BACKGROUND: Leaflet modification (LM) techniques are of interest in redo transcatheter aortic valve replacement (TAVR). However, factors impacting the efficacy of LM are poorly understood. OBJECTIVES: The aim of this study was to investigate the effects of valve constraint, redo-TAVR sizing, flow, and valve pathology on LM efficacy. METHODS: Naive and degenerated transcatheter heart valves (THVs) were used to evaluate BASILICA (bioprosthetic or native aortic scallop intentional laceration to prevent iatrogenic coronary artery obstruction), balloon-assisted BASILICA (BA-BASILICA), and balloon laceration. Conditions included: 1) degree of annular constraint of the index THV (modeled using different annular sizes); 2) flow conditions during deployment of the second THV (pulsatile vs continuous flow in a simulated deployment system); 3) leaflet modification pattern; 4) degree of oversizing of the second THV; and 5) extent and type of leaflet degeneration in the index THV. Quantitative outcomes included splay area, functional splay area, and variability in splay geometry across repeated deployments. RESULTS: When unconstrained, BASILICA created a splay in SAPIEN, Evolut, and ACURATE valves but not in Navitor valves, with BA-BASILICA improving splay for ACURATE and Evolut valves. Annular constraint at the minimum diameter per the THV instructions for use reduced splay in all THVs, with the greatest effect in Evolut valves. Nonbasal and angled leaflet splits impacted leaflet splay. Simulated redo-TAVR under flow conditions caused variability of leaflet splay for all THVs. Balloon laceration resulted in highly variable splay patterns ranging from complete clearance to obstruction. Oversizing of the second THV resulted in larger splay areas. Leaflet fibrosis diminished functional splay area. Increasing calcification resulted in irregular cuts and decreased splay. CONCLUSIONS: Leaflet splay following LM for redo-TAVR is highly dependent on THV type and constraint, implantation of the second THV, laceration pattern, and the nature of index THV degeneration.