Four important component of CTO should be taken into consideration during percutaneous intervention of CTO.

1. The proximal fibrous cap
CTO is caused by organization of coronary artery thrombus, but the process of organization is not uniform.1-3 Briefly, it occurs rapidly at the edges of the occlusion, but slowly in the central portion. Therefore, hard tissue rich in fibrous and calcified components, called the proximal and distal fibrous caps, often forms at the proximal and distal borders where organization is more rapid. Penetration through these caps is often difficult. The proximal border of a CTO lesion is classified as abrupt or tapered. At a bifurcation, no protrusion is noted in the former type, while a cone-shaped protrusion is observed in the latter type. From the standpoint of the wiring technique, the latter type is classified with respect to the presence or absence of an entrance into the lesion, which is called a stump. In abrupt and tapered lesions without a stump, there is a hard proximal fibrous cap at the margin that can be seen by angiography. It is usually impossible to penetrate this cap with a moderately stiff wire, such as an Intermediate or a Miracle 3gr (Asahi Intec). Therefore, a very stiff wire, such as the Conquest Pro or Miracle 12gr (Asahi Intec) is considered as the first wire. An abrupt type CTO at a side branch is a kind of bifurcation lesion. It has been shown by intravascular ultrasonography that plaques are mainly distributed on the side opposite to the side branch. Based on this rule, the site of penetration is narrowed to the commencement of the side branch. In the tapered type with a stump, a proximal fibrous cap is not necessarily present at the margin that can be seen by angiography. Instead, resistance (presumably due to the proximal fibrous cap) is usually felt after the wire has been advanced a few millimeters into the lesion. If the duration of occlusion is several months, penetration is usually possible using a moderately stiff wire. However, if the duration of occlusion is unknown or if it is estimated to be longer from the extent of development of collateral vessels, a moderately stiff wire may be redirected by the proximal fibrous cap and enter the subintimal space. In such cases, the wire should be immediately changed to a Conquest Pro. When the wire needs to be changed, the parallel wire technique mentioned below should be used in principle.4

2. Calcification5
Relatively recent occlusion had little calcification, and gentle rotation of the wire would enable antegrade progression. Heavy calcification however posed a barrier to progression of the wire. Unless a very stiff wire was used, calcium could deflect the wire tip potentially into the sub-intimal space. Calcification was best evaluated by CT angiography rather than angiography, although CT angiography is not routinely carried out. Therefore, practically any calcification on CT angiography or conventional angiography could suggest that an initial strategy using very stiff wire might be more effective than hydrophilic wire or moderate stiff wire.

3. Microvessels
Neovascularization within CTO may have both advantages and disadvantages to percutaneous strategy. If well-developed microvessels run parallel within the occlusion, then this may facilitate passage with a guidewire, especially hydrophilic coated wire including Crosswire NT, Shinobi wire, Whisper wire, and Pilot wire. Tapered tip (Fielder XT) wire may also be able to pass through these microvessels. However, micorvessels tend to be friable and when superficial, may increase the risk of perforation with the wire. Bridging collaterals can be deceitful particularly when very tortuous and angulated, as even though they may appear to be of a reasonable size, wires may frequently perforate through the friable thin wall. In such chronic lesions, it is often better to take a relatively stiff wire and direct through the central part of the occlusion.

4. The distal fibrous cap
Penetration through the distal end of the occlusion may also be difficult at times particularly in very long or tortuous occlusions as torque control of the wire becomes relatively reduced. Up to date, parallel wire techniques and retrograde approach were considered as novel techniques for penetration of distal cap.