The impact of UV cross-linking on corneal stromal cell migration, differentiation and patterning.

Previous studies have demonstrated that UV cross-linking (CXL) increases stromal stiffness and produces alterations in extracellular matrix (ECM) microstructure. In order to investigate how CXL impacts both keratocyte differentiation and patterning within the stroma, and fibroblast migration and myofibroblast differentiation on top of the stroma, we combined CXL with superficial phototherapeutic keratectomy (PTK) in a rabbit model. Twenty-six rabbits underwent a 6 mm diameter, 70 µm deep phototherapeutic keratectomy (PTK) with an excimer laser to remove the epithelium and anterior basement membrane. In 14 rabbits, standard CXL was performed in the same eye immediately after PTK. Contralateral eyes served as controls. In vivo confocal microscopy through focusing (CMTF) was used to analyze corneal epithelial and stromal thickness, as well as stromal keratocyte activation and corneal haze. CMTF scans were collected pre-operatively, and from 7 to 120 days after the procedure. A subset of rabbits was sacrificed at each time point, and corneas were fixed and labeled in situ for multiphoton fluorescence microscopy and second harmonic generation imaging. In vivo and in situ imaging demonstrated that haze after PTK was primarily derived from a layer of myofibroblasts that formed on top of the native stroma. Over time, this fibrotic layer was remodeled into more transparent stromal lamellae, and quiescent cells replaced myofibroblasts. Migrating cells within the native stroma underneath the photoablated area were elongated, co-aligned with collagen, and lacked stress fibers. In contrast, following PTK + CXL, haze was derived primarily from highly reflective necrotic "ghost cells" in the anterior stroma, and fibrosis on top of the photoablated stroma was not observed at any time point evaluated. Cells formed clusters as they migrated into the cross-linked stromal tissue and expressed stress fibers; some cells at the edge of the CXL area also expressed α-SM actin, suggesting myofibroblast transformation. Stromal thickness increased significantly between 21 and 90 days after PTK + CXL (P < 0.001) and was over 35 µm higher than baseline at Day 90 (P < 0.05). Overall, these data suggest that cross-linking inhibits interlamellar cell movement, and that these changes lead to a disruption of normal keratocyte patterning and increased activation during stromal repopulation. Interestingly, CXL also prevents PTK-induced fibrosis on top of the stroma, and results in long term increases in stromal thickness in the rabbit model.

Characterizing the Structural Integrity of Endotracheal Tube Taping Techniques: A Simulation Study.

BACKGROUND: Endotracheal tubes (ETTs) are commonly secured with tape to prevent undesirable tube migration. Many methods of taping have been described, although little has been published comparing various methods of taping to one another. In this study, we evaluated several methods for securing ETTs with tape. We hypothesized a difference in mean peak forces between the methods studied during forced extubation. METHODS: Five methods of securing an ETT with tape were studied in a variety of contexts including cadaver and simulation lab settings. Testing included measurement of peak force (Newton [N]) during forced extubation, durability of taping following mechanical stress, effects of tape length-width variation, and characterization of failure mechanisms. RESULTS: We found several significant differences in mean peak extubation forces between the 5 methods of taping, with mean peak forces during forced extubation ranging from 20 N to 156 N. In separate tests, we found an association between mean peak forces and total surface area as well as geometric configuration of tape on the face. Long thin strips of tape appeared to provide surprising durability against forced extubation, a phenomenon that was associated with minimization of the "peel angle" as tape was removed. CONCLUSIONS: We found evidence of differential structural integrity between the 5 taping methods studied. More generally, we found that increased peak extubation forces were associated with increased total surface area of tape and that minimization of the "peel angle" by lateral application of tape is associated with surprisingly high relative peak extubation forces.