Material Characterization of Ligaments Using Non-contact Strain Measurement and Digitization

Area and strain measurement pose special problems in experiments designed to characterize the constitutive response of ligamentous tissue. Contact area measurements tools yield inaccurate results because the specimen is easily deformed. In this study, a 3D non-contact optical digitizer that relies on the principle of structured lighting is used to develop geometric models of all the ligaments of the knee joint, from which accurate calculations of cross-sectional area can be made. Similarly, it is important that the strain measurement technique used does not disturb the strain state of the specimen. Furthermore, full-field strain measurements are necessary because ligamentous tissue is inhomogeneous and local strain measurements can be misleading. Thus, this study demonstrates the application of a full-field 3D non-contact strain measurement system that relies on principles of photogrammetry and pattern recognition to compute the surface strain distribution on the Medial Collateral Ligament (MCL) of the human knee joint. INTRODUCTION esigning experiments for material characterization of knee ligaments poses several problems and has therefore been a subject of much debate. Two issues, in particular, are the measurement of specimen cross sectional area for the computation of stress and the measurement of surface strain. Consider, for instance, the results shown in Figures 1 and 2. In these tests, boneligament-bone (BLB) specimens of knee ligaments were loaded in anatomical distraction at loading rates ranging from 1/60 mm/sec to 1600 mm/sec. The force-displacement response shows many characteristics reported in past experimental investigations (Noyes et al., 1974(a), Noyes et al.,1974(b), Woo et al., 1990, Crowninshield et al., 1974, Schenck et al., 1999) and provide useful insight into the rate dependent response of the various knee ligament structures. However, in order D Injury Biomechanics Research 184 to develop a constitutive model, the stress-strain response of the tissue is required. Thus, methodologies capable of measuring ligament cross-sectional area and surface strain distribution are necessary. Traditional area measurement methods, such as using calipers and projection techniques, cannot be used because the ligament is easily deformed by contact, and these approximations do not account for the complex cross-sectional ligament geometry. In this study, an optical noncontact digitizer is used to build high resolution solid models of the ligament surface from which cross-sectional geometry and area are easily obtained. Similarly, conventional strain measurement techniques, such as using extensometers, disturb the strain state in the ligament and result in point measurements that do not account for the vastly varying strain distribution resulting from material in-homogeneity. This study addresses these issues by making full-field measurements using a 3D non-contact strain measurement system. In the following sections, the non-contact area and strain measurement systems are described first. This is followed by a review of the literature for ligament strain and area measurement techniques and an application of the non-contact methods. 0 50 100 150 200 250 300 0 2 4 6 8 10 Actuator Displacement (mm) L ig am en t L o ad ( N ) 1/60 mm/s 160 mm/s 1600 mm/s Figure 1: Force-displacement response of the LCL of the human knee joint at varying strain rates 0 20 40 60 80 100 120 140 160 0 2 4 6 8 10 12 Actuator Displacement (mm) Li ga m en t Lo ad ( N ) aPCL LCL pACL 0 100 200 300 400 500 600 700 800 0 2 4 6 8 10 12 Actuator Displacement (mm) Li ga m en t Lo ad ( N ) MCL Figure 2: Force-displacement response for the LCL, antero-lateral bundle of the PCL (aPCL), postero-lateral bundle of the ACL (pACL) (left) and MCL (right) at quasi-static loading (1/60 mm/sec) Material characterization of ligaments using non-contact strain measurement and digitization 185 NON-CONTACT DIGITIZATION AND STRAIN MEASUREMENT In this study, non contact optical methods have been used for ligament area and strain measurement. A three dimensional non-contact surface digitization system (ATOS: Advanced TOpometric Sensor) was used to digitize the ligament surface and obtain cross-sectional geometry. The principle of structured lighting is used to obtain explicit information about the object. The system has been widely used for digitization of non-biological objects for use in CAD and rapid prototyping (Capture 3d, 2002). A 3D measurement volume is first calibrated using calibration templates. Then a series of phase shifted fringe patterns are projected on the ligament and imaged using two CCD (Charged Coupled Devices) cameras to yield spatial coordinates of the surface. The specimen is then rotated and imaged again. Reference markers on the ligaments are used to ensure overlap between successive measurements. The schematic of the set-up is shown in Figure 3. The system yields detailed point cloud data, which can be processed to develop a solid model of the ligament. The measurement volume of the system ranges from 45 X 36 X 20 mm to 200 X 160 X 150 mm. The resolution of point spacing in the digitized model ranges from 0.03 mm to 0.15 mm. Fixture for ligament testing Area of the specimen viewed by the cameras Reference Markers Bone Cups Rotation direction CCD Cameras Computer