Silicon nitride (Si₃N₄) is a non-oxide ceramic with excellent material properties such as high toughness and strength, high-temperature stability, and good wear and chemical resistance. Due to its good osseointegration and stimulated cell differentiation, as well as its osteoblastic activity and anti-infective behaviour, it can also be used as a medical implant. Its excellent biocompatibility and simultaneous mechanical strength are superior to calcium phosphates and make Si₃N₄ very attractive as scaffolds for bone regeneration in biomedical engineering. [1]

Digital light processing-based vat polymerization (DLP) is an additive manufacturing technology and provides the possibility to produce complex-shaped, dense ceramic structures with high resolution. The ceramic component is mixed with a photosensitive formula to form a ceramic-filled resin (slurry) that is cured layer-by-layer with layer thicknesses between 10 and 100 µm. The structured body, the so-called green body, must then be further processed to remove the organic components by evaporation or decomposition during debinding and densification by fusing the ceramic particles during sintering.

In contrast to oxide ceramics, Si₃N₄ as a non-oxide ceramic is more challenging to process as it absorbs and scatters light due to its dark colour and high refractive index. Therefore, properties such as curing and penetration depth, as well as critical energy of Si₃N₄ slurries must be examined and tuned. Further requirements for good processability of a slurry are deagglomeration and stability, as well as suitable rheological properties. The agglomerates must not be larger than the layer thickness. Both, the agglomerates and the rheological properties can be influenced by dispersants. For this reason, we have investigated the effect of different polymeric dispersants in a 39 vol% Si₃N₄ slurry.

[1] A. A. Altun, T. Prochaska, T. Konegger, und M. Schwentenwein, „Dense, Strong, and Precise Silicon Nitride-Based Ceramic Parts by Lithography-Based Ceramic Manufacturing“, 2020

Introduction:

Primary implant stability is critical for osseointegration and subsequent implant success. Small displacements on the screw/bone interface are necessary for implant success, however, larger displacements can propagate cracks and break anchorage points which causes the screw to fail. Limited information is available on the progressive degradation of stability of an implanted bone screw since most published research is based on monotonic, quasi-static loading [1]. This study aims to address this gap in knowledge.

Methods:

A total of 100 implanted trabecular screws were tested using multi-axial loading test set-up. Screws were loaded in cycles with the applied force increasing 1N in each load cycle. In every load cycle, Peak forces, displacements, and stiffness degradation (calculated in the loading half of the cycle) where recorded.

Results:

The stiffness degradation vs displacement results show a total displacement at the point of failure between 0.3 and 0.4 mm while an initial stiffness reduction close to 40%. It is also shown that at a displacement of ~0.1 mm, the initial stiffness of every sample had degraded by 20% (or more) meaning that half of the allowable degradation occurred in the first 25-30% of the total displacement.

Discussion:

Other studies on screw overloading [1] suggests similar results to our concerning initial stiffness degradation at the end of the loading cycle. Our results also show that the initial stiffness degrades faster with relatively small deformations suggesting that the failure point of an implanted screw might occur before the common failure definition (pull-out force, for example). These results are of great significance since primary implant stability is better explained by the stiffness of the construct than by its failure point.

References:

[1] B. Voumard et.al.,“‘Peroperative estimation of bone quality and primary dental implant stability,’” Journal of the mechanical behavior of biomedical materials, vol. 92, pp. 24–32

A prosthetic limb aims to support its user by restoring missing functionality. The acceptance of a prosthesis is highly influenced by the timely fitting as the timeframe between amputation and prosthetic limb restoration is crucial. A “Golden Period”, the first month after the amputation, has been identified as the optimal time when an upper-limb prosthesis should be fitted to an amputee in order to maximise their chances for a fast and successful return to their daily life.

Unfortunately, in most healthcare systems this time window is surpassed. Reasons for this inefficiency can be many, such as administrative hurdles or the many steps necessary in conventional socket fabrication, but the case is made ever more complex due to the fact that the design of each prosthetic socket has to be personally customized to fit an individual amputee. All the more the residual limb often changes its shape in the first weeks after the amputation. To better capitalize on the “Golden Period” and allow for early prosthetic training, we propose an adaptable and versatile temporary transradial socket design capable of multidimensional adjustments and easy user fitting for below elbow amputees.

A bone structure inspired design is 3D printed and assembled as to host all relevant myoelectric prosthetic hand components in a wearable prototype. This prototype, called the BeneFit socket, was evaluated in a monocentric, non-interventional explorative study with both experts and users.

The investigation shows that the proposed design is able to appropriately accommodate its diameter and length to user needs. Moreover, according to the conducted survey, it is perceived as satisfactory with respect to both user needs and expectations of the experts. While promising, further improvements towards even more versatile and robust design are needed before wider clinical application.

Magnesium implants appear as an interesting innovative solution in orthopedics, as they combine mechanical stability with the ability to dissolve over a several months period, giving way to newly grown mechanically stable bone tissue. We investigate pin shaped Mg-implants in femurs of rats, which we model mathematically, as a system of intersecting cylinders onto which a higher-order 3D-analytical beam theory is applied. The latter model is mapped onto a set of experimental data (3D-small angle x-ray diffraction tensor tomographies, computed tomography [1]),by establishing equivalence, in terms of volume-inertia properties, of the simple geometrical shapes associated with the analytical beam model, and the imaging data representing the actual anatomy.First result suggests correlation of principal stress directions and directions obtained from 3D small angle X-ray tensor tomography - underlining the role of mechnical forces driving the micro-morphology of bone tissue.

Reference: M. Liebi, V. Lutz-Bueno, M. Guizar-Sicairos, B. M. Schönbauer, J. Eichler, E. Martinelli,J. F. Löffler, A. Weinberg, H. Lichtenegger, and T. A. Grünewald. “3D nanoscale analysis of bone healing around degrading Mg implants studied by X-ray scattering tensor tomography”.In: (2020). doi: 10.1101/2020.11.09.375253.