Determinants of Femoral Tunnel Length in Anterior Cruciate Ligament Reconstruction: CT Analysis of the Influence of Tunnel Orientation on the Length

PURPOSE: The purpose of the study was to identify the femoral tunnel orientation that consistently results in a tunnel length of more than 35 mm in anterior cruciate ligament reconstruction. MATERIALS AND METHODS: Computed tomography (CT) scans were obtained from 30 patients who did not have any pathology in the distal femur. Virtual tunnels were marked on 3D (3-dimensional) reconstructed CT images after determining the orientation defined by sagittal inclination and axial angle. The length of a femoral tunnel in 64 different combinations of orientations (between 30° and 65° in 5° increments in two planes) was measured on 3D reconstructed images in both knees in 30 patients. Reliability of measurement was assessed with correlation coefficient for intra-observer and inter-observer measurements. A one-way analysis of variance (ANOVA) was used for statistical analysis. RESULTS: The mean femoral tunnel length varied significantly with each 10° change in orientation in both planes (p<0.05, ANOVA). A femoral tunnel of more than 35 mm in length could be obtained only with a sagittal inclination of more than 50° and axial angle of 30°-45°. When the axial angle was kept constant, the tunnel length increased with the increase in sagittal inclination. Higher sagittal inclinations consistently resulted in longer tunnels (r>0.9) and reduced the incidence of posterior cortical breakage. CONCLUSIONS: A tunnel orientation with an axial angle between 30°-45° and a sagittal inclination of more than 50° can result in a tunnel length of more than 35 mm.