A method to obtain surface strains of soft tissues using a laser scanning device

A three-dimensional laser scanning device was developed allowing surface digitization of musculoskeletal and soft tissue structures under different loads. Image-processing algorithms were formulated for image registration. These were used to determine displacement mapping and then surface strains. Various validation experiments were performed. Accuracy was obtained on a test cylinder after rigid rotation and on a silicon cylinder compressed in four loading steps. The system accuracy (including the scanning and the data evaluation) was +/-0.10% strain in vertical and +/-0.16% strain in shear and circumferential direction for the rigid rotation exhibiting the zero-strain situation. Silicon cylinder compression showed that the accuracy was best for small strains, whereas strains >5% evoked a slight underestimation increasing further with higher strains (error of 0.54% for 7.22% vertical strain). It was possible to increase the accuracy by performing the strain measurements via sub-steps. This had a remaining error of 0.41% for 7.22% vertical strain. A further experiment was carried out in order to acquire the surface strain of a human lumbar intervertebral disc while it was forced to flexion and extension. This study introduced a laser-based scanning method to obtain soft tissue surface strains. It is important to know the strain distribution of musculoskeletal structures and soft tissues. This could help to better understand the mechanical loading of biological structures e.g. the processes in fracture healing. These data could also be used to assist in the validation process for finite-element models.