Summary
We describe the novel use of Digital image correlation (DIC) an optical non-contact industrial technique, to provide detailed multi-dimensional (unlike linear strain gage) surface strain field mapping of the native ACL undergoing cyclic loading, and found that strain increased with cycling especially in the midportion of the anteromedial bundle.
Abstract
Introduction
Digital image correlation (DIC) is an optical non-contact technique used in industrial materials testing used to obtain shape, displacement and surface strain measurements of loaded objects, and has been utilised for the assessment of some biologic tissues such as blood vessels. In this technique, unique speckle pattern is created on the test object surface, and digital images are successively acquired during testing which allows the calculation of strain in multiple dimensions, unlike linear strain gauges which need to be carefully embedded or attached to tissue. We describe the first use of this technology to provide detailed strain field mapping of the native ACL undergoing cyclic loading.
Methods
For this study, 20 adolescent porcine stifle joints were meticulously prepared. All non-ACL ligaments were removed, and a small osteotomy of the medial condyle of the femur was performed to expose the ACL, ensuring optimal viewing and accurate results. To enhance tissue registration for Digital Image Correlation (DIC), the ACLs were painted white and covered with an irregular speckled pattern in black. The first and final cycles of each test were isolated to assess the progression of damage within the ACL. Testing was recorded with a commercial off-the-shelf Android smartphone, and the footage was processed using HitFilm. The images were subsequently analyzed using MATLAB and Ncorr DIC software to determine strain and displacement patterns. Each sample underwent cyclic tensile testing for 100 cycles, with 10 samples subjected to 300N and 10 samples to 600N, both well below the mean ultimate load of 1266±250N. The displacement rate was maintained at 2.00mm/s throughout the testing. Force and displacement were recorded using an MTS System tensile loading machine.
Results
During the cyclic tensile tests, tension was increased to the set point and then reversed to zero, not necessarily returning to the original starting position. Initial cycles at 300N required a displacement of 4.16±1.45mm, while those at 600N required 8.17±2.14mm. In the final cycles, the 300N group exhibited a displacement of 2.65±1.70mm, while the 600N group showed a displacement of 7.30±6.92mm. These results highlight the behavior of the entire ACL during cyclic loading. The highest relative localized yy-strain along the loading axis was 0.208±0.204 for the first cycle at 300N and 0.364±0.097 for the final cycle. At 600N, the yy-strain increased from 1.013±0.609 in the first cycle to 1.509±1.382 in the final cycle. The strain data consistently indicated that the highest strain occurred in the anteromedial bundle of the ACL, near the midline, a finding consistent with previous clinical observations of ACL failure.
SIGNIFICANCE/CLINICAL RELEVANCE: DIC was able to effectively assess surface strain fields of the ACL in multiple dimensions. DIC can provide detailed information non-invasively and can map out detailed strain behavior over an entire surface in multiple directions, replacing thousands of strain gauges. The continued validation of DIC against clinically expected behavior for high-strain biological tissues can significantly enhance material characterization techniques in medical applications. This study’s findings underscore the potential of DIC techniques to assess strain behavior of the ACL, and also the adolescent porcine stifle joint as a reliable model for ACL research.