In mandibular reconstructions using plates, the greatest stresses occur at the bone-screw interface, especially in the screw that is located closest to the bone defect.[10] The loads are greater on the screw closest to the defect on the side contralateral to the force exerted. The Von Mises stress gradually decreases with increasing distance from the loading point in the models with segmental defects. Force mapping showed that the stresses are concentrated around the screws at the ends.[5]
In this study, the greatest concentration of stress occurred in the screws inserted at the ends of the bone segment in the segmented atrophic mandible, as it is a fragile structure, and the plate does not provide good rigidity to the assembly.
When analyzing the 2.4 reconstructive plates in the segmented atrophic mandible, the area of greatest fragility was near the bone defect and independent of the number of screws. Several studies used three screws for fixation; however, more than three screws were used in the distal and proximal stumps in our study.[11],[12],[13]
Significant stresses occurred at the screw attachment-bone interfaces during chewing, and screw loosening may develop as a long-term complication after mandible reconstruction with plates.[14],[15],[16] Although factors other than stresses may be involved, such as local infection and screw wear, this finding provides a basis for screw loosening from a biomechanical aspect.[17]-[18]
In this study, the design of the plate, especially in terms of the width and arrangement of the screws, showed a significant difference in the site of resistance and stress of the plates and screws in atrophic mandibles and segmented atrophic mandibles with bone defects of up to 5 cm. In the atrophic mandible and segmented atrophic mandible, the reconstruction plate had a greater width, and the screw insertions were distant from the midline of the plate. The geometry of the plate and the positioning of the screws were predominant along the distribution of stresses, such that the greatest stress concentration in the plate occurred at the mandibular angle, close to the load/force applications. The maximum stress value tends to decrease when the plate width increase; however, the maximum value of screw stress tends to increase.[17]-[18]
Knoll et al. and Wong et al. reported that the stresses exerted on the plates lead to fatigue fractures in the plate and loosening of the screws in the plate and bone, given the high dynamic chewing stress. The stresses were reduced by more than half when the screw diameter increased by 50%. The use of larger screw diameters is generally not possible. A reconstruction plate with a flat design has more favorable stress characteristics and could be better adapted to the geometry of the jaw pre-operatively. Despite the use of appropriate tools, the standard plate allows limited flexion. However, these pre-formations do not have a negative effect on the geometric stability of the plate.[19]-[20]
The maximum stress value tends to decrease when the plate width increases; however, it tends to increase the maximum bolt stress value.[21]
This study had several strengths. First, custom plates were fabricated with new soft construction materials, allowing 3D plate planning according to the anatomy and bar reinforcement at individual locations. Second, sensitivity analyses determined the most optimal locations for plate reinforcement, thereby considering all possible width-bar arrangements. Third, biomechanical competency analyses included empirical (mechanical) tests and computational (finite element) analyses, confirming the benefits of plate reinforcement in two separate ways.
The difficulties arising from the design and development of the reconstruction plate include: (1) the repetitive loads produced during mastication, (2) the complex shape of the mandible, (3) the unfavorable environment within the tissues and the sequentially changing material behavior, (4) foreign material inside the body functioning flawlessly and not damaging the tissues, and (5) rejection of foreign materials by the body. The mechanical loading of the mandible and the reconstruction plate in the maximal bite make the upper edge of the mandibular ascending ramus and the internal curvature of the plate the most vulnerable sites for fracture.[22]-[23]
Prototyping reconstruction plates is a new tool for surgeons and has many applications in mandibular reconstruction. While it is not necessarily cost-effective or required in all cases, it is extremely useful in cases with primary bone malignancy and cases with involvement of the outer contour of the mandible, which makes it impossible to perform direct adaptation of the plate prior to resection. Technological advances in medical imaging and rapid prototyping would enable the production of 3D models.[24],[25],[26]