Failure Mode Analysis of an Exeter Stem Fracture Initiated at the Introducer Hole: Time for a Design Change?

The fracturing of a hip prosthesis stem at its neck, in the absence of a trauma, is an extremely rare but serious adverse event. The patient in our case was young, active, and tall, thereby putting high mechanical loads on the prosthesis. Radiographs of the initial procedure and blood and synovium analysis showed no abnormalities. Analysis of the stem revealed niobium-rich precipitates, that is, alloy artifacts, at the introducer stud hole. The mechanically vulnerable location of the introducer stud hole, combined with alloy artifacts at that location and high mechanical stress, ultimately led to failure of the prosthesis. As younger and heavier patients will demand hip arthroplasty in the future, simple stem design adaptations should be considered to prevent stem fractures at the introducer stud hole.

The Mechanical Contribution of Vimentin to Cellular Stress Generation.

Contractile stress generation by adherent cells is largely determined by the interplay of forces within their cytoskeleton. It is known that actin stress fibers, connected to focal adhesions, provide contractile stress generation, while microtubules and intermediate filaments provide cells compressive stiffness. Recent studies have shown the importance of the interplay between the stress fibers and the intermediate filament vimentin. Therefore, the effect of the interplay between the stress fibers and vimentin on stress generation was quantified in this study. We hypothesized that net stress generation comprises the stress fiber contraction combined with the vimentin resistance. We expected an increased net stress in vimentin knockout (VimKO) mouse embryonic fibroblasts (MEFs) compared to their wild-type (vimentin wild-type (VimWT)) counterparts, due to the decreased resistance against stress fiber contractility. To test this, the net stress generation by VimKO and VimWT MEFs was determined using the thin film method combined with sample-specific finite element modeling. Additionally, focal adhesion and stress fiber organization were examined via immunofluorescent staining. Net stress generation of VimKO MEFs was three-fold higher compared to VimWT MEFs. No differences in focal adhesion size or stress fiber organization and orientation were found between the two cell types. This suggests that the increased net stress generation in VimKO MEFs was caused by the absence of the resistance that vimentin provides against stress fiber contraction. Taken together, these data suggest that vimentin resists the stress fiber contractility, as hypothesized, thus indicating the importance of vimentin in regulating cellular stress generation by adherent cells.

Surface metrology and 3-dimensional confocal profiling of femtosecond laser and mechanically dissected ultrathin endothelial lamellae.

PURPOSE: To determine the feasibility of confocal profiling in measuring surface roughness and obtaining 3-dimensional reconstructions of mechanically dissected and femtosecond (fs)-laser photodisrupted endothelial lamellae. To determine the predictability of single-pass dissection of ultrathin endothelial lamellae using a novel motor-driven linear microkeratome. METHODS: Thirty (n = 30) human corneas were harvested using a motor-driven linear microkeratome (n = 20); a hand-driven rotatory microkeratome (n = 6); and a 60-kHz fs laser (n = 4). Surface roughness was measured using an optical profiler operated in confocal microscopy mode followed by environmental scanning-electron-microscopy. RESULTS: Mean surface roughness for the fs laser, motor-driven linear microkeratome, and hand-driven rotatory microkeratome measured 1.90 ± 0.48 µm, 1.06 ± 0.42 µm, and 0.93 ± 0.25 µm, respectively. Femtosecond photodisrupted lamellae were significantly rougher than mechanically dissected lamellae (P < 0.001). Mean (±SD) cutting depth with the motor-driven linear microkeratome measured: 552 ± 11 µm (550-µm head); 505 ± 19 µm (550-µm head); 459 ± 19 µm (450-µm head); and 392 ± 20 µm (400-µm head). CONCLUSIONS: Confocal microscopy allows quantitative surface roughness analysis and 3-dimensional reconstruction of human corneal lamellae. Femtosecond-laser photodisruption at 60 kHz results in rougher surfaces compared with mechanical dissection. The motor-driven linear microkeratome allows single-pass dissection of ultrathin endothelial lamellae with a standard deviation ≤20 µm.