The safe establishment of a transseptal portal in the posterior knee

Purpose The aims of this study were as follows: (1) to determine the dimensions of the posterior compartments and the location of the popliteal artery with regard to the posterior septum by magnetic resonance arthrography; (2) to assess the effect of demographic factors on the measurements taken; and (3) to find a safe means of establishing the transseptal portal. Methods Thirty magnetic resonance arthrographies taken in 90 of flexion were evaluated to simulate knee position during arthroscopic surgery. Coronal and axial images were obtained in positions parallel and perpendicular to the long axis of the tibia, respectively. The anterior-to-posterior dimensions of the posteromedial and posterolateral compartments and popliteal artery locations were measured in the safe zone to establish transseptal portals. These dimensions were measured in axial images corresponding to the mid-PCL level. Results The average anterior-to-posterior dimension was 11.7 ± 2.9 mm in the posteromedial compartment versus 7.9 ± 1.8 mm in the posterolateral compartment. The anterior-to-posterior dimension was consistently greater in the posteromedial compartment, and the average ratio of the two dimensions was 1.5 ± 0.3. The average posterior deviation of the popliteal artery from the PCL was 27.1 ± 4.8 mm. The popliteal artery was always located lateral to the septum, and the lateral deviation on averaged measured 10.2 ± 4.0 mm. Conclusions The safe margin of the posteromedial compartment is consistently wider than that of the posterolateral compartment, and the popliteal artery is consistently located lateral to the posterior septum. Therefore, injury to the popliteal artery can be avoided when penetrating the septum lateral to medial direction, even if the penetrating stick slips posteriorly. Level of evidence Diagnostic study, Level IV.