## 2 Materials and Methods  
Recordings of 614 knee arthroscopies performed in the (blinded for review) clinic between April 2015 to March 2020 were retrospectively evaluated for this study. A total of 38 patients (11 females, 27 males; mean age 37.0 years, SD = 12.1, range 17-53) were operated on due to clinically significant posterior knee instability and were included in the study group. The indications for arthroscopy were symptomatic posterior knee instability with at least 5 mm of increased posterior tibia translation in physical examination and at least 6 months post-injury. Preoperative MRI was performed in all cases. However, MRI appearance of PCL in cases of chronic PCL injury was not used as inclusion criteria due to the fact that, as reported in the literature, sometimes PCL tears heal in unfunctional elongation even though their continuity is maintained [1; 4; 5]. The remaining 576 patients had an intact PCL (203 females, 373 males; mean age 43.7 years, SD = 14.9, range 14-81). The PCL was observed for the presence of an \"PCL envelope\" as a part of our standard arthroscopic inspection protocol. In cases of a suspected chronic PCL tear, we assessed for the presence of the PCL envelope lack sign (PELS). Inclusion criteria was the possibility to evaluate PELS presence on recordings. Within the group of patients with intact PCL, it was impossible to evaluate the presence of the PCL \"envelope\" in 22 cases (9 females, 13 males; mean age 63.9 years, SD = 11.0, range 24-74) due to large osteophytes on the medial femoral couple or medial tibial eminence with no indications to extend the surgery by partial medial tibial eminence resection. Therefore, 554 PCL-intact patients (194 females, 360 males; mean age 42.9 years, SD = 14.4, range 14-81) were included in the control group. All recordings were analyzed by a single author with more than 20 years of experience in knee mini-invasive and arthroscopic surgery. In 2013, during arthroscopy in PCL-efficient knees, we started to observe an anatomic sign that we called a \"PCL envelope\", defined as the \"soft tissue cuff coursing around the PCL tibial attachment, visible with arthroscope positioned between the PCL, medial femoral condyle and posterior horn of medial meniscus at the level of its shiny white fibers\" (Figure 1). The shiny white fibers of the posterior horn of medial meniscus are important arthroscopic landmark in PCL surgeries [3; 9]. To visualize the PCL envelope in a PCL-intact knee, we performed the arthroscopy through standard anterolateral and anteromedial portals using a 30\({}^{\circ}\) arthroscope. After a thorough inspection of the whole joint, the PCL was assessed anteriorly and then the arthroscope was positioned between the PCL, MFC, and over the posterior horn of the medial meniscus (MM) and inserted into the posteromedial recess using a trans-notch maneuver. Initially, visualization was directed radially toward the \"ramp\" of the MM. Then, the arthroscope was gradually withdrawn with rotational movements until the moment when the MFC and posterior root of MM and its shiny white fibers were observed. In this position, a soft tissue cuff coursing around the PCL tibial attachment could be observed (Figure 1a), which was the aforementioned PCL envelope. Next, the arthroscope was withdrawn a few millimeters more to visualize the PCL tibial attachment and the soft tissue envelope surrounding this attachment was observed (Figure 1b). Then, we started to analyze this finding in knees with a suspected chronic PCL insufficiency, concluding that PCL envelope is something that usually cannot be observed in PCL-deficient knees. In the case of suspected chronic PCL insufficiency, when the arthroscope was positioned in the place where the PCL envelope could normally be visualized, it was not visible. Instead, phenomenon that we called the \"PCL envelope lack sign\" (PELS) could be seen, which we theorized could be a direct sign of chronic PCL insufficiency.