In terms of anatomy, the term “coronal” means any vertical separation between the back and the front (dorsal and ventral) sections of the body. This is also sometimes referred to as the “coronal plane”. In terms of hair restoration, coronal incisions refers to a type of incision created in the recipient area. These slits point roughly to the left and to the right of the scalp but by definition they are created perpendicular to the intended direction of growth of the new graft that is to be placed, which will vary depending on where the recipient zone is located.
The difference between the two incisions is how they are angled in relation to the direction of hair and collagen growth. Collagen is a connective protein fiber that grows in a uniform direction. It can be denoted using a medical technique called Langer’s lines. Langer’s lines signify the directional growth patterns of collagen, and any incision that follows these lines tends to heal more neatly, and with less scarring.
A sagittal incision is always made parallel to the direction of collagen and hair growth. Since they follow the collagen, a sagittal incision does cause slightly less damage, though it does not necessarily heal faster. When it comes to determining healing time, the type of surgical tool matters much more than the angle of the incision.
Unlike a sagittal incision, a coronal incision is perpendicular to the direction of hair growth, but not opposite. That is to say, these incisions are created at a 90-degree perpendicular angle – so if the direction of the hair growth is forward, for example, then the coronal incision must run side to side. These types of incisions may happen to follow Langer’s lines, but this is a coincidence, not a requirement. The main difference is that the coronal incision is at a 90-degree angle when compared to the direction of the hair’s growth.
The name “lateral slit” was coined by Dr. Wong, though it has been heavily misunderstood since that time. Many people mistakenly refer to all coronal incisions as “lateral slits”, although this is often not the case. In order for a coronal incision to classify as a true “lateral slit”, three components must be satisfied:
Angling a needle 90-degrees from the direction of hair growth is considered a coronal incision, but it is not a lateral slit. It is impossible to create a perfectly flat slit with a needle!
Sometimes, after a hair transplant, the scalp will still be visible when the hair is viewed under a strong light source. Coronal angles created with flat blades allows less light through the hair, as the “flat” nature of the incision forces the multi-haired grafts to spread out (much like a peacock’s tail). When the graft is prepared ahead of time with this effect in mind, it is usually “nicked” with a scalpel or razor along the vertical axis of the tissue to allow the graft to “splay” when inserted into the flat incision. If thousands of grafts are placed at a coronal angle, this illusion is greatly compounded. In this sense, less grafts can create the appearance of a greater overall coverage.
Coronal incisions or “Lateral Slits” can create more trauma to the scalp tissue as it is argued that such incisions cut across more blood vessels than incisions created sagittally. This is confirmed my many doctors including Dr. Ron Shapiro however it is also acknowledged that because the incisions from lateral slits are inherently more shallow than sagittal slits that this additional potential for trauma is lessened to the point that there is no quantifiable difference with either incision orientation. Another disadvantage is that coronal incisions or lateral slits are more difficult to master thus the potential for problematic results is higher unless the doctor making the incisions has years of experience.