BackgroundKnee extensor structure disruption is sometimes combined with multi-ligament knee injury (MLKI). These cases could not be classified into the previous classification for knee dislocation or MLKI. To propose a new classification of MLKI that addresses both femorotibial and patellofemoral (PF) factors.MethodsThe present study involved patients diagnosed with MLKI at our institute. Cases with two or more injured structures diagnosed clinically and by MRI and X-ray examinations were included. Details of injured structures were defined as follows: ‘cruciate ligaments’, including the ACL and PCL; ‘collateral ligaments’, including the MCL and the posterolateral corner (PLC); and the ‘PF joint factor’, including the medial patellofemoral ligament (MPFL), quadriceps tendon, and patellar tendon. A total of 65 cases met the inclusion criteria from 2007 to August 2020 and were enrolled in this study.Based on the number of injured structures, cases were categorized as: Type A, two structures injured; Type B, three structures injured; Type C, four structures injured; and Type D, five structures injured. Depending on the combination of injured structures, they were subdivided into 1 to 5 in Types A and B and into 1 to 3 in Type C. Additional injuries were subdivided as follows: MLKI with fracture was defined as X, with neurovascular injury as Y, and both fracture and neurovascular injury as Z. MLKI cases were divided into both Schenck’s KD classification and the present classification.ResultsFourteen of 65 cases (21.5%) could not be categorized using Schenck’s KD classification; two cases of PLC+MPFL (Type-A5 in new classification), ACL+PLC with periarticular fracture (Type-A1-X), and single cases of ACL+MCL+PLC (Type-B2), ACL+PCL+MCL+PLC+MPFL (Type-D), ACL+MPFL (Type-A4), ACL+MCL+MPFL (Type-B3), PCL+PLC+ patellar tendon with periarticular fracture (Type-B3-X), and PCL+ patellar tendon (Type-A4), ACL+PLC with neurovascular injury (Type-A1-Y), and three cases of PCL+PLC with complications (Type-A1-X, -Y, -Z). With the new classification for MLKI and complications, all cases were successfully assigned to each category.ConclusionIn the present classification, every combination of MLKI cases containing both tibiofemoral and patellofemoral factors, with or without complications, was successfully classified into a suitable category.