The natural orientation of the Anterior Cruciate Ligament compared to the tibial plateau on magnetic resonance imaging scans

Purpose: Remnant preserving Anterior Cruciate Ligament (ACL) reconstruction requires the graft to be passed through the remnant ACL stump. This requires knowledge of the native alignment of the ACL. We investigated the alignment and orientation compared directly to the tibial plateau on Magnetic Resonance Imaging. We assessed if current equipment is adequate for anatomical reconstruction. Methods: 50 MRI scans with intact ACL and PCL were reviewed. Measurements were taken of; a) angle of ACL and tibial plateau in the sagittal plane, b) angle of ACL and tibial plateau in the coronal plane, c) position of ACL insertion into tibia as percentage of anterior-posterior width, d) position of ACL insertion into tibia as percentage of medio-lateral width. Statistical analysis included intra and interobserver agreement. Commonly used tibial guides were assessed for range of angles possible in use. Results: The mean results for each measurement (+/standard deviation) were; a) 45 degrees(4.4 degrees), b) 69 degrees(5.5 degrees), c) 37.2%(5.5%) d) 47.4%(1.5%). Intra-observer and inter-observer reliability were measured with satisfactory values. Not all tibial guides were able to produce an angle <45 degrees. Conclusion: Our study is the first comparing the angle of inclination of the ACL directly with the tibial plateau. We demonstrate the angle in the sagittal plane is lower than reported previously. This data can be used in planning the tibial tunnel using remnant preserving techniques, to orientate in the natural alignment of the ACL. Not all reconstruction systems include options to align at the lower than previously thought natural angle of the ACL.