Elastographic Imaging of Acl-Bone Insertions in Vitro

The anterior cruciate ligament (ACL) functions as a mechanical stabilizer in the tibiofemoral joint. Over 250,000 Americans each year suffer ACL ruptures and tears, making the ACL the most commonly injured knee ligament. Methods which permit the in situ monitoring of changes in ACL graft mechanical properties during healing are needed. A long term goal in ACL reconstruction is to regenerate the ACL-bone interface. To this end, an understanding of mechanical properties of the ligament-bone interface is needed. However, experimental determination has been difficult due the small length scale (<1 mm) involved and limited resolution of standard techniques. The current study uses elastography to characterize the functional properties of the ACL and the ACLbone interface under applied load. In a first experiment, bovine joints were excised, cast in an agar gel matrix and externally compressed. In a second experiment, tibiofemoral joints were mounted on a MTS 858 Bionix Testing System. The ACL was loaded at different strain rates and tested to failure while RF data was collected at 5 MHz. For both tensile and compression testing, axial elastograms between successive RF frames were generated using cross-correlation and recorrelation techniques. When the ACL-bone complex was tested in the tibial alignment on the MTS system, compressive strains were found to dominate at the tibial insertion. Compressive strains were observed in the ligament proper when the transducer beam was aligned with respect to the insertion during loading. The distribution of tensile and compressive strain varied as a function of strain rate during testing and between loading and unloading. These preliminary results agree with those of prior FEA model predictions. In addition, a narrow band of high strain in the middle of the ACL was detected during compression that is considered to be a softer region of the ACL containing a highly collagenous structure. These preliminary results on ACL geometry and function indicate that elastography can provide important information in understanding the structure and function of both the ACL and the ACL-bone insertion. Ongoing studies focus on in-depth evaluation of the mechanical properties existing at the ACL and ACL-bone insertions.