This double-locking technique allows for some wound eversion while concomitantly taking advantage of the pulley effect of spreading the wound tension over multiple loops. Its central advantage is the increased strength of this approach over the standard locking horizontal mattress suture, and it is therefore best used in closures under significant tension, such as the back and shoulders, or when a transepidermal approach is desired and no deeper sutures have been placed, such as the closure of punch biopsy sites.
The double-locking technique confers three important advantages over the traditional horizontal mattress suture. First, the standard horizontal mattress suture does not typically permit the same degree of wound-edge apposition as can be accomplished with other transepidermal sutures, since the everting effect of the suture technique may even be associated with a small degree of gaping at the center of the horizontal mattress suture. Double locking the suture material brings the knot, as well as the parallel external rows of suture, to the center of the wound, thus improving wound-edge approximation.
Second, suture removal with the standard horizontal mattress technique may be challenging, particularly if sutures are left in situ for an extended period of time and some of the suture material has been overgrown by the healing epidermis, as the knot may be buried in the context of a ridged everted repair. Bringing the knot, along with the parallel rows of external suture material, centrally with the locking technique allows the knot to be more easily grasped at the time of suture removal.
Finally, the double-locking approach adds a pulley effect, thus increasing the strength of the suture and permitting closure under higher-tension environments. As an added benefit, the double-locking technique incorporates an extra throw of the horizontal mattress suture, meaning that each suture extends further along the wound, thus necessitating fewer knot ties for a given incision length.
As with most transepidermal techniques, it is important to enter the epidermis at 90 degrees, allowing the needle to travel slightly laterally away from the wound edge before fully following the curvature of the needle when utilizing this technique. This will allow for maximal wound eversion and accurate wound-edge approximation.
As with the simple interrupted suture, care should be taken to avoid skimming the needle superficially beneath the epidermis. This results from failing to enter the skin at a perpendicular angle and to follow the curvature of the needle. This may result in wound inversion as the tension vector of the shallow bite pulls the wound edges outward and down.