SAN DIEGO Some closures may be too complex for inexperienced Mohs surgeons, but even novices can successfully close most of their cases if they have an understanding of how skin flaps move and a willingness to see the possibilities in the two simple skin movementsthe fusiform and the z-plastythat underlie all flap closures, Dr. Kenneth G. Gross said at a meeting sponsored by the American Society for Mohs Surgery.
It helps to think of flap movement as occurring in two dimensions.
"The majority of skin flaps can be explained simply by exploring what makes a fusiform work," Dr. Gross said.
Fusiform flaps generally have 30-degree angles at their tips, based on principles of plane geometry, but the skin is not a flat plane, and whether a lesion will close with 30-degree angles depends on the anatomic location.
"The purpose of making 30-degree angles at the ends of the fusiform is to allow closure without a standing cone," said Dr. Gross, a dermatologic surgeon in private practice in San Diego.
The fusiform consists of three components: the central defect to be closed (depicted as a circle) and two Burow's triangles.
The short axis of the defect usually forms the short axis of the fusiform. The surgeon uses the Burow's triangles at each end of the fusiform to bring the defect to 30-degree angles or to the angles necessary to close the wound without creating a standing cone at the ends of the excision.
The Burow's triangles also can be rotated at the ends of the fusiform as needed to achieve the best cosmetic result. "The Burow's triangle can even be rotated 90 degrees," Dr. Gross pointed out. "You should do whatever type of Burow's triangle is needed to make the best possible closure."
If the location of the defect prevents the use of a Burow's triangle at one or both ends of the fusiform, the surgeon can offset the Burow's triangles, Dr. Gross explained.
"This will result in the creation of what we call an advancement or a rotation flap. If the Burow's triangle is offset to two sides, the result will be a bilateral flap, such as the bilateral advancement or the A to T flap," he commented.
"If the surgeon is in doubt about how to close a wound, he or she may start by drawing a fusiform and then [assessing] whether the fusiform lies in the relaxed skin tension lines and whether it causes distortion of surrounding important anatomic structures," Dr. Gross advised.
If the fusiform does interfere with important anatomy, the surgeon must decide where to offset the Burow's triangles at one or both ends of the fusiformthereby creating an advancement or rotation flap that doesn't interfere with important anatomy or degrade the cosmetic result.
He noted that about 90% of his Mohs closures are variations of rotation flaps.
Although rotation flaps are generally larger than other types of flaps, Dr. Gross believes that they yield better cosmetic results than transposition flaps because the resulting scar lines can usually be placed in junctions between anatomic units.
Transposition flaps are based primarily on z-plasty movement.
"If you understand the movement of a z-plasty, you will understand how all transposition flaps move and why they move the way they do," Dr. Gross said at the meeting.
Think of a z-plasty as two Burow's triangles sharing a common side, called the central limb. When planning a z-plasty, think about using imaginary skin hooks to pull the central limb longer, Dr. Gross said.
While the central limb lengthens and rotates, there is concomitant shortening of tissue approximately 90 degrees from the axis of the central limb.
The amount of central limb lengthening, and the amount of rotation and shortening in the other axis, is determined by the total of both angles of the Burow's triangles in the z-plasty.
"You can combine 45-degree and 90-degree angles in the same z-plasty, and in fact the Burow's triangles of transposition flaps never have the same-sized angles," Dr. Gross said.
This is the reason some surgeons don't recognize the z-plasties that lie at the heart of all transposition flaps: They are accustomed to seeing z-plasties in textbooks that are drawn with equal angles.
As long as there is sufficient tissue in the opposite direction to allow for shortening without disrupting the surrounding anatomy, as well as enough skin laxity, the surgeon can use whatever size z-plasty angles are needed to produce the amount of central limb lengthening that will close the defect.