• Alinejad, Mona
• Oconnor, Jj
• Murray, David
• Pegg, Elise Catherine

Abstract

It is essential when performing anterior cruciate ligament (ACL) reconstruction that the replacement ligament has sufficient mechanical properties to function during all activities. In order to mechanically test a synthetic ligament and assess how well it will function, it is necessary to know how the device will be loaded both axially and in torsion for different activities. <br/><br/>We have developed a novel method by which the 3D movement of ACL attachment sites can be calculated from standard MRI data; which can then, in turn, be processed to estimate torsional and tensional loading data. The 3D shape of an individual patient knee was determined from standard MRI images using image segmentation techniques. The knee was then moved into position for each degree of flexion (ranging from -10deg to 140deg). The positions were known from published interventional MRI data which has recorded bone positions of ten healthy patients during flexion when weight bearing. Finally, the locations of the attachment sites throughout flexion were determined and the torsional and tensional loading within the synthetic ligament calculated. The ligament data were analysed assuming that the synthetic ligament was implanted at 90deg of flexion without pre-tension.<br/><br/>The results demonstrated significant twisting during hyperextension of the synthetic ligament (maximum 95.5deg), which would result in an approximate torsional force of 3.9N and a torque of 0.01Nm. This correlates with previous work by Zavatsky et al. which examined the mechanics behind ligament twisting [1]. Maximum ligament elongation (3.46mm) was found at 50deg of flexion, which would represent axial loading of 336N. For the calculations the synthetic ligament was assumed to be comparable to the native ACL, with a stiffness of 97Nmm-1[2], diameter 6mm and shear modulus 0.00172GPa. <br/><br/>This study highlights the importance of considering both ligament twist as well as elongation when testing synthetic ligaments for ACL reconstruction. Some in vivo trials of synthetic ACL replacement devices have found fibre wear to be an issue; it is possible that cyclic torsional loading tests could have predicted these problems.<br/><br/>[1] Zavatsky et al. J Eng Med (1994) 208 p229<br/>[2] Hosseini et al. J Orthop Sci (2009) 14 p298

Topics

• impedance spectroscopy