Anterior cruciate ligament reconstruction and determination of tunnel size and graft obliquity

Increase in ACL (anterior cruciate ligament) reconstructions has led to a higher prevalence of patients with postoperative symptoms which require investigation. We aimed to investigate the utility of magnetic resonance imaging (MRI) and computer tomography (CT) in determining tunnel size and graft obliquity after single bundle ACL reconstruction.

A retrospective comparison was made on 29 symptomatic knees after anatomic single bundle (trans AM) and transtibial ACL reconstructions which had both MRI and CT scans at an average of 1.3 years postoperatively (2 months-5.7 years). We compared CT and MRI (T2 sequence) tunnel size and graft obliquity estimates using Pearson correlation and t-test. We also compared MRI's of ACL reconstructed knees with hamstrings or patellar autografts, which were confirmed by operative protocol as either antero-medial (AM) technique (n=21) or trans-tibial (TT) technique (n=19). The surgeries were performed for an average of 6.29 (4-10) years for the TT group and 1.3 (0-3) years for the AM group, respectively. The graft inclination was measured relative to the tibial plateau using DICOM software. Statistical analysis used the mean value for each case and the data were processed using the non-parametric Kruskal-Wallis test to determine the difference in graft obliquity and tunnel placement.

Tunnel size estimates correlate well between CT and MRI on axial scans: R2=0.795 and 0.630 for femur and tibia respectively. The position of the tunnels and graft obliquity were found to differ on MRI images in both coronal and sagittal planes. Coronal graft obliquity averaged 72.38° (ranging from 69° to 76°) using the AM technique and 75.47° (ranging from 72° to 78°) with TT technique. Sagittal graft inclination angle was 54.5 (51-58.5) and 63.68 (59-69.5) respectively. MRI proves to be the most useful imaging method in determining outcome after ACL reconstruction. However, for a better revision of the ACL reconstructions, CT can offer a clearer image of tunnels and bone stock. A more anatomical graft positioning increases obliquity in coronal and sagittal planes and, thus, becomes difficult to assess both tunnels in a single slice.

The anatomic single bundle reconstruction technique has been found to more accurately reproduce the femoral footprint and the orientation of the graft compared to the TT technique where the appropriate tibial tunnel placement resulted in a more vertical graft.