Introduction Photogrammetry is an optical technique for determining the object-space coordinates of a 3-d surface. In this study, we demonstrate this technique using accessible and inexpensive photographic equipment and computer resources, examine the error inherent to the reconstruction process, and apply this technique to study the deformation of a surface.

Methods The photogrammetry apparatus consists of two 35mm SLR cameras with 50mm lenses mounted on a tripod and a calibration frame. After digitizing a set of 2-d images of an area of interest, the direct linear transform (DLT) is used to reconstruct its 3-d position. With the reconstructed 3-d information, a surface interpolation technique (Smith et al, 2000) was used to calculate the stretch ratio distribution for the deformed surface. This study consisted of two parts: 1) determination of the residual error inherent to the apparatus 2) application of this technique to determine the stretch ratio distribution of a deformed region of interest (ROI). This technique was applied in two scenarios a) in vitro on a deformed cylindrical surface b) in vivo on several different random pattern skin flaps created on the flanks of swine.

Results Respectively for each camera, the residual errors are 1.28+0.12mm and 0.85+0.60mm. The residual stretch ratio error for the deformed cylindrical surface was 0.001+0.006. This technique allowed us to quantitatively assess in vivo skin flap strain in skin flaps created on pig flanks.

Conclusion Photogrammetry is powerful technique to determine the object space coordinates of a 3-d region of interest. We have employed this method utilizing equipment and methods that are easily obtainable and inexpensive. We have calculated the errors inherent to this technique and in its use in calculating the stretch ratio distribution of a deformed surface. We have applied this technique towards the analysis of skin strain in vivo and find it an effective tool in the analysis of skin flap design. Photogrammetry is a powerful technique that potentially can be a ubiquitous tool in the analysis of surface deformation crucial to the study of the role of soft tissues in plastic surgery.

Reference Smith DB. Sacks MS. Vorp DA. Thornton M. Surface geometric analysis of anatomic structures using biquintic finite element interpolation. Annals of Biomedical Engineering. 28(6):598-611, 2000 Jun.