Effect of Bone Quality and Leg Depth on the Biomechanical Performance of a Nitinol Staple.

The use of Nitinol compression staples has increased in foot and ankle procedures due to their ease of delivery and ability to offer sustained, dynamic compression. Prior biomechanical studies have predominantly examined mechanical performance in healthy bone models without investigating the effect of unicortical versus bicortical fixation. The purpose of this study was to examine the effect of bone quality and staple leg depth on the biomechanical performance of Nitinol staples in a bicortical bone model. Two-legged Nitinol staples were implanted in bicortical sawbone of 2 densities. Two different leg depths were tested to simulate unicortical versus bicortical fixation. Interfacial compressive forces, interfacial compression area, torsional strength, and shear strength were measured for each group. The effect of leg depth was minimal compared to the effect of sawbone density on the mechanical performance of Nitinol staples. Interfacial compressive force and interfacial compression areas were greater in the low density bone model, while torsional strength and shear strength were greater in the normal density bone model. Nitinol staple's mechanical performance is highly dependent upon bone quality and less dependent on whether staple legs terminate in cancellous versus cortical bone. Low density bone allows for a higher compressive interfacial area to be imparted by the staple. Staples in normal density bone are able to resist torsion and shear deformation more readily than staples in low density bone. Bone density may have a greater effect on the Nitinol staple's stability and compressive capability in vivo as compared to unicortical versus bicortical leg fixation.

The effect of surface topography and porosity on the tensile fatigue of 3D printed Ti-6Al-4V fabricated by selective laser melting.

Additive manufacturing (3D printing) is emerging as a key manufacturing technique in medical devices. Selective laser melted (SLM) Ti-6Al-4V implants with interconnected porosity have become widespread in orthopedic applications where porous structures encourage bony ingrowth and the stiffness of the implant can be tuned to reduce stress shielding. The SLM technique allows high resolution control over design, including the ability to introduce porosity with spatial variations in pore size, shape, and connectivity. This study investigates the effect of construct design and surface treatment on tensile fatigue behavior of 3D printed Ti-6Al-4V. Samples were designed as solid, solid with an additional surface porous layer, or fully porous, while surface treatments included commercially available rotopolishing and SILC cleaning. All groups were evaluated for surface roughness and tested in tension to failure under monotonic and cyclic loading profiles. Surface treatments were shown to reduce surface roughness for all sample geometries. However, only fatigue behavior of solid samples was improved for treated as compared to non-treated surfaces Irrespective of surface treatment and resulting surface roughness, the fatigue strength of 3D printed samples containing bulk or surface porosity was approximately 10% of the ultimate tensile strength of identical 3D printed porous material. This study highlights the relative effect of surface treatment in solid and porous printed samples and the inherent decrease in fatigue properties of 3D printed porous samples designed for osseointegration.