The keystone flap: not an advance, just a stretch

The keystone flap is being promoted as an advance in oncological surgical reconstruction. Wound closure with this island flap involves two V-Y advancements toward the center and along the long axis of the island, at right angles to the line of maximum tension when the wound is closed. It is implied that the long-axis advancements enable closure by relaxing the entire flap of skin, allowing it to be stretched more easily along the short axis. We undertook a study to test this hypothesis.

We measured tension and extension along perpendicular axes in nine freshly excised specimens of human skin. We held the longitudinal axis fixed while stretching the skin along the transverse axis. We then released the longitudinal axis and measured the resultant drop in transverse tension. Finally, we increased the transverse tension to approximately its previous level, and measured the new transverse extension.

There was significant interdependence between longitudinal and transverse tensions. The fall in transverse wound tension associated with longitudinal release varied with starting tensions and with the original site and orientation of the specimen. In the five cases where the longitudinal release was from the in vivo length, the mean increase in transverse stretch attributable to the release was 0.6 mm (95% confidence interval 0.1-1.1 mm).

The increase in transverse stretch that can be achieved by releasing skin from its longitudinal in vivo length appears trivial, raising questions about the rationale for the use of the prototypical (type 1) keystone flap in wound closure.