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.