No abstract

The Co-based alloy ASTM (Co -28.5 wt.-% Cr -6.3 wt.-% Mo) is widely used for medical implants, e.g. knee prostheses, and is commonly processed by investment casting. Selective laser (SLM) melting is supposed to be an efficient alternative for the production of individually designed knee implants regarding production time and production costs. The mechanical properties, in particular the fatigue strength, of the material have been studied in different states of the material. The mechanical properties of investment casted ASTM F75 and PM-SLM produced ASTM F75 were investigated. The focus in this study was on the PM-SLM material, the specimens were initially produced by selective laser melting and a part of the specimens were further processed by hot isostatic pressing (HIP). The PM-SLM material was mechanically tested in the as-SLM state as well as in the SLM þ HIP state. It was found that the mechanical properties of the as-SLM material did not reach the level of the fatigue strength of as cast material. The post-densification treatment by HIP offers distinct improvements regarding the fatigue strength compared to the as-SLM material.

Validation of musculoskeletal models for application in preoperative planning is still a challenging task. Ideally, the simulation results of a patient-specific musculoskeletal model are compared to corresponding in vivo measurements. Currently, the only possibility to measure in vivo joint forces is to implant an instrumented prosthesis in patients undergoing a total joint replacement. In this study, a musculoskeletal model of the AnyBody Modeling System was adapted patient-specifically and validated against the in vivo hip joint force measurements of ten subjects performing one-leg stance and level walking. The impact of four model parameters was evaluated; hip joint width, muscle strength, muscle recruitment, and type of muscle model. The smallest difference between simulated and in vivo hip joint force was achieved by using the hip joint width measured in computed tomography images, a muscle strength of 90 N/cm2, a third order polynomial muscle recruitment, and a simple muscle model. This parameter combination reached mean deviations between simulation and in vivo measurement during the peak force phase of 12% ± 14% in magnitude and 11° ± 5° in orientation for one-leg stance and 8% ± 6% in magnitude and 10° ± 5° in orientation for level walking.

A consideration of the patient-specific biomechanical situation in the context of the surgical planning of total hip arthroplasty is highly recommended and may have a positive impact on the therapeutic outcome. In current clinical practice, surgical planning is based on the status of the individual hip and its radiographic appearance. Several authors proposed different biomechanical modeling approaches for the calculation of the resultant hip force R on the basis of parameters gathered from plain radiography. The comparative study presented in this paper shows that the biomechanical models by Pauwels, Debrunner, Blumentritt and Iglič provide a good approximation of the magnitude of R when compared to the in vivo data from instrumented prostheses. In contrast, the Blumentritt model resulted in abnormally high values. However, the computational results for the orientation of R show a high variability of all modeling approaches and seem to depend more on the model used than on patient-specific parameters.

The J-Curve in the native knee as well as the femoral component’s J-Curve after total knee arthroplasty are known to have a high influence on kinematics. Furthermore, the J-Curve’s shape affects ligament strain and tension and consequently already slight changes may strongly alter knee forces and stability. To optimize current implants’ J-Curve design with regard to the population’s morphology, information about the main sources of contour variation is necessary.In this study, a principal component analysis (PCA) was performed on the medial and lateral femoral J-Curves of 90 cadavers without history of osteoarthritis. The J-Curves’ mean shapes were further investigated by geometric parameter analysis and effect sizes were calculated for the first three principal components (PCs). In addition, a combined PCA for both sides was performed and evaluated qualitatively. The results were compared with the variation in standard implants’ J-Curve shape.The isolated PCA of medial and lateral J-Curves resulted in PCs involving changes in contour orientation, arc length, scaling and circularity. The combined PCA of both sides resulted in PCs comprising combinations of the individual variations together with changes in relative position. In contrast, the qualitative evaluation of J-Curves from 2 different standard implant systems revealed no visible changes in shape but only changes in size.Limitations of this study were the restriction to a 2-dimensional contour derivation and the sole consideration of the femoral contours. Nevertheless, the sagittal variability in the medial, lateral and combined femoral J-Curves should be considered in implant design.