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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.

To increase the applicability of ultrasonic linear motors, we have developed a holding mechanism for the transducer using parallel leaf springs. The present mechanism is compact since the parallel leaf spring structure has functions for both holding and preloading at the same time. Also, a special design for the spring characteristics provides a non-linear load-displacement region with buckling phenomena. These results in a constant preload for some range of the displacement, then the motor performance does not alter even after abrasion of contact surfaces. In this report, the relationship between the dimension of the leaf spring and the force-displacement curves was investigated in order to obtain the design appropriate for constant preloading. Moreover, the effect of the vibration reduction by the holding mechanism was investigated theoretically and experimentally. The vibration reduction was as small as less than 10 % since the stiffness in terms of the vibration direction was relatively small. Finally, the dynamic responses of the motor with the holding mechanism were measured for a sinusoidal control signal. No prominent peak was observed in the motor's responses, however, the motor velocity change was distorted near the natural frequency of the holding mechanism.

Background and purpose Many clinical reports have indicated that polished hip stems show better clinical results than rough stems of the same geometry. It is still unknown, however, what the mechanical effects are of different surface finishes on the cement at the cement-bone interface. We compared mechanical effects in an in vitro cemented hip arthroplasty model. Methods Two sizes of double-taper polished stems and matt-processed polished stems (rough stems) were fixed into composite femurs. A 1-Hz dynamic load was applied to the stems for 1 million cycles. An 8-h no-load period was set after every 16 h of load. Stem subsidence within the cement, and compressive force and horizontal cement creep at the cement-bone interface, were measured. Results Compared to rough stems, stem subsidence, compressive force and cement creep for polished stems were a maximum of 4, 12, and 7-fold greater, respectively. There was a strong positive correlation between stem subsidence and compressive force for polished stems. In contrast, a strong negative correlation was found between stem subsidence and compressive force for rough stems. There was also a statistically significant relationship between compressive force on the cement and cement creep for the polished stems, but no significant relationship was found for rough stems. Interpretation This is the first evidence that different surface finishes of stems can have different mechanical effects on the cement at the cement-bone interface. Stem subsidence in polished stems resulted in compressive force on the cement and cement creep. The mechanical effects that polished taper stems impart on cement at the cement-bone interface probably contribute to their good long-term fixation and excellent clinical outcome.

In the hip replacing arthroplasty, to make the stem shape of an artificial hip joint fit to the morphology of proximal femoral canal is generally effective to the fixation in an initial stage. Furthermore, it contributes to reduce a stress shielding at the proximal femur. Therefore, this paper proposed the design method of the custom-made stem corresponding to each patient in order to improve the fill ratio of the stem to the proximal femoral canal. This modeling is mainly achieved by means of a Boolean operation (logical AND operation) of the cross sectional contours of the proximal femoral canal along the circular insert axis, which is decided by the bow of the femoral canal morphology with 3D X-ray CT images and a CAD system. As a result, the fill ratio of the stem to the proximal canal by the proposed method was higher than that by a method with the simple straight type as an insert axis. Also, the fill ratio and the spatial distributions of it in the femoral canals with this proposed method were so similar between the different femoral morphology. The result is important for a design of the custom-made model corresponding to variable femoral morphologies.

The aim of this study was to evaluate the cytotoxicity of cubic boron nitride (cBN), a component of surgical cutting tools. The small quantities of cBN that typically remain on implants as a result of the manufacturing process may act as abrasives, injuring tissues surrounding the implant. To determine how cBN affects cells, we treated human neuroblastoma cells (NB-1) and human articular chondrocytes (nHAC-kn) with different concentrations of cBN powder and assessed cell growth and cell survival using the methyl-thiazol-tetrazolium (MTT) assay and a fluorescence probe assay. We also assessed the effects of tungsten carbide (WC) and cobalt (Co), two common components of joint implants, on cell growth and cell survival. Both cBN and WC moderately inhibited NB-1 and nHAC-kn cell growth. However, cBN and WC did not affect cell survival, even at high concentrations (40 microg/ml). By contrast, Co affected cell survival, inducing cell death in both cell types at increasing concentrations. These results suggest that cBN may be less toxic than WC alloys containing Co.