Novel finite element-based plate design for bridging mandibular defects: Reducing mechanical failure

A method for accuracy of placement analysis on radiolucent polyether-ether-keton facial implants: A case series

Facial asymmetry is defined as a bilateral difference between facial components. Correction, often desired by the patient, can be performed with the aim of bone born patient-specific solid implants designed using 3D CAD software. This treatment is embedded in the daily practice of today's healthcare. However, an analysis of the implant's accuracy of placement has not been reported. This case series describes the accuracy analysis of bone born aesthetic facial implants manufactured out of polyether-ether-ketone (PEEK). The accuracy analysis was based on postoperative (cone beam) computed tomography ((CB)CT) data and preoperative 3D planning. The analysis showed a median entry point error of 0.7 mm (min: 0.1, max: 3.3, interquartile range: 0.78). The median maximal orientation error was 5.5° (min: 0.1, max: 36.8, interquartile range: 7.13). Both parameters showed an excellent intraobserver and interobserver agreement with an ICC above 0.84. The described cases show that the analysis method is an objective approach for determining the accuracy of PSI placement and indicates that these implants can be placed accurately on the osseous face.

Finite element analysis of low-profile reconstruction plates for atrophic mandibles: a comparison of novel 3D grid and conventional plate designs

OBJECTIVES

This study aimed to compare the reconstruction with type 2.4 and three-dimensional (3D) grid plates relating the areas of weakness to anatomical regions of force incidence in atrophic mandibles with and without segmentation using finite element analysis (FEA).

MATERIALS AND METHODS

Strength and force quality in the mandible were also determined. The atrophic mandible models with and without segmentation were divided into four groups, and the behavior of each plate was analyzed using finite element analysis. In the atrophic mandible without segmentation using the type 2.4 reconstruction plate, the highest stress concentration in the plate was observed in the posterior region of the mandibular body and in the grid type 3D reconstruction plate was observed in the mandibular angle.

RESULTS

In the segmented atrophic mandible with the 2.4 reconstruction plate, higher stress concentration was observed in the mandibular segment defect. Analysis of the 3D grid-like reconstruction plate revealed that the geometry of the plate conferred greater stiffness to the assembly, as the most significant stress concentration was observed at the mandibular angle.

CONCLUSION

The width of the plate design may influence the strength, not the thickness.

Alloplastic reconstruction of the mandible after subtotal mandibulectomy for medication-related osteonecrosis of the jaw: An update of the method

BACKGROUND

Reconstruction of continuity defects following osteonecrosis in multimorbid patients is challenging. In all cases of the predescribed palliative treatment method for alloplastic mandible reconstruction, plate fractures were detected in follow-up. We hypothesized that a modification could avoid these fractures, leading to stable long-term results.

METHODS

This retrospective study compares the original method with a modification using single, laser-sintered CAD-CAM plates instead of manually bent miniplates. The predescribed shuttering technique was used to reconstruct the mandible in its original shape with bone cement.

RESULTS

Uneventful wound healing was observed in 86% of the cases after modification of the method. No implant or plate fracture occurred.

CONCLUSIONS

The presented method should be considered as a treatment option for mandible reconstruction in elderly, multimorbid patients in palliative situations. The results of this study suggest that the modification of the method leads to significantly improved long-term stability. Donor site morbidity is avoided with this method of palliative surgery.

Mandibular Reconstruction after Resection of Ameloblastoma by Custom-Made CAD/CAM Mandibular Titanium Prosthesis: Two Case Reports, Finite Element Analysis and Discussion of the Technique

Virtual surgical planning for CAD/CAM mandibular reconstruction by titanium prosthesis was recently reported for resected cases. Even if some advantages are evident, difficulties that may arise for TMJ function after reconstruction originate from prosthesis contamination through oral mucosa dehiscence. In these two cases reported of mandibular reconstruction after resection of ameloblastoma by custom-made CAD/CAM titanium prosthesis, the procedures were aimed to preserve the TMJ glenoid cavity and articular disc avoiding functional problems for hemi-mandibular resections that included the condyle (as in case #1) or with condylar preservation (as in case #2) and avoiding intraoral incisions in both cases. The entire surgical planning and prosthetic fabrication were explained with specifications and the sequence of the surgical procedure. Finite elements analysis (FEA) was performed to check the force distribution and efficacy of the prosthetic device (case 1 with hemi-mandibular resection and rehabilitation). Although successful in these two cases, surgical reconstruction of the mandibular defect after resection by a CAD-CAM custom-made prosthesis still shows some drawbacks and failure risks. Several advantages of this technique and the surgical success in these two cases were presented, but limitations and side effects must be considered when cases are selected.

Comparative evaluation of a patient-specific customised plate designs and screws for partial mandibular reconstruction

Mandibles with odontogenic tumors are often partially reconstructed with a metallic bone graft analogue with dental roots, crowns, along with a customized plate fixed with monocortical or bicortical screws, following resection of the tumor. In this study, two different designs of patient specific customized Ti reconstruction plates, solid and plate with holes, were considered. Fixation through both bicortical and monocortical screw types were investigated. FE models of the reconstructed mandibles were developed to analyse the influence of the plate-screw type combination on the load transfer across the mandibles under a mastication cycle. The effective homogenized orthotropic material properties of the lattice structures with 0.6 mm fibre diameter with 0.5 mm inter-fibre space were assigned to material properties for the bone graft analogue. The study shows that the combination of plate and screw types influences the state of stresses in the reconstructed mandible. Based on the results of this patient specific study, following resection of the tumor, either solid Ti plate with bicortical screws or Ti plate with holes along with monocortical screws may be used for partial mandibulectomy. It should also be noted that stresses in none of the plates or screws exceeded the yield limit for Ti under the mastication cycle indicating that the components are safe for mandibular reconstruction. However, the choice of this combination of reconstruction plates and screws is dependant on the condition and severity of the tumor in the diseased mandible.

On interaction between fatigue of reconstruction plate and time-dependent bone remodeling

BACKGROUND AND OBJECTIVE

The fibula free flap (FFF) has been extensively used to repair large segmental bone defects in the maxillofacial region. The reconstruction plate plays a key role in maintaining stability and load-sharing while the fibula unites with adjacent bone in the course of healing and remodeling. However, not all fibula flaps would fully unite, and fatigue of prosthetic devices has been recognized as one major concern for long-term load-bearing applications. This study aims to develop a numerical approach for predicting the fatigue life of the reconstruction plate by taking into account the effect of ongoing bone remodeling.

METHODS

The patient-specific mandible reconstruction with a prosthetic system is studied in this work. The 3D finite element model with heterogeneous material properties obtained from clinical computerized tomography (CT) data is developed for bone, and eXtended Finite Element Method (XFEM) is adopted for the fatigue analysis of the plate. During the remodeling process, the changing apparent density and Young's modulus of bone are simulated in a step-wise fashion on the basis of Wolff's law, which is correlated with the specific clinical follow-up. The maximum biting forces were considered as the driving force on the bone remodeling, which are measured clinically at different time points (4, 16 and 28 months) after reconstruction surgery.

RESULTS

Under various occlusal loadings, the interaction between fatigue crack growth and bone remodeling is investigated to gain new insights for the future design of prosthetic devices. The simulation results reveal that appropriate remodeling of grafted bone could extend the fatigue life of fixation plates in a positive way. On the other hand, the rising occlusal load associated with healing and remodeling could lead to fatigue fracture of fixation plate and potentially cause severe bone resorption.

CONCLUSION

This study proposes an effective approach for more realistically predicting fatigue life of prosthetic devices subject to a tissue remodeling condition in-silico. It is anticipated to provide a guideline for deriving an optimal design of patient-specific prosthetic devices to better ensure longevity.

A Contemporary Approach to Non-Invasive 3D Determination of Individual Masticatory Muscle Forces: A Proof of Concept

Over the past decade, the demand for three-dimensional (3D) patient-specific (PS) modelling and simulations has increased considerably; they are now widely available and generally accepted as part of patient care. However, the patient specificity of current PS designs is often limited to this patient-matched fit and lacks individual mechanical aspects, or parameters, that conform to the specific patient’s needs in terms of biomechanical acceptance. Most biomechanical models of the mandible, e.g., finite element analyses (FEA), often used to design reconstructive implants or total joint replacement devices for the temporomandibular joint (TMJ), make use of a literature-based (mean) simplified muscular model of the masticatory muscles. A muscle’s cross-section seems proportionally related to its maximum contractile force and can be multiplied by an intrinsic strength constant, which previously has been calculated to be a constant of 37 [N/cm2]. Here, we propose a contemporary method to determine the patient-specific intrinsic strength value of the elevator mouth-closing muscles. The hypothesis is that patient-specific individual mandible elevator muscle forces can be approximated in a non-invasive manner. MRI muscle delineation was combined with bite force measurements and 3D-FEA to determine PS intrinsic strength values. The subject-specific intrinsic strength values were 40.6 [N/cm2] and 25.6 [N/cm2] for the 29- and 56-year-old subjects, respectively. Despite using a small cohort in this proof of concept study, we show that there is great variation between our subjects’ individual muscular intrinsic strength. This variation, together with the difference between our individual results and those presented in the literature, emphasises the value of our patient-specific muscle modelling and intrinsic strength determination protocol to ensure accurate biomechanical analyses and simulations. Furthermore, it suggests that average muscular models may only be sufficiently accurate for biomechanical analyses at a macro-scale level. A future larger cohort study will put the patient-specific intrinsic strength values in perspective.

Titanium or Biodegradable Osteosynthesis in Maxillofacial Surgery? In Vitro and In Vivo Performances

Osteosynthesis systems are used to fixate bone segments in maxillofacial surgery. Titanium osteosynthesis systems are currently the gold standard. However, the disadvantages result in symptomatic removal in up to 40% of cases. Biodegradable osteosynthesis systems, composed of degradable polymers, could reduce the need for removal of osteosynthesis systems while avoiding the aforementioned disadvantages of titanium osteosyntheses. However, disadvantages of biodegradable systems include decreased mechanical properties and possible foreign body reactions. In this review, the literature that focused on the in vitro and in vivo performances of biodegradable and titanium osteosyntheses is discussed. The focus was on factors underlying the favorable clinical outcome of osteosyntheses, including the degradation characteristics of biodegradable osteosyntheses and the host response they elicit. Furthermore, recommendations for clinical usage and future research are given. Based on the available (clinical) evidence, biodegradable copolymeric osteosyntheses are a viable alternative to titanium osteosyntheses when applied to treat maxillofacial trauma, with similar efficacy and significantly lower symptomatic osteosynthesis removal. For orthognathic surgery, biodegradable copolymeric osteosyntheses are a valid alternative to titanium osteosyntheses, but a longer operation time is needed. An osteosynthesis system composed of an amorphous copolymer, preferably using ultrasound welding with well-contoured shapes and sufficient mechanical properties, has the greatest potential as a biocompatible biodegradable copolymeric osteosynthesis system. Future research should focus on surface modifications (e.g., nanogel coatings) and novel biodegradable materials (e.g., magnesium alloys and silk) to address the disadvantages of current osteosynthesis systems.

Novel finite element-based plate design for bridging mandibular defects: Reducing mechanical failure

INTRODUCTION

When the application of a free vascularised flap is not possible, a segmental mandibular defect is often reconstructed using a conventional reconstruction plate. Mechanical failure of such reconstructions is mostly caused by plate fracture and screw pull-out. This study aims to develop a reliable, mechanically superior, yet slender patient-specific reconstruction plate that reduces failure due to these causes.

PATIENTS AND METHODS

Eight patients were included in the study. Indications were as follows: fractured reconstruction plate (2), loosened screws (1) and primary reconstruction of a mandibular continuity defect (5). Failed conventional reconstructions were studied using finite element analysis (FEA). A 3D virtual surgical plan (3D-VSP) with a novel patient-specific (PS) titanium plate was developed for each patient. Postoperative CBCT scanning was performed to validate reconstruction accuracy.

RESULTS

All PS plates were placed accurately according to the 3D-VSP. Mean 3D screw entry point deviation was 1.54 mm (SD: 0.85, R: 0.10-3.19), and mean screw angular deviation was 5.76° (SD: 3.27, R: 1.26-16.62). FEA indicated decreased stress and screw pull-out inducing forces. No mechanical failures appeared (mean follow-up: 16 months, R: 7-29).

CONCLUSION

Reconstructing mandibular continuity defects with bookshelf-reconstruction plates with FEA underpinning the design seems to reduce the risk of screw pull-out and plate fractures.