  Background: Since their discovery by Karl Langer in the 19th Century, Skin
Tension Lines (STLs) have been used by surgeons to decide the location and
orientation of an incision. Although these lines are patient-specific, most
surgeons rely on generic maps to determine their orientation. Beyond the
imprecise pinch test, there remains no accepted method for determining STLs in
vivo.
  Methods: (i) The speed of an elastic motion travelling radially on the skin
of canine cadavers was measured with a commercial device called the
Reviscometer. (ii) Similar to the original experiments conducted by Karl
Langer, circular excisions were made on the skin and the geometric changes to
the resulting wounds and excised samples were used to determine the orientation
of STLs.
  Results: A marked anisotropy in the speed of the elastic wave travelling
radially was observed. The orientation of the fastest wave was found to
correlate with the orientation of the elongated wound (P<0.001, R^2 = 74%).
Similarly, the orientation of fastest wave was the same for both in vivo and
excised isolated samples, indicating that the STLs have a structural basis.
Resulting wounds expanded by an average area of 9% (+16% along STL and -10%
across) while excised skin shrunk by an average area of 33% (23% along STL and
10% across).
  Conclusion: Elastic surface wave propagation has been validated
experimentally as a robust method for determining the orientation of STLs
non-destructively and non-invasively. This study has implications for the
identification of STLs and for the prediction of skin tension levels, both
important factors in both human and veterinary reconstructive surgery.
