On the orthogonal anisotropy of human skin as a function of anatomical region.

Skin samples were obtained from 8 anatomical sites of 6 human deceased at ages ranging from 30 to 80 years 24 hours post mortem. As shown by biochemical analysis the collagen content varied between 71% and 78% depending on the anatomical location of the skin samples. The content of collagen type III was in the range of 19.2% to 22.2% of the total collagen concentration. As to the biomechanical analysis the axes of minimum and maximum shrinkage after excision were determined and correlated with Langer cleavage line drawn on the specimen with a marker after incision. Two-dimensional biomechanical tests were conducted with a multiaxial tensile testing device consisting of 12 loading axes. The in vivo configuration was a circle with 30 mm diameter. The in vivo stresses were determined by restoring the original shape of the specimen. According to the nonlinear stress-strain relationship incremental strains were applied to the sample with the in vivo configuration and states of uniform extension as reference. The corresponding stresses were recorded after stress relaxation was completed and the equilibrium stresses were regarded as the elastic contribution to the viscoelastic biomechanical behavior. The elastic parameters as a function of the initial strain level were calculated using a set of different incremental strains and stresses. The highest in vivo stresses were found in patella, and upper and lower back. The maximum deviation of the direction of maximum in vivo stress from the Langer cleavage line was found in upper back, the volar part of thigh, and sternum. In vivo orthogonal anisotropy was most pronounced in patella and hollow of the knee.