A systematic comparison of the tensile and compressive response of glass ceramic Macor, with zero porosity and low density, is carried out by using flattened Brazilian disk and cylindrical specimen from quasi-static to dynamic loading conditions. The experiments were performed on a screw driven Zwick machine and an in-house built split Hopkinson bar synchronized with a high speed photographic system. Likewise, the loading rate dependent fracture toughness is also investigated by using a notched semi-circular Brazilian disk. A digital image correlation technique is adopted to assist in the monitoring of strain field, crack initiation and propagation under dynamic loading conditions. Both tensile and compressive strength show loading rate dependencies, however, the static and dynamic tensile strengths are only 20% of the compressive strengths without confinement and less than 10% of the confined compressive strength. The microstructural characterization reveals the fracture mechanisms in unconfined Macor are predominantly transgranular with mica platelets and cleavage planes, which are influenced by the loading mode and loading rate. However, the Macor with confinement shows ductile fracture micrographs with a shear localization zone consisting of fine particles. With the use of Macor ceramic as a model material, the paper presents an economical approach to investigate the loading mode and pressure dependent failure of ceramic materials. This will support the characterization of dynamic properties of current and future developed advanced ceramics for demanding applications in the aero engine.
DJ Screw P s And Q s (Chapter 022)
A minimally invasive technique with various screw configurations without open surgery is currently used for the fixation of transverse patellar fractures. Percutaneous crossing screw configuration has been reported to have a good bone union rate in patellar fractures. However, the difference in mechanical stability of the fractured patella between different screw-included angles has not been fully investigated. Hence, this study aims to compare the mechanical stability of parallel and crossing screw fixations with different screw-included angles for the fixation of transverse patellar fractures during level walking.
A finite element knee model containing a patella with a transverse fracture is created. Two headless compression screws with different angles (0, 30, 60, and 90) are used to fix the fracture. The loading conditions of the knee joint during level walking are used to compare the stability of the fractured patella with different fixation screw configurations.
The results indicate that the maximum fracture gap opening distance increased with an increase in the included angle. Two parallel screws yield the smallest gap distance among all screw configurations. The maximum gap opening distances at the anterior leading edge of the fractured patella with two parallel screws and two screws having an included angle of 90 are 0.73 mm and 1.31 mm, respectively, at 15% walking cycle.
Based on these results, the superior performance of two parallel screws over crossing screw fixations in the fixation of transverse patellar fractures is established. Furthermore, the smaller the angle between the crossing screws, the better is the stability of the fractured patella.
To date, several surgical approaches have been proposed for the management of patellar fractures, including tension band wiring, cerclage, modified tension bands, pins or screws, and combined approaches [6,7,8,9,10]. According to the literature, more than half of those cases with patellar fracture accepted tension band wiring from 2003 to 2015, while the combined approach has increased recently [11]. The major advantage of the combined approach, such as cannulated screws along with an anterior wire, is its excellent stability [12, 13]. Cannulated screws along with an anterior wire were also reported to be superior to tension band wiring in the literature [14]. However, the major disadvantage of cannulated screws with an anterior wire is delayed healing and pain after surgery due to damage to the soft tissue because of open surgery [15,16,17]. Hence, several studies have focused on minimally invasive surgery without the anterior wire and with a specific cable-pin system, two and three parallel headless compression screws, and full-thread screws to enhance the healing process and reduce pain [12, 13, 18, 19].
A percutaneous crossing screw configuration without an anterior wire has also been proposed to achieve a good union rate for patellar fractures [20]. However, the difference in mechanical stability of the fractured patella between the two parallel screws and two crossing screws during daily walking is unclear. Hence, the aim of this study is to compare the stability of the fractured patella with different screw-included angles in the fixation of transverse patellar fractures in level walking by using finite element (FE) simulation. The reason for using FE simulation is the strength in solving such a highly nonlinear lading of the patella during walking. FE modeling has been used in many biomechanical studies, particularly with complex geometries and loadings [21].
A knee joint model containing the patella, distal femur, proximal tibia, and fibula was developed based on the computer tomographic images of a healthy man with a body weight of 70 kg and height of 170 cm. The images were obtained when the subject was lying supine; hence, the knee model was created in the full extension position. The areas of the cortical and cancellous bone in the images were isolated out by the higher gray values than the surrounding soft tissues. The 3D model of the bones was then developed by stacking the isolated bony areas. It was subsequently imported into the CAD software, SolidWorks 2019, to create the cartilage and meniscus. The thickness of the cartilage of the patella, distal femoral, and proximal tibia was set at 1 mm based on the literature [22]. The space between the cartilage was created as a meniscus. The transverse fracture type AO 34-C1 at the middle of the patella was used in this study (Fig. 1a). The transverse fracture was created by a virtual plane; hence, no gaps existed after the section. After the fracture was created, a 4.5 mm headless compression screw (HCS, DePuy Synthes, Raynham, MA, US) was used to fix the fractured patellar fragments (Fig. 1b).
To investigate the effect of the included angle of the screws in crossing fixations, three crossing angles were considered: 30, 60, and 90. Furthermore, in the crossing screws, there are two different screw configurations: the anterior screw inward and anterior screw outward. In addition to cross-fixation, traditional parallel screw fixation was used for comparison. Two parallel screws were placed in the middle third of the patella in the front plane. The proximity of the screws was approximately half of the thickness of the patella. Additionally, the currently used anterior wire in a figure of eight with two parallel screws was used for validation. The diameter of the anterior wire was set at 1 mm. The length of the screws ranged from 35 to 40 mm, and the thread length was 12 mm. The thread was controlled to avoid passing through the fracture site; hence, compression and separation of the fractured patellar fragments under loading were allowed. In total, eight different configurations were used (Fig. 2): two parallel screws (Para), two parallel screws with anterior wire (Para&Wire), crossing 30with the anterior screw inward (X30-AI), crossing 30with the anterior screw outward (X30-AO), crossing 60with the anterior screw inward (X60-AI), crossing 60with the anterior screw outward (X60-AO), crossing 90with the anterior screw inward (X90-AI), and crossing 90with the anterior screw outward (X90-AO).
The model was imported into ANSYS Workbench 2022 for simulation. A quadratic tetrahedron element (solid 187) was used to mesh the complex model, including the bone, cartilage, screw, and wire. The ligaments of the knee joint were reconstructed using tension-only springs in the ANSYS Workbench. In total, five ligaments (Table 1), namely the patellar, medial collateral, lateral collateral, anterior cruciate, and posterior cruciate ligaments, were created in the FE model. The stiffness of the spring was defined based on literatures [23,24,25,26]. The material properties of the bone, cartilage, and meniscus were set according to literature (Table 2) [27,28,29,30,31]. The material properties of the metal were used based on the engineering database in the ANSYS Workbench. All the materials were as assumed to be linear elastic, isotropic, and homogeneous.
The fractured patella separated at the anterior surface, and a large gap was developed, while the posterior aspect remained in contact with each other during walking (Fig. 5). Parallel screw fixation, regardless of whether the anterior wire was used, yielded a smaller gap opening distance than crossing screw fixation. Furthermore, the maximum gap opening distance during walking increased with an increase in the included angle in the crossed screws. In stance phase, the maximum gap opening distance of X90.AO was 1.25 (Fig. 6), 1.01 (Fig. 7) and 0.63 mm (Fig. 8) at anterior, medial, and lateral site of the gap, respectively, at 15% of the walking cycle. In swing phase, the maximum gap opening distance of X90.AO was 0.57, 0.4 and 0.16 mm at the anterior, medial, and lateral site of the gap, respectively, at 60% of the walking cycle. In general, the anterior gap opening distance was larger than the medial and lateral gaps during the walking cycle.
This novel study demonstrates loading on a fractured patella with different screw configurations in level walking. The present simulation could demonstrate the difference in fracture gap deformation between the stance and swing phases of the walking cycle. The loading conditions of the patella differ during walking, and the direction of the muscle force changes during knee flexion and extension. In most previous studies, a worst-case loading condition of the fractured patella was assumed, and the gap opening distance was demonstrated at specific knee flexion angles [12, 32, 34]. In this simulation, the loading condition was extended to the entire walking cycle. Determining the gap opening of the fractured patella with different screw fixations is helpful to surgeons and physical therapists in the decision-making process of postoperative care. 2ff7e9595c
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