The Blumensaat’s line morphology influences to the femoral tunnel position in anatomical ACL reconstruction

Purpose The purpose of this study was to reveal the influence of the morphological variations of the Blumensaat’s line on femoral tunnel position in anatomical anterior cruciate ligament (ACL) reconstruction. Methods Thirty-eight subjects undergoing anatomical single-bundle ACL reconstruction were included in this study (22 female, 16 male: median age 45: 15–63). Using a trans-portal technique, the femoral tunnel was targeted to reproduce the center of antero-medial bundle. Following Iriuchishima’s classification, the morphology of the Blumensaat’s line was classified into straight and hill types (small and large hill types). Femoral ACL tunnel position was evaluated using the quadrant method. When the quadrant method grid was applied, the baseline of the grid was matched to the anterior part of the Blumensaat’s line, without considering the existence of a hill. Using pre-operative 3D-CT data, the axial and sagittal morphology of the knee was also compared, establlishing straight and hill types. Results There were 12 straight type knees and 26 hill type knees (7 small hill type knees and 19 large hill type knees). The femoral tunnel position in straight type knees was 23.6 ± 3.7% in the shallow–deep direction, and 41.3 ± 8.2% in the high–low direction. In hill type knees, the tunnel position was 27 ± 4.7% in the shallow–deep direction, and 51 ± 10.1% in the high–low direction. The femoral tunnel was placed significantly more shallow and lower in hill type knees when compared with straight type knees. Conclusion Femoral ACL tunnel placement was significantly influenced by the morphological variations of the Blumensaat’s line. As detecting morphological variation in arthroscopic surgery is difficult, surgeons should confirm such variations pre-operatively using radiograph or CT so as to avoid making extremely shallow and low tunnels in hill type knees. Level of evidence Case-controlled study, III.