stated that condylar fractures are unlikely to be associated with angular fractures. Likewise, angular fractures are associated with a lower risk of condylar fractures.
Several treatment approaches are used for the management of subcondylar mandibular fractures in adults. Primarily the treatment could be either closed reduction with maxillomandibular fixation (MMF) or open reduction with internal fixation (ORIF). The treatment of choice remains a controversial issue (Kyzas et al., 2012, Vesnaver et al., 2012, Shiju et al., 2015), and some surgeons still prefer the closed reduction with MMF (Neff et al., 2014, Kommers et al., 2015). However, ORIF could be necessary and is indicated in fractures with a high degree of displacement (Hackenberg et al., 2014, Cranford et al., 2016). Another controversial issue, which is the focus of the current study, concerns the best design and arrangement of titanium miniplates to be used in ORIF in order to achieve maximum stability of fixed bony fragments.
Previous studies have shown that the performance of one straight miniplate is suboptimal and that the use of two straight miniplates with nonparallel configuration performs better in biomechanical models (Choi et al., 1999, Wagner et al., 2002, Meyer et al., 2006, Tominaga et al., 2006, Parascandolo et al., 2010, Aquilina et al., 2013). However, the limited surgical access to the condyle area and the small dimensions of bone fragments require the use of minimum hardware. Two straight miniplates, each fixed with four screws, may significantly weaken bone fragments, and would be more difficult to handle during surgery. These limitations have encouraged the emergence of new single miniplates with three-dimensional designs that distribute the fixing screws in a nonparallel fashion. Such miniplates are the Delta miniplate (Medartis, Basel, Switzerland), trapezoid miniplate (Synthes CMF, PA and Medartis), and Lambda miniplate (Synthes CMF and Medicon, Tullingen, Germany). Several studies selected some of these miniplates and compared their performances, and sometimes yielded contradictory results due to the different methodologies and conditions between the studies (Lauer et al., 2007a, de Jesus et al., 2014, Hakim et al., 2014, Zrounba et al., 2014, Murakami et al., 2017). The most inclusive and recent study, conducted by Murakami et al. (2017), did not present information related to the displacement of fracture fragments. Therefore, it is difficult to conclude from that study which miniplate provides the most rigid fixation. They investigators focused on stresses in miniplates, which indicates only whether the miniplate will fracture before the other miniplates under certain loads, a problem that is rarely encountered in practice.
In the current study, we aimed to assess the performance of five single three-dimensional (3D) miniplates for fixation of subcondylar mandible fractures using finite element (FE) analysis. Assessment measures included displacements of fracture fragments, strains in bone, and stresses in titanium miniplates.
Section snippets
Construction of 3D models
Computed tomography (CT) of a living human was used to generate a surface model of mandibular bone. Segmentation was performed using the segmentation module in Mechanical Finder V7.0 (RCCM, Osaka, Japan). The left part of the mandible was used for simulations, and the condylar base was cut with a space of 0.25 mm between the fragments. Three-dimensional surface models were developed for five commercially available miniplates using the computer-aided design software Autodesk Inventor
Hardware volume
The analyzed miniplates are significantly different in total volume of miniplate and screws (Fig. 3). Delta and rhombic miniplates have noticeably smaller volumes than other miniplates. These two small miniplates are almost the half of the volume of lambda or strut miniplates. On the other hand, the lambda and strut miniplates have the largest volumes among all miniplates.
Rigidity of osteosynthesis
The FE models yielded significant differences in rigidity among the miniplates. For assessment of rigidity, displacement
Discussion
The mandible is subject to heavy functional forces that generate internal tensile and compressive strains. It is advocated that the successful osteosynthesis of mandibular fractures can only be achieved by moderating the injurious tensile strains (Champy et al., 1978). Efforts have been made to determine the tension and compression lines at the condyle area (Meyer et al., 2002), and it was found that tension lines run below and parallel to the sigmoid notch of the ramus. These biomechanical
Conclusion
The findings of the current study suggest that the trapezoid plate has the best performance among the available three-dimensional miniplates. Further comparative mechanical and clinical studies are needed to confirm this observation.
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