Tunnel position in anterior cruciate ligament (ACL) reconstruction is critically important for successful surgery as it determines the orientation of the graft and its degree of anisometry. If the length of the graft changes significantly over the range of movement of the knee, the risk of graft failure is increased.
Previous cadaveric studies have investigated the anisometry of ACL grafts by reconstructing the ACL with various tunnel positions and measuring graft length changes during knee movement. This is limited by the number of tunnels that can be made in any given specimen. Using a 4D CT scan, hundreds of virtual femoral and tibial points can be generated to create thousands of virtual grafts. The aim of this project is to calculate the change in length between multiple virtual graft attachment points on the femur and tibia during knee movement and to define a safe zone for ACL graft placement that minimises graft anisometry.
6 fresh-frozen cadaveric knees were chosen. Knees were only included in the study if they had no surgical scars or deformities, and were stable on clinical testing of the cruciate and collateral ligaments. An arthroscopic examination was performed to evaluate the chondral surfaces and integrity of the ACL. Tantalum markers, used as reference points, were implanted into the femur and tibia. The knees were then attached to a machine that moved the joints through a range from 0-90 degrees, whilst the 4DCT scan was performed. The 4DCT takes multiple scans per second and was therefore able to capture the knees at several points during the range of movement.
Using the DICOM data, virtual points were created in and around the femoral and tibial ACL footprints. The coordinates of these points were extrapolated for each position of the knee recorded by the 4DCT. The distance from each femoral point to every tibial point was then calculated, and from this the variability of the length of the virtual graft during the range of movement was derived. The graft length variability was then plotted for each femoral and tibial virtual point and then overlaid onto the specimen’s CT scan. The grid by Bernard and Hertel was drawn over the plot to describe the positions of the femoral and tibial points.
The femoral points resulting in the least graft anisometry had an average depth of 30% in the shallow-deep direction and 10% in the high-low direction, according to Bernard-Hertel. On the tibial side, points within the ACL footprint did not affect the length variability of the graft by more than 5mm.
The degree of graft anisometry is more dependent on the position of the femoral tunnel than the tibial tunnel. The least anisomteric femoral points are mostly outside of the ACL footprint. Small changes in the position of the femoral tunnel can result in large changes in graft length variability. Siting the femoral tunnel deeper and more superior on the lateral wall results in a more isometric graft. Tibial tunnels placed more anteriorly also improves graft isometry.