Background: The plantar fascia is a primary arch supporting structure of the foot and is often stressed with high tension during ambulation. When the loading on the plantar fascia exceeds its capacity, the inflammatory reaction known as plantar fasciitis may occur. Mechanical overload has been identified as the primary causative factor of plantar fasciitis. However, a knowledge gap exists between how the internal mechanical responses of the plantar fascia react to simple daily activities. Therefore, this study investigated the biomechanical responses of the plantar fascia during loaded stance phase by use of the finite element (FE) modeling. Methods: A 3-dimensional (3-D) FE foot model comprising bones, cartilage, ligaments, and a complex-shaped plantar fascia was constructed. During the stance phase, the kinematics of the foot movement was reproduced and Achilles tendon force was applied to the insertion site on the calcaneus. All the calculations were made on a single healthy subject. Results: The results indicated that the plantar fascia underwent peak tension at preswing (83.3% of the stance phase) at approximately 493 N (0.7 body weight). Stress concentrated near the medial calcaneal tubercle. The peak von Mises stress of the fascia increased 2.3 times between the midstance and preswing. The fascia tension increased 66% because of the windlass mechanism. Conclusion: Because of the membrane element used in the ligament tissue, this FE model was able to simulate the mechanical structure of the foot. After prescribing kinematics of the distal tibia, the proposed model indicated the internal fascia was stressed in response to the loaded stance phase. Clinical Relevance: Based on the findings of this study, adjustment of gait pattern to reduce heel rise and Achilles tendon force may lower the fascia loading and may further reduce pain in patients with plantar fasciitis.

To investigate the effects of backward adjustable thoracic support on spinal curvature and back muscle activation during wheelchair sitting. Twenty elderly people were recruited for this study. The backward adjustable thoracic support sitting posture was compared with the slumped, normal, and lumbar support sitting postures. Spinal curvatures (pelvic, lumbar, and thoracic angles) and muscle activations of 4 back muscles on both sides (maximal voluntary isometric contraction of the lumbar multifidus, lumbar erector spinae, iliocostalis lumborum pars thoracis, and thoracic erector spinae at T9) were measured and compared between the different sitting postures using one-way analysis of variance with repeated measures. The backward adjustable thoracic support sitting posture showed a relatively neutral pelvic tilt (−0.32±4.80°) when compared with the slumped (22.84±5.27°) and lumbar support (−8.97±3.31°) sitting postures (P<0.001), and showed relatively higher lumbar lordosis (−23.38±6.50°) when compared with the slumped (14.77±7.83°), normal (0.44±7.47°), and lumbar support (−16.76±4.77°) sitting postures (P<0.05). It also showed relatively lower back muscle activity when compared with the normal and lumbar support sitting postures (P<0.05). The backward adjustable thoracic support sitting concept was suggested because it maintains a more neutral pelvic tilt, higher lumbar lordosis, and lower back muscle activation, which may help maintain a better sitting posture and reduce the risk of back pain.

The dynamic hip-screw (DHS) system is a common implant for fixation of proximal femur fractures. During assembly, it has been recommended to remove the compression screw after initial compression has been obtained; however, related complications had been reported. So far, the role of compression screw in the reconstructed stability of hip fractures as well as the mechanical strength of the DHS system has rarely been mentioned. This study investigated the function of this screw in the DHS system during fracture healing. Based on the FE method, six numerical models of proximal femur were employed to analyze the mechanical response of a DHS implant with various fracture types and different fixation strategies (with or without a compression screw). The displacement of the femur head and peak von Mises stress were selected as indices of the stability of a fractured femur stabilized by a DHS device and of the risk of implant failure, respectively. Our results showed that a retained compression screw increased reconstructed structural stiffness, reducing the displacement of the femur head. This screw also helped to lessen mechanical failure of side plate by reducing the peak von Mises stress around the connection between the barrel and side plate. Both findings were evident in the proximal femur fracture involving the intertrochanteric part, and even more obvious in the setting of bony defects. Thus, we recommend the maintenance of compression screw in the DHS system while treating the intertrochanteric fracture, particularly in cases with bony defects.

Elastic nails made of the nickel-titanium shape memory alloy (Nitinol) have been reported to control bone modeling in animal studies. However, the mechanical stability of the Nitinol nail in the fixation of long bone fractures remains unclear. This study compared mechanical stability among nails made of three materials, namely Nitinol, titanium, and stainless steel, in the fixation of long bone fractures. These three materials had identical shapes (arc length: π/2 and radius: 260 mm). A cylindrical sawbone with a 10-mm gap and fixed with two C-shaped elastic nails was used to examine the stability of the nails. A finite element (FE) model was developed based on the sawbone model. The end cap for elastic nails was not used in the sawbone test but was considered based on a constraint equation in FE simulation. The results of stability tests appeared to depend on the presence or absence of the end cap. In the sawbone test, the titanium nail yielded a higher ultimate force against the applied load than did the stainless steel and Nitinol nails before the gap completely closed; the difference in linear stiffness between the nails was nonsignificant. In FE simulation, the titanium nail produced smaller gap shortening than did stainless steel and Nitinol nails without the end cap; the difference in gap shortening between the nails was minor with the end cap. The titanium elastic nail should be a better choice in managing diaphyseal long bone fractures when the end cap is not used. For Nitinol and stainless steel nails, the end cap should be used to stop the nail from dropping out and to stabilize the fractured bone.

To maintain the corrected alignment after high tibial osteotomy (HTO), fixation with titanium locking plate and screws is widely used in current practice; however, screw breakage is a common complication. Thus, this study was to investigate the mechanical stability of HTO with locking plate and various screw fixations, including the length as well as the type. A finite element (FE) model involving a distal femur, meniscus, and a proximal tibia with HTO fixed with a titanium locking plate and screws was created. The angle of the medial open wedge was 12°, and bone graft was not used. Two types of screws, namely conventional locking and far-cortical locking screws, with various lengths and configurations were used. At the proximal tibia, conventional locking screws with different lengths, 30 and 55 mm, were used; at the tibia shaft, different screw fixations including one-cortical, two-cortical, and far-cortical locking screws were used. The use of far-cortical locking screw generated the highest equivalent stress on the screws, which was four times (from 137.3 to 541 MPa) higher than that of the one-cortical screw. Also, it led to the maximum deformation of the tibia and a greater gap deformation at the osteotomy site, which was twice (from 0.222 to 0.442 mm) larger than that of the one-cortical screw. The effect of different locking screw length on tibia deformation and implant stress was minor. Thus, far-cortical locking screws and plates increase interfragmentary movement but the screw stress is relatively high. Increasing the protection time (partial weight duration) is suggested to decrease the risk of screw breakage in HTO through fixation with titanium far-cortical locking screws and plates.