The primary findings of this study are as follows: 1) in the RA procedure, the factors that affected the safety mechanism to avoid damage to the elastic material included a larger minimum turning radius of the approaching curve, higher burr rotational speed, and higher viscosity of the filled fluid; 2) in the OA procedure, the safety mechanism for elastic material was less effective than RA.
The EHL theory was based on the classical one-dimensional Reynolds equation, \(Ps=6\eta u/h\). In this equation, P is the pressure between the rotational material and the opposite wall, η is the viscosity, u is the speed of the moving surface, and h is the distance between the rotational material and the opposite wall . Currently, several tenets, including elastic deformation caused by fluid pressure, have been included in the theory. According to the EHL theory, laminar fluid flow caused by rotational forces creates pressure between the rotational material and the opposite wall. During RA, this pressure causes micro-elastic deformation in the elastic vessel wall and maintains the distance between the rota-burr and the vessel wall. This phenomenon seems to be the key mechanism within the safety mechanism of the rotablator; therefore, the present in vitro study has attempted to demonstrate this logic. We found that a higher burr speed and a higher viscosity were effective for preventing elastic damage; however, a larger burr size was not effective. Subsequently, we must pay attention to fluid viscosity and control optimal rotational speed during the RA procedure. The viscosity of H2O at 20oC is approximately 1.0 mPaཥs, and that of low-molecular weight dextran is 3.5-4.5 Paཥs. In real-world RA procedure, fluid viscosity represents the blood viscosity. Low-molecular weight dextran is reported to be the same as 50% hematocrit blood viscosity . With respect to the correlation between blood viscosity and hematocrit and between blood viscosity and blood protein levels , patients with severe anemia and low blood protein levels may not be expected to have a good DC effect during RA. In such cases, transfusion and/or supplementation of albumin before PCI may be required.
Logically, a larger burr size results in a higher speed of the moving surface and increases the DC effect. However, the shape of the rota-burr depends on the burr size, and the approaching degree of the vessel wall is more coaxial in the 1.25-mm burr than in the 2.0-mm burr (Online Resource 1). As suggested in a previous study , the difference in shape results in a higher perpendicular reaction force of the opposite wall in a 2.0-mm burr than in a 1.25-mm burr. This might have caused a missing DC effect in the larger rota-burr (Online Resource 2). Moreover, the surface point of the rota-burr attached to the vessel wall is not necessary at the maximum radius point; therefore, the theory that a larger burr size increases the safety for elastic material is not necessarily true. For the same reason, a tighter approaching curve results in a higher perpendicular reaction force between the rota-burr and the vessel wall; therefore, a tighter curve may decrease the safety for elastic material. However, a higher perpendicular reaction force teaches procedural operators the power limitation of pushing the burr controller; thus, well-experienced operators might avoid vessel perforation before the perpendicular reaction force becomes excessive. In clinical practice, PCI operators should evaluate the strong wire bias for the tight curve in the normal vessel wall via angiography and imaging devices such as optical coherence tomography and/or intravascular ultrasound when the rota-burr must pass through the tortuous point in the rota-mode. Regarding the burr rotational speed, the Boston Scientific formally recommended a speed of 140,000-190,000 rpm . With respect to the safety effect for the elastic vessel wall, this speed is adequate; however, excess burr speed might result in thrombus formation. Therefore, operators should consider the appropriate settings for each rotational atherectomy case.
Khan et al. reported that the incidence of coronary perforation was significantly higher in OA patients than in RA patients . In the orbital atherectomy system, the center of gravity of the asymmetric crown is far from the wire, which is the axis of rotation; therefore, the rotational surface is not continuous. This makes it difficult to produce a laminar fluid flow and apply the EHL theory. In the RA in vitro experiment, the rubber latex could not be damaged in the loose curve system under various conditions, even at low rotational speeds. However, OA could easily damage the same model without 80,000 rpm in low-molecular weight dextran. Although a higher viscosity might confer the benefit of possibly avoiding vessel damage in OA, a strong wire bias point for the elastic vessel wall should be mentioned for severe vessel perforation in OA more than RA. Moreover, perforation caused by OA may be more complex than that caused by RA.
This study has several limitations. First, this was an in vitro assessment. To evaluate the damage of the rubber latex that might have been tougher than the real vessel wall, we prepared a strong wire bias by pulling the wire using a weight. Second, the curve models were not duplications of the real coronary artery. Thus, the approaching degree for the wall of the rota-burr or OA catheter might differ from real-world practice. Moreover, in the present study, we investigated the damage on the strong forced inner side of curve. In the RA practice, the perforation was experienced at the outer side of curve just after the tortuous vessel route. This might be related burr manipulation. However, it seemed to be essentially the same principle of the safety mechanism for elastic material in RA. Therefore, the results of the experiment should be interpreted as a comparative assessment in various conditions, including rotational speed of the rota-burr or OA catheter, rota-burr size, fluid viscosity, approaching curve, and RA or OA.
From a clinical perspective, RA has an advantage in its safety mechanism compared to OA. Moreover, RA operators could decrease the possibility of vessel perforation with controlling factors, higher rotational speed, and blood viscosity.
In conclusion, a higher rotational speed, coaxial approach for the wall, and higher viscosity contribute to the safety mechanism of RA. The safety mechanism for elastic material in OA proved less effective.