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Undermining
Undermining
Undermining serves several purposes. First, it permits closure of large (based on defect size) or tight (based on lack of elasticity, such as on the scalp and lower leg) defects under tension that would otherwise not close easily. Second, it reduces tension across moderately tense defects by recruiting adjacent skin for the closure. Third, it frees up underlying attachments that would otherwise lead to both lateral and downward pull, potentially leading to wound inversion.
It is important to undermine the apices as well as the lateral portions of the wound.11 In fact, wide apical undermining may in some cases be more important than lateral undermining given its potential to partially mitigate dog-ear formation.
Disadvantages of undermining include its potential impact on blood supply, as wide undermining essentially creates a small local random pattern flap.54 Undermining also increases intraoperative morbidity, by creating a potential space for hematoma or seroma formation.
The plane of undermining has a profound effect on surgical outcomes, as it may affect the risk of bleeding (and the resulting need for more aggressive and time-consuming hemostatic measures), as well as the risk of nerve damage. Remaining in a uniform plane is important, and may be aided by retracting back the skin with uniform pressure.
Novice dermatologic surgeons often prefer to err on the side of remaining
superficial, and have a tendency to attempt all undermining at the level of the superficial fat, fearing an increased risk of bleeding or nerve compromise with deeper undermining. Understanding the vascular supply of various anatomic areas may be helpful, as overly superficial undermining may also increase the risk of damage to perforating vessels supplying the dermal plexus, decreasing vascular supply and increasing the risk of necrosis. Deeper undermining may permit the reapproximation of deeper, well-vascularized structures rather than only the relatively thin dermis and epidermis. This phenomenon may be seen when performing a linear closure on the nose, where deep submuscular undermining affords a robust vascular supply to the advancing wound edge, while superficial subdermal undermining leads to a significant risk of postoperative necrosis.
When undermining, keep in mind that a scalpel and scissors work from opposite directions: the scalpel cuts from the distal point to the more proximal point, while scissors begin cutting proximally and extend the incision distally.
One recent study evaluated the relative merits of wide undermining and imbrication.54 In this porcine study, both undermining and imbrication were studies to assess their effect on wound tension and perfusion. Not surprisingly, both techniques led to decreased tension across the surface of the wound, and both led to a marginal drop in wound perfusion, though imbrication was associated with a slightly more dramatic decrease in perfusion. Notably, the experimental wounds were undermined a full 4 cm, significantly beyond the point that most surgical wounds are undermined in dermatologic surgery.
When feasible, undermining should be minimized in patients on thrombolytic agents such as aspirin or in those with very ruddy skin who are prone to numerous small wound-edge bleeding vessels.
Pragmatically, undermining is best performed under direct visualization. Sharp dissection may lead to less tissue trauma as it avoids shearing forces that may be seen with blunt or spread undermining. While many surgeons prefer blunt-tipped surgical scissors, such as blepharoplasty scissors (for almost all cases) or Metzenbaum, facelift, or Mayo scissors (for larger cases), on a practical level sharp undermining may also be performed with a scalpel, or using electrocautery on either a pure-cut or blend setting. Radiofrequency cutting devices, as well as newer cold plasma devices, may also be used. These confer the advantages of reduced thermal injury though they are also associated with a significantly increased cost that makes them impractical for most linear excisions and repairs.