Search

Purpose: Subchondral insufficiency-type fracture is associated with rapidly progressive osteoarthritis of the hip, but are these fractures caused by a single load event such as an injury from a fall or by stress (fatigue) from repeated loading during the activities of daily life? Our biomechanical study applied repeated loading to a simulated osteoporotic bone model to investigate the relationship between an inverted acetabular labrum and the incidence of subchondral insufficiency-type fracture of the femoral head.Methods: We performed fatigue load testing on 15 osteoporotic composite femurs, applying 2000 N at 1 Hz, 1 million times or to failure, on 5 composite femurs without silicone rubber and 10 with 10x5 mm silicone rubber. Silicone rubber was inserted as a surrogate for inversion of the acetabular labrum between the acetabulum and femoral head.Results: No breakage was noted in the 5 specimens without silicone rubber, even after the application of 1 million individual loads at 1 Hz. Fractures appeared in the anterior femoral head in all 10 specimens with 10×5 mm silicone rubber, and mean number of loads to fracture was 6389. Conclusion: Our research selected an osteoporotic bone model to investigate the effects of loading tests that were designed to mimic typical repeated loads in daily activities, using silicone rubber as a surrogate for inversion of the acetabular labrum and resulting in the development of insufficiency-type stress fracture on the femoral head.Concept diagram or graphical abstract

Although studies suggest that subchondral insufficiency fracture of the femoral head may cause rapidly progressive osteoarthritis of the hip, the mechanism of that relationship remains unclear. Our biomechanical study aimed to provide more data in this area by quantifying pressure distribution on the femoral head for normal and inverted hips and by determining the effects of labral inversion on pressure distribution across the joint, focusing on types of fracture under load. We tested mid-sized fourth-generation composite femurs at 15° of adduction, and applied 1 mm/min of axial compressive force to the femoral heads until failure. Additionally, single loads (3000 N) were applied using Prescale film to investigate pressure distribution on the femoral head, with or without silicone rubber representing entrapment of an inverted acetabular labrum. In tests with an external load of 3000 N, the mean pressure for 10 × 5 mm of silicone rubber was 11.09 MPa, significantly greater (about 5.7-fold) than 1.94 MPa without silicone rubber. Different fracture patterns were observed with and without the 10 × 5 mm silicone rubber; when the 10 × 5 mm silicone rubber specimens were used, all eight cases showed fractures in the anterior femoral head. When silicone rubber representing an inverted acetabular labrum was placed between a hemispherical metallic platen and a composite bone model, the silicone rubber areas were subjected to extreme concentration of stress. The fractures that developed at the silicone rubber areas clearly represented subchondral fractures of the femoral head, rather than fractures of the femoral neck.

In this paper, cooling performance of conformal cooling channel in plastic injection molding (PIM) is numerically and experimentally examined. To examine the cooling performance, cycle time and warpage are considered. Melt temperature, injection time, packing pressure, packing time, cooling time, and cooling temperature are taken as the design variables. A multi-objective optimization of the process parameters is then performed. First, the process parameters of conformal cooling channel are optimized. Numerical simulation in the PIM is so intensive that a sequential approximate optimization using a radial basis function network is used to identify a pareto-frontier. It is found from the numerical result that the cooling performance of conformal cooling channel is much improved, compared to the conventional cooling channel. Based on the numerical result, the conformal cooling channel is developed by using additive manufacturing technology. The experiment is then carried out to examine the validity of the conformal cooling channel. Through numerical and experimental result, it is confirmed that the conformal cooling channel is effective to the short cycle time and the warpage reduction.