ABSTRACT
Objective To compare the postoperative stability of unstable distal radius fractures fixed by locking screws with different length and analyze the stress distributions of distal screws and callus at different healing periods, so as to provide biomechanical references for choosing appropriate length of screws for treating distal radius fractures. Methods The three-dimensional finite element models of unstable distal radius fractures with fracture section and callus were established, respectively, and fixed by volar locking plates and locking screws with different length. Then different periods of fracture healing were simulated by assigning callus with different material properties. Stress distributions on callus and distal screws at different postoperative periods were analyzed, and compression stiffness of the whole fixation system was calculated according to the maximum axial displacement at fracture section. Results Under the same axial loads, the compression stiffness was basically the same by using unicortical screw with over 75% length or bi-cortical screw. With the screw length increasing, the maximum stress of callus was decreased gradually during the period of early healing; while the maximum stress of distal screws was increasing gradually with the increase of screw length at middle and last period of fracture healing, and the stress of distal bi-cortical screw was the largest. Conclusions Using the unicortical distal locking screws with at least 75% length can not only produce early stability， but also avoid extensor tendon injuries due to dorsal screw prominence.
ABSTRACT
Objective To study the influence from different assigned gradients of material attributes on mechanical properties of the vertebral finite element model. Methods An adult human spine (T12-L5) was CT scanned, and the 3D models of each vertebra were reconstructed in MIMICS, which were then modified in Geomagic and imported into ANSYS for meshing. The element models were imported back to MIMICS and assigned with material properties by separating into 8 kinds of gradients (2, 4, 8, 10, 50, 100, 200, 400 divisions). These models were then imported to ANSYS again for finite element analysis under the same loading condition. Results Significant differences were found in stresses from models with 2, 4, 400 gradients, but the deviations between 8, 10, 50, 100, 200 gradients were not obvious. Conclusions The material attributes of finite element model should be appropriate, and the assigned gradient of 10 divisions could be better guarantee the accuracy of calculation and enhance the calculation speed as well, which is suitable for personalized rapid finite element modeling in clinic.
ABSTRACT
Objective To investigate changes in plantar pressure distributions on forefoot and hindfoot region during the support phase under different Achilles tensile loads. Methods Six fresh frozen human below-knee specimens were used and placed on the material testing machine. The ankles were kept in neutral position and the axial load of 350 N was applied on the specimens. Achilles tensile loads varied from 0 kg to 80 kg (0, 100, 200, 300, 400, 500, 600, 700, 800 N）were applied by weights. Two scales were placed under the forefoot and hindfoot region of the specimen, respectively, to record the pressure under the Achilles tensile loads. Results With the Increasing Achilles tensile force, the pressure on forefoot region was increasing, while the pressure on the hindfoot was decreasing. The percentage of average plantar pressure in forefoot and hindfoot region over the total plantar pressures presented a linear relationship with the Achilles tensile force (P<0.01, R2=0.996). Conclusions The support phase of foot in vitro was simulated and the plantar pressure distributions on forefoot and hindfoot region was investigated in this paper. The result can provide theoretical evidences for some diseases (such as diabetic foot ulcers, metatarsalgia) caused by plantar pressure changes due to Achilles tensile contracture in clinic.