Background
Incomplete stent apposition (ISA) is defined as a separation of at least one stent strut from the vessel wall, not related with a side branch. The relevance of this phenomenon derives from intravascular ultrasound (IVUS) studies suggesting a potential relationship between ISA and stent thrombosis. The mechanism by which ISA can contribute to stent thrombosis remains unknown, but it has been suggested that the characteristics of the flow in the ISA region may facilitate the deposit of fibrin and platelets acting as a nidus for thrombus formation. Some evidence also suggests that malapposed struts have a higher probability of delayed endothelialization, another mechanism involved in stent thrombosis.
The high resolution of OCT (axial 10 μm) makes it the more sensitive tool currently available in the clinical practice for the evaluation of ISA. Several studies have demonstrated that OCT is able to detect small areas of ISA not visible with IVUS. Kubo et al., in a sample of 55 patients, found that the detection of malapposition by OCT was 47 %, versus 18 % for IVUS.1
ISA can be acute, occurring at the time of stent deployment, or late, in which ISA is observed on follow-up.2 Univariate predictors of malapposition include implantation of sirolimus-eluting stents, the presence of overlapping stents, longer stent lengths and a type C lesion.3 Regardless of the pathophysiological mechanism, the key concern of ISA is based on the assumption that areas of strut malapposition cause non-laminar and turbulent blood flow characteristics, which in turn trigger platelet activation and thrombosis.
The methodology to evaluate stent apposition with OCT requires some considerations, especially when evaluating DES. Stent struts can be identified in OCT as highly reflective surfaces that cast shadows on the underlying vessel wall. Most DES are built with a metal body covered by a polymer that contains and controls the release of the drug. The light is not able to penetrate through metal; therefore, only the endoluminal surface of the strut is visible with OCT, and the rest of the metal and polymer are hidden by a shadow. As a result, the complete thickness of the strut (including the metal and polymer) should be taken into account to assess strut apposition. The strut can be considered malapposed only if the distance from the endoluminal surface of the strut to the vessel wall is higher than the sum of the metal and polymer thickness. The strut thickness depends on the stent type. In research, different methodologies have been used to evaluate stent apposition with OCT. Some authors have used a different cut-off point for each stent type, depending on their strut thickness. Others, however, have chosen a single cut-off point that is above the strut thickness of all types of stents used currently. From a clinical point of view, using a single cut-off point (e.g. 200 μm) can facilitate the assessment of stent apposition, without losing any clinically relevant area of ISA.
The publication of this information was supported by St. Jude Medical.